Advances in LWD Sigma and a Novel Approach for Saturation Evaluation by Combining Resistivity and Capture Sigma for Low-Resistivity Pay Reservoirs

Conventional resistivity-based water saturation estimates using Archie's equation depend on petrophysical parameters strongly related to reservoir rock texture, pore structure, wettability, and formation salinity. These parameters are often not readily available and can vary significantly within and across depositional sequences. At the same time, for high-angle wells, the different bed boundary effects mask the true formation resistivity only to add to the grim of saturation evaluation. This makes formation sigma particularly interesting because it offers saturation estimation independent of resistivity measurement and empirical Archie exponents. However, the method has not been extensively used as the unknown invasion effect in single-depth sigma measurement can lead to erroneous interpretation.

With the availability of multiple-depth-of-investigation (MDOI) formation sigma, the latest logging-while-drilling (LWD) service provided means to invert for undisturbed formation sigma (Mauborgne et al. 2013). An invasion-free sigma and the resistivity log can be used together to simultaneously solve for the Sw and formation water salinity with a reasonable knowledge of the porosity and Archie parameters (m and n). The method solves for a salinity value that is consistent with the S_w calculated from resistivity and from sigma.

This technique was used for the first time to evaluate S_w in the Gamba-sand and Dentale, a reservoir with very low resistivity. Following the success, the method was used in another field (Robin) to evaluate similar reservoirs. The technique provided accurate oil saturations in the oil leg and proved its applicability in these fields. This paper will illustrate the methodologies of deriving true formation sigma and the advanced steps for combining sigma and resistivity to derive a precise answer.

The availability of LWD sigma opened new frontiers of applications beyond the conventional baseline monitoring logs. The capability of deriving true formation sigma along with the novel technique of integrating two different measurements, sigma and resistivity, to derive one single answer was the key to the success.