| At present,the development of offshore oil and gas resources has become the world oil industry development trend,and the deep-sea oil and gas development still have many technical problems to be overcome,the wellbore integrity of gas hydrates formation is one of the technical problems need to overcome.Based on this,this paper studies the stability of casing and wellbore integrity in the wellbore with the problem of instability of gas hydrate formation.The safety of the wellbore is discussed by orthogonal test method.This paper mainly completed the following work:(1)Based on the theory of multi layer thick wall cylinder,the mechanical model of casing-cement-formation in gas hydrate formation is established.The numerical analysis of the casing stress in the wellbore is carried out.By comparing with the existing experimental results,it is proved that the numerical model is feasible.(2)Based on the validated numerical model,the temperature field and stress field of the complete wellbore in gas hydrate formation are calculated.The variation of casing stress with casing wall thickness is obtained by numerical calculation.(3)Under the condition that the pore pressure and temperature are not coupled,the influence of the shape of the cement ring(fan-shaped,rectangular and semi-circular)on the casing stress is analyzed.The influence of the defect depth,the defect size and the number of defects on the casing stress is analyzed.The wellbore temperature field and casing stress in the three cases of the absence of fluid or water or wet soil in the circumferential defect of cement ring are calculated.The influence of the fluid properties of the circumferential defect area on the wellbore temperature field and casing stress is studied.(4)A numerical model for the coupling of pore pressure and temperature in gas hydrate formation is established.The stress field of casing is analyzed and compared with the case of no coupling.At the same time,the influence of drilling fluid temperature and the heat transfer characteristics of cement material on the casing stress is studied.In addition,two cases of water and no water in the circumferential defect of cement ring were analyzed.(5)Using the method of orthogonal test analysis,the sensitivity of the factors affecting the stress field of casing is analyzed.Based on the MATLAB computing platform,the quantitative expression function of casing stress is calculated.Design-Expert is used to analyze the corresponding cloud images.Finally,the safe combination of various factors under the limit condition is obtained.It can provide theoretical guidance for the safe use of casing in field construction.This paper provides a method of gas hydrate formation in wellbore integrity assessment and development of deep-sea oil and gas well,thus reducing the development cost of deep-sea oil and gas well;provide a theoretical basis for the safety evaluation of casing.It is of guiding significance for the prevention of casing damage in gas hydrate formation. |
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