变有效应力条件下致密砂岩声波特性实验研究

Acoustic characteristics of tight sandstone under different variable effective stress

致密砂岩气藏在钻完井、生产过程中有效应力会发生变化,致密砂岩孔隙结构复杂,微裂隙发育,当有效应力增大时,微裂缝发生闭合,孔隙被压缩,使得声波在致密砂岩中的传播特性发生变化。通过探究变有效应力条件下声波传播特点对预测致密砂岩地层压力以及对岩石动态力学参数响应特征的认识具有重要意义。以鄂尔多斯盆地上古生界二叠系典型致密砂岩为研究对象,测定了有效应力增加时,干岩样纵波、横波波速和渗透率。结果表明,当有效应力低于临界有效应力时(0~15 MPa),致密砂岩主要表现为微裂缝的闭合,纵、横波波速与有效应力均呈幂指数关系;当高于临界有效应力时(15~50 MPa),岩石变形特征由线弹性阶段逐渐向塑性变形阶段过渡,致密砂岩发生层间错动,孔隙被压缩,纵、横波波速随有效应力线性变化。同时,渗透率和波速对有效应力的响应具有一致性,当小于临界有效应力时,渗透率随有效应力呈幂指数递减。

During the process of drilling,well completion and production,the effective stress changes,the pore structure of tight sandstone is complex,and the micro-fracture is developed.When the effective stress increases,the micro-fracture will close,and the pores volume will decrease,which make a difference in the propagation characteristics of sound waves in tight sandstone.The researches on the characteristics of acoustic wave propagation under variable effective stress are significant for the prediction of formation pressure of tight sandstone and the cognition of response characteristics of dynamic mechanical parameters of rock.Taking the typical tight sandstone of upper Paleozoic Permian in Ordos basin as the research object,velocity of P-wave and S-wave and permeability of dry rock samples with the increasingly effective stress were measured.It was proved that when the effective stress(between 0~15 MPa)was lower than the critical value,the tight sandstone was mainly characterized by the closure of micro-fractures,and there was an exponential relation between effective stress and velocity of P-wave and S-wave respectively;when the ef?fective stress(between 15~50 MPa)was higher than the critical value,the deformation characteristics of rock gradually transited from linear elastic stage to plastic deformation stage,the interlayer dislocation occured in tight sandstone,the pores volume decreased,and the velocity of P-wave and S-wave changed with the effective stress in a liner relation.Meanwhile,the response of permeability and wave velocity on effective stress matched with each other.When it was below the critical value,it went down exponen?tially with the effective stress.