湘西北下寒武统牛蹄塘组页岩气储层可压裂性评价

摘要

The effect of macro and micro parameters on fracability of Lower Cambrian Niutitang shale of northwestern Hunan was evaluated based on field investigations and sample analysis. The quantitative evaluation of fractured rate of shale was achieved by combining SEM technique and ImageJ. A mathematical model based on the combination weight method was established to quantitatively evaluate the shale fracability. The results show that the Lower Cambrian Niutitang formation shale in the study area has great content of brittle mineral, high brittleness index, high degree of thermal evolution and well developed micro-fractures and pores, possession of advantageous conditions for the fracturing of shale gas reservoir. The fracability of the shale Lower Cambrian Niutitang is 0.504 0. And the optimum area for fracturing in the Niutitang Formation shale is Xianfeng, Longshan, Sangzhi and Cili due to their relatively higher fracability indexes. Besides, shale fracability can be divided into three levels according to the reservoir parameters. The shale with low fracability (fracability index is less than 0.277 4) cannot be stimulated effectively. Shale with the medium fracability (fracability index range is [0.277 4, 0.436 3] can be stimulated effectively but the effect is actually modest. Shale with high fracability (fracability index range (0.436 3, 0.653 8] is the best interval to be fractured. It is better to choose shale with fracability index above 0.436 3.%通过对湘西北地区实地勘察及采样分析,对湘西北下寒武统牛蹄塘组页岩储层的宏观和微观参数对可压裂性的影响进行评价;基于SEM技术与ImageJ软件相结合的方法,实现页岩孔裂隙率的定量评价;采用组合赋权法建立可定量评价研究区页岩储层可压裂性的可压裂系数数学模型.研究结果表明:研究区下寒武统牛蹄塘组页岩具有脆性矿物质量分数高、脆性系数高、热演化程度高、微孔裂隙较发育的特点,具备页岩储层压裂的有利条件;运用可压裂系数模型计算研究区页岩储层的可压裂系数为0.5040,咸丰、龙山、桑植和慈利的可压裂系数较高,是研究区页岩较理想的压裂区域;基于储层各参数特征,将研究区的页岩可压裂性分为3个级别,即可压裂性差(可压裂系数小于0.2774,压裂效果差)、可压裂性中等(可压裂系数为[0.2774, 0.4363],压裂效果一般)和可压裂性好(可压裂系数为(0.4363, 0.6538],缝网压裂效果较理想),应选择可压裂系数大于0.4363的页岩层进行水力压裂.