Study On Bottom Hole Dynamic Pressure,reservoir Dynamic Stress And Fracture Propagation Law Of Variable Flow Hydraulic Fracturing

Hydraulic fracturing,as the main stimulation measure for increasing production and injection in oil and gas fields,plays an important role in improving the permeability of reservoirs and increasing the output of oil and gas wells.Traditional hydraulic fracturing generally adopts constant flow injection mode,which faces problems such as excessive operation pressure,single fracture morphology and poor stimulation effect in field applications.In contrast,variable flow hydraulic fracturing can effectively solve the above shortcomings.Although laboratory experiments and field tests have been carried out for this technology,its mechanism cannot be systematically explained at present.Its theoretical research needs to be carried out urgently.Therefore,a series of models are established,including the simulation model of fluid pressure transfer law in fracturing tubing under variable flow injection,the simulation model of reservoir stress response law under excitation of fluctuating fluid pressure,and the simulation model of fluid-solid-fatigue coupling hydraulic fracturing.On the basis of the above models,a coupled simulation model of fluid flow in the tubing and hydraulic fracture propagation is established.The accuracy of the simulation model is verified through laboratory experiments and field tests.Based on the unsteady flow theory,the simulation models of fluid pressure transfer law in fracturing tubing under the conditions of bottomhole non-seepage boundary and seepage boundary are established respectively.The influence of bottomhole non-seepage boundary and seepage boundary on the model simulation results is analyzed.The effect of variable flow injection scheme on the fluctuating pressure of the bottom hole fluid is studied.The simulation results show that it is necessary to consider the effect of bottom hole seepage for the reservoirs with permeability greater than 1m D.Moreover,the variable flow injection scheme can form significant fluid fluctuation pressure in the bottom hole.The mathematical models of dynamic stress of homogeneous continuous reservoir and porous media reservoir under the excitation of fluctuating fluid pressure are established respectively.Based on the finite difference method and analytical method,numerical calculation models of dynamic stress are established.The applicable conditions of different models are compared.Furthermore,the influence of fluid pressure fluctuation parameters on the response characteristics of reservoir dynamic stress is analyzed.The simulation results show that it is necessary to use the porous medium model to analyze the reservoir stress for high Biot coefficient or high permeability reservoirs.Dynamic stress will be generated in the reservoir under the excitation of fluctuating fluid pressure.Considering the fatigue damage effect of fluctuating fluid pressure on the reservoir,a parallel bond fatigue damage model based on dissipative energy is established.It is embedded in the particle flow code software.The software is redeveloped to establish a fluid-solid-fatigue coupling hydraulic fracturing simulation model.The simulation results show that compared with the conventional constant flow injection scheme,the variable flow injection scheme can effectively reduce the breakdown pressure and improve the complexity of the fractures after fracturing.Based on the axial flow model of fluid in the tubing and the fluid-solid-fatigue coupling model of the reservoir,a coupling mathematical model of fluid flow in the tubing and hydraulic fractures propagation is established.Moreover,the simulation algorithm problem of the coupling mathematical model is solved.The effects of different injection schemes on the fluid pressure in the tubing and the propagation law of hydraulic fractures are analyzed.The simulation results show that the maximum operation pressure can be effectively reduced by a single rectangular pulse variable flow injection scheme before the hydraulic fracture initiation.In the stage of hydraulic fracture propagation,the continuous rectangular pulse variable flow injection scheme can improve the complexity of fractures.Based on the GCTS RTX-3000 test system,hydraulic fracturing experiment is carried out in the laboratory.The crack initiation and propagation in rock samples are monitored by acoustic emission technology.It is proved that the effect of fluctuating fluid pressure on the fatigue damage of rock samples.According to the field test data,the accuracy of the coupling simulation model of fluid flow in the tubing and hydraulic fractures propagation is verified.The application effect of variable flow hydraulic fracturing is proved.