Well Parameters
Load Template
Well Information
m TVD
days
m
Enter 0 for onshore
Formation Tops
| Formation | Depth (m) | ± Unc. | Lithology | PP (ppg) | FG (ppg) |
|---|
Casing Strings
| String Type | Depth (m) | Size (in) | Rationale |
|---|
Geohazards & Risks
Well Prognosis
Theory: Well Prognosis
Purpose of Well Prognosis
A well prognosis is a comprehensive pre-drill document that integrates geological, geophysical, and engineering data to predict:
- Formation tops: Predicted depths of key geological markers
- Pressure regime: Pore pressure and fracture gradient profiles
- Casing design: Number and depths of casing strings
- Hazards: Drilling risks and mitigation strategies
- Operational parameters: Mud weights, time estimates, costs
Formation Depth Prediction
From Seismic Data:
- Time-depth conversion using velocity models
- Correlation with offset wells
- Seismic horizon interpretation
- Uncertainty quantification (P90/P50/P10 cases)
Uncertainty Sources:
- Velocity model accuracy
- Lateral velocity variations
- Seismic resolution limits
- Structural complexity
Pressure Window Analysis
Pore Pressure (PP):
- Predicted from seismic velocities, offset wells
- Normal gradient ~0.465 psi/ft or 10.5 ppg EMW
- Overpressure mechanisms: undercompaction, hydrocarbon generation, tectonic loading
Fracture Gradient (FG):
- Estimated using Matthews-Kelly, Eaton, or LOT correlations
- Typically 0.7-0.9 psi/ft or 13.5-17.3 ppg EMW
- Controls maximum safe mud weight
Mud Weight Window:
- Minimum MW: PP + kick margin (typically +0.5 ppg)
- Maximum MW: FG - loss margin (typically -0.5 ppg)
- Narrow windows (<2 ppg) require intermediate casing or MPD
Casing Design Principles
Casing Setting Criteria:
- Conductor: 50-150 m, protect shallow unconsolidated formations
- Surface casing: Below shallow gas zones, protect aquifers
- Intermediate casing: Set when pressure window closes
- Production casing: Set above reservoir, isolate pay zone
Casing Size Selection:
- Work backwards from production tubing size
- Each string typically 2-4 inches smaller OD than previous
- Common sizes: 30", 20", 13⅜", 9⅝", 7"
Geohazard Identification
Key Drilling Hazards:
- Shallow Gas: Uncompacted sand with gas at shallow depth, blowout risk
- Overpressure: PP > normal gradient, kick risk
- Lost Circulation: Losses into fractured/cavernous formations
- H₂S/CO₂: Sour gas, corrosion and safety hazard
- Unstable Formations: Shales, evaporites - wellbore stability issues
Depth Uncertainty (P90/P50/P10)
Probabilistic depth estimates account for seismic and velocity uncertainties:
- P90: 90% probability formation is deeper (optimistic)
- P50: 50% probability (best estimate / median)
- P10: 10% probability formation is deeper (pessimistic)
Example: Reservoir P90=2900m, P50=3000m, P10=3100m
Uncertainty = ±100m, useful for rig planning and casing inventory
Best Practices
- Integrate all offset well data (formation tops, pressures, LOTs)
- Use multiple pressure prediction methods (seismic, basin modeling, correlations)
- Include depth uncertainty ranges for critical formations
- Plan casing points for worst-case (P10) depths
- Identify all potential hazards and mitigation plans
- Update prognosis as drilling progresses (real-time updates)