Modeling of Emission Behavior of Shale Gas Based on Canister Desorption Testing
Understanding of emission behavior of shale gas for the drilled cores will help us evaluate the emitted shale gas in the boreholes and oil and gas fields, which can help further manage effects of emitted shale gas (mostly greenhouse gas, methane) in the petroleum plays on the natural environment. A total of 33 fresh shale cores were collected from two wells, Q14 and Q25, which targeted the Upper Carboniferous Benxi and the Lower Permian Shanxi formation in the Ordos basin, NW China. A series of canister desorption testing (CDT) were conducted in-situ to investigate the emission mechanism of shale gas, besides fundamental geochemical and petrological measurements, including organic geochemistry, X-ray diffraction (XRD) mineralogy, and pore structure analysis using low-pressure CO2 and N2 isotherms. Geochemical results show that Type III organic matter (OM) that comprised mainly of vitrinite originates from a transitional environment, lagoon/delta setting. The enrichment of OM that proxied by total organic carbon (TOC) content range from 0.488 wt % to 13.68 wt %. The deep gas shale (>3000 m) is at over mfature stage, with an averaging vitrinite reflectance of 2.6%. Clay (25.4-97.0 wt %, average 58.8 wt %) and quartz (1-62.1 wt %, average 33.3 wt %) are dominant in the two gas shales. Plagioclase and siderite are common, and feldspar, carbonate, and pyrite were observed as well. The pore system of the two gas shales follow a bi-disperse trend. And clay minerals contribute most to the absolute micropore and mesopore volume, while, OM contribute most to the pore specific surface area. The CDT results show that temperature plays an important role in emission behavior of shale gas. Increasing the temperature by 15-20°C from the reservoir condition, the emitted gas volumes increase by an average of 43%. We modeled the emission data of all 33 cores in two ways, using an emission-occupation equilibrium derived from an extended Langmuir Theory and a traditional desorption-diffusion model. The best fitting performance comes from the emission-occupation equilibrium model with a coefficient of determination, R2, of up to 0.99, due to the complex emission process includes not only desorption and diffusion, but also water and vapor imbibition, which results in replacement of shale gas by water molecules on the surface adsorption sites, especially of clay minerals. In addition, the correlation between emitted capacities and minerals also confirm that the emitted shale gas mainly comes from the clay minerals during the CDTs, with a R2 of 0.66.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019