Advanced Numerical Simulation of CO2 Storage in Depleted Unconventional Reservoirs with Enhanced Embedded Discrete Fracture Modelling (EDFM)

Abstract The development of unconventional reservoirs has been powered by drilling more lateral wells per pad and by fracturing more stages. As these fields mature and production declines, enhanced oil recovery (EOR) methods, such as CO2 injection, have been adopted to boost oil recovery. Simultaneously, CO2 injection is a critical component of achieving net-zero emissions. While prior studies have focused on the EOR benefits of CO2 injection in unconventional reservoirs, the feasibility of long-term CO2 storage during the continuous injection phase remains underexplored. This study bridges the gap by modelling the CO2 huff-and-puff phase for recovery enhancement and the subsequent continuous injection phase to evaluate CO2 storage potential in depleted unconventional reservoirs. Embedded Discrete Fracture Modelling (EDFM) has been introduced to explicitly represent fractures in simulation models about a decade ago. Good amount of research has been conducted on EDFM to evolve its algorithm, which leads to further improvement in its performance. Moreover, EDFM is flexible and can be applied to both complex fractures and planar fractures. In this paper, we studied CO2 storage in a layer-cake sector model with EDFM representing hydraulic fractures. The hydraulic fractures are generated from a numerical simulation, sequentially, the geometry and properties are converted to embedded discrete fracture model (EDFM) for the reservoir simulation. The pad is primarily depleted for 5 years, followed by a huff-and-puff CO2 injection EOR process for 5 years, and concluded with a continuous CO2 injection and storage period. A series of sensitivity analysis were performed, examining factors such as fracture geometry and matrix permeability (0.0005-0.1 mD) to gain deeper insights into CO2 storage potential. The simulation results demonstrate EDFM's superior performance and scalability. The EDFM case completed the 20-year simulation in approximately 1 hour, while the unstructured grid case struggles to proceed. The EDFM case is 637 times faster to reach the same simulation time. Other key findings include: Total CO2 injection volume is comparable between complex fractures and planar fractures, with the planar fracture case accommodating about 1.8% more CO2.Increasing matrix permeability from 0.0005 mD to 0.005 mD enhanced cumulative CO2 injection by 6.2%. Beyond 0.005 mD, further permeability increases do not increase cumulative CO2 injection because the injection is not restricted by the bottom hole limitation anymore.As matrix permeability increases from 0.0005 mD to 0.005 mD, 0.05 mD, and 0.1 mD, the cumulative oil production increases by 120%, 279.6%, and 328.5% respectively. Few literatures modelling the continuous CO2 injection and storage period for hydraulically fracture unconventional reservoirs. This paper fills this gap by studying the topic with the novel methodology. The integration of advanced EDFM techniques with comprehensive reservoir modelling provides valuable insights. This work highlights the potential of unconventional reservoirs as viable options for CO2 sequestration, contributing to both enhanced oil recovery and climate goals.