Resolving Water Saturation Uncertainty in a Thin Oil Rim Reservoir, Using Sigma While Drilling, Case Study from Onshore Abu Dhabi

Abstract

Objectives This study focuses on resolving the water saturation (Sw) uncertainty in a small oil rim that consist of thin high-permeability layers (super K). Resistivity-based water saturation techniques suffer from multiple uncertainties including thin layers with distinct capillarity profile, inaccurate resistivity due to high-angle bed effects and unknown Archie parameters. Accurate saturation using alternate methods than resistivity based is critical for the calibration of saturation height model for reserve estimate and to design the perforation program away from the mobile water zone. Methods To resolve the issue of accurate Sw, an alternate method of computing saturation from sigma measurements while drilling is being investigated. Data from multiple wells has been analyzed- saturation from sigma being computed and compared to resistivity based saturation. The resulting saturation is benchmarked against other known sources of saturation like core data and cased-hole measurements. The input parameters used to compute the sigma-based saturation were validated by well test and PVT lab data. Observations A general assumption for LWD logs is that invasion is so shallow that LWD data acquired during drilling may be sufficient to evaluate uninvaded zone properties However, this premise need to be confirmed by other sources of information, in this case Multi-Depth of Investigation (MDOI) sigma data. It provides valuable insight into the status of formation invasion, at the time of acquiring data and of measurement quality control. In situations where the resistivity data does not suffer from any high-angle bed effects, there is immense value in combining the information from resistivity and sigma. When there are two measurements free from invasion and reading the same volume of rock, this can be used for not only calculating water saturations but also to fine-tune the Archie parameters, which might be of immense value in situations, for use on offset wells with resistivity data only. Novel Information Sigma while drilling is another weapon in the arsenal of the petrophysicist and the multi-depth of investigation sigma measurements in a LWD environment contributes a new opportunity to evaluate fluid saturations in reservoirs with unknown or variable Archie parameters.