Sensitivity analysis for fault reactivation in potential CO₂-EOR site with multi-layers of permeable and impermeable formations

Abstract

CO₂-EOR is considered as a promising solution for enhanced oil recovery (EOR) and is attracting attention as being a more economical CO₂ geological sequestration solution along with oil recovery enhancement. However, injecting CO₂ at high pressure may cause many geomechanical changes and potential instabilities in surrounding formation such as ground uplift, caprock fracturing, and nearby fault reactivation. Such instabilities could significantly influence the stability of both surface facilities and subsurface structures. Especially, miscible CO₂-EOR, by which recovers more oil than immiscible one but, uses less CO₂, requires an injection pressure exceeding the minimum miscible pressure (MMP), which is determined by characteristics of reservoir conditions and oil compositions. Thus, for successful and safe CO₂-EOR operation, injection pressure interval between MMP and the maximum pressure that could be tolerated from geomechanics safety concerns should be appropriately designed considering site-specific reservoir conditions. In this study, we perform a numerical simulation of coupled multiphase fluid flow and geomechanical analysis using TOUGH-FLAC simulator for the potential CO₂-EOR site in Indonesian oil field, and demonstrate how much fault reactivation is sensitive to fault structure, slip-weakening property of faults, reservoir permeability, and in situ stress conditions. The model site consists of impermeable shale and permeable sandstone reservoir units so that the potential for fault slip through this multilayered formation is highlighted in the simulations. Our simulation results showed that fault slip initiation can be reached earlier period when in situ stress is anisotropic and reservoir is more permeable, because the stress state at the faults is near the frictional strength limit and the pore pressure buildup reaches to the fault much faster. The analysis shows that multilayered formations with high- and low-permeability layers are advantageous in CO₂-EOR since intense pore pressure buildup and subsequent fault reactivation could be impeded by pressure dissipation in high-permeability layers. However, we noted that fault reactivation may become substantial when the fault has a slip-weakening property and the residual frictional coefficient of the site-specific fault is very low.

Keywords::

• CO₂-EOR
• CO₂ geological sequestration
• multilayered formation
• geomechanical instability
• fault reactivation
• slip-weakening model

Acknowledgements

We thank Samudra Energy in Indonesia for providing us with the information on geological layers for this study. We also thank Mr Suranto, Ahmad Muraji and Mr Madi, Naser at Sejong University for extracting and processing the data for constructing the analysis model.

Additional information

Funding

This work was supported by the grant from Energy Resources R&D Program of the Korea Institute of Energy Technology Evaluation and Planning [KETEP, grant number 2012T100201728] funded by the Ministry of Trade, Industry and Energy of Korea. Dr Hyung-Mok Kim was also supported by the Korea Institute of Geoscience and Mineral Resources through their basic research program [KIGAM, 14-3214].