Well-to-seismic tie is essential for accurate seismic interpretation and inversion, aligning reflections with well logs to calibrate time-depth relationships and extract wavelets. It ensures proper horizon mapping, rock property positioning, and reduces exploration and drilling risks. Well ties underpin reservoir models and enable confident, data-driven decisions in subsurface analysis.
Data Requirements
- Well logs (sonic, density, check-shot/VSP). If density is missing, derive it using Gardner’s rule from Vp or assume a constant. If no TD chart, create one from integrated DT log with a manual time shift.
- Seismic data (migrated and stacked). For deviated wells, extract seismic traces along the well trajectory.
- Commercial tools (e.g., Kingdom) automatically calibrate logs, generating impedance and reflection coefficient logs.
Generate Wavelets
- Wavelets are used to generate synthetic traces for comparison with seismic near the well. Their quality directly affects tie accuracy.
- Start with theoretical wavelets like Ricker or Ormsby (bandwidth matching seismic).
- A zero-phase statistical wavelet derived from seismic for realistic amplitude spectrum.
- A deterministic wavelet (Roy White, 1980) combining seismic and logs for accurate phase and amplitude.
Perform Matching
- Bulk Shift: Align a key seismic and synthetic event using a bulk time shift.
- Stretch/Squeeze: Fine-tune by stretching/squeezing events (±5–10% TWT). Limit velocity edits to ±5–7% to maintain realistic intervals consistent with TD and regional trends.
- Phase Rotation: Optimise by rotating wavelet phase (±10°); phase is sensitive, so adjust cautiously.
Validate and Refine
- Validation improves match accuracy through correlation and adjustments.
- Compute cross-correlation; aim ≥0.6.
- Visually confirm reflector alignment across nearby wells and seismic.
- Ensure velocity edits match sonic logs and regional trends.
- Re-estimate deterministic wavelet using the updated TD chart.
- Adjust bulk shift or wavelet phase as needed.
- Skip wells with poor data, bad logs, or complex geology (faults, salt).
Is Stretching/Squeezing Justifiable?
Stretching and squeezing align synthetic and seismic traces by correcting log errors, check-shots, anisotropy, wavelet mismatches, or seismic artifacts. Minor adjustments (±5–10% TWT) are justifiable for matching events and improving inversion. However, modified TD curves may distort velocities, so don’t use them to represent true subsurface properties.
Conclusions
Well ties are the backbone of inversion, enabling wavelet estimation, rock property mapping, and robust reservoir models. Mastering this skill supports reliable horizons, reduces risk, and improves decisions, whether exploring new plays or refining mature fields.
Written by:
Frank (Xin-Quan) Ma (PhD) Senior R & D Geophysicist at Equipoise Software Limited