An Integrated Petrophysics and Geomechanics Approach for Fracability Evaluation in Shale Reservoirs

The identification of the fracture barrier is important for optimizing horizontal-well drilling, hydraulic fracturing, and protecting fresh aquifer from contamination. The word "brittleness" has been a prevalent descriptor in unconventional-shale-reservoir characterization, but there is no universal agreement regarding its definition. Here, a new definition of mineralogical brittleness is proposed and verified with two independent methods of defining brittleness. Formation with higher brittleness is considered as a good fracturing candidate. However, this viewpoint is not reasonable because brittleness does not indicate rock strength. For instance, the fracture barrier between upper and lower Barnett can be dolomitic limestone with higher brittleness. A new fracability index (FI) is introduced to overcome the shortcoming of brittleness by integrating both brittleness and energy dissipation during hydraulic fracturing. This FI considers that a good fracturing candidate is not only of high brittleness, but also requires less energy to create a new fracture surface. Therefore, the formation with lower FI is considered as a bad fracturing candidate, whereas that with higher fracability is considered as a better target. Logging data from one well in the Barnett shale are applied (1) to verify the principle of the new brittleness definition and FI model and (2) to demonstrate the process of screening hydraulic-fracturing candidates with the FI model.