Unique Test Process Resolves Junction Difficulties in a Multilateral Completion on the North Sea Statfjord Field
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Stability of highly inclined non-circular wellbores in isotropic formations
The shear and tensile stabilities of highly inclined non-circular wellbores are investigated in this study. Using the equivalent-ellipse hypothesis, the non-circular geometry was approximated as an ellipse, and the corresponding stress concentration equations are presented. With the new set of stress concentration equations, a comprehensive study of the tensile and shear stabilities of an elliptical borehole was conducted, including the impact of well inclination and azimuthal angles, horizontal stress difference, degree of ellipticity, and orientation of the maximum horizontal stress to the major axis of the ellipse. Using five commonly used shear failure criteria, we observed that both Mohr–Coulomb and Drucker Prager (inscribed) failure criteria predicted higher collapse pressures, relative to the others including Drucker Prager (inscribed), Mogi-Coulomb, and Modified Lade. While Drucker Prager's (circumscribed) failure criterion underestimates the collapse pressure. Both the linear elastic and poroelastic models were used in investigating the fracture initiation orientation and pressure of highly inclined elliptical boreholes. The prediction from the poroelastic model is always less than the linear elastic model. In some instances, they predict different fracture initiation orientations. From this study, we observed that generally, a near-circular wellbore is more stable than elliptical borehole in both shear and tension. Nevertheless, there are some well inclination and azimuthal angles than can make an elliptical borehole have more shear and tensile stabilities than a near-circular wellbore.
Stability of highly inclined non-circular wellbores in isotropic formations
The shear and tensile stabilities of highly inclined non-circular wellbores are investigated in this study. Using the equivalent-ellipse hypothesis, the non-circular geometry was approximated as an ellipse, and the corresponding stress concentration equations are presented. With the new set of stress concentration equations, a comprehensive study of the tensile and shear stabilities of an elliptical borehole was conducted, including the impact of well inclination and azimuthal angles, horizontal stress difference, degree of ellipticity, and orientation of the maximum horizontal stress to the major axis of the ellipse. Using five commonly used shear failure criteria, we observed that both Mohr–Coulomb and Drucker Prager (inscribed) failure criteria predicted higher collapse pressures, relative to the others including Drucker Prager (inscribed), Mogi-Coulomb, and Modified Lade. While Drucker Prager's (circumscribed) failure criterion underestimates the collapse pressure. Both the linear elastic and poroelastic models were used in investigating the fracture initiation orientation and pressure of highly inclined elliptical boreholes. The prediction from the poroelastic model is always less than the linear elastic model. In some instances, they predict different fracture initiation orientations. From this study, we observed that generally, a near-circular wellbore is more stable than elliptical borehole in both shear and tension. Nevertheless, there are some well inclination and azimuthal angles than can make an elliptical borehole have more shear and tensile stabilities than a near-circular wellbore.
In this study, we formulate the Finite Element model (FEM) of the stress-strain state for a three-dimensional (3D) borehole section with a sidetracked hole in fluid saturated elastoplastic formation. We then investigate the dependence of the safe mud weight range on the build angle while considering the plastic rock deformation behavior. The objective here is to showcase the unique drilling recommendations that can be generated for the sidetrack initiation or junction point, especially when compared to those of the mother-bore. 3D meshes are developed to analyze sidetracking scenarios in terms of orientation and initiation points with respect to the mother-bore. We utilize a 3D poro-elastoplastic model to take into account the pore pressure, using the concept of Biot-Terzaghi effective stresses. The Galerkin weak formulation is used to obtain the equations in discrete form. A robust solution of the static problem for elastoplastic body with stress boundary conditions is ensured by adding the Lagrange multipliers to set the constrains on rotation and translation of the solid as a rigid body. We utilize the Newton-Rapson method to get the solution of the equilibrium equation by providing the non-linear iterations. We demonstrate the ability to determine a safe mud weight range by computing several scenarios for in-situ stresses in a sandstone formation. The scenarios consider changes in the borehole orientation angles and the junction or initiation point. The scenarios also analyze the dependence of the stress-strain state near the sidetracked borehole section on the existing mother-bore. The shape and thickness of the plastic zone strongly depends on the orientation of the borehole and applied pore pressure distribution. Utilizing the elastoplastic model, we implement the procedure to find the optimal build angle for the design of sidetracking sections considering the hole orientation and geometry. The developed code for poro-elastoplastic formations is realistic alternative to conventional geomechanics models that fail to consider the influence of the mother-bore on the drilling conditions of the sidetrack.
Placement of the multilateral junction is one of the most critical elements in the design of a multilateral well. A misplaced or mal-performing junction can have an adverse effect on an otherwise successful multilateral project. The optimum placement of the junction is a function of reservoir, geological, drilling, and completion criteria. The list may include such criteria as reservoir development strategy, pressures & heterogeneity, geological structure, lithology, wellbore stability, multilateral junction integrity, directional drilling requirements, logging requirements, artificial lift design, and completion type. These combined factors have a direct economic impact on the success and viability of the project. This paper describes a process whereby the proper junction selection criteria can be evaluated in light of pertinent technical and economic criteria and examines lessons learned from case histories with junction problems. Introduction A screening analysis is generally performed prior to having all of the data, offset information, and production information. Consequently, the purpose of this paper is to provide some Rules of Thumb meant primarily to narrow the focus area. Several assumptions are made in the generation of these Rules of Thumb:The beds are assumed to be laminar and horizontal for the base caseDogleg Severity (DLS) is a function of build rate and turn rate. In this paper, the 2D trajectory, where the DLS turn rate component is zero, is considered the base case.Correlations with respect to DLS and equipment capabilities are given with the assumption that the casing exit is from the vertical well.Directional tools, crossflow, formation damage, stimulation, and cleanup are design factors in all multilateral levels. However, discussion of optimal junction placement is primarily focused on TAML Levels 3–6. Levels 1 and 2 have junctions that are typically installed in the same horizontal plane in carbonates or very consolidated sands, where placement is not dependent on a correlation between the type of multilateral system and wellbore geometry. Conventional Well Design Criteria Many of the aspects that are discussed in this paper are not unique to multilateral wells. A cost/benefit analysis will always be performed prior to undertaking any oil field operation; therefore, many of the parameters mentioned are not pertinent just to multilateral well design alone. Issues such as placement of the wellbore in the reservoir, reservoir exposure length, completion and production types are all relevant to conventional well design. The "Morphing" of a Multilateral Well Multilateral wells metamorphose away from the process of conventional well design by creating more options. Instead of one reservoir entry point, it is possible to have two or more. Instead of one set of reservoir conditions, there may be two or more (including fluid types). Target geometries become more convoluted and 3-dimensional, and differences in approach and mindset come to bear with multilateral wells (Figure 1.). Issues Unique to Multilateral Wells The placement and construction of the multilateral junction are the key design differentiators between conventional and multilateral wells, and are the most critical elements in a successful multilateral well project.
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