Linking the Chemical and Physical Effects of CO2 Injection to Geophysical Parameters Investigators:

This project aims to demonstrate techniques for quantitatively predicting the combined seismic signatures of CO2 saturation, chemical changes to the rock frame, and pore pressure. This will be accomplished (i) by providing a better understanding the reaction kinetics of CO2-bearing reactive fluids with rock-forming minerals, and (ii) by quantifying how the resulting long-term, CO2-injection-induced changes to the rock pore space and frame affect seismic parameters in the reservoir. This research involves laboratory, theoretical, and computational tasks in the fields of both Rock Physics and Geochemistry. Ultrasonic Pand S-wave velocities are being measured over a range of confining pressures while injecting CO2 and brine into the samples. Pore fluid pressure will be varied and monitored together with porosity during injection. The measurement of rock physics properties will be integrated and complemented by those obtained via geochemical experiments to link the physical and chemical processes underlying the mechanisms triggered by CO2 injection. In this report, we begin with a study aimed at better understanding how chemomechanical processes associated with CO2 injection alter the pore-space attributes. The agreement between the evolution of velocity and permeability found in the laboratory and that expressed by the natural diagenetic trends shows that there are basic rules of pore space evolution dictated by the coupling between chemical and mechanical processes acting on a depositional-dependent microstructure. Carbonate facies determine the initial microstructure, which closely controls pore stiffness and the proneness to volumetric compaction during diagenesis. Depending on the effectiveness of compaction, changes in the rock may lead either to reduction in contact stiffness (high compaction) or connectivity enhancement (low compaction); eventually leading to two evolutionary patterns of velocity and permeability of carbonates experiencing underground circulation of aggressive fluids. A second study was aimed at understanding the effects of injecting carbon dioxide rich brine on the elastic and transport properties of the Lower Tuscaloosa sandstone of Cranfield, Mississippi. We measured compressional and shear wave velocities before and after injecting one sandstone sample with carbon dioxide rich synthetic Tuscaloosa brine at various confining pressures. The bulk and shear moduli decreased from 19 GPa and 12.4 GPa by roughly 9% and 6.5%, respectively, immediately following the first injection of 30 pore volumes of CO2 rich synthetic Tuscaloosa brine. After injecting a total of 160 pore volumes, the rigidity remained constant and the bulk modulus decreased by a total of 13%, which is detectable on the seismic scale. The decrease in elastic properties is likely due to the dissolution of iron-bearing minerals and calcite that have formed at grain contacts as testified by the negligible change in porosity measured after injection. Decreasing the differential pressure acting on the core plug also decreased both P and S-wave velocities by 60 and 20 m/s respectively. Introduction: Monitoring, verification, and accounting (MVA) of CO2 fate are three fundamental needs in geological sequestration. The primary objective of MVA protocols is to identify and quantify (1) the injected CO2 stream within the injection/storage horizon and (2) any leakage of sequestered gas from the injection horizon, providing public assurance. Thus, the success of MVA protocols based on seismic prospecting depends on having robust methodologies for detecting the amount of change in the elastic rock property, assessing the repeatability of measured changes, and interpreting and analyzing the detected changes to make quantitative predictions of the movement, presence, and permanence of CO2 storage, including leakage from the intended storage location. This project addresses the problem of how to interpret and analyze the detected seismic changes so that quantitative predictions of CO2 movement and saturation can be made. The main goals are: (a) linking the chemical and the physical changes occurring in the rock samples upon injection; (b) assessing the type and magnitude of reductions caused by rock-fluid interactions at the grain/pore scale; (c) providing the basis for CO2-optimized physical-chemical models involving frame substitution schemes.