Translated Abstract
Heavy oil is one of the important oil and gas resources in China. And the main process is cyclic steam huff and puff. Due to the large variation of wellbore temperature and the axial displacement of casing string, cumulative damage is generated in casing operation, resulting in a large number of failures. The traditional design and evaluation method of casing based on stress cannot meet the requirements of casing service condition, and the design and evaluation method of casing string based on strain is being developed internationally. In the thermal cycle of middle temperature range, the creep, fatigue behavior and damage mechanism of the casing have not been systematically studied wtich seriously restricts the development of strain based design and evaluation of casing string theory.
In this paper, multi factor low cycle fatigue behavior, steady creep behavior and damage mechanism of 80SH and N80Q steels of 80 steel grade casing material in middle temperature range (from 300℃ to 450℃) are systematically studied under variable temperature and load conditions in thermal recovery wells. A long life prediction model of casing material under working conditions of thermal recovery wells is established, which provides a strong theoretical basis for service model.
The regularity of creep steady state rate of 80SH and N80Q steel under medium temperature range are revealed. The results show that the creep behavior of 80SH and N80Q steel under medium temperature and high stress conditions is obvious. And the steady-state creep rate presents an exponential relationship with the increase of stress amplitude. The steady-state creep rate of 80SH steel is significantly smaller than that of N80Q steel which presents good creep resistance. In the process of creep, the dislocation structure at the grain boundary of N80Q steel is stacking, and the dislocation substructure evolves into a network subgrain structure in steady state. Dislocation structure of 80SH steel evolves from carbide precipitates cut by slip dislocation to stacking fault and micro twin. The good creep resistance of 80SH steel is due to the solid solution strengthening caused by alloying elements, as well as the interaction of multi-scale M23C6 and MX type precipitates.
The low cycle fatigue behavior of 80SH and N80Q steel at room temperature was studied. The results show that the peak stress response of 80SH and N80Q steel shows the characteristics of first hardening and softening, and there is a significant mean stress relaxation behavior. And the strain life model of asymmetrical low cycle fatigue under the influence of mean strain was proposed. The strain life model of asymmetric low cycle fatigue under the influence of mean strain is related to the maximum value of absolute value of strain, and shows a linear relationship in the double logarithmic coordinate system. The fatigue life of 80SH and N80Q steel affected by strain amplitude is consistent with Manson-Coffin model.
The low cycle fatigue behavior of 80SH and N80Q steels in middle temperature range has been studied and revealed. And the revised Manson-Coffin model of low cycle fatigue life and strain under the influence of dynamic strain aging (DSA) was proposed. In medium temperature range, there is a significant DSA effect on low cycle fatigue of 80SH and N80Q steel, resulting in the secondary hardening phenomenon of stress response curve. The hardening exponent characterizing secondary hardening is proposed. The two factors of temperature and strain both have a significant effect for the hardening exponent which increased with the increasing temperature. When the strain is low, the material exhibits a significant hardening phenomenon and the hardening exponent has a maximum value. Because of the DSA effect, the fatigue life is significantly different from room temperature. The ratio of elastic strain to total strain increases due to medium temperature, which affects plastic damage accumulation and makes the low cycle fatigue life obviously changed. When the DSA is not significant, the fatigue life and elastic strain show a linear relationship in the double logarithmic coordinate system. When DSA has a significant influence on the elastic strain, the fatigue life and the elastic strain show a bilinear relationship in the double logarithmic coordinate system.
The effect of dynamic strain aging on the evolution of dislocation substructures during low cycle fatigue at middle temperature range was observed. Under the influence of temperature, the low cycle fatigue behavior of 80SH and N80Q steels changed significantly due to the influence of DSA. The DSA effect on dislocation evolution is observed for the first time in the low cycle fatigue process by observing the microscopic substructure of TEM in different cycles of low cycle fatigue. The mechanism of DSA effect on dislocation substructure evolution is explained. And the effect of DSA on the low cycle fatigue properties of high temperature provides a new idea for the study of fatigue resistance of materials, which is from lath substructure to a smaller cell substructure at high temperature. The analysis shows that the reason for the secondary hardening is mainly because of DSA which affects the evolution of dislocations.Fracture observation indicates that the new dislocation structure promotes the crack propagation resistance of the material.
The correlation between creep and low cycle fatigue of 80SH and N80Q steels is revealed. The effect of short-term creep on low cycle fatigue is mainly the influence of creep variables, and the effect of short-term creep on low cycle fatigue life is equivalent to the mean strain for 80SH and N80Q steel. Then the effect of low cycle fatigue on creep is mainly in the early stage of low cycle fatigue (Nf＜0.4). Low cycle fatigue loading increases the time of creep into the second stage which reduces the total creep strain of the short-term creep to a certain extent, and the degree of influence decreases with the increase of low cycle fatigue cycles.
The service safety factors and life prediction criteria of 80kSi steel casing material under steam thermal recovery conditions are studied and put forward. The research shows that the strain limit and low cycle fatigue life are the two core issues of service life. Life prediction need to satisfy two criteria. The first is the strain criterion, the strain limit should be lower than the total strain during the long service life. The second is the low cycle fatigue criterion, which satisfies the expectation of low cycle fatigue life under the three conditions of strain, mean strain and temperature.
Translated Keyword
[Energy storage behavior, Lead-free ferroelectric ceramics, Relaxor behavior, Strontium barium niobate, Tungsten bronze structure]
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