Correlation Between In-Situ Stress Variations and Mineralogy Along Horizontal Wells in the Woodford Shale, Oklahoma, USA

Abstract

We conducted a case study to investigate the effectiveness of hydraulic fracturing and formation properties in the Woodford shale. The analysis involves two parallel horizontal wells, each is about 10,000 feet long with ∼15 frac stages. Analysis of Instantaneous Shut-In Pressure (ISIP) of each frac stage yielded values for variations of the minimum horizontal principal stress (Shmin) magnitude along the length of the wells. We found significant ISIP variations along the length of both wells. Although there is no direct correlation between the number of micro-earthquakes and Shmin magnitude of a given stage, in general, the lower Shmin values correlates to further distribution of microseismic events away from the well both vertically and laterally. We utilized Elemental Capture Spectroscopy (ECS) logs to determine the content of clay and organic matter over the length of the horizontals. We observed that ISIP by stage rises with the content of clay and organic matter. By combining the analysis of the ECS log and the well steering data, we found that the wellbore traveled in and out of the target zone and penetrated different facies of the Woodford shale along its path, resulting variation of mineralogy along the well length. The value of Shmin is a measure of the pressure needed for hydraulic hydraulic fracture. The variation of Shmin along the length of the wells appears to correlate with variations of rock composition and mechanical properties of the formation close to the wellbore. Low values of Shmin also reflect greater stress anisotropy is this an indicator of relatively, brittle (or less ductile) formations resulting from variations of clay and organic matter. A plausible explanation is by stress relaxation. According to Sone and Zoback (2014, JPSE), the viscous creep of ductile materials over geological time is responsible for deviations from expected at certain depth. We plan to quantitatively analyze the connection between mineralogy and stress variation using the viscous stress relaxation model.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016