Shale Deformation Experiments Toward an Understanding of Elastic and Fracture Anisotropy

Abstract:

The significance of shales as unconventional hydrocarbon reservoirs has opened new research frontiers in geosciences. Among many of its unique physical properties, elastic anisotropy in shales has long been investigated by both experimental and computational approaches. Shales is highly anisotropic due to texture (crystallographic preferred orientation) of their constituent clay minerals. Moreover, texturing is known to cause fracture anisotropy, which can affect both fracture toughness and fracture orientations in metals, ceramics, and polyphase materials. However, the relationship between texture and fracture anisotropy in shale has not been explored. In this study we use the multi-anvil deformation tool (D-DIA) to deform shales with a range of clay and silt contents to failure while collecting x-ray diffraction and radiography images. Diffraction images are used to extract to texture and lattice strain evolution while radiography are used to measure macrostrain and determine failure. Since clay mineral have shear moduli in the range of 6-17 GPa, our stress resolution is in the range of 30 -100 MPa respectively, within the range of unconfined compressive strengths of shales. Our results show that the orientation of clay minerals become more prominent in all samples upon deforming the sample at 100 MPa. Recovered samples are investigated with SEM to document microstructural changes. A second deformation experiment will be coupled with ultrasonic and acoustic emissions measurements to make direct comparisons of elastic anisotropy and understand the role of fracture on anisotropy. Acoustic emissions allows us to locate damage initiation and determine fracture orientations in-situ. This information will be compared with texture data to determine fracture anisotropy in our samples.