超临界二氧化碳对煤增透作用微观机理研究

Coal-bed gas reservoirs permeability is poor and the recovery rate of coal-bed methane is low in China. The micro mechanism research of coal is carried out after the effect of different temperature and pressure supercritical carbon dioxide by using experimental research, theoretical analysis and numerical simulation. Using Micro CT experiment to study the microstructure evolution law of pore-fracture morphology and size after the effect of supercritical carbon dioxide; Conducting the CT image processing to extract the coal pore feature distribution map, get the box dimension analysis chart of pore distribution, analyze coal pore, fissure development degree quantitatively after improving the permeability; Purse the effect law of micro pore, fissure evolution on permeability. Carrying out the mercury injection experiment to study pore structure characteristic parameters of coal and the characteristics of capillary pressure curve, get the influence law of pore throat structure on the permeability of the coal. Based on the effect of coal skeleton adsorption deformation, temperature and gas pressure, to establish dynamic evolution model of increasing permeability coal sample. According to the porosity dynamic evolution model, based on the effective stress, temperature，gas pressure and pore-fracture，to build the permeability dynamic evolution model of increasing permeability of coal sample. Combined with macroscopic and microscopic method, to reveal the microcosmic mechanism of permeability improvement coal sample. The project will provide important scientific basis for improving the coal seam permeability and the recovery ratio of coal bed methane by using supercritical carbon dioxide.

针对我国煤层气储层渗透性差致使煤层气采收率低的现状。采用实验研究、理论分析和数值模拟相结合的方法，进行不同温压超临界二氧化碳作用后煤微观机理研究。利用显微CT实验研究超临界二氧化碳作用后煤孔隙、裂隙系统形态、大小等微观结构演化规律;进行CT图像处理，提取煤样孔隙特征分布图，得到孔隙分布盒维数分析图,定量分析增透后煤孔隙、裂隙发育程度;寻求微观孔隙、裂隙结构演化对渗透性的影响规律。开展压汞实验研究煤孔隙结构特征参数和毛细管压力曲线形态特征，得到煤孔隙孔喉结构特征对渗透性的影响规律。建立基于煤体骨架吸附变形、温度和瓦斯压力影响的增透煤样孔隙率动态演化模型；依据孔隙率动态演化模型，建立基于有效应力、温度、瓦斯压力,孔隙、裂隙影响的增透煤样渗透率动态演化模型。宏微观结合揭示超临界二氧化碳对煤增透作用微观机理。项目将为利用超临界二氧化碳改造低渗透煤层渗透性及提高煤层气采收率提供重要科学依据。

针对我国绝大多数高瓦斯煤层渗透性低和当前低渗透煤层强化抽采瓦斯效果不理想的现状，结合超临界二氧化碳强扩散和溶解增透孔隙介质等独特优点，进行超临界二氧化碳对煤增透微观机理研究。自制三轴加载超临界二氧化碳增透实验装置，开展不同条件超临界二氧化碳作用后煤的宏观增透实验。结果表明：超临界二氧化碳增透低渗透煤层效果显著，增透后煤的渗透率较增透前和非超临界二氧化碳作用后均提高一个数量级。得到不同体积应力、温度和孔隙压力超临界二氧化碳作用后煤的渗透率变化规律：随体积应力增加，煤的渗透率呈负指数变化规律；随孔隙压力增加，煤的渗透率呈正指数变化规律；定孔隙压力不同温度超临界二氧化碳作用后，煤的渗透率得到提高，但在二氧化碳超临界温度范围内，煤的渗透率随温度增加呈降低变化规律。超临界态二氧化碳的孔隙压力对煤的增透效果起主控作用。在宏观实验基础上进行煤微观扫描成像实验，得到了超临界二氧化碳作用后煤微观孔裂隙演化规律及对宏观渗透性影响规律：增透后煤微观孔隙率是增透前的10倍；随孔隙压力增加，蜂窝状孔隙逐渐演化形成连通网络，孔隙分形维数逐渐增大，孔隙率呈正指数变化规律；恒定孔隙压力不同温度超临界二氧化碳作用后煤孔隙率明显提高，但超临界态下孔隙率随温度增加呈递减变化规律；超临界二氧化碳作用后煤宏观渗透率随微观孔隙率增加呈正指数变化规律；宏微观结果一致：超临界二氧化碳增透促进了煤微观孔裂隙演化，提高了低渗透煤层渗透性。结合超临界二氧化碳作用后煤微观孔裂隙演化特征，建立考虑体积应力、温度、孔隙压力及超临界二氧化碳溶解增透作用影响的热流固耦合力学模型。依据增透煤微观孔裂隙演化特征建立反映煤体内部真实细观结构的孔裂隙随机分布的三维实体模型，利用ABAQUS软件提供的场变量子程序，结合PYTHON脚本和子程序二次开发功能，开展不同注气条件下低渗透煤层注超临界二氧化碳增透规律数值模拟。