A Numerical Study on the Propagation Mechanisms of Hydraulic Fractures in Fracture-Cavity Carbonate Reservoirs

Field data suggests that carbonate reservoirs contain abundant natural fractures and cavities.The propagation mechanisms of hydraulic fractures in fracture-cavity reservoirs are different from conventional reservoirs on account of the stress concentration surrounding cavities.In this paper,we develop a fully coupled numerical model using the extended finite element method(XFEM)to investigate the behaviors and propagation mechanisms of hydraulic fractures in fracture-cavity reservoirs.Simulation results show that a higher lateral stress coefficient can enhance the influence of the natural cavity,causing a more curved fracture path.However,lower confining stress or smaller in-situ stress difference can reduce this influence,and thus contributes to the penetration of the hydraulic fracture towards the cavity.Higher fluid viscosity and high fluid pumping rate are both able to attenuate the effect of the cavity.The frictional natural fracture connected to the cavity can significantly change the stress distribution around the cavity,thus dramatically deviates the hydraulic fracture from its original propagation direction.It is also found that the natural cavity existing between two adjacent fracturing stages will significantly influence the stress distribution between fractures and is more likely to result in irregular propagation paths compared to the case without a cavity.