During multi-stage fracturing operation for tight reservoirs with cluster perforation, a main fracture will initiate from each perforation cluster position. By adjusting spacing of fractures to change the interference intensity between multiple fractures, fracture network can be formed to achieve better results effect. Based on the plane strain theory of semi-infinite fracture, the characteristics of hydraulic fracture-induced stress are analyzed; according to effect that the superposition of induced stress can effectively reduce the horizontal stress difference (σH-σh), formation of complex branched fractures network becomes easier. In this paper, optimization of spacing of fractures—spacing of perforation clusters—is made for stimulation under two different kinds of circumstances, so that the guidance for parameters of the perforation clusters technology can be obtained.
At present, the shale gas industry has developed rapidly worldwide. No matter in china or out of china, one of the commonest exploitation methods for shale gas is multi-stage fracturing technology with clustering perforation in horizontal wells. The operation separates the horizontal well section into several fracturing segments with bridge plugs and each segment contains 2–5 perforation clusters. During the process of hydraulic fracturing, fractures will initiate simultaneously and extend from each perforation cluster, as shown in Figure 1. Mutual interference among different fractures will promote the formation of more complicated fractures network, namely, volume fracturing. For ultra-low permeability and tight shale reservoirs, volume fracturing possesses larger flow area than conventional fracturing, which can achieve better effect of reservoir stimulation (Jaeger 1959).
During the fracturing operation, the opening of each fracture will squeeze the surrounding formation to generate induced stress. Induced stress itself brought about by multiple fractures will yet superimpose to influence the extension direction of each fracture itself, so "stress shadow" effect will be formed (Zoback 2007, Roussel & Manchanda 2012, Manchanda et al. 2012, Cheng 2012).
Currently, scholars from home and abroad have done some research on the ambient induced stress field around a single hydraulic fracture. While research on superimposed effect of induced stress among fractures is not enough, and practical calculation method of fracture spacing optimization design is not provided either. Based on the analysis of induced stress field in a single fracture, this paper further illustrates the stress inversion phenomenon existed around the fracture. In addition, according to the characteristic that induced stress field can reduce the horizontal stress difference of shale reservoirs, this paper optimizes the designed value of perforation cluster spacing and proposes relative practical design method. Theoretical foundation for further efficient development of shale gas can be provided.