Impact of hydraulic perforation on fracture initiation and propagation in shale rocks

To enhance the oil and gas recovery rate, hydraulic fracturing techniques have been widely adopted for stimulation of low-permeability reservoirs. Pioneering work indicates that hydraulic perforation and layout could significantly affect fracture initiation and propagation in low-permeability reservoir rocks subjected to complex in-situ stresses. This paper reports on a novel numerical method that incorporates fracture mechanics principles and the numerical tools FRANC3 D and ANSYS to investigate the three-dimensional initiation and propagation behavior of hydro-fracturing cracks in shale rock. Considering the transverse isotropic property of shale rocks, the mechanical parameters of reservoir rocks attained from laboratory tests were adopted in the simulation. The influence of perforation layouts on the 3D initiation of hydro-fracturing fractures in reservoir rocks under geo-stresses was quantitatively illuminated. The propagation and growth of fractures in three dimensions in different perforating azimuth values were illustrated. The results indicate that: 1) the optimal perforation direction should be parallel to the maximum horizontal principal stress, 2) the crack plane gradually turns toward the direction of the maximum horizontal principal stress when they are not in parallel, 3) compared with the linear and symmetric pattern, the staggered perforation is the optimal one, 4) the proper perforation density is four to six holes per meter, 5) the optimal perforation diameter in this model is 30 mm, and 6) the influence of the perforation depth on the fracture initiation pressure is low.