Low Resistivity Pay Identification in Lower Cretaceous Carbonates, Onshore UAE

Abstract

The presence of Low Resistivity Pay (LRP) in reservoirs has been widely reported worldwide for both, clastic and calcareous formations. By definition, a LRP is not identified by the resistivity log, as its electrical beam is short circuited by the microporosity water bearing. This effect results in lower responses than expected for the conventional resistivity log, and hence, in higher estimations of water saturation that observed from production data. There are many factors that could create a LRP effect such as water invasion, conductive minerals, fracturing, thin bedding, fresh formation water or rock fabric (i.e. microporosity).

Regarding to rock fabric as driver for the LRP effect, the most common is the presence of the microporosity. This microporosity can be created by diagenetical processes (i.e. micritization) or whatever other component or process which preserves a microporous system inside the total porosity system of the rock. The presence of microporosity in the porous system, as a water bearing system, acts as an electrical shortcut for the electrical current, and it increases the general water content as calculated by the resistivity tool. The result is that SW due to microporosity contributes as Irreducible Water Saturation (SWirr) and not as Free Water Saturation (SW), with a direct effect on reserves assessment (Static Model), but with no significant effect to the field production (Dynamic Model).

The aim of this study is to identify the eventual presence of a LRP, and to determine its origin. By the above, a new fluid saturation assessment was performed based on both, available core and log data, together with a new set of core data focused on the available cored oil wells, in order to identify and recalculate the Gas, Oil and Water saturations for building a the new Static and Dynamic models. The new set of core analysis includes not only Conventional Core Analysis (CCAL), such as air porosity, grain density, Ka, Kk, Kg, CT Scanning or thin sections, but also Special Core Analysis (SCAL) as Mercury Injection Capillary Pressures (MICP) and Nuclear Magnetic Resonance (NMR).