Research On Mechanism And Characteristics Of Sustained Casing Pressure In Gas Well

China’s oil and gas resources are abundant.With the development of oil and gas exploration,the depth of gas wells is also deepening.However,with the increase of depth,these high temperature and high pressure gas wells generally have sustained casing pressure in the production process.Some of these gas wells,their tube-casing annulus and the casing annulus both performed annular pressure phenomenon.Sustained Casing Pressure（SCP）usually refers to the phenomenon that the gas well annulus pressure returns to the pressure level before the pressure release in a short time after the pressure is released.Downhole gas is continuously accumulated in the annulus of top well,which causes annulus pressure.Meanwhile,the corrosive gases such as CO₂and H₂S in natural gas will corrode the strings,which weaken the strength of the strings,and directly threaten the integrity of the gas wellbore.All of these will affect the safe production of gas wells.In addition,the SCP will adversely affect the subsequent operations such as acidification and fracturing of the gas well.When the SCP is not obvious,the pressure monitoring and pressure relief operations will increase the production cost;in severe cases,the shut-in or even the entire well will be scrapped.In order to reduce the occurrence of SCP in gas wells,ensure the safe production of existing SCP gas wells and reduce the management costs of annulus pressure,it is necessary to study the mechanism,influencing factors and related characteristics of the SCP of gas wells.Based on a large number of literatures,this paper has carried out research on SCP in tube-casing annular and casing annular,then an annulus pressure management method has been developed.For SCP in tube-casing annulus,based on the principle of pressure balance,the depth of the leaking point is determined by the temperature and pressure field of the gas wellbore,and the gas leakage equation of the well is solved by the method of simplification and equivalent calculation combined with the leak depth.Then studied the process of gas generation in the form of bubbles,migration,and finally gathering at the wellhead.Taking the wellhead column as the research object,combined with the mass conservation equation and the gas PVT equation,the mathematical prediction model of SCP in tube-casing annulus is established.For SCP in casing annulus,by studying the seepage law of gas in casing cement sheath,the continuity equation and motion equation of gas seepage are established,and the seepage flow rate of gas in cement sheath is solved by the Darcy flow law.Based on the gas percolation velocity rate and the gas PVT equation of the wellhead,a mathematical prediction model of SCP in casing annulus is established.According to the well structure and related fluid data of the gas well on site,combined with the established mathematical prediction models of SCP in tube-casing annulus and casing annulus,the case calculation is carried out separately.Compare the measured pressure data with the calculated results,the results show that the calculated values of the annulus pressure are consistent with the field measured data,which verifies the accuracy of the model.Sensitivity analysis of relevant parameters affecting SCP in tube-casing annulus and casing annulus using established mathematical models.Considering the safety of high-pressure gas well annulus corresponding to all well barrier components under different working conditions,a calculation method for the maximum allowable pressure and minimum reserve pressure of annulus is developed.Then,a set of high pressure gas well annulus pressure management chart was designed.The prediction model of SCP in tube-casing annulus and casing annulus established in this paper has important theoretical value.The research content is of great significance for the diagnostic evaluation of SCP and the control of annulus pressure.