ABSTRACT
Reduced stage- and cluster-spacing have been widely used in placing multiple closely-spaced hydraulic fractures (HFs) for horizontal well multi-stage fracturing. However, the effectiveness of this technology may be weakened due to the unclear relationship between dominant operation parameters and the resulting HF geometries. In this study, a novel experimental process was proposed to model tight-spacing fracturing, and 3D reconstruction of HF geometries based on CT scanning was conducted to study the effects of fluid pressure, cluster-spacing and cluster-number on stress interference. Results demonstrate that the high fluid pressure in the created HF may cause the subsequent HFs to deflect at a large angle. Multiple fractures propagating simultaneously within a stage tend to coalesce under small cluster-spacing, with a short independent growth distance for a single fracture. On the basis of real-time injection pressure, three to five clusters per stage may facilitate the decrease in the initiation pressure but aggravate the asymmetric and uneven propagation of fractures, leading to low perforation efficiency.
Horizontal well multi-stage fracturing with small stage- and cluster-spacing (tight-spacing fracturing) has been widely used in the development of shale gas in China (Xu et al., 2018; Lei et al., 2018; Tan et al., 2019; Fan et al., 2019). However, the uneven initiation and asymmetrical propagation of hydraulic fractures (HFs) within a stage may be induced by the strong stress interference among multiple closely spaced HFs and the heterogeneity of reservoirs, causing a low stimulated reservoir volume (Liu et al., 2018; Chen et al., 2020). Thus, the interference law amid the competent propagation of multiple closely spaced HFs must be clarified to provide theoretical support for the optimization of operation parameters in horizontal well tight-spacing fracturing.
Earlier research (Neil et al., 2011) has suggested that the optimal cluster-spacing usually ranges from 20 to 30 m for multiple clusters in a stage. In case three clusters are placed within a stage, the length of each stage (stagespacing) is generally designed to 60∼90 m. However, the production might be significantly increased by decreasing fracture- or cluster-spacing in horizontal well multi-stage fracturing even for reservoirs with extremely low permeability such as shale gas (Mayerhofer et al.,2006; Zhu et al., 2017). Several field tests have indicated that multi-stage fracturing with small stage- and cluster-spacing is also applicable for the reservoirs with high horizontal stress difference and strong plastic deformation characteristics, which have difficulty in creating complex fracture geometries (Maxwell et al., 2002; Cipolla et al., 2010; Desroches et al., 2014; Lecampion et al., 2015).
