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Collaborating Authors
Fredrich, J.T.
Abstract Lack of consideration of the geomechanical interaction between salt bodies and surrounding formations has led to documented drilling failures adjacent to salt diapirs, in some cases resulting in individual well abandonment costs of tens of millions of dollars. To address this issue, a three-dimensional non-linear finite element geomechanical simulation effort was initiated to analyze the in situ stress state existing in and adjacent to salt bodies before drilling as well as under producing conditions. This work leverages unique expertise in salt mechanics and computational geomechanical modeling. Non-linear finite element geomechanical models were developed for four idealized deepwater Gulf of Mexico geometries including a spherical salt body, a horizontal salt sheet, a columnar salt diapir, and a columnar salt diapir with an overlying tongue. The analyses reveal that at certain locations for specific geometries: shear stresses may be highly amplified; horizontal and vertical stresses may be significantly perturbed from their far-field values; principal stresses may not be vertical and horizontal (i.e., the vertical stress may not be the maximum stress); and anisotropy in the horizontal stresses may be induced. For some geometries, the vertical stress within and adjacent to the salt is not equal to the gravitational load; i.e., a stress-arching effect occurs. Analogously, the assumption that the horizontal stress within a salt body is equal to the lithostatic stress is shown to be incorrect sometimes. The modeling also suggests an alternative explanation for the so-called rubble zones thought to occur beneath and/or adjacent to salt diapirs, in that they may be an intrinsic consequence of the equilibrium stress field needed to satisfy the different stress states that exist within the salt body and in the non-salt surrounding formations. We demonstrate with an example how this work can enable more rigorous planning of well locations and trajectories by providing more accurate estimates of the vertical and horizontal stresses around and within salt bodies for wellbore stability analyses so as to avoid areas of potential geomechanical instability, and to enable accurate fracture gradient prediction while entering, drilling through, and exiting salt bodies. Introduction The deepwater Gulf of Mexico (GoM) is the most active deepwater region in the world, currently providing some of the greatest challenges in scope and opportunity for the industry. The region is estimated to contain undiscovered recoverable resources of at least ~13 billion boe, and is known to harbor exceptional reservoirs such as the Thunder Horse discovery at a water depth exceeding 6,000 ft and with estimated recoverable reserves of at least 1 billion boe. However, the complex salt tectonics and the extreme water and reservoir depths necessitate high development costs, and innovative technology is required to bring these fields on stream. Integral to successful economic development is a well service lifetime of 15–30 years. Many of the significant deepwater GoM objectives are subsalt, with several thousand feet of salt not being uncommon. At the edge of the industry's experience are salt sections of 10,000-ft thickness that overlie targets at depths of 25,000(30,000 ft below mudline. The cost of drilling these deepwater subsalt wells is substantial, and, in some cases, operators have been forced to sidetrack or even abandon wells after experiencing drilling difficulties, with losses running to several tens of millions of dollars. The zone lying immediately below the salt section is notoriously difficult, and there are well known difficulties in accurately predicting fracture gradient and pore pressure immediately upon exiting the salt body. There are necessarily two key components in assuring the considerable investment that must be made to develop these deepwater subsalt fields. First, the planning of well locations and trajectories needs to consider the large-scale geomechanical loading conditions that exist in and around massive salt bodies, and second, the well casing designs need to consider the long-term casing loading that will occur because of salt creep. In this paper we present some of our work addressing the first area that relates to the large-scale geomechanical setting, and specifically, to unique geomechanical effects associated with the presence of massive salt bodies. Our other work that addresses the second component focuses on wellbore-scale finite element modeling to assure the long-term integrity of through-salt well casings and is presented elsewhere.
- Geology > Structural Geology > Tectonics > Salt Tectonics (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Viosca Knoll > Block 990 > Pompano Field (0.99)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Green Canyon > Block 826 > Mad Dog Field (0.99)
- North America > United States > Gulf of Mexico > Central GOM > East Gulf Coast Tertiary Basin > Green Canyon > Block 825 > Mad Dog Field (0.99)
- (4 more...)
Summary Assuring the integrity of subsalt wells in the deepwater of the Gulf of Mexico throughout the field's life is a major drilling engineering challenge. The consequences of well failures may result in billions of dollars in remedial costs and lost production. On the other hand, the costs associated with overly conservative well design are significant, which motivates systematic analysis of casing loading for scenarios of interest. Simplified hole-closure and casing-design guidelines for salt, many developed for the western U.S. Overthrust Belt, are not appropriate for the relatively pure, slow-moving halite found along the Gulf Coast. Instead, our work applies knowledge gained by Sandia Natl. Laboratories from technical research and development (R&D) investigations of the Waste Isolation Pilot Plant (WIPP) and the Strategic Petroleum Reserve (SPR) to determine the magnitude and timing of salt loading on well casings in the Gulf of Mexico. If hole quality can be assured, the analyses presented show that it is not always necessary to cement the casing/borehole annulus through the salt because the subsequent uniform loading is insufficient to substantially deform the casing. This poses no threat to drilling operations or impingement on the inner casing string in the long term and results in considerable cost savings. However, if hole quality is poor, a cemented annulus is necessary, as the cement effectively transforms the potentially nonuniform loading situation into one of uniform loading. Significant benefits can accrue from quantifying the magnitude and timing of salt loading. Difficult cementing jobs and liner tiebacks can be omitted and a more aggressive well design adopted. The simplified well design, and the elimination of potentially troublesome operations, leads to millions of dollars in cost savings in individual wells. Introduction The Gulf of Mexico is the most active deepwater region in the world, currently providing some of the greatest challenges in scope and opportunity for the industry. Undiscovered, recoverable resources are estimated to be at least ~13 billion barrels of oil equivalent (boe). However, the complex salt tectonics and extreme water and reservoir depths necessitate very high development costs, in addition to requiring innovative technology to bring these fields on stream. A well lifetime of 10 to 25 years is integral to successful economic development (where the cost of a single well can be from U.S. $20 to 60 million). A significant majority of the wells will potentially penetrate considerable salt thicknesses, with 1,000 to 6,000 ft of salt not uncommon. Therefore, assuring the longevity of well casings drilled through salt is a major casing-design requirement for these subsalt developments. Though the behavior of salt is well described from a geologic standpoint, our knowledge of the influence of salt deformation on both a well and reservoir scale (both temporally and spatially) is poor. However, the nature of the deformation occurring during field life is considered more likely to be detrimental than beneficial. In subsalt reservoirs in which the salt is laterally extensive and in close vertical proximity to the reservoir formations, there may be a tendency for the salt to flow laterally to fill "subsidence bowls" formed by compaction of the reservoir interval. This lateral movement could jeopardize the integrity of well casings drilled through the deforming salt because of anisotropic loading and induced shears at the bounding formation interfaces. It is important, therefore, that loading by salt is properly defined and incorporated in any casing design. Because salt may typically be encountered at relatively shallow depths below mudline in deepwater wells, well failure caused by salt loading (e.g., collapsed or ruptured casings) may require redrilling the entire well. Unforeseen failures can, in deep water, make entire field development projects uneconomical. Casing Design Adjacent to Salt - Historical Perspective Within the petrochemical industry and the geomechanics community, previous assessments of salt behavior pertaining to well design have principally fallen into one of four categories, addressed in the following sections. Experimental. These investigations are principally concerned with the constitutive behavior of salt deformation without necessarily relating the mechanics to a specific engineering problem; good examples of these approaches include Refs. 1 through 6. The work undertaken in support of the SPR and WIPP projects is particularly thorough. Of particular relevance to casing design, the work by Cheatham and McEver, though published in 1964, is still a pioneering work. Their experimental work, subjecting copper tubes to various loading conditions and transmitting by blocks of rock salt (from Hockley, Texas), set the ground rules for three subsequent decades of assessing casing design adjacent to salt.It is not economically feasible to design casing for the most severe situations of nonuniform loading. When the annulus is completely filled, by cement or salt, casing is subjected to a nearly uniform loading approximately equal to the overburden pressure. Incorporating Nonuniform Loading Effects Into the Casing Design. Recognizing that salt has the potential to apply nonuniform loading, several studies have proposed methodologies for including this in the casing design. Incorporation of nonuniform loading has application beyond salt loading, in tectonically stressed areas, for example. The principal thrust of these studies has been to include the effects of a unidirectional load, an opposed line load, or loading by means of a limited contact arc (see Fig. 1). With some modification in parameter values to achieve field-specific calibration, these methodologies have proven to be largely successful. As discussed later, several of the models assume a simplistic (and in some circumstances, a potentially unrealistic) mode of salt deformation that bears little correspondence to present-day knowledge of the mechanics of salt flow around underground openings.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral > Halide > Halite (0.70)
- Geology > Structural Geology > Tectonics > Compressional Tectonics > Fold and Thrust Belt (0.49)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock (0.47)
- Energy > Oil & Gas > Upstream (1.00)
- Materials > Chemicals > Commodity Chemicals > Petrochemicals (0.74)
- South America > Colombia > T Formation (0.99)
- North America > United States > South Dakota > Williston Basin (0.99)
- North America > United States > North Dakota > Williston Basin (0.99)
- (3 more...)
Abstract In the next decade significant new oil field developments will be brought on-stream in the deep water Gulf of Mexico. A large number of these wells will be drilled through salt. Assuring the integrity of these wells over the life of the field is a major drilling engineering challenge, as the consequences of well failures may result in billions of dollars remedial costs and lost production. To address these challenges for BP's deepwater developments, advanced numerical finite element analyses of salt / casing interaction were undertaken by Sandia National Laboratories using state-of-the-art computational modeling developed as part of extensive research supporting the Strategic Petroleum Reserve (SPR) and the Waste Isolation Pilot Plant (WIPP) projects. The paper summarizes, briefly, the salient features of salt mechanical behavior of relevance to well integrity. The results of numerical modeling of casing loading are presented which focus on the importance of assessing the possibility of both uniform and non-uniform loading by the salt; and the impact that production-induced heating of the salt has on the rate and magnitude of casing loading. Based on the findings of these analyses, strategies for minimizing the magnitude of loading on well casings are discussed. The relative merits of cementing the annulus through salt are compared with the option of leaving the annulus open in order to defer the time at which salt loading occurs. Introduction The deepwater Gulf of Mexico (GoM) is the most active deepwater region in the world and currently provides some of the greatest challenges in scope and opportunity for the industry. The deepwater GoM is estimated to contain undiscovered recoverable resources of at least ~13 billion boe, and is known to harbor some exceptional reservoirs such as the recent Crazy Horse discovery at over 6,000 feet water depth with estimated recoverable reserves of at least 1 billion boe. However, the complex salt tectonics and extreme water and reservoir depths necessitate very high development costs, in addition to requiring innovative technology to bring these fields on stream. Integral to the successful economic development of these fields (where the cost of a single well can be $20-$60 MM) is a well lifetime of 10 to 20 years. A significant majority of these wells will penetrate potentially considerable thicknesses of salt, 1000 ft to 6000 ft of salt not being uncommon. Therefore, assuring the longevity of well casings drilled through salt is a major requirement in the casing design for these sub-salt developments. Though the behavior of salt from a geologic standpoint is quite well described, our knowledge of the influence of salt deformation on both a well scale, and reservoir scale (both temporal and spatial), is poor. However, the nature of the deformation occurring over field life is considered more likely to be detrimental than beneficial. In sub-salt reservoirs, where the salt is laterally extensive and in close vertical proximity to the reservoir formations there will be a tendency for the salt to flow laterally to fill 'compaction bowl's formed by production from the reservoir interval. This lateral movement of the salt could jeopardize the integrity of well casings drilled through the deforming salt as a consequence of anisotropic loading and induced shears at the bounding formation interfaces. It is important, therefore, that the loading by salt is properly defined and included in the casing design. The consequences of well failures can be very severe. As salt may typically be encountered at relatively shallow depths below mudline in deepwater wells, failure of the well due to salt loading (e.g. collapsed or ruptured casings) may require the re-drilling of the entire well. Unforeseen failures could, in deepwater, make the field development project uneconomic were they to occur.
- Geology > Structural Geology > Tectonics (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral > Halide > Halite (0.69)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock (0.47)
- Government > Regional Government > North America Government > United States Government (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- South America > Colombia > T Formation (0.99)
- North America > United States > South Dakota > Williston Basin (0.99)
- North America > United States > North Dakota > Williston Basin (0.99)
- (8 more...)
- Well Drilling > Wellbore Design > Wellbore integrity (1.00)
- Well Drilling > Drilling Operations (1.00)
- Well Drilling > Drilling Fluids and Materials (1.00)
- (2 more...)
ABSTRACT: 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. INTRODUCTION 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.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock (0.34)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.46)
- North America > United States > California > San Joaquin Basin > San Joaquin Valley > Tulare Formation (0.99)
- North America > United States > California > San Joaquin Basin > Lost Hills Field (0.99)
- North America > United States > California > San Joaquin Basin > Belridge Field (0.99)
- (3 more...)
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. 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. 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. 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. Analyses of the field-wide database indicated that two-dimensional approximations 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. 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.
- Geology > Rock Type > Sedimentary Rock > Siliceous Rock > Diatomite (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > California > San Joaquin Basin > South Belridge Field > Tulare Formation (0.99)
- North America > United States > California > San Joaquin Basin > South Belridge Field > Diatomite Formation (0.99)
- North America > United States > California > San Joaquin Basin > San Joaquin Valley > Tulare Formation (0.99)
- (9 more...)
ABSTRACT ABSTRACT: At low mean stresses, porous geomaterials fail by shear localization, and at higher mean stresses, they undergo strain-hardening behavior. Cap plasticity models attempt to model this behavior using a pressure-dependent shear yield and/or shear limit-state envelope with a hardening or hardening/softening elliptical end cap to define pore collapse. While these traditional models describe compactive yield and ultimate shear failure, difficulties arise when the behavior involves a transition from compactive to dilatant deformation that occurs before the shear failure or limit-state shear stress is reached. In this work, a continuous surface cap plasticity model is used to predict compactive and dilatant pre-failure deformation. During loading the stress point can pass freely through the critical state point separating compactive from dilatant deformation. The predicted volumetric strain goes from compactive to dilatant without the use of a non-associated flow rule. The new model is stable in that Drucker's stability postulates are satisfied.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type (0.95)
- Energy > Oil & Gas > Upstream (1.00)
- Government > Regional Government > North America Government > United States Government (0.93)
- North America > United States > California > San Joaquin Basin > San Joaquin Valley > South Belridge Field > Tulare Formation (0.99)
- North America > United States > California > San Joaquin Basin > San Joaquin Valley > South Belridge Field > Diatomite Formation (0.99)
- North America > United States > California > San Joaquin Basin > Lost Hills Field (0.99)
- North America > United States > California > Monterey Formation (0.99)
ABSTRACT ABSTRACT: Energy production, deformation, and fluid transport in reservoirs are linked closely. Recent field, laboratory, and theoretical studies suggest that, under certain stress conditions, compaction of porous rocks may be accommodated by narrow zones of localized compressive deformation oriented perpendicular to the maximum compressive stress. Triaxial compression experiments were performed on Castlegate, an analogue reservoir sandstone, that included acoustic emission detection and location. Initially, acoustic emissions were focused in horizontal bands that initiated at the sample ends (perpendicular to the maximum compressive stress), but with continued loading progressed axially towards the center. This paper describes microscopy studies that were performed to elucidate the micromechanics of compaction during the experiments. The microscopy revealed that compaction of this weakly-cemented sandstone proceeded in two phases: an initial stage of porosity decrease accomplished by breakage of grain contacts and grain rotation, and a second stage of further reduction accommodated by intense grain breakage and rotation.
- Geology > Mineral (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.92)
- Government > Regional Government > North America Government > United States Government (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Tor Formation (0.99)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Ekofisk Formation (0.99)
- North America > United States > Texas > Permian Basin > Martin Field > Ellenburger Formation (0.98)
Three-Dimensional Geomechanical Simulation of Reservoir Compaction and Implications for Well Failures in the Belridge Diatomite
Fredrich, J.T. (Sandia National Laboratories) | Arguello, J.G. (Sandia National Laboratories) | Thorne, B.J. (Sandia National Laboratories) | Wawersik, W.R. (Sandia National Laboratories) | Deitrick, G.L. (Shell E&P Co.) | de Rouffignac, E.P. (Shell E&P Co.) | Myer, L.R. (Lawrence Berkeley National Laboratory) | Bruno, M.S. (Terralog Technologies)
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. P. 195
- Geology > Rock Type > Sedimentary Rock > Siliceous Rock > Diatomite (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > California > San Joaquin Basin > South Belridge Field > Tulare Formation (0.99)
- North America > United States > California > San Joaquin Basin > South Belridge Field > Diatomite Formation (0.99)
- North America > United States > California > San Joaquin Basin > San Joaquin Valley > Tulare Formation (0.99)
- (8 more...)
- Well Drilling > Wellbore Design > Wellbore integrity (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Integration of geomechanics in models (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (1.00)
INTRODUCTION ABSTRACT: A suite of experiments was performed to investigate the effect of solution transfer processes on the frictional properties of faults. Triaxial experiments were conducted at an effective confining pressure of 175 MPa and temperature of 235°C on sawcut cylinders of Sioux quartzite which contained a layer of simulated quartz gouge. Some samples were "healed" at elevated confining and pore pressures and temperatures up to 800øC for varying periods of time prior to loading. Unhealed samples slide in a stable mode and approach a quasi steady-state strength at shear strains of 3-4%. Healed samples, slid at identical pressures and temperature, are stronger (i.e. higher ?.) and exhibit unstable stick-slip behavior. A quasi steady-state strength is eventually established after the initial stick-slip event in samples healed at 636°C; however, samples healed at 817øC stick-slip repeatedly. Experiments in which the healing time was varied indicate that the bulk of the strength recovery occurs during the first 60 minutes at elevated temperature. Unhealed samples for which the time of stationary contact was varied do not exhibit differences in either strength or stability. No strength recovery is observed in a sample healed without pore fluids at identical effective pressures. SEM of fracture surfaces of the healed gouge reveals a marked absence of fines as compared to the starting powder, smooth grain faces, rounded grain shapes, and grain interpenetration. Seismologic observations suggest that faults may recover strength during the interseismic period of the earthquake cycle. Larger values of average stress drop are associated with crustal faults with longer repeat times (Kanamori & Allen 1986). Also, stiess drops associated with intraplate earthquakes are typically larger than those of interplate earthquakes, perhaps owing to the longer recurrence times associated with the former (Scholz et al. 1986; Kanamori & Anderson 1975). Although laboratory observations of the variation of stress drop amplitude with loading vel .ocity and the time of stationary contact at room temperature are in qualitative agreement with the seismological observations, the observed variations are significantly less than expected, thus suggesting that other healing mechanisms operate during the interseismic phase (Scholz et al. 1986; Wong & Zhou 1990). Field observations also suggest that the physical properties of fault zones change with time. For example, fluid flow localized along fault zones may cause variations in the density, permeability, and mineralogy of rocks and gouge in the fault zone which may, in turn, affect the mechanical properties of the fault (Angevine et al. 1982; Chester & Logan 1986; Parry et al. 1988). In this paper we present results of a suite of triaxial experiments performed to investigate the effect of solution transfer processes on the mechanical properties of simulated fault gouge. To study such thermally activated processes in the laboratory, it is necessary to accelerate the kinetics by elevating temperature. We conducted frictional sliding experiments at confining pressures, Pc, of 250 MPa, pore pressures, Pp, of 75 MPa, and temperatures of 250°C on samples which were "healed" at temperatures up to 800°C for varying periods of time prior to axial loading. Our results indicate that the thermally activated processes which occur in the presence of aqueous pore fluids strongly affect both the frictional strength and stability of sliding.
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)