A sizable amount of literature exists analyzing the effect of confining stress on fracture conductivity in sandstones. This report attempts to answer similar questions with regard to shale formations. The low Young's Moduli and Brinell hardness values characteristic of many prospective shale formations may lead to a great deal of embedment and fines production which can drastically reduce fracture conductivity. Furthermore, shales exhibit sensitivity to aqueous fluids which may cause them to be weakened in the presence of certain fracturing fluids. Previous work analyzing shale fluid sensitivity has failed to preserve the shales’ formation properties by allowing the shale to dry out.
This paper presents a study of propped fracture conductivity experiments at reservoir temperature and pressure using various North American shale reservoir cores. Exposure to the atmosphere can alter the mechanical properties of the shale by either drying or hydrating the samples, so care was taken to preserve these shales in their native state by maintaining constant water activity (relative humidity). Variations in applied closure stress and aqueous fluid exposure were analyzed and in certain cases altered the propped fracture conductivity by crushing proppant, embedding the proppant into the fracture face, and producing fines. The damage to fracture conductivity is correlated to mineralogy for the various shale samples. These findings show that a one-size-fits-all frac design will not work in every shale formation, rather a tailored approach to each shale is necessary.
In the future, the results of this work will be analyzed alongside easier to perform Brinell hardness tests, swelling tests, and other characterization techniques incorporated into the UT Shale Characterization Protocol (Zhou et al., 2013). Correlations were developed to relate the simpler tests to the fracture conductivity experiments will yield an easier method to determine the role embedment and fluid sensitivity have on post treatment fracture conductivity in shales.