Mechanical Properties of Longmaxi Black Organic-Rich Shale Samples from South China under Uniaxial and Triaxial Compression States

Abstract

Abstract:With the exploitation of shale gas booming all over the world, more and more studies are focused on the core technology, hydraulic fracturing, to improve commercial exploitation. Shale gas resources in China are enormous. In this research, a series of tests were carried out with samples of black organic-rich shale from the Lower Silurian Longmaxi formation, south China. Samples were drilled from different directions and were subjected to uniaxial and triaxial condition with various confining pressures, aiming at studying its rock mechanics properties, so as to provide basis for research and breakthrough of hydraulic fracturing technology. According to the results of the study, the development and distribution of shale's bedding planes significantly impact its mechanical properties. Shale samples show obvious brittle characteristics under low confining pressure, and its mechanical behavior begins to transform from brittle to plastic characteristics with increasing confining pressure. Shale samples with different inclinations (β) have different sensitivities to the confining pressure. As a result, samples with 45 • inclinations (β) are least sensitive. The strength of bedding planes is significantly lower than that of shale matrix, and tensile failure and shear failure generally tend to occur along the bedding planes. When hydraulic fracturing was conducted in shale formation with depth less than 2.25 km, corresponding to original in-situ of 60 MPa, cracks will preferably occur at first along the inclination (β) angle of 45 • from the maximum principal stress, and the failure mode is most likely to be shear failure without volumetric strain. And, different modes of failure will occur at different locations in the reservoir, depending on the orientation of bedding inclined from the principle stress, which can probably explain the phenomenon why there are fractures along and cross the bedding planes during hydraulic fracturing treatment. When hydraulic fracturing was conducted in shale formation with depth greater than 2.25 km, hydraulic fractures may not crack along the bedding surfaces to some extent.

Study on the Progressive Failure Characteristics of Coal in Uniaxial and Triaxial Compression Conditions Using 3D-Digital Image Correlation

“…Moreover, the lower and upper limits of long-term strength can be obtained from the crack initiation stress and crack damage stress [6]. Accordingly, the progressive failure characteristics and crack propagation in rocks have been studied by many researchers in detail [7][8][9]. However, investigations on progressive failure and stress thresholds of coal are scarce, especially in triaxial compression.…”

A Study of Nonlinear Elasticity Effects on Permeability of Stress Sensitive Shale Rocks Using an Improved Coupled Flow and Geomechanics Model: A Case Study of the Longmaxi Shale in China

“…Equation (2) represents the gas phase pressure This irreversible permeability damage is the hysteresis phenomenon for stress-sensitive permeability. A conceptual hysteresis model of stress-sensitive permeability in the literature also depicted similar phenomenon [17]. This hysteresis phenomenon is attributed to the nonlinear consolidation in the stress loading/unloading cycling [14].…”

“…In the study of stress sensitivity related problems, the involved numerical simulators usually implement the coupling between the fluid flow problem and the geomechanics problem. Wu et al incorporated an analytical correlation into their numerical simulator coupling flow and geomechanics problems to model the stress distribution in a field-scale problem considering stress sensitive reservoir rock properties [17]. This lab-validated analytical correlation represents the stress sensitive permeability hysteresis curve depicting permeability changes due to stress changes.…”

“…As a complement to analytical approaches, numerical models are known for their capability of handling complex temporal and spatial evolutions of reservoir characteristics, as numerical methods can efficiently address the heterogeneity in space and the nonlinear behaviors of flow and geomechanics problems [17,18]. In the study of stress sensitivity related problems, the involved numerical simulators usually implement the coupling between the fluid flow problem and the geomechanics problem.…”

Permeability Change Caused by Stress Damage of Gas Shale

“…Hydraulic fracturing is needed during the development of shale gas reservoirs [5] combined with CO 2 -enhanced oil recovery [6,7] and other new methods to increase the productivity. Meanwhile, borehole instability is an important problem restricting shale gas drilling [8][9][10]. Shale gas reservoirs generally have high strength and strong brittleness [11][12][13][14][15] and are a typical hard brittle shale.…”