Anisotropic Shale Research Articles

Evaluating the mechanical properties of anisotropic shale containing bedding and natural fractures with discrete element modeling

Natural fracture data from one of the Carboniferous shale masses in the eastern Qaidam Basin were used to establish a stochastic model of a discrete fracture network and to perform discrete element simulation research on the size effect and mechanical parameters of shale. Analytical solutions of fictitious joints in transversely isotropic media were derived, which made it possible for the proposed numerical model to simulate the bedding and natural fractures in shale masses. The results indicate that there are two main factors influencing the representative elementary volume (REV) size of a shale mass. The first and most decisive factor is the presence of natural fractures in the block itself. The second is the anisotropy ratio: the greater the anisotropy is, the larger the REV. The bedding angle has little influence on the REV size, whereas it has a certain influence on the mechanical parameters of the rock mass. When the bedding angle approaches the average orientation of the natural fractures, the mechanical parameters of the shale blocks decrease greatly. The REV representing the mechanical properties of the Carboniferous shale masses in the eastern Qaidam Basin were comprehensively identified by considering the influence of bedding and natural fractures. When the numerical model size is larger than the REV, the fractured rock mass discontinuities can be transformed into equivalent continuities, which provides a method for simulating shale with natural fractures and bedding to analyze the stability of a borehole wall in shale.

Stress-sensitivity of fracture conductivity of Tuscaloosa Marine Shale cores

Production decline from wells producing from shale rocks is typically quite rapid. One possible factor responsible for such decline is loss in fracture conductivity. This is true in the case of the Tuscaloosa Marine shale reservoir. In this work, the stress-dependent fracture conductivity of TMS core samples is investigated using five propped cores under varying stress conditions. In addition, the effects of static rock mechanical properties and rock mineralogy on conductivity decline are investigated. The mineralogy of each core sample was determined by X-ray Diffraction analysis. Fracture conductivity was measured using a Hassler-type pressure core holder with a pressure rating of 15,000 psi (103.42 MPa). Estimated rock mechanical properties showed that shale anisotropy could affect fracture conductivity. The time decline in conductivity revealed two possible decline trends, a) decline from the reduction in induced fracture width, and b) decline from the healing of developed micro-cracks. An exponential decline in fracture conductivity with increasing confinement pressure was observed with an average decline rate constant of 3.15 × 10 − 10 mPa −1 . While rock mineralogy did not have any clear relationship with conductivity decline, the effects of rock mechanical properties were only important at 10.34 MPa confinement pressure and above. The observation showed that a high Young's modulus correlate to high conductivity, whereas a high Poisson's ratio correlate to low conductivity. This study provides useful observations for optimizing fracture design to enhance well productivity in the TMS. • Fracture conductivity decline followed an exponential trend. • Micro-cracks formed in the rock samples which later contributed to fluid flow. • Reduction in fracture width is controlled by proppant embedment for the TMS. • Fracture width reduction and healing of micro-cracks causes conductivity decline.

Impact of Stress-Induced Rock Damage on Elastic Symmetry: From Transverse Isotropy to Orthotropy

In this article we explore the change in elastic symmetry and anisotropy of two geological materials with stress-induced damage. Two independent uniaxial deformation experiments on two layered and natural geomaterials, a shale and a sandstone, support this analysis. Both samples were loaded along their bedding planes at a constant strain rate up to mechanical failure. During deformation, an array of ultrasonic P- and S-wave transducers were employed to monitor the evolution of the ultrasonic velocities up to and beyond sample failure. We report here the impact on elastic symmetry and anisotropy of a uniaxial load applied parallel to the bedding plane of the transversely isotropic shale and sandstone samples. Based on symmetry considerations we analyse whether this load and the resulting stress-induced damage preserve the original transverse isotropy of the rock prior to loading, or lead to orthotropy (orthorhombic symmetry). Our results show that the two transverse isotropic samples retained their transverse isotropy in the initial stages of loading/damage. However for the more anisotropic shale sample the main failure mode was splitting of the bedding planes. In contrast the sandstone sample failed along a shear plane inclined to the bedding plane. In addition, the experimental data are further analysed using continuum damage mechanics to identify the evolution of the general fourth order anisotropic damage tensor during loading. We found that the highest damage variable of the damage tensor was D_{11} for the shale and D_{55} for the sandstone sample. The next highest damage variables for the shale sample were related to the bedding plane (x_2–x_3 plane) symmetry: D_{23}, D_{32}, D_{44}. However the damage variables obtained for the less anisotropic and more brittle sandstone sample were harder to interpret due to the mixed mode of fracturing present.

Effect of Velocity Anisotropy in Shale on the Acoustic Emission Events Matching and Location

Accurate source event location is important in fracturing monitoring and characterization. Velocity anisotropy has a great influence on both events matching and events location. Failure to take into account the velocity anisotropy can lead to huge errors in locating events. In this article, we have presented an experimental study on lower Silurian shale from the Sichuan Basin. The experimental observations include ultrasonic measurements, acoustic emissions (AEs) in a three-point bend experiment, and CT scanning of the original sample and the fractured sample. The ultrasonic measurements show that the shale sample has strong velocity anisotropy. Initially, AEs are analyzed using the conventional event-matching method and event location method (Geiger’s method), and the detected events are compared to the X-ray image of the fracture. Event-matching aims to obtain AE signals from the same source event and thus assists in selecting valid AE signals that come from the same source and are received by at least four sensors, to determine the location of the source. Although many reliable signals are obtained by isotropic event-matching, fewer sources were located than expected, and the event location results did not match the fracture distribution. To address this problem, an improved event-matching method is proposed using a stricter matching threshold based on directional velocity rather than a single threshold same for all directions. In addition, we propose an improved Geiger’s method using the anisotropic velocity model. The new methods located more sources that better match fracture distribution than the results of the isotropic method. We have concluded that both event-matching and the source location of the fracturing are largely influenced by velocity anisotropy, and thus in practice, the velocity anisotropy information obtained from various measurements (e.g., laboratory measurements, well logs, VSP, and velocity analysis of reflected seismic surveys) should be involved in both processing procedures. This study can be useful to provide some background for monitoring and predicting dynamic geo-hazards in relation to the AE method.

Elastic anisotropy of shales: The roles of crack alignment and compliance ratio

Cracks, broadly defined as compliant discontinuities, are a major cause of elastic anisotropy. However, few models are available for quantifying crack properties relevant to anisotropy. We developed a rock-physics model to quantify crack angular distribution and normal-to-tangential compliance ratio from pressure-dependent acoustic velocities measured in the laboratory. Our model uses a rectangular function of variable width and amplitude to extract the maximum dip angle of cracks, which is a direct quantification of crack alignment relative to the bedding plane. We tested the model on an organic-rich shale data set and confirmed that crack alignment and compliance ratio strongly impact Thomsen anisotropy parameters, thus demonstrating the model as a useful tool for better understanding how cracks affect elastic anisotropy.

Anisotropy characterization of the elasticity and energy flow of Longmaxi shale under uniaxial compression

The inherent anisotropy of shale makes it exhibit different mechanical properties when measured in different directions. Generally, the measurements of the anisotropic mechanical properties of shale are generally conducted on cylindrical samples, and the computation of elastic constants is usually an empirical approximation. To accurately estimate the elastic parameters of shale rocks, an iterative method for approaching the crack initiation stress threshold was proposed in this work. Several uniaxial compression experiments were performed on cuboid Longmaxi shale samples with different bedding plane orientations. With a newly presented method, the apparent elastic modulus and Poisson’s ratios and five elastic constants were simultaneously determined. By examining the volumetric strain reversal point, the crack damage stress was obtained for each sample. The results demonstrate that the elastic regime, which was the range in which the elastic parameters were determined, significantly varies with bedding plane orientation. A transverse anisotropy ratio of 1.28 between in-plane modulus E1 and out-of-plane modulus E3 was determined for Longmaxi shale. Additionally, with the obtained elastic parameters, the energy releasing and dissipating behaviors of the samples during uniaxial loading were quantitatively characterized. The energy dissipation was found to rapidly increase once the crack damage stress was reached. However, before rock failure occurred, the total dissipated energy in each sample was very limited, indicating that microcrack propagation and rock damage were quite limited before rock rupture and that the shale samples were essentially destroyed by rapid energy release.

Effect of formation strength anisotropy on wellbore shear failure in bedding shale

The shale gas/oil is playing an important role in the current energy resources. However, the wellbore instability problems bring a high risk to drilling engineering especially in the formation with bedding planes. A normal wellbore stability analysis approach is based on elasticity, which is not enough to recognize the mechanisms. In this paper, the poroelasticity was adopted to present the hydro-mechanical coupling effect in shale formation, meanwhile, the permeability anisotropy which could affect the effective stresses around the well was considered. A three-dimensional finite difference model was developed to simulate arbitrary directional well in shale by the coordinate transformation method. The jointed material model was proposed to character the strength anisotropy and plastic behavior of bedding shale. Additionally, the effects of well trajectory, bedding plane orientation and mud weight on shale wellbore stability were discussed, thus some implications for improving wellbore stability in bedding shale formation were obtained from the proposed model. • The jointed material model is proposed to analyze the shear failure region in bedding shale. • The poroelastic theory is adopted to represent the hydro-mechanical coupling effect in the formation. • A finite difference three-dimensional model is established to simulate the wellbore failure behavior. • The effects of well trajectory, bedding plane orientation and mud weight on wellbore stability are evaluated.

Elastic properties of fractures in transversely isotropic media

A fracture may be characterized by its normal and shear compliances that relate the discontinuity in displacement of the fracture faces to an applied traction. To assess the feasibility of using elastic waves for monitoring natural and hydraulic fractures in anisotropic shale reservoirs, an understanding of how these compliances relate to the properties of the background rock and any fracture infill material is required. Three models of vertical fractures in a VTI (transversely isotropic medium with vertical axis of rotational symmetry) background suitable for feasibility studies are discussed. The first two models are suitable for open vertical fractures and represent the fractures as aligned low aspect ratio elliptic cylindrical inclusions with long axis of the ellipse either horizontal or vertical. In these models the horizontal and vertical shear compliance differ in a way that depends on the anisotropy of the background medium and the orientation of the fractures. The third model is suitable for estimating the properties of hydraulic fractures containing proppant. In this model, proppant is represented as a random packing of identical elastic spheres subject to a uniaxial compression due to the normal stress acting on the fracture. The results are expected to help improve the reliability of fracture models used for assessing the feasibility of elastic waves for characterizing natural and hydraulic fractures.

How does structure affect the evolution of cracking and the failure mode of anisotropic shale?

How does structure affect the evolution of cracking and the failure mode of anisotropic shale?

The Influence of Lamina Density and Occurrence on the Permeability of Lamellar Shale after Hydration

The characteristics of laminae in lamellar shale oil reservoirs have important influences on reservoir parameters, especially permeability. In order to explore the influence of lamina density and occurrence on the permeability of lamellar shale after hydration, we studied the lamellar shale in the Chang 7 member of the Yanchang Formation of Triassic in Ordos Basin. By comparing the permeability of bedding shale and lamellar shale with different densities of laminae, it was found that the permeability anisotropy of lamellar shale was stronger. In the direction parallel to the lamina, the permeability increased approximately linearly with an increase in lamina density. The effect of hydration on rock micropore structure and permeability was studied by soaking shale in different fluids. Most of the microfracture in the lamellar shale was parallel to the lamina direction, and hydration led to a widening of the microfracture, which led to the most obvious increase in permeability parallel to the lamina. Collectively, the research results proved that lamina density, occurrence, and hydration have a significant influence on the permeability anisotropy of lamellar shale.

The Investigation on Initiation and Propagation of Hydraulic Fractures in Shale Reservoir

Hydraulic fracturing is a necessary technique for shale gas exploitation. In order to have efficient stimulation treatment, a complex fracture network has to be developed, whereas with rich bedding planes and natural fractures, the mechanism of forming a fracture network is not fully understood and it is so tricky to predict propagation and initiation of hydraulic fracture. Therefore, in this paper, considering the strong anisotropy of shale reservoir, numerical simulation has been conducted to analyze fracture propagation and initiation on the basis of finite element and damage mechanics. Simulation results indicate that hydraulic fracture is not merely controlled by in situ stress due to strong anisotropy in shale. With plenty of bedding planes, hydraulic fracture tends to have initiation and propagation along the bedding plane. In particular, this influence becomes stronger with low strength and high development density of bedding planes. Additionally, in combination with natural fracture and bedding plane, the initiation point is usually on a natural fracture plane, causing relatively small breakdown pressure. In the process of fracture propagation, hydraulic fracture connects with natural fractures and bedding planes, forming dendritic bifurcation and more complicated paths. Numerical simulation proves that bedding plane and natural fracture are vital factors of hydraulic fracture. Compared to natural fracture, the bedding plane has a stronger impact on hydraulic fracture propagation. For the initiation of hydraulic fracture, natural fracture is the major effecting factor. The outcome of this study is able to offer theoretical guidance for hydraulic fracturing in shale.

The Effect of Hydraulic Fracture Geometry on Well Productivity in Shale Oil Plays with High Pore Pressure

We propose three idealized hydraulic fracture geometries (“fracture scenarios”) likely to occur in shale oil reservoirs characterized by high pore pressure and low differential in situ stresses. We integrate these geometries into a commercial reservoir simulator (CMG-IMEX) and examine their effect on reservoir fluids production. Our first, reference fracture scenario includes only vertical, planar hydraulic fractures. The second scenario has stimulated vertical natural fractures oriented perpendicularly to the vertical hydraulic fractures. The third fracture scenario has stimulated horizontal bedding planes intersecting the vertical hydraulic fractures. This last scenario may occur in mudrock plays characterized by high pore pressure and transitional strike-slip to reverse faulting stress regimes. We demonstrate that the vertical and planar fractures are an oversimplification of the hydraulic fracture geometry in anisotropic shale plays. They fail to represent the stimulated volume geometric complexity in the reservoir simulations and may confuse hydrocarbon production forecast. We also show that stimulating mechanically weak bedding planes harms hydrocarbon production, while stimulated natural fractures may enhance initial production. Our findings reveal that stimulated horizontal bedding planes might decrease the cumulative hydrocarbon production by as much as 20%, and the initial hydrocarbon production by about 50% compared with the reference scenario. We present unique reservoir simulations that enable practical assessment of the impact of varied hydraulic fracture configurations on hydrocarbon production and highlight the importance of constraining present-day in situ stress state and pore pressure conditions to obtain a realistic hydrocarbon production forecast.

Co-effects of bedding planes and loading condition on Mode-I fracture toughness of anisotropic rocks

The mechanical and fracture behavior of rocks are mainly determined by the discontinuous defects inside the rock and the external force state. The focus of this research is to clarify the co-effects of bedding planes and loading conditions on the fracture behavior of anisotropic rocks. Notched semi-circular bend (NSCB) tests are performed on anisotropic rocks with varied bedding angles (from 0° to 90°) and loading conditions (from static loading to impact loading). By analyzing the failure characteriacs recorded throught the experiments and fracture toughness of anisotropic rocks (shale and coal), a critical loading rate, two failure modes, three empirical relations, upper and lower envelopes are provided. In addition, bedding effects, laoding rates effects and co-effects between them are further investigated. The experimental results indicate that the bedding effects is the most obvious under static loading and becomes weaker as loading rates increase. At this time, the loading rates effects plays a dominant role. Under the co-effects of bedding plane and loading conditions, the failure modes become more complex and the fracture toughness exhibits inconsistent characteristics for anisotropic shale and coal. Due to the different internal discontinuities, the maximum loading rate that shale and coal can withstand differs greatly, which further affects their mechanical and failure behaviors. The results have sightful implications to the response of anisotropic rocks to engineering activities.

New methodology to determine the upper limit of safe borehole pressure window: Considering compressive failure of rocks while drilling in anisotropic shale formations

Shales are frequently encountered in oil and gas drilling operations that accompanied by severe borehole stability issues now and then. It is of primary importance to determine the accurate limits of safe borehole pressure window (SBPW) while drilling in anisotropic shale formations. To our knowledge, a convention is followed by nearly all the researches in the literature that the lower limit of SBPW is determined by collapse failure while the upper limit of SBPW is determined by fracture failure of rocks. Nevertheless, this paper aims to present a unique method through which the upper limit of SBPW is proved to be also affected by collapse failure of surrounding rocks. Based on transformations of different coordinate systems and the elastic anisotropic theory, the stresses around directional boreholes are first calculated. Then, shear failure criteria of rock matrix and weak planes as well as tensile failures are incorporated to analyze the two limits of SBPW. The influences of in-situ stress, relative orientations and anisotropy ratio are also investigated. The results show that the order change of principal stresses in rocks can result in change of shear failure modes on the borehole wall and this will lead to the appearance of a new upper limit of SBPW. Under many conditions, the SBPW determined by the new method can be greatly narrower than that defined by traditional methods, whether considering the failure of rock matrix or weak planes. The collapse area reappears when the borehole pressure is high enough and the failure extension direction may also change. A more uniformed in-situ stress state makes it more necessary to apply the new method to determine the SBPW. Besides, the elastic anisotropy characteristics of rocks can make the SBPW narrower.

Experimental investigation on failure behaviors and mechanism of an anisotropic shale in direct tension

Experimental investigation on failure behaviors and mechanism of an anisotropic shale in direct tension

Experimental study the viscoelastic properties of deep shale gas reservoirs in Sichuan Basin

In recent years, the deep shale gas reservoirs has been a major shale gas target in Sichuan Basin. Usually, the burial depth of this target formation is more than 3500m, which is receiving increasing interests to get industrial natural gas. The deep shale gas reservoirs, characterized by fine-grained nature and low matrix permeability as the common shale, require good understanding of geomechanical properties to optimize drilling and fracturing strategies. Meanwhile, the shale reservoirs are anisotropic and inhomogeneous, exhibiting a nonlinear feature under stress loading. However, very limited studies are devoted to characterization of the mechanical properties of deep shale, as opposed to other shale formations. Due to an increasing demand for fundamental properties of this formation to meet the needs of development of this deep shale gas, a comprehensive characterization studies is needed. One of the most important investigation is to obtain the mechanical properties based on an integration of unconfined/triaxial compression tests and creep tests. In this study, the time-dependent, elastic, hysteresis, and strength properties of deep shale specimens, retrieved from a deep well in Sichuan Basin, are evaluating by performing a series of triaxial creep experiments. With respect to mineral composition, the shale cores are found to be organic-rich. The response of these shale specimens are found to be influenced by the planes of weakness and the presence of micro-cracks. The results of creep experiments show that shale tends to significantly creep under the applied stress. The time-dependent axial strain increase linearly with differential stress. And the Power-Law creep model can be used to describe the time-dependent behaviour of shale well. Moreover, the existence of bedding planes results in the creep anisotropy of shale. It is observed that the overall time-dependent axial strain normal to bedding is higher than the parallel to bedding. But the axial strain at 45° angle with respect to the bedding plane is highest. These research results are of great significance for safe and fast drilling in deep shale gas reservoirs. Also, it can be used to guide hydraulic fracturing.

Numerical study on the in-situ pyrolysis process of steeply dipping oil shale deposits by injecting superheated water steam: A case study on Jimsar oil shale in Xinjiang, China

Taking the oil shale deposits in the Jimsar area of Xinjiang, China as an example, this paper proposes an in-situ pyrolysis method using superheated steam suitable for steeply dipping oil shale. This method involves drilling parallel to the bedding and fracturing fractures perpendicular to bedding direction. High-temperature fluid is injected from the injection well to develop a larger heat exchange area through fracturing fractures and oil shale deposits. In this study, a thermal-hydraulic-mechanical coupling mathematical model considering oil shale anisotropy was established to study the in-situ conversion process. The results show that the new proposed method (hydraulic fracture number = 3) has the same heating efficiency as the traditional well layout method (drilling well perpendicular to beddings, fracture number = 3), but has the advantage of preventing shear damage to the wellbore and reducing the drilling cost; For the optimization of hydraulic fracture number and length, when the hydraulic fracture number is increased to 4 (fracture spacing = 20 m), the heating efficiency no longer significantly increases. When the fracture length increased, the heating efficiency does not significantly change. The results of this paper can serve as a reference for the study of in-situ pyrolysis of steeply dipping oil shale deposits.

Influence of the Weakening Effect of Drilling Fluid on Wellbore Stability in Anisotropic Shale Formation

For horizontal wells in Longmaxi Formation, oil-based drilling fluid that soaks for a long time is more likely to cause a wellbore collapse. Therefore, in this paper, the downhole core method of shale formation, in Longmaxi Formation, was adopted. First, rock samples were selected from different sampling angles and soaked with field drilling fluid. Second, a triaxial mechanics experiment was carried out. Based on the anisotropic wellbore stress distribution model, the stability of shale wellbore was calculated and analyzed. The results show that the compressive strength and cohesion of the shale are reduced after soaking in the drilling fluid. Hence, the reduction range of various sampling angles obviously differs as well. Shear failure occurs in vertical stratification; shear slip failure occurs along the weak plane, showing strong anisotropy. Combined with the experimental results, the collapse pressure is calculated, and it is found that the weakening effect of drilling fluid makes the overall collapse pressure rise by about 0.2 g/cm3. Finally, the shale bedding dip and dip direction have a great influence on the collapse pressure. The lower critical mud weight always takes the minimum value when the borehole axis is perpendicular to the bedding.

Rock Physics Model of Shale: Predictive Aspect

AbstractShales often show strong elastic anisotropy that originate from the alignment and platy nature of its constituent minerals. Despite its impact on amplitude variation with offset response and seismic time‐shifts, elastic anisotropy of shales is, however, often ignored since it is difficult to measure enough parameters in the field. Being able to correctly estimate anisotropy parameters of shales can therefore significantly improve seismic reservoir characterization. A predictive model is developed by combining existing theories. Properties of locally aligned clay platelets, called domains, are calculated using a rock physics model based on the anisotropic Hashin‐Shtrikman estimates. The effect of domain orientation is then accounted for by the orientation distribution function of domains. The applicability of the model was investigated using existing core measurements. Interesting findings include: (a) most of modeled anisotropy parameters are consistent with the measured values even though only limited information was used for the model parameters optimization, and (b) most of optimized interplatelet medium properties are consistent with the saturated fluid and small interplatelet medium shear modulus suggested by existing studies. These findings imply that the model can be used to predict anisotropy parameters from limited information.

Anisotropic characteristics of the energy index during the shale failure process under triaxial compression

Energy conversion is an essential driving force of shale anisotropic deformation, damage initiation and evolution. In this study, theoretical calculations of five energy indexes (total input energy Ut, releasable energy Ue, dissipated energy Ud, dilatational energy UV and distortional energy UD) during the shale failure process considering the inherent anisotropy of shale are presented in detail for the first time. The anisotropic features of the five energy indexes are systematically discussed. The results indicate that bedding structure considerably influences these energy indexes. Ue, UV and UD all decrease to a minimum as the bedding orientation reaches 60° and then increase with the bedding degree. The relationships between Ue, UV and UD and the confining pressure can be described by linear functions, similar to the variation in the compressive strength with either bedding inclination angle or confining pressure. As the axial strain increases, the rates of increase in Ue, UV and UD first increase, then remain constant, and then decrease before failure; however, the rates of increase in Ut and Ud continue to increase until failure. Ue, UD and UV have strong relationships with the peak compressive strength but weak relationships with the peak axial strain, whereas Ut and Ud exhibit the opposite relationships with the peak strength and strain. The anisotropic characteristics of the energy indexes can help to further elucidate the failure mechanisms of shale and are of great significance to hydraulic fracturing design.