Research On Mechanics Mechanism In Refracturing And Its Applications

It is known to us that low permeability reservoirs occupy a considerable proportion inChina’s oil reserves. And so far, hydraulic fracturing is the important technology in thesekinds of oilfield. If hydraulic fracturing fails, refracturing will be used in most oil-water wells.However, there are often a large number of defects in low-permeability reservoirs, such ascracks or inclusions (referred to herein as defective reservoirs). Due to the complexity of therefracturing mechanism, the theoretical researches are far behind the field practices.Ground stress distribution is one of the major factors which decide fracture propagationafter refracturing. In fact, this stress distribution is more intricate in the defective reservoirsbecause of their particularities and production. In this paper, based on stress distribution,stress field is analyzed in natural fracture or line inclusion reservoirs, and crack propagationafter refracturing is simulated. Major works proceed as follows:(1) Based on fluid-solid coupling theory of oil-water two-phase flow, the couplingmathematical model of artificial fracture-reservoir system in homogeneous reservoirs isestablished. By taking a typical five point production as an example, this paper discusses theinfluences of artificial fracture and injection-production on stress field.(2) Combined with basic solution of stress distribution in infinite area with a fracture, themodel of interference between natural fracture and artificial fracture is established based onsuperposition principle. And the interference of a natural fracture and artificial fracture underinternal pressure is reduced to a set of Cauchy singular integral equations. Consequently, thestress intensity factor of artificial fracture tip is well analyzed.(3) This paper views line inclusion as the thin bar of elastic mechanics and applies thebasic solution of line inclusion in infinite plane. Influenced by single line inclusion, stressfield around artificial fracture is deduced. Similarly, the interference of single line inclusionand artificial fracture under internal pressure is reduced to a set of Cauchy singular integralequations. Consequently, the stress intensity factor of artificial fracture tip in the simplified model is resolved.(4) In order to reduce the size effect on the experimental results, this paper firstlysimulates the refracturing in defective reservoirs with large size of true triaxial experimentalequipment, which was developed by engineering mechanics laboratory in China University ofPetroleum (East China). According to the results, how random defects and ground stressimpact on the crack propagation has been analyzed.(5) Based on the fluid-solid coupling theory of incompressible single-phase fluid, thestress fields both in natural fracture reservoir and line inclusion reservoir are simulated beforerefracturing. This paper also discusses some influences on stress field distribution, includingthe orientation, the density of distribution and relative position to oil-water well. In addition,the dynamic propagation of artificial fracture is simulated by the extended finite element.(6) Based on the flow theory of oil-water two-phase in the reservoir and theory ofequivalent filtrational resistance in the crack, the impact of reservoir defects on theproductivity after refracturing is studied. By establishing five point methodinjection-production model, this paper explores whether cracks would steer after refracturing,and discusses some influences on well productivity, including the orientation, the density ofdistribution and relative position to oil-water well.In this paper we conclude the ground stress distribution in defective reservoirs beforerefracturing, and analyze crack propagation after refracturing theoretically and experimentally.It can provide certain references for the future research of refracturing mechanism in complexreservoirs and guide for field analysis of refracturing in defective reservoirs.