A Viscoplastic Stress Relaxation Model for Predicting Variations of the Least Principal Stress With Depth in Unconventional Reservoirs

Abstract

In this paper we extend the viscoplastic stress relaxation model of Sone and Zoback (Jour. Petrol. Sci. and Eng., 2014) for predicting variations of least principal stress with depth and its impact on the vertical propagation of hydraulic fractures. Viscoplastic stress relaxation makes the stress field in the reservoir more isotropic. In normal faulting and strike-slip faulting environments, this causes the least principal stress to increase. Thus, formations with more viscoplastic stress relaxation are more likely to be frac barriers. In order to predict the magnitude of viscoplastic stress relaxation in different unconventional formations, we generalize the constitutive law developed from a wide range of creep experiments in our lab over the past several years and apply it to an area where stacked pay is being developed by drilling multiple horizontal wells at different depths in the Permian Basin of west Texas. Using frac gradients measured from DFIT (Diagnostic Fracture Injection Test), we empirically calibrate one of the viscoplastic parameters that would ideally be determined by creep experiments and find that it correlates well with the mineral composition. In the formations tested, low QFM (Quartz, Feldspar, and Mica) content tends to correlate with greater viscoplasticity and less anisotropy between the two horizontal principal stresses. The viscoplastic model does a good job of explaining vertical hydraulic fracture propagation, as indicated by the distribution of microseismic events recorded during stimulation.

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