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.

The two major concerns during drilling and completion of wellbores in unconventional reservoirs are: wellbore instability and proppant embedment. Both of these issues are controlled by the interaction of water-based fluids with the organic-rich shale. Understanding the interaction of water-based fluids with organic-rich shales is the first step towards developing screening tests for their compatibility with the shale. In this paper, we present a systematic approach to investigate the interaction of water-based drilling and fracturing fluids with organic-rich shales. We performed a series of measurements to determine shale mineralogy, native water activity, swelling parallel and perpendicular to bedding planes, Brinell hardness, P-wave and S-wave velocities and compressive strength. Changes in hardness and acoustic wave velocities were measured before and after shale samples came into contact with water-based fluids. Shale swelling was determined in two directions simultaneously with our unique 2-D strain indicators. The dynamic Young's modulus and Poisson's ratio were calculated based on the wave velocities. It is shown that these measurements can be used to screen drilling and completion fluids that may interact unfavorably with the shale. The shale mineralogy was obtained using XRD. The shale native water activity was determined with controlled humidity environments. Multi-scale durometers were used to measure the shale hardness. Anisotropic swelling was observed for all shale samples characterized. The results showed a good correlation with the mineralogy, hardness and wave velocities of the shale. In general, the water adsorption was greater with fresh water than with 4% NaCl and 2% KCl. Changes in hardness, P-wave and S-wave velocities, dynamic Young's modulus and Poisson's ratio after shale's contact with water-based fluids can be used as indicators for wellbore stability and fracture conductivity impairment in these shales. The measurement techniques and procedures presented in this paper can be used as standard screening tests to evaluate the interaction between organic-rich shales and water-based drilling and fracturing fluids. If this testing protocol shows incompatibility of the fluids with the shale, additional fracture conductivity tests should be conducted. The screening protocol presented here minimizes the number of fracture conductivity measurements (that are hard to perform) that need to be conducted to check for shale-fluid compatibility.