Experimental Study On Mechanical Damage Mechanism And Material Modification Of High Pressure Gas Well Cement Ring

Cement sheaths are a key part of borehole integrity in oil and gas wells and have a direct influence on the life cycle of oil and gas wells. Especially for the high pressure gas well, the role of the cement sheaths is more outstanding, once cement sheaths sealing ability in failure, which led to happen the phenomenon of gas migration, not only affect the gas well cementing quality, but also threaten the safety of gas well production. Therefore, to carry out cement sheaths the study of failure mechanism is imperative. Current research about the integrity of cement sheaths is mainly through the establishment combination model of casing, cement sheaths and surrounding rock, combining with the finite element software to carry on the theoretical derivation or design simple simulator, but for the cement sheaths in the actual complex of downhole environment (formation temperature, pressure, casing pressure),how to bearing failure, which is few research.Therefore, established is suitable for the high pressure gas well cement sheaths the integrity of mechanical model, designed cement sheaths damage mechanics simulator, explored cement sheaths failure mechanisms, and the material mechanics experiment of cement sheaths, to improve the high pressure gas well cement annulus packer ability, effectively guarantee the integrity of wellbore has very important significance.High pressure gas well is presented in this paper based on the analysis of geological characteristics and engineering difficulties, considering the actual condition of borehole, the initial stress, casing pressure and reservoir stress by the stress superposition, establishing the combination model, and designing simple simulator, which is developed and used to simulate downhole environment, including formation temperature, pressure, the change of casing pressure, to create an bearing process equivalent to that of the actual cement sheath under changing working conditions, combined with the model to design an equivalent physical experiment of cement sheaths, to explore cement sheath failure mechanisms.Based on equivalent physical experiment, to explore cement sheath failure mechanisms, selecting a Well, cement sheaths as experiment object in high and low stress environment. A study on simulation experiment using the simulator, and according to the CT scan and micro experimental phenomena and results of the scan, to explore cement sheaths failure mechanism under different stress environment. Based on the failure mechanisms, which were selected for four kinds of high elastic modulus, high toughness, high stress intensity and high ability of fracture toughening materials, to improve the mechanical properties of the cement sheaths, reached the requirements of cement sheaths effectively the packer, efficient for high pressure gas well production development and provide a strong guarantee. In this paper, on the basis of theoretical research and experimental research, form the conclusion is as follows:(1) In this paper, in combination with the practical situation of borehole, considering the actual condition of borehole, the initial stress, casing pressure and reservoir stress by the stress superposition, establishing the combination model, and based on the principle of geometric similarity and stress equivalence, a well bore simulator was designed to simulate the actual casing program, down-hole temperature and pressure environments, continuous changes in the internal casing pressure.(2) From the perspective of theory combining with practice of thought, the integrity model of cement sheaths combined with the simulator, to design an equivalent physical experiment of cement sheaths, to create a bearing process equivalent to that of the actual cement sheath under changing working conditions in order to explore cement sheath failure mechanisms.(3) According to the simulator test and CT scan found that under the condition of low stress environment, ensile fractures under low confinement pressures. Under the condition of high stress environment, a reduction in its pore volume and total volume and the appearance of a micro-annulus on its cementation plane with the casing. Based on the results of this experiment, it can be concluded that there were two primary reasons for the bearing failure of the cement sheath:on the one hand, tensile fractures under low confinement pressures. On the other hand, the separation of the interface under high confinement pressures, the microscopic mechanism of which was that the pore volume was reduced and that the skeleton particles of the interface were crushed or migrated under load. The study provides significant guidance that will be useful for guaranteeing cement sheath integrity in the future.(4) Based on cement sheaths failure mechanisms under the high pressure gas well in different stress conditions, selecting for four kinds of toughening materials. Under low stress environment:polyvinyl alcohol fiber and polyester fiber; High stress environment:Carbon fiber and calcium carbonate whisker. Under low stress environment plus the amount of 0.2% polyester fiber cement modified effect is best, high stress environment added amount to 0.2% of the carbon fiber cement modified effect is best, and through the equivalent physical experiment evaluation, the integrity of cement sheaths was guaranteed in the same experimental conditions.