In this paper we present the results of a coupled nonlinear finite element geomechanics model for reservoir compaction and well-to-well interactions for the high-porosity, low strength diatomite reservoirs of the Belridge field near Bakersfield, California. We show that well damage and failures can occur under the action of two distinct mechanisms: shear deformations induced by pore compaction, and subsidence, and shear deformations due to well-to-well interactions during production or water injection. We show such casing damage or failure can be localized to weak layers that slide or slip under shear due to subsidence. The magnitude of shear displacements and surface subsidence agree with field observations.
The subject of this paper is the effect of ground subsidence and pore compaction on the damage and failure of oil wells in the Belridge field in Southern California. It is well known that subsurface rock formations can compact under the reduction of pore pressure, that is when interstitial fluids are removed (Poland, J.F. and Davis, G.H., 1969, Geertsma, 1973). Under large loads or a significant reduction of pore pressure, rock grains can fracture or become crushed and undergo translations and rotations thus filling pore space, a mechanism referred to as pore collapse(see, Wong, et al., 1992 and Wong, et al., 1996). Pore compaction is expressed at the surface of the earth as subsidence, or sinking of the ground. The magnitude of subsidence can be large, as much as 30 feet in the case of the San Joaquin Valley groundwater withdrawal (Poland, J.F. and Davis, G.H., 1969), 29 feet in the Wilmington oil fields in southern California (Mayuga and Allen, 1969) and at least 13 feet in Ekofisk in the North Sea and continuing to subside (Boade, et al., 1988). The forces associated with pore compaction and subsidence can be large and on the scale of an oil field, say several thousand feet in width, length and depth, a steel casing string that is 7 inches in diameter offers little resistance. In this paper we report the results of analysis of subsidence in the Belridge Field, located near Bakersfield, California. We constructed a two- dimensional plane strain finite element model of a slice of the field using a commercially available general-purpose finite element program (Hibbitt, et al., 1994). A geoemechanical model consisting of mixed displacement-pore pressure elements was coupled to a two-dimensional reservoir simulator to capture the constitutive behavior of the formation rocks during oil and gas production and water injection. To model failure of weak layers, such as weak shales, we used contact elements. In Section 2 of this paper we describe the relevant aspects of the Belridge field and the subsurface environment. In Section 3 we describe the coupled geomechanics reservoir model and modeling procedures. In Section 4 we present results obtained to date (this project is ongoing and we expect to report on further developments in the near future). In Section 5 we discuss observations and conclusion made from a review of the modeling results.
