变围压循环下低渗透致密砂岩有效应力方程研究

采用了修正的析因设计方案,对低渗透致密砂岩进行了有效应力方程的实验研究。实验方案中包含多个循环,每个循环都是在孔隙流体压力不变,增加和降低围压的方式下进行。用稳态法采集各测点的渗透率,并用响应面法进行数据处理分析。结果表明,有效应力系数随着围压的增加而减小,随着孔隙流体压力的增加而增大。在实验研究范围内,有效应力系数的最大值不超过0.88,当围压大、孔隙流体压力小时,有效应力系数远小于1.0,表明孔隙流体压力对渗透率变化的影响变得很小。用响应面方法获取的有效应力系数确定了有效应力,建立了渗透率与有效应力的关系,与传统有效应力的概念相吻合。

Research on calculation of effective stress in low-permeability sandstone rock under cyclic loading and unloading

Laboratory study on the effective stress of low-permeability sandstone rock was carried out using the modified factorial design program. The design program includes several cycles under the conditions of loading and unloading confining stress at constant pore-pressure. The experimental data were collected with the steady-state method and analyzed with the response-surface method. The analysis results showed that the permeability and the effective-stress coefficient decreased as increasing confining stress and increased as increasing pore-pressure. The maximum of the effective coefficient is below 0.88 in the range of experiment condition. The effective stress coefficient is far less than 1.0 at high stress and low pressure of fluid flow in pores, which suggests that the pore pressure has a small effect on permeability. The relationship between permeability and effective stress was established. The value of effective stress was determined by using the effective stress coefficient obtained from the response-surface method. It is agreeable with the traditional concept of effective stress.