Geomechanical Modeling of Reservoir Compaction, Surface Subsidence, and Casing Damage at the Belridge Diatomite Field

Summary Geologic, and historical well failure, production, and injection data were analyzed to guide development of three-dimensional geomechanical models of the Belridge diatomite field, California. The central premise of the numerical simulations is that spatial gradients in pore pressure induced by production and injection in a low permeability reservoir may perturb the local stresses and cause subsurface deformation sufficient to result in well failure. Time-dependent reservoir pressure fields that were calculated from three-dimensional black oil reservoir simulations were coupled unidirectionally to three-dimensional nonlinear finite element geomechanical simulations. The reservoir models included nearly 100,000 gridblocks (100 to 200 wells), and covered nearly 20 years of production and injection. The geomechanical models were meshed from structure maps and contained more than 300,000 nodal points. Shear strain localization along weak bedding planes that causes casing doglegs in the field was accommodated in the model by contact surfaces located immediately above the reservoir and at two locations in the overburden. The geomechanical simulations are validated by comparison of the predicted surface subsidence with field measurements, and by comparison of predicted deformation with observed casing damage. Additionally, simulations performed for two independently developed areas at South Belridge, Secs. 33 and 29, corroborate their different well failure histories. The simulations suggest the three types of casing damage observed, and show that, although water injection has mitigated surface subsidence, it can, under some circumstances, increase the lateral gradients in effective stress that in turn can accelerate subsurface horizontal motions. Geomechanical simulation is an important reservoir management tool that can be used to identify optimal operating policies to mitigate casing damage for existing field developments, and applied to incorporate the effect of well failure potential in economic analyses of alternative infilling and development options. Introduction Well casing damage induced by formation compaction has occurred in reservoirs in the North Sea, the Gulf of Mexico, California, South America, and Asia.1–4 As production draws down reservoir pressure, the weight of the overlying formations is increasingly supported by the solid rock matrix that compacts in response to the increased stress. The diatomite reservoirs of Kern County, California, are particularly susceptible to depletion-induced compaction because of the high porosity (45 to 70%) and resulting high compressibility of the reservoir rock. At the Belridge diatomite field, located ~45 miles west of Bakersfield, California, nearly 1,000 wells have experienced severe casing damage during the past ~20 years of increased production. The thickness (more than 1,000 feet), high porosity, and moderate oil saturation of the diatomite reservoir translate into huge reserves. Approximately 2 billion bbl of original oil in place (OOIP) are contained in the diatomite reservoir and more than 1 billion bbl additional OOIP is estimated for the overlying Tulare sands. The Tulare is produced using thermal methods and accounts for three-quarters of the more than 1 billion bbl produced to date at Belridge.5 Production from the diatomite reservoir is hampered by the unusually low matrix permeability (typically ranging from 0.1 to several md), and became economical only with the introduction of hydraulic fracturing stimulation techniques in the 1970's.6 However, increased production decreased reservoir pressure, accelerated surface subsidence, and increased the number of costly well failures in the 1980's. Waterflood programs were initiated in the late 1980's to combat the reduced well productivity, accelerated surface subsidence, and subsidence-induced well failure risks. Subsidence rates are now near zero; however, the well failure rate, although lower than that experienced in the 1980's, is still economically significant at 2 to 6% of active wells per year. In 1994 a cooperative research program was undertaken to improve understanding of the geomechanical processes causing well casing damage during production from weak, compactable formations. A comprehensive database, consisting of historical well failure, production, injection, and subsidence data, was compiled to provide a unique, complete picture of the reservoir and overburden behavior.7,8 Analyses of the field-wide database indicated that two-dimensional approximations9–11 could not capture the locally complex production, injection, and subsidence patterns, and motivated large-scale, three-dimensional geomechanical simulations. Intermediary results for Sec. 33 that used preliminary reservoir flow and material models were reported earlier.8 This paper presents results for best-and-final simulations that used improved reservoir flow models, more sophisticated material models, and activated contact surfaces. The simulations were performed for two independently developed areas at South Belridge, Secs. 33 and 29.