致密砂岩高围压和高孔隙水压下渗透率演化规律及微观机制

Permeability evolution of tight sandstone under high confining pressure and high pore pressure and its microscopic mechanism

深部岩石工程具有高地应力和高水头压力的特点。为了研究岩石在高围压和高孔隙水压条件下渗透率演化规律,选取致密砂岩开展不同围压条件下变孔隙水压的渗流试验。研究结果表明:(1)在所研究的围压范围内(0~50 MPa),随孔隙压力增加,渗透率依次呈现3种不同的变化趋势,即快速增长阶段(围压为10~20 MPa)、缓慢增长阶段(围压为30~40 MPa)和保持恒定阶段(围压为50 MPa);在围压卸载时,由于高围压作用使试样内部产生不可逆变形,导致渗透率具有明显的不可恢复现象,且随围压降低,渗透率恢复存在滞后效应。(2)渗流试验过程中,体积应变和渗透率演化具有较好的一致性。(3)在围压加卸载过程中,高孔隙水压力条件下渗透率对应力的敏感程度和恢复程度均大于低孔隙水压力。(4)偏光显微镜图像从微观角度揭示了试样在围压加卸载过程中产生不可逆变形的内在机制:骨架颗粒相互挤压、错动导致原有微裂隙压缩、孔隙减小甚至坍塌,引起渗透率不可恢复。渗流试验后,纵波波速增大,说明岩石致密性提高,与试样内部微观结构变化具有较好的一致性。

The deep rock engineering is characterised by high ground stress and high head pressure. To study the permeability evolution of rock under high confining pressure and high pore pressure conditions, the seepage tests for the variable pore pressure were conducted on tight sandstone under different confining pressure conditions. The results show that within the studied confining pressure range(0~50 MPa), the permeability presents three different variation trends with the increase of pore pressure: rapid growth phase(confining pressure at 10~20 MPa), slow growth phase(confining pressure at 30~40 MPa) and constant phase(confining pressure at 50 MPa). During the unloading process of confining pressure, irreversible deformation occurs in the sample due to high confining pressure, which results in a significant irreversible permeability and hysteresis effect on the restitution of permeability. During the seepage test, volumetric strain and the permeability evolution have a good consistency. In the loading and unloading process of confining pressure, the sensitivity and recovery capabilities of permeability to stress under high pore water pressure are stronger than those under low pore water pressure. Polarising microscope images reveal the inherent mechanism of irreversible deformation during the confining pressure loading and unloading process: the mutual extrusion and movement of skeleton particles cause the compression of the original micro-fissures, the decrease or even collapse of the pores, resulting in the irreversible permeability. After the seepage test, the P-wave velocity increases, which indicates that the compactness of the rock is improved and has a good agreement with the change of the internal microstructure.