Burst Induced Stresses in Cemented Wellbores

Abstract

The industry acknowledges that there is an increase in the burst resistance of cemented pipe vs. uncemented pipe; but the effects of cement and formation mechanical properties, and insitu stresses are not well understood. This paper presents the results of a finite element study of the resistance of casing to internal burst pressure under a variety of conditions. This will provide for better design understanding of the stress conditions developed in casing under burst loading. Two-dimensional stress distribution model results are presented in graphical and tabular format for a variety of geometrical and mechanical material properties of formations, cement slurries, and casing combinations. A better understanding of the true stress profile in cemented pipe will allow for less expensive decisions concerning casing design parameters and safety factor criteria. Applications of using the burst resistance of the cemented pipe as a system as opposed to using the burst resistance of free pipe can include deeper drilling with thinner walled pipe, smaller rigs, and better casing integrity decisions for refracturing candidates. Introduction In casing design, the standard practice is to design the casing while ignoring the cement effects. This is depite the industry's acknowledgement that there is indeed a positive cement effect on the required strength of casing. A primary reason for this method of casing design is that here-to-fore, no method was available to determine the magnitude of this positve effect. This paper focuses a method for determining the magnitude of the stresses in the casing-cement-formation as a system and shows how cement does enhance the ability of a casing string to resist burst pressures. There is no standard casing design criteria for burst resistance. The American Petroleum Institute (API) does publish a bulletin for the formulae and calculations for casing properties (API Bulletin 5C3). This bulletin defines internal yield resistance as the lowest of the internal yield resistance of the pipe or the internal yield resistance of the coupling. API's burst pressure rating for the casing body is based upon Barlow's equation (Equation (1)), relying on the minimum yield stress of steel, the physical dimensions of the pipe and a minimum tolerance to calculate a burst pressure rating. Equation (1) The objective of this paper is to accurately model the cased borehole environment, and simulate the effects of realistically constraining the ballooning of cemented casing caused by internal burst pressure. A better understanding of these stresses acting upon casing and the surrounding cement sheath will help better quantify the risks so that more informed casing and cement job designs can be made. The risks associated with stimulation operations involving existing casing and high treating pressures will also be better understood. Finite Element Analysis To study the effects of constraining the expansion of casing due to internal burst pressures, the finite element analysis method (FEA) was chosen. The finite element analysis method of analysis is a numerical technique to obtain approximate solutions to partial differential equations (Reddy 1984). The method is applied to a system by spatially discretizing the system and solving the FEA mathematics simultaneously across the geometry. The resulting matrix of equations describes the physical interactions at specified points called nodes, based on the relevant material mechanical properties and the applied boundary conditions. Computer hardware and software has rapidly advanced to the point that complex modeling can realistically be accomplished with a desktop computer in a reasonable length of time. This allows practical analysis of problems in multiple dimensions. The method is not tied to any specific discipline, but can be applied to many types of problems. Thermal analysis, structural and fluid mechanics are a few of the many applications.

Effect of Dynamic Loading on Wellbore Leakage for the Wabamun Area CO2-Sequestration Project

“…Several numerical methods have been used previously to study wellbore-leakage problems in CO 2 -sequestration wells (Bois et al 2011;Jo and Gray 2010;Ferla et al 2009;Shahri et al 2005;Rodriguez et al 2003;Fleckenstein et al 2000;Thiercelin et al 1998). Most of the previous numerical approaches placed an emphasis on studying cement integrity when cement had been set and the mechanical and/or thermal loadings were applied to the model (Jo and Gray 2010;Ferla et al 2009;Shahri et al 2005;Rodriguez et al 2003;Fleckenstein et al 2000).…”

“…Most of the previous numerical approaches placed an emphasis on studying cement integrity when cement had been set and the mechanical and/or thermal loadings were applied to the model (Jo and Gray 2010;Ferla et al 2009;Shahri et al 2005;Rodriguez et al 2003;Fleckenstein et al 2000). One notable simulation effort, Bois et al (2011), presents a comprehensive mechanistic analysis to simulate cement-sheath microchannels while considering several configurations, including temperature and pressure changes, stress variations, and cement hydration.…”