Evaluation of the geologic CO[subscript 2]sequestration potential of the Morrow B sandstone in the Farnsworth, Texas hydrocarbon field using reactive transport modeling
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The objectives of this research were to examine the long term fate of CO[subscript 2] injected into the Morrow B Sandstone, a hydrocarbon reservoir in the Farnsworth Unit in northern Texas. A suite of numerical multiphase reactive transport models was constructed simulating continuous and water-alternating-gas (WAG) CO[subscript 2] injection through nine wells into an initially water-saturated reservoir. The simulations predicted little change in the pressure or temperature of the reservoir as a result of fluid injection. However, the injectedCO[subscript 2] temporarily reduced the pH of the formation water from near neutral to as low as 4.7, which induced further changes in the formation water composition and reservoir mineral matrix. Much of the injected CO[subscript 2] also initially existed as a separate immiscible gas phase, but this CO[subscript 2] appears to have leaked vertically through the overlying Morrow Shale cap rock and disappeared from the Morrow B Sandstone within 30-50years. However, some of the injected CO[subscript 2] dissolved into the Morrow B formation water and migrated westward. This acidified CO[subscript 2]-enriched formation water initially dissolved native calcite and ankerite in the reservoir but later in the simulations calcite, dolomite, and siderite began to precipitate, serving as minor sinks for CO[subscript 2]. Among non-carbonate minerals, albite and illite, both initial constituents of the Morrow B Sandstone matrix, were predicted to dissolve, whereas quartz, kaolinite, and smectite were predicted consistently to precipitate. However, these changes in mineral precipitation and dissolution caused negligible changes in porosity on the order of only 0.001%, causing correspondingly negligible changes in the patterns of fluid flow and CO[subscript 2] storage capacity of the Morrow B Sandstone. Based on the results of the simulations, because of significant leakage of CO[subscript 2] through the overlying Morrow Shale, the Morrow B Sandstone is judged to be a minor reservoir for CO[subscript 2] sequestration for at least decadal to century time scales, where the CO[subscript 2] is sequestered primarily in aqueous solution in the Morrow B formation water. However, petroleum, a likely major sink for injected CO[subscript 2], was not considered in the models and likely would significantly improve the CO[subscript 2] sequestration capacity of the Morrow B Sandstone.
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