Potential implementation of geologic carbon sequestration in southern Colorado
Herman, David N.
Herman, David N.
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2024
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Abstract
Deep saline aquifers provide isolated reservoirs for geologic carbon sequestration. It is necessary to minimize the risk of CO2 injection within these deep saline aquifers. This is done through reservoir characterization, static modeling and injection modeling of CO2 within the saline aquifer. Modeling the time-lapse seismic response from this injection allows for better monitoring of the CO2 plume when injection commences.
Utilizing available data, a static model of the Lyons saline aquifer is built. Post stack seismic inversionis used within a 3D seismic volume to determine the acoustic impedance of the saline aquifer and calculate subsequent properties including porosity and permeability. With this model, injection of CO2 is simulated for 30 years with 70 years of shut-in simulated afterwards. The resulting pressure and saturation plumes are used to model the change in P-impedance (IP) and S-impedance (IS) to create synthetic seismic and study the change due to CO2 injection. Changes in IP and IS are modeled using pressure dependent velocity testing of the Lyons sandstone to find the elastic moduli of the dry rock as effective pressure changes and the pressure dependence of the pore fluids. Variations in porosity in the reservoir rock are accounted for by calculating the pore space stiffness and rigidity and integrating that into the rock physics model. Resulting IP and IS within the aquifer are used to calculate synthetic angle stacks to simulate the change in Amplitude Versus Offset as a function of the change in IP and IS.
From the simulation of CO2 injection into the static model a decrease in IP and IS is calculated. CO2 replaces brine in the pore space lowering fluid bulk modulus (Kfluid). Increased pore pressure reduces the effective pressure, softening the rock matrix and lowering dry rock bulk modulus (Kdry) and dry rock shear modulus (µdry). Additionally, density (ρ) decreases as the density of the pore fluid drops. The Lyons sandstone is not sensitive to pressure changes at the initial effective pressure or after injection. The changes in the elastic moduli from effective pressure decrease does not contribute significantly to the decrease in IP and IS. IP and IS within the CO2 plume decrease as pore pressure increases and CO2 replaces the brine in the pore space, with a maximum decrease of 4.31% and 1.13% respectively. This change creates a decrease in intercept and gradient from the simulated amplitude versus offset.
Evaluation of the reservoir properties, geologic structure, simulation results and synthetic AVO data allows us to conclude that the Lyons saline aquifer would be suitable for geologic sequestration of CO2. Utilizing the limited data a reliable static model is able to be constructed. The aquifer is strongly sealed by alternating anhydrites and shales. Synthetic angle stacks allow for modeling of the extent of the simulated CO2 plume as CO2 saturation and pore pressure change the elastic moduli of the saturated rock.
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