Fracability evaluation of lacustrine shale in the Yanchang Formation of Southeastern Ordos Basin

摘要

As a typical lacustrine shale in China, the Chang 7 shale in the Yanchang Formation of the Ordos basin shows great potential for development. Buried less than 2000 m deep, the shale is always characterized as a tight reservoir with well-developed bedding and cracks. Many studies have been performed on Chang 7 shale to determine its depositional systems, mineral compositions, and fracability. The fracability studies are particularly significant for developing lacustrine shale gas in the Ordos basin. This paper evaluates the fracability of Chang 7 shale and presents our discoveries regarding the brittle mineral content and rock mechanics parameters. First, a new mineralogical evaluation method was used to determine that the average brittleness index for Chang 7 shale was 39.5 percent. This benefits complex fracture network formation. Second, important rock mechanics parameters were obtained for Chang 7 shale via triaxial compression tests, Brazil splitting experiments, and Kaiser experiments. The Rickman brittleness index was 39.31%, the differential horizontal stress was 3.509 MPa, and the differential horizontal stress ratio was 0.133. The experimental results indicate that fractures extend along natural cracks and can easily form complex fracture networks during SRV fracturing in Chang 7. The field microseismic monitoring results indicate that the shale did form fracture networks after hydraulic fracturing were also verified, which is consistent with the experimental results given in this paper.