Letter Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX
http://pubs.acs.org/journal/aesccq
13
C Nuclear Magnetic Resonance Spectroscopy of Methane and Carbon Dioxide in a Natural Shale
Geoffrey M. Bowers,*,† H. Todd Schaef,‡ Quin R. S. Miller,‡ Eric D. Walter,§ Sarah D. Burton,§ David W. Hoyt,§ Jake A. Horner,∥ John S. Loring,‡ B. Peter McGrail,‡ and R. James Kirkpatrick⊥ †
Department of Chemistry and Biochemistry, St. Mary’s College of Maryland, St. Mary’s City, Maryland 20686, United States Physical and Computational Sciences Directorate, §William R. Wiley Environmental and Molecular Sciences Laboratory, and ∥ Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ⊥ Departments of Earth and Environmental Sciences and Chemistry, Michigan State University, East Lansing, Michigan 48824, United States ACS Earth Space Chem. Downloaded from pubs.acs.org by WEBSTER UNIV on 02/24/19. For personal use only.
‡
S Supporting Information *
in 2017, representing ∼20% of global production and positioning the U.S. as the world’s largest natural gas producer; this productivity is expected to make the U.S. a net energy exporter by 2022.3 The U.S. is also poised as a technological leader, because its estimated proven reserves of 309 Tcf are only a small fraction of the estimated 6867 Tcf of the technically recoverable global resource.3,4 This high-tech production portfolio includes horizontal drilling techniques to increase footage capacity by 60% over vertical drilling,5 multi-stage hydraulic fracturing to stimulate production of natural gas and oil, and the deployment of CO2-, N2-, and CH4-based hydraulic fracturing approaches as potential nonaqueous stimulation fluids.6,7 Currently, fracture stimulation occurs in stages and costs millions of dollars per well. The specific techniques used are typically based on operator experience, and fracturing fluid selection is often centered on the experience of the field operator and the well service provider. Key factors that influence production that are less well-understood and more difficult to probe include the hydrocarbon transport and storage mechanisms,8 pore network connectivity in the complex pore structures,9 and native fluid and injectate behavior.10 Shales (mudstones) are fine-grained rocks that exhibit complicated pore structures on many length scales11 as well as a wide range of mineralogical compositions, including clays, quartz, carbonates, Fe oxides and sulfides, and organic matter (kerogen).12 Shale porosity typically varies from 3 to 15% and occurs between mineral grains, in expandable clay interlayers, and within the kerogen. Because of the small size of the mineral grains, the average interparticle pore size is often in the mesopore range of a few to tens of nanometers, and the nanopores in clay interlayers are smaller (
