Abstract

Currently, staged multi-cluster perforation fracturing was proved to be an efficient method to achieve the maximum stimulated reservoir volume in unconventional reservoir stimulation. However, an extensive work was conducted to study the influences of net fracturing pressure, fractures spacing and in-situ stresseses. However, the pore-pressure distribution, may affecting hydraulic fracture growth, was not well understood. Therefore experimental and numerical simulation are used to study the influence of pore-pressure distribution on sequential and simultaneous fracturing in tight sandstone.

In this paper, the experiments are divided into two categories, to investigate the interference of staged fracture propagations in porous sandstone subjected to true tri-axial stresses. Two independent fluid pumps are used to control the growth of two neighboring fractures. After the experiment, the specimens are sliced to analyze fracture paths. Then 3D fluid-solid coupling model is applied to simulate fracture propagation under the influence of pore-pressure distribution. Both experimental results and numerical modeling with pore pressure show attraction between the adjacent fractures, instread of repulsion in both sequential and simultaneous fracturing in experiments. More specifically, when the ratios of fracture spacing to fracture half-length are small, the fractures turn towards and coalesce with each other. And when the ratios of fracture spacing to fracture half-length are large, there is no obvious interference between the fractures. Consequently, among the parameters, other than net fracturing pressure, the fracture spacing and in-situ stresses, pore-pressure distribution plays a vital role on the fracture propagation, which leads to non-planar and asymmetric bi-wing fractures. The influences of pore pressure can attribute to shear failure of rocks, caused by reduced effective stress. Thus, increasing the perforation clusters interval can lower the possibility of deflection or coalescence of the multi-fractures, which promotes the performance of staged multi-cluster perforation hydraulic fracturing in tight sandstone.

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