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ABSTRACT The goals of this work include reviewing the phenomena of stress rotation in the presence of faulting, salt bodies, and non-uniform distribution of physical rock properties, pore pressure, and tectonic deformation; numerically simulating the stress-field distribution for these cases at the field scale by using a 3D finite element method (the tectonic stress factor and gravity loads are included in the loading used in the analysis); and conducting a numerical analysis of the variation of stress orientation within formations caused by existence of salt body and/or pore pressure depletion. Examples from a field model which consists of a salt body of 7-km in diameter along with the Ekofisk field in the North Sea have been analyzed numerically. A 3D calculation of deformations of the formation matrix is combined with porous flow. Non-uniform initial stress field and non-uniform initial pore pressure field are constructed by means of user subroutines included in commercial finite element software. This paper also includes suggestions regarding stress orientation related topics, such as trajectory optimization and safe mud weight window design. 1. INTRODUCTION The orientation of the principal horizontal stress has an important influence on completion design, i.e., casing direction and hydraulic fracturing. Several tectonic and depositional mechanisms influence the orientation of the principal stresses: 1) relaxation of stresses adjacent to faults; 2) accumulation of stresses adjacent to faults prior to slippage; 3) halo kinetics, i.e., movement of salt masses; 4) slumping; and 5) rapid deposition of sediments on top of a subsurface environment dominated by strike-slip or reverse faulting. Stress rotation has been reported by several authors in various drilling environments that share common complex geological structures, including active tectonic regions, complex fault and joint systems, salt bodies, and depleted reservoirs. Stress rotation can be observed within one well or from one well to another well. It causes extremely expensive and difficult wellbore stability problems during drilling, completion, or production, and represents a challenge for the oil industry in both operation and modeling. As a result, a large effort has been expended to study and fully understand this phenomenon. Martin and Chandler reported a maximum horizontal stress rotation near two major thrust faults that were intersected during the excavation of the Underground Research Laboratory (URL) in the Canadian Shield [1]. In this region, the fault system divides the rock mass into varying stress domains. Above the fault system, the rock mass contains regular joint sets, in which the maximum horizontal stress is oriented parallel to the major sub-vertical joint set. Below the fault system, the rock is massive with no jointing; the maximum horizontal stress has rotated approximately 90° and is aligned with the dip direction of fracture zone. Stress rotation is commonly observed where the block of rock above the fault has lost its original load because of displacement above the fault; this results in considerably less maximum horizontal stress magnitude than the magnitudes below the fracture zone, where the maximum horizontal stress magnitude is fairly constant [1].
- North America > United States (1.00)
- Europe > Norway > North Sea > Central North Sea (0.35)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Structural Geology > Tectonics > Plate Tectonics (0.74)
- Geology > Structural Geology > Fault > Dip-Slip Fault > Reverse Fault (0.68)
- Geology > Structural Geology > Tectonics > Salt Tectonics (0.46)
- South America > Colombia > Casanare Department > Llanos Basin > Cusiana Field > Mirador Formation (0.99)
- 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 > Viosca Knoll > Block 827 > Tahoe Field (0.99)
- (8 more...)
ABSTRACT Early during construction of Tekeze Dam, unfavorable geologic and topographic conditions were encountered on the left abutment, prompting an urgent program to evaluate foundation stability, expected behavior under all loading conditions, and to develop solutions for implementation within the construction contract and schedule. Potential failure mechanisms were identified and an approach was adopted to explore these and to evaluate design options. Block theory, kinematic, and FEM analyses were performed, results showing that lack of rock mass confinement was the controlling scenario. Results indicated potential for large foundation displacements and that dam thrust loads would not be dissipated into the rock mass. Large-scale support and treatment measures were designed and constructed to reinforce and stabilize the rock mass. These included a large concrete thrust block anchored with high capacity tendons, and a combination of anchors and concrete buttress support in the area immediately downstream. Lessons learned include the importance of checking accuracy of topography used for final design and construction before excavation starts, the necessity of immediate responsive action to evaluate the severity of a condition once it is identified, and to reach acceptable design solutions which can be accommodated into an ongoing construction contract with minimal disruption to the schedule. INTRODUCTION This technical paper presents the results of geological and geotechnical analyses and evaluations performed at the upper left abutment of Tekeze Dam, located on the Tekeze River in Northern Ethiopia, East Africa. Construction of the 300-MW Tekeze Hydropower Project, of which the dam is an integral part, was completed in 2009. The dam is a variable radius, thin-arch type with a structural height of 188 m and crest length of about 460 m. The dam crest is at elevation 1145 m, the maximum water level is expected to be at elevation 1140 m, and the dam foundation at river level is at elevation 957 m. The purpose of these foundation studies was to evaluate the stability and behavior of the left abutment following dam construction and to develop design recommendations for stabilizing and reinforcing the rock mass to substantially resist the expected loads imposed by thrust from the arch dam. The evaluation was prompted by construction staff who had expressed concerns in late 2004 about unfavorable foundation conditions being revealed in excavations for the left abutment thrust block and keyway, and in initial construction of the grouting/drainage gallery at El. 1105 on the left bank. One issue was the presence of major joints subparallel to the slope that, combined with the effects of weathering, was contributing to a serious and unstable toppling condition (Figs. 1 and 2) that affected construction safety and schedule though was mostly at elevations above the dam and its foundation. However, during field study of this condition a more worrisome concern evolved from recognition that there was a substantial disconnect between the pre-design and actual topography of the upper left abutment area. The pre-excavation surface topography did not adequately reflect the actual bedrock topography exposed immediately downstream of the left abutment which is more deeply incised and closer to the abutment keyway than the available topographic maps and surveys showed (Figures 3 and 4).
- Africa (1.00)
- North America > United States > Colorado (0.28)
- North America > United States > California (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral (0.69)
ABSTRACT We present a rigorous validation of the analyticalAmadei solution for the stress concentration around arbitrarily orientated borehole in general anisotropic elastic media. First, we revisit the theoretical framework of the Amadei solution and present analytical insights that show that the solution does indeed contain all special cases of symmetry, contrary to previous understanding, provided that the reduced strain coefficients ß11 and ß55 are not equal. It is shown from theoretical considerations and published experimental data that the ß11 and ß55 are not equal for realistic rocks. Second, we develop a 3D finite-element elastic model within a hybrid analyticalnumerical workflow that circumvents the need to rebuild and remesh the model for every borehole and material orientation. Third, we show that the borehole stresses computed from the numerical model and the analytical solution match almost perfectly for different borehole orientations (vertical, deviated and horizontal) and for several cases involving isotropic and transverse isotropic symmetries. It is concluded that the analytical Amadei solution is valid with no restrictions on the borehole orientation or elastic anisotropy symmetry. 1 Introduction The calculation of stresses and displacements around cavities is required in some of the most important subsurface geotechnical engineering problems such as for boreholes, tunnels and mine excavations. For example, the presence of a borehole in a stressed subsurface rock formation alters the local principal stress directions and magnitudes around the borehole and away from it over a distance of several borehole diameters. For isotropic elastic homogeneous rocks, borehole stresses are given by the classical elastic solution by [1] or its generalized version for nonaligned borehole and stress directions by [2, 3] and [4]. Borehole stresses depend on the far-field stress, the orientation of the borehole with respect to the stress field directions, the wellbore pressure and the material Poisson’s ratio. These solutions are very convenient for practical purposes as all the borehole stress components except the axial component are independent of the material elastic properties, and the axial component is only dependent on the Poisson’s ratio by the virtue of the plane strain assumption. Most wells drilled for the purpose of natural oil and gas extraction encounter anisotropic shale formations during the drilling process, either in the overburden for conventional reservoirs or in the reservoir itself for unconventional shale reservoirs. For conventional clastic reservoirs, it has been reported that shales constitute about 75% of the clastic fill of sedimentary basins [5] and for unconventional reservoirs, the recent exploration and production of US gas shale reservoirs has put a renewed focus on drilling and hydraulic fracturing in shale formations [6]. Shales are known to exhibit anisotropic properties not only for their elastic behavior [7, 8, 9, 10, 11] but also for their strength due to their laminated structure [12, 13]. [14] gives a thorough review of existing experimental data in shales. Today, most wells drilled in highly deviated or horizontal directions are penetrating strongly transverse isotropic formations or lower symmetries such as orthorhombic or monoclinic if fractures are present.
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- North America > United States > South Dakota > Williston Basin (0.99)
- North America > United States > North Dakota > Williston Basin (0.99)
- North America > United States > Montana > Williston Basin (0.99)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Shale gas (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.88)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Open hole/cased hole log analysis (0.88)
ABSTRACT When an underground excavation is cut in a discontinuous rigid rock mass, instabilities may occur mainly due to block failure. Isolated rigid block methods were developed since the 80’s to locate critical blocks and evaluate their stability but have major drawbacks: ignoring in-situ stresses, mechanical behavior of joints and rotational movements. Other methods included those variables improperly and were limited to simple cases. This paper presents a critical review of previous approaches then a more complete model to study isolated rock blocks is proposed. It is based on the fact that stresses on the block faces are known before excavation and once a face is freed, the block moves as a rigid body in translation and rotation. Applying equilibrium and rock joint behavior equations, the stresses on the faces after excavation can be calculated and stability evaluated using a Mohr- Coulomb criterion. Any block geometry can be studied by partitioning the block faces into simple elements. Numerical integration is done on elements using Gauss points. The method is applied on a case study and comparisons are made with other simplified methods. Finally, a parametrical analysis shows the important influence of in-situ stresses and joint stiffnesses on the block’s stability. 1. INTRODUCTION When underground excavations are made in rigid rock masses intercepted by several discontinuities, blocks may form at the free surface and present the risk of sliding or falling into the open space causing damage. Modeling such phenomena is an essential requirement in order to predict a degree of instability and evaluate the support needed. Adopting a continuum model without discontinuities is not appropriate in this case since the main deformation occurs due to the displacements about the joints rather than to the deformation of the rock matrix. Furthermore, applying a complete discontinuous method, including all joints, is computationally hard because of the complexity of the three dimensional geometry. Additionally, the uncertainties concerning the distribution of joint sets require performing multiple simulations in order to cover all the possibilities. From the necessity to overcome this complexity, came the idea of studying separately only the blocks formed at the surface of the tunnel. Supporting the unstable blocks is assumed to assure stability for all the rock mass. This approach is simple and can provide the engineer with an easy tool to evaluate the stability of the excavation or to choose its optimum direction. The generation of blocks can be made by studying all the combinations of discontinuities that may form removable blocks at the surface of the tunnel. This approach does not require information of spacing between discontinuities or their exact location (ubiquitous approach). Another approach consists of generating all blocks by using distribution of joint sets or introducing joints one by one (specific approach). This article does not consider the problem of generation of the rock blocks in the rock mass. It focuses only on the stability analysis of a rock block once it is generated by whatever method. Isolated rock block methods have been widely used.
- North America > United States (1.00)
- South America > Chile (0.85)
- Geology > Mineral (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Materials > Metals & Mining (0.95)
- Well Completion > Hydraulic Fracturing (1.00)
- Data Science & Engineering Analytics > Information Management and Systems (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.69)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Naturally-fractured reservoirs (0.51)
ABSTRACT In order to clarify the mechanisms of hydraulic fracturing in unconsolidated formation, we carried out laboratory tests in this study. For a simulated borehole, a casing pipe was buried vertically in a cubical specimen of unconsolidated sands with moderate permeability of 5 mD. After the specimen was subjected to anisotropic triaxial compressive stresses of a few MPa, a fracturing fluid of a viscous machine oil injected into the casing pipe, and it came out through an axial slit of the casing pipe into sands. After the test, the specimens were cut off bit by bit in order to check fracture formation and invasion of the fracturing fluid. Then we observed that fractures were induced actually and the fracture pattern obviously changed with azimuthal orientation of the slit relative to azimuth of the maximum horizontal stress. A single, double or multiple fractures were induced by combination of the slit orientation and a manner of pressurization. It is inferred from the elasto-plastic FEM analysis that such fracture patterns were caused by variation of stress state around the casing pipe due to sliding of pre-induced fractures and yielding. Those result suggest the possibility to control not only length and volume but also patterns of the fractures hydraulically induced in unconsolidated formation. 1. INTRODUCTION In petroleum industry, the technique of hydraulic fracturing has been used routinely to enhance well productivity of hydrocarbon through creation of flow pathway in reservoirs. In such background, a number of theoretical and experimental works have been done for understanding the behavior of hydraulic fractures, which are borehole pressures required for initiation and propagation of fractures, orientation and geometry of induced fractures etc. Those works have led to the conventional theory of hydraulic fracturing based on linear elastic fracture mechanics (LEFM). This theory assumes low permeability competent rocks and states that fracture initiation and propagation should be governed by the fracture tip effects such as fracture toughness, the non wetted zone (fluid lag) and the process zone etc. From the early 1990’s, moderate-to-high permeability unconsolidated sand (permeability k > 10 mD [1]) have become a target of fracturing stimulation in petroleum engineering. Then hydraulic fracturing is performed to bypass formation damage around a borehole and to mitigate sand production problems, and such a treatment is known as FracPack completion. Unlike competent rocks, unconsolidated sand has little or no tensile strength and it is naturally in a condition as already fractured at particle scale, i.e. micro scale. Hence no material bond exists between particles and no new surface is created at the particle scale. Accordingly, in unconsolidated sand, the aforementioned fracture tip effects should not be present, and fracture formation at macro scale should occur in a manner fundamentally different from the conventional theory of LEFM as defined for competent rocks. In order to clarify the mechanisms of hydraulic fracturing in unconsolidated formation, we have been examining the phenomena by an approach of laboratory experiment [2, 3].
ABSTRACT An analytical solution on the uncertainty in the estimates of volumetric fraction of different compositions in a Bimrock is presented in this paper. The derivation was developed based upon the concept of representative volumes. To tackle the anisotropic orientation of blocks, this study developed the concept of a parallelogram representative volume element that contains an ellipse. Our results demonstrate that the uncertainties in the estimates depend upon the aspect ratio, orientation, and diameter of blocks and the level of volumetric fraction. We further simplified the analytical expression in terms of intercept length. Our results were verified through numerical simulation albeit preliminary. 1. INTRODUCTION We have previously developed an analytical solution to the uncertainty of volumetric fraction estimates of isotropic, Block-in-Matrix, ( Bimrocks) [1]. Bimrocks are defined as “a mixture of rocks, composed of geotechnically significant blocks within a bonded matrix of finer texture” [2]. Bimrocks thus encompass a wide range of geologic materials including, for example, melanges, fault rocks, landslide debris, and glacial till. Their overall mechanical behaviors are highly dependent on their volumetric block fraction [2][3][4]. Three categories of measurement methods have been used in estimating the volumetric fraction, Vf , of Bimrocks, namely, one-dimensional (linear measurement and borehole), two-dimensional (image analyses and window mapping) and three-dimensional (sieve analyses). Although sieve analysis is the most accurate method for laboratory-scale studies, separation of blocks from the weaker matrix is not always possible, affecting by factors such as the number and size of blocks, and the degree of contact strength between blocks and matrix [1]. According to the basic principles of stereology, if the sampling is under IUR - isotropic, uniform and random conditions- such that all portions of the structure are equally represented (uniform), there is no conscious or consistent placement of measurement regions with respect to the structure itself to select what is to be measured (random), and all directions of measurement are equally represented (isotropic) [5], namely, the results be the same regardless of the dimension of a measurement method. Or, simply put Vf = Af = Lf = Pf, where Vf is volumetric fraction, Af is area fraction, Lf is linear fraction, and Pf is point-count fraction. Thus, in this paper we interchange the use of Vf, Af and Lf. Scanline is one of the most efficient and economical method for estimating Vf , and may be the only way such as in the case of sampling through drilling. Scanlines estimate Vf by dividing the total cumulative intercept length, or block length, with the total scanlin length. Thus its operation and processing is rather straightforward. But scanline use has a caveat: How does one determine what constitute an adequate cumulative length of scanlines?
ABSTRACT In this paper, the stress-dependent permeability in fractured rock masses have been investigated considering the effects of nonlinear normal deformation and shear dilation of fractures using a two-dimensional distinct element method program, UDEC, based on a realistic discrete fracture network (DFN) realization . Also simple estimates concerning the effects of in-situ stresses on fracture aperture variations have been used in this study. Considering that Hydro-Mechanical coupling with realistic fracture network is not well investigated and Fractures have a big impact on reservoir production but are inherently difficult to quantify. This condition should be interpreted as stress condition with mechanical loading.. A new analytical and numerical model was proposed to determine the relationship between fracture dip angle, aperture and permeability. The numerical work were conducted in two ways: (1) increasing the overall stresses with a fixed ratio of horizontal to vertical stresses components; and (2) increasing the differential stresses while keeping the magnitude of vertical stress constant. The results shown that at the stress ratio of 1 the significant shear dilation occurs at an approximately low stress and mean fracture angles. For the differential stresses case, the shearing process can result in breakage of the asperities, resulting in the decrease of the dilation rate and strain softening of the fracture. Therefore pressure changes and productivity index of production reservoirs extremely affected by this phenomenon. 1. INRODUTION In general, the understanding of the fracturing process has recently progressed from an empirical to a more scientific approach, .and therefore, reservoir description and reservoir modeling has benefited. It is, however, necessary to remember that the evaluation of fracturing is far more complex than the evaluation of porosity and permeability in a conventional reservoir. In fact, the fracturing depends on the pattern of mechanical stresses of the rock material and rock properties. Hence, the results of fracturing, such as fracture openings, size, distribution, orientation, etc., will be related to stresses and type of rock (brittle or ductile), structural conditions, depth (overburden stress). In reservoir conditions an elementary rock volume is in a state of stress provoked by overburden (geostatic) pressure, confining pressure, fluid (pore) pressure and, in addition, tectonic forces. Adopting the usual representation of forces by three normal directions and designating the three normal vectors as the principal stresses. However, the characterization of fluid flow through fractured rocks is one of the most challenging problems faced by the geotechnical engineers. This difficulty largely comes from the fact that rock is a heterogeneous material contains various natural fractures in different scales [1]. Apertures of fractures can change due to normal stressinduced closures or openings and due to shear stressinduced dilations.Hence, the permeability of fractured rock masses is stress dependent. This process can be viewed as an ‘indirect’ hydro-mechanical coupling that occurs when the applied stresses produce a change in the hydraulic properties, whereas a ‘direct’ coupling occurs when the applied stresses produce a change in fluid pressure and vice versa [2].
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Geological Subdiscipline > Economic Geology > Petroleum Geology (0.51)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Unconventional and Complex Reservoirs > Naturally-fractured reservoirs (1.00)
- Reservoir Description and Dynamics > Reservoir Fluid Dynamics > Flow in porous media (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
ABSTRACT Hydraulic fracturing of transversely isotropic reservoirs is of major interest for reservoir stimulation and in-situ stress estimations. Rock fabric anisotropy not only causes changes in near-wellbore stress concentration but also affects fracture initiation from the wellbore. Analytical solutions are developed and used to investigate stress distributions around horizontal wells in shale and to study the variation of fracture initiation pressure and angle with degree of anisotropy. After verification of the solutions, a sensitivity analysis is carried out to consider the impact of different mechanical properties including Young’s modulus and Poisson’s ratio on the fracture initiation pressure and fracture initiation angle. Results show that changing the degree of anisotropy affects the fracture initiation pressure. The fracture initiation position may also change, contributing to near wellbore fracture tortuosity. The sensitivity analyses also show that Poisson ratio anisotropy does not significantly change the fracturing pressure and angle for low anisotropy ratios. However, it does change the effect of anisotropy in elastic modulus. Key Words: fracture initiation, fracturing pressure, transverse isotropy, wellbore failure 1. INTRODUCTION Unconventional petroleum resources are among the most important sources of energy and tend to occur in formations with elastic and hydraulic anisotropy. The different mechanical properties in different directions can cause difficulty in accurately estimating the safe mud weight while drilling and the pressure required for stimulation during fracturing treatment. Although, consideration of general anisotropy is impractical, the commonly encountered case of transverse isotropy lends itself to analytical treatment. A transversely isotropy rock is one with mechanical properties that are symmetric about an axis (called axis of rotation). Laminated sedimentary rocks such as shales can be classified as transversely isotropic. Very low permeability is the main constraining factor in gas production from shales so that stimulation from inclined or horizontal wells becomes necessary. Prediction of fracture initiation pressure in inclined or horizontal wellbores is essential for safe drilling and efficient hydraulic fracture stimulation. To predict the pressure requirements in these activities, the stress distribution around the wellbore should be assessed. This is often achieved using Kirsch[1] solution for elastic and isotropic rocks. Haimson and Fairhurst [2] proposed analytical equations for stress state around the borehole in vertical elastic rock to estimate the fracturing pressure. Hossain et al [3] also estimated the fracture initiation pressure for openhole wells drilled in isotropic rocks with different trajectories. Other equations have also been proposed for stress distribution around a wellbore [4-6] in anisotropic elastic and poroelastic rocks. Stress analysis assuming isotropy can be inaccurate in unconventional resources with degrees of anisotropy and often underestimates/overestimates fracturing pressure[7]. Amadei [8] and Lekhnitskii [9] solved the stress distribution around inclined boreholes in transverse isotropic rocks. Aadnoy [10-12] simplified Amadei and Lekhnitskii’s methods to estimate the stress distribution around horizontal wells but he neglected the effect of Poisson ratio difference in vertical and horizontal directions for transversely isotropic rocks. His simplified equations may result in erroneous conclusions, especially during back analysis to estimate the in situ stresses and rock mechanical parameters.
- North America > United States > Oklahoma (0.46)
- North America > United States > Texas (0.28)
- Research Report > New Finding (0.48)
- Research Report > Experimental Study (0.34)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.65)
ABSTRACT This paper summarizes the collected geomechanical properties of the Bakken Formation screened from 4000+ wells in North Dakota’s portion of the Williston Basin. Contents f all the geomechanical parameters are rather scattered, and they are not correlated with the depth and location directly. The orientation of the maximum horizontal principal stress, determined using anelastic strain recovery method, acoustic anisotropy velocity method, and induced tensile fractures in oriented cores and well walls, is N30ºE to N70ºE in the Bakken formation. In addition, the fracture gradient, estimated with laboratory tests, ranges from 0.75 to 0.85 psi/ft. The vertical stress (overburden) is the maximum principal stress. The stress conditions are favorable for normal faulting, and the ratios among three principal stresses, i.e. the vertical, the maximum horizontal and the minimum horizontal stresses are 1:0.95-0.85:0.85-0.75. 1. INTRODUCTION The Williston Basin is an oval-shaped structural down-warp centered at Williston, North Dakota. The Bakken Formation is a relatively thin unit limited in the deep part of the Williston Basin[1]. The organic-rich shales in the upper and lower Bakken Formation have been documented as excellent petroleum source-rocks. The Bakken Formation is believed to contain tremendously amount of oil, estimated from 200 to 400 billion barrels. Knowing the geomechanical properties of the Bakken Formation is important for well design and hydraulic fracturing treatment. However, so far the Bakken Formation has not been investigated very much in comparing to the rapidly increasing development activities. As a first step to fill this gap, the authors screened most available well completion reports obtained from North Dakota Department of Mineral Resources and searched many publications to collect the geomechanical parameters of the Bakken Formation. This paper summarizes the screened geomechanical properties of the Bakken Formation in North Dakota’s portion of the Williston Basin. The goal of this work is to increase the success rate of horizontal drilling and hydraulic fracturing to improve the ultimate recovery of this unconventional crude oil resource. 2. GEOLOGIC BACKGROUND 2.1. The Williston Basin The Williston Basin underlies most of North Dakota, eastern Montana, northeastern South Dakota, southeastern Saskatchewan and a small section of southwestern Manitoba (Fig. 1). Deposition in the Williston Basin occurred during all periods of the Phanerozoic. All sedimentary systems from Cambrian through Quaternary are present in the basin, with a rock column more than 15,000 ft thick in the deepest section near Williston, North Dakota [1,2]. The basin is considered neither structurally complex nor tectonically active. 2.2. Structural geologyThe basement of Williston Basin is dissected into blocks (Fig. 2) by a series of tectonic features referred to as lineaments [3, 4]. Lineaments are best defined as zones of structural weakness. Similar to faults and, possibly, the sites of faulting, lineaments are believed to be responsible for the origin of structures and depositional patterns within the basin. Lineaments formed in response to external basinal stresses and, once formed, served as a conduit to transmit and release stresses. The Williston Basin may have formed as a tensional sag on the craton in response to a left lateral shearing movement between two regional lineaments:
- North America > United States > South Dakota (1.00)
- North America > United States > North Dakota (1.00)
- North America > United States > Montana (1.00)
- (2 more...)
- North America > United States > Wyoming > Wind River Basin > Madison Formation (0.99)
- North America > United States > South Dakota > Williston Basin > Bakken Shale Formation (0.99)
- North America > United States > North Dakota > Williston Basin > Bakken Shale Formation (0.99)
- (7 more...)
ABSTRACT Systematic experiments on ice under increasing levels of confinement have established two distinct modes of brittle-like shear faulting. Similar modes of shear faulting appear to operate in rocks and minerals as well, and possibly in ceramics. One kind, termed a Coulombic (C) or frictional fault, is oriented at ~30° to the direction of shortening and is comprised of a narrow band of microcracks that nucleate prior to terminal failure and then link up to create the fault. The orientation of the fault is governed by the coefficient of internal friction, as predicted by the Mohr-Coulomb relationship. C-faulting is characterized by pressure hardening, grain size dependent strength, and the creation of fault gouge. The other, termed a plastic (P) or nonfrictional fault, is oriented at ~ 45° (i.e. sub-parallel to planes of maximum shear stress) and is comprised of a band of recrystallized grains. P-faults form once the degree of triaxial confinement is sufficient to suppress frictional sliding. P-faults are characterized by pressure and grain size independent strength. We show, for the first time, direct measurements of localized heat production during faulting. We observe that the level of heating is higher for P-faulting than C-faulting. For C-faults, the observed rise in temperature can be explained by the generation of frictional heat though sliding across the faces of the faults. In contrast, measured temperature rises in P-faults are too great to be explained by the generation of heat from frictional sliding. Instead, we find that the degree of heating in P-faults is consistent with heat generation associated with localized plastic flow. 1. INTRODUCTION Using insight into the micromechanics of failure learned from using ice as a model material, Renshaw and Schulson [1] demonstrated that the transition from brittle to ductile failure (BD transition) in a variety of crystalline materials, including crystalline rocks, occurs when the applied strain rate is sufficiently low to allow creep deformation to relax stress concentrations at flaw tips or when the confinement is sufficient to prevent frictional sliding along flaws. To investigate the mode of failure at high strain rates under high confinement, we present here a systematic comparison of the structural and mechanical characteristics of polycrystalline ice rapidly deformed to terminal failure under triaxial compression with increasing degrees of confinement. Emphasis is placed on the physical processes underlying terminal failure. The results unambiguously confirm two distinct modes of compressive shear faulting under triaxial loading. Based on our earlier success in using ice as a model material for rock, we believe that the lessons learned from systematic experiments on ice can provide insight into similar processes likely occurring within crystalline rocks. 2. METHODS 2.1. The Ice Cubes 100 mm on side were machined from laboratory grown freshwater granular and columnar S2 ice following standard laboratory procedures described in Golding et al., 2010. The resulting material was free from cracks and completely transparent. Porosity was maintained less than 0.5 % for columnar ice and less than 1.0 % for granular ice.