What is the impact of formation evaluation on well design and construction?

In summary, formation evaluation is a critical component in well design and construction. It enables engineers to make informed decisions regarding well placement, drilling fluid selection, wellbore stability, formation damage prevention, reservoir characterization, and economic considerations. By leveraging the insights gained from formation evaluation, operators can improve the overall success rate, efficiency, and profitability of their well operations.

Formation evaluation is critical to drilling wells for the mere simple fact that you just can’t drill blindly. You need to have a very good understand of where you are going underground. It’s like having a GPS system to guide you to a destination you have never been to. It is extremely important. You must understand where you are navigating to, what fluids you will encounter underground, the rock properties, whether you will hit shallow gas, depleted zones or over pressures zones. That will help you design your drilling mud to ensure the sands don’t “kick in” on you. It is CRITICAL else you might have blowouts, kicks which can lead to the entire loss of your drilling platform.

After data QC pass do the full evaluation and plan based on: 1. new well location for your purpose 2. casing sizes and depth set on the base of problems 3. reset mud properties until reaching the target depth with lest problems 4. replan the logging tools to estimate reserve, get more info on the reservoir /well performance 5. plan proper perfs and stimulation methods 6. replan well monitoring.

Formation evaluation is not restricted to hydrocarbon exploration and production. It is also being used for geothermal energy production, carbon hydrates production, gas storage, oil storage, carbon storage, and in the mining industry. Soon, white hydrogen production will be added to the list of applications. Formation evaluation is also an important contribution to Earth's sciences.

Formation evaluation simply put is the different techniques used in "evaluating" the "formation" just as the name implies, the formation being oil and gas reservoirs. The different subsurface disciplines: geology/geophysics petrophysics, reservoir engineering etc collaborate to deliver robust formation evaluation. Data to be evaluated varies in scale from large scale like seismic to very small scale like rock thin sections studied by X-ray diffraction, it also varies by mode of collection like geophones for seismic acquisition, well Logging (LWD & Wireline), coring and pressure/fluid sample collection. All this data is analysed by different specialists with aim of evaluating the formation for hydrocarbons.

One way I like to put is that the formation is like a patient needing treatment and the different subsurface disciplines are the Doctor, Radiographer, Lab Scientist, Pharmacist etc all working together to help restore the patient "formation" to health

Formation evaluation makes use of all available data: geological records, geophysical information, mud logs, wireline and/or LWD logs, fluid samplings, core sampling (whole cores & sidewall cores), productions tests, production logs. Pre-drill evaluation involves the use of local or regional data for modeling purposes. Models get continuously updated while drilling.

Formation evaluation goes with the assessment of risks. For example, it is good to estimate the potential of over-pressures before drilling. At times, available data is insufficient to anticipate the risk of drilling through shallow gas or shallow carbon hydrates. This is the case when deep invasion by mud-filtrate masks the type of fluids in a reservoir. Scenarios, good and bad, have to be envisaged and discussed with all involved in drilling.

Formation evaluation (Petrophysics) is an input to geomechanical studies (e.g. wellbore stability, pore pressure estimation, fracturing) and geophysical studies (modeling of AVO, fluid/matrix substitution effects).

Cost is a dangerous criterion for Formation evaluation. Especially when considering the use of tools and methods with no immediate use in the evaluation (case of sonic logs). An example: highly porous carbonate beds frequently require the use of dipole sonic logging to accomodate slow sonic waves. This is more expensive than standard monopole sonic logging. In view of reducing costs, the dipole option is sometimes replaced arbitrarily by the monopole option. As a result, the juiciest part of the carbonate reservoir is not-loged due to the monopole sonic signal being obscured by mud arrivals. This is dramatic, especially in an exploration well. Better not record any sonic than waste money for a useless product.

Allow me to add that in some climes, Formation Evaluation is also known as reservoir characterization. In this discipline, there is such a thing as "too much data". If the value of a particular dataset doesn't outweigh the cost, it is always advisable not to include it in the acquisition program. This is because often times, there are different types of data that infer the same results in one way or another, so it is not necessary to acquire all data to tick boxes. Sometimes also, multiple different data types may infer conflicting results and it is left to the evaluator to prioritise what datasets are most important to achieving set goals and objectives. Uncertainty analysis is key here.