Doctor of Philosophy (PhD)
Shales play an essential role in petroleum exploration and production because they can occur either as unconventional reservoir rocks for hydrocarbon extraction via hydraulic fracturing or as caprocks for conventional reservoirs and subsurface gas/ waste storage. For both extraction and storage applications, the success rate is directly depending on the rigorous candidate selection. Conventional rock characterization techniques normally measure rock properties by seismic/logging at the reservoir scale, and on drilled/outcrop cores at the core scale. However, shales are highly heterogeneous in composition, containing a large number of reactive minerals in micro/nanoscale with significantly different properties. The structural features and properties of these minerals at the micro/nanoscale can impact the durability performance of rock materials at the macroscale.
This study compares properties of several types of shales measured at mineral scale, with the intent to predict the rock susceptibility to fracturing at a larger scale since all fractures are initiated at atomic scale imperfections. Indentation tests were conducted at micro and nanometer scales on sections cut from drilled core samples to obtain the mechanical properties of the bulk and individual mineralogical phases. Their durability performances under stress were combined with the differences observed in depth, lamination, composition, as well as microstructure to give the final conclusion. High-resolution microscopy and Energy Dispersive X-Ray Spectroscopy (EDS) analysis were combined to provide a spatial link between geochemistry and geomechanics at micro and nano scale.
Results from this study indicated 1). Shale caprocks had the higher bulk mechanical properties, more uniform grain size, higher rigid grain content, and lower degree of anisotropy than the shale reservoir rocks; 2) Fractures were more likely initiated at the boundary of two mechanically different mineral grains, and the fracture/deformation within the caprocks can be self-healed within few months.
This study provides a time and cost efficient way for rock geomechanical evaluation to help the identified the optimal target/candidate for subsurface applications by evaluating the mechanical properties of shale and their susceptibility to fracturing. Also, the non-destructive nature of testing makes it possible for the dynamic study of the rock in contact with different fluids and dry/wet cycling conditions.
Du, Hui, "Experimental Evaluation of How Mineralogy and Microstructure Impact Micro-Geomechanics of Shale Rocks" (2020). LSU Doctoral Dissertations. 5134.