Oil-Based Mud Contamination in Gas-Condensate Samples

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Presentation transcript:

Oil-Based Mud Contamination in Gas-Condensate Samples R&D Results to Date March 2008 Work Completed 2005-2006 Norsk Hydro / ConocoPhillips Open Research Results Curtis H. Whitson Robert Mott Øivind Fevang

r-z-θ Sensor EOS Compositional Model

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.

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

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.

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.