高低温循环冲击作用下砂岩孔裂隙结构的演化特征

Evolutionary characteristics of sandstone pore-fracture structure under the action of high and low temperature cyclic impact

我国煤系气储量丰富,但煤系气藏复杂的地质条件使得对单一含气系统的开采成本高、效率低,对多种含气系统难以合层共采。如何提高煤系储层的渗透性与兼容性是开发煤系气的关键问题。为探讨改善煤系储层结构的有效方法,研究了在高温200℃、低温-196℃循环冲击作用下煤层顶板砂岩孔裂隙的演化特征,使用工业显微CT对高低温冲击作用前后砂岩试样孔裂隙的发育过程进行了表征。利用数字图像和三维重构技术,对冲击前后试样的CT数据进行了滤波和均值化等处理,定性和定量分析了0、1、5、10、15次高低温度冲击后砂岩孔裂隙体积、表面积、孔隙率和孔裂隙分布复杂度分形维数的变化。研究结果表明:(1)高低温循环冲击促使砂岩试样孔裂隙发育、扩展、连通和次生裂隙的萌生,冲击作用有助于微小孔裂隙扩展形成较大的宏观裂隙。(2)砂岩孔裂隙体积、表面积、孔隙率和孔裂隙分布复杂度分形维数随温度冲击次数的增加呈对数规律变化,其增长幅度呈先增大后减小的趋势,在第5次冲击作用后其面积和孔隙率增幅达到最大,分别为15.48%,20.98%。(3)高低温循环冲击作用减弱了砂岩基体间的束缚力,打破了其原有的静态平衡,试样孔隙率随扫描层数的变化曲线演变为类“C”状,表明砂岩两端的束缚力衰减幅度更大。

China is rich in coal-measure gas reserves.However,the complex geological conditions of coal-measure gas reservoirs make it costly and inefficient to mine a single gas-bearing system,and difficult to co-mine multiple gas-bearing systems.How to improve the permeability and compatibility of coal reservoirs is the key issue in development of coal-measure gas.In order to explore effective methods to improve the structure of coal reservoirs,the evolution characteristics of sandstone pore-fracture in the top plate of coal seams under the cyclic impacts of high temper-ature(200℃)and low temperature(−196℃)were investigated.The development of sandstone pore-fracture before and after high-and low-temperature impacts was characterized using industrial micro-CT.The CT data of the specimens before and after impact were filtered and homogenized using digital image and three-dimensional reconstruction techniques.The changes of volume,surface area,porosity and fractal dimension of distribution complexity of sandstone pore-fracture after 0,1,5,10 and 15 high-and low-temperature impacts were qualitatively and quantitatively analyzed.The results shown that:①High-and low-temperature cyclic impacts promoted the development,expansion and connectivity of pore-fracture,and the sprouting of secondary fracture in sandstone.The impacts contributed to the expansion of micro pore-fracture to form larger macro fracture;②The volume,surface area,porosity and fractal dimension of distribution complexity of sandstone pore-fracture varied logarithmically with the increase of the number of temperature impacts,and the growth tended to increase first and then decrease.The increase of its area and porosity reached a maximum of 15.48%and 20.98%,respectively,after the 5th impact effect;③High-and low-temperature cyclic impacts weakened the binding force between the sandstone matrix and broke the original static equilibrium.The variation curve of sandstone porosity with the number of scanned layers evolved into a“C-like”shape,which indicated that the binding force at the ends of the sandstone was more attenuated.