A Newly Developed Approach to Evaluate Rock Brittleness and Fracability for Hydraulic Fracturing Optimization in Shale Gas

摘要

To economically develop the unconventional reservoirs, it is essential to understand the petrophysical and reservoir property variations throughout the entire field in helping to identify sweet spot areas and guide both completion and hydraulic fracturing design. The brittle shales are more likely to be naturally fractured responding well to hydraulic fracturing. Ductile shales, on the contrary, are more plastic, absorb energy, and are not considered good producers. In such shales, the formation tends to heal any natural or induced fractures. Thus, formation sections with high brittleness are considered good candidates for hydraulic fracturing. However, many researchers argue that this viewpoint is not reasonable, because rock brittleness is not an indicator of rock strength and the current brittleness indices are based on elastic modulus or mineralogy. Brittle rock just has shorter plastic deformation, and it is not certain that it is easier to fracture brittle rock than ductile rock since brittle formation may have greater strength than ductile formation. So, rock fracability term is introduced to correct the shortcoming of rock brittleness. Fracability is defined as the rock failure under the ultimate rock strength in either brittle or ductile formation. The higher the fracability of the formation, the smaller formation strength. Therefore, the good hydraulic fracture candidate is the formation that does not only have high brittleness but also high fracability thus, less energy is required to induce fracture. A fracability model integrating the rock elastic properties, fracture toughness and confining pressure is presented in this paper. Geomechanical rock properties derived from analysis of full-wave sonic logs and core samples were combined to develop sophisticated models to verify the principle of brittleness and fracability indices and to screen hydraulic fracturing candidate intervals.