Three-Dimensional Geomechanical Simulation of Reservoir Compaction and Implications for Well Failures in the Belridge Diatomite
Authors: Fredrich J.T. , Arguello J.G. , Thorne B.J. , Wawersik W.R. , Deitrick G.L. , de Rouffignac E.P. , Myer L.R. , Bruno M.S. 
Published: December 2007

Abstract
This paper describes an integrated geomechanics analysis of well casing damage induced by compaction of the diatomite reservoir at the Belridge Field, California. Historical data from the five field operators were compiled and analyzed to determine correlations between production, injection, subsidence, and well failures. The results of this analysis were used to develop a three-dimensional geomechanical model of South Belridge, Section 33 to examine the diatomite reservoir and overburden response to production and injection at the interwell scale and to evaluate potential well failure mechanisms. The time-dependent reservoir pressure field was derived from a three-dimensional finite difference reservoir simulation and used as input to three-dimensional non-linear finite element geomechanical simulations. The reservoir simulation included 200 wells and covered 18 years of production and injection. The geomechanical simulation contained 437,100 nodes and 374,130 elements with the overburden and reservoir discretized into 13 layers with independent material properties. The results reveal the evolution of the subsurface stress and displacement fields with production and injection and suggest strategies for reducing the occurrence of well casing damage.

Introduction
Well casing damage induced by formation compaction has occurred in reservoirs in the North Sea, the Gulf of Mexico, California, and Asia. 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 (45-70%) and resulting high compressibility of the reservoir rock.
The Belridge Field, located about 45 miles west of Bakersfield, California, recently attained billion-barrel status and is currently the fifth most productive field in the US. The thickness (more than 1000 feet), high porosity, and moderate oil saturation of the diatomite reservoir translate into huge reserves, with more than 3 billion bbl of original-oil-in-place estimated for the Belridge Field. However, 75% of production to date has been from the overlying Tulare sands.
Production from the diatomite reservoir is hampered by the unusually low matrix permeability (-0.1 mDa or less). Although the Belridge Field was discovered in 1911, production from the diatomite reservoir became economical only with the introduction of hydraulic fracturing stimulation techniques in the mid-1970's. 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 2-5% of active wells per year.
In 1994 a cooperative research program was undertaken to improve understanding of the geomechanical processes leading to well casing damage during production from weak, compactable formations. The study focuses on the Belridge Field and represents a significant extension of earlier work in two regards. First, a comprehensive data base was compiled to provide a unique, complete picture of the reservoir and overburden behavior. Second, the results of the field-wide analysis motivated large-scale three-dimensional numerical simulations.
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