超临界CO_2增透煤微观图像重构及三维数值模拟

Microscopic image reconstruction and three-dimensional numerical simulation of supercritical CO_2 permeability improvement coal

针对我国低渗透煤储层渗透性差的特点,以超临界CO_2作用后煤的微观成像为基础,构建三维实体模型;考虑煤的热损伤效应,孔隙流体流动及有效应力对渗透规律的影响,利用增透后煤微观孔隙率的变化定义损伤变量,推导出损伤函数关系式,建立超临界CO_2增透煤在温度-渗流-应力耦合作用下的力学模型,模拟超临界CO_2在低渗透煤层内部的渗流运移规律。结果表明:由于煤体各处煤基质和矿物的密度、弹性模量和热膨胀系数等物理参数的不同及其分布的随机性,超临界CO_2注入过程中应力场、温度场、渗流场均呈现明显的非均匀性;在一定压力和温度超临界CO_2注入过程中,煤体内部各处孔隙、裂隙的变形、开裂以及扩展等的不均一性,使得煤体各处的损伤量不同。在其他注入参数一定的条件下,增加CO_2的注入时间,有利于提高煤体的渗透性。

Aiming at the characteristic of poor permeability in the low permeability coal seam of China,based on the microscopic imaging of coal after the effect of supercritical CO2,a three-dimensional reconstruction program was developed and a three-dimensional solid model was constructed by combining the image processing technology and the finite element method.Considering the influence on the permeability laws by the thermal damage effect of coal,the pore fluid flow and the effective stress,the damage variable was defined by the change in microscopic porosity of coal after permeability improvement,and the damage function relation was deduced. A mechanical model of supercritical CO2 permeability improvement coal under the coupling effect of temperature,seepage and stress was established,and the seepage migration law of supercritical CO2 in the low permeability coal seam was simulated. The results showed that all the stress field,temperature field and seepage field are obviously nonhomogeneous in the injection process of supercritical CO2 as the result of the difference and the random distribution of density,elastic modulus,thermal expansion coefficient,and other physical parameters of the coal matrix and mineral around the whole coal body. During the injection process of supercritical CO2 with a certain pressure and temperature,the deformation,cracking and expansion of each pore and fracture around the whole coal body are heterogeneous,hence the damage amount around the whole coal body is different. Increasing the injection time of CO2 will benefit the permeability improvement of coal under the fixed conditions of other injection parameters.