Collapse phenomenon behavior has been broadly studied in the petroleum industry but few changes have been implemented in this subject. Since the early 1960's when the American Petroleum Institute (API) published the Recommended Practice 5C3 for casing design equations under burst, collapse and tension, these equations have remained unchanged until the present. These equations ignore the effects of the cement sheath on collapse resistance and assume uniform collapse loading of the casing. Incorporating the effects of a cemented wellbore improves collapse casing design. The study presented in this paper describes the effects on the collapse loading conditions of the mechanical properties of cement and rock formation surrounding the casing strings. In order to investigate these effects, the Finite Element Analysis technique was used. Three separate cases are studied: unsupported casing; casing with a cement sheath bonded outside of it; and casing, cement sheath, and consideration of the surrounded rock formation. For all of them, the same constant value of stress was applied at the outer boundary of each model. According to the results yielded by this study, improvements of up to 62% in casing collapse resistance can be achieved, depending on the mechanical properties of the cement systems and rock formation properties. Introduction Casing strings serve one of the most important functions in a well. Casing isolates the wellbore fluids from the subsurface fluids and formation fluids. Casing also provides a high-strength flow conduit to the surface, permits safe control of formation pressure, and prevents collapse of the borehole during drilling. Traditionally, casing string design has been accomplished in accordance with the string type and function. Generally, there are four types of casing strings: conductor, surface, intermediate drilling, and production casing. For each one, a combination of loads is applied to the pipe body in order to select the best tubular characteristic that agrees with burst, collapse, tension/compression, and triaxial resistance. The traditional procedure takes into account the worst isolated load for burst, collapse and tension. A relatively new methodology, presented by Klementich and Jellison in 1983 (1), treats each drilling, completion and production event as a load case, and depending on the situation and severity, could be considered as burst, collapse, or a combination of both, with the presence of tension or compression. This last methodology is known as Service Life Model. Studies have shown that cemented pipe under burst loads have better performance than uncemented casing. Fleckenstein, et al. (2) demonstrated in their studies that a casing string constrained with a hard cement sheath supports 58.4% less triaxial stress than an uncemented pipe for the 5–1/2", 23# casing studied. The collapse of steel pipe from external pressure is a much more complex phenomenon than pipe burst from internal pressure. A simplified free-body diagram analysis does not lead to useful results; however, a more complex, classical elasticity could be used to establish the radial stress and tangential hoop stress in the pipe wall. For both collapse and burst conditions, the resultant induced tangential stress will be the maximum (Bourgoyne et al. 1991) (3). The main objective of this paper is to model several collapse loading conditions for different cement and rock formation mechanical properties in order to simulate the stress transmissibility through different media in the wellbore. This will allow an estimate of the resultant induced stresses in the casing under a variety of collapse loading conditions.

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The United States National Science Foundation, engaging 29 researchers at nine institutions, has funded a Sustainability Research Network (SRN) focused on natural gas development. The mission of this Sustainability Research Network is to provide a logical, science-based framework for evaluating the environmental, economic, and social trade-offs between development of natural gas resources and protection of water and air resources and to convey the results of these evaluations to the public in a way that improves the development of policies and regulations governing natural gas and oil development.Currently, there are a wide range of estimates of the probability of shallow aquifer contamination. There are a series of independent events that must occur to allow hydrocarbon migration and estimates were made of these probabilities. An analysis of data from drilling in the Wattenberg field, CO was made to quantify the probability of contamination.It has been determined that there are five events that must each independently happen to allow the migration of fracturing fluids, and there are three events that must occur independently for the migration of hydrocarbons. The lower number of independent events, which must arise for hydrocarbon migration to occur, explains the infrequent, but well publicized natural gas migrations in poorly constructed wellbores, and the lack of such publicized events of hydraulic fracturing fluid contamination, which was confirmed by our analysis.The significance of these results is to help quantify the risks associated with natural gas development, as related to the contamination of surface aquifers. These results will help shape the discussion of the risks of natural gas development and will assist in identifying areas of improved well construction and hydraulic fracturing practices to minimize risk.

Summary This paper presents the results of a finite element study of the resistance to burst pressure. Results from the 2D model quantify the effects of various mechanical properties of cement on a cemented wellbore. Comparison of the predicted stresses with experimental results demonstrated that ductile cement is far less likely to crack radially from high internal burst pressures than a brittle cement. It is demonstrated that the in-situ formation stresses acting on the cemented wellbore greatly affect the burst resistance of the cemented wellbore. 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 in-situ 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. 2D 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 allows for less expensive decisions concerning casing design parameters and safety-factor criteria. Applications 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, standard practice is to design the casing while ignoring the cement effects, despite the industry's acknowledgment that there is a positive cement effect on the required strength of casing. A primary reason for this method of casing design is that previously, no method was available to determine the magnitude of this positive effect. This paper focuses on a method for determining the magnitude of the stresses in the casing, cement, and formation as a system and shows how cement enhances the ability of a casing string to resist burst pressures. There are no standard casing design criteria for burst resistance. The American Petroleum Institute (API) publishes a bulletin1 of the formulas and calculations for casing properties that defines internal yield resistance as the lower 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 on Barlow's equation, 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 model accurately 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 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 be better understood. Finite Element Analysis To study the effects of constraining the expansion of casing caused by internal burst pressures, the finite element analysis (FEA) method was chosen. The FEA method of analysis is a numerical technique to obtain approximate solutions to partial differential equations.2 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 advanced rapidly to the point that complex modeling can 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 and structural and fluid mechanics are a few of the many applications. The FEA method has the advantages of versatility and general applicability. Various shapes and sizes of objects can be described mathematically, and interactions between those objects can be solved. Irregular shapes can be approximated, allowing shapes with ill-defined boundaries to be analyzed. Several different materials with separate mechanical properties can be modeled easily. In FEA, a continuous physical system is discretized into a series of finite elements. These elements are composed of a series of nodes at specified intervals. At the location of these nodes in structural mechanics, deflections and stresses are calculated. A series of equation matrices are solved, allowing each node to affect the deflection and stress at each other node. As the mesh becomes finer in this analysis, the increments between nodes become smaller, increasing the size and complexity of the system of matrices that must be solved. A larger number of nodes increases the number of calculations necessary to solve the system of equations.3 FEA Assumptions 3D analysis is the most intuitive method for analysis. However, it is also computationally complex and prone to errors if the boundary conditions are not applied correctly. 2D and 1D models are less computationally intense and less prone to application error. However, they can lead to misunderstanding of the solution except under certain conditions. There are three cases when 2D analysis is appropriate for the elastic analysis of solids: plane strain, plane stress, and axisymmetry. These problems are simplifications of 3D elasticity problems under the following assumptions.Body forces, if any, cannot vary in the direction of the body thickness.Applied boundary forces do not have axial components, and the forces must be uniformly distributed across the thickness.Loads may not be applied across the parallel planes bounding the top and bottom surfaces.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractConventional collapse design is based upon uniform external pressure loading. Conventional design considers pore pressure and does not account for the effects on casing stresses from the external cement sheath and surrounding formation. In addition, the conventional design does not predict the collapse load of non-uniform loaded casing that arises from imperfect cement jobs and formation voids. This paper presents the results of a finite element study of casing subjected to external, non-uniform loading. These loads include the effects of cement channels and voids, surrounding formation voids, and pore pressure decline in those voids. A better understanding of these common downhole conditions will help to better understand the resultant stress fields and enable a more accurate determination of the necessary collapse strength of casing.