Simulation of fracture propagation and optimization of ball-sealer in-stage diversion under the effect of heterogeneous stress field

Abstract

Ball-sealer in-stage diversion in horizontal wells is a key technique to realize the uniform stimulation of fractured sections in tight oil and gas reservoirs. So far, however, there are fewer research results on the propagation morphologies of multi-cluster fractures after the implementation of different ball-sealer in-stage diversion processes during the fracturing treatment, which results in less theoretical support for the preparation of field ball-sealer in-stage diversion process and measures and impacts its application effects in the fracturing field. To deal with this situation, this paper established a fully coupled “wellbore–perforation–fracture propagation” model of horizontal wells on the basis of the boundary element method. Then, a method for calculating the allocation of ball sealers was proposed. Finally, the number of ball sealers, diversion time and number of diversions during the intra-stage temporary plugging and diversion under the condition of initial heterogeneous stress field and their effects on the propagation of multi-cluster fractures were simulated. And the following research results were obtained. First, the flow restriction of perforation friction can counterbalance the intake difference caused by the induced stress interference so that the friction difference applied on the fluid flow in each fracture cluster is reduced. Second, when the effect of the heterogeneous distribution of the initial stress field is taken into consideration, the liquid intake of each fracture cluster changes greatly and even ineffective perforation clusters without liquid incoming appear in the high-stress region. And after the ball is injected, new fractures are initiated from the ineffective perforation clusters. Third, when the initial minimum horizontal principal stress difference (Δσh) is higher than 3 MPa, it is beneficial to reduce the non-uniform propagation of each fracture cluster by increasing the number of ball sealers appropriately in the middle stage of the construction (over half of the total perforations of each stage) or carrying out temporary plugging in the early–middle stage (including ball injection in batches in the early–middle stage). Fourth, when Δσh is lower than 2 MPa, it is necessary to reduce the number of ball sealers or inject balls in the middle–late stage, or the non-uniform propagation of each fracture cluster will be aggravated.