Evolution of propped fractures in shales : The microscale controlling factors as revealed by in situ X-Ray microtomography

The behavior of proppant at the microscale during fracture closure in oil and gas shales is not yet well understood. In this context, we used a combination of dynamic synchrotron X-ray micro computed tomography with morphometric analysis and flow modeling to provide new insight into the dominant physical processes acting at the microscale during fracture closure, and understand their impact on hydraulic properties of the fracture. The roles of three variables have been investigated in proppant monolayers: i) Shale mineralogy and microstructure; ii) Shale bedding orientation with respect to the fracture plane; iii) Proppant characteristics. Unsurprisingly, all three variables have an impact on the permeability evolution during closure and their extent has been quantified via Stokes flow simulations. For the fracture geometry considered, proppant rearrangement during loading is the leading cause of permeability loss. The mechanical strength of proppant and shale becomes important at the later stages of the fracture closure. Bedding orientation has an impact on the mechanical response of the proppant-shale contact areas. The more regular morphology and higher mechanical strength of ceramic proppant allows to maintain a better permeability throughout the whole fracture evolution cycle when compared to quartz sand. This combined analysis approach allowed us to understand and quantify the processes involved during the closure of a propped fracture and to directly link them to the evolution of morphology and hydraulic properties. A better understanding of the contributions of these processes could ultimately help in the design and optimization of proppants, enabling more efficient extraction of hydrocarbons from unconventional systems.