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Characterization of anisotropy in organic-rich shales: shear and tensile failure, wave velocity, matrix and fracture permeability
Mokhtari, Mehdi
Mokhtari, Mehdi
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2015
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Lamination and fractures are common features in organic-rich shales. The objective of this thesis is to determine the impact of lamination and fractures on elastic properties, shear and tensile failures, and the permeability of organic-rich shales. The elastic properties of rocks determine the hydraulic fracture geometry. Elastic properties are independent of direction in isotropic rocks, thus they can be calculated with only two static or dynamic measurements. However, thin laminations at millimeter scale and bentonite layer at foot-scale cause directional dependency (anisotropy) in the elastic properties of Eagle Ford shale. Throughout the literature, it is usually assumed that the three oriented samples have identical properties when they are drilled closed to each other. However, this assumption is not valid for some Eagle Ford shale facies due to the presence of irregular thin laminations which can create heterogeneity in one-inch samples. We explain how the heterogeneity can generate error in the estimation of anisotropy, thus we recommend that characterization of heterogeneity is essential for anisotropic studies in shale formations. At the well scale, the estimation of the minimum horizontal stress in Eagle Ford shale was improved by including the effect of lamination in Eagle Ford shale and the characterization of a bentonite layer at the boundary of upper and lower Eagle Ford shale with high Poisson's ratio. Breakout is used to estimate the insitu stress orientation which is eventually used to determine the orientation of lateral landing for optimum hydraulic fracture performance. However, the compressive strength of Mancos and Green River shales at various lamination angles show dependency on lamination angle. It was found that the Mancos shale has the weakest compressive strength at the lamination angle of 60°. Moreover, the post-failure radial gauge recording and visual inspection of the shale samples show that the failure mechanism at 60° is by sliding on the lamination. The shear fracture pattern can impact wellbore stability and hydraulic fracture analyses. A planar tensile fracture is expected perpendicular to minimum horizontal stress based on the assumption of isotropic mechanical properties. However, complex fracture network can be created in organic-rich shales due to the presence of anisotropic and heterogeneous features. We examined how lamination and natural fractures can impact tensile failure pattern using Brazilian testing on Green River, Mancos and Niobrara shales. The tensile strength of calcite-filled fractures was obtained to be one-third of the matrix and the tensile strength along the lamination is almost half the tensile strength across the lamination in Green river shale. The tensile strength of Eagle Ford shale along the lamination is negligible due to the presence of microfractures generated during the maturation process. Moreover, calcite-filled fractures or laminations can be activated if the approaching angle is less than 30°. Including the tensile strength anisotropy and tensile fracture pattern in hydraulic fracture modeling can improve the estimation of fracture geometry which can eventually improve the production forecast. The measurement of permeability of shale formations is challenging due to both the tight nature of shales and the presence of anisotropic features (lamination and microfractures). As an alternative to pulse decay and crushed sample method, we explain the measurement of permeability in nano-Darcy range by complex transient method. We determined that errors can be generated as a result of heterogeneity when three oriented samples are used to examine the effect of lamination on permeability or due to the artifact of the induced fractures at low effective stress. The path of microfractures in Eagle Ford shale can become more tortuous due to the deviation or termination toward foraminiferas, thus fracture permeability can be impacted. One of the major questions in shale hydraulic fracturing is about the low water back-flow recovery. Comparing the fracture permeability testing of a granite sample with a Niobrara shale sample, we conclude that water trapping in microfractures can be partially responsible for low backflow water recovery. Moreover, the fracture geometry was numerically created using the surface topographic map by laser profilometry measurements. The numerical simulations of fluid flow show that fracture permeability decrease as a result of roughness and the impact of roughness on fracture permeability is higher as fracture aperture decreases. Moreover, 50% anisotropy in fracture permeability was obtained as a result of fracture roughness. Incorporation of intrinsic fracture permeability anisotropy and fracture surface roughness can improve the modeling of fluid flow in shale reservoirs, fluid migration in faults and proppant transport and efficiency in hydraulic fractures. Altogether, this thesis improves the characterization of organic-rich shales by incorporating the effect of lamination and fractures on failure and flow properties as well as elaborating the impact of heterogeneity on laboratory studies of organic-rich shales.
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