Perforating cluster space optimization method of horizontal well multi-stage fracturing in extremely thick unconventional gas reservoir

X5 reservoir is a 3000 m deep tight sand-shale interbedded thick reservoir. The horizontal principle stress ratio is around 0.45. The minimum horizontal principle stress is 75-80 MPa, and natural fractures are not abundant. Those conditions limit the generation of fracture network. Treatments based on previous fracturing design have not received good production. Due to extremely thick formation and lack of significant stress barrier, it is hard to maintain the fracture height in expected section. This paper established a 3D fracture propagation model based on finite-element method to simulate multi-cluster staged fracturing, and proposes a cluster space optimization method which considers the fracture geometry of each cluster and the induced stress field between the fractures. This model is used to study three fractures propagating simultaneously from three clusters. Simulation shows that when the cluster space is 10 m, 20 m and 30 m, the middle fracture is severely restricted, and is enwrapped by the side fractures. When cluster space is 40 m and 50 m, three fractures propagate in a balanced way. In the cases with enwrapping phenomenon, there is stress slightly diverting area existing between side fractures, which will reduce the possibility of generating fracture network. Results show that in the 40 m and 50 m cases, minimum stress diverted in the whole regime between the three fractures and no 'stress slightly diverting area' exists, which makes fractures more efficient in generating fracture network. But small cluster space will make the stress changes more severe. By comprehensively considering the geometry of three fractures and the induced stress field between fractures, the optimized cluster space is 40 m. Based on this optimization, a 7-stage fracturing treatment of X5-5H well was designed and conducted. The treatment process was smooth and its production was 1.7 times the average production of the adjacent horizontal wells. (C) 2015 Elsevier B.V. All rights reserved.