True Stratigraphic Position TSP
Understanding True Stratigraphic Position (TSP)
In geosteering, knowing your depth is not enough. You must understand your exact placement within the geological layers. True Stratigraphic Position (TSP) is the fundamental metric that defines where the drill bit is located relative to the top or bottom of the target reservoir.
While “True Vertical Depth” (TVD) tells you how far down you are in the earth, TSP tells you how far you are from your geological “marker.” If the reservoir is 10 meters thick, your TSP indicates if you are at the 2-meter mark (near the top), the 5-meter mark (the middle), or the 8-meter mark (near the bottom).
Why TSP Matters
Geosteering is essentially the act of managing your TSP. By maintaining the desired TSP, the engineer ensures the wellbore stays within the “sweet spot” of the reservoir.
- Predicting Exits: If your TSP approaches the boundary of the reservoir, you know you are about to exit the pay zone.
- Correlation Accuracy: You compare your real-time LWD data against a pre-drilled type log to calculate your current TSP.
- Proactive Steering: By tracking the rate of change in your TSP, you can correct the well path before you accidentally drill into non-productive shale or water-bearing zones.
Calculating TSP
Engineers calculate TSP by correlating the formation dip—the angle at which the rock layers tilt—with the wellbore path. The formula essentially accounts for both the vertical thickness of the bed and the inclination of the well.
When the wellbore inclination matches the formation dip, your TSP remains constant. When the wellbore crosses the formation at an angle, your TSP changes over time.
Challenges in TSP Interpretation
Calculating TSP is not always straightforward due to several geological factors:
- Formation Dip Changes: If the rock layers tilt unexpectedly, your calculated TSP will be wrong, even if your TVD is correct.
- Faulting: Crossing a fault can cause a “jump” in your TSP, making it appear as if the formation has suddenly disappeared or repeated.
- Thin Beds: In very thin reservoirs, minor errors in TSP calculation can be the difference between staying in the pay zone or exiting it entirely.
What You Will Learn in This Lesson
This lesson focuses on the math and logic behind stratigraphic correlation. You will learn:
- How to convert TVD into TSP using dip and azimuth.
- How to identify “picks” or stratigraphic markers in your LWD logs.
- How to handle faults and stratigraphic thickening/thinning.
- How to use software to model your real-time TSP.
- How to adjust your steering plan when the actual TSP deviates from your geological model.