Fracture Behavior of the Longmaxi Shale with Implications for Reservoir Evaluation

Abstract:

Fracture mechanical properties of shales have gained continued interest due to their critical role in many shale-related applications such as unconventional petroleum systems. However, due to their strong reactive nature to fluids, fracture mechanical properties of shales interacted with fluid remain unknown. Using the double torsion test, we measure fracture mechanical properties of Longmaxi shale outcrop samples in Sichuan Basin, China, under different environments of ambient air, de-ionized water, saline fluids of NaCl and KCl at 0.5 M concentration, and acidic HCl fluid at pH of 5. All aqueous fluids tested show strong weakening effects on fracture propagation compared to the air environment, with fracture toughness reduced by 75% and subcritical fracture growth index reduced by 50%. Microstructural analysis reveals predominantly grain boundary opening cracking mode for all tests, but fracture traces branch more in aqueous fluids. Natural fractures are comparable with artificial fractures in morphology. Bedding-perpendicular opening mode fractures with multiple stages fracture fillings of calcite, quartz, and organic matters develop well in natural fractures. Our results suggest that clay mineral hydration and expansion are the main cause for the fluid weakening effect in the Longmaxi shale, which has substantial implications for subsurface evaluation of shale reservoirs.