Monitoring Reservoir Fluids with Surface and Borehole Gravity Sensors

The article addresses the challenge of tracking a gas/water contact by an inversion of time-lapse gravity data. The objective of the study was to evaluate benefits of using the combination of surface and borehole gravity for monitoring fluid movements in the reservoir. The expectation was that borehole gravity measurements performed much closer to the reservoir would provide a higher resolution for detecting a gas/water contact. A combination of surface and borehole gravity measurements was considered. All results and conclusions are based on 3D forward and inverse mathematical modeling of the surface and borehole gravity fields. Software tools for the forward modeling and joint inversion of surface and borehole gravity data in a 3D environment have been developed. The conventional way to implement regularization in the solution of the inverse gravity problem is based on the class of inverse models with a smooth distribution of model parameters. Better results were obtained when the method of constrained focusing inversion was implemented. This inversion provided stable solutions for inverse gravity models with sharp boundaries.