Study On Numerical Simulation Of CO₂-ECBM For Shizhuangnan Demonstration Project In Qinshui Basin

CO₂ geological sequestration and enhanced coalbed methane recovery（CO₂-ECBM）technology can not only store CO₂ in coal reservoirs for reducing greenhouse gas emissions,but also promote CH₄ production rate to improve coalbed methane recovery.In this work,the 3^#coal reservoir of Shanxi Formation for CO₂-ECBM demonstration project in Shizhuangnan Block of Qinshui Basin was taken as the research object,the three-dimensional（3D）heterogeneous geological model of coal reservoir from Petrel was reconstructed in COMSOL Multiphysics using the split-reconstruction method,a multi-physical field coupling of adsorption-hydro-thermo-mechanical-chemical（AHTMC）mathematical model for CO₂-ECBM was established on the basis of the extended physical model of the dual-pore medium for coal,The investigation on numerical simulation for CO₂-ECBM was carried out based on the 3D heterogeneous coal reservoir geological model and AHTMC multi-field coupling mathematical model,the changes of each physical field in the coal reservoir during CO₂-ECBM process,CO₂ storage effect and CH₄ production increase effect were analyzed and the CO₂ injection pressure for coal reservoir was optimized.The main understanding and achievements of this work are as follows:（1）The split-reconstruction method of corner grid geological model reconstruction in COMSOL Multiphysics was proposed.Based on the COMSOL Multiphysics,the 3D heterogeneous geological model for the coal reservoir in the study area of CO₂-ECBM demonstration project in Shizhuangnan,Qinshui Basin was reconstructed.The split-reconstruction method splits the structural model and attribute model of the corner grid geological model,converts them into text files by programming,imports the text files into COMSOL Multiphysics,and uses the built-in linear interpolation function,cube modeling,Boolean operation and segmentation and deletion of entities to reconstruct the 3D heterogeneous geological model of the coal reservoir.The reconstructed 3D heterogeneous geological model of coal reservoir in the study area of CO₂-ECBM Demonstration Project in Qinshui Basin can accurately represent its 3D spatial distribution and the heterogeneous distribution characteristics of CH₄ content,CH₄ adsorption time,porosity,permeability,elastic modulus and Poisson’s ratio.（2）The physical model of dual-pore medium for coal was extended,and the AHTMC multi-field coupling mathematical model for CO₂-ECBM was constructed.The expanded physical model of dual-pore medium for coal includes not only matrix and fracture,but also minerals distributed between matrix and fracture,which can more truly describe the material composition and pore-fracture structure characteristics of underground coal.The CO₂-ECBM mathematical model derived from the extended physical model is an AHTMC multi-field coupling model.The model includes adsorption field,hydraulic field,thermal field,mechanical field,chemical field and the coupling equations for porosity and permeability.In addition to accurately displaying gas adsorption,fluid seepage,temperature and stress change in coal reservoirs,it can also quantify the impact of temperature change on the diffusion of CO₂ and CH₄ in coal matrix and fracture water during CO₂-ECBMsand a series of geochemical reactions,such as the ionization of carbonic acid and bicarbonate ion and the dissolution/precipitation of calcite.（3）The variety rules of adsorption field,hydraulic field,thermal field,mechanical field and chemical field in the coal reservoir of the demonstration project during the CO₂-ECBM were revealed,and the influence of CO₂ injection pressure on CO₂ storage and CH₄ stimulation and the influence of reservoir heterogeneity on gas production rate of wells were identified.Affected by the heterogeneous distribution of gas adsorption capacity,initial porosity and permeability,elastic modulus and Poisson’s ratio in coal reservoir,the CH₄and CO₂ content,permeability,temperature,in-situ stress and ion concentration of the coal reservoir show certain heterogeneous distribution characteristics during the CO₂-ECBM process.With the prolongation of CO₂ injection time and the increase of CO₂injection pressure,the CH₄ and CO₂ contents in the reservoir gradually decrease and increase respectively,the reservoir pressure and principal stress gradually increase and decrease respectively,and the high temperature range of the reservoir extends from the injection well to the surrounding area in an irregular circular halo.In addition,reservoir permeability decreases with the increase of gas injection pressure.With the extension of CO₂-ECBM time,the concentration of HCO₃^-and H⁺in the water of the coal reservoir fracture gradually increases,and the calcite presents a complex process of dissolution-precipitation-dissolution in the coal reservoir near the injection well and shows the characteristics of continuous dissolution in the coal reservoir near the production well.By the end of 15 years,within the injection pressure range of 5.5～9.5MPa,with the increase of CO₂ injection pressure,the CO₂ storage capacity in coal reservoir and the cumulative production and yield-increase rate of CH₄ in the production well group increase,but the CO₂ storage rate in the reservoir decreases.During the CO₂-ECBM,the weaker the adsorption capacity and the better the percolation capacity of the coal reservoir around the production well,the higher the production rate of CO₂ and CH₄.When CO₂-ECBM is carried out in the unit of well group,the stimulation effect of single well CH₄ is mainly affected by the seepage capacity of its surrounding coal reservoirs.（4）The optimization method of gas injection process for CO₂-ECBM was established,the CO₂ injection pressure for four engineering objectives under the simultaneous and sequential shut-in modes was optimized,and the influencing factors of CO₂ injection process optimization were pinpointed.The optimization of CO₂-ECBM gas injection process needs to establish the objective function for evaluating the engineering effect,determine the shut-in mode of production wells,establish the engineering objectives of CO₂-ECBM,and build the sample space for numerical simulation.The objective function for CO₂-ECBM injection process optimization is composed of engineering time,CH₄ production,CO₂storage capacity,CO₂ storage rate and their corresponding weight coefficient.The optimal CO₂ injection pressure for the engineering objectives of time saving,CH₄stimulation,CO₂ storage and comprehensive consideration of the three are 8.05,3.50,6.35 and 6.25 MPa for the Simultaneous Shut-in,respectively,and 9.50,9.50,3.50 and9.50 MPa for the Sequential Shut-in,respectively.With the optimal CO₂ injection pressure,the Sequential Shut-in mode can achieve better CH₄ stimulation and CO₂storage effects by extending the engineering time.The factors affecting the determination of the optimal CO₂ injection pressure include the shut-in conditions of production wells（CO₂ concentration threshold）,the objective function and its internal weight coefficient combination.There are 74 figures,22 tables and 192 references in this doctoral thesis.