The Feasibility of CO2 Injection for IOR in Shale Reservoirs
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Improved oil recovery (IOR) methods for shale-oil reservoirs are considered relatively new concepts compared with IOR for conventional oil reservoirs. Different IOR methods—including CO2, surfactant, natural gas, and water injection—have been investigated for unconventional reservoirs using laboratory experiments, numerical simulation studies, and limited pilot tests. For a variety of reasons, CO2 injection is the most-investigated option. In this paper, numerical simulation methods of compositional models were incorporated with logarithmically spaced, locally refined, and dual-permeability reservoir models and local grid refinement (LGR) of hydraulic-fracture conditions to investigate the feasibility of CO2 injection in shale oil reservoirs.
Advancements in horizontal drilling and hydraulic fracturing enabled unconventional liquids-rich reservoirs (ULRs), such as shale and source-rock formations and very tight reservoirs, to change the oil industry. ULRs are characterized by pore throats of micro- to nanomillimeters and an ultralow permeability. Although different studies reported that these ULRs contain billions of recoverable oil barrels in place, it is estimated that less than 7% of the original oil in place can be recovered during the primary depletion stage. Production sustainability is the main problem behind the low oil recovery in these unconventional reservoirs. Oil wells in ULRs typically start with a high production rate, but show a steep decline rate in the first 3–5 years of production life because of the rapid depletion in the natural fractures combined with a slow recharge from the rock matrix.
The logical steps of academic research such as experimental investigation, simulation studies, and pilot tests for examining the applicability of different unconventional IOR methods have just begun in the past decade. Applying one of the feasible IOR methods in most oil and gas reservoirs should be mandatory to increase the oil-recovery factor. However, the mechanisms of IOR methods in unconventional reservoirs are not necessarily the same as those in conventional reservoirs. The primary characteristics of unconventional reservoirs that might impair performing IOR operations are low porosity and ultralow permeability. As a result, finding IOR methods that are insensitive to these very small pore throats is a priority.
The authors reviewed more than 70 reports and studies in investigating the applicability of different IOR methods in North American unconventional formations (Fig. 1). The reviewers recommended that the most-feasible IOR techniques to be applied in ULRs are miscible gases, surfactant, and low-salinity waterflooding. CO2-EOR was recommended as the most-promising EOR method to be applied in shale oil reservoirs. In core-scale studies, some researchers reported that the main mechanism by which CO2 extracts oils from shale core samples is molecular diffusion. However, other researchers reported that oil swelling, viscosity reduction, and repressurization are the main mechanisms by which CO2 extracts oils from conventional core samples. This study tried to combine numerical-simulation methods with some of the CO2-EOR pilot tests in shale formations to understand the primary mechanisms that are more likely to control CO2-EOR performance in the field-scale level of unconventional formations. The study also investigated the differences of CO2-EOR mechanisms in laboratory scale vs. field scale.
Different mechanisms for CO2 interactions with organic surface, shale brine, and shale oil were implemented in different scenarios of numerical models. Molecular-diffusion mechanisms, adsorption effects, and aqueous solubility effects were simulated. Additionally, linear elastic models and stress-dependent correlations were used to consider geomechanics-coupling effects on production and injection processes of CO2-EOR in shale oil reservoirs. Some of the results for this simulation study were validated by matching the performance of CO2 field pilots performed in the Bakken Formation.
The complete paper uses a variety of charts, tables, equations, and screenshots in presenting a detailed discussion of the mechanistic study. Topics include fluid-flow mechanisms, coupling between the geomechanics module and the reservoir flow model, compositional models for the formation fluids, and reservoir modeling.
- The mechanistic study described in the complete paper simulated different mechanisms for CO2 interactions with organic surface, shale brine, and shale oil. The investigated mechanisms have been implemented in different scenarios of numerical models.
- The study revealed that CO2 molecular diffusion mechanism has a clearly positive effect on CO2-EOR in huff ’n’ puff protocol. However, this mechanism has a relatively negative effect on the continuous flooding mode of CO2-EOR.
- Both dissolution and adsorption mechanisms have a negative effect on CO2 performance in terms of enhancing oil recovery in unconventional formations.
- The CO2 molecular-diffusion mechanism has the dominant role among other CO2 mechanisms to control success or failure of CO2-EOR in ULRs.
- Geomechanics coupling has a clear effect on CO2-EOR performance. However, different geomechanics approaches have a different validity in these shale plays. Stress-dependent correlations give the best match in CO2-EOR pilots in the Bakken, while linear elastic models provide the best match in the Eagle Ford.
- General guidelines have been provided in the study to enhance the success of CO2-EOR in these types of reservoirs.
The Feasibility of CO2 Injection for IOR in Shale Reservoirs
01 July 2019
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09 July 2019