1. Oil-Based Mud Contamination in Gas-Condensate Samples
R&D Results to Date
March 2008
Work Completed
Norsk Hydro / ConocoPhillips
Open Research Results
Curtis H. Whitson
Robert Mott
Øivind Fevang

2. r-z-θ Sensor EOS Compositional Model

3. Main Conclusion
Standard GC-based decontamination methods can provide accurate estimates of true in-situ composition.
Even when BHFP drops below the dewpoint.
Independent of relative permeability effects.
For any type of OBM.
For any richness of the condensate.
Even though the measured PVT properties are not representative of in-situ fluids.

4. Sensitivities Studied
Reservoir Fluid
Initial pressure
OBM type
Grid
Relative permeability
Horizontal permeability
kv/kh
Production rate
Clean-up time
OBM volume
How to introduce OBM
Rate for sampling
Tool contact area
Lean & Rich Condensates
Near-saturated
2-component & Diesel
Heterogeneities
Rock & Linear Curves
10 & 1 md
1 & 0.1
0.5 – 1 L/min
6 – 24 hr
5 – 13 cm invasion
Initialize & Injection
Constant & Variable / Shut-in
5 cm x 5 cm & 10 cm x 10 cm

5. Obervations Key to degree of OBM contamination:
Initial OBM depth of invasion.
Cumulative production before sampling.
kv/kh ratio.
Less important parameters:
Heterogeneity.
Drawdown.
Flow area into tool.
Relative permeability.

6. Observations (continued)
Leaner gas condensates have higher OBM contamination, all else being the same.
Dewpoint may either rise or drop because of OBM contamination.
Recommended method for extrapolating OBM contamination uses mol-% vs time on log-log plot.
Avoid large rate changes just prior to sampling.
It is still possible to obtain in-situ representative samples when FBHP drops below dewpoint.