Research On Casing String Integrity In Offshore Heavy Oil Thermal Recovery Horizontal Wells

Offshore heavy oil reserves are abundant in China,but due to the constraints of special offshore environment and platform space,the difficulty of thermal recovery is far greater than that of onshore heavy oil fields.Wellbore integrity easily fails due to the sharp rise of operating temperature in the process of thermal recovery,seriously restricting the development of heavy oil resources.Focusing on the integrity of casing string in offshore heavy oil thermal recovery horizontal wells,a systematic study was carried out in this paper mainly on the prediction of wellbore temperature and pressure distribution,annular pressure buildup(APB),wellhead growth and casing thermal buckling mechanism.The main achievements are as follows:Considering the physical property change,friction effect,gravity effect and heat absorption and release effect caused by phase change of multi-component thermal fluid,the wellbore heat transfer and pressure prediction model of multi-component thermal fluid stimulation in offshore heavy oil thermal recovery horizontal wells was established.According to the characteristics of heat injection,soaking and production stage,a prediction method of wellbore temperature and pressure suitable for the whole period production of thermal recovery well was established with the finite difference method,and a case verification was carried out.On this basis,the sensitivity analysis of wellbore thermal loss and its influencing factors were further conducted.Based on the P-V-T relationship and the equation of state of fluid,the mechanism of annulus pressure buildup was analyzed.Considering the nonlinear change of fluid properties,annular volume change in the cementing section and the open hole section,and the multi-annulus coupling effect,an integral prediction model of coupling annular pressure buildup for multiple annuli was established.A multi-annulus pressure buildup simulation experimental setup was independently designed and manufactured and laboratory experiments of multi-annulus pressure buildup were carried out to verify the validity of the theoretical model.Based on an actual well,a sensitivity analysis of the influencing factors of multi-annulus pressure buildup was carried out.The debonding mechanism of casing and cement sheath under high temperature in heavy oil thermal recovery well was analyzed theoretically,and the calculation model of debonding length of casing and cement sheath was obtained.Considering the friction resistance of cement sheath after debonding and the multi-casing coupling effect,the prediction method of wellhead growth was further established.The simulation experimental setups for casing and cement sheath debonding load and wellhead growth were designed and manufactured for casing and cement sheath debonding load test under cyclic heating and wellhead growth laboratory experiments.The theoretical model was verified with experimental results and case study.The sensitivity analysis of influencing factors of wellhead growth was carried out.Considering casing cyclic stress and strain characteristic,kinematic hardening characteristics and low cycle fatigue characteristics,numerical simulation method of casing thermal buckling under borehole constraint conditions was established.The casing thermal buckling and thermal fatigue characteristics under multi-cycle injection-soakingproduction were analyzed.The casing thermal stress and strain distribution and residual stress and strain changing with the temperature alternating cycles were obtained.At the same time,sensitivity analysis of influencing factors of casing thermal buckling and thermal fatigue was carried out.Based on casing safety evaluation,the management methods of annular pressure buildup in heavy oil thermal recovery wells were summarized.At the same time,the application conditions,implementation effects and limitations of prevention and control measures for annular pressure buildup and casing damage in thermal recovery wells were analyzed by comprehensive application of research achievements.