Multi-cluster staged fracturing is an effective method to exploit shale gas. Field observations reported some clusters did not generate fractures. X5 formation in Sichuan Basin is a 3000m deep shale reservoir. The horizontal stress difference is so high, therefore it is difficult to generate fracture network. How to enable all fractures propagate effectively from each cluster and generate enough stress interference to enable fracture network is of critical concern.

This paper established a 3D fracture propagation model based on finite-element method to simulate multi-cluster fracturing of X5 reservoir. The fracture propagation model couples seepage-stress-damage theories to simulate fracture propagation. The cohesive element is used to simulate the forming of fracture, and the filtration from fracture to matrix is taken into consideration. This model is used to study three fractures propagating simultaneously from three clusters. Six different cluster spacing cases were simulated to investigate the fracture geometry and the stress field.

The research finds that when the cluster spacing is 10m, 20m, 30m and 35m, the length of the middle fracture is severely restricted; but for the side fractures, the length is over propagated. When the cluster spacing is 40m and 50m, balanced propagation of all the three fractures is achieved. In the 10m, 20m, 30m, 35m and 40m cluster spacing cases, the horizontal stress ratio is low and could generate fracture network, but the 50m case could not. By considering the fracture geometry and stress ratio, the optimized cluster spacing is 35m-40m. Based on this optimization, a 7-stage fracturing treatment of X55 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.

This paper presented a method to optimize the cluster spacing by both considering the fracture geometry and stress ratio. Cluster spacing optimized by this method could enable the effective propagation of all main fractures and increase the possibility of generating fracture network.

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