NICHE 2014 Annual Conference – Care. Collaborate. Change.

Abstract

Using the double torsion method, we investigated the fracture mechanical properties of clay-rich Woodford and Mancos Shales, and carbonate-rich Marcellus Shale under conditions of varying fluid salinity, pH, and temperature. Both fracture toughness and subcritical fracture growth properties for clay-rich Woodford and Mancos Shales are sensitive to fluid salinity but not sensitive to pH. With increasing salinity, mean fracture toughness increased up to 34% and 39% for Woodford and Mancos Shale, respectively, and mean subcritical fracture growth index increased up to 99% for Woodford Shale. In contrast, fracture toughness for carbonate-rich Marcellus Shale is unaffected by fluid chemistry, but subcritical fracture growth index increases with increasing pH. Subcritical fracture growth index is reduced by 51% with a decrease in pH from 7 to 1.2. Clay–water interaction is identified as the mechanism for enhanced subcritical fracture growth in clay-rich Woodford and Mancos Shales while enhanced calcite dissolution is accountable for the positive correlation between pH and subcritical fracture growth index in carbonate-rich Marcellus Shale. Increasing solution temperature from 23 °C to 63 °C shifts the K–V curves toward lower stress intensity and amplifies the salinity dependence in the shift of the K–V curves for Woodford Shale. The increase in subcritical fracture growth rate at elevated temperature observed in both clay-rich and carbonate-rich shales is interpreted to reflect faster diffusion rates of fluid from the propagating fracture into the matrix at elevated temperature. Our results emphasize the dependence of critical and subcritical fracture behavior on fluid chemical environment with implications for hydraulic fracture growth in shale reservoirs, and for caprock integrity of wastewater and CO2 storage reservoirs. In these applications, fluid chemistry can both strengthen and weaken caprocks depending on caprock lithology and induced changes in formation water chemistry.