Gravity and Gravity Gradient Monitoring of Fluid Migration in SAGD Reservoirs: A modeling study

English Abstract:

Steam assisted Gravity Drainage (SAGD) is a process to mobilize heavy oil or bitumen through steam injection into a reservoir for increased production. Consequences of the injection of steam include a change in the subsurface density distribution, which outlines the fluid migration processes in the reservoir. Monitoring the status and changes of reservoirs is critical for optimizing production as well as mitigating socio-environmental risks associated with developing a reservoir. Time-lapse gravity is a recent approach to measure gravity or gravity gradient over time to infer subsurface mass redistribution. For SAGD reservoir monitoring, major obstacles include the required sub-microGal sensitivity to sense the small density changes as well as the significant noise level originating from other processes in the proximate environment. We develop forward models of two operational SAGD reservoirs and calculate the gravity and gravity gradient signals considering confirmed density effects of 0.05 to 0.25 g/cm³ in the growing steam chamber. Results outline the importance of measuring gravity gradients for varying baselines in order to exploit its higher spatial focusing ability compared to gravity observations alone. At present, field-deployable superconducting gravimeters are the only relative gravimeters capable of performing such observations at the required sub-microGal level. The forward modeling results enable the development of optimized survey geometry and observation schedule. The models include realistic density distributions as well as randomly distributed variations of reservoir thickness and morphology of geological interfaces. Inversion of simulated gravity gradient data constrained by sedimentology and SAGD operational parameters will be presented.