致密砂岩储层抗拉强度评价方法研究

Study on prediction method of tensile strength for tight sandstone formation

致密砂岩储层抗拉强度直接影响压裂过程中裂缝起裂、水力裂缝扩展以及水力裂缝与天然裂缝的沟通能力.基于巴西劈裂实验圆盘测定了14组致密砂岩抗拉强度,同时在围压条件下测定了相同取心位置的14块岩心的纵横波波速和密度.基于上述实验结果,并与岩心所处井深测井数据对比分析,建立了基于地球物理测井数据(声波时差测井和密度测井)的致密砂岩抗拉强度的预测模型,模型相关系数大于0.93.由预测模型可知,抗拉强度与岩样密度和纵波波速呈正比(与声波时差呈反比).利用所建立的抗拉强度评价方法在鄂尔多斯盆地M16井致密砂岩储层进行了应用分析.得到了全井段抗拉强度,并基于抗拉强度预测了储层断裂韧性,其评价结果与压裂后产能监测数据相一致.本研究可为压裂设计、可压裂性评价及工程"甜点"选择提供支撑.

The tensile strength of tight sandstone reservoir rocks directly affects fracture initiation, hydraulic fracture propagation, and the ability of hydraulic fractures to communicate with natural fractures during fracturing. Based on the Brazilian disk test, the tensile strength of 14 groups of tight sandstones was measured. At the same time, the longitudinal and transverse wave velocity and density of 14 cores at the same core location were measured under confining pressure. Based on the above experimental results and comparative analysis with the depth logging data of the core, a prediction model for the tensile strength of tight sandstone based on geophysical logging data(acoustic time difference logging and density logging) was established. The model correlation coefficient was greater than 0.93. It can be seen from the prediction model that the tensile strength is proportional to the rock sample density and the longitudinal wave velocity(inversely proportional to the acoustic time difference). The established tensile strength evaluation method was applied to the tight sandstone reservoir of M16 well in Ordos Basin. The tensile strength of the whole well section was obtained, and the fracture toughness of the reservoir was predicted based on the tensile strength. The evaluation results were consistent with the productivity monitoring data after fracturing. This study can provide support for fracturing design, fracturing evaluation and engineering "sweet spots" screening.