Chemical effects of carbon dioxide sequestration in the Upper Morrow Sandstone in the Farnsworth, Texas, hydrocarbon unit Bulbul Ahmmed, Martin S. Appold, Tianguang Fan, Brian J. O. L. McPherson, Reid B. Grigg, and Mark D. White ABSTRACT Numerical geochemical modeling was used to study the effects on pore-water composition and mineralogy from carbon dioxide (CO 2 ) injection into the Pennsylvanian Morrow B Sandstone in the Farnsworth Unit in northern Texas to evaluate its potential for long-term CO 2 sequestration. Speciation modeling showed the present Morrow B formation water to be supersaturated with respect to an assemblage of zeolite, clay, carbonate, mica, and aluminum hydroxide minerals and quartz. The principal accessory minerals in the Morrow B, feldspars and chlorite, were predicted to dissolve. A reaction-path model in which CO 2 was progressively added up to its solubility limit into the Morrow B formation water showed a decrease in pH from its initial value of 7 to approximately 4.1 to 4.2, accompanied by the precipitation of small amounts of quartz, diaspore, and witherite. As the resultant CO 2 -charged fluid reacted with more of the Morrow B mineral matrix, the model predicted a rise in pH, reaching a maximum of 5.1 to 5.2 at a water–rock ratio of 10:1. At a higher water–rock ratio of 100:1, the pH rose to only 4.6 to 4.7. Diaspore, quartz, and nontronite precipitated consistently regardless of the water–rock ratio, but the carbonate minerals siderite, witherite, dolomite, and calcite precipitated at higher pH values only. As a result, CO 2 seques- tration by mineral trapping was predicted to be important only at low water–rock ratios, accounting for a maximum of 2% of the added CO 2 at the lowest water–rock ratio investigated of 10:1, which corresponds to a small porosity increase of approximately 0.14% to 0.15%. AUTHORS Bulbul Ahmmed ~ Department of Geological Sciences, 101 Geological Sciences Building, University of Missouri, Columbia, Missouri 65211; bulbul_ahmmed@baylor.edu Bulbul Ahmmed received his B.S. in geology from the University of Dhaka, Bangladesh, in 2011 and his M.S. in geological sciences from the University of Missouri –Columbia in 2015. He is currently a Ph.D. student in geology at Baylor University, where he is studying hy- draulic fracture networks in low-permeability rocks using the electrical resistivity method. Martin S. Appold ~ Department of Geological Sciences, 101 Geological Sciences Building, University of Missouri, Columbia, Missouri 65211; appoldm@missouri.edu Martin Appold is an associate professor in the Department of Geological Sciences at the University of Missouri–Columbia. He received a B.A. in geology from Washington University, an M.S. in economic geology from the University of Michigan, and a Ph.D. in hydrogeology from Johns Hopkins University. His main research interests are the physical and chemical behavior of subsurface fluids. Tianguang Fan ~ Petroleum Recovery Research Center, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801; tfan@nmt.edu Tianguang Fan is a research chemist in the Petroleum Recovery Research Center at the New Mexico Institute of Mining and Technolo- gy. He earned a B.S. in engineering from the East China University of Science and Technol- ogy and an M.S. in chemistry from the New Mexico Institute of Mining and Technology. Brian J. O. L. McPherson ~ Department of Civil and Environmental Engineering, University of Utah, 110 Central Campus Drive, Suite 2000, Salt Lake City, Utah 84112; bmcpherson@egi.utah.edu Brian McPherson is a professor in the Department of Civil and Environmental Engineering at the University of Utah and coprincipal investigator of the Southwest Regional Partnership on Carbon Sequestration. He received a B.S. in geophysics Copyright ©2016. The American Association of Petroleum Geologists/Division of Environmental Geosciences. All rights reserved. Manuscript received May 26, 2015; provisional acceptance September 3, 2015; revised manuscript received March 21, 2016; final acceptance May 13, 2016. DOI:10.1306/eg.09031515006 Environmental Geosciences, v. 23, no. 2 (June 2016), pp. 81–93 81