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.

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. P. 195

Shell Development Co.; and S.K. Hara," Shell Offshore Inc. q SPE Members Copyright1995, Sodety of PetroleumEngineers,Inc. Thw paper wea preperedfor presentationat the Western RegionalMeethg held in Bakersfield,CA, U.S.A., S-10 March 1SS5. Th@PSPWwss selaotedfor preeentatbnby an SPE ProgramOommittaefolfowingreviewof informationcontainedIn an abatrsoleubmittadby the authef(s).Contentsof the paper, es presented,have not bean reviewedby the Sooiity of PetroleumEngineersand era sub@t to oormotionby the author(s).The material,se pWSSntW$ dOSenOtn--lY re~of enY~tl~Ofthe -kfy of PStdeUM Engineers,its officers,or members.Paperspresentedat SPE meetingsare aubjeotto publication ravkw by EditorialCommHtees of the .%cIefy of Pafm$aum Engineers. Permission toqy !srestricted toan stmtrect ofnotmorethan300 words.Illustmfiona mayI-Wb oapiad.The sbatrsct shouldcontainconapicucua acknowkdgmant of where end by whomthe pspar is presented.Write Librarian,SPE, P.0, Sox SSSS3S, Rkhardaon, TX 750s3-SS3S,U.S.A., Telex 1SS24.5SPEUT. ABSTRACTWithdrawal of fluids from shallow, thick and low strength rock can cause substantial reservoir compaction leading to surface subsidence and wel! failure. This is the case for the Diatomite reservoir, where over 10 ft of subsidence have occurred in some areas. Well failure rates have averaged over 3°/0 per year, resulting in several million dollars per year in well replacement and repair costs in the South Belridge Diatomite alone. A program has been underway to address this issue, including experimental, modeling and field monitoring work. An updated elastoplastic rock law based on laboratory data has been generated which includes not only standard shear failure mechanisms but also irreversible pore collapse occurring at low effective stresses (< 150 psi). This law was incorporated into a commercial finite element geomechanics simulator. Since the late 1980s, a network of level survey monuments has been used to monitor subsidence at Belridge. Model predictions of subsidence in Section 33 compare very well with field measured data, which show that water injection reduces subsidence from 7-8 inches per year to 1-2 inches per year, but does not abate well failure.The model has been used to infer potential well failure mechanisms resulting from fluid withdrawal policies and to estimate the impact of alternative operating policies for a 5/8-acre waterflood project; results show that aggressive operation of a newly completed producer can potentially damage a nearby well and that 1/1 producer/injector ratio results in significantly less subsidence and lower shear strains than a 3/1 ratio.

A joint Shell/General Electric field experiment is described in which a new in-situ thermal desorption soil remediation process (ISTD-Thermal Wells) is shown to remove high-boiling-point contaminants from deep soils. For this pilot, a sand pit was prepared with surrogate soil contaminants placed in a cylindrical region 9 feet in diameter and 7 feet deep. Twelve heater/vacuum wells were completed in a triangular array with a 7.25-foot well spacing. During the remediation, electrical resistance heating and vacuum were applied to the wells for a period of 70 days. Soil temperatures were monitored throughout the experiment, and soil samples were taken with a Geoprobe coring unit to observe the removal of contaminants. Energy and material balance data were also collected to improve understanding of process mechanisms. Temperatures above 500 F were achieved in the interwell regions, and contaminants were completely removed despite large inflows of ground water that resulted from heavy rains. A second test at a PCB-contaminated site avoided most of the water influx problems and demonstrated effective heating to over 1000 F and complete removal of the PCBs. Introduction The difficulty in remediating the large number of sites contaminated by toxic, carcinogenic, or radioactive chemicals has generated interest in developing improved processes for cleaning these sites. At present, the typical clean-up response is either (1) capping with an impermeable surface seal to reduce direct exposure of contaminants to human contact and leakage to aquifers, or (2) excavation of the contaminated soil, followed by ex-situ treatment or disposal at another site. In-situ processes, which either destroy contaminants in place or remove them without disturbing the soil, offer advantages over those requiring excavation, by minimizing disturbance at the surface and by reducing costs of full remediation. One of the most versatile and effective of these in-situ processes is In-Situ Thermal Desorption (ISTD), in which heat and vacuum are applied simultaneously to subsurface soils. For shallow soil contamination, an ISTD method using surface heater blankets has been developed. Recently, ISTD-Thermal Blankets have been demonstrated to he highly effective in removing PCBs from soils, and commercial remediation services are now available. For deep soil contamination, a similar thermal vacuum process using heater wells (ISTD-Thermal Wells) has been proposed. As with the thermal blanket, this process is a clean, closed system that is simple and fast. It destroys pollutants in place without having to move the soil. It can be used under roads, foundations and other fixed structures. If required, the thermal wells can be slanted or drilled horizontally. The operations are low profile, quiet, and cause little disruption of adjoining neighborhoods. The process possesses a high removal efficiency because the narrow range of soil thermal conductivities provides excellent sweep efficiency and because its high operating temperature assures complete displacement efficiency of contaminants in the gas phase. Unlike fluid injection processes, ISTD is applicable to tight soils, clay layers, or in soils with wide variations in permeability and water content. In homogeneous soils, fluid injection can be used along with heat injection to speed up the process, provided a vacuum is maintained throughout the heated region. The ISTD-Thermal Wells process is an adaptation of two oil field production methods: vacuum pumping wells to enhance light oil recovery, and well heating for increasing heavy oil production. In one application of technology, an array of heater/vacuum wells is placed vertically in the ground in triangular patterns. The wells are equipped with high-temperature electric heaters and connected to a vacuum blower. P. 905^

Steam foam is a hybrid and novel method of the thermal and chemical flooding to improve the sweep efficiency of steam for producing heavy crude oils. Steam injection is a mature process to substantially reduce the oil viscosity in heavy oil reservoirs to increase its mobility. Steam flooding is an unstable displacement since the gravity of steam causes poor vertical sweep efficiency due to the gravity override in thick high permeability pay zones and poor areal sweep efficiency in high permeability channels with high connectivity. On the other hand foam reduces the mobility of steam by stabilizing the liquid lamellae that cause some or all of the steam to exist as a discontinuous phase. Therefore, foam plugs large pores to divert the flow into the low permeability zones and controls gravity override. Foam increases the pressure gradient slightly in the steam swept regions and leads to heating oil more efficiently when steam diverts into the cold unswept regions. Furthermore, surfactant mobilizes the high viscous oil by emulsification and reduction of interfacial tension. The synergy of steam, surfactant, and foam has the potential to greatly improve the recovery of heavy oil reservoirs.Based on a literature survey, steam foam injection has been conducted in both laboratory corefloods and few field pilots. On the other hand, existing numerical simulators have not been able to capture the mechanisms involved in such a process. In this paper, we present the development and implementation of a new robust steam formulation in a four phase chemical flooding reservoir simulator (UTCHEM) to model and understand the contribution of each mechanism such as viscosity reduction, emulsification, and foam for mobility control. Results illustrate that the steam foam process controls the mobility of steam to avoid incomplete vertical sweep due to gravity segregation. Formation of the emulsion phase by condensing steam along with the presence of water leads to an increase in the emulsion viscosity and thereby decreases water production. The presence of surfactant and emulsification of oil either as water in oil or oil in water emulsions can also impact the displacement and propagation of viscous oil.The mechanistic understanding of steam foam process and improvement of the heat transfer compared to conventional steam flooding is a key finding in this research to optimize the technology that unlocks heavy oil reservoirs with favorable economics.