| In recent years, as for the increase of research on CO2geologic storage (CGS), oil andnatural gas underground storage, deep resources exploitation, high-level radioactive wastedisposal, and enhanced geothermal systems(EGS), more and more scholars found thermal-hydro-chemical-mechanical (THCM) multi-physical coupling problems have veryimportant effects on the normal operation and on the safety in the engineering design,construction and operation in these projects, which has very important research significance.Mechanics effect and the hydraulic conductivity characteristics of porous media areaffected by temperature, pore pressure, stress, and the influence of chemical forces. Open orclosed natural fracture have very important effects on CO2geological storage and disposal ofnuclear waste underground. Expansion or contraction caused by temperature change and thecorresponding thermal stress can affect the deformation characteristics of the structure orporous medium. In CGS projects, supercritical CO2is injected into the underground salineaquifer, which will result in the high pore pressure of reservoir and cap rock skeletondeformation, cause fracture as a new escape path, and endanger the stability of the caprock.And supercritical CO2may react with the surrounding rock for a long time, cause mineraldissolution or precipitation, change porosity and permeability of medium. In EGSengineerings, coupling of fluid and mechanics is the main consideration in the process offracturing. Cold water injection results in the decrease of temperature around the well atruntime, energy migration through the fluid flow in the fracture and pore, heat exchangebetween fluid and rock, rock and rock at the same time.There are both heat conduction, andheat convection, the change of the local stress field due to temperature varition also need to beconsidered in the whole process. In many geological science and engineering fields, therefore,people need to understand the coupling problems of its environment and to design andanalyze based on this.In the process of multi-physics problem calculation, however, due to the mutualrelationship between main factors is interactive and restrained, the coupling effect is verycomplex, it’s tough to parse through a simple means of analysis. In view of its computationalcomplexity, related research has just started and under exploration. The main method is bytechnology of numerical simulation at home and abroad so far.Multi-phase multi-physics coupling numerical analysis has been an important technical means and tool to study CO2geological sequestration and enhanced geothermal systems. Dueto the deep drilling for CGS and EGS engineering and monitoring is difficult and costly, sonumerical simulation can be used as a kind of method that is very effective and economical.For CGS projects, through simulation we can study the physical and chemical changes effectsof supercritical CO2injection, and the capacity and penetration ability of storage formation.We can also study the mechanics change in the reservoir and cap rock by supercritical CO2injection which may lead to a series of safety issues,. For EGS projects, hydro-thermo-mechanical coupling numerical simulation are key points and difficulties in engineeringdesign and operation as well. Numerical simulation can be used to evaluate distribution anddevelopment of seepage and temperature field, to predict the heat transfer capacity andoperation efficiency, to analyze the characteristics of fluid stress, temperature stress andmutual coupling strength, as well as mechanical effects such as strain and displacement ofunderground and surface etc.Accordingly, aiming at multi-physics problems in geological science such as geologicalstorage of CO2, and enhanced geothermal systems etc, by the collection of multiphaseTHCM coupling theory, an independent simulator links TOUGHREACT with FLAC3Dwasdeveloped using FISH language plus FORTRAN90-95. They are linked because both thecodes have been widely used and have high recognition in their respective fields.Coupling program of the simulator based on loose coupling principle, the first step is totransform grid form of different files into same distribution in space, then the total time willbe divided into several simulation time steps according to the calculation precision andefficiency of computer, control equation are calculated respectively in turn within one timestep, the next step is to adjust and transfer the parameters according to the related equations,then start the calculation of next time step until the total simulation time is reached. Theparameter types and the adjustment way to coupling are considered more comprehensive inour simulator. The correction of porosity due to modulus change is considered, as well aspressure solution. Simultaneously the synchronization of porosity, permeability in FLAC andTOUGH is realized, which will enhance the accuracy of calculation, and perfect couplingtheory. There are two ways for the transfer of coupling parameters, direct transfer orinterpolation. In directly transfer, the grid number and software version has certainrequirements, but leaves out the difference calculation. When interpolation is used, programusing three dimension Lagrange method, values of27points around a three-dimensionalinternal node are needed for calculation, which makes less amount of calculation and ensureshigh precision difference. Calculated results of the two kinds of parameter passing way are basically identical through a lot of model simulations.Because database of chemical and physical properties of CO2are embedded in the CO2module of TOUGHREACT code, it can simulate multi-phase multi-component reaction solutetransport, its temperature module supports high temperature phase change of the water. So oursimulator can realize THCM coupling simulation in engineering of CGS and EGS. Theprogram TOUGHREACT-FLAC3Dcan realize external data input and output, using openoutside control file, it has a good readability, generality and operability. Grid generation arecalculated by use of a one-click automatic identification, which avoids the complexity ofFLAC internal non-uniform grid generation. The development and testing of a primaryinterface for beginners was also completed.When the procedure is finished, some numerical models of CO2geological storage wereundertaken including theory and Salah site, by compared with the monitoring data andpredecessors’ achievements, the accuracy, applicability and portability of the simulator thecorresponding interface in CGS projects were verified. Based on a coupled wells example ofenhanced geothermal systems simulation, to further verify the applicability of the couplingcalculation in geotechnical heat extraction of high temperature.Further analysis were done based on the results of above numerical simulations. Take amassive CO2geological storage engineering as the research point, we study the developmentrules of key factors of rock physics, mechanics, seepage evolutionary from the perspective ofgeology and mechanics. We also analyzed the influence of caprock fracture channels onmigration of CO2in storage environment, the variation of porosity, permeability under theinjection pressure, as well as the possibility of rock damage and the distribution of plasticzone. For enhanced geothermal systems, numerical simulations were done to study the hydro-thermal-mechanical coupling process of a coupled pumping and injection wells, to analyzethe distribution and evolution of seepage field and temperature field distribution, to reveal therule and influencing factors of coupling strength. These studies can provide reference forrelated theory research and actual projects. |
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