Research on casing damage mechanism and prevention and control under creep behavior of salt-gypsum layer

Abstract

As a good cap rock, the salt-gypsum layer often contains abundant oil and gas resources, but it has strong plastic flow and creep properties, resulting in frequent casing damage problems. Therefore, a new method is proposed to prevent and control casing failure in creep formation by using solid expandable tubular (SET). Based on the Heard model, the creep behavior of formation was described. The expansion experiment of the SET, the material tensile experiment of casing and SET with different expansion rates were carried out. The three-dimensional mechanical model and finite element model of casing-cement sheath- (SET) -formation were established. Numerical simulation of creep behavior of salt-gypsum layer under non-uniform in-situ stress was carried out. Comparative study of casing damage mechanism in two cases with or without SET prevention and control. The influence of sensitive factors such as SET wall thickness and expansion rate on the failure behavior of casing prevented and controlled by SET were analyzed. The results show that (1) In the direction of the maximum horizontal in-situ stress, the stress on the outer surface of the casing is large. In the direction of the minimum horizontal in-situ stress, the stress on the inner surface of the casing is the largest, and the material yield failure occurs first at this position of the casing. The compression deformation of casing is greater than the tensile deformation. The casing is oval after deformation. (2) By increasing steel grade and wall thickness of casing, the service time of the casing under creep behavior of salt-gypsum layer can be prolonged. However, while increasing cost and reducing diameter, it has not played a role in preventing casing failure. (3) By using the method of SET prevention and control, the casing stress is more evenly distributed, the maximum stress is effectively reduced, the stress growth rate is slowed down, and the external collapse resistance ability is significantly enhanced. The maximum stress of the casing is reduced by up to 24.96 %. The larger the wall thickness of the SET, the better the prevention and control effect, and the longer the service time of the casing. The increase of the ultimate wall thickness of the SET with high expansion rate can effectively make up for its poor prevention and control effect. (4) The new method of adding the SET between salt-gypsum layer and cement sheath has the advantages of increasing casing inner diameter, optimizing wellbore structure, reducing cost and wellbore risk. The research results can provide some reference value for the prevention and control of casing failure in creep salt-gypsum layer.