Geomechanical analysis applied to geological carbon dioxide sequestration, induced seismicity in deep mines, and detection of stress-induced velocity anisotropy in sub-salt environments.

摘要

The role of geomechanical analysis in characterizing the feasibility of CO2 sequestration in deep saline aquifers is addressed in two investigations.; The first investigation was completed as part of the Ohio River Valley CO2 Storage Project. We completed a geomechanical analysis of the Rose Run Sandstone, a potential injection zone, and its adjacent formations at the American Electric Power's 1.3 GW Mountaineer Power Plant in New Haven, West Virginia. The results of this analysis were then used to evaluate the feasibility of anthropogenic CO2 sequestration in the potential injection zone. First, we incorporated the results of the geomechanical analysis with a geostatistical aquifer model in CO2 injection flow simulations to test the effects of introducing a hydraulic fracture to increase injectivity. Then, we determined that horizontal injection wells at the Mountaineer site are feasible because the high rock strength ensures that such wells would be stable in the local stress state. Finally, we evaluated the potential for injection-induced seismicity.; The second investigation concerning CO2 sequestration was motivated by the modeling and fluid flow simulation results from the first study.; The geomechanics-based assessment workflow follows a bottom-up approach for evaluating regional deep saline aquifer CO2 injection and storage feasibility. The CO2 storage capacity of an aquifer is a function of its porous volume as well as its CO2 injectivity. For a saline aquifer to be considered feasible in this assessment it must be able to store a specified amount of CO2 at a reasonable cost per ton of CO 2. The proposed assessment workflow has seven steps. The workflow was applied to a case study of the Rose Run sandstone in the eastern Ohio River Valley. We found that it is feasible in this region to inject and store 113 Mt CO2/yr for 30 years at an associated well cost of less than 1.31 US{dollar}/t CO2, but only if injectivity enhancement techniques such as hydraulic fracturing and injection induced micro-seismicity are implemented.; The second issue to which we apply geomechanical analysis in this thesis is mining-induced stress perturbations and induced seismicity in the TauTona gold mine, which is located in the Witwatersrand Basin of South Africa and is one of the deepest underground mines in the world.; In the first investigation, we developed and tested a new technique for determining the virgin stress state near the TauTona gold mine. This technique follows an iterative forward modeling approach that combines observations of drilling induced borehole failures in borehole images, boundary element modeling of the mining-induced stress perturbations, and forward modeling of borehole failures based on the results of the boundary element modeling. The final result was a well constrained range of principal stress orientations and magnitudes that are consistent with all the observed failures and other stress indicators.; In the second investigation, we used this constrained stress state to examine the likelihood of faulting to occur both on pre-existing fault planes that are optimally oriented to the virgin stress state and on faults affected by the mining-perturbed stress field, the latter of which is calculated with boundary element modeling. We made several recommendations that could potentially increase safety in deep South African mines as development continues.; Finally, the third issue addressed in this thesis is the detection of stress-induced shear wave velocity anisotropy in a sub-salt environment. In this study, we tested a technique proposed by Boness and Zoback (2006) to identify structure-induced velocity anisotropy and isolate possible stress-induced velocity anisotropy. The investigation used cross-dipole sonic data from three deep water sub-salt wells in the Gulf of Mexico. First, we determined the parameters necessary to ensure the quality of the fast azimuth data used in our analysis. We then characterized the q