Investigation of the Influence of Natural Cavities on Hydraulic Fracturing Using Phase Field Method

In this paper, the influence of natural cavities on the propagation of hydraulic fractures is investigated using the phase field method. The deflection behaviour of a fracture during its propagation is firstly verified against published experimental data. Then, a sensitivity analysis on the mechanical behaviour of fracture propagation near a cavity is conducted. The fracture deflection is quantified in terms of the deviation distance and deflection point. The influence of the Young's modulus ratio between the cavity and rock mass (E-r= E-cave/E-rock), the differential stress (S-d= S-x - S-y) and the relative spatial position of the fracture and cavity (l(r)) on the propagation trajectory are considered. Simulation results show that with the decrease in E-r, crack path deviation becomes more prominent. With the increase in S-d, hydraulic fractures tend to propagate along the direction of maximum horizontal geostress. As l(r) varies, the deflection of the hydraulic fracture can be classified into three regimes: (1) the deflection is negligible; (2) the hydraulic fracture deflects and approaches the natural cavity, but does not connect with it; (3) the hydraulic fracture deflects and connects with the natural cavity. The results could be used as guidance for field design of stimulation scheme in carbonate oil/gas reservoirs.