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At Cranfield field, Mississippi, a monitored carbon dioxide (CO2) sequestration and enhanced oil recovery project provides a unique opportunity to study sealing properties of a marine shale as a CO2-confining zone. The reservoir is in the amalgamated fluvial basal sandstone of the lower Tuscaloosa Formation at depths of more than 3000 m (9843 ft). The marine mudstone of the middle Tuscaloosa forms a continuous regional confining system of approximately 75 m (246 ft).
A 6-m (20-ft) core was retrieved from the middle Tuscaloosa marine mudstone approximately 70 m (230 ft) above the CO2 injection zone. We conducted a series of characterizing analyses on the core that would enable us to assess with high confidence seal performance over geologic time. The core displays considerable heterogeneity at centimeter to decimeter scales, with lithology varying from silt-bearing clay-rich mudstone to siltstone and very fine grained sandstone. In total, nine microfacies are recognized in the core. Petrographic, mineralogical, and chemical analyses (scanning electron microscopy, x-ray diffraction, and x-ray fluorescence) show that calcite cements preferentially form in coarser grained beds and have greatly reduced porosity and permeability, making silty and sandy beds less permeable than mudstone. Mercury intrusion capillary pressure tests show desirable sealing capacity for all samples capable of retaining a CO2 column of 49 to 237 m (161–778 ft) at 100% water saturation. Permeability and porosity of all facies are less than 0.0001 md and 4%, respectively. Pores in the samples are at nanometer scales, with modal pore-throat sizes less than 20 nm. Scanning electron microscopic imaging on ion-milled surfaces confirms that nanopores are scarce and generally isolated.
Jiemin Lu is a geologist who received degrees from Nanjing University, Peking University, and the University of Edinburgh. He has been working on reservoir and seal properties associated with geologic sequestration of carbon dioxide since his Ph.D. years at Edinburgh.
Kitty Milliken has degrees in geology from Vanderbilt University (B.A.) and the University of Texas at Austin (M.A., Ph.D.) and is currently a senior research scientist at the Bureau of Economic Geology. Her research interests revolve largely around the challenges of integrating various types of petrography with chemical analysis to decipher the chemical and mechanical histories of rocks.
Rob Reed is a research scientist associate at the Bureau of Economic Geology. He received his B.S. degree and his Ph.D. in geological sciences from the University of Texas at Austin and his M.S. degree in geology from the University of Massachusetts. His current research focuses on various aspects of the microstructure of rocks.
Susan D. Hovorka has degrees in geology from Earlham College and the University of Texas. She has worked on diverse topics related to water quality protection, waste storage, and reservoir characterization. Hovorka is the principal investigator of the Gulf Coast Carbon Center (www.gulfcoastcarbon.org), an industry/academic partnership working on economically viable approaches to geologic sequestration of carbon dioxide.