Numerical Simulation for Compaction and Subsidence During Production Period for a Large Carbonate Gas Field

Many gas fields were discovered in carbonate build-ups located in Sarawak, Malaysia. One of the most challenges and well-known issue to operate these fields is the reservoir compaction and surface subsidence. In the new discovered field, which was recently discovered in 2019, the data related to stress magnitude and mechanical rock properties is very limited. The available cores were used to perform the essential rock mechanics laboratory test. The coupling between reservoir dynamic simulation and mechanical earth model (MEM) was selected as an approach to predict the episodic compaction and subsidence through the field life. The 1D mechanical modeling was then performed, calibrated with rock mechanics laboratory test, and used as the key input in 3D mechanical earth modeling. The coupling steps between dynamic and 3D-MEM were selected throughout the production lifetime when the pressure depletion is significant compared to the previous coupling step. Based on the literature review, the reservoir compaction and subsidence occur in this region was caused not only from the reversible elastic deformation but also from the effect of pore collapse in some classes of the limestone. This pore collapse during pore pressure depletion and increase in effective stress tends to have more impact to the subsidence than the normal elastic deformation. This behavior was also captured and incorporated using critical state criterion. The results of the coupling will strongly have the impact on how the platform will be designed to account for potential subsidence throughout 20 years of production. Due to the limitation of input parameters in the 3D-MEM, the sensitivity analysis was performed to assess the impact of each mechanical properties on the magnitude of the reservoir compaction and subsidence. The use of ready coupled 3D-MEM can also be further extended to other applications such as wellbore integrity, wellbore stability analysis, and mud weight optimization at any given period of time. Moreover, the impact of the compaction and subsidence in this study could leads to better planning in data acquisition such as rock mechanics data, laboratory test, and logging acquisition program to narrow down the uncertainty of the subsidence analysis and other geomechanical applications.