fax 01-972-952-9435. AbstractRemoval of water and hydrocarbon liquids from gas wells is increasingly recognized as an important topic for mature gas reservoirs. Accumulation of these liquids in the bottom of a gas well is often referred to as liquid loading. Liquid loading limits current productivity of 90% of the natural gas wells in the USA. Liquid loading first appears in the casing below the end of tubing (EOT). One way to reduce loading below the EOT is to install dead-end production tubing to the bottom of the perforations and force the gas to flow from the perforations through the tubing-casing annulus up to a cross-over connection near or above the top of the perforated interval. We conducted tests in a flow loop to evaluate such flow.The primary objective of our tests was to determine the critical flow rates for two phase flow through tubingcasing annulus using two different tubing sizes (2.88-inch-OD and 3.50-inch-OD) in a 4.00-inch-ID casing. Secondary objectives were to develop a method to predict critical flow rate, to identify the flow regimes that exist at the critical flow rate, and to evaluate the mode of liquid transport.For gas-water flow in vertical tubing, the Turner-Hubbard-Dukler (1969) prediction for critical flow rate (without the 20% correction) is very close to what we observe in our flow loop. However, the critical rates for flow in the tubing-casing annulus were found to be 20 to 50% less than predicted by multiplying the Turner-Hubbard-Dukler (THD) critical velocity and the annular cross-sectional area. It was observed from the tests that two types of flow regimes could occur at the critical flow rate: annular flow regime and transitional annular flow regime.In flow through an annulus, the film thickness on the casing wall is larger than the film thickness on the tubing wall. Theoretical analysis for one-phase flow shows that the maximum velocity in tubing-casing flow is closer to the tubing. We believe that this observation also applies to two-phase flow and that the higher velocity near the tubing pushes liquid toward the casing, which results in the observation of thicker liquid films on the casing wall.
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.
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.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThis paper presents an algorithm to optimize the production and shut-in periods of plunger-lift based on reservoir performance. The algorithm combines analytical models of reservoir and wellbore fluid flow. The implementation of the algorithm requires a relatively simple and inexpensive electronic control system. The new system may be connected to existing wellheads with minimal modification. One of the advantages of the new algorithm is the ability to automatically incorporate the changes in the line pressure. An example application is presented to indicate that the new algorithm may increase plunger-lift production by a factor of two.
An educational model exists at the Colorado School of Mines (CSM) that responds to current industry objectives, incorporates the SPE Professional Competency Matrices 1 (hereinafter, Competency Matrices) and satisfies the Accreditation Board of Engineering and Technology (ABET) 2 requirements. GEGN/GPGN/PEGN439 Multidisciplinary Petroleum Design (hereinafter, 439 Capstone Course) is the cross-discipline, senior-level, capstone design course in the Geological Engineering, Geophysical Engineering and Petroleum Engineering Departments. Historically, the 439 Capstone Course has met ABET requirements and industry objectives by providing students with multidisciplinary problem solving and integrated team experience prior to entering the work force. Teamwork experiences focused on the integration of data, information, and people from the disciplines of geology, geophysics, and petroleum engineering. However, the need exists to move beyond "integration" with a step forward to "implementation." Using the Competency Matrices as a guideline and responding to industry feedback, the revised 439 Capstone Course is a novel educational approach that accomplishes this goal.• Recent shifts in petroleum industry emphasis with focus on unconventional reservoirs and new technologies necessitated incorporating these elements in the 439 Capstone Course.• Elements of the instruction phase including brainstorming, group decision-making, and conflict resolution are now introduced in freshmen and sophomore level courses.• Students are now asked to work in teams, to write technical papers and to present them in a variety of underclassmen courses.Thus, the need existed to move the 439 Capstone Course forward, maintaining a focus on multidisciplinary integration, but with a goal of incorporating the latest industry trends and technologies.
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