Study On Mechanism Of And Prevention And Control Measures Against Casing Failure Of Heavy Oil Thermal Recovery Wells In Gudong Oil Field

Casing failure is a great problem in the process of developing a heavy oil reservoir in unconsolidated sandstone. Firstly, casing failure leads to the reduction in the degree of reserves recovery and the deterioration in development effect due to intensified planar diversity, thus presenting a great threat to the stable oil production and oil recovery enhancement. Secondly, casing failure results in huge economic losses, and annually, the state has to invest heavily in drilling some new wells for this reason and repairing casing-damaged wells. This means that casing failure restricts increase in economic benefits of oil field development. Gudong Oil Field is a typical unconsolidated sandstone reservoir of heavy oil. Each year, its casing-damaged wells amount to 40 or so, the drilled new wells amount to 30 or so, and the total cost incurred due to this exceeds 100 million Yuan. A large amount of data and study shows that, casing failure is caused mainly by one or more geological, engineering, and/or corrosion factors. Many factors including underground construction, steel quality, sand control method, perforation mode, etc. involved in drilling to production may induce casing failure. In a thermal recovery environment based on steam injection, there are more complex factors behind casing failure. So, from the practical perspective, to study the mechanism of casing failure and take appropriate prevention and control measures to reduce the number of casing failure cases and to extend the lifetimes of the casings in use is of the utmost importance to ensuring the oil field's stable oil production and increasing its economic benefits.Experts at home and abroad have carried out a large amount of work in the aspect of study on casing failure mechanism of heavy oil fields. The study on the single-field casing failure mechanism is currently mature; however, the multi-field casing failure mechanism of heavy oil thermal recovery wells has yet to be well understood. On the basis of analysis of a large amount of on-site casing failure data, the paper presents a systematic study of factors behind casing failure of thermal recovery wells based on steam injection in Gudong Oil Field's No.9 District as well as an in-depth study of the casing failure mechanism of thermal recovery wells, and furthermore, puts forward pertinent suggestions as to casing failure prevention and control. The paper is mainly based on study contents as follows:(1) the thermal-hydrological-mechanical coupling theory for heavy oil reservoirs, which involves comprehensive knowledge in terms of thermodynamics, permeation fluid mechanics, and elastic-plastic mechanics, is used to make a mathematical model for heavy oil reservoirs, which involves three interactive fields:temperature field, permeation field, and stress field; (2) No.9 district's steam flooding well batteries are taken as examples to establish the comprehensive criteria for the casing failure under thermal stress, and the numerical simulation method is adopted to study how casing failure takes place in thermal recovery wells under different portfolios of steam injection parameters; (3) on the basis of the results of the above numerical simulation, fine numerical simulation of individual wells is carried out to study how the heat insulation measures affect near-wellbore temperature and reservoir stress distributions; (4) the impact of changes in downhole steam temperature on casing string stresses is studied in regard to different near-wellbore temperature and reservoir stress distributions; (5) on the basis of the residual stress creation mechanism, a theoretical model is made to calculate reservoir stresses and study how residual stress affects casing condition in the multicycle steam injection process; (6) the casing failure mechanism of thermal recovery wells is found out according to the simulation calculation results above, and in addition, concrete prevention and control measures against casing failure are put forward for the thermal recovery wells based on steam injection according to the oil field's on-site production conditions.