Investigation of Methane/Carbon Dioxide Partitioning onto Woodford Shale and Kerogen Analogs Using In Situ Near-Infrared Spectroscopy

Abstract:

Production of natural gas from shale gas reservoirs is curtailed by sorption of methane (CH₄) onto minerals and organic constituents in the shale matrix. Recently, supercritical CO₂ (scCO₂) has been proposed as a working fluid that could be utilized to enable recovery of unproduced CH₄ from these low permeability environments. This strategy depends upon a competitive adsorption process in which CO₂ drives desorption of CH₄ from the shale matrix. However, relative sorption of gases on organic-rich shales is not well understood and appears to depend on several factors, such as clay mineral composition, total organic carbon content, microporosity, and moisture content. Moreover, little is known about the migration mechanisms for CH₄ transport from kerogen sources to a production well and the enhanced transport rates that result from formation changes induced by the injection of CO₂. In this work, a newly developed high-pressure titration system with in situ spectroscopic detection was used to investigate the sorption behavior of CH₄ and CO₂ onto Woodford shale and kerogen analogs. The apparatus consists of a custom-designed titanium reactor fitted with quartz windows, switching valves, a fixed-volume injection loop, stainless steel tubing, and a near-infrared spectrometer capable of monitoring gas-phase concentrations at pressures up to 190 bar and temperatures to 100 °C. Quantitative CH₄ sorption measurements were conducted in two steps. First, the empty cell was titrated with CH₄ to identify a correlation between concentration and the near-infrared absorbance of the CH combination and overtone bands. Next, the process was repeated with a sorbent present in the cell, and the difference between the integrated absorbance from the two titrations was used to determine the sorbed concentration of CH₄. Additional experiments were conducted in which the sorbent was initially exposed to CH₄, and then scCO₂ was titrated into the reactor to desorb the CH₄. The results from these investigations will be discussed in the context of optimizing CH₄ recovery from shale-gas reservoirs.