Uses of Cl-RSA (Chlorine Residual Salt Analysis): A New Technique for the Investigation of Waters from Cores

Woulé Ebongué, V.¹, N. Jendrzejewski¹, F. Walgenwitz², F. Pineau¹, M. Javoy¹ (1) Institut de Physique du Globe de Paris, Université Paris 7, Paris, France (2) Total, Pau, France

Formation waters characteristics are commonly used in oilfield studies. They allow the detection of permeability barriers, the calibration of resistivity, etc... Changes in composi­tions occur during the evolution of the reservoir in response to water-rock interactions, fluid flow processes, mixing, etc... During the filling, formation waters are displaced as the WHC (Water Hydrocarbon Contact) moves down progressively. Irreducible waters trapped at the WHC are isolated from the underlying aquifer and record the evolution of formation waters. The Cl-RSA technique (Chlorine Residual Salt Analysis) has been developed on the model of the Sr-RSA technique. As chlorides are not involved in diagenetic reactions, chlorine con­tents and ‰³⁷Cl values allow the investigation of mixing and transport processes such as ion filtration and diffusion. The Cl-RSA technique can be performed on dried cores. It pro­vides an additional tool for the investigation of formation waters. The validity of ‰³⁷Cl measurements (± 0.05 per mil relative to SMOC) performed on salts extracted from core samples has been tested on sandstones and shales from the North Sea and West Africa oil­fields. The West Africa study is based on 85 samples of shales collected in turbidite series of Miocene age. The range of ‰³⁷Cl values varies from -1.93 to +2.40 per mil. Reservoir compartmentalization is evidenced by discontinuities in the ‰³⁷Cl depth profiles. In addi­tion, ‰³⁷Cl profiles for different wells are established. When considered with traditional tracers (Cl/Br, NaCl, etc…) these isotopic profiles help characterize the oilfields compart­mentalization and reconstruct the filling history of the reservoirs.