Numerical simulation and optimization of CO_2 sequestration in saline aquifers

摘要

With heightened concerns on CO_2 emissions from coal fired electricity generation plants, there has been major emphasis in recent years on the development of safe and economical Carbon Dioxide Capture and Sequestration (CCS) technology worldwide. Saline reservoirs are attractive geological sites for CO_2 sequestration because of their huge capacity for long term sequestration. Over the last decade, numerical simulation codes have been developed in US, Europe and Japan to determine a priori the CO_2 storage capacity of a saline aquifer and to provide risk assessment with reasonable confidence before the actual deployment of CO_2 sequestration can proceed with enormous investment. In US, the 2nd version of Transport of Unsaturated Groundwater and Heat (TOUGH2) numerical simulator has been widely used for this purpose. However at present, it does not have the ability to determine optimal parameters such as injection rate, injection pressure, injection depth for vertical and horizontal wells, etc. for optimization of the CO_2 storage capacity and for minimizing the leakage potential by confining the plume migration. This paper describes the development of a "Genetic Algorithm (GA)" based optimizer for TOUGH2 that can be used by the industry with good confidence to optimize the CO_2 storage capacity in a saline aquifer of interest. This new code including the TOUGH2 and the GA optimizer is designated as "GATOUGH2". It has been validated by conducting simulations of three widely used benchmark problems by the CCS researchers worldwide: (a) study of CO_2 plume evolution and leakage through an abandoned well, (b) study of enhanced CH_4 recovery in combination with CO_2 storage in depleted gas reservoirs, and (c) study of CO_2 injection into a heterogeneous geological formation. The results of these simulations are in excellent agreement with those of other researchers using different codes. The validated code has been employed to optimize the proposed water-alternating-gas (WAG) injection scheme for (a) a vertical CO_2 injection well and (b) a horizontal CO_2 injection well, in order to optimize the CO_2 sequestration capacity of an aquifer. The optimized calculations from GATOUGH2 are compared with the brute force nearly optimized results obtained by performing a large number of calculations. These comparisons demonstrate the significant efficiency and accuracy of GATOUGH2 as an optimizer compared to using TOUGH2 in a brute force manner. This capability holds a great promise in studying a host of other problems in CO_2 sequestration such as how to optimally accelerate the capillary trapping, accelerate the dissolution of CO_2 in water or brine, and immobilize the CO_2 plume.