Effects of High Pressure-Dependent Leakoff and High Process-Zone Stress in Coal-Stimulation Treatments

Hydraulic fracturing in coals has been studied extensively over the last two decades; however, there are factors that were often ignored or incorrectly diagnosed, resulting in screenouts. Assuming that a majority of the perforations are open and there are no problems with the stimulation fluids, screenouts during coal hydraulic-fracture treatments can be attributed to either high pressure-dependent leakoff (PDL), high process-zone stress (PZS) or in some cases both. The objective of this work is to discuss, help identify, and present solutions to address these reservoir-related issues such that screenouts can be avoided in optimized refracture treatments and new well Stimulations. The tools for identifying these reservoir-related parameters include a diagnostic fracture-injection test (DFIT) and a grid-oriented fully functional 3D fracture simulator with shear decoupling. An example for each respective case is presented in this paper. In the first example, in which high PZS was considered to be the dominant reason for screenout or pressure out, the well was restimulated successfully by implementing the solutions presented in this paper. In the second example, in which high PDL was considered to be the main reason for screenout, there were several wells in the same project area that exhibited the same behavior resulting in screenouts. After implementing the solutions presented in this paper to address high PDL, all new wells were stimulated successfully without any issues.