| According to the statistics, since the21st century exploitable reserve of the world conventional crude oil is evaluated at1272×108m3; heavy oil and extra heavy oil’s exploitable reserve is approximately1510×108m3, which exceeds the former. Because the crude oil has entered mid-late period, heavy oil resource working as an important supplementary energy has become the chasing goal for countries all over the world. Steam injection method conducts as a main means for present heavy oil recovery. It can greatly enhance the heavy oil recovery ratio, while leading to very serious casing damage, which presents the fast growing tendency. Many factors, i.e. erosion caused by the hot steam, creep rupture, CO2, H2S, electrochemical environment and corrosive substances, can induce casing to corrode and bring about the local reduction of casing thickness. And worse still, these will result in the effective strength of casing decreasing, the casing life shortening and even the whole well scrapping. Hence studying the casing residual strength and remaining lifetime under corrosion condition is of vital importance to the safe operation of casing in service, as well as providing reliable theoretical basis for periodic maintenance of casing.This topic research mainly involves the elastic-plastic mechanics, finite element simulation and many other disciplines, applies the way of combining the theoretical analysis and numerical calculation, from the perspective of three corrosion morphologies, i.e. uniform corrosion, local corrosion and pitting. By the way of building a three dimensional model of thermo-structural coupling on corrosive casing system to research the residual strength law of N80, TP90H and P110under a single corrosion parameter and loading. Based on conclusions of the former approaches, birth and death element method is put forward to simulate the process of casing corrosion by establishing a mathematical model of remaining life on casing corrosion, in order to assess the service life of corrosive casing.The main contents are shown in the following aspects:(1) This thesis studies the casing damage mechanism of thermal recovery wells and research work of casing residual strength and remaining life at home and abroad, and collects the casing research background and the related progress under steam injection wells at home and abroad.(2) The failure mechanism of casing corrosion in thermal recovery wells is researched, and simultaneously the three basic forms caused casing corrosion failure are analyzed, namely uniform corrosion, local corrosion and pitting, ultimately establishing casing failure criterion.(3) Considering the casing special working conditions such as high temperature and high pressure during the heavy oil exploitation, the thermo-structural coupling theory of casing is studied. Through the establishment of the thermo-structural coupling model of casing corrosion system, the residual strength rules of N80, TP90H and P110casing under different defect parameters and loads are summarized by large amount of simulation calculations, many useful conclusions are drawn by carrying on the detailed analysis of the calculation results, i.e. corrosion depth for three different types casing residual strength is the primary factor, the influence of the defect length will have little impact on the local corrosion casing residual strength, etc.(4) According to the characteristics of this topic research, birth and death element method is presented to predict the residual life of corrosive casing by establishing a mathematic model. Thus this method is verified possessing a high reliability and practical value in investigating the remaining life of casing by operating on Changqing Longdong Oilfield, which has extremely general corrosion taken place. Finally this new method is promoted to predict the casing lifetime under local corrosion in heavy oil steam injection wells. |
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