Numerical Simulation Of Hydraulic Fracturing In Tight Sand Reservoirs

Tight sand reservoirs usually featured low-porosity and low-permeability.The combination of multistage hydraulic fracturing and horizontal well drilling plays a key role in the economic development of these unconventional reservoirs.For efficiency reasons,each stage comprising of multiple perforation clusters in order to initiate multiple hydraulic fractures simultaneously.However,the performance of stimulation is generally below-expectation.This is because,on the one hand,approximately uniform fractures are seldom obtained during hydraulic fracturing treatment due to the stress interference among multiple fractures and reservoir variability,which may result in many non-producing perforation clusters.On the other hand,the large scale,complex fracture network may not be formed during stimulation.Generally,the complexity of fracture network is determined by the interaction between hydraulic fracture(HF)and natural fracture(NF).To improve the performance of stimulation,the propagation of multiple fractures and the interaction between HF and NF have been investigated.In this work,a new fully coupled,parallel-planar 3D hydraulic fracturing model which considering the perforation pressure drop,stress shadowing,and fracturing fluid partitioning among fractures was developed based on finite element method and then utilized it to investigate the competition growth of multiple fractures within a stage.The simulation results demonstrate that it is possible to obtain more-uniform fracture growth and improve the total fracture area by increasing the perforation friction pressure.In addition,uneven perforation number strategy and high pump rate can also obtain similar effect with increasing perforation friction while the effect of fracturing fluid viscosity on the uniform growth of multiple fractures is relatively small.Next,a new fully explicit hydro-mechanical method,Explicit-PPCZ,was proposed based on explicit temporal integration scheme finite element method,which is well suited for highly non-linear problems of convergence difficulties.One of the key features of this approach is that the hydraulic fracture can propagate along an edge between any two neighboring triangular bulk elements,which is similar to the discrete element method(DEM).As it embraces the advantages of both the FEM and DEM,the Explicit-PPCZ method has proven to be an effective method in simulating the evolution of complex fracture network.A series of numerical cases were performed to validate the Explicit-PPCZ method.The agreement of numerical results and analytical results is very good,suggesting that the Explicit-PPCZ method is accurate for simulating hydraulic fracturing in naturally fractured reservoirs.Finally,the Explicit-PPCZ method was utilized to understand the mechanism of complex fracture network in naturally fractured reservoir.A series sensitivity analyses were performed by varying aforementioned parameters to investigate how sensitive the hydraulic fracture path are to the values of these parameters.The simulation results suggest that the path of hydraulic fracture in naturally fractured reservoir was heavily influenced by differential stress,fluid viscosity,friction coefficient,distribution of natural fractures.The numerical results also suggest that natural fracture of initial hydraulic aperture could significantly complicate the geometry of fracture network.