Some Basic Understandings About Fracturing

Section 1: Fracturing Equipment

1. Fracturing truck

The fracturing truck is the main equipment for fracturing. Its function is to inject high-pressure and large-displacement fracturing fluid into the well, to open the formation and squeeze the proppant into the fracture. The fracturing truck is mainly composed of four parts: carrying, power, transmission and pump body. The fracturing pump is the working host of the fracturing truck. Field construction requires the high technical performance of the fracturing truck. The fracturing truck must have high pressure, large displacement, corrosion resistance and wear resistance.

2. Blender truck

The function of the sand mixing truck is to mix sand according to a certain proportion and procedure, and supply the sand mixing liquid to the fracturing truck. Its structure is mainly composed of three parts : transmission, liquid supply and sand conveying system.

3. Balanced car

The function of the balance car is to keep the pressure difference between the upper and lower packers within a certain range, and to protect the packer and casing. In addition, when sand plugging, sand jam and other accidents occur in the construction, the balance vehicle can also immediately carry out backwash or back pressure well to eliminate the fault.

4. Instrument car

The function of the instrument car is to remotely control the fracturing car and the sand mixing car in the fracturing construction, collect and display the construction parameters, carry out real-time data acquisition, construction monitoring and crack simulation, and analyze the whole process of construction.

5. Manifold

The role of the manifold is to transport the manifold, such as: high-pressure three-way, four-way, single-flow valve, control valve, etc.

Section 2: the basic procedure of fracturing construction

1. Cycle

The fracturing fluid is driven from the tank truck to the fracturing truck and then returned to the tank truck. The cycle route is liquid tank truck-mixed sand truck-fracturing pump-high pressure manifold-liquid tank truck, which aims to check the water supply of fracturing pump and the connection of pipeline. During the cycle, it should be carried out by car and gear, and the normal discharge of the outlet is qualified.

2. Pressure test

Close the main gate of the wellhead and pressurize the ground high-pressure pipelines, wellheads, connecting threads, oil pipes, etc. to 30-40Mpa. If there is no puncture or leakage for 2-3 minutes, it is considered qualified.

3． Trial squeeze

After passing the pressure test, open the main gate and use 1-2 fracturing trucks to squeeze the reagent liquid into the oil layer until the pressure is stable. The purpose is to check whether the downhole pipe string and downhole tools are normal, and to understand the water absorption capacity of oil and water.

4． fracture

After the test extrusion pressure and displacement are stable, all vehicles are started to inject fracturing fluid into the well at the same time, causing the bottom hole pressure to rise rapidly. When the bottom hole pressure exceeds the formation fracture pressure, fractures will form in the formation.

5. Proppant

The sand mixing ratio should be small at first. When it is judged that the sand has entered the cracks, the sand mixing ratio should be increased accordingly.

6． Substitute

After the expected sand addition is completed, the displacing fluid is immediately pumped in to displace all the sand-carrying fluid in the surface pipelines and wellbore into the cracks to prevent residual sand from accumulating at the bottom of the well to form sand stuck.

7． Backwash or moving column

Immediately after replacement, backwash the well or move the pipe string to prevent residual sand from remaining within the wellbore packer clamping distance, causing sand stuck.

Section 3： Principles of Fracturing Fluid

The essence of fracturing is to use a high-pressure pump unit to inject liquid with a certain viscosity into the formation at high speed. When the injection speed of the pump is greater than the absorption speed of the formation, the formation will crack or the original tiny gaps will open to form larger cracks. As the liquid continues to be injected, the cracks that have formed extend inwards. In order to prevent the crack from re-closing under the gravity of the upper rock layer after the pump is stopped, proppant should be added to the injected liquid so that the proppant can be filled in the opened crack to support the fracture surface.

According to the role of fracturing fluid at different stages in the fracturing process, it can be divided into pre-fluid, sand-carrying fluid and displacement fluid.

1. Pre-fluid

The function of the pre-position fluid is to break the formation and create cracks of a certain geometric size to prepare for the entry of subsequent sand-carrying fluid. In strata with higher temperatures, it can also have a certain cooling effect.

2. Sand carrying liquid

The role of the sand-carrying liquid is to bring the ground proppant into the fracture and carry it to a predetermined position in the fracture. It also has the function of extending the fracture and cooling the formation.

3. Replacement fluid

The function of the displacement fluid is to send the sand-carrying fluid to a predetermined location and replace all the sand-carrying fluid in the wellbore into the fractures.

4． Proppant

Proppant refers to the substance that is brought into the fracture with fracturing fluid and used to support the fracture after the pressure is released.

5. Destroying agent

Destroying agents include gel breakers, demulsifiers, viscosity reducers, etc. Gel breaker is used to destroy the cross-linked structure of gel. Demulsifiers are used to destroy the stability of emulsions, and viscosity reducers are used to reduce the viscosity of thickened liquids.

6． Drag reducer

Drag reducers reduce the flow friction of fracturing fluids by reducing turbulence and energy loss during flow.

7． Fluid loss agent

It is used to reduce the leakage of fracturing fluid from fractures to the formation, thereby reducing the contamination of the formation by the fracturing fluid and causing the pressure to increase rapidly during fracturing.

8. Anti-emulsifier

Prevent the formation of emulsion between crude oil and fracturing fluid. Can be divided into surfactants and mutual solvents.

9. Clay Stabilizer:

Stabilizes clay

10． Drainage aid

Divided into two types: surfactants and energizers. The former can effectively reduce the interfacial tension and make the residual liquid easily discharged from the formation. The latter injects a slug of energizing agent into the formation before acid injection to increase the pressure near the well and make the residual liquid easily discharged from the formation.

11. Wetting reversal agent

Divided into two categories: surfactants and mutual solvents. The surfactant can adsorb the second adsorption layer on the surface of the formation according to similar polarity rules and play a role in wetting reversal. The mutual solvent can desorb the corrosion inhibitor adsorbed on the surface of the formation and restore the hydrophilicity of the formation surface.

Section 4： Basic knowledge of fracturing

1. What is hydraulic fracturing of oil reservoirs?

The method of using hydraulic pressure to form cracks in the oil layer buried deep in the formation is called oil layer hydraulic fracturing.

2. What is the basic principle of hydraulic fracturing of oil reservoirs?

Hydraulic fracturing of oil reservoirs generally refers to the use of the principle of liquid pressure transmission, using high-pressure and large-displacement pumps on the ground to inject liquid with a certain viscosity into the oil reservoir at a level greater than the capacity of the oil layer to absorb, so that the wellbore pressure gradually increases. When the pressure When the pressure is greater than the pressure required for the oil layer to rupture, one or several horizontal or vertical cracks will form in the oil layer. After the crack is formed, with the continuous injection of liquid, the crack will continue to extend and expand until the speed of liquid injection is equal to the speed that the oil layer can absorb. At this time, if the external force is removed, the crack will close again. In order to keep the fracture in an open state, a certain proportion of solid particles with higher strength are mixed into the fracturing fluid as proppant to support the fracture. Because the support is strictly screened, it has good granularity and strength and is deposited in the fractures, changing the conductivity of the formation near the wellbore, thereby reducing the resistance of liquid flowing from the formation into the wellbore.

3. What is the purpose of hydraulic fracturing of oil reservoirs?

The purpose of hydraulic fracturing of an oil layer is to transform the physical structure of the oil layer and artificially form one or several channels with high permeability in the oil layer to reduce the flow resistance near the well, increase the seepage capacity, and increase the production of the oil well.

4. What are the functions of hydraulic fracturing of oil reservoirs?

Hydraulic fracturing of oil reservoirs has the following effects:

① Remove oil layer pollution and blockage caused by killing fluid during drilling or workover.

② Improve the intra-layer contradictions caused by uneven permeability up and down the thick oil layer.

③ Improve the permeability of low-permeability oil layers, adjust the interlayer and plane conflicts of oil wells, and improve development results;

④ Expand and communicate the original fractures and channels of the oil layer to improve the oil production capacity of the oil well and the water absorption capacity of the water injection well.

5. What are the types of oil reservoir cracks formed by hydraulic fracturing?

The cracks formed after hydraulic fracturing of oil layers can generally be summarized into two basic types, namely horizontal cracks and vertical cracks. The cracks parallel to the oil layer are called horizontal fractures, and the cracks perpendicular to the oil layer are called vertical fractures.

6. How many types of fracturing fluid are there?

The liquid squeezed into the well during the fracturing process is collectively called fracturing fluid. There are many types of fracturing fluids, which can be roughly divided into two categories: oil-based fracturing fluids and water-based fracturing fluids. Oil-based fracturing fluids usually use crude oil or refined oil as the base fluid; water-based fracturing fluids usually use clean water, sesbania gum, seaweed gum, thickened water, and living oil-containing emulsions.

7. What basic properties should fracturing fluid have?

The fracturing fluid plays a role in transmitting pressure, splitting fractures, and carrying proppant into the oil layer. Therefore, it must have the following five basic properties:

① The filter loss is small and difficult to leak into the oil layer, which is conducive to joint creation;

② The friction resistance is small to reduce the power loss of the equipment;

③ It has good suspension ability and can carry a large proportion of proppant into the oil layer;

④ Does not react chemically with the oil layer, is easy to flow back after fracturing, and does not pollute or block the oil layer;

⑤ Convenient sources of materials, simple preparation and low cost.

8. What is proppant? How many types are there?

The solid particles used to support and fill oil reservoir fractures during fracturing are called proppants. Commonly used proppants include: quartz sand, ceramsite, glass balls and other types.

9. Which parts of fracturing fluid are used in fracturing construction? What is the role of each?

During the fracturing process, the fracturing fluid is divided into three parts: pre-fill fluid, sand-carrying fluid, and displacement fluid:

① Pre-fluid is generally the amount of liquid used before adding sand, including the entire amount of liquid used in the two processes of trial extrusion and fracturing during fracturing construction. The task of the pre-positioning fluid is to build up the bottom hole pressure, gradually reach the fracture pressure of the formation, and press the formation to open fractures.

② Sand-carrying liquid, as the name implies, refers to the total amount of liquid required to transport proppant to the fractures of the oil layer.

③Displacing fluid is the total amount of liquid used to replace the sand-carrying fluid entering the cracks of the oil layer in the final stage of construction.

10. What are the common basic methods for hydraulic fracturing of oil and water wells?

Hydraulic fracturing of oil and water wells is divided into two basic methods: ordinary fracturing and selective fracturing.

① Ordinary fracturing can be divided into: casing fracturing, smooth tubing fracturing, annular fracturing, and annular and tubing mixed fracturing due to different fluid injection channels.

②Selective fracturing is when the thickness of the oil layer is large or there are many layers, the oil layer with a large difference in permeability between layers is fracturing in layers with a packer or a temporary plugging agent is used to block the high permeability layer at a certain displacement The blast holes will open the pre-pressure laminate.

11. What investigations should be done on oil and water wells before fracturing construction?

Before fracturing construction, the construction wells must be investigated according to the well numbers and plans provided by the geological design. The purpose is to have a comprehensive understanding of the current status of the oil and water wells and provide a basis for construction. The contents of the investigation generally include:

① The road conditions that can be used by fracturing vehicles;

② Whether the well site can accommodate the required fracturing vehicles;

③ Check the model of the Christmas tree to see whether it can withstand the maximum pump pressure during fracturing operations and whether it can be matched with the snubbing operation device;

④ Check the structure of the pipe string in the well and whether there are any falling objects;

⑤ Check whether the casing is in good condition and whether downhole tools for layered fracturing can be run.

12. What preparations should the operation team make after receiving the well number for fracturing construction?

When the operation team receives the construction design, they must prepare item by item for the construction requirements. Generally, there are 8 preparations that must be done:

① Prepare to deepen the oil pipe to deepen the sand exploration surface of the original well string;

② Prepare special flanges to match various Christmas trees for snubbing operations;

③ Prepare sand blasting tools and water hoses, and carry out sand blasting operations when sand blasting is required;

④ Prepare fishing tools and salvage when it is necessary to salvage fallen objects;

⑤ Check whether the original well string meets the fracturing requirements, and replace the entire well string if necessary;

⑥ Prepare wellhead ball valves, ball throwers and steel balls required for layered fracturing;

⑦ Prepare enough clean water for balancing, sand washing and backwashing;

⑧ Prepare manifold accessories, including various oil pipes, elbows, large and small heads, etc.

13. Which oil pipes cannot be used as fracturing strings?

Ordinary oil pipes that are bent, deformed, cracked, or have blisters cannot be used for fracturing. Ordinary oil pipes that have been acidified or corroded by other reasons cannot be used for fracturing. Glass-lined oil pipes and paint oil pipes cannot be used for fracturing. Threaded oil pipes cannot be used for fracturing. Damaged and old oil pipes cannot be used for fracturing.

14. Why is it necessary to inspect the string first before fracturing oil and water wells?

Because the wellbore must withstand high pressure during the fracturing process, if the cementing quality is not good and cross-grooving occurs, the high-pressure sand-carrying liquid will form cracks in non-target layers, which may easily cause the interlayer to be opened or the water layer to be opened. The unfavorable situation of being pressed open. Therefore, it is necessary to check the situation of any oil layer with string groove display so that measures can be taken to seal it.

15. What are the requirements for well site layout for fracturing construction?

The well site layout for fracturing construction should be divided into four areas, namely, high-pressure area, low-pressure area, wellhead area and auxiliary area. The wellsite layout should be centered around the high-pressure area. First, the location of the high-pressure manifold should be determined and should be as far away from the wellhead area as possible.

16. What are the requirements for high-pressure manifolds during fracturing construction?

No matter which manifold is used in fracturing construction, the following requirements must be met:

① Must be able to withstand high pressure 1.5-2.0 times higher than the highest rupture pressure in the region;

② The connecting part between the manifold and each fracturing vehicle must be equipped with a gate valve or one-way valve to facilitate single-vehicle control;

③ Right-angle elbows are not allowed on the manifold;

④ There should be enough interfaces on the manifold so that after the fracturing truck is filled, high-pressure pipeline venting and other operations can be carried out;

⑤ All valves on the manifold must be flexible and easy to open and close quickly.

17. What are the basic processes of fracturing construction?

Although there are many methods of fracturing construction, the basic process is the same, which can be roughly divided into seven steps: circulation, pressure test, test extrusion, fracturing, sand addition, replacement extrusion, movable pipe string or backwash.

18. Why is there a recycling process during fracturing construction?

Because before fracturing construction, we must first check the working performance of each equipment to see whether the water supply of the pump is good and whether the manifold is smooth. At the same time, the fracturing fluid in the storage tank must be stirred to make its viscosity and temperature uniform. . During winter construction, it is also necessary to check whether pipelines or gate valves are frozen, so circulation is a must.

19. Why is it necessary to test the pressure of ground pipelines before fracturing construction?

Because each stage of fracturing construction is carried out continuously, there can be no pauses in the middle. Therefore, it is required that all connecting parts of the pipeline must be firmly connected without puncture or leakage, in order to ensure the safe operation of the pipeline under the conditions of burst pressure, so the pressure test of the ground pipeline must be carried out.

20. What is the purpose of trial extrusion during fracturing construction?

The purpose of test extrusion is to check whether various parts of the downhole pipe string, such as packers, sandblasters, hydraulic anchors, etc., are working properly. Check whether the pipe string is lowered into the correct position. More importantly, the fluid absorption capacity of the formation can be understood through test extrusion of the oil layer in order to estimate the highest fracture pressure.

21. How to judge whether cracks have formed in the oil layer during fracturing construction?

① Determine whether cracks form in the oil layer based on changes in pressure and displacement: During fracturing, as the pressure increases, the displacement also increases, and there is a certain ratio between the two. But when cracks form, the original ratio between them is broken. At this time, two situations may occur: First, the pump pressure drops rapidly and the displacement increases. Second, the pressure remains unchanged and the displacement increases. At this time, it can be judged that the oil layer has been pressed to open cracks.

② Determine whether cracks have formed in the oil layer based on changes in mechanical equipment: Before cracks form in the oil layer, due to the relatively high pressure, the diesel engine on the fracturing truck has a heavy load, making a heavy sound and emitting thick smoke. It can be observed from the sand tank of the sand mixer that the liquid displacement is not large and the liquid churning in the sand tank is also small. However, once a crack forms, the diesel engine will immediately change its sound, and the liquid displacement in the sand tank will suddenly increase, causing a large churning spray. At this time, it is judged that the oil layer has been pressed to open cracks.

③ Use changes in the formation water absorption index to determine whether the oil layer has been cracked: When no cracks are formed in the formation, since the permeability surface and permeability of the formation itself are fixed, the water absorption index is also a fixed value. When the oil layer is cracked, new crack surfaces appear, which increases the fluid absorption capacity of the formation. At this time, the water absorption index increases significantly, and it is judged that the oil layer has been cracked.

22. What issues should be paid attention to during the sand-adding process?

Adding sand is a key process for the success of fracturing. Before adding sand, you should quickly check whether the working conditions of each vehicle are normal, and whether there are any problems with the pipelines and wellhead equipment. If any problems are found, you must stop and deal with them before adding sand.

When adding sand, the amount should be small first and then large. The sand mixing ratio can be controlled at about 5-7% at the beginning. At this time, it can be further verified whether cracks are really formed in the oil layer. If the oil layer does not fracture, sand will deposit in the wellbore causing pressure to rise. If this happens, stop adding sand immediately and perform venting and backwashing.

During the sand-adding process, the sand content ratio should be gradually increased to maintain uniform sand adding. It is strictly forbidden to downshift and stop the pump at this time.

23. Why does it need to be replaced after fracturing construction?

In order to squeeze all the sand-carrying liquid into the cracks of the oil layer, replacement squeezing is required. The amount of displacing fluid is very critical. Excessive displacing fluid will push the proppant deep into the formation and close the formation fractures near the wellbore, thereby affecting the fracturing effect. Insufficient squeezing will cause proppant to remain in the wellbore, easily causing sand plugging accidents. Therefore, after adding sand during fracturing construction, an appropriate amount of replacement must be carried out.

24. Why should the well or movable pipe string be backwashed immediately after fracturing?

Most of the current fracturing processes use layered pressure-selective strings based on packers. After the construction is completed, some proppant will inevitably remain in the wellbore, which will often cause deposition between the two-stage packer clips. If not eliminated in time, it will easily lead to pipe string accidents. Therefore, after extrusion, the pipe string must be moved quickly to allow the packer to contract and release the sand blockage or the backwash well to wash up the settled sand to prevent sand from blocking the pipe string.

25. Why does it sometimes fail to open?

There are several reasons why the pressure cannot be opened:

① There is an error in the setting position of the pipe string, and the packer is stuck in the unperforated well section;

② The perforation quality is poor, the oil well is imperfect, and the formation resistance to liquid absorption is large;

③ There is a blockage in the oil pipe or ground pipeline, causing obstruction;

④ The formation permeability is poor and the liquid absorption capacity is too low;

⑤ During the drilling process, mud contaminates the oil layer, and the mud cake blocks the formation channels;

⑥ Downhole tools fail.

26. Why does the pressure sometimes rise suddenly during fracturing? How to deal with this situation?

The pressure was normal at the beginning of fracturing, but the pressure surged after adding sand. The main reason was that the sand was added unevenly, causing sand blockage in the pipe string. Or it is caused by blockage caused by too high sand mixing ratio. It is also caused by the accumulation of impurities near the blasthole or the fibrous items in the fracturing fluid blocking the pores of the oil layer. After this happens, you should immediately stop adding sand, reduce the displacement, and let the pump pressure drop to within the safe load first, but do not stop the entire pump to avoid more serious sand blocking accidents. At this time, we need to further observe the changes in pump pressure and displacement. If the sand blockage is relieved, we can consider continuing to add sand.

27. Why does the casing pressure sometimes rise suddenly during fracturing construction?

The sudden rise in casing pressure during fracturing construction is due to the following reasons:

① The opening pressure of the sandblaster and packer does not match well;

② There are string grooves outside the casing;

③ Oil pipe leakage;

④ The upper packer is damaged.

28. Why does the pressure sometimes drop suddenly during fracturing?

During the fracturing process, when the wellhead pressure rises to a certain height, the pressure suddenly drops significantly. The cause may be due to casing damage, packer failure or string troughing. At this time, the machine should be stopped immediately for on-site analysis. At the same time, there should be a rise in casing pressure; if the packer is damaged or the lower oil pipe is disconnected, there should be a phenomenon of the pipe string rising at the same time.

29. What is the reason for the sudden drop in displacement during fracturing?

The reasons for sudden decrease in displacement during fracturing construction are:

① The water valve packing on the fracturing pump is damaged;

② The water supply line is blocked;

③ The fracturing fluid storage tank level is too low;

④ Evacuate the fracturing pump.

30. How to deal with pipeline leakage during fracturing?

When encountering leakage in fracturing pipelines, you must handle it with caution as appropriate. If a leak occurs in the connecting pipeline between a certain fracturing truck and the high-pressure manifold, the truck can be stopped, and the gate on the high-pressure manifold can be closed before processing while reducing the sand ratio. If there is serious leakage at the wellhead, etc., you should stop adding sand for backwashing, and then proceed with the treatment after removing the sand in the well. In short, the treatment of pipeline leakage must be carried out while ensuring that the well is not blocked by sand.

31. What are the requirements for packers in layered fracturing strings?

The structural requirements for the fracturing packer are: high pressure resistance, sand jam prevention, good sealing performance, backwash capability, strong adaptability to casing, and easy operation.

32. How to inspect the fracturing packer before going down the well?

Before running the fracturing packer down the well, mainly check whether the threads at both ends are intact, whether the surface of the rubber barrel is damaged, and whether the packer number matches the certificate. If problems are found, the packer cannot be used downhole.

33. What is the structure and working principle of the spring-type sliding sleeve sandblaster?

A spring is installed between the upper and lower joints of the sandblaster to generate a pre-pressure on the valve. The size of the pre-pressure can be controlled by an adjusting ring. When the oil pipe is filled with high-pressure liquid, the fracturing fluid enters the inner cavity of the valve seat through the hole in the central tube. As the pressure increases, the valve can be pushed upward to separate the valve from the valve seat. At this time, the fracturing fluid can be sprayed out from around Ver and injected into the oil layer. Due to the action of the constant pressure spring, the valve will only open when the pressure reaches a certain value. When the pressure of the fracturing fluid drops to a certain value, the valve will close itself under the action of the constant pressure spring.

During multi-stage layered fracturing, sliding sleeves with different diameters need to be installed on the sandblaster using fixed pins (the lowest stage sandblaster does not have sliding sleeves). When the first layer of fracturing is completed, a steel ball with a diameter matching the sliding sleeve is thrown into the well. Under the action of the fracturing fluid, the steel ball will sit on the sliding sleeve and hold up high pressure, shearing the fixing pin, and causing the upper The sliding sleeve of the first-level sandblaster falls into the next-level sandblaster, causing it to close the liquid outlet channel. In this way, the second stage sandblaster will start working like the first stage sandblaster. Repeating this can achieve multi-layer continuous fracturing.

34. What are the common methods for dealing with fracturing sand plugging accidents?

There are four commonly used methods to deal with fracturing sand plugging accidents:

① Smoothly squeeze the cracking liquid to remove the blockage: When the pump pressure increases sharply and the displacement drops, and it is confirmed that there is a sand blockage, you should immediately stop adding sand and use a large-displacement method to steadily squeeze the cracking liquid to remove the blockage. Wait until it normalizes. Continue to add sand;

② Backwashing to remove blockage: After sand blockage occurs, immediately stop adding sand and perform backwashing. After flushing out all the sand in the pipe string, try adding sand again and continue fracturing;

③Water hammer unblocking: Water hammer unblocking uses a small number of fracturing trucks to pressurize the oil pipe positively within a safe pressure range, and then releases the pressure violently, forming a water hammer phenomenon, resulting in shock and vibration. Through the reciprocating action of positive impact and reverse vibration, the impact or vibration of the sand settling section releases the sand blockage.

④ Backwashing of movable pipe string to remove blockage: This method combines movable pipe string with backwash well to relieve sand blockage. When the sand blockage is severe, the fracturing string is quickly moved to remove the sand settling in the annular space of the oil casing. Then, backwash or water hammering method is used to resolve the sand settling in the oil pipe.

35. How to prevent fracturing sand plugging accidents?

The most important thing to prevent fracturing sand plugging accidents is to do the following tasks on the ground:

① Strict quality control is required for downhole tools from processing and assembly, pressure testing to delivery to the site;

② When running downhole pipe strings, "three measurements" and "three matchings" must be achieved. In particular, the stratified fracturing string of the lower packer must be stuck accurately;

③ The high-pressure manifolds on the ground, wellhead controllers, pitchers, ball gates, and high-pressure oil pumps on pipelines must be regularly inspected and maintained in a timely manner to ensure safety, reliability, and flexible operation;

④ Various containers for storing fracturing fluid must be cleaned regularly;

⑤ The proppant must be clean and free of impurities to prevent mud, sand and other dirt from being mixed in;

⑥ Fracturing equipment must be maintained regularly and inspected in time. Sick equipment cannot be put into the well and incorporated into the vehicle crew for risky construction;

⑦ Fracturing commanders must use equipment rationally in strict accordance with construction design requirements, make accurate judgments, and command appropriately. For special wells, preparations must be made in advance to prevent sand plugging accidents.

36. Under what circumstances can down-stroke be used to release jams to deal with sand blockage?

After the sand jam occurs, if the movable pipe string is ineffective, the downstroke method can be used to deal with it. When handling, please note that if the pipe string does not move, it is best to use a 24-meter-high derrick when lowering, use a larger diameter drill pipe as a heavy object, and use a sliding rod centralizer to connect the heavy object to the stuck pipe string. Then lift the drill and smash it down to destroy the sand bridge and release the sand jam. Although this method has a better effect of unblocking the jam, it should be noted that when there is oil and gas leakage at the wellhead, this method cannot be used to unblock the jam to prevent fire.

37. What are the reasons for sand plugging and sand jam caused by fracturing?

The main reasons for sand plugging and sand jam caused by fracturing are:

① The poor sand suspension performance of the fracturing fluid causes sand to precipitate;

② The sand content ratio of the fracturing fluid is too high;

③ The pump is stopped for too long during fracturing;

④ The clamping distance of the pipe string is too large or a failure occurs, resulting in a large amount of sand settling;

⑤ Insufficient replacement fluid.

38. How should the pipe string be moved when sand is jammed?

After fracturing sand gets stuck, the pipe string should be moved in time to shrink the packer in the well, causing the sand bridge near the stuck point to loosen. When moving the pipe string, do not lift it suddenly. Instead, lift it slowly and release it quickly to avoid jamming the pipe string or pulling out the oil pipe.

39. What are the characteristics of broken pipe strings during fracturing construction?

When pipe strings are broken during fracturing construction, there are generally three characteristics:

① The pipe string breaks off near the wellhead: the pump pressure suddenly drops, the casing pressure rises sharply, there is large vibration and noise at the wellhead, and at the same time, the oil pipe rises;

② The oil pipe is broken above the packer and deep from the wellhead: the pump pressure suddenly drops, the casing pressure rises rapidly, there is vibration at the wellhead, and the pipe string is slightly lifted;

③ The pipe string breaks off between the stuck points of the two packers: the pump pressure fluctuates slightly, the wellhead suddenly rises, and the casing pressure may rise. If the pump is stopped and then started slowly, the oil and sleeve must be connected.

⑤ Generally speaking, when the pipe string above the packer breaks off, the hanging weight decreases. When the upper pipe string breaks off, the backwash cycle is short.

40. How to deal with the accident of fractured pipe string?

If the pipe string breaks above the packer, especially near the wellhead, the general treatment method is as follows:

Deepen the pipe string and perform buckling. If the buckling can be successful, backwash the well immediately to flush out the sand in the well. If the backwash fails, try lifting the column to remove the jam. If the oil pipe is broken, the broken oil pipe should be poured out with a female cone, and then backwashed and moved.

If the pipe string is disconnected from the packer, the following measures can be taken:

① Immediately start the pump to backwash, flush out the sand, and then try to move the pipe string;

② Kill the well and pull out the upper pipe string;

③ Salvage the downhole pipe string.

41. Why is it necessary to install a special wellhead ball valve during fracturing construction?

After fracturing construction, it is required to move the pipe string as soon as possible to promote the packer rubber barrel to be retracted as soon as possible. In the event of sand stuck, the most advantageous way to relieve the jam is to lower the pipe string to release the jam. Therefore, a special ball valve with an outer diameter of 114 mm must be installed at the wellhead. This kind of ball valve has the same outer diameter as the downhole packer. It can not only be opened and closed quickly when necessary, but also can be released by lowering the pipe string through the wellhead controller.

Section 5： Conventional Fracturing

1. How many common methods are there for using packers for layered fracturing?

The construction method of using packers to perform multi-layer fracturing at one time has been widely adopted. There are three methods:

① Pressurized layered fracturing method;

② Lift packer layered fracturing method;

③ Sliding sleeve layered fracturing method.

2. What is pressure-suppressed layered fracturing?

The pressure-suppressed stratified fracturing method uses several packers to separate each section of the layer to be fracturing. A single-flow Val ball seat is connected below each upper packer, and the packer below the ball seat Connect a sub-joint with a nozzle to the top (the closer to the packer, the better), and the nozzle is sealed with copper sheet (the purpose is to ensure that the packer opens to work first, and then the nozzle works through the copper sheet).

When fracturing, press the bottom layer first, then ball the ball to seal the bottom layer, then press the pressure to open the copper skin of the second layer of nozzles, and press the second layer. Press each layer apart in turn. The pressure-holding method can compress 3 layers at a time. The main technical key of this method is the cooperative use of copper skin and ball valve. From bottom to top, the valve seat should be from small to large, and the copper skin should be from thin to thick.

3. What is packer lifting layered fracturing?

This fracturing method uses a packer to pre-lock the bottom to two layers that need to be fracturing underground according to the construction design. After the fracturing is completed, the pump is stopped to shrink the packer rubber barrel, and then the packer is pressed according to the pre-designed When the lifting depth is good, part of the oil pipe is lifted out, the pump is started, and then the fracturing construction of the previous layer is carried out. This method is relatively simple, but the piping column is more troublesome.

4. What is shallow well sliding casing layered fracturing?

Using the sliding sleeve fracturing method in shallow or medium-deep wells can achieve multi-layer fracturing without moving the string. This method is fracturing from the bottom up, that is, the bottom sandblaster is not equipped with a sliding sleeve, and the bottom layer is cracked first. After pressing, keep pumping, and use a wellhead ball thrower to throw a steel ball so that it sits on the bottom level. After pressing, keep pumping, and use a wellhead ball thrower to throw a steel ball so that it sits on the sliding sleeve of the upper-level sandblaster of the bottom sandblaster. After holding down the pressure and shearing the pin, the sliding sleeve will fall to the bottom. On the first-level sandblaster, the liquid outlet channel is blocked, so that the second layer can be fractured. By analogy, multi-layer fracturing can be achieved.

5. What is the role of the wellhead ball thrower in the sliding sleeve layered fracturing string?

During the layered fracturing construction of the sliding sleeve, steel balls of different diameters need to be thrown in without stopping the pump to block the liquid supply channel, and generate an instant high pressure to break the pin on the sandblaster, causing the upper sliding sleeve to fall into the Next level sandblaster. In order to simplify the operation and achieve continuous construction without stopping the pump, a wellhead pitcher is used.

6. What information is required for conventional fracturing?

Eight pieces of information must be collected during fracturing construction:

① Fracturing fluid: name, properties, total fluid volume;

② Proppant: name, particle size, total dosage;

③ Reagent process: time, number of vehicles, model, grade (displacement), pressure (pump pressure, casing pressure), rupture pressure;

④ Sand adding process: time, gear (displacement), pump pressure, casing pressure, sand mixing ratio;

⑤ Replacement extrusion process: time, gear (displacement), pressure (casing pressure, pump pressure), total amount of replacement liquid;

⑥ Downhole pipe string structure: packer depth, sandblaster depth, liner depth, total length of tubing (number of pipes), and pipe string structure schematic diagram;

⑦ Oil well construction process: casing production time, nozzle, blowout time, blowout volume;

⑧ Water well construction process: overflow release time, overflow volume, injection time, water injection method, pump pressure, oil pressure, casing pressure, and injection volume.

7. What is the reasonable sand adding procedure for fracturing construction?

In fracturing, a reasonable sand adding procedure is to use an electronic computer to process various oil layer parameters, thereby optimizing a reasonable construction plan. Compared with ordinary fracturing, it mainly solves two problems in terms of technology. One is to determine the reasonable amount of proppant for tailings with large particle size (generally using 0.8-1.2 or 1.2-1.6mm ceramsite or coarse particles with the same particle size). sand); the second is to determine a reasonable displacement amount, the purpose of which is to maximize the conductivity of the near-wellbore zone.

8. Why does the reasonable sand adding procedure require strict control of the displacement amount?

The purpose of a reasonable sand adding procedure is to improve the conductivity of the fracture opening of the fracturing layer. The required ideal displacement amount is to ensure that the last batch of tailings just enters the fracture opening and then stops the construction. Therefore, the amount of replacement liquid must be strictly controlled, otherwise the crack may be partially closed due to excessive replacement or the pipe string may be blocked by sand due to dirty replacement. The construction method of reasonable sand adding procedure requires that the amount of replacement fluid should be 1.1-1.2 times the volume of the wellbore fracturing string, which is more appropriate.

Section 6: Flow-limiting method and pitching method fracturing

1. What is flow-limiting fracturing?

Strictly perform low-density perforation on the target layer for fracturing. Under the condition of maintaining the rupture pressure of the oil layer, the surface fracturing equipment increases the construction displacement as high as possible and utilizes the energy generated when the first fracturing layer absorbs the fracturing fluid. The abrasion of the blasthole greatly increases the bottom hole pressure, forcing the fracturing fluid to divert, causing other layers with similar fracture pressures to be pressed open one after another. The fracturing technology that can simultaneously open several oil layers with similar properties in one operation is called flow-limiting fracturing.

2. What are the special process requirements for flow-limiting fracturing construction?

Since the current limiting method is different from ordinary fracturing in principle, the construction process has the following special requirements:

① Dredge the blasthole: Since the flow-limiting fracturing method requires low-density perforation, generally only one or two holes are perforated in each small layer. Therefore, it is crucial to keep the blastholes clear. Therefore, negative pressure perforation is required in clear water, and acidification pretreatment is required before construction. The main purpose is to remove the blockage of the oil layer in the blasthole and near the well area.

② High-pressure test extrusion: Due to the large differences in the physical properties of the pretreatment layer, the rupture pressure when cracks occur in the oil layer also has large differences. In order to ensure that more oil layers can be pressed open in one construction, a pump is required during the test extrusion process. close to the allowable pressure limit.

③ Increase the amount of pre-fluid: Since the main modification target of flow-limiting fracturing is thin oil layers or poor oil layers with poor liquid absorption capacity, increasing the amount of pre-fluid can prevent desanding during the sand adding process. Pre-flush is generally required in the design. The liquid volume should reach about one third of the total liquid volume.

④ Gradually increase the sand ratio: In order to prevent sand blocking accidents, the sand ratio must be gradually increased and the concentration of tailings must be increased. This will help improve the permeability of the near-wellbore zone.

⑤ During construction, it is required to reduce the friction resistance of pipelines and pipe strings as much as possible. Therefore, when the fracturing equipment capacity is insufficient, smooth oil pipe or casing fracturing construction methods are often used.

⑥ Strictly control the amount of displacement: the amount of displacement is controlled within 1.2 times of the wellbore volume;

⑦ Controlled blowout after construction: In order to prevent sand from the formation and blocking the seepage channels near the well, it is generally not allowed to raise the pipe string immediately after fracturing or uncontrolled blowout.

3. What principles are used to determine the perforation plan for flow-limiting fracturing construction?

Formulating a reasonable perforation plan is the core content that determines the process effect. Therefore, the perforation plan should be formulated based on the following principles:

① Ensure sufficient blasthole friction value. Under this premise, make full use of the maximum displacement of the fracturing equipment to crush as many target layers as possible;

② Distribute the number of blastholes as proportionally as possible according to the specific conditions of each layer. Geometric dimensions also need to be considered. The intensity of treatment of layers with visible water or easy water infiltration on a flat surface should be strictly controlled. When thick layers and thin layers are treated together, the intensity should be different.

③ Generally, the perforation is selected in the position with the best permeability in the layer and the most guaranteed oil production. When there is a thin interlayer of lithology or physical properties in the layer, it may be considered to distribute holes above and below the interlayer.

④ When there may be a non-target layer that colludes with it during fracturing near the target layer, attention should be paid to increasing the distance between the perforation point and the interlayer. This should be paid more attention to when the thickness of the interlayer is less than 3 meters. And reduce the number of holes to avoid the occurrence of cross grooves.

⑤ Taking into account the influence of the fracture zone, when there are many layers in the treatment section and the total number of blast holes is limited and is less than the number of layers to be treated, several closely adjacent small layers without obvious interlayer obstruction can be Holes in the middle.

⑥ Due to the current limitations of actual perforation levels, clogging of individual blast holes is unavoidable. Therefore, the actual number of holes is allowed to be greater than the theoretical calculation value.

⑦ The perforation plan should comprehensively consider the needs of fracturing construction, and determine the fracturing construction method while determining the perforation plan.

4. What construction methods are used for flow-limiting fracturing?

Four construction methods are usually used for flow-limiting fracturing:

① Sliding casing string layered segmented fracturing method: This method is suitable for dispersed combinations of multiple oil layers, that is, the target layer in a well is vertically distributed into several small sections, and each section has several adjacent small layers. By utilizing the thicker layers existing between layers, more is reduced to less, thereby reducing the requirements for fracturing displacement.

② Blast eyeball classification flow restriction method: When the target layer is numerous and dense, cannot be separated by packers, is restricted by equipment or ground conditions, and cannot provide sufficient construction displacement, the method of shooting eyeballs can be used , the current-limiting method construction is divided into several sections, and each section processes several layers at the same time.

③Oil-casing pressure combining method: This method injects fracturing fluid from the oil and casing at the same time, thereby greatly reducing the friction value and being beneficial to large-displacement construction.

④Casing fracturing method: There is no oil pipe in the well, and the fracturing fluid is injected directly from the casing. In this way, the pipe loss is very small, and it is suitable for construction wells with high displacement requirements.

5. What are the main advantages of flow-limiting fracturing?

Flow-limiting fracturing has the following three advantages:

① Ensure that cracks occur in the most favorable parts of the oil layer. Conventional long-section perforating methods cannot guarantee the location of fractures, while the flow-limiting method uses fixed-point perforating methods to only perforate the most favorable parts of the oil layer, ensuring that cracks form in this part first. For oil layers with longitudinal heterogeneity within the layer, the fracturing effect is very obvious.

②In the design of the current limiting method, the distribution of blastholes is based on the needs of each oil layer and distributed as proportionally as possible. Therefore, while pressing open multi-layer cracks, the treatment intensity received by each layer is different. Usually Thicker layers are treated more intensively.

③ Since the current limiting method strictly limits the number and diameter of blastholes and the characteristics of local shooting within the layer and simultaneous pressure opening between layers, this process causes less damage to the casing and cement sheath, and generally does not cause cross grooves.

6. Why does flow-limiting fracturing achieve better reservoir transformation effects?

Flow-limited fracturing requires low-density perforation. During fracturing, the flow rate of each single-hole blasthole reaches 0.4-0.6m3/min, and the flow rate at the blasthole outlet can reach nearly 100 meters per second. Under the action of high-speed sand-carrying liquid flow, not only cracks will occur in the near-wellbore area, but also permanently deformed grooves will be formed, making it impossible for the cracks to be completely closed, thus greatly improving the oil conductivity.

7. What conditions limit the flow-limiting fracturing process?

① For production wells that have been perforated according to conventional methods, flow restriction cannot be achieved and cannot be used;

② It should not be used in wells with thick oil layers and good permeability;

③ The fracturing equipment used is required to have high pressure and large displacement. During construction, multiple conventional fracturing trucks must be connected in parallel, occupying a large area of the well site.

8. What is pitching fracturing?

Ball ball fracturing technology utilizes the characteristics of the pressure-opening layer's large liquid absorption capacity and low starting pressure. After a layer is pressed, a high-strength temporary plugging agent ball is squeezed in at a lower pressure to block the pressure-opening layer, forcing high pressure The fracturing fluid is diverted and enters other oil layers. When the pump pressure rises significantly, all fracturing trucks are started to crack the second layer, and then the ball is fired to seal it, and then the third layer is pressed, and multiple layers are opened successively. After fracturing construction, the temporary plugging agent ball will automatically unblock at the temperature of the oil layer.

9. What are the applicable ranges of pitching fracturing?

Ball ball fracturing is not only suitable for oil layer stimulation of multi-layer wells that have been put into production, but also can be used as a completion measure for oil wells with many and thin layers, small interlayers, poor permeability and low productivity, and can be used in the same well, the same It can be used repeatedly within a layer. In addition, it is also suitable for layers with poor cementing quality that cannot be isolated by packers. It is also suitable for reservoir stimulation in open-hole wells.

10. How to achieve the purpose of blocking when adding temporary plugging agent during fracturing construction using the ball method?

After squeezing, use a displacement of about 0.5m3/min to deliver the ball, and keep the displacement stable. When the pressure rise exceeds 14.7Mpa, other pump trucks can be started for fracturing, otherwise it needs to be blocked again. At this time, the amount of temporary plugging agent is half of the amount.

11. What is the principle of selective fracturing of wax balls?

The principle of wax ball selective fracturing is to take advantage of the characteristics of high permeability, large water absorption, and low starting pressure of water-containing parts of oil layers or layers or repeatedly fracturing layers. The fracturing fluid is used to carry wax balls during fracturing construction. The temporary plugging agent is squeezed into the well under a certain pressure and displacement to block the blast holes in the high-permeability layers, allowing the fracturing fluid to act on the medium-low permeability layers until the fractures are opened. After construction, the temporary plugging agent will dissolve automatically under well temperature conditions, and there is no need to take other plugging measures when opening the well for production.

12. What problems can selective fracturing solve?

Selective fracturing can solve the following problems:

① After local water breaks out in the heterogeneous oil layer, pressure selection technology can be used to press open the oil-bearing parts instead of the water-bearing parts;

② When there is an aquifer in the fracturing section, the aquifer can be opened without opening the aquifer, but other oil layers that do not contain water can be opened;

③ For layers that have been fractured, pressure selection technology can be used to create new fracture locations.

Therefore, pressure selection technology can solve the problem of fracturing and increasing production in aquifers and repeatedly fracturing sections that cannot be solved by ordinary fracturing technology, and can better play the role of medium and low permeability layers. However, selective fracturing technology is only applicable when the fracture shape is horizontal, and the purpose of selective fracturing can only be achieved when there is a large vertical difference between high water content parts and non-water and low water content parts.

Section 7: Fracturing Tools

1. Packer

The purpose of packers in fracturing is to perform layered fracturing and protect casing. There are two main types of fracturing packers used in various oil fields in my country, namely hydraulic differential pressure packers and hydraulic mechanical packers. The working principle of the hydraulic pressure differential packer is that after the packer is lowered into the predetermined position in the well, the surface pump begins to inject liquid into the well to increase the pressure. When the liquid passes through the filter screen of the packer and enters the annular space between the rubber barrel and the central pipe, the pressure of the liquid causes the rubber barrel to expand outward until it contacts the inner wall of the casing, thus isolating the annular space of the oil and casing from the top and bottom. As the pressure increases, the sealing performance of the rubber cartridge becomes more and more reliable. When the pressure in the oil pipe is relieved, the rubber barrel relies on its own elastic contraction force to drain the liquid between the rubber barrel and the center tube into the oil pipe and return to its original state.

2. Sandblaster

Sandblasters are mainly divided into two types: spring type and nozzle type. The spring-type sandblaster is composed of upper and lower joints, adjustment ring, spring seat, spring, spring sheath, valve, valve seat, sliding sleeve, center tube, weighted head and other parts. During fracturing construction, the sandblaster is used in conjunction with the packer, so that the fracturing fluid is sprayed toward the well wall through it, and fracturing is performed step by step. The amount of sandblasting used is determined based on the number of production layers to be pressed.

3. Hydraulic anchor

In order to prevent the packer from creeping up and down due to pressure fluctuations during fracturing, and to prevent the oil pipe from rising due to uncoordinated upper and lower packers or damage to the lower packer, hydraulic anchors can be lowered to fix the downhole pipe string. Ensure that construction proceeds normally. When the oil pipe is pressurized, as the pressure rises, the hydraulic anchor begins to compress the spring and push outward until the outer teeth of the hydraulic anchor contact the casing wall. The higher the pressure, the tighter the embedding, which prevents the downhole pipe string from moving up and down in the well. When the pressure is relieved, the spring pushes the hydraulic anchor body, causing the outer teeth to leave the inner wall of the casing and return to their original position. When using hydraulic anchors, attention should be paid to the lowering position within the height range of the cement sheath to prevent casing deformation due to excessive pressure. If the hydraulic anchor has a sand control device, it can be lowered into the bottom of the pipe string.

If there is no sand control device, the hydraulic anchor should be placed above the uppermost packer to prevent the hydraulic anchor body from being stuck by sand.

Shandong Yitai Hydraulic Technology Co., Ltd is one of the leading manufacturers of fracturing hoses. Welcome for your comments:

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