CCS and Power Systems
Carbon Storage - Monitoring, Verification, Accounting, and Assessment
Advanced Technologies for Monitoring CO2 Saturation and Pore Pressure in Geologic Formations: Linking the Chemical and Physical Effects to Elastic and Transport Properties
Performer: Stanford University
Project No: FE0001159
To date, researchers have performed a series of laboratory experiments and high-resolution images assessing the changes in microstructure, transport, and seismic properties of fluid-saturated sandstones and carbonates injected with CO2. The following summarizes the results to date:
Injecting CO2-rich brine into carbonate rocks results in decreased ultrasonic P- and S-wave velocities and increased porosity and permeability. Initial measurements on carbonate samples reveal as much as 30 percent decrease in elastic velocity, 5 percent increase in porosity, and 4,000 mD increase in permeability, all associated with permanent dissolution of minerals and flushing of mineral fines. Experiments illustrate that the magnitude of the changes correlates with the rock microtexture – tight, high surface area samples showed the largest changes in permeability and smallest changes in porosity and elastic stiffness compared to those in rocks with looser texture and larger intergranular pore space. In contrast, computer simulations show that mineral precipitation in the pore space can cause significant decreases in porosity and permeability and increases of elastic moduli.
Injecting CO2 into brine-saturated-sandstones induces salt precipitation primarily at grain contacts and within small pore throats (Figure 2). High porosity samples exhibited less permeability change with precipitation than the low porosity samples. In rocks with porosity lower than 10 percent, salt precipitation reduces permeability and increases P- and S-wave velocities of the dry rock frame. High resolution images revealed that salt precipitated preferentially in thin cracks and grain boundaries, leading to relatively large increases in elastic stiffness.
Injecting CO2-rich water into micritic carbonates induces dissolution of the microcrystalline matrix, leading to porosity enhancement and chemo-mechanical compaction under pressure. In this situation, the elastic moduli of the dry rock frame decreased.
Injecting CO2-rich brine into a sample of the Lower Tuscaloosa sandstone of Cranfield, Mississippi affected the formation’s elastic and transport properties. Iron concentration, compressional and shear wave velocities were measured before and after injecting the sandstone sample with carbon dioxide rich synthetic Tuscaloosa brine at various confining pressures.
Studying the heterogeneities of microstructure in several rock types to understand distributions of flow paths and pore surface area yielded fairly uniform pore size distributions within each rock sample but variation from one sample to the next on the core plug scale. Sandstones had the narrowest range of pore sizes, carbonates had a broader range of sizes, and the tightest carbonate had a very broad distribution of sizes.
Injecting CO2 into oil-bearing sandstones and carbonates demonstrated that oil coating reduces the overall reactivity associated with CO2 injection. The increases in porosity and decreases in ultrasonic velocity were smaller in an oil-bearing sample than in a very similar clean sample.
Developed a preliminary model that theoretically predicts the changes in rock elastic bulk modulus upon replacement of one pore fill with another. The computational work used finite element methods to predict the frequency-dependent stiffness of rocks containing mixtures of free CO2 and brine. The model is an important step towards understanding the seismic signature of dissolved and precipitated minerals. It is anticipated that the model will provide a framework that can include chemical changes to the pore space.