Research On The Sealing Integrity Of Casing Cement Sheath For High Temperature And High Pressure Gas Wells

The seal integrity of casing and cement sheath is the weak link of the wellbore integrity.The seal integrity failure of casing and cement sheath is easily caused especially under demanding conditions of high temperature and pressure,which leads to security incidents such as the annular pressure and wellhead uplifting posing a severe threat on rockshaft safety and efficient production.This paper discusses the influence of the mechanical properties of the cement sheath interface on the sealing integrity of the casing cement sheath and the damage of the pressure and temperature to the sealing integrity of the cement sheath under the alternating cycle conditions,which is particularly important to improve the sealing property of the casing cement sheath and prolong the production life of the high temperature and high pressure well.At present,the research on the sealing integrity of casing cement sheath is mostly concentrated in the theoretical stage,which is done through the establishment of the corresponding mechanical model for theoretical and simulation analysis.As lack of corresponding laboratory research,the cement sheath interface micro-annulus generation and interfacial mechanical properties of the indoor full-size experimental study is almost blank.Therefore,based on the self-developed device,a new full size test and evaluation method of cement sheath seal integrity is proposed in view of the actual working conditions of high temperature and high pressure downhole,and a series of corresponding experiments are carried out,the specific researches are as follows:(1)For the purpose of basic mechanical parameters of two cement slurry systems to be tested and for later experimental analysis and model calculation,their compressive strength,modulus of elasticity and Poisson ratio under different compressive conditions were measured by uniaxial and triaxial cement stone compression test.(2)In the hope to estimate the size of the micro-annulus,a theoretical model based on Mohr-Coulomb yield criterion which considers the interaction among casing,cement sheath and formation is built.The plastic behavior of the cement sheath and the bonding strength at two interfaces are taken into account.(3)A new method for testing the sealing integrity of casing cement sheath is proposed,and the results of alternating internal pressure and alternating temperature on the sealing integrity of cement sheath are studied by using the self-developed cement sheath seal integrity test device,aiming at the change of downhole pressure and temperature respectively.6 groups of secondary alternating cycle pressure test experiments and 6 groups of secondary alternating cycle temperature test experiments were carried out.The relationship between mechanical properties,alternating internal pressure,alternating temperature and micro-annulus generation of cement sheath is compared and analyzed.(4)Using the principle of "pressure out method",the interfacial mechanical properties of the casing cement sheath after the action of alternating pressure and alternating temperature are tested,and the damage degree of the sealing integrity of the cement sheath of the two systems under different working conditions is evaluated and analyzed.The relationship between compressive strength,interfacial mechanical properties and sealing integrity of cement sheath is studied.