Can We Predict Primary Creep and Least Principal Stress S_hmin at Depth Either from Specific Surface Area or Weak Phase of Gas Shales?

Recently short-term laboratory primary creep i.e., time-dependent deformation under triaxial in situ stress condition of ultra-low permeable gas shales have been utilized to work out geomechanical impacts of field development cycle such as modification of in situ stress state, prediction of production induced deformation, and understanding of fracture closure mechanism. However, obtaining creep data from the laboratory method is tedious, time-consuming, and costly. A simple power law model as a function of time involving instantaneous elastic compliance of the studied material B, and time dependent component n is used to describe creep and stress relaxation owing to the superposition principle of linear viscoelastic materials. Gas shales usually have a large specific surface area (SSA) because of the dominance of clay minerals (Illite, Smectite, Kaolinite, and Chlorite) and/or total organic carbon (TOC). Low-pressure nitrogen gas adsorption is a quick and cost-effective method to derive specific surface area value S_N2 on powdered gas shale samples. From the observed strong empirical correlation between creep parameters and S_N2value as well as with weak phase fraction ClayTocPHI (combination of clay, porosity, and TOC), a novel indirect approach is proposed to predict primary creep constitutive parameters either from the specific surface area (SSA) value S_N2 or weak phase fraction ClayTocPHI of multiple gas shales at deeper subsurface formations (Figure 1). These gas shales cover a broad range of mineralogy, maturity, porosity, and depositional history. Through a case study, empirically derived creep parameters from S_N2 are utilized to predict the least principal stress S_hmin magnitude at depth of a six lithological layered gas shale formation with a viscoelastic stress relaxation approach. Direct field measurement validated the layered variation of the predicted S_hmin magnitude.