Lithology-Controlled Stress Variations: A Case Study of the Woodford Shale, Oklahoma

In this paper, we investigate the variations of in situ stress with lithology through a case study of the Woodford shale (WDFD) in Northern Oklahoma. The geomechanics aspects of this study has been detailed by Ma and Zoback (2017) in relation to the effectiveness of hydraulic fracturing stimulation affected by stress variations and fault presence. Here we follow up and concentrate on the stress variations along two WDFD horizontal wells. The available stress data in the study area comes primarily from two sub-parallel horizontal wells (Wells B and D) in the WDFD and a vertical well located between and near the center sections of the two horizontals. Each horizontal well is approximately 5,500 feet long and fractured by around 14 stages, and the two wells are about 1,500 feet apart. A limited number of stress measurements (via diagnostic fracture injection tests (DFIT's)) performed in the three formations along the vertical well generally established a S_hmin gradient slightly above what is permitted under frictional equilibrium when assuming S_v is the maximum principal stress (Ma and Zoback, 2017) (Figure 1). However, the instantaneous shut-in pressures (ISIP's) measured during each hydraulic fracturing stage depict significant variations in the estimated S_hmin magnitude along both WDFD wells (Figure 1, Figures 2b and 3b). The estimated S_hmin values generally scatter between the lower bound set by the frictional equilibrium and the overburden stress (S_v).