Time-lapse gravity: A numerical demonstration using robust inversion and joint interpretation of 4D surface and borehole data

Abstract

There are a number of ongoing developments in the 4D gravity method for time-lapse production and sequestration problems. Complex model construction is an essential component for meaningful feasibility studies and data interpretation in 4D. In the case of oil reservoirs, for example, the 4D gravity method must deal with very small density contrasts at depth, and overly simplistic model representations of a field site may not properly guide monitoring efforts for effective reservoir management decisions. The tracking of injected fluid through inversion can be significantly improved with the joint interpretation of surface and borehole gravity data. This is particularly relevant for time-lapse gravity problems where subtle changes in density contrast at depth may not produce surface data that alone contain the necessary information to recover the boundaries of fluid movement. Complex model construction and robust inversion show that time-lapse gravity surveys may contribute to improved production efficiency and reservoir management in-between the more traditional, and expensive 4D seismic surveys. Results hint that the true value of time-lapse gravity as an additional tool for production efficiency and reservoir management may be greatly undervalued. Data simulated within current and ongoing sensor technologies, combined with advances in computing power and robust inversion, can extract much more meaningful information about fluid movement in reservoirs that are geometrically complex and relatively thin and deep. To illustrate these points, simulations are performed using a representation of the published Jotun Field in the Norwegian North Sea, a well-studied site demonstrating successful application of the time-lapse seismic method, and reconstructed for 4D gravity understanding.