MODELING THE FATE AND IMPACT OF CO2 INJECTED INTO THE MORROW-B SANDSTONE THROUGH NON-ISOTHERMAL REACTIVE TRANSPORT MODELING: A CASE STUDY OF THE FARNSWORTH UNIT, TEXAS

The objective of this study was to track the behavior and impact of CO₂ within the Farnsworth Unit (FWU), a hydrocarbon field in northern Texas. The study uses a multiple fluid phase, non-isothermal, numerical reactive transport model to simulate the sequestration potential of large-scale CO₂ injection into the Morrow B Sandstone in the western half of the FWU as part of an active CO₂-enhanced oil recovery (EOR) operation. The model in this study reflects the field development pattern, simulating the individual injector wells with the exact mass rate of the fluid under water-alternating gas (WAG) schemes. The model simulates the effects of formation fluid dissolution, mineral trapping, and hydrodynamic trapping. The model was calibrated with eight years of available historical field data. In the calibrated model, CO₂ was injected on a WAG scheme for 25 years , after which injection ceased until a final simulation time of 1000 years..

The model results showed a change in the formation water composition that leads to changes in the formation mineralogy. The model predicts a sharp initial drop in pH reflecting the impact of the large mass of CO₂ that dissolves in the immediate vicinity of the injectors. Once CO₂ injection has ceased, the pH gradually gradually increases, approaching its initial value. In addition to dissolving into the formation water, much of the injected CO₂ dissolves into petroleum, with the remainder persisting as an immiscible gas phase. The pH remains low enough throughout the simulation for calcite, a native reservoir mineral, to dissolve continuously. With increasing time, other non-native carbonate minerals, dolomite, ankerite, and siderite, begin to precipitate, becoming significant mineral sinks for CO₂ by the end of 1000 year simulation. The changes in mineral abundance, however, do not lead to significant changes in porosity. The results of this study are encouraging for the feasibility of large-scale CO₂ sequestration in sandstone hydrocarbon reservoirs.