Pre-injection reservoir characterization for CO₂ storage in the inner continental shelf of the Texas Gulf of Mexico

Abstract

The injection of CO₂ into the subsurface (carbon capture and storage; CCS) is the most viable approach to significantly reduce industrial emissions of greenhouse gasses to the atmosphere. The inner continental shelf of the northern Gulf of Mexico has incredible potential for CO₂ storage. This study quantitatively evaluates the CO₂ storage capacity of the Lower Miocene brine-filled sandstones in the inner continental shelf of the Texas Gulf of Mexico using 3D seismic and well log data. The first part of this work investigates the relationship between elastic properties and reservoir properties (e.g., porosity, mineralogy, and pore fluid) of the Lower Miocene section using rock physics modeling and simultaneous seismic inversion. The elastic properties are related to porosity, mineralogy and pore fluid using rock physics models. These rock physics transforms are then applied to the seismically derived elastic properties to estimate the porosity and lithology away from the wells. The porosity and lithology distribution derived using this quantitative method can be interpreted to predict the best areas for CO₂ storage in the inner continental shelf of the Texas Gulf of Mexico. The second part of this work studies the effect that CO₂ has on the elastic properties of the Lower Miocene rocks using fluid substitution, amplitude variation with angle (AVA), and statistical classification to determine the ability of the seismic method to successfully monitor CO₂ injected into the subsurface. The velocities and density well logs were modeled with different fluid saturations. To characterize the seismic properties corresponding to these different fluid saturations, the AVA responses and probability density functions were calculated and used for statistical classification. The AVA modeling shows a high sensitivity to CO₂ due to the soft clastic framework of the Lower Miocene sandstones. The statistical classification successfully discriminates between brine and CO₂ saturation using Vp/Vs and P-impedance. These results shows that the Lower Miocene sandstones have the capacity to host CO₂, and that the CO₂ injected in these rocks is likely to be successfully monitored using seismic methods.