Shallow Geohazard Screening

Systematic checklist and register of shallow geohazards at proposed well locations. Assess risks from shallow gas, overpressure, weak formations, faults, and seabed features. Generate hazard registers, risk scores, and go/no-go recommendations based on industry guidelines.

Geohazards Shallow Gas Wellbore Stability Risk Assessment

Geohazard Assessment

Well Location

1. Shallow Gas

2. Gas Hydrates

3. Shallow Overpressure

4. Weak/Unconsolidated Formations

5. Salt/Mobile Shale

6. Faults

7. Seabed Features (Offshore)

8. Wellbore Stability

Assessment Results

Shallow Geohazards: Background & References

Common Shallow Geohazards

  • Shallow Gas: Trapped gas at depths less than 1000m, often biogenic. Can cause blowouts if not properly managed. Identified by amplitude anomalies, velocity pushdowns, and chimney structures on seismic.
  • Gas Hydrates: Ice-like solids formed from gas and water at low temperatures and high pressures. Stability zone typically > 300m water depth. Dissociation can cause wellbore instability and shallow flows.
  • Shallow Overpressure: Abnormally high pore pressure in shallow sections, often due to undercompaction, rapid sedimentation, or gas generation. Can lead to kicks, lost circulation, and wellbore collapse.
  • Weak Formations: Unconsolidated sands, silts, clays with low strength. Prone to washouts, tight spots, differential sticking, and hole cleaning issues.

Geohazard Identification from Seismic

Shallow Gas:

  • Bright spots, acoustic blanking/masking
  • Velocity pushdown (deeper reflectors pulled down)
  • Gas chimneys (vertical disturbances)
  • Amplitude anomalies conforming to shallow structure

Gas Hydrates:

  • Bottom-simulating reflector (BSR)
  • High-amplitude reflections at base of stability zone
  • Reverse polarity relative to seabed

Weak Zones:

  • Low seismic velocities (< 1800 m/s)
  • Continuous low-amplitude reflections
  • Interval velocity inversions

Mitigation Strategies

Shallow Gas:

  • Surface casing set below gas-bearing zone
  • Diverter system or subsea BOP
  • Riserless drilling (offshore)
  • Reduced circulation rates when penetrating

Overpressure:

  • Pre-drill pore pressure prediction
  • Managed pressure drilling (MPD)
  • Real-time PWD/LWD monitoring
  • Casing shoes placed to isolate zones

Weak Formations:

  • Optimized mud weight (narrow window)
  • Wellbore strengthening additives
  • Controlled ROP and ECD management
  • Short bit trips to minimize exposure time

Regional Considerations

Deepwater Gulf of Mexico:

  • Shallow water flows (SWF) in Mississippi Canyon
  • Gas hydrates in stability zone (> 300m WD)
  • Salt overhangs and weak sediments
  • Rapid sedimentation → shallow overpressure

North Sea:

  • Shallow gas in Quaternary/Tertiary sands
  • Pockmarks and seabed gas seeps
  • Unconsolidated sands (weak formations)
  • Active faults and gas migration pathways

Onshore (Conventional):

  • Shallow aquifers (freshwater zones)
  • Coal seams with methane
  • Lost circulation in fractured/karst zones
  • Wellbore stability in shales

Regulatory Requirements

  • IADC/SPE Guidelines: Shallow hazard assessment mandatory before drilling in new areas
  • BSEE (US): Shallow hazard survey and analysis required for all offshore wells
  • UK HSE: Site survey with high-resolution seismic, geotechnical sampling
  • NORSOK (Norway): Detailed geohazard report, risk assessment, and mitigation plan
  • ISO 19901-8: Marine soil investigations for offshore structures

Minimum site survey requirements (offshore):

  • High-resolution 2D/3D seismic over drilling location
  • Bathymetry and side-scan sonar
  • Shallow cores (geotechnical properties)
  • Shallow hazard interpretation report