Effect Of Thermal Expansion Annulus Pressure On Cement Sheath Integrity In Deep Water Gas Wells

During the testing and production process of deep water gas wells,due to the flow of high-temperature fluid in the wellbore,the closed-loop hollow fluid between the free-sleeve casings of each layer is heated and expanded,causing the annulus pressure to rise,so that the seal integrity of the deep-water gas well cement ring is faced.There is a huge challenge,so it is necessary to carry out related research on the stress and failure of cement ring cementing surface by annulus pressure,and provide more scientific basis for the sealing integrity of cement ring.Based on the research work on the integrity of cement ring seal under the influence of deep water wellbore temperature field,annulus pressure and temperature and pressure,the relationship between the annulus pressure and temperature and the thermal expansion pressure of the annulus on the first and second cemented surfaces of the cement ring were explored.The effects of stress and failure are as follows: the thermal expansion pressure of the annulus will increase the tangential tensile stress of the cemented joint surface of the cement ring,and the excessive tangential tensile stress will cause the cement ring to fail.At the same time,the tangential tensile stress generated by the increase in wellbore temperature also increases the risk of cement ring failure.The greater the thermal expansion pressure of the annulus,the lower the safety factor of the cement ring,and the effect of the temperature effect on the safety factor is small.The simulation experiment of two-ring empty trap pressure in deep water wells was carried out.The effects of cycle temperature,circulation flow and circulation time on the annulus trap pressure were studied.The results show that the annulus pressure has a nonlinear relationship with the temperature difference,and the annulus pressure The experimental results differ greatly from the theoretical results.After discussion and analysis,in the field experiment,the casing annulus liquid may contain a small amountof gas,or the gas is evolved during the process of continuous circulation heating,resulting in a sudden drop in the annulus pressure.Based on a gas well data from the Liwan gas field in the South China Sea,the influence of temperature and pressure changes on the integrity of the cement ring was analyzed.The results show that when the annulus pressure is 40.00 MPa and the temperature changes by 30 °C,the tangential tensile stress of the cement ring-casing cemented surface Has reached 4.06 MPa,exceeding the tensile strength of the cement ring of 3.9 MPa,at this time the cement ring tensile damage.When the annulus pressure is less than 30.00 MPa,the wellbore temperature change has a significant impact on the safety factor of the cement ring.As the wellbore temperature increases,the safety factor of the cement ring gradually decreases.