Lithology-controlled stress variations and pad-scale faults: A case study of hydraulic fracturing in the Woodford Shale, Oklahoma

We have conducted an integrated study to investigate the petrophysical and geomechanical factors controlling the effectiveness of hydraulic fracturing (HF) in four subparallel horizontal wells in the Mississippi Limestone-Woodford Shale (MSSP-WDFD) play in Oklahoma. In two MSSP wells, the minimum horizontal stress Sh min indicated by the instantaneous shut-in pressures of the HF stages are significantly less than the vertical stress Sv. This, combined with observations of drilling-induced tensile fractures in the MSSP in a vertical well at the site, indicates that this formation is in a normal/strike-slip faulting stress regime, consistent with earthquake focal mechanisms and other stress indicators in the area. However, the Sh min values are systematically higher and vary significantly from stage to stage in two WDFD wells. The stages associated with the abnormally high Sh min values (close to Sv) were associated with little to no proppant placement and a limited number of microseismic events. We used compositional logs to determine the content of compliant components (clay and kerogen). Due to small variations in the trajectories of the horizontal wells, they penetrated three thin, but compositionally distinct WDFD lithofacies. We found that Sh min along the WDFD horizontals increases when the stage occurred in a zone with high clay and kerogen content. These variations of Sh min can be explained by various degrees of viscous stress relaxation, which results in the increase in Sh min (less stress anisotropy), as the compliant component content increases. The distribution of microseismic events was also affected by normal and strike-slip faults cutting across the wells. The locations of these faults were consistent with unusual lineations of microseismic events and were confirmed by 3D seismic data. Thus, the overall effectiveness of HF stimulation in the WDFDwells at this site was strongly affected the abnormally high HF gradients in clay-rich lithofacies and the presence of preexisting, pad-scale faults.