Using surface deformation to image reservoir dynamics

The inversion of surface deformation data such as tilt, displacement, or st rain provides a noninvasive method for monitoring subsurface volume change. Reservoir volume change is related directly to processes such as pressure variations induced by injection and withdrawal. The inversion procedure is illustrated by an application to tiltmeter data from the Hijiori test site in Japan. An inversion of surface tilt data allows us to image how processe s in a fractured granodiorite. Approximately 650 barrels of water, injected 2 km below the surface, produces a peak surface tilt of the order of 0.8 m icroradians. We find that the pattern of volume change in the granodiorite is very asymmetrical, elongated in a north-northwesterly direction, and the maximum volume change is offset by more than 0.7 km to the east of the pum ping well. The inversion of a suite of leveling data from the Wilmington oi l field in Long Beach, California, images large-scale reservoir volume chan ges in 12 one- to two-year increments from 1976 to 1996. The influence of v arious production strategies is seen in the reservoir volume changes. In pa rticular, a steam flood in fault block ZI in the northwest portion of the f ield produced a sudden decrease in reservoir volume.