Laboratory tests were conducted on cement blocks with a size of 30cm×30cm×30cm subjected to true triaxial stresses of 30-28-22MPa to investigate effects of cyclic injection on sub-parallel fracture propagation in horizontal well. Variable injection schemes, constant-rate, cyclic constant-rate and stepwise-rate, were used to initiate three clusters of perforation. It was observed that under case of constant-rate injection, two fractures initiate from perforations and subsequently intersect with each other. While a T-shaped fracture is created by cyclic constant-rate injection, characterized with a transverse fracture perpendicular to the minimum principal stress and an axial fracture along the wellbore. The fatigue damage due to cyclic loading is prone to reopen microcrack and induce a slippage along the pre-existing cracks. Stepwise and layered fractures are induced by shear fracturing under combined effects of fracture net pressure and in-situ stress difference. Sub-parallel fractures are likely to be created in cyclic constant-rate injection attribute to poroelastic effects and fatigue damage.
Multiple hydraulic fractures consist of an array of multi-staged or multi-stranded fractures from multi-clusters. It is first inferred from in-site microseismic monitoring by Fisher et al. (2004). Lately, a combination of distributed acoustic and temperature sensing (DAS/DTS) monitoring demonstrates that significant stress shadow exists (Dohmen et al., 2014; Jin and Roy, 2017). Several critical parameters, such as fracture net pressure, fracture spacing, in-situ stresses and pore pressure distribution, commonly affect multi-fracture initiation and propagation.
In the early stage, extensive experimental works have been done on horizontal fracturing to investigate multi-fractures (El Rabaa, 1989; Weijers et al., 1994; Abass et al., 1996; Kear et al., 2013). A propagating fracture competition means that there exists a balance between attraction and repulsion forces. It is well known the stress shadow induced by created fractures can suppress the subsequent fracture propagation or even deviate away from each other (Rahman et al., 2002). A higher fracture net pressure is needed than a single fracture to counteract work done by nearby hydraulic fractures. Bunger et al. (2011) presented experimental results of sequential fracturing, i.e., injection into one cluster at a time, in gabbro blocks, showing a subsequent fracture curve towards a previous fracture when the minimum stress perpendicular to the fracture is small. In addition to the influence of elastic deformation induced by fluid flow, Zhou et al. (2016) investigated the effects of poroelasticity on the multi-fracture propagation in the porous sandstone. A subsequent fracture directly curves towards and merges into a previous fracture, implying a strong attraction enhanced by pore pressure around the multiple fractures.