有效应力对高煤级煤储层渗透率的控制作用

Control of effective stress on permeability in high-rank coal reservoirs

为了探究有效应力对高煤级煤储层渗透率的控制作用及其应力敏感性的各向异性,对5块高煤级煤样进行了覆压孔渗实验,揭示了有效应力对煤储层渗透率的控制机理。以3.5 MPa模拟原始地层压力发现,煤岩在平行主裂隙和层理面方向具有最高的初始渗透率,垂直层理面方向初始渗透率最低;有效应力从3.5 MPa增加到15.5 MPa的过程中,渗透率呈现出良好的幂函数降低趋势;渗透率伤害/损失的各向异性表明平行主裂隙方向渗透率伤害率和损失率最大,且不同方向应力敏感性受裂隙的宽度及其展布方向的控制;裂隙压缩系数随应力的增加呈现降低趋势,但由于高煤级煤岩压缩难度大,裂隙压缩系数的各向异性不明显。有效应力对渗透率控制的实质为通过减小煤储层孔裂隙体积降低渗透率,从而对各个方向上的渗透率均造成较大的不可逆伤害。

In order to discuss the anisotropy of stress sensitivity of permeability in the high rank coal reservoir, five high-rank coal samples were measured under overburden pressure to reveal the control mechanism of effec-tive stress. The coal mass has the highest permeability in the direction parallel to the face cleat and bedding plane by using 3.5 MPa to simulate the original formation pressure, while it has the lowest permeability in the direction perpendicular to the bedding planes. The coal permeability declines（or increase） in power function with the increase（or decline） of effective stress. The permeability damage/loss anisotropy indicates that in the direction parallel to the face cleat, coal mass has the highest stress sensitivity, and the stress sensitivity in dif-ferent directions is controlled by the width of the crack and the direction of its distribution. High rank coal is of high density, poor development of pore and fracture, and it is very difficult to compress, the anisotropy of cleat compressibility is not obvious. With the increase of effective stress the cleat compressibility showed a decreas-ing trend. The essence of effective stress on permeability is that the reduction of the coal reservoir pore fracture volume results in the reduction of permeability and leads to larger irreversible damage of permeability in all di-rections.