Proper reservoir evaluation in carbonate environments has always been challenging. Petrophysicists are constantly struggling with complex lithology, thin bed effects and water salinity uncertainty in evaluation.

A recent integrated Logging-While-Drilling (LWD) tool, which delivers capture spectroscopy and formation sigma in addition to the conventional "triple-combo" measurements of density, neutron and resistivity, was applied in a very mature field and has permitted:

  • Improved formation lithology evaluation through the availability of elemental capture spectroscopy. Porosity evaluation is enhanced by the use of formation grain density as derived from the elemental composition.

  • Cross-validation of the water saturation from both Archie and Sigma methods, significantly increasing the level of confidence in each.

  • Thin bed evaluation with minimal shoulder effect. In these thinly bedded carbonates, the relatively high shoulder bed resistivities compared to the reservoir can cause over-estimation of oil saturation if the shoulder effects on the resistivity are not taken into account. The availability of resistivity-independent water saturation with the pre-invasion Sigma measurement removes much of the uncertainty associated with these effects.

  • Simultaneous estimation of water salinity and saturation. By solving for the water salinity that gives a match between the Sigma- and Archie-derived saturations, the complex problem of saturation evaluation in unknown water salinity conditions can be addressed.

The limitation of the interpretation methodology is also addressed, such as where rapid invasion invalidates the use of the LWD Sigma measurement for uninvaded zone water saturation evaluation. How this was detected is demonstrated and the implications outlined.

The downhole data acquisition strategy in this highly mature field has clearly benefited from this application of a new LWD technology. This has eventually enabled additional field development, and consequently maximized associated reserves.

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