Casing Shear: Causes, Cases, Cures
- Maurice B. Dusseault (Porous Media Research Inst., U. of Waterloo) | Michael S. Bruno (Terralog Technologies Inc.) | John Barrera (Terralog Technologies Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 2001
- Document Type
- Journal Paper
- 98 - 107
- 2001. Society of Petroleum Engineers
- 3 Production and Well Operations, 5.1.2 Faults and Fracture Characterisation, 5.8.5 Oil Sand, Oil Shale, Bitumen, 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 1.2.3 Rock properties, 6.5.2 Water use, produced water discharge and disposal, 5.4.6 Thermal Methods, 5.3.4 Integration of geomechanics in models, 4.3.4 Scale, 2.2.2 Perforating, 5.8.7 Carbonate Reservoir, 5.9.2 Geothermal Resources, 5.3.9 Steam Assisted Gravity Drainage, 4.1.5 Processing Equipment, 1.14 Casing and Cementing, 4.2.3 Materials and Corrosion, 1.6 Drilling Operations, 1.10 Drilling Equipment, 1.14.1 Casing Design, 5.1.1 Exploration, Development, Structural Geology, 1.1.6 Hole Openers & Under-reamers, 1.2.2 Geomechanics
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Casing impairment leads to loss of pressure integrity, pinching of production tubing, or an inability to lower workover tools. Usually, impairment arises through shear owing to displacement of the rock strata along bedding planes or along more steeply inclined fault planes. These displacements are shear failures. They are triggered by stress concentrations generated by volume changes resulting from production or injection activity. Volume changes may arise from pressure changes, temperature changes, or solids movement (solids injection or production).
Dominant casing-deformation mechanisms are localized horizontal shear at weak lithology interfaces within the overburden; localized horizontal shear at the top of production and injection intervals; and casing buckling within the producing interval, primarily located near perforations.
Mitigating casing damage usually means reducing the amount of shear slip or finding a method of allowing slip or distortion to occur without immediately affecting the casing. Strengthening the casing-cement system seldom will eliminate shear, although in some circumstances it may retard it. Proper well location or inclination, underreaming, special completions approaches, reservoir management, and other methods exist to reduce the frequency or rate of casing shear.
Casing shear is caused by rock shear. Rock shear is caused by changes in stress and pressure, induced by typical petroleum-recovery activities such as depletion, injection, and heating.
Rock Mechanics and Formation Shear.
Because geomaterials are not homogeneous and isotropic, and because they display strain-weakening, rock-mass shear deformation tends to be concentrated in planes, rather than occurring as uniform shear distortion. Rock shear occurs as relative lateral displacement, often across a planar feature such as a bedding plane, joint, or fault. Even if there are no obvious pre-existing planar features, large shear strains will induce slip along specific planes as rock yields (fails) in response to large induced shear stresses. Earthquakes, landslides, and fault movements are expressions of induced shear stresses large enough to overcome natural material strength.
In cases of reservoir rock or overburden shearing, slip planes tend to develop either along interfaces between materials of different stiffness, or on existing discontinuities or weakness planes. A particularly weak stratum may be a high-porosity smectitic-shale zone or a bedding plane or surface which previously has slipped and therefore is presheared. In homogeneous intact rock subjected to large shear stresses, slip will occur on single planes, almost always near the interface between two materials of different stiffness because the shear strains responsible for slip tend to concentrate naturally where a contrast in deformation properties occurs. This concentration, for example, is responsible for delamination of composites such as plywoods or laminates. In a sand-shale sequence, shear slip will occur in the shale because it is weaker than the sandstone, but near an interface with the sandstone, where the shear strains are focused by deformation. Exceptionally, a particularly weak bed or slickensided zone in a shale sequence will shear before an interface because of the low intrinsic strength.
Slip Criteria in Geomaterials.
Formation shear is analyzed in terms of stress/strain behavior and rock strength. The critical mechanical factors are the geomaterial deformation parameters, the shear strength of the various units and interfaces, and the changes in stress, temperature, and volume to which the strata are exposed. These changes arise because of injection and production activity.
Fig. 1 depicts three basic stress definitions. Fig. 1a shows the disposition of the principal compressive stresses at a point. Seven independent parameters are needed to fully specify fully the stress state: the orthogonal major, s1, intermediate, s2, and minor, s3, principal stresses, the orientation of these stresses (stipulated as three direction cosines), and the pore pressure, po. Fig. 1b shows the stress terms commonly used in petroleum engineering, defined with respect to the ground surface. It is usually assumed that the vertical stress, sv, is one of the principal stresses; therefore, the other two are the larger and smaller horizontal stresses, sH and sh, respectively. Stresses may be estimated or measured by various methods outlined in a number of articles.1,2 The natural shear stresses, t, are highest on planes 45° from the principal-stress planes, and the maximum shear stress, tmax, is defined as (s1-s3)/2. Thus, the larger the natural difference in the major and minor principal stresses, the greater the shear stress, and the closer a rock is to a state of failure or shear slip. However, a rock generally will not slip along the plane of maximum shear stress, but at an angle to it, as shown in the triaxial test schematic in Fig. 1c.
A common slip criterion for a geomaterial (or for a plane of weakness in the strata), called the Mohr-Coulomb (MC) criterion, is expressed in terms of effective stresses as shown in Fig. 2. The effective (or matrix) stress, s', is defined by Terzaghi's law, s'=s-p, often expressed tensorially as s'ij=sij-pdij. Simply stated, the important factor in formation shear is the effective stress that is transferred by grain-to-grain (matrix) forces, and this is affected not only by the boundary loads and the depth of burial, but also by the fluid pressure, p. Higher fluid pressures mean lower effective stresses.
The slip criterion is referred to by many different terms: shear strength, failure criterion, yield criterion, or Coulomb criterion. It is expressed often as an equation that stipulates the maximum permissible shear stress along the slip surface being analyzed. The simplest linear form, the linear MC criterion, may be written as
Here, tmax=the maximum shear stress that the plane can sustain before slip; c'=the cohesion of the rock; s'n=the normal effective stress across the slip plane; and f'=the internal friction angle. The material parameters c' and f' are determined empirically from testing and are defined in Fig. 2.
The normal effective stress across potential slip surfaces, s'n, is determined by calculations, usually with a finite element numerical model that can account for different strata properties, boundary conditions, and changes in volume, temperature, pressure, and stresses. Often, nonlinear (curvilinear) MC criteria are used because a linear MC criterion is insufficient, but all MC criteria are nevertheless based on laboratory test results. Finally, one may note that many failure criteria of different forms have been published, but they all serve the same function: to relate the maximum permissible shear stress to the effective normal stress in a geomaterial.
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