Geomechanical and Decision Analyses for Mitigating Compaction-Related Casing Damage

Abstract

Summary Reservoir compaction and its associated bedding plane slip and overburden shear have induced damage to hundreds of wells in oil and gas fields throughout the world. Critical casing damage mechanisms observed in a variety of structural settings include overburden shear damage on localized horizontal planes, shearing at the top of production and injection intervals, and compression and buckling damage within the production interval, primarily around perforations. Analytical solutions are readily available to estimate compaction, subsidence, and casing damage risks. These should be applied as initial screening tools at an early stage in reservoir development planning. They also can be applied to estimate relative risks for various well locations and trajectories. Geomechanical models of increasing complexity, including 2D and 3D finite element type techniques, have been used with good success to assess formation deformation and casing damage risks in several reservoirs and are described herein. Three-dimensional geomechanical models at the wellbore scale are required to evaluate shearing deformation on specific well designs and are used to assess damage mitigation effectiveness for varying completion strategies. An economic decision tree model is applied to compare the costs and benefits of alternative well designs while taking into account inherent uncertainties in model input data, the well's damage location, and the effectiveness of various mitigation strategies. In some instances, the appropriate action is not to change the completion design, but simply to accept damage risk. Introduction Significant subsidence and casing damage have occurred in several fields throughout the world, including the North Sea, Gulf of Mexico, California, Canada, South America, and southeast Asia.1–10 Problems can be particularly acute in deep offshore operations, where individual well costs often exceed U.S. $10 million and specific wells often target individual sand formations or fault blocks. Therefore, the loss of even one or two wells may significantly impact recoverable reserves for the field. Compaction-related casing damage can include compression and buckling, localized shearing deformation, tension damage, and even distortion damage to internal completion assemblies, such as prepacked screens. The appropriate mitigation strategy depends on the most likely location of casing damage, the expected type of damage, and the magnitude. This requires combining geomechanical analysis of deformations in the reservoir, overburden, and casing- cement formation assembly with quantitative decision analyses that compare the costs of various mitigation strategies to the economic benefit of reducing damage risk, given the inherent uncertainty and variability in reservoir deformation, damage type and location, and mitigation effectiveness. This paper describes casing damage observations worldwide and geomechanical analysis techniques applied to evaluate casing damage mitigation strategies. We further describe a quantitative decision-analysis process to estimate the economic value of various completion designs to mitigate casing damage. Compaction and Subsidence Overview The weight of sediments above an oil-and-gas-bearing geologic formation is partially supported by the rock matrix and by the pressurized fluid or gas within the rock pore space. When fluid pressure is reduced, more of the load is transferred to the rock matrix, and the pressure-depleted formation compacts slightly. Subsurface compaction, if significant or if the formation is relatively shallow, can produce measurable surface subsidence. Formation compaction can induce compression and buckling damage within the producing interval. More importantly, significant formation compaction also induces small-scale slip on bedding planes and faults within the reservoir and overburden material, causing severe shear damage to wells. Formation Compaction. The effective stress on a porous material is equal to the external stress applied to the material minus the internal pore pressure. For geologic formations, the vertical external stress is the weight of overburden material, while the lateral stress depends on the tectonic setting. External stresses generally remain constant, while the internal pore pressure declines during fluid withdrawal or increases with fluid injection. A change in effective stress leads to volume compression, which is equal to the change in pore pressure times the compressibility property for the material. A reservoir formation subject to pore pressure change compresses uniformly or uniaxially depending on the loading pattern and geometry. A general change in bulk volume, V, is related to the bulk compressibility, Cb, and pore pressure change, ?p, through the following expression.Equation 1 For most oil and gas formations, however, the lateral dimensions of the formation are large relative to the thickness. Furthermore, while the surface above the formation is completely free to displace downward, lateral deformation is constrained by adjacent material. Therefore, most of the compression associated with pressure decline in relatively thin, flat-lying geologic formations occurs in the vertical direction. The magnitude of this vertical compaction or pressure-induced change in formation thickness, ?H, can be estimated by the following equation.Equation 2 in which H=the original formation thickness, Cm=the uniaxial compaction coefficient for the material, and ?p=the change in pore pressure. For elastic and isotropic materials and assuming grain compressibility is small relative to bulk compressibility, the uniaxial compaction coefficient is related to the bulk compressibility through the following expression.Equation 3 in which ?=Poisson's ratio for the material, ?=the bulk density, and Vc=the compressional wave velocity for the formation material.

Real-Time Fiber-Optic Casing Imager

“…Similar to Equation 6, the axial location of each sensor along the tubular, n z , can be calculated from its spatial location along the fiber optic cable, n S : θ tan n n S z = (7) Analysing the shift in wavelength of laser light reflected on Bragg-gratings that are attached to the tubular, Equations 4 to 7 allow characterizing any bend or buckle of the tubular with respect to its location and orientation, as well as the determination of the corresponding axial strain the tubular has experienced.…”

“…3 Historically, direct measurement of compaction effects in the reservoir have been made by installing radioactive tags on the casing or by placing radioactive bullets in the formation. 7,8 More recently, reservoir changes have been analysed using time-lapse (4D) seismic techniques. More recently, acoustic imaging tools and high-resolution multi-finger calipers have been available and have significantly improved the awareness of well tubular deformations.…”

Fluid-driven fractures in granular materials

“…Low mechanical strength, high pore pressures and large leakoff of fluid often make stimulation operations more difficult and less efficient. The problems of reservoir compaction and the associated bedding plane slip have caused severe damage to hundreds of wells around the world [16][17]. Additionally, the created fractures are not always planar, and their geometry and dimensions may become very difficult to predict, as shown in Fig.…”

Well Integrity Evaluation for Methane Hydrate Production in the Deepwater Nankai Trough

“…Casing integrity, or more generally, well integrity, has been extensively evaluated previously (El-sayed and Khalaf, 1992;Hamilton et al, 1992;Fredrich et al, 1996;Hilbert et al, 1996;Wagg et al, 1999;Fleckenstein et al, 2000;Bruno, 2002;Willson et al, 2003). Previous evaluations show that compaction and subsidence can trigger excessive casing deformation and that can eventually cause the casing to fail and result in loss of the wellbore.…”