When Water Turns to Oil: Low Resistivity Pay Characterization through Integrated Open-Hole/Cased-Hole Log Interpretation

Abstract

Zubair is a giant oil field located in the South of Iraq. The production started in 1951 and current oil production is around 450 kbopd achieved through 150 wells completed in two main formations: Mishrif (carbonate) and 3rd Pay (sandstone). The scope of this paper is to show how an integrated methodology based on core analysis, open-hole and cased-hole logs unlocked the underneath potential of a sand layer (L1) with an anomalous resistivity. Multiple wells, indeed, show resistivity curves in the L1 interval with surprising low values with respect to the average of other levels of the same sandstone reservoir. Therefore, fit-for-purpose open-hole (OH) and cased-hole (CH) log acquisitions have been integrated with information from cores and dynamic data (i.e. production logging) in order to better understand the phenomena behind the low resistivity scenario. As a consequence, several perforation extensions have been performed with L1 as the main target, providing an overall improvement of hydrocarbon deliverability without any increase in water production. In details, routine and special core analyses in L1 samples delineate the typical setting of a fine-grained low resistivity pay sandstone, able to host a large quantity of irreducible water. However, such behavior is not always present among L1 cores. Therefore, a methodology aimed at characterizing this sandstone behavior was mandatory. Nuclear magnetic resonance logging, commonly used to identify low resistivity pays, was not a suitable option due to bad-hole problems. Hence, an approach based on a detailed integration of OH resistivity and CH pulsed neutron logging (PNL) is used to recognize and characterize such low resistivity pay. This method mainly relies on the fact that formation water is very conductive and strongly affects the resistivity, while its effects on PNL measurements are not so pronounced. Such intuition is confirmed by multi-rate PLT interpretations that dynamically describe the L1 sandstone with fair productivity index and high reservoir pressure, together with a significant dry production contribution. In conclusion, a clear geological trend of L1 resistivity behavior is revealed and associated to the decreasing cementation of the matrix and its coarsening in the same direction. The integrated OH/CH methodology allows characterizing low resistivity intervals as pay zones. Such achievement represents an important milestone for the perforation strategy of new and existing wells in Zubair. As a natural consequence, the overall field production has been enhanced by widely applying the new technique without any increase in water-cut.