Diverting Mechanism And Its Law Of Temporary Plugging In Hydraulic Fracturing

Reservoir stimulation is the core technology of unconventional oil and gas development such as shale gas,tight oil and gas reservoirs.The goal of hydraulic fracturing construction is to achieve SRV(stimulated reservoir volume)fracturing through the formation of complex fracture network.When the reservoir has poor fracability or large stress difference,it is difficult to form a complex fracture network.In such cases,it is important to enhance the complexity of fracture network propagation by temporary plugging to force the crack to divert.Although the field practice has achieved good results,the mechanical mechanism of fracture by temporary plugging,crack propagation law and regulation methods are still in the exploratory stage.Therefore,there is an urgent necessity to carry out the study of diverting mechanism and its law of temporary plugging in hydraulic fracturing.This thesis explores a new experimental method and develops the numerical calculation method of hydraulic fracturing.The fracability and diverting ability of unconventional reservoir,temporary plugging rule of fracture on diverter,and crack propagation law of diverting fracturing are studied using the core test and physical and numerical simulation.The main results of this PhD thesis are as follows:(1)Test and Evaluation of Fracability of Fractures in Tight Reservoirs.The ability of the reservoir to form fracture network(fracability)is the basis and important influencing factor of crack diversion.It is found that:(1)Shale has a strong microscopic heterogeneity,which affects the crack diversion capacity of shale reservoirs together with the mineral composition,natural fissures and TOC(Total Organic Carbon)content.(2)The fluid has a significant effect on the mechanical properties of shale,and it is related to the low water saturation,clay content,TOC and micro-nano pores of shale reservoir.(3)A new method for comprehensive evaluation of fracture diversion capacity in tight reservoirs is established.This method is based on the matrix brittleness,natural fissure density and AE(acoustic emission)activity and it is feasible for field application.(2)Experimental Study on the Mechanism and Regularity of Fracture Diversion by True Triaxial Simulation.Based on the experimental device of true triaxial hydraulic fracturing simulation,the diverting extension law of fractures is studied in the fiber-assisted temporary plugging fracturing.The main control factors of fracture diversion are: the fracability of reservoir and its heterogeneity,the horizontal principal stress difference,the natural fracture distribution,the primary crack width,the fiber concentration,the viscosity and the flow rate.The condition and the pattern of temporary plugging are obtained,and the critical stress difference is given when fracture diversion occurs.An evaluation model of fracture diverting ability is established,in which the main parameters are the inclination angle of artificial crack,in-situ stress difference,fracability and flow pressure in the fracture.(3)A crack propagation model of hydraulic fracturing with PGD(Proper Generalized Decomposition)method.This method is established under full coupling conditions.The presented formulation can predict crack diversion and network extension in hydraulic fracturing.PGD alorithm is suitable in this type of problem,which is non-linear,transient and coupled.Moreover,the computational cost is less than the traditional FEM(finite element method).(4)Numerical simulation of crack diversion is carried out by using PGD algorithm.The simulation results agree with good agreement the true triaxial physics simulation.The main factors of fracture diversion are the reservoir properties,the horizontal principal stress difference,the interference among fractures,the temporary plugging efficiency of fracture,the viscosity and the flow rate.Under the condition of low stress difference and shorter fracture space,the interference between the cracks is strong and the crack tip is more likely to turn.The shear slip of natural fracture has remarkable influence on the crack propagation path.The crack is easier to divert in the effect of shear stress and flow pressure.In this case,the crack width and net pressure are abruptly changed,i.e.the fracture opening is becoming narrower and the net pressure is becoming lower.The results of this thesis will provide theoretical basis and technical guidance for diverting fracturing of unconventional oil and gas.