Numerical Simulation Of The CO₂-ECBM Fluid Continuous Process In Anthracite Reservoirs Based On Digital Petrophysical Technology

CO₂ geological storage and enhanced coal bed methane recovery technology?CO₂-ECBM?can improve CH₄ recovery efficiency while reducing CO₂ emissions.In this study,the anthracite reservoirs in southern Qinshui basin was taken as the research object.The combined application of the X-ray computed tomography technology?X-ray CT?and the focused ion beam scanning electron microscopy technology?FIB-SEM?has realized the digital reconstruction of the multi-scale pore and fracture structures.The joint application of the MATLAB and COMSOL software has realized the numerical simulation of the CO₂-ECBM fluid continuity process on the laboratory and engineering scales.The main results of this study are as follows:?1?The characterization method of pore and fracture structure in coal reservoir was condensed,the multi-scale results and multi-scale upscaling of pore and fracture structure were characterized,and the relationship between storage and seepage capacity of coal reservoir and pore and fracture structure was discussed:Median filtering can smooth the contacting area between pore and matrix.Threshold selection of normalized distribution histogram based on the gray value can realize the segmentation of the organic matter,pore and inorganic mineral.According to the Maximum Sphere Algorithm,the pore and fracture network model can be constructed.Based on the image registration method,the upscaling research of multi-resolution pore and fracture structures was studied.Pore and microfracture are developed in organic matter,inorganic mineral and their contacting area,and there are mainly organic matter pore,inorganic mineral pore and differential shrinkage pore in pore.Based on the pore and fracture structures with high-resolution scale,the image registration can be carried out on the same position of the scanning image with low-resolution scale,and the threshold range of the pore and fracture structure can be obtained in a low-resolution scale.Applying the threshold range to the pore and fracture structures with low-resolution scale,the upscaling of the multi-scale pore and fracture structure can be completed.The pore and fracture structures have great influence on the storage and seepage ability of CBM.When the pore size is between 0 and 50nm,the number of connected pores decreases as the pore size increases.The number of connected pores with pore sizes ranging from 50-200nm accounts for 77.27%of the total number of pores within the pore size range.All pores with pore sizes greater than200nm are connected pores.The throat area is mainly determined by the throat with the size greater than 20nm,and the global connection state between pore and throat indicates that the sample has a strong fluid migration ability.?2?The structural parameters of pore fracture were analyzed,the reservoir and gas parameters needed for numerical simulation were tested,the numerical model of co2-ecbm fluid continuous process was derived at laboratory scale,and the numerical calculation was carried out and the numerical results were analyzed:The distribution of gas pressure field of CO₂ and CH₄ in 3D,2D and 1D shows the pressure injected and diffusion coefficient of CO₂ will affect CO₂-ECBM process.For CO₂,with the increase of CO₂ pressure injected in the same time,the CO₂ pressure shows an increasing trend,and the CO₂ pressure in the reservoir is significantly different under each injected pressure of CO₂.Under different injected pressure of CO₂,the CO₂ pressure in reservoir varies greatly,and the change of CO₂ pressure in the center of the slice is relatively small,while the change of CO₂ pressure at the edge of the slice is relatively large.?3?Dynamic characteristic,controlling factor and migration mechanism of the CO₂-ECBM fluid continuity process at laboratory scale are discussed:The CO₂ injected mainly continuously flowed along the macro-fracture and the microscopic fracture to the matrix.The CO₂ injected first replaces the CH₄ adsorbed by covering the inner surface of the macropores and mesopores to form the CO₂ monolayer adsorption,and then migrates to the micropores by means of Fick type diffusion,slip flow and surface diffusion.Furthermore,CO₂ replaces the CH₄ adsorbed by volume filling in the micropores and forms the multi-molecular layer adsorption of CO₂.The migration path of CH₄ is the opposite of that of CO₂.The micropore and mesopore in coal are the main storage sites of CH₄ and CO₂.On the scale of pore and fracture,the migration path of CH₄ is as follows:micropore?mesopore?macropore?micro-fracture?endogenic fracture?macroscopic fracture?fracturing fracture.Macroscopically,the three-level flows experienced by CH₄ production are:pore?natural fracture?fractured fracture?wellbore.The migration path of CO₂ is opposite in the pore and fracture,and macroscopic scales.The adsorption capacity of matrix to CO₂ is about twice that of CH₄,and the adsorption and desorption of CH₄ and CO₂ are always in a dynamic equilibrium state.The gas diffusion is mainly affected by the concentration difference,which follows the Fick's law.The gas flow is mainly affected by the pressure difference,which follows the Darcy's law.?4?The geophysical model was analyzed and basic assumptions were derived,and the multi-field full-coupling mathematical models for numerical simulation of CO₂-ECBM fluid continuity process at engineering scale were derived,the geological model,numerical parameters and numerical schemes were analyzed,and the numerical simulation study of CO₂-ECBM stimulation effect evaluation was carried out:The CH₄ pressure decreases with time,and the decreasing trend slows down and becomes steady during direct exploitation.The gas pressure increases with time at a fast rate in the initial stage and a low rate in the late stage during the CO₂ injection exploitation.The CO₂ injection has an enhancement effect on CH₄ cumulative output.The differential pressure between the CO₂ injection well and the production well is the driving force of the seepage for CO₂ and CH₄.Increasing CO₂ pressure injected can raise the differential pressure and improve the gas energy in a short time.The faster the CO₂ diffused from fractures to pores,the larger the range of CH₄ driven out of coal,which improves the rate of CO₂ injection and CH₄ production.Increasing CO₂ pressure injected has a positive effect on improving CH₄ cumulative output and CO₂ cumulative storage.The differential pressure between the reservoir and the boundaries,and the competitive adsorption of gas are the main factors affecting reservoir permeability.When CO₂ injected does not reach the production well,the permeability near the CO₂injection well is mainly affected by the competitive adsorption of gas,and the permeability near the production well is mainly affected by the differential pressure.When CO₂ injected spreads to the production well,the effect of the competitive adsorption will gradually replace the effect of the differential pressure on permeability near the production well.There are 82 figures,16 tables and 266 references in this doctoral thesis.