Study On Variation Of Shut In Wellbore Pressure After Gas Invasion In Fractured Formation

With the deepening of exploration and development of oil and gas resources,the conventional oil and gas resources are less and less,and the exploration and development of oil and gas is gradually moving towards complex strata.In the actual drilling process,fractured formation is often encountered,and gas invasion occurs from time to time.Improper treatment will lead to blowout and blowout out of control accidents.Compared with the conventional porous formation,the coupling law of multiphase flow between fractured formation and wellbore is more complex.The establishment of multiphase flow model and accurate solution is an important demand for wellbore pressure control.In this paper,the formation wellbore wellhead is regarded as a whole system,and the wellbore pressure variation law is studied when gas invasion occurs in fractured formation and after well shut in,which provides a theoretical basis for the realization of wellbore pressure safety control.This paper mainly studies the gravity displacement gas cut and negative pressure gas cut.The governing equation of gas-liquid two-phase flow in the fracture during gas cut by gravity displacement is established and solved by finite difference method.The variation of bottom hole pressure and gas influx in the process of gravity displacement and negative pressure gas influx was simulated by numerical simulation method.The coupling model of reservoir wellbore multiphase flow is established and solved by numerical method.The wellbore pressure model of fractured formation after gas invasion and shut in is established,and the shut in pressure build-up curve is obtained.For the formation,wellbore and wellhead,the following conclusions are obtainedIn the cyclic state,the calculation and analysis show that when the gravity displacement gas invasion occurs in the formation,the larger the bottom hole positive pressure difference,the lower the fracture height or the smaller the fracture width,the occurrence of gravity displacement gas invasion will be more inhibited,resulting in only leakage phenomenon in the fracture.The numerical simulation shows that when gravity displacement gas invasion occurs,the gas invasion rate rises sharply at the initial time,then falls back quickly,and tends to be stable when it decreases to a lower value.The numerical simulation shows that for negative pressure gas invasion,when the bottom hole negative pressure difference increases or the fracture length,height,width and permeability increase,the gas invasion speed will accelerate and the gas invasion amount will increase.In the process of gas invasion by gravity displacement,the bottom hole pressure drops slowly and the mud pit increment is small;With the decrease of bottom hole pressure,the gravity displacement gas cut changes into negative pressure gas cut,and the change of bottom hole pressure and mud pit increment will appear obvious turning point.When negative pressure gas invasion occurs,the larger the fracture length,height,width or permeability,the faster the bottom hole pressure drops,the more mud pit increment,the higher the gas invasion rate and the greater the wellbore gas content.During the period of gravity displacement gas invasion,the bottom hole pressure increases linearly and slowly after shut in.After shut in with negative pressure gas invasion,the bottom hole pressure first increases rapidly with time,and then the growth rate slows down.In the case of negative pressure gas invasion,the increase of fracture size will lead to the increase of gas invasion rate during overflow,the decrease rate of gas invasion rate and the rise rate of riser pressure after shut in,and the time for balancing bottom hole pressure and formation pressure will be shortened.The calculation results of shut in vertical pressure,casing pressure and mud pit increment model are in good agreement with the actual well data,which verifies the reliability of the established mathematical model.