塔巴庙低渗致密砂岩渗透率有效应力定律实验研究

为了从实验角度探索低渗砂岩是否存在很小的ESCK值及重新认识低渗砂岩ESCK的变化规律，用两种修正的析因设计方案开展了塔巴庙低渗致密砂岩渗透率有效应力定律实验研究.一种方案包含了3个循环，每个循环是在孔隙流体压力不变，通过加载和卸载围压完成的；另一种方案包含4个循环，每个循环是在围压不变，通过降低和增加孔隙流体压力完成的.采用稳态法采集每个测点的渗透率值，并选择合适的经验模型拟合实验数据.为了使模型更好地拟合实验数据，本文采用最大似然函数法确定的转换系数转换实验得到的渗透率数据，使拟合得到的经验模型计算的渗透率值与实验值偏差的联合概率密度趋于极大值且残余平方和最小.拟合得到的经验模型可以用渗透率-围压-内压响应面直观地表示，再用Bernabe的ESCK计算式将这一响应面转换成渗透率有效应力系数ESCK-围压-内压响应面.ESCK响应面的响应特征表明，ESCK随围压和孔隙流体压力的变化而变化，随围压的增加而降低，随内压的增加而增加，反之亦然；实验研究还表明，ESCK的变化范围在0.0~1.02之间，这一变化范围和以往实验的结果存在巨大的差异，为此，文中分析了产生这一差异原因，同时提出一新的机理模型解释了实验获取的ESCK的变化规律是低渗岩石中微裂缝和孔隙变形共同作用的结果.

Laboratory study of the effective pressure law for permeability in Ta-Ba-Miao low-permeability sandstones

To obtain new knowledge on the effective pressure law for permeability, i. e.,P_(eff)=P_c-κP_p.and to see if there exist very small κ values and its variability, laboratory experiments with two modified factorial designs were performed to determine the effective pressure law for permeability of two samples from Ta-Ba-Miao low permeability sandstone formation. One modified factorial design for one sample included three cycles with different pore pressures. Each cycle was run through loading and unloading confining pressure (P_c) in constant-pore-pressure condition. Another design for the second sample contained four cycles under different confining pressures. Each cycle was run through raising and lowering pore pressure (P_p) in constantconfining-pressure condition. Permeability data were taken with the steady-state method. The response-surface method was used which supposed that nothing was known about the material behavior, and a model was built empirically by matching an approximate k-P_c-P_p surface to the data. The coefficients describing the surface reflected the information about material behavior and were transformed into κ-P_c-P_p response surface. The κ-P_c-P_p surfaces showed that at intervals along the pressure path "local" values of the coefficient κ in the effective pressure law varied with pore and confining pressure and its range is from 0 to 1.02. The values of κ of two samples were significantly less than 1.0 and even approached zero at the high confining pressure and low pore pressure conditions. These experimental observations indicate that the variability of κ is not in agreement with present theories and other published laboratory work. The data suggest that κ values decrease with increasing confining pressure and are small at high confining pressure and low pore pressure. This is interpreted in terms of the changes in the geometry of the micro-cracks during closure and the elastic deformation of the rock particles after the closure of micro-cracks with increasing confining stress in the paper.