What are the most common CO2 sequestration methods in reservoir engineering?

At first sight CO2 injection is a significant EOR method but it depends on the availability of CO2 and reservoir characteristics. It can range from a relatively small portion to a dominant method. we need more field pilot projects, and economic assessments to have better understanding of effectiveness of CO2 sequestrations

CO2-enhanced oil recovery represents a promising approach to both increasing oil production and mitigating greenhouse gas emissions, and it is likely to continue playing a significant role in the effort to address climate change while meeting global energy demands.

EOR has been widely used to recover residual oil after the primary or secondary oil recovery process. Therefore, EOR can't be applied in primary and secondary recovery. Tertiary recovery, the third stage of production, was that obtained after waterflooding (or whatever secondary process was used). Tertiary processes used miscible gases, chemicals, and/or thermal energy to displace additional oil after the secondary recovery process became uneconomical. Because of such situations, the term “tertiary recovery” fell into disfavor in petroleum engineering literature and the designation of “enhanced oil recovery” (EOR) became more accepted.

Depending on the origin of the CO2 being used for the Enhanced oil recovery process, the entire exercise can be deemed a form of carbon sequestration. If the origin of the CO2 being used is natural i.e. recovery from any subsurface accumulation or it is being synthetically generated with the sole purpose of oilfield injection, then the process is not carbon sequestration. However, if the CO2 is captured from any other industrial process or is direct air capture, then depending on the carbon footprint of the produced hydrocarbons, then, the entire process can be deemed to be carbon utilization or if stored to produce carbon ratio is more than 1-1.5, carbon sequestration.

In my point of view, CO2 injection into the reservoir is an important process that has the benefits of both: enhancing the oil recovery and storing the CO2 into the formation. Better results can be achieved when injecting CO2 to reach miscibility with the oil, reducing the viscosity and improving the oil mobility. WAG (Water alternating gas) has been used to achieve better performance of the process, controlling the adverse mobility ratio between CO2 and oil, generating a more stabilized front, and pushing more oil towards producing wells. CO2 is a proven and promising technology to improve oil recovery as a tertiary enhanced oil recovery as well as storage CO2 and contribute with net zero 2050 target.

The principle of CO2 storage with EGR in shales is that CO2 adsorbs preferentially to most rock surfaces vs methane (natural gas) so injecting CO2 should help release more natural gas molecules for production while CO2 molecules are permanently sequestered through adsorption.

The efficiency of CO2 enhanced gas recovery, identified by CO2-EGR, is mainly determined by various reservoir properties and engineering design parameters. However, due to the heterogeneous nature of carbonate and shale reservoirs and the complexity involved in modeling the CO2-CH4 displacement process, there still exist uncertainties in identifying the main factors that govern CO2 sequestration and enhanced CH4 recovery in such reservoirs. Hence, further studies are required for any specific project to address these uncertainties.

Many high GOR reservoirs are retrograde condensate reservoirs. Ergo, small pressure drops can initiate condensation and subsequent skin effects in the reservoir. CO2 lowers this impact by acting on the irreducible saturation via surface tension and viscosity augmentation.

In reservoir engineering, enhanced gas recovery (EGR) is a method similar to enhanced oil recovery (EOR), but it focuses on the recovery of natural gas. CO2 sequestration in the context of EGR involves injecting CO2 into gas reservoirs to enhance the recovery of natural gas while storing the injected CO2 underground. This dual-purpose approach contributes to both increased energy production and carbon capture. The effectiveness of EGR as a CO2 sequestration method depends on the specific characteristics of the gas reservoir and the geological formations involved.

The injecting CO2 into natural gas reservoirs is one of the Enhanced gas recovery (EGR) as well: to maintain the pressure and displace the gas, thus enhancing its production. This method can also store CO2 permanently in the reservoir, as some of the injected CO2 dissolves in the gas or remains trapped in the pore spaces of the rock3

The methane recovery rate may be increased (∼30%), according to field, experiments and numerical simulations of CO2 injection compared against natural depletion.

This method is also affiliated with large stratigraphic reservoir structures that could be shallow in nature. They provide a formation that can be easily coupled with industrial processes that have a significant carbon footprint like cement plant production. From a CAPEX and OPEX standpoint that provides a hybrid hydrocarbon storage standpoint.

CO2-ECBM is quite an interesting method to have CO2 utilization in both worlds (enhancing methane production and sequestration) due to the uniqueness of coal properties (adsorption and desorption capabilities, as well as coal deformation to its permeability). However, it requires a lot of studies (both experimental and numerical study) to make this method beneficial. The good thing about CO2-ECBM compared to CO2-EOR or typical CO-EGR is CO2 can be sequestrated while increasing methane production at the same time without any CO2 recycling process.

Saline aquifers can store large amounts of CO2 and can be found in many more locations than depleted reservoirs. However, much less data on saline aquifers has been recorded as these reservoirs have not been a target of exploration and research until recently. This lack of data makes starting a new project costly as the risk of leakage needs to be clearly defined before large-scale sequestration can commence.

Saline aquifers compared to depleted reservoirs have less penetrations and this reduces loss of containment risk from legacy wells. However storage efficiency is kept low initially due to limited data available for characterisation. From a measurement monitoring and verification stance saline aquifer which already contain aqueous fluids are likely to provide contrast with injection of CO2 as against injection into depleted reservoirs. They provide an excellent opportunity for CO2 sequestration and a path to quick win decarbonisation strategies.

The CO2 is generally stored in large saline aquifers. However, its storage capacity depends upon how big is the “subsurface tank.” The bigger tank allows a greater room for pressure to dissipate through and thus a larger volume of CO2 can be stored and vice versa. In addition, how dense and viscous ( which depends upon TDS/ Salinity) the resident brine is. Viscous the brine, difficult it is for injected CO2 to replace it ad thus lower storage efficiency is. Another important parameter is the depth of aquifer. Greater depth means denser CO2. The greater depth generally means High allownace for fracture pressure.

While saline aquifers are more numerous than depleted fields, the CO2 storage efficiency within them is lower. This is due to how CO2 flows in water but also due to pressure limits. The rock pores are already full of water which is relatively incompressible and injecting CO2 increases the pressure. You need to keep the pressure in the rock below the fracture pressure of the sealing rock of the store to ensure containment and this sets your storage limit.

Saline reservoirs exists in both onshore and offshore areas around the globe. In some cases the areas may overlap with existing Oil and gas Fields. If so, the other existing wells in the area shall be evaluated regarding integrity.

Recent studies have demonstrated that the implementation of the carbon dioxide enhanced oil recovery (CO2-EOR) technique could recover the remaining hydrocarbon. Experimental studies on the CO2-EOR technique showed success in unconventional reservoirs and could be applied in formations with porosity as low as 4.1% and reservoir permeabilities as low as 0.0004mD.

For unconventional storage, the CO2 sequestration into basaltic rock reservoir is playing significant role in CCS industry in recent years. Most successful project for this study is Carbfix Iceland. Recent study from Carbfix result, they’ve successfully injected CO2 with sea-water into the basaltic rock.

A six method of CO2 sequestration is in depleted oil and gas reservoirs, that have many advantages: - adequately characterized by development wells and reservoir studies - seal is known to have trapped hydrocarbons for long periods of time - existing infrastructure on the surface (pipelines) and underground, which could be reused for CO2 storage with only minor modification. - traditional evaluation methods can be applied Volumetrics, material balance, reservoir simulation - capacity may be reasonably well defined. - subsurface data and information collected during the hydrocarbon reservoir development could reduce the uncertainty in a CO2 storage One of the major challenge is well integrity

Indeed, estimating the capacity of CO2 storage in saline formations involves considering various factors, including impurities that may come with CO2 captured from facilities. Non-condensable impurities like N2, O2, CH4, and Ar can impact storage capacity by occupying space in the reservoir and affecting the overall injectivity and efficiency of the storage process. The presence of impurities can influence the behavior of CO2 in the subsurface and may affect the overall storage integrity. Efforts are being made to develop technologies that address impurities and enhance the efficiency of CO2 storage in saline formations. Continuous research and advancements aim to improve our understanding of these factors and refine capacity estimates.

Bio digestion using Algae. Biological CO2 fixation by algae is another such form; i.e. sunlight being used to reduce CO2 to carbon.

it's important to also consider factors like the geological suitability of the storage site, the potential for CO2 leakage, and the long-term monitoring and verification of the stored CO2. These aspects ensure the effectiveness and safety of the sequestration process.

Another method that can be used for Co2 sequestration is Geological Storage which involves Injection of CO2 into deep underground geological formations like depleted oil and gas reservoirs. Key considerations for assuring proper material selection for the injection “tubing” string and the other flow “injection” wetted equipment to maintain well integrity.