(215a) Investigation of Effects of Subcritical Water As Fracturing Fluid on Hydraulic Fracturing and Fracture Permeability of Shale

Hydraulic fracturing is a promising technique to produce unconventional oil and gas. However, with the current hydraulic fracturing practice, only 10-30 % of the hydrocarbon is recoverable from unconventional shale according to the U.S. Energy Information Administration (EIA). Low porosity (<5 %) and low permeability (2-50 nD) of Utica and Marcellus shale are the primary reasons for the low hydrocarbon recovery. Hydrocarbon recovery can be enhanced significantly by connecting the pores and creating new fracture branches which will result in enhanced porosity and enhanced permeability respectively.

Hydraulic fracturing uses water (>90%) as fracturing fluid at temperatures in the range of 50-95 °C. Physical and chemical properties of hot compressed water (T=100-374 °C and corresponding saturation pressure) and supercritical water (T>374 °C and P>22.4 MPa) are very different than water below 100 °C. Thermodynamic properties of water (e.g., density, ionic product, and dielectric constant) changes significantly with the increase of water temperature. As a result, sub-and supercritical water behaves like a strong nonpolar solvent. Supercritical water also becomes highly reactive and corrosive depending on the density and temperature. Therefore, using subcritical water as fracturing fluid is likely to enhance the hydrocarbon recovery from shale reservoirs by enhancing the shale porosity and permeability.

In previous study, shale samples were treated in subcritical water environment which was named as hydrothermal deformation (HTD). With HTD, shale porosity was enhanced significantly. For example, HTD at 300°C for six hours of residence time shale porosity increased up to four times than raw shale. A positive correlation of shale porosity with HTD reaction time and temperature was observed. Evolution of numerous micro and mesopores were also observed.

In this study, HTD was applied in Utica shale samples to observe the changes in shale fracture. It was observed that fracture width of shale was increased significantly with HTD. Besides, additional fractures were also observed. Along with the HTD of shale, subcritical water was also injected into the pre-drilled Utica shale core plug in a core holder with certain confining pressure. Confining pressure was allowed to vary within a range of 2500-3500 psig. Consistent changes in the shale morphology were also found from the SEM-EDS analysis. Digital microscope and micro computerized tomography scanning (CT-scan) images were used to observe the changes in fracture have occurred across the shale core.