Theoretical And Experimental Research On Pre-Expansion Cementing For Preventing Thermal Recovery Well Casing From Failure

Heavy oil is an important constituent of hydrocarbon resources. In China, it takes up 20% to 25% of the total with proven reserves of over 1.3 billion tons. Technologies of heavy oil recovery include steam drive, steam stimulation, in-situ combustion, etc., and steam stimulation is adopted the mostly in China. However, after several rounds of steam injection, casing failure often occurs, which causes huge economic losses. Based on analysis of current situations of casing failure and preventive measures at home and abroad, this paper carries out a comprehensive study of pre-expansion cementing technology in preventing casing failure in thermal recovery wells.The paper first introduces methods of heavy oil recovery and the wellbore configuration of steam injection thermal recovery wells. Based on analysis of the stress and damage of casing, it is found that the main cause of casing failure of steam injection thermal recovery wells is continuous thermal stress on the section at oil reservoir. Therefore, the research model of present study is simplified as a combination of casing at oil reservoir section, cement sheath and the formation.By analyzing the interaction model of the casing and the formation with strata stress in an initial state, respective formula for the external pressure on casing is derived under conditions of rigid formation, elastic formation and creep formation. Based on the thick cylinder plane stress theory, a mechanical model of stress calculation is established for casing in the state of free expansion during the cementing process. By adopting the thick cylinder plane strain theory, the study specifically establishes a mechanical model for stress calculation of casing during the production process of steam injection thermal recovery wells. According to the compatibility equation of displacement between casing, cement sheath and the formation, calculating methods are elaborated for the external pressure on the casing in different formations. The study also figures out the effective stress intensity of casing during the production process and analyzes the mechanical mechanism of casing failure in thermal recovery wells by Mises yield criterion.Based on aforementioned work, the study improves the theoretical model of pre-expansion cementing. It also establishes mechanical models of stress calculation on the casing at different stages and under different formation conditions with pre-expansion cementing technology. With these models the mechanical mechanism of casing protection is further analyzed. Apart from theoretical analysis, by adopting data from actual oil wells, the study also calculates respectively the stress and state of casing during the whole production process with conventional cementing, half pre-heating cementing with different temperatures, and pre-pressurization cementing with different stress. The results help in deriving appropriate ranges of pre-heating temperature and pre-pressurization stress for prevention of casing failure.Finally, to verify the rationality of the theory, an indoor simulation of pre-expansion cementing experimental platform is designed according to the simplified research model and requirements of pre-expansion cementing theory. Performance tests of the cement slurry and the experimental Al pipe are carried out as well as corresponsive simulated production experiments including conventional cementing, half pre-heating cementing, pre-pressurization cementing and steam injection.