Research On Prediction And Prevention Measures Of Trap Pressure In Deep Water Test Wells

Offshore deep water oil and gas resources are one of the important components of the global oil resources.With the continuous development of offshore oil and gas exploration technology,the development and utilization of oil and gas resources has gradually shifted from inland areas to deep-sea areas in China.During the deep water testing,the testing conditions of deep water wells are worse than those of onshore mining.The problem of annular trap pressure caused by the change of temperature and volume of fluid in closed-loop space is an important problem in the testing process,which is caused by the heat transfer from high temperature fluid in test string to outer casing.Foreign scholars on the prevention measures of annular trap pressure has been relatively mature,while domestic research is lagging behind,and a complete set of measures to alleviate and prevent trap pressure has not yet been formed.Therefore,it is of great significance to study the prediction of annular trap pressure and prevention measures in deep water test wells.According to the characteristics of multi-annuli in deep water test wellbore,a semisteady wellbore temperature field calculation model is established for subsection calculation.Considering the axis symmetry of deep water test wellbore,based on the basic theory of cylinder heat transfer model and energy conservation law,combined with steady heat transfer in wellbore and transient heat transfer from casing to formation,the convection heat transfer between fluid and casing in wellbore and cement ring are considered.Heat loss such as heat conduction,formation/seawater heat conduction and heat transfer is calculated,and the variation law of closed-loop air temperature is obtained.Based on the distribution of wellbore temperature field,a prediction model of multicasing annular trap pressure is established considering the variation of thermal expansion coefficient and allowable compressibility coefficient of fluid in trap annulus with temperature and pressure.According to the three methods of determining the thermal expansion coefficient and the allowable compression coefficient of the fluid in the closed loop space,three trap pressure prediction models are established.Taking the B trap annulus as an example,the calculation results are analyzed from the aspects of formation thermal gradient,well depth,test output and test time.The results show that the annulus trap pressure model established in this paper is more accurate.The effects of formation thermal gradient,test output,test time and well depth on trap annulus temperature and annulus trap pressure of each annulus are analyzed.The results show that the trap annulus temperature and the annulus trap pressure of the wellbore increase linearly with the increase of the geothermal gradient.With the increase of the well depth,the temperature and pressure of each trap annulus also increase gradually.The annular trap pressure increases mainly in the initial stage of production,and tends to be gentle with the increase of production time.With the increase of test production,the temperature and pressure of each trap annulus will also increase,but the rate of increase gradually slows down.The prevention measures of trap pressure were investigated,and the working principle,advantages and disadvantages of each pressure relief tool were analyzed and compared.A trap annulus pressure relief tool was designed.The working threshold of the trap annulus pressure relief tool was determined from the wellhead device pressure,casing string strength and packer pressure difference,and the finite element strength analysis of the trap annulus pressure relief tool is carried out to meet the design requirements.The results can provide guidance for the prevention and control of trap pressure.