Research On Wellbore Integrity Control Of Staged-fracturing Horizontal Well In Tight Oil Reservoir

The exploitation practice of tight oil indicates that the combination of horizontal well and multi-stage fracturing is the key to a successful commercial exploitation of tight oil,and that both the Open Hole Multi-Stage(OHMS)method and the Plug&Perf(P&P)method are the most popular hydraulic fracturing technologies.However,wellbore integrity will be destroyed during the processes both of drilling or hydraulic fracturing of horizontal well in tight oil reservoir,which not only limits the selection of completion methods,but also affects the performance of horizontal well after multi-stage fracturing.In order to improve the performance of multi-stage fracturing horizontal well in tight oil reservoirs,a theoretical study for controlling the wellbore integrity of horizontal well during drilling and hydraulic fracturing,including OHMS and P&P methods,is conducted in this paper.The main research contents and research results are as follows:(1)According to thermo-chemical poroelastic constitutive equation,the thermal diffusion equation,the solute diffusion equation and the fluid continuity equation,the pore pressure transfer equation considering thermo-chemical-fluid coupling effects is estabilished.Both the effect of thermal filtration on solute concentration and the effect of thermal osmosis on fluid volume are considered in this transfer equation.Solving the transfer equation,the analytical expressions of pore pressure field and stress distribution around the wellbore are obtained.The comparation of pore pressure with the results of finite difference method shows that the derived analytical expression of pore pressure field is correct.Compared with the solution of the pore pressure field in the Laplace domain,the analytical solution does not need to be inverted by Stehfest method,which is convenient for application.Based on the analytic expression of stress distribution around wellbore,a model is established to predict the failure degree of horizontal wellbore during drilling process.The analysis results of this model show that shortening drilling time,increasing the internal wellbore pressure,using a drilling fluid with low temperature and high solute concentration,and optimizing drilling direction can reduce the failure degree of wellbore and improve the integrity of the horizontal wellbore drilled in tight reservoirs.Considering that an accurate in-situ stress is crucial for predicting wellbore integrity,a model and method for calculating the maximum horizontal stress using failure information from the wellbore of tight reservoirs are developed.Using the method,a C#program is developed to calculate the maximum horizontal stress,and the calculation result is consistent with the hypothetical data.(2)The failure mechanism of horizontal well during hydraulic fracturing using the OHMS method is analyzed.Considering that the squeezing effect of hydraulic fracture may affect the integrity of open-hole wellbore,the calculation model of the stress distribution in the vicinity of hydraulic fracture is deduced using complex function.Taking into account the effects of the induced stress of hydraulic fractures and the temperature difference between fracturing fluid and formation,a model is established to analyze the wellbore integrity of horizontal well during hydraulic fracturing using the OHMS method.The influences of packer location,fracture net pressure and fracturing fluid temperature on the shear failure and tensile failure of wellbore are simulated.The analysis results indicate that the shear failure degree of non-fractured section will increase with the half-length of hydraulic fractures,and that the induced stress caused by the hydraulic fractures will lead to the closing of the tensile cracks on the wellbore wall.Optimizing the location of open-hole packer can make the interval isolation more effective during hydraulic fracturing.Reasonable wellbore pressure and net pressure of hydraulic fracture will help to avoid the failure of open-hole wellbore.(3)Considering the stress induced by hydraulic fractures and the thermal stress,a model is established to analyze the stress distribution of casing-cement sheath-formation system and the integrity of the cement sheath during hydraulic fracturing using the P&P method.The influences of plug location,fracturing fluid temperature,cement stone mechanical properties and casing thickness on the integrity of cement sheath are simulated.The results show that,if there is no initial effective stress in the cement sheath,the cement sheath is prone to tensile failure and is not prone to shear failure during hydraulic fracturing.Decreasing the distance between plug and hydraulic fracture,the induced stress will restrain the development of the radial tensile cracks or promote the closure of radial tensile cracks in cement sheath at the location of plug.Reduce the temperature difference between fracturing fluid and formation,the cement sheath is less prone to shear failure and tensile failure.Using cenment slurry with low Young's modulus,high Poisson's ratio and tensile strength,and increasing the thickness of casing will contribute to the integrity of cement sheath during hydraulic fracturing.Using the core test data and drilling history information of the Baikouquan formation in the northern slope of the Mahu tight oil reservoir,several integrity control schemes of horizontal wellbore during drilling and multi-stage fracturing are put forward.We find that horizontal well MH-XX5H did not surfer shear failure because it was drilled in the direction of the minimum horizontal stress and with drilling fluid density higher than 1.27 g/cm3.During the process of hydraulic fracturing of horizontal well MH-XX5H using the OHMS method,the open-hole wellbore kept integrity because the fracture net pressure is not higher than 12.5 MPa and the wellbore pressure is not higher than 89.5 MPa.The mechanical properties of cement slurry are so excellent to enable the horizontal well MH-XX6H to avoid tensile failure of cement sheath during the process of hydraulic fracturing using the P&P method.The fracturing and drilling effects of horizontal wells indicates that the proposed control schemes can ensure the integrity of horizontal wells of tight oil reservoirs in this area.This paper conducted an analysis and put forward control methods for the wellbore integrity of horizontal well during the processes both of drilling and hydraulic fracturing in tight reservoirs,which have a certain theoretical guidance for avoiding the fracturing fluid channeling caused by the wellbore failure and for improving the effect of multi-stage fracturing of horizontal well in tight reservoirs.