ABSTRACT: The coupled fluid flow and geomechanical simulator TOUGH-FLAC was employed to study the mechanisms of depletion-induced reservoir compaction and its impact on hydrocarbon gas production. For consideration of compaction-drive in the sequential coupling between fluid flow and geomechanics, we developed and applied a new alternative approach of linking volumetric strain to the fluid mass balance through a correction of rock compressibility in the fluid flow simulator. Using this approach, we conducted model simulations for understanding the impact of porosity change on deformation and gas production, including sensitivity studies with regard to material properties and operation parameters for the optimization of gas production. The model simulations showed that the reservoir compaction can increase or decrease the gas recovery depending on the specific porosity and the permeability changes in the reservoir. This result shows that the interaction between fluid flow and geomechanics should be considered for optimal reservoir management and TOUGH-FLAC with the implemented coupling approach can be an effective tool for such analysis.
Biogenic gases have become increasingly attractive targets of oil and gas exploration and production activities in the worldwide. However, production of the gases from poorly consolidated or unconsolidated soft sediments in shallow reservoirs can be technically challenging operations because of depletion-induced reservoir compaction. Fluid production from such reservoirs and associated pressure drop may cause reservoir compaction and consequent surface subsidence (Settari, 2002), potentially resulting in surface facility damage (Mayunga, 1969), fault reactivation (Segall, 1989, Odonne et al., 1999), or wellbore instability (Bruno, 1992, Rutqvist et al., 2012). Pore collapse of weak sediments during compaction could drastically degrade reservoir quality by significantly reducing porosity and permeability. Meanwhile, the reservoir compaction can maintain reservoir pressure being an important driving mechanism enhancing oil and gas production (compaction-drive). Therefore, understanding the mechanisms of and impact of reservoir compaction is essential for reservoir management and risk control. However, such interaction between fluid flow and geomechanics may not be properly handled with conventional reservoir simulators where compaction is calculated as a function of pore pressure only, while neglecting stress changes due to deformation.