METHANE ADSORPTION AND RELEASE MECHANISM IN OVER-MATURE KEROGEN BY MOLECULAR MODELING

Shales are characterized by low permeabilities as they contain extremely small pores with sizes ranging from 1 to 200 nm. Whether or not shale can be targeted for oil and gas exploration depends largely on the amount and type of kerogen present. Literature data show that kerogen can adsorb gas and store significant amounts of free gas. Using molecular modeling we investigate the adsorption of methane on type II post-mature kerogen, the extraction of methane from nanopores, and the properties of methane in porous kerogen. We discover that methane release from kerogen nanopores is characterized by two stages, each with a distinct release mechanism. At the early stage of gas extraction, when the gas pressure is still high, methane molecules withdrawn from the system are mainly from the pressurized free gas, and the migration of methane is driven by the gas pressure gradient. At the late stage, when the gas pressure is low, gas molecules extracted from the system are adsorbed gas. Methane desorption coupled with diffusion becomes the time-limiting step of the whole extraction process. At this stage, a significant amount of methane remains inside the kerogen nanopores, but the extraction rate is very small. The pore network connectivity can significantly affect the ultimate recovery. Our results shed a new light on mechanistic understanding gas release in unconventional reservoirs.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.