The study reveals how to improve natural gas production in shale

LOS ALAMOS, NM, November 12, 2020 – A new hydrocarbon study contradicts conventional wisdom about how methane is trapped in rock, revealing a new strategy to more easily access the precious energy resource.

“The most challenging problem for the shale energy industry is the very low recovery rate of hydrocarbons: less than 10% for oil and 20% for gas. Our study has produced new insights into the fundamental mechanisms governing transport. of hydrocarbons within shale nanopores, ”said Hongwu Xu, an author in the Division of Earth and Environmental Sciences at Los Alamos National Laboratory. “The results will ultimately help develop better pressure management strategies to improve recovery of unconventional hydrocarbons.”

Most of US natural gas is hidden deep in shale fields. The low porosity and permeability of shale make it difficult to recover natural gas in tight reservoirs, especially in the advanced stage of the well life. The pores are tiny – typically less than five nanometers – and little known. Understanding the retention mechanisms of hydrocarbons in the subsoil is essential to increase the efficiency of methane recovery. Pressure management is an inexpensive and effective tool available to control production efficiency that can be readily adjusted during well operation, but the study’s multi-institutional research team found a trade-off.

This team, including lead author Chelsea Neil, also of Los Alamos, integrated molecular dynamics simulations with the novel in situ high pressure diffusion of small angle neutrons (SANS) to examine the behavior of methane in shale. of Marcellus in the Appalachian Basin, the largest natural gas field, to better understand the transportation and recovery of gas when the pressure is changed to extract the gas. The investigation focused on the interactions between methane and organic (kerogenic) content in the rock that stores most of the hydrocarbons.

The study results indicate that while high pressures are useful for recovering methane from larger pores, dense gas is trapped in smaller, more common shale nanopores due to kerogen deformation. For the first time, they present experimental evidence for the existence of this deformation and have proposed a methane release pressure range that has a significant impact on methane recovery. This information helps to optimize strategies for increasing natural gas production, as well as to better understand fluid mechanics.

The behavior of methane was compared during two pressure cycles with peak pressures of 3000 psi and 6000 psi, as it was previously believed that the increase in pressure from fluids injected into the fractures increases gas recovery. The team found that unexpected methane behavior occurs in very small but prevalent nanopores in kerogen: pore uptake of methane was elastic down to the lower peak pressure, but it became plastic and irreversible at 6,000 psi, trapping dense methane clusters that have developed in the submarine -2 nanometer pores, comprising 90 percent of the measured shale porosity.

Led by Los Alamos, the multi-institutional study was published in Nature’s new Earth and environment communications diary of this week. Partners include the New Mexico Consortium, the University of Maryland, and the National Institute of Standards and Technology Center for Neutron Research.

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Financing:

This work was supported by the Department of Energy’s Office of Fossil Energy, the National Energy Technology Laboratory, and the Los Alamos National Laboratory Laboratory’s Directed Research and Development (LDRD) program.

The paper:
“Reduced methane recovery at high pressure due to methane trapping in shale nanopores”; Chelsea W. Neil, Mohamed Mehana, Rex P. Hjelm, Marilyn E. Hawley, Erik B. Watkins, Yimin Mao, Hari Viswanathan, Qinjun Kang and Hongwu Xu;
Earth and environment communications; DOI: https: //doi.org /10.1038 /s43247-020-00047-w

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