高压油气藏对砂岩力学特性影响的试验研究

 首先采用声波纵、横波测量方法，进行岩样筛选。然后根据高压油气藏地质构造特征，设计模拟高压油气藏内部孔隙压力变化条件下岩石力学特性测试的方案。在GCTS–1000型三轴压缩试验机上进行高温高压三轴岩石力学测试，结果表明：随着砂岩内部孔隙压力增加，外部围压保持不变的条件下，岩石的强度与围压不呈单调上升的变化趋势，而是随着孔隙压力的增加，净围压减小，岩石强度先随净围压减小而逐渐减小，之后则表现出反常的增大现象。在地压梯度为2.20 MPa/(100 m)时，产生最低强度值。随着地压梯度的增大，岩石强度值反而升高，形成一个V形曲线。砂岩的弹性模量为一波浪形曲线，上下波动范围最大差值为2 909 MPa。泊松比的值从低向高；在地压梯度大于2.00 MPa/(100 m)时，泊松比接近0.5。重复试验揭示了岩石三轴强度特性的这一特殊现象。该结果对于高压油气藏、水泥环和套管系统的真三维套管变形与损坏的模拟有着重要的参考价值，而且是必不可少的基础数据。

EXPERIMENTAL STUDY OF MECHANICAL BEHAVIORS OF SANDSTONE UNDER HIGH-PRESSURE OIL AND GAS RESERVOIR

Sandstone samples are selected by the compressive wave velocity and the shear wave velocity. Then test scheme has been designed to simulate rock mechanical behaviors under the altering pore pressure of high- pressure oil and gas reservoir according to the geological structure. All the triaxial tests have been performed on the triaxial compression test system GCTS–1000 with high temperature and high pressure. The test results show that：rock strength doesn¢t increase monotonically with confining pressure while the internal pore pressure is increasing and confining pressure keeps constant；but the rock strength will decrease with the net increasing confining pressure first and increase abnormally then. The minimum value appears when the geostress gradient is equal to 2.20 MPa/(100 m)；but the sandstone compressive strength grows up with the increase of geostress gradient. The relationship curve between rock strength and pore pressure is V-shaped. The relationship curve between elastic module and pore pressure appears as a wave shape and the maximum difference among elastic module is 2 909 MPa. In the experiment，the Poisson¢s ratio increases gradually and approaches 0.5 when the geostress gradient coefficient is larger than 2.00 MPa/(100 m). This particular phenomenon of sandstone triaxial strength has been observed during repeated tests. The test result is significant to simulate the true three- dimensional deformation and failure of the system composed of high-pressure gas and oil reservoir，cement sheath and casing.