Formation Pressure: The Pretest
Learning objectives
- Read a pretest as mud column, drawdown, and buildup to the formation pressure
- Identify the formation pressure as the buildup plateau, not the mud pressure
- Relate mobility (k/mu) to how fast the buildup stabilizes
- See that a tight zone may not stabilize, so the read comes in low
A Pressure at a Point
Everything in this chapter starts with a single measurement: the pressure of the fluid in the rock at one depth. A wireline formation tester gets it by pushing a probe against the borehole wall, withdrawing a small volume so the pressure draws down, then shutting in and letting it build back up. The plateau the pressure climbs to is the formation pressure.
Drawdown, Buildup, Mobility
The shape of the curve carries more than one number. The depth of the drawdown for a given withdrawal rate measures the mobility , the ease with which fluid moves: a high-mobility sand barely dips and recovers in seconds, a tight streak draws down hard and recovers slowly. Pull harder and the drawdown deepens, but the buildup plateau, the answer, is unchanged.
When the Read Is Wrong
In a tight zone the buildup may not reach the true pressure inside the time the tool waits, so the recorded value comes in low. The opposite error, supercharging, happens when mud filtrate has pressurized a low-permeability zone and the read comes in high. A good interpreter reads the curve, not just the final number, before trusting a pressure point.
References
- Schlumberger (2006). Fundamentals of Formation Testing. Schlumberger.
- Dake, L. P. (1978). Fundamentals of Reservoir Engineering. Elsevier.