Shear

Key Takeaways

• The Mohr-Coulomb failure criterion is the foundational model for predicting shear failure of rock under subsurface stress conditions, stating that a rock or soil will fail in shear when the shear stress on any plane exceeds the cohesive strength of the material plus the frictional resistance to sliding (proportional to the normal stress on the plane times the coefficient of internal friction); on a Mohr circle diagram, failure is predicted when the circle representing the stress state (with the maximum and minimum principal stresses as its diameter endpoints) touches the failure envelope (a line defined by the cohesion and friction angle of the material); in drilling engineering, the Mohr-Coulomb criterion is used to predict wellbore stability by determining whether the stress concentrations around the borehole (caused by removal of the rock supported by the overburden and horizontal stresses) will exceed the rock's shear strength, causing shear failure that manifests as breakouts (elongation of the wellbore in the direction of minimum horizontal stress) or as tight hole conditions from rock caving into the wellbore; the difference between the in-situ stress state and the rock's failure envelope determines how much overbalance (excess mud weight) is needed to prevent shear failure, and the appropriate mud weight window is calculated by applying the Mohr-Coulomb criterion across the full stress state around the borehole using finite element or analytical methods.
• Shear wave (S-wave) velocity in reservoir rock is a key petrophysical parameter because, unlike P-wave (compressional wave) velocity which is strongly affected by pore fluid (since fluids transmit compressional stress but not shear stress), S-wave velocity is relatively insensitive to pore fluid type and saturation; this differential fluid sensitivity of P-waves and S-waves is the physical basis of AVO (amplitude versus offset) analysis and of the Vp/Vs ratio as a fluid indicator: a gas-saturated sand has a significantly lower Vp and a similar Vs compared to the same sand saturated with brine, resulting in a distinctly lower Vp/Vs ratio (and higher Vp/Vs for brine) that can be detected in the seismic data; S-wave velocity is measured directly by dipole sonic logs (also called shear sonic logs or DSI — dipole shear imager — logs) that generate monopole and dipole acoustic pulses in the borehole fluid and detect the shear wave energy that travels along the borehole wall; in addition to fluid detection, Vp/Vs ratios and the derived Poisson's ratio (which is directly computed from Vp/Vs) characterize the rock's brittleness, with lower Poisson's ratios (higher Vp/Vs inverse, more brittle rock) indicating rock that will fracture more effectively in hydraulic fracturing operations.
• Shear thinning (pseudoplastic behavior) in drilling mud and cement is described by power-law or Herschel-Bulkley rheological models and is the property that allows these fluids to be effectively both pumpable at high flow rates (where high shear rates reduce apparent viscosity and minimize friction losses) and capable of suspending drill cuttings and weighting materials at low or zero flow rates (where low shear rates allow the viscosity to increase and provide suspension capacity); the shear rate in a drill pipe ranges from hundreds to thousands of reciprocal seconds during circulation, where the low apparent viscosity provided by shear thinning allows efficient pumping with modest friction pressure; in the annulus and in wide wellbore cavities, the shear rate drops significantly and the apparent viscosity increases, improving cutting suspension and transport; Fann VG meter readings at 600 RPM and 300 RPM (which correspond to representative high and low shear rates in the wellbore) are the standard measurements of drilling fluid rheology and are used with the Bingham plastic or power-law models to calculate circulating pressure losses and evaluate cutting transport efficiency across the range of shear conditions in the wellbore.
• Fault shear displacement (shear offset) is the net distance one side of a fault has moved relative to the other in the direction of slip (horizontal for strike-slip faults, vertical for normal and reverse faults, or oblique for oblique-slip faults), and measuring this displacement is critical for understanding the fault's impact on reservoir connectivity and hydrocarbon trapping; a fault with large shear displacement (hundreds to thousands of meters) may juxtapose reservoir units against sealing shale or salt, creating a fault seal that traps hydrocarbons against the fault plane; conversely, a fault with small shear displacement may only partially offset reservoir beds, maintaining some degree of connectivity across the fault that allows hydrocarbons to migrate or water to breakthrough during production; shale gouge ratio (SGR), the proportion of shale in the fault zone predicted from the volumetric shale content of the displaced sequence and the fault displacement, is the most commonly used proxy for fault seal capacity and is computed from the detailed displacement history of the fault mapped on 3D seismic data.
• Shear bond strength of cement in casing annuli is a critical well integrity parameter that is distinct from tensile bond strength and is the mechanical property that determines whether the cement sheath can prevent the casing from sliding (shear movement) relative to the formation under differential loads imposed by temperature changes, pressure cycling, and mechanical loads during perforation and completion operations; shear bond strength is typically 5-15 times the tensile bond strength for set oilwell cements (because cement is much stronger in compression and shear than in tension), but can be reduced by mud contamination at the cement-casing interface, by excessive free water during the hydration process (which settles to create a weak water film), or by casing movement during cement placement; API RP 10B-2 provides test procedures for measuring cement shear bond strength under simulated downhole conditions, and the results are used to qualify cement blend designs for completions where strong casing-cement mechanical bonding is required to maintain well integrity under high cyclic thermal or pressure loads.