Research On Parallel Numerical Simulation Of THM Multi-physical Coupling Based On CO₂ Geologic Storage

CGS（CO₂ Geologic Storage） is a kind of technology in storage of greenhouse gas recently. As a new type of geotechnical engineering technology, CGS has the characteristics of large storage capacity, low cost, and also has little impact to the surface ecological environment, so it has been widely used and developed in reducing the CO₂ content in the atmosphere to slow down global warming. Safety and reliability of CO₂ geologic storage is related to human life. In order to ensure the safety of CO₂ storage area, a large number of demonstration work is needed before any CGS project. Taking into account the complexity of geological and mechanical problems and the lage scale of time and space in CGS projects, it is hardly to reappraise safety of CGS engineering in practical operation. However, numerical simulation, as a very effective and economical technical method, is necessary in studying CO₂ geological storage.The CGS numerical simulation process is a complex multi- physical coupling problem which relates to thermal- hydro- mechanical（THM）. As an important basic theory in geotechnical engineering, many researches have been down on THM multi-physical coupling problems and on CGS engineering, but there are still some deficiencies when deal with problems in numerical simulation of CGS Engineering. At present, a stand-alone T-H-M coupling program has been developed by linking TOUGH with FLAC3 D. However, limited by the number of grids, models must be simplified by that simulation program when dealing with large scale and complex formation conditions of CGS project. Obviously, the accuracy is not nearly enough in the simulation of CGS projects.On the basis of previous studies, we developed an independent simulator to link TOUGH-MP with FLAC3 D using FORTRAN95 and FISH language. Resource codes developed to form the parallel THM coupling program. To verify the applicability of parallel THM simulator, the CGS model of In Salah site in Algeria area was established. The simulation results show that CO₂ continuous perfusion after three years, comparison of pore pressure and CO₂ migration, parallel version of the THM program simulation results and stand-alone version of THM simulation results basically close to the emergence of small differences are caused by the thickness of grid division of; in terms of the amount of surface uplift and parallel THM version of the program simulation results 1.49 mm phase compared to the stand-alone version of THM program simulation results 1.55 mm, closer to monitor the earth is 1.5mm. Therefore, the parallel version of THM coupling program in the numerical simulation of the CGS project is better than the stand-alone version of the THM coupling program.Physical and mechanical properties of reservoir and cover rock determine the CO₂ storage capacity. We have done some laboratory experiments to the rock samples taken from the Ordos Area in this paper. Analysis and researches have been done to physical properties and mechanical properties such as the porosity, permeability, thermal conductivity, specific heat capacity and density, which are basics of numerical simulation to CO₂ geological storage in Ordos.In this paper, a three-dimensional geological model of the 130 thousands grids was established at the size of 8000 m × 8000 m × 2800 m using parallel THM coupling program. According to the actual situation of the project, an injection well with a depth of about 2500 m was set in the model, which covers 13 groups of reservoirs composed of Liujiagou group, Shiqianfeng Formation and Shihezi group and Shanxi group. CO₂ was injected at the rate of 105t/a for three years, after stopping perfusion, simulation continued to run for seven years. An analysis is made on the change of strata in these 10 years. Besides, on the basis of this model a new simulation was made with the CO₂ injection rate of 105t/a for ten years. A prediction is made on the migration of CO₂ and the change of the mechanical properties. While a lot of CO₂ was injected into the aquifer, the original stress balance would be broken, which made the connection between the solid skeleton becomes loose. Thus porosity and permeability would increase. CO₂ injection may cause the uplift of surface. After Injection stoped for a period of time, due to the role of self weight pressure, solid skeleton will become dense, the uplift of surface will gradually decline until the balance recoved.