PETROLOGICAL CONTROLS ON DENSITY OF ADSORPTION PHASE IN SHALES UNDER RESERVOIR CONDITIONS: A CASE STUDY FROM THE LATE MISSISSIPPIAN CANEY SHALE, SOUTHERN OKLAHOMA, USA

Shale gas continuously plays an important role in the natural gas market to satisfy the growing demand of energy. To estimate the shale gas-in-place (GIP), adsorbed gas (up to 85% of the total shale gas content) needs to be accurately assessed. Measured excess adsorption of methane is often used to mimic the adsorption of subsurface shale gas under geological conditions. In the correction from excess to absolute adsorption, adsorption phase density (APD) is critical in the process of conversion. However, APD of subsurface shale gas and the effects of mineralogy on APD remains poorly understood.

A series of methane excess adsorption isotherms on Caney Shales are collected from this work and other US and Chinese shales from the literature at a wide temperature of 35 °C to 125 °C and a pressure up to 15 MPa to investigate the effects of mineralogy on APD of shale gas. And a method assuming a constant volume of adsorption is utilized to derive the temperature and pressure dependent APDs coupled with low-pressure nitrogen and high-pressure methane adsorption isotherms. Brunauer-Emmett-Teller and Barrett-Joyner-Halenda analysis via low-pressure nitrogen adsorption isotherms and micropore analysis by the Horvath-Kawazoe method are conducted to study the pores structure as well as Scanning Electron Microscopy. X-ray diffraction and organic geochemistry are combined to reveal the mineralogy. Results show that APD of shale gas increases with organic matter (OM) proxied by total organic carbon, and decreases with clay minerals and the sum of quartz and feldspar. OM dramatically contributes to the APD as a multiple-layer adsorption exist and the APD for OM could be 1.4-8.5 times of that for clay minerals. Other inorganic minerals contribute limitedly to APD. And the nature and geometry of surface area instead of the quantity contributes to the APD in shales. The APD does not show an obvious relationship with micropore volume, likely related to the ratio of micropore volume to the total pore volume.

This work provides a significant and comprehensive study of petrological factors that impact the APD of subsurface shale gas, which will improve estimation of supercritical adsorption and shale GIP under geological conditions. Besides, the findings in this work provide implication for subsurface carbon dioxide trapping due to adsorption.