(644a) Enhancing Unconventional Shale Porosity and Permeability By Subcritical Water Treatment

Utica and Marcellus shales are two large unconventional shales in the U.S. With the advance horizontal drilling technology along with state-of-the-art fracking fluid, only 28% of the reserve is recoverable from unconventional shales according to the U.S. Energy Information Administration (EIA). Among other challenges, low porosity (<5 %) and low permeability (2-50 nD) of Utica and Marcellus shales are the primary reasons for the low recovery. Increasing porosity and permeability without compromising the mechanical strength are necessary for enhanced gas recovery. Therefore, the fracking fluid needs to react with the shale minerals and kerogen in order to connect nano-pores and increase the porosity, permeability, and recovery of the shale.

Hydraulic fracturing uses water (~90%) as fracking 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, sub-and supercritical water are promising to react with kerogen as well as shale minerals.

In this study, hydrothermal treatment of both Utica and Marcellus shales were performed in a batch reactor. The reaction temperatures were set between 200-300 °C and the reaction times were tested for 1-6 h. The treated shales were analyzed with BET surface area analyzer, mercury induced capillary pressure (MICP) porosimeter, FE-SEM, XRD, and TEM. The process water was analyzed with ICP-OES, pH, and IC. Mass recovery was reduced with the increase of treatment temperature and time. Porosity, calculated from BET, showed a positive correlation with hydrothermal treatment temperature. For instance, porosity increases from 4.0 % to 10 %, when Marcellus shale was treated at 250 °C for 3 h. FE-SEM and TEM revealed the increase of pore sizes up to 100 nm, while XRD shows the decrease of several minerals from the solid matrix of the treated shales. ICP-OES confirmed the mass loss in the form of inorganics due to the hydrothermal treatment.