Altered Zone

Key Takeaways

• Stress-relief microcracks in the altered zone reduce V_p by 5 to 30% within 2 to 15 cm of the borehole wall, creating a radial velocity gradient that causes short-spacing sonic tools to under-read the true formation velocity, biasing synthetic seismograms and seismic-to-well tie calibrations toward slower-than-actual velocity picks: The magnitude of stress-relief velocity reduction depends on the in-situ differential stress (maximum minus minimum horizontal stress, σ_H - σ_h), the rock's crack density (vuggy or micro-fractured carbonates are more susceptible than tight siliceous siltstones), and the time elapsed since drilling (alteration increases as near-wellbore pore pressure equalises with borehole pressure over hours to days). In Devonian Wabamun carbonate at 3,200 m depth in the Alberta Foothills (σ_H - σ_h approximately 18 MPa), sonic check-shot measurements at 30 m depth intervals show that the long-spacing sonic V_p (3.6 m transmitter-receiver) averages 5% higher than the short-spacing V_p (0.9 m) in the same formation, consistent with the short-spacing tool sampling partially within the stress-altered 8 to 12 cm zone while the long-spacing tool reads primarily the undisturbed formation. Failure to correct for this altered-zone bias produces a synthetic seismogram that ties the field seismic at the wrong time-depth relationship by 4 to 12 ms at the reservoir level, introducing depth uncertainty of 8 to 24 m at 4 km depth.
• Filtrate invasion creates a three-zone radial resistivity profile — mudcake on the borehole wall, flushed zone (Rxo) invaded by mud filtrate, and undisturbed formation (Rt) — each requiring a different resistivity measurement for accurate water saturation calculation in the invaded formation: The microspherically focused log (MSFL) or micro-laterolog measures Rxo (resistivity of the flushed zone) at 5 to 10 cm depth of investigation, the medium laterolog (LLM) measures Ri (intermediate invasion zone) at 60 to 90 cm, and the deep laterolog (LLD) measures Rt (true formation resistivity) at 100 to 200 cm. In the Cardium D2 sandstone at Pembina (typical permeability 60 to 120 mD, formation brine resistivity 0.4 ohm·m, oil viscosity 5 cP), the filtrate invasion front from a 1.10 g/cm³ water-based mud at 1,400 m depth (mud hydrostatic 15.4 MPa, Cardium pore pressure 12.2 MPa, overbalance 3.2 MPa) extends approximately 35 to 50 cm after 48 hours of wellbore exposure before logging. The LLD reading (deep, sampling beyond the invasion) gives Rt = 22 to 65 ohm·m in pay zones; the MSFL gives Rxo = 8 to 18 ohm·m (lower, because the 11,000 mg/L NaCl mud filtrate is more conductive than in-situ oil). Water saturation calculated from Archie's equation using only LLD avoids alteration error; using MSFL would over-estimate S_w by 15 to 35% in pay intervals.
• The altered zone must be quantitatively characterised for geomechanical wellbore stability analysis because the mechanical weakening from stress-relief microcracking increases the compressive strength failure risk and the tensile fracture gradient at the borehole wall relative to the undisturbed formation, affecting casing setting depths and mud weight programme design: In geomechanical models used to predict wellbore breakout (compressive failure producing dog-ear-shaped cavities oriented in the σ_h direction) and drilling-induced fractures (tensile failure oriented in the σ_H direction), the rock strength parameters (unconfined compressive strength UCS, friction angle φ, tensile strength T₀) must reflect the mechanical state at the borehole wall, not the undisturbed laboratory values from core samples. The altered zone typically has UCS reduced by 15 to 40% relative to undisturbed formation (from microcrack coalescence and opening), which means the mud weight window (minimum required to prevent collapse, maximum before fracture initiation) is narrower at the actual borehole wall than geomechanical models based on undisturbed-formation core strength would predict. For horizontal Montney wells at high overbalance (1.5 to 2.0 MPa) and extreme horizontal stress anisotropy (σ_H/σ_h = 1.5 to 2.0 in the Peace River area of northeast British Columbia), the altered zone can reduce the mud weight window from a predicted 0.12 g/cm³ margin (undisturbed formation) to a practical 0.06 to 0.08 g/cm³ margin, requiring more precise mud weight control during lateral drilling to avoid both wellbore breakout and mud losses.
• Ultrasonic borehole imaging tools (UBI, CAST-V) image the altered zone directly by measuring acoustic reflectivity variations around the borehole wall at 1 to 5 mm spatial resolution, providing the primary quantitative measurement of altered-zone extent and the opening aperture of stress-relief fractures, which are qualitatively visible as dark (low amplitude) arcs on the borehole image perpendicular to the minimum horizontal stress: The ultrasonic borehole imager transmits a rotating 250 to 500 kHz pulse and records its reflection amplitude and two-way travel time from the borehole wall (the mudcake/rock interface). Regions where the borehole wall has stress-relief microcracks produce lower reflection amplitude (10 to 30 dB lower than intact formation) because the open cracks scatter or absorb the acoustic pulse before it can return coherently to the receiver. The spatial pattern of low-amplitude regions on the UBI image defines the altered zone extent around the circumference: in wells with high differential stress (σ_H - σ_h > 15 MPa), the stress concentration is highest at the borehole wall in the σ_h direction, and the altered zone is thicker (8 to 20 cm) in these two opposing sectors of the borehole than in the σ_H direction (2 to 5 cm). This directional altered-zone asymmetry, mapped from UBI amplitude images, provides one of the best-quality indicators of in-situ stress orientation available without performing dedicated minifrac tests.
• Alteration-corrected sonic velocities (using long-spacing sonic arrays, check-shot calibration, or Biot fluid-substitution to remove filtrate effects) are required for accurate time-to-depth conversion and AVO analysis in WCSB tight formations where the 2 to 10% velocity bias from the altered zone causes reflector mis-positioning of 50 to 200 m at 3,000 to 5,000 m target depths: The Biot-Gassmann fluid substitution equation relates the elastic moduli of the rock frame at the original formation fluid (V_p,original) to those measured after filtrate invasion (V_p,invaded): the substitution requires knowledge of the bulk modulus of the dry frame (K_dry, determined from V_p and V_s through Biot poroelastic theory), the pore fluid bulk moduli (K_fl of original and invading fluids), and the formation porosity (from the density log). For a gas-saturated Duvernay siltstone at 4,000 m (porosity 6%, gas K_fl = 0.08 GPa, filtrate K_fl = 2.2 GPa), the filtrate invasion of 30% pore volume replacement raises V_p by approximately 180 m/s in the invaded zone (from 4,820 to 5,000 m/s) due to the higher bulk modulus of water-based filtrate versus gas. The short-spacing sonic reading the invaded zone over-estimates V_p by 180 m/s (3.7%), which on a 3.0 s TWT at the Duvernay reservoir depth (100 ms per 250 m two-way at 5,000 m/s average velocity) produces a 27 ms time-depth error — equivalent to 67 m depth mis-tie at the reservoir. The Biot correction removes this filtrate-invasion bias to restore the pre-invasion V_p that should be used for seismic depth conversion.