Differentiating gas condensate from light oil

Conventional logs would not be able to readily identify changes in hydrocarbon type in a shaly sand in a North Sea exploration well drilled with oil-base mud. Because the economic value of the expected gas condensate was significantly different from that of the light oil that could also be present, the operator needed to recognize which fluids were present and determine the contact between them.

Acquiring NMR data at multiple DOIs

To meet the challenge in identifying fluid type, Schlumberger added MR Scanner expert magnetic resonance service to the logging program for radial NMR acquisition. Two passes for continuous NMR fluid characterization were made at multiple DOIs. The data were processed with a simultaneous 4D inversion that constrained the bound-fluid components because the bound-fluid volume does not exhibit radial variation. With this approach, variation in the fluid saturation resulting from invasion by oil-base mud is characterized to distinguish formation hydrocarbons from mud filtrate and accurately quantify fluid volumes.

Confirming fluid types and pinpointing their contacts

As shown on the triple-combo log, a hydrocarbon zone was identified, but no change in fluid type could be discriminated. However, the plots to the right of the log of molecular diffusion (D) and longitudinal relaxation time (T₁) measured by MR Scanner service at a 4-in [10-cm] DOI clearly show the change in response from light oil to gas condensate. The 4-in DOI is ideal for this purpose because the response is deep enough to not be dominated by oil filtrate.

Although it was now known that both gas condensate and light oil were present, the depth-pressure plot constructed from conventionally measured pressure data only indicates a change in fluid gradient, not a definitive depth for the contact. The 4D fluid analysis of MR Scanner service's radial data over the condensate-oil-water interval placed the condensate/oil contact at X,838 ft MD and the oil/water contact at X,877 ft MD. The latter depth was critical information because the resistivity log indicated residual hydrocarbon below X,877 ft MD.

The contact depths determined by MR Scanner service were used to target a formation tester sampling program, which successfully acquired samples of the gas condensate, oil, and water. The samples, along with a subsequent drillstem test, confirmed the fluids identified by 4D processing of MR Scanner service data.