Wellbore Integrity Analysis Of Complex Deep Wells Based On The Reliability Theory

The research of wellbore system integrity has been given increasing importance in recent years,following large accidents like the Macondo blow-out that damaged the deepwater horizon rig,killed eleven people and caused a large oil spill in the Gulf of Mexico.Deep wells have so long well lengths that they may be dilled through many different formation types,such as salt gypsum beds,salt beds,sparse sand strata,tight sandstone,and high temperature and high pressure(HTHP)beds.It is not easy to get wellbore system stress distributions by theory method under the influence of many factors.But some stress theory solutions that apply to some particular problems can be gotten by making a few logical simplifying assumptions.Since well construction materials are prone to degradation with age and upon exposure to down-hole fluids,pressures and temperature variations,the number of well integrity problems tends to increase as the wells age.It means that mechanical parameters of well construction materials have much probability of random distribution.In order to solve these problems,some work must be done.Based on the reliability theory,this paper establishes an evaluation method of the wellbore system integrity,gets an optimization method of well construction materials parameters that influent the wellbore system integrity by multi-objective optimization genetic algorithm,and writes a software of wellbore system integrity to serve the construction scene.The main conclusions are listed as below:(1)Based on the theory of elastic mechanics,the stress theory solutions of the wellbore system that contains single-layer casing-cement sheath-formation and multi-layers casing-cement sheath-formation are derived,and the theory solutions of the temperature stress of the wellbore system are also derived.The three-dimensional(3D)stress distributions in the different radius from the wellbore center are drawn by software written by VB language.The influence rules of many material parameters are studied.The 3D stress field distributions of inclined wellbore system are derived by some reasonable plane assumptions.(2)Through comparative analysis of the different failure criterions of casing,cement sheath,formation and cementation surface,and revising the criterion parameters in consideration of the complex deep wells environment,the most suitable failure criterions are found.And the integrity evaluation method of wellbore system based on the failure of multiple weak layers is established.The failure factor of the weak layer is defined,and the curves of failure factor influenced by different influencing factors are gotten.(3)By analyzing the influencing factors of complex deep wellbore system,it is found that the influencing factors are subject to a certain probability distribution.Through analysis of the probability distributions of influence factors,the characteristics of the probability distribution are received.The equations of the probability distribution,the values of the average and the variance are obtained by mathematical fitting.(4)Through the analysis of field data of drilling,the wellbore systems can be divided into two types: one is the regular wellbore system,and the other one is the irregular wellbore system.In view of the probability distributions of the influence factors of the wellbore system and study of all kinds of reliability analysis methods,the integrity assessment theories of the regular and irregular wellbore systems are established based on the theory of reliability.(5)The integrity evaluation method of the wellbore system is founded by the integrity evaluation theories established by the theory of reliability.An optimization design method of material parameters of the wellbore system is gotten by using the genetic algorithm of multi-objective optimization.And the wellbore integrity evaluation software is built,which has some main modules such as numerical borehole trajectory modular,ground stress calculation and analysis of stress field around the wellbore modular,crustal stress analysis of the layered rock modular and the evaluation of wellbore system integrity modular.