We describe coupled reservoir-geomechanical modeling of production at the Lost Hills oil field. We first review a database that was developed to discern the depth distribution of casing damage and to motivate model development. We then describe the development of constitutive models for the overburden formations, reservoir formations, and underlying strata. 3D reservoir fluid flow and geomechanical models that cover portions of Sections 4, 5, 32, and 33 were developed. Black oil reservoir simulations were performed that included 229 wells and modeled 17 years of primary production and waterflooding. The timedependent reservoir pressures were then used as input to non-linear finite element geomechanical simulations. The simulations predict surface subsidence and shearing displacements in the subsurface that can result in well casing damage. Multiple simulations were performed to assessensitivity to the initial stress state and to the friction coefficient at the contact surfaces that are used to model the behavior of thin claystones.
The shallow diatomaceous oil reservoirs located in Kern County, California are susceptible to depletion-induced compaction because of the high porosity (45-60%) and large vertical extent of the producing formation. More than a thousand wells at the Belridge and Lost Hills fields have experienced severe casing damage as the reservoir pressures have been drawn down. The purpose of this study is to improve understanding of the geomechanical behavior by integrated analyses of field data, experimental measurement of rock mechanical behavior, and numerical simulation of the reservoir and overburden behavior during primary and secondary recovery. In our work, time-dependent reservoir pressures derived from three-dimensional (3d) black oil reservoir simulations are coupled unidirectionally and applied as loads in 3d nonlinear finite element geomechanical simulations. Central to the numerical modeling is the use of sophisticated material models that accurately capture the nonlinear deformation behavior of the reservoir rock, including inelastic compaction (yield) at stress states below the shear failure surface.
Fredrich et al. (2000) described development of 3d geomechanical models for the Belridge field and historical simulations that were performed of sections 33 and 29 using the quasi-static largedeformation structural mechanics finite element code JAS3D. The historical simulations served to validate both the conceptual model that was formulated for casing damage and the modeling approaches. That work also showed that geomechanical simulation could be applied as a reservoir management tool to optimize production and injection policies, and to identify the most economical density and spacing of production and injection wells during infill drilling.
Following the successful application at Belddge, attention shifted to Lost Hills. The Lost Hills field is at an earlier stage of development, and therefore, development and application of geomechanical models were expected to be particularly useful in regard to future infill drilling and expansion of the waterflood.
