Shear Strength

Key Takeaways

• The Mohr-Coulomb failure criterion is the standard geomechanical model for rock shear failure, expressed as tau_failure = C + sigma_n * tan(phi), where tau_failure is the shear stress at failure, C is the cohesion (in psi or MPa), sigma_n is the effective normal stress on the failure plane (total normal stress minus pore pressure), and phi is the internal friction angle (typically 20-45 degrees for sedimentary rocks): on a Mohr-Coulomb plot (shear stress versus normal stress), the failure envelope is a straight line with y-intercept equal to C and slope equal to tan(phi), and any stress state with a Mohr circle that touches or exceeds the failure envelope will result in shear failure on the plane corresponding to the tangent point; the two critical parameters — cohesion C and friction angle phi — are determined by triaxial testing of core samples at multiple confining pressures, with each test defining one point (the peak of a Mohr circle) that together constrain the best-fit failure envelope line; for wellbore stability analysis, the Mohr-Coulomb criterion is applied to determine whether the stress concentrations at the borehole wall (calculated from the far-field principal stresses, pore pressure, and mud weight using elastic stress concentration theory) will exceed the rock's shear strength and cause breakout or collapse; the mud weight window (the range of mud weights that maintain the wellbore without shear failure on one side or tensile fracture on the other) is computed from the shear strength parameters and the in-situ stress state, providing the primary input to mud weight selection in wellbore stability analysis.
• Unconfined compressive strength (UCS, also called uniaxial compressive strength) is the most widely measured and applied rock strength parameter in petroleum geomechanics, representing the maximum axial stress a rock can withstand in the absence of confining pressure (zero lateral stress), and is used as a proxy for the overall strength of the rock in empirical correlations and as a calibration point for the Mohr-Coulomb parameters: UCS is measured by loading a cylindrical core plug axially in a mechanical testing frame without any confining stress until the sample fails in shear, and is reported in MPa or psi; typical UCS values for sedimentary reservoir rocks range from less than 5 MPa (very weak, unconsolidated sand or chalk — likely to sand-produce and prone to borehole instability) to over 150 MPa (very strong, tight sandstone or hard carbonate — resistant to breakout and hydraulic fracturing requires high pressures); empirical correlations between UCS and wireline log measurements (compressional wave velocity Vp, sonic transit time DT, acoustic impedance, Young's modulus from sonic) are used to estimate UCS continuously along the wellbore from wireline log data, providing a pseudo-continuous strength profile that complements the discrete core-based UCS measurements; the log-derived UCS profile is used to construct the mechanical earth model (MEM) that characterizes rock strength variations with depth, essential for wellbore stability analysis, hydraulic fracture design, and sand production prediction in wells where complete core coverage is not available.
• Wellbore breakout and drilling-induced tensile fractures are the two primary borehole failure modes controlled by the shear strength and tensile strength of the formation, and their presence in image logs (electrical or acoustic borehole image logs that provide high-resolution images of the borehole wall) provides a direct in-situ measurement of the rock's failure state and the principal stress orientations: wellbore breakout (stress-induced compressive failure of the borehole wall in the direction of the minimum horizontal principal stress) appears in image logs as symmetric zones of borehole enlargement oriented 180 degrees apart in the direction of SHmin, because the stress concentration at the borehole wall is maximum in the SHmin direction where the compressive tangential stress is Shmax + SHmin - 2Pp (assuming plane stress, where Shmax and SHmin are the horizontal principal stresses and Pp is the pore pressure), and the rock fails in shear when this concentration exceeds the UCS; drilling-induced tensile fractures (DIF) appear in image logs as thin, vertical fractures oriented in the direction of maximum horizontal stress (SHmax), created when the tangential stress at the borehole wall becomes tensile (negative) in the SHmax direction and exceeds the tensile strength of the rock (typically 1-10% of UCS); the systematic orientation of breakouts (perpendicular to SHmax) and DIF (parallel to SHmax) in image logs provides the current-day stress orientation, which is essential for optimizing the azimuth of horizontal wells (drilling parallel to SHmin to maximize hydraulic fracture cross-cutting of the wellbore) and for understanding the direction of natural fracture propagation in the reservoir.
• Sand production prediction and management uses shear strength to identify the conditions under which reservoir sand grains disaggregate and flow into the wellbore with the produced fluids: sand production is triggered when the effective stress at the perforation face (which increases as pore pressure is depleted and decreases as drawdown increases) exceeds the shear strength of the formation rock, causing the weakly cemented sand grains to fail in shear and be carried into the wellbore by the flowing formation fluids; the critical drawdown pressure (CDP) at which sand production begins is calculated from the UCS or the cohesion of the formation sand (from core testing or log correlations), the perforation geometry, the formation porosity, and the current pore pressure; if the required production drawdown to achieve economic production rates exceeds the CDP, sand production is expected and sand control (gravel pack, screen, or frac pack) must be installed; the UCS of unconsolidated and weakly cemented sands in tertiary deltaic reservoirs (Gulf of Mexico, Niger Delta, Mahakam Delta) is typically less than 5-10 MPa, making virtually all production rates above a threshold value capable of triggering sand production; UCS values above 20-30 MPa (better-cemented or more deeply buried sands) typically allow production without sand control at economic rates, with sand production prediction based on the UCS and stress state used to determine whether sand control is required before the completion is designed.
• Fault reactivation and induced seismicity from oilfield operations are governed by the shear strength of pre-existing fault surfaces, which are typically weaker than intact rock and prone to slip when the effective normal stress is reduced (by pore pressure increase from injection) or the shear stress is increased (by poroelastic stress transfer from production or injection): the stability of a fault under a given stress state is assessed using the Coulomb stress change (delta CFS = delta tau - mu * delta sigma_n, where delta tau is the shear stress change on the fault, mu is the friction coefficient of the fault surface, and delta sigma_n is the effective normal stress change on the fault), with positive Coulomb stress change indicating increased failure potential and negative change indicating increased stability; injection of produced water into disposal wells (Class II UIC) increases pore pressure in the injection zone, which reduces the effective normal stress on nearby faults according to the poroelastic relationship, increasing the Coulomb stress and potentially reactivating faults that were previously stable under the pre-injection stress state; the shear strength of fault surfaces in sedimentary basins is characterized by friction coefficients (mu) of 0.6-0.85 for most mineral fault gouge materials (Byerlee's law), lower values for clay-rich fault gouges (0.2-0.4 for smectite-dominated gouge), with the lower-friction clay-rich faults being both easier to reactivate under stress perturbation and less likely to produce strong seismic events because the low-friction aseismic slip dissipates stress gradually rather than in sudden brittle rupture.