Determination of Sulphur Isotopic Ratio to Identify the Source for Sulphate in Reservoir Chalks and Formation Water

Abstract

Reservoir chalk samples have earlier been found to contain low concentration of sulphate. These concentrations are higher than shown to alter the wettability of outcrop chalks to more water-wet. The source of the sulphate in the reservoir chalk samples can either be the original reservoir or contaminations. The sulphate source should be identified to allow preparation of core plugs with the correct wettability. The sulphur isotopic ratio 34S/32S of sulphate in the global ocean has changed with geological time. In the presented study, the sulphate source has been identified by determination of the sulphur isotopic ratio of the sulphate in the reservoir chalk samples and formation water samples. Reservoir chalk samples from two different chalk fields were selected for the study. Crushed reservoir chalk was dissolved in acidic solution, and the filtrate was mixed with BaCl2 solution to precipitate sulphate as BaSO4. Real formation waters from the same chalk fields contain low concentrations of sulphate. Sulphate from produced water samples from wells without breakthrough of injected sea water, was also precipitated as BaSO4 by adding BaCl2 solution. The BaSO4 filter cakes were dried and analyzed for the sulphur isotopic ratio (32S/34S) using the EA-IRMS (Elemental Analysis – Isotope Ratio Mass Spectrometry) method. The sulphur isotopic ratio for sulphate both in reservoir chalk samples and produced water samples, was found to be in the same range as for the sulphate in the global ocean during sedimentation of the chalk reservoirs. The sulphate found in reservoir chalk samples and in real formation water was therefore from the original chalk reservoirs, i.e. the field samples were not contaminated by sulphate. Reservoir chalk core plugs should therefore be restored with the same amount of initial sulphate as in the area where these reservoir chalk samples are taken, both in Special Core Analysis (SCAL) and enhanced oil recovery (EOR) studies. The amount of sulphate in the original rock and in the rock after restoration can be determined by analysis of effluent samples for sulphate.