High‐stress triaxial direct‐shear fracturing of Utica shale and in situ X‐ray microtomography with permeability measurement

Abstract

The challenge of characterizing subsurface fluid flow has motivated extensive laboratory studies, yet fluid flow through rock specimens in which fractures are created and maintained at high‐stress conditions remains underinvestigated at this time. The studies of this type that do exist do not include in situ fracture geometry measurements acquired at stressed conditions, which would be beneficial for interpreting the flow behavior. Therefore, this study investigates the apparent permeability induced by direct‐shear fracture stimulation through Utica shale (a shale gas resource and potential caprock material) at high triaxial stress confinement and for the first time relates these values to simultaneously acquired in situ X‐ray radiography and microtomography images. Change in fracture geometry and apparent permeability was also investigated at additional reduced stress states. Finite element and combined finite‐discrete element modeling were used to evaluate the in situ observed fracturing process. Results from this study indicate that the increase in apparent permeability through fractures created at high‐stress (22.2 MPa) was minimal relative to the intact rock (less than 1 order of magnitude increase), while fractures created at low stress (3.4 MPa) were significantly more permeable (2 to 4 orders of magnitude increase). This study demonstrates the benefit of in situ X‐ray observation coupled with apparent permeability measurement to analyze fracture creation in the subsurface. Our results show that the permeability induced by fractures through shale at high stress can be minor and therefore favorable in application to CO2 sequestration caprock integrity but unfavorable for hydrocarbon recovery from unconventional reservoirs.

Scalable En Echelon Shear‐Fracture Aperture‐Roughness Mechanism: Theory, Validation, and Implications

“…In nature, faults are typically segmented with step‐overs and fault jogs linking into en echelon structures (Segall & Pollard, ). In laboratory direct‐shear tests, fractures are often nonplanar and rough relative to the direct‐shear plane (Carey et al, ; Frash, Carey, Lei, et al, ; Park & Song, ; Potts et al, ). In both of these cases, fractures are coalesced along a nominal plane by the geometry of loading and the location of preexisting local weaknesses or stress concentrators (Hoek & Brown, ).…”

“…Of these, fracture direct‐shear testing is perhaps the most direct method for measuring coupled anisotropic mechanical and hydraulic parameters (Gentier et al, ; Carey, Lei, et al, ; Carey, Rougier, et al, ). Direct‐shear tests systemically produce nonplanar fracture roughness when rocks are fractured under constant normal stress (Carey et al, ; Frash, Carey, Lei, et al, ). The causes of this nonplanar roughness are thought to be related to complexity in the shear stress state, misalignment of the direct‐shear plane relative to the material's preferred fracturing plane, total amount of shearing displacement, and/or material heterogeneity and anisotropy (Park & Song, ; Potts et al, ; Frash et al, ).…”

Simulation of Fracture Coalescence in Granite via the Combined Finite–Discrete Element Method

“…Natural rock contains a large number of natural fractures that can range in scale from sub-micron to kilometers in length [1]. A thorough understanding of crack initiation, propagation, interaction, and eventual coalescence emanating from pre-existing fractures is key to characterizing geologic risk in engineering design and to improving the safety and effectiveness of subsurface hydrologic, energy, and waste disposal activities [2][3][4]. In an effort to better understand the fracture processes within brittle solids (i.e., rock-like materials), crack coalescence has been extensively studied, both experimentally and numerically [5][6][7][8][9][10][11][12][13][14][15][16].…”

“…In earlier work, we conducted 3-D FDEM analysis of split-Hopkinson pressure bar experiment on granite in which the temporal evolution of strain and tensile stress was determined including a model of strain softening following initial specimen fracture. We have also applied 2-D FDEM analysis to the interpretation of triaxial direct shear permeability experiments on shale in which we have reproduced fracture patterns [2] and analyzed stress development and stress-strain history to specimen failure [3]. In these projects, HOSS is used to interpret mechanisms of fracture formation and fluid flow observed in experiments and to provide fracture patterns for use in larger-scale discrete fracture network models.…”

Integrated production logging tools approach for convenient experimental individual layer permeability measurements in a multi-layered fractured reservoir

“…Wireline logging tools may confidentially provide a reasonable solution Huang et al 2013) for obtaining the proper value of porosity and fluid saturation; nevertheless, in the case of permeability measurements, it may be debatable. Furthermore, core plugs are not the best candidate tools to measure the characterization of fractured and vuggy intervals (Ali Ahmadi et al 2013;Frash et al 2016;Sullivan 2007). Evaluating the appropriate properties of each layered reservoirs is considered as the underlying issues of engineering and geological perspectives.…”