| dc.description.abstract | Understanding our observations of the subsurface and its behavior over time requires quantifying both the in-situ conditions and the intrinsic material properties. Because of the difficulty in directly quantifying the relevant rock properties, in many cases the rock characteristics are often assumed while emphasis is put on determining the in-situ conditions. These assumptions are problematic, especially since the material properties are often used to model and predict subsurface response to short- and long-term perturbations in the in-situ conditions. Accurately modeling the subsurface structure and stability requires the relevant rock properties and their variability with in-situ conditions be quantified. This work utilizes a laboratory-based approach to analyze the rock properties of several lithologies in two areas of interest: 1) the crystalline basement rocks of Oklahoma and Kansas and 2) deeply buried caprocks from the northern Sichuan Basin.
The recent surge of seismicity in Oklahoma and Kansas has been attributed to wastewater injection in the subsurface reactivating previously dormant faults in the crystalline basement. Research has primarily focused on factors related to in-situ stress changes and basement structure, but little attention has been given to the basement rock properties that could affect seismicity. In the first study, several different basement rocks were characterized using a suite of mechanical and petrophysical laboratory tests. Laboratory experiments were conducted with granite, rhyolite and diabase samples collected from southern Oklahoma. Evolution of compressional and shear wave velocity with increasing confinement was measured through a series of ultrasonic velocity tests. A suite of uniaxial and triaxial tests were conducted to measure the elastic and inelastic deformation behavior of the basement rocks. Deformation data was evaluated using the Mohr-Coulomb criterion and compared with additional preexisting deformation data of igneous basement rocks. Dynamic and static elastic properties compare favorably with available field measurements and demonstrate the role physical properties can play in varying mechanical behavior. Water-weakening in the basement rocks may indicate fluid-assisted processes such as stress corrosion cracking enhance deformation in the crystalline basement.
In the next study, work was focused on incorporating laboratory-based observations into modeling the geophysical behavior of the crystalline basement. The construction of accurate velocity models remains a key step in seismological studies and subsurface imaging. As the vertical or 1D velocity structure is often difficult to determine through field and well log observations, we measured the ultrasonic velocity in vertically oriented basement samples from Oklahoma and Kansas to synthesize 1D velocity models from different lithologies. The results were compared with well log measurements and 1D velocity models developed through seismologic observations. The agreement between the laboratory-based models and seismic models depends heavily upon the properties of the basement rocks in different locations and the assumptions used to develop each seismic velocity model. Changes in VP/VS ratios of basement samples with pressure suggest that the constant VP/VS assumed in many 1D velocity models for the crystalline basement is incorrect.
Following the previous work, the next study examined the 3D velocity anisotropy inherent in several basement rocks from Oklahoma and Kansas. Velocity anisotropy and particularly shear-wave splitting is a powerful tool for determining the in-situ stress orientations in the subsurface. Factors other than the stress field are capable of generating velocity anisotropy, including fracture orientations and mineral alignment. For the crystalline basement, rocks are often assumed as isotropic and thus observed anisotropy is attributed solely to the stress orientations. Two sets of laboratory tests were used to measure the horizontal and vertical velocities of several basement rocks from Oklahoma and Kansas. Tests were conducted under hydrostatic stress conditions (i.e., σ1 = σ2 = σ3) where any velocity anisotropy observed could not be attributed to the stress orientations. Microstructural observations were used to quantify the inherent anisotropy attributed to fractures in five basement rock sample in the vertical and horizontal orientations. All rocks were shown to exhibit velocity anisotropy both in the vertical and horizontal planes, though velocity anisotropy was found to be greatest in the horizontal plane (relative to surface). Sample anisotropy was highly variable between different regions and depths. The results were compared with well log and seismically measured anisotropy to show that the agreement between the velocity polarizations and stress orientations depends upon 1) whether the stresses are aligned with anisotropic structural features such as faults; 2) the degree of deformation in the basement that can induce velocity anisotropy; and 3) the scale at which velocity anisotropy is measured in the basement.
The last study focuses on characterizing several identified caprocks from the northern Sichuan Basin. Caprocks are a crucial component of petroleum systems as they act as impermeable barriers to upward migration of hydrocarbons. Their impermeability or integrity depends upon their lack of features such as fractures that enhance fluid flow. As a result, the geomechanical properties in determining whether a lithology may act as an efficient seal. Laboratory mechanical tests were conducted to measure various strength, elastic, and hardness properties of several evaporite and carbonate caprocks. The results were compared with other laboratory caprock tests and used to develop a map of the ideal caprock characteristics. Strength data from the deformation tests was combined with stress magnitudes determined for the region to identify the stresses required to induce failure in each caprock and quantify the potential risk of caprock failure. | en_US |
