Stress, seismicity, and microseismic source mechanisms at the Hydraulic Fracture Test Site-1 in the Midland Basin, TX

Abstract:

We investigated the relationship between borehole stress measurements and microearthquakes resulting from hydraulic stimulation at the Hydraulic Fracture Test Site-1 in the Midland Basin, TX. The project is comprised of two sets of horizontal wells in the Upper Wolfcamp (UWC) and Middle Wolfcamp (MWC) shale units. Following hydraulic fracturing, a slant well was drilled to core the area where hydraulic fracturing and microearthquakes occurred.

Regional patterns of stress indicate that the area is in a normal/strike slip faulting regime. Diagnostic formation integrity tests (DFITs) in vertical and horizontal wells constrain the pore pressure and the minimum principal stress, S_hmin. Above the UWC, the S_hmin gradient (hydraulic fracture gradient) is close to the drilling mudweight, creating a normal faulting regime. In the UWC, S_hmin increases with depth, resulting in a transition to strike slip faulting, then decreases with depth, returning to a value consistent with the gradient in overlying units. In the MWC, S_hmin varies between the high and low hydraulic fracture gradients, indicating lithofacies with alternating high and low differential stress. Instantaneous shut-in pressures from hydraulic fracturing stages corroborate the vertical stress variations reflected by DFITs.

In the UWC and MWC units, the fracture gradient decreases with depth over the length of the ‘toe up’ horizontal wells. For the heel stages in each well, the vertical extent of seismicity is limited to the UWC and MWC, while for the toe stages, seismicity extends into the overlying units. Downward propagation of seismicity is limited in both cases by the highest S_hmin value at the base of the MWC. We performed full waveform inversion to constrain the microearthquake focal mechanisms and then inverted the focal mechanisms to estimate the directions and relative magnitudes of the principal stresses. We compared the results to Mohr-Coulomb faulting analysis based on observations of stress, wellbore failure, and fracture orientations from image logs and the post-stimulation core. The focal planes in each lithofacies are consistent with the fractures expected to slip in the measured stress state, which indicates that the observed variations in stress control both the distribution and mechanisms of microearthquakes induced by fluid injection.