我国不仅煤炭资源丰富，而且拥有丰富的煤层气（瓦斯）资源。煤层气资源的开发利用既能有效降低矿井瓦斯灾害事故，减小排放对环境的污染，也能推进我国能源转型，实现“2030碳达峰和2060碳中和”的远景目标。我国煤层瓦斯压力高，渗透率低，气体吸附性强，常规瓦斯预抽措施效率低。CO₂驱替煤层CH₄技术是当前的研究热点之一。注入煤体的CO₂包含“渗流-扩散-吸附”过程，煤体中CH₄呈现“解吸-扩散-渗流”过程，本文针对CO₂驱替煤层CH₄过程，从孔道内气体运移特征、竞争吸附特性和气体分布特征等角度展开分析，研究了CO₂驱替煤层CH₄非线性渗流机制及演化规律，并基于渗流特征从驱替“稳定性、能力和效果”三方面开展了CO₂渗流驱替煤层CH₄演化规律研究，以期为CO₂驱替煤层CH₄技术提供理论基础。

本文采用理论分析、实验研究和现场试验等方法，对孔道内单组分CO₂/CH₄气体运移特征及影响因素、多组分气体竞争吸附规律、不同渗透性煤体单组分CO₂/CH₄气体渗流规律、CO₂渗流驱替煤层CH₄理论、现场试验效果评价等方面展开系统研究，取得主要成果如下：

（1）单组分（100% CO₂+0% CH₄、0% CO₂+100% CH₄）气体流量与压力梯度变化曲线特征可分为两个阶段：低流速或低压力梯度段呈非线性渗流特征，随着流速或压力梯度的增加，仍然存在非线性特征，但逐渐向线性渗流特征转变；根据CO₂/CH₄在孔道内的分布特征，分析了粘滞阻力、边界层厚度、体相-边界流体相互作用力及影响因素，构建了基于动态阻力梯度的气体非线性渗流数学表达式，通过理论流量值与实验值的对比分析，验证了压力梯度与动态阻力梯度的表征式，实现了不同压力梯度煤体内CO₂与CH₄运移难易程度的定量表征。

（2）煤对气体的吸附特性及气体可压缩性，决定了孔道内气体组分的分布特征，改变了不同区域（孔壁周围、孔道中线）气体的动力粘度系数，影响气体在孔道内的渗流过程，煤体孔道内气体组分分布特征引起不同区域气体动力粘度系数的变化是气体非线性渗流形成的直接原因；煤层渗透性及CO₂驱替煤层CH₄气体组分变化是影响气体流量与压力梯度曲线非线性偏离度的重要原因；煤层渗透性主要通过煤体孔隙结构特征影响曲线的非线性偏离度，是煤体的固有属性，而驱替过程中气体组分是通过改变孔道内气体动力粘度系数影响曲线的非线性偏离度。

（3）CO₂驱替煤层CH₄注气压力小于临界压力值时，孔道内二元气体CO₂组分越大，气体渗流启动压力梯度值越大，压力梯度与流量曲线斜率增长越缓慢，流量增长滞后性越明显，曲线偏离度越高，非线性特征越显著；注气压力大于临界压力值时，压力梯度与流量曲线斜率趋于稳定，孔道内CO₂气体组分越大，曲线斜率越小，偏离度越高，出口端气体流量越小；随着煤体渗透性的升高，气体渗流启动压力梯度越小，压力梯度与流量曲线斜率增长越快，气体流量增长越显著，曲线偏离度越低，线性段与非线性段临界压力值越小。

（4）基于分子动力学进行CO₂渗流驱替煤层CH₄能力理论分析，提出了分子碰撞频率和分子平均自由程两个表征驱替能力的指标，获得了影响指标的关键参数气体分压（气体组分）；通过恒温条件下不同注入压力（大于临界压力值）CO₂渗流驱替煤层CH₄实验，获得了驱替比与驱替压力的线性关系，随驱替压力升高而减小；驱替效率随驱替压力升高而增大，通过增加驱替压力，可减小驱替比，增大驱替效率。

（5）CO₂渗流驱替煤层CH₄过程中，一方面，孔道内CO₂与吸附态CH₄发生竞争吸附或CO₂与吸附态CH₄分子碰撞发生动能传递（驱替能力），促进吸附态CH₄解吸；另一方面，游离态CO₂不断与孔道内解吸后的CH₄发生分子碰撞，驱赶出煤层CH₄，宏观表现为两种气体在孔道内的渗透能力，即流度。引入驱替相（CO₂）与被驱替相（CH₄）流度比来表征驱替稳定性，相同条件下CO₂动力粘度值大于CH₄，被驱替相和驱替相动力粘度比值小于1；基于CO₂与CH₄分子结构、气体物理特性及相同条件下分子平均自由程对比分析，发现同一尺度孔隙CH₄渗流能力强于CO₂；通过出口端气体流量、启动压力梯度及运移阻力判定煤体内CH₄渗流能力强于CO₂，驱替相渗透率与被驱替相渗透率比值小于1，说明驱替相与被驱替相的流度比小于1，波及效率高，能够形成稳定驱替，并通过现场试验考察孔CH₄浓度变化对驱替稳定性进行了验证。

（6）提出了一种CO₂渗流驱替煤层CH₄抽采达标等效量化评估方法，并通过此方法对工程试验数据进行汇总计算，抽采期60天，K1和K3区域煤层可解吸瓦斯含量与半径呈类指数增长，K1和K3区域的有效达标驱替半径分别为15m和12m，驱替效率分别为20.52%、21.81%，波及效率分别为60%和48%。此外，综合评估此次煤层注CO₂渗流驱替CH₄试验有效达标驱替半径为9m，驱替效率为23.95%，波及效率为36%。

China is not only rich in coal resources, but also rich in coalbed methane resources. The development and utilization of gas resources can not only effectively reduce mine gas disaster accidents and reduce environmental pollution caused by emissions, but also promote China’s energy transformation and realize the vision goal of “2030 carbon peak and 2060 carbon neutralization”. China’s coal seam has high gas pressure, low permeability, strong gas adsorption, and low efficiency of conventional gas pre-drainage measures. CO₂ displacement of coal seam CH₄ technology is one of the current research hotspots. The CO₂ injected into the coal body contains the process of “ seepage-diffusion-adsorption ”, and the CH₄ in the coal seam presents the process of “ desorption-diffusion-seepage ”. In this paper, the process of CH₄ displacement by CO₂ in coal seam is analyzed from the perspectives of gas migration characteristics, competitive adsorption characteristics and gas distribution characteristics in the channel. The nonlinear seepage mechanism and evolution law of CH₄ in coal seam by CO₂ displacement are studied. Based on the seepage characteristics, the evolution law of CH₄ in coal seam by CO₂ seepage displacement is studied from three aspects of “ stability, ability and effect ”, in order to provide the basis for the technology of CH₄ displacement by CO₂ in coal seam.

In this paper, theoretical analysis, experimental research and field test methods are used to systematically study the migration characteristics and influencing factors of single-component CO₂/CH₄ gas in the channel, the competitive adsorption law of multi-component gas, the gas seepage law of coal with different permeability, the theory of CO₂ seepage displacement CH₄, and the evaluation of field test results. The main results are as follows :

（1）The characteristics of gas flow and pressure gradient curves of single component ( 100 % CO₂ + 0 % CH₄, 0 % CO₂ + 100 % CH₄ ) can be divided into two stages : low flow rate or low pressure gradient section shows nonlinear seepage characteristics. With the increase of flow rate or pressure gradient, there are still nonlinear characteristics, but they are becoming less and less obvious, and gradually show linear seepage characteristics. According to the distribution characteristics of CO₂/CH₄ in the channel, the viscous resistance, the thickness of the boundary layer, the action resistance of the bulk fluid and the boundary fluid and the influencing factors are analyzed. The mathematical model of gas nonlinear seepage based on dynamic resistance gradient is constructed. Through the comparative analysis of the theoretical calculation flow value and the experimental value, the characterization formulas of pressure gradient and dynamic resistance gradient are verified, and the quantitative characterization of the difficulty of CO₂ and CH₄ migration in coal with different pressure gradients is realized.

（2）The adsorption characteristics of coal to gas and the compressibility of gas determine the distribution characteristics of gas components in the pores, change the dynamic viscosity coefficient of gas in different regions ( around the pore wall and in the middle line of the pores ), and affect the seepage process of gas in the pores. The change of gas dynamic viscosity coefficient in different regions caused by gas composition distribution in coal pores is the direct cause of gas nonlinear seepage. The permeability of coal seam and the gas composition change of CO₂ displacement coal seam CH₄ are the important reasons that affect the nonlinear deviation of gas flow and pressure gradient curve. The permeability of coal seam mainly affects the nonlinear deviation of the curve through the pore structure characteristics of coal which is the inherent property of coal body. The gas composition in the displacement process affects the nonlinear deviation of the curve by changing the gas dynamic viscosity coefficient in the channel.

（3）When the gas injection pressure of CO₂ displacement coal seam CH₄ is less than the critical pressure value, the larger the CO₂ gas component in the binary gas in the channel, the slower the slope of the pressure gradient and flow curve increases, the more obvious the hysteresis of flow growth, and the more significant the nonlinear characteristics. When the injection pressure is greater than the critical pressure value, the slope of the pressure gradient and flow curve tends to be stable. The larger the CO₂ gas composition in the channel, the smaller the slope of the curve, the higher the deviation, and the smaller the gas flow at the outlet. With the increase of coal permeability, the slope of pressure gradient and flow curve increases faster, the gas flow increases more significantly, and the critical pressure of displacement pressure in nonlinear and nonlinear sections decreases.

（4）Based on the theoretical analysis of the ability of CO₂ flooding to displace CH₄ by molecular dynamics, two indexes of molecular collision frequency and molecular mean free path were proposed to characterize the displacement ability, and the key parameters affecting the indexes, temperature and gas partial pressure ( component concentration ), were obtained. Through the experimental analysis of CH₄ flooding by CO₂ under different injection pressures ( higher than the critical pressure ), the linear relationship between displacement ratio and displacement pressure is determined, which decreases with the increase of displacement pressure. The displacement efficiency increases with the increase of displacement pressure. By increasing the displacement pressure, the displacement ratio can be reduced and the displacement efficiency can be increased.

（5）In the process of CO₂ flooding coal seam CH₄ mass transfer, on the one hand, the competitive adsorption of CO₂ and adsorbed CH₄ in the channel or the collision kinetic energy transfer ( displacement ability ) between CO₂ and adsorbed CH₄ molecules promotes the desorption of adsorbed CH₄. On the other hand, the free CO₂ continuously collides with the desorbed CH₄ in the channel, and drives out the CH₄ in the coal seam. The macroscopic performance is the permeability of the two gases in the channel, that is, mobility. The mobility ratio of the displacement phase ( CO₂ ) to the displaced phase ( CH₄ ) is introduced to characterize the displacement stability. Under the same temperature and pressure conditions, the dynamic viscosity of CO₂ is greater than that of CH₄, and the dynamic viscosity ratio of the displaced phase to the displaced phase is less than 1. Based on the comparative analysis of CO₂ and CH₄ molecular structure, gas physical properties and molecular mean free path under the same conditions, it is found that the seepage capacity of CH₄ in the same scale pore is stronger than that of CO₂. Through the gas flow rate, starting pressure gradient and migration resistance at the outlet end, it is determined that the seepage capacity of CH₄ in coal is stronger than that of CO₂, and the ratio of displacement phase permeability to displacement phase permeability is less than 1, indicating that the mobility ratio of displacement phase to displaced phase is less than 1, and the sweep efficiency is high, which can form stable displacement. The displacement stability was verified by field test of CH₄ concentration change in the hole.

（6）An evaluation method of CH₄ equivalent standard effect of CO₂ seepage displacement coal seam is proposed, and the field data are aggregated and calculated by this method. The desorption gas content and radius of K1 and K3 coal seams increase exponentially in 60 days of extraction period. The effective standard displacement radius of K1 and K3 are 15m and 12m respectively, the displacement efficiency are 20.52% and 21.81% respectively, and the sweep efficiency is 60% and 48%, respectively. In addition, it is comprehensively evaluated that the effective standard displacement radius of the CO₂ seepage displacement CH₄ test is 9m, the displacement efficiency is 23.95%, and the sweep efficiency is 36%.

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