Numerical Investigation Of Non-uniform Fracture Growth In Multi-stage Hydraulic Fracturing

Multi-stage hydraulic fracturing is one of the most effective techniques used in horizontal well to enhance the hydrocarbon production for low-permeability unconventional reservoirs.With utilizing this technique,engineers want to generate numerous closely-spaced hydraulic fractures,by adopting smaller perforation intervals and more fluids,to increase the production.However,monitoring data indicate that serious non-uniform fracture growth always occurs in multi-stage hydraulic fracturing.During the fracturing treatment,only a very small portion of fractures preferentially take most injected fluid,meanwhile other fractures cannot obtain fluid and hence gradually stop their growths.This non-uniform fracture growth phenomenon significantly hurt the effectiveness of the aggressive fracturing treatment.Focus on the severe problem,in this paper we have constructed a numerical model for simulating the multiple fracture growth.By using this model,we have performed the numerical investigation of the non-uniform fracture growth phenomenon in multi-stage hydraulic fracturing.In addition,we have also implemented the optimization study on the technologies widely-used for promoting the uniform fracture growth.The main contents of our study are listed as follows:(1)Based on displacement discontinuity method,finite volume method and implicit level set method,we have constructed a fully solid-fluid coupled numerical model for simulating the multiple fracture growth in 2D,pseudo-3D and 3D cases,which couples the stress interaction,the dynamically flux partition into fractures,the pressure loss caused by perforation,the perforation erosion and the transport of solid particles in fractures.In this model,a universal tip asymptotic solution,reflecting the multi-scale tip behavior for hydraulic fractures,is adopted as a propagation criterion to locate the fracture front,which can also relax the requirement of mesh refinement in the tip zone.Particularly,in this paper a stress correction factor T is established for correcting the numerical results of stress field in the case of reservoir with multiple rock layers.The stress correction factor T can be used for the construction of pseudo-3D multiple fracture growth model.The numerical model in this paper,can be effectively applied in numerical study and optimization for the multiple fracture growth in multi-stage hydraulic fracturing.(2)With adopting dimensional analysis and proper scaling,we have performed parametric analysis to seek the favorite conditions for uniform fracture growth by using our fully-coupled numerical model.The results show that,only in the case of viscosity-dominated regime(the dimensionless toughness ?<1),multiple fractures with similar initial sizes can grow relatively uniform in the early-stage.It means that,in the early-time of multi-stage hydraulic fracturing,higher viscosity,higher injection rate and closer initial fracture lengths are beneficial to the promotion of uniform fracture growth.In addition,we have discussed the mechanism of non-uniform fracture growth related to the principle of least action.In most case,preferential fracture growth is energetically favorable than uniform fracture growth,leading to the non-uniform fracture growth phenomenon.(3)By using fully-coupled numerical model,we have performed the numerical study on the technologies widely-used for promoting the uniform fracture growth.First,we have investigated the effects of limited-entry method on multiple fracture growth.The results show that higher perforation friction can effectively balance the fluid influx,leading to more uniform fracture growth.Accordingly,a convenient method is established for the optimization of perforation parameters,which can be used to select proper perforation parameters by estimating required pressure loss based on geology and engineering parameters.Then,we have studied the influences of fracture spacing and non-uniform fracture placement on the multiple fracture growth.Finally,we have performed the numerical investigation on the diverting agents.The numerical results indicate that,higher injection rate and higher fluid viscosity will cause wider fracture opening and then it will require larger and more diverting agents to block the predominant fracture.In the case of dimensionless particle size ?<6 and dimensionless volume 0.24<?<0.4,the diverting agents can successfully block the predominant fracture,leading to more uniform growth.In addition,the results show that the variation of wellbore pressure is not an accurate indicator for the effectiveness of diverting treatment.(4)By using fully-coupled numerical model,we have investigated the influence of fracture spacing,well spacing and fracture scheme(simultaneous-frac and zipper-frac)on the multiple fracture growth for multi-well case in a well pad.Based on numerical results,the suggestions for promoting uniform fracture growth are presented.The numerical simulation results show that,in multi-well case,the asymmetric fracture growth occurs due to the inter-well stress interference.The inter-well stress interaction prevents the lateral propagation of stunted fractures.In the case of low differential stress between rock layer,the stunted fractures start to propagate vertically into adjoining layer,resulting in excessive height growth.The results also reveal that,a lower limitation of well spacing exists in successful design of treatment.When the well spacing is below to the critical value,generated fracture surface may be ruinously reduced.Compared to simultaneous fracturing scheme,results show that zipper fracturing scheme obtains a better fracturing performance in both the fracture complexity and the fracturing surface area.