This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 206162, “4D Seismic in Subsurface CO2 Plume Monitoring: Why It Matters,” by Pankaj K. Tiwari, SPE, Debasis P. Das, and Parimal A. Patil, SPE, Petronas, et al. The paper has not been peer reviewed.
Carbon dioxide (CO2) sequestration in depleted carbonate reservoirs requires the incorporation of comprehensive and innovative monitoring technologies. 4D time-lapse seismic is necessary for monitoring, measurement, and verification (MMV) planning to demonstrate the migration of CO2 plumes within geological storage. An adaptive, site-specific MMV plan for monitoring CO2 plumes is critical to minimize possible subsurface and project-integrity risks. In the complete paper, the authors explore the migration of CO2 plumes within depleted carbonate reservoirs and the expected effects of CO2 saturation and pressure buildups during injection on time-lapse 4D seismic for conformance monitoring.
In the current study, a depleted carbonate reservoir gas field was identified for near-future CO2 sequestration offshore Sarawak. The storage site has been studied thoroughly for both overburden and reservoir integrity by adopting work flows comprising integration of subsurface integrity analysis, pore-pressure analysis, and subsidence modeling, along with a health check of existing wells. All parameters were cross-correlated. Qualitative risk categorization was performed to determine the robustness of the reservoir for long-term CO2 storage.
Integrated reservoir dynamic simulation was performed by coupling compositional reservoir dynamic/geomechanics/geochemistry 3D models to assess the integrity of the field as a CO2 storage site and to demonstrate the periodic development of CO2 saturation and plumes within the gas-depleted carbonate reservoir. Computation of the cumulative effect of changes occurring in all three models was essential. Changes to reservoir rock properties caused by dissolution and precipitation, as well as compaction during injection, were interpreted by 3D coupled modeling simulation and integrated with seismic modeling.
Apart from storage capacity, results from the coupled modeling study showed that reservoir pressure at the end of the injection period was approximately 400 psia lower than the initial reservoir pressure. Modeling also showed that the entire incoming permeate stream rate could be injected with three injection wells. Fig. 1 shows the CO2 plume migration after 2, 10, and 20 years of injection from all three injectors. Because injection is planned from a predrill liner in the full reservoir column, the CO2 plume migrates upward in a funnel shape around the well first. In the initial years of injection, most of the CO2 migrates laterally in the shallower formation of the reservoir, then migrates downward once the shallower formation of the reservoir section is fully CO2-saturated.