Modeling injection induced fractures and their impact in CO₂ geological storage
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Large-scale geologic CO₂ storage is a technically feasible way to reduce anthropogenic emission of green house gas to atmosphere by human beings. In large-scale geologic CO₂ sequestration, high injection rate is required to satisfy economics and operational considerations. During the injection phase, temperature and pressure of the storage aquifers may vary significantly with the introduced CO₂. These changes would re-distribute the in-situ stresses in formations and induce fracture initiation or even propagation. If fractures are not permitted by regulators, then the injection operation strategies must be supervised and designed to prevent fracture initiation, and the storage formations should be screened for risk of fracturing. In more flexible regulatory environment, if fractures are allowed, fractures would strongly influence the CO₂ migration profile and storage site usage efficiency depending on fracture length and growth rate. In this dissertation, we built analytical heat transfer models for vertical and horizontal injection wells. The models account for the dependency of overall heat transfer coefficient on injection rate to more accurately predict the borehole temperature. Based on these models, we can calculate temperature change in formation surrounding wellbores and thus evaluate thermo-elastic stress around borehole as well as its impact on fracture initiation pressure. By considering the impact of thermo-elastic effect on fracturing pressure, we predicted maximum injection rate avoiding fracture initiation and provided injection and storage strategies to increase the maximum safe injection rate. The results show that thermo-elastic stress significantly limits maximum injection rate for no-fractured injection scenario, especially for horizontal injectors. To improve injection rate, partial perforation and pre-heating CO₂ before injection have been designed, and results shows that these strategies can strongly negate thermo-elastic influence for various injection scenarios. On the other hand, the model provides parametric analysis on geological and operational conditions of CO₂ storage project for site screening work. In the case of permitting fracture occurrence, a semi-analytical model was built to quantitatively describe fracture propagation and injected fluid migration profile of a fractured vertical injector for storage systems with various boundary conditions. We examined the correlation between fracture growth and CO₂ migration in various injection scenarios. Two-phase fractional flow model of Buckley-Leverett theory has been extended to account for the CO₂-brine three-region flow system (dry CO₂, CO₂-brine, and brine) from a fractured injector. In the sensitivity study, fracture growth and fluid migration greatly depend on Young's modulus of the formation rock and storage site boundary conditions. Consequently, the results show that fast growing, long fractures may yield a flooding pattern with large aspect ratio, as well as early breakthrough at the drainage boundary; in contrast, slow growing short fractures provides high injectivity without changing flooded area shape. We studied the physics for issues related to injection induced fractures in geologic CO₂ sequestration in saline aquifers, assessed risk associated to them and developed low cost and quick analytical models. These models could easily provide predictions on maximum injection rate in no-fracture regulation CO₂ storage projects as well as estimate fracture growth and injected fluid migration under fracture allowable scenarios. "Preferred storage aquifers" have following properties: larger permeability, deep formation, no over pressure, low Young's modulus and low Poisson's ratio and open boundaries. In many practical cases, however, injection strategies have to be designed if some properties of formation are out of ideal range. Besides applications in CO₂ storage, the approach and model we developed can also be applied into any injection induced fracture topics, namely water/CO₂ flooding and wasted water re-injection.