Study On The Mechanism Of Casing Damage In Marine Formations In Northern Sichuan

The marine strata of north Sichuan is characterized by ultra deep(6000m),high temperature(150?),high pressure(110MPa)and containing gypsum creep formation.More than 20 wells production casing of the marine strata is damaged in this area,part of the gas wells have the appearance of casing deformation,and casing deformation up to 19.5mm,the casing deformation section mainly concentrated in fourth period of Jianglingjiang salt-gypsum layer.Part of gas well casing in marine strata has found the B annulus migration pressure caused by casing threaded leakage.Thus the problem of casing degree is significant in marine strata of north Sichuan.As part of the "twelfth five-year" national key research of oil and gas project topic 21-6(contract number:2011ZX05021-006),the analysis of this article has been made as follows:(1)According to Jianglingjiang formation casing deformation data of A4 well in northern Sichuan X block,established the model of the salt-gypsum layer casing deformation non-uniform load inversion.This paper which uses finite element optimization module calculated the maximum non-uniform load was 98.93 MPa and the minimum non-uniform load was 92.85 MPa.The result which satisfied fourth Jianglingjiang period,provides the basic data for casing deformation damage mechanism and special thread contact stress analysis.(2)The paper established the casing mechanics model aiming at salt-gypsum layer characteristics of fourth Jianglingjiang period in marine strata of north Sichuan,The stress state of 0177.8mm casing is calculated under current non-uniform load by programming calculated and finite element simulated,calculated the outside equivalent load of casing is 119.68 MPa,revealed the mechanism of casing deformation damage casing in well is unable to bear the non-uniform outside loading by salt-gypsum layer creeped.(3)Using ProE software established(?)168.28mm casing special thread connection three-dimensional model,simulated the contact pressure of special thread distribution under the effects of no extrusion load outside of wellbore,uniform extrusion load and the non-uniform load state underground by using the finite element.Through the comparison and analysis,under the effect of non-uniform load,compared with the uniform external extrusion load underground and no extrusion load outside the wellhead,the contact pressure of thread sealing cone was reduced respectively by 10.13%and 19.95%,the contact pressure of thread torque shoulder was reduced respectively by 9.06%and 16.70%.Under the effect of non-uniform in-situ stress underground,the probability of thread leak and abnormal annular pressure increases greatly.(4)Through the research of casing damage mechanism and special thread contact stress analysis in marine strata of north Sichuan,puts forward the solution and plan which suitable for the casing damage in marine strata of the region:1)Using the method of equivalent external collapse load to calculate the external extrusion stress of the casing under non-uniform loads,so as to carry out the casing strength design.2)By increasing the thickness and tightening torque of the casing thread coupling to prevent possible leakage of the casing thread and the abnormal pressure in the ring under non-uniform loads.The results of theoretical research and engineering measures have certain reference significance to casing design,material selection and field construction in marine strata of northern Sichuan.