Fracability evaluation of shale gas reservoir-A case study in the Lower Cambrian Niutitang formation, northwestern Hunan, China

摘要

Four main fracability-related factors, brittleness, quartz content, diagenesis and natural fracture, are analyzed for the shale in Lower Cambrian Niutitang formation, Northwestern Hunan. The quantitative evaluation of fissure ratio of shale was achieved by combining SEM technique and Image J. It shows that the Lower Cambrian Niutitang formation shale in the study area has large content of brittle minerals, high brittleness index, high degree of thermal evolution, and well developed fractures, which therefore indicate advantageous conditions for the fracturing of shale gas reservoir. In addition, a mathematical model integrates the four factors is proposed based on the combination weight method to quantitatively evaluate the fracability of the shale in the study area. It was found that the optimum area for fracturing in the study area is Xianfeng, Longshan, Sangzhi and Cili due to their relatively high fracability indexes. Also the shale fracability in this area can be divided into three levels according to the characteristics of reservoir parameters and it is suggested to select shale with fracability index above 0.4354 to be fractured. The lab hydraulic fracturing tests of the shale samples are conducted to verify the accuracy of the quantification model. The experiment results agree with the fracablity index predicted by the proposed quantification model.