--> Fluid Substitution Modeling to Determine Sensitivity of Time-Lapse 3-D Vertical Seismic Profile Data to Injected CO<sub>2</sub>

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Fluid Substitution Modeling to Determine Sensitivity of Time-Lapse 3-D Vertical Seismic Profile Data to Injected CO2

Abstract

For commercial scale carbon capture and storage projects it is essential for site operators and regulators to understand the fate of injected CO2. The Southwest Regional Partnership on Carbon Sequestration is testing monitoring technologies at Farnsworth field, TX, and of particular interest is the use of 4D vertical seismic profiles centered on CO2 injection wells to cost effectively monitor plume growth and distribution. The reservoir interval is a Morrowan age fluvial sandstone deposited in an incised valley at about 2800m depth, with porous sands between 10 to 25m thick. Farnsworth field was first developed in 1958, and was converted to a water flood in the late 1960's. Chaparral Energy took the field over in 2011 and instituted a CO2 flood using 100% anthropogenic CO2 generated at an ethanol plant and a fertilizer plant. Ultimately, the majority of injected CO2 will be sequestered. In order to detect and quantify the CO2 plume in time-lapse VSP data it is important to understand the seismic sensitivity of the interval to the replacement of fluids with CO2. Through 2015, three wells have had baseline 3D VSP data acquired, and one injection well had a repeat survey after seven months of CO2 injection. 3D VSP surveys offer a higher level of detail at reservoir intervals since active source energy from surface shot points only needs to travel through near surface layers in the downward direction to receivers in the well-bore, reducing the effect of attenuation in near-surface formations. 3D VSP surveys at Farnsworth can image up 1.5km away from the center well, which allows examination of full injection patterns. To optimize timing of repeat VSP acquisition, the sensitivity of the 3D VSP surveys to CO2 injection was analyzed to determine at what injection volumes a seismic response to the injected CO2 is observable. Static geologic models were generated for pre-CO2 and post-CO2 reservoir states by interpreting, and populating fine scale geologic models using baseline 3D VSP data, and then by history matching pre and post CO2 models. These generated static states of the model, where CO2 replaces oil and brine in pore spaces, allow for the generation of impedance volumes which when convolved with a representative wavelet, are used to generate synthetic seismic volumes used to contrast synthetic and actual time lapse 3D VSP datasets. Funding for the project is provided by DOE's National Energy Technology Laboratory (NETL) under Award No. DE-FC26-05NT42591.