(a) determine the scope of the study
Generally speaking, an independent natural hydrogeological system is the best choice for the calculation area. It has natural boundary, so it is convenient to use its real boundary conditions more accurately and avoid the difficulties and errors in providing data caused by artificial boundary. However, in practical work, it is often impossible to make full use of natural boundaries, and it is necessary to make full use of investigation and long-term observation data to establish artificial boundaries. When determining the calculation area, in addition to ensuring a large enough range, the assumed boundary conditions should be as close to the real state as possible.
The delineation of calculation scope should fully consider the research purpose, regional geological structure, reservoir lithology, reservoir rock mineral composition and groundwater chemical composition. The numerical simulation time has different time scales according to different research purposes. As far as the numerical simulation of CO2 geological storage is concerned, if the geochemical effect is not considered, the storage system has basically reached equilibrium or stability within the simulation time of 1000. The diffusion distance of CO2 in the reservoir should also be considered when delimiting the boundary, which is closely related to the parameters such as porosity and permeability of the geological model in the study area. In order to ensure that the range boundary of the selected model will not affect the simulation results during the simulation period, we try to make analogy through natural gas CO2 gas fields (reservoirs) with the same geological conditions to determine the approximate boundary of the calculation range. If geochemical reaction is considered, the water-rock-gas reaction caused by CO2 injection significantly changes the lithology of surrounding rock, and restricts the CO2 injection speed and radial migration distance.
(B) a clear research purpose
Before numerical simulation, it is necessary to make clear what kind of problems numerical simulation technology should solve. For the CO2 geological storage project, the main purpose of numerical simulation is to optimize the site selection and scheme design of the project through numerical simulation technology, and to predict the technical guidance during the project implementation, monitoring during the operation and risk assessment of CO2 leakage in the later period, so as to guide the scientific and reasonable implementation of the project and minimize the risk of CO2 leakage.
The purpose of the study determines the type of data acquisition in the early stage, the focus of geological modeling, the accuracy of geological model dispersion and the treatment of initial and boundary conditions.
(3) Collection and arrangement of data
1) Obtain information and data on lithology, geological structure, hydrogeology, hydrogeochemistry, rocks and minerals of deep strata in the site by means of remote sensing, comprehensive geological survey, geophysical exploration, drilling and various sample tests and analysis;
2) Collect and analyze the geological lithology, regional structural pattern, active faults and seismic activity of the CO2 geological storage site;
3) Investigate the spatial distribution, buried depth, thickness and scale of target reservoirs and caprocks in CO2 geological storage sites by using drilling cores, logging and seismic reflection methods;
4) Study and analyze the mineral composition, pore structure characteristics and physical and chemical properties of the rocks in the sealed place by X-ray diffraction, scanning electron microscope and other methods;
5) By analyzing the total water quality of water samples from shallow aquifer and deep aquifer, the initial hydrochemical composition of reservoir overburden and shallow aquifer formation water is obtained.
Different numerical simulation software has different numerical solutions to its mathematical model, different spatial discretization methods and different model parameters. Table 9- 1 is the main parameter needed for numerical simulation of toughness.
Table 9-1Main parameters required for CO2 geological storage simulation (taking TOUGHREACT as an example)
(D) the establishment of a conceptual model
According to the topography, meteorology, hydrology, stratigraphic lithology, geological structure, hydrogeology and hydrogeochemistry of the study area, the prototype of geological model is initially established according to the research purpose. In order to solve practical problems, it is often necessary to generalize the model, initial conditions and boundary conditions, including:
1) Structure generalization of aquifer system: According to the type, lithology, thickness and permeability coefficient of aquifer, the internal structure is generalized as homogeneous, heterogeneous, isotropic or anisotropic aquifer;
2) Generalization of lateral boundary and top-bottom boundary; According to the demarcation of the boundary of the study area, the horizontal boundary and the upper and lower boundaries are generalized into one, two and three types of boundary conditions;
3) According to the difficulty of the problem to be solved, generalize the geological model into one-dimensional, two-dimensional or three-dimensional models, and generate the grid with reasonable accuracy;
4) Treatment of source and sink projects: The storage is generally located below 800 meters below the surface, and it is covered by dense caprock, so it is difficult to supplement and discharge CO2 through overflow. Under ideal conditions, CO2 in the whole geological storage system is supplemented and discharged by radial convection or extraction wells.
(5) Simulation scheme design
According to different research problems, different schemes can be designed according to the designer's technical level and field experience. Using numerical simulation technology to simulate and analyze various schemes, evaluate the feasibility of scheme implementation, and optimize the scheme, and finally get an economic and reasonable scheme. For example, when choosing the CO2 injection site, we should simulate the CO2 injection capacity, storage potential, diffusion speed and distance in the reservoir and the CO2 leakage risk in several target sites, and then post-process the simulated output data. Through the analysis and statistics of these data, the best pouring location is determined, and the horizon and thickness of reservoirs and caprocks are reasonably divided. The evaluation of on-site perfusion capacity and storage potential is another difficult problem in the field of CO2 geological reservoirs, which can be solved by numerical simulation technology. Different injection methods lead to different velocity and flow rate of CO2 entering the reservoir. According to all feasible injection modes designed by the designer, the injection capacity and injection quantity under different schemes are simulated to determine the best injection mode.
The design of simulation scheme depends on different research problems, and the rationality of scheme design depends on the designer's own theory and actual field experience. Different researchers may design different schemes for the same problem. We can establish different models for these schemes, judge their rationality and feasibility through simulation technology, and finally determine the best scheme.
(6) Selection of numerical model and simulation software
The key of numerical simulation is the generalization, calculation accuracy and calculation speed of geological model. Because the accuracy of calculation depends on the degree of discretization, and the degree of discretization determines the speed of calculation, which is a contradiction. We should choose the degree of discretization and the speed of calculation according to the needs of solving problems.
The migration and dissolution of CO2 in the reservoir and the chemical reaction with surrounding rocks form a multi-phase and multi-component reaction system, and the main mathematical equations involved include the motion control equation of supercritical CO2- water two-phase fluid, solute migration control equation and chemical reaction equation. When establishing numerical model, the commonly used methods are finite difference method, finite element method and integral finite difference method.
In practical application, the existing numerical simulation software is mostly used to simulate the whole process of CO2 geological storage, and does not involve the development of software and the compilation of program code. Just choose the appropriate software to simulate and predict according to the research needs, and once the software is selected, the mathematical model and numerical model will be basically determined. Taking TOUGHREACT as an example, a mathematical model is established based on the above conceptual model. The unified partial differential equation of gas phase and liquid phase is (9- 1), the partial differential equation of brine is (9-2), and the supercritical equation is (9-3). See Table 9-2 for related characters and corner marks involved in the equation.
Introduction of geological storage technology and method of carbon dioxide
Introduction of geological storage technology and method of carbon dioxide
Table 9-2 Symbol Meaning Involved in Mathematical Model
(7) establishing a numerical model
1. Grid generation
After the geological model is established, the study area should be discretized, that is, divided into grids. Firstly, the discrete points are determined, that is, the research area is divided into grid systems according to a certain geometric shape (such as rectangle, arbitrary polygon, etc.). ). The boundary of the study area can be approximately represented by the grid line closest to it. When the grid is small enough, the zigzag grid can also describe the shape of the boundary well. This process is also called discretization (subdivision) of the research area. Discretization should follow the following two basic principles.
1) geometric similarity. The physical simulation model is required to be close to the real simulation object in geometry.
2) Physical similarity. The characteristics of discrete elements are required to be similar to the physical properties of the real structure in this area in terms of physical properties (aquifer structure and flow state).
Grid types can be roughly divided into regular and irregular types. Regular grids include regular figures such as rectangles and triangles (Figure 9-3), and irregular grids include irregular polygons. The shape of the grid mainly depends on the shape of the study area.
Figure 9-3 Grid Generation
Grid generation has a great influence on the accuracy and efficiency of calculation. The higher the accuracy, the more detailed the simulation results will be, but the greater the amount of data calculation, the higher the requirements for computers. It is suggested that the geological model should be divided into coarse grids. If the simulation results are reasonable, then fine grid division should be carried out to describe the simulation results in more detail.
2. Parameters and initial conditions
The initial condition refers to the initial value of the main state variables of the mathematical model in the research area at the initial moment (t=0). The number and types of state variables required by different applications are different. For example, the initial main state variables required for TOUGHREACT include the spatial distribution of pressure, temperature and component concentration. Geological parameters include porosity, permeability, density, pressure, temperature, capillary pressure and other parameters. Some of these values are measured by indoor experiments, and the other part is based on the empirical values of references. The initial value of chemical composition of formation water adopts the chemical analysis of actual formation water, which mainly includes the concentration, salinity and pH value of eight main ions. If it is difficult to obtain deep water samples (such as caprock) in the study area, the static equilibrium method is adopted, and the brine with the same salinity as the reservoir reacts with the formation rocks containing primary minerals in the in-situ formation environment to obtain the initial value of formation hydrochemical composition in the equilibrium state; By means of rock and mineral analysis, electron scanning and X-ray diffraction, the initial value of the volume content of primary mineral components that constitute the cover of CO2 geological reservoir is obtained, and the secondary minerals are reasonably judged according to the composition of primary minerals.
In principle, the initial time can be determined arbitrarily, as long as the parameters and state variable values needed at this time are known. Therefore, we should not understand the initial conditions as the initial state of the research system. How to get it depends on the needs of the problem, the source of data, the convenience of calculation and other factors.
3. Boundary conditions
Boundary condition is one of the necessary conditions for the mathematical model of practical problems to have a definite solution. The boundary conditions of groundwater flow problem and solute transport problem are not the same, but they are generally summarized into the following three types.
(1) A Class of Boundary Conditions (Dirichlet Condition)
When solving the flow problem, this kind of boundary condition is that the head of all points on the boundary is given; For solute transport, a kind of boundary conditions means that the solute concentration distribution on the boundary of the study area is known. When solving the problem of CO2—- water two-phase flow, this boundary condition is the given pressure at all points on the boundary.
(2) The second boundary condition (Neumann condition)
When the inflow or outflow per unit area of the boundary is known, it can be regarded as the second kind of boundary to solve the flow problem; Relative to solute transport, this boundary is also called given dispersion flux boundary, that is, the dispersion flux on the boundary is known with time.
(3) Three kinds of boundaries (Cauchy condition)
When one part of the study area satisfies Dirichlet condition and the other part satisfies Neumann condition, this kind of problem is called mixed boundary problem, which is called three kinds of boundary. For solute transport, the law of solute flux changing with time on this kind of boundary is known.
In the process of numerical simulation of CO2 geological storage, because the reservoir strata are mostly below 800m, the top and bottom of the geological model can be treated as impermeable boundaries according to actual needs. In order to avoid the influence of the boundary on the simulation results, the research area is generally much larger than the range where the actual CO2 can move. Therefore, when dealing with the four peripheral boundaries, it is generally set as an infinite boundary or an impermeable boundary. When determining the boundary conditions, hydrogeological conditions and existing data should be considered comprehensively.
4. Source and receiver project processing
In porous media flow and solute transport, convection, hydrodynamic dispersion and solute source or/and sink are two main factors that determine the time-varying rate of solute mass at any point in aquifer. The source-sink term plays an important role in the calculation of water quality and quantity, and in correctly handling the convection-dispersion equation and the basic differential equation of seepage. There are many ways as source and sink items, such as overflow recharge, aquifer elastic release recharge and pumping (injection) well recharge.
For CO2 geological storage system in deep salt aquifer, the top of the system is generally mudstone and shale with low permeability and porosity, so it is difficult to make overflow recharge. The source and sink items of the whole CO2 geological storage system mainly refer to convection (such as lateral boundary) and pumping (injection) wells.
(viii) Model calibration and verification
Model identification is one of the most important steps to establish a numerical model of underground fluid, and correct understanding and fitting is very important to improve the simulation of numerical model. In the case of measured results, such as demonstration projects, the simulation results can be compared with the measured results, and the relevant parameters can be adjusted appropriately and reasonably to make the simulation results coincide with the measured results within a given error range. If the error is large, it is necessary to re-test or even re-establish the reliability of the conceptual model. After identification and correction, the corrected model should be used to continue calculation, and compared with the actual data not used for identification and correction to verify the accuracy and reliability of the model. If the error is large, the previous process must be repeated. In the absence of measured results, the reliability of the numerical model can be judged by analogy with relevant data or according to personal experience and theory.
(9) Simulation and prediction
Model prediction is the main purpose of implementing numerical simulation technology. As for the geological storage project of CO2, it is too early to put forward the geological storage technology of CO2, and the research on the migration and diffusion of CO2 in deep saline water, the chemical reaction with formation water and surrounding rocks, and the changes of physical and chemical properties of reservoir caprock caused by CO2 perfusion are all in the research and development stage. Therefore, in the process of project implementation, tools with technical guidance are urgently needed to avoid investment waste and CO2 leakage.
Using the identified, corrected and verified numerical model, the geological sequestration process of CO2 is simulated and predicted, and the simulated data are processed pertinently, such as statistical analysis and comparison, and the results are interpreted, so as to achieve the purposes of site selection optimization, evaluation of the perfusion capacity and sequestration potential of the target reservoir, detailed description and simulation of CO: diffusion migration path and speed, sequestration capacity of different capture methods and their time-space transformation. At the same time, it can predict the possibility and time of CO2 escaping from existing, reactivated or newly generated fractures, evaluate the risk of CO2 leakage, and evaluate the impact of CO2 leakage on the quality and quantity of shallow groundwater and the surface environment.
The analysis of the above results is only the tip of the iceberg that numerical simulation technology can solve the problem. The processing of numerical simulation results should be extracted and interpreted according to the research purpose. By summarizing and analyzing the processed data, we can find out the problems and solve them, and master the internal laws, which can provide theoretical support and scientific and technical guidance for the early design, project implementation and mid-term monitoring and management of CO2 geological storage projects, predict the risks in advance, make plans as soon as possible, and prevent possible hidden dangers in the implementation and operation of CO2 geological storage projects.