Multi-frac propagation in unconventional shale
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In recent years, the Zipper-Frac technique has become one of the most widely used stimulation techniques in the oil and gas industry. The efficiency in this technique lies in minimizing stress shadows between adjacent stimulated fractures while maximizing fracture network and surface area in order to increase fluid production. The Zipper-Frac technique stimulates two parallel horizontally drilled wellbores, alternating between perforation clusters, while maintaining pressure in the previously fractured wellbore or perforation cluster. This study analyzes and discusses multi-fracture experiments in the laboratory that mimic Zipper-Frac results in unconventional shale. The experiments were conducted with two intended outcomes: (i) to examine how time-dependent pressure decay limits stress shadow effects and (ii) to investigate fracture complexity developed in Zipper-Fracs. To achieve these objectives, laboratory experiments were conducted on synthetic blocks (gypsum cement) of three layers (hydrostone, plaster and hydrostone, respectively). The experiment was conducted on 12 samples. Six samples were fractured with a pressure hold-up technique, where the in-situ stress after fracturing was above the fracture closure stress (FCS); and the other six were fractured with a pressure bleed-off technique, where the pressure was bled-off below the FCS. The results indicate is that greater well spacing and bleeding off pressure in fractures post-treatment result in longer and straighter fractures, hence minimal stress shadow, while closer well spacing and maintaining pressure in fractures post-treatment caused more non-planar fracture paths and less created fracture length, hence stronger stress shadow. Consequently, it can be inferred that less complexity would result with the bleed-off method, but this gives greater fracture surface area because of the greater achieved lengths.