CH4 Adsorption on Moisturized Oil-Bearing and Oil-Free Shales and Its Implication to Oil and Gas Storage in Organic-rich Shales

Abstract

Unconventional natural gas and liquids production from organic-rich shales have been drawing increasing attention and changing the fossil energy resources supply picture throughout North America. Oil and gas storage in organic-rich shales is an important issue for oil and gas deliverability and flow modeling. CH4 adsorption studies of moisture-equilibrated oil-bearing and oil-free shales, combined with vapor adsorption and pore characterization, provides a way to understand oil, gas, and water distribution in nanometer scale pores in organic-rich shales. Artificially-matured Woodford Shale rock samples having different thermal maturities were prepared by hydrous pyrolysis under five different time-temperature conditions. Soluble hydrocarbons were recovered from a second set of artificially-matured samples using Soxlet extraction. Both extracted and un-extracted samples were moisturized under stable relative humidity conditions. CH4 sorption isotherms were measured for the moisture-equilibrated samples at three different temperatures (35, 50, and 65 deg C). Nitrogen and vapor adsorption experiments were also conducted for the extracted and un-extracted samples to characterize pore size, surface area, and surface wettability. CH4 sorption capacity of oil-bearing samples is about 30 to 50% lower than that of oil-free samples, indicating oil covers some pore surfaces and reduces the sorption capacity. Minor differences in CH4 sorption for oil-contained samples were observed with and without the presence of moisture, while substantial decrease of CH4 sorption was observed for oil-free samples. These results, combined with vapor adsorption results that indicate different surface wettability for the oil-bearing and oil-free samples, suggest that the presence of oil precludes water adsorption and that oil is mainly distributed in organic-hosted pores and water is associated with clay minerals. In oil-free samples, water is distributed in both clay and organic-hosted pores therefore decreasing CH4 sorption substantially.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014