Influence of carbonate reservoir mineral heterogeneities on contact angle measurements

The wettability of reservoir rocks is one of the critical, advanced petrophysical parameters affecting oil production, during the field production life, from primary to enhanced recovery projects. This paper addresses the effect of mineral heterogeneities on wettability behaviour by measuring the contact angle (CA) on carbonate rock core samples from a Brazilian pre-salt reservoir. Mineralogical composition and mineral maps of samples extracted from reservoir plugs were characterised by X-ray based image analysis (scanning electron microscope coupled with automated mineral liberation analysis - SEM-MLA). The results indicated that around 99% of the samples were composed of calcite, dolomite, and quartz with completely different mineral distribution and texture. Also, the attained mineral maps directed the obtention of samples containing different mineral contents in which CA measurements were carried out by using the sessile drop method. CA experiments were run at a temperature of 60 °C and a pressure of 4000 psi with no-aged and aged samples in dead oil. Self-organised maps, generated by machine learning, were used to analyse the relationships between the variables. The results indicate that the predominant mineral constituents and their distribution influence the contact angle values and emphasise the importance of using reservoir core samples and their closest analogues when approaching rock/fluid interaction studies. Despite the samples with higher quartz content showing a greater water-phase affinity than carbonate ones, when both phases are inter-grown, wettability was changed to neutral and oil-wet. In this way, the mineral proportions and texture play a key role in the measured CA. Dolomite contributes to increasing the oil-wet tendency of the rock samples. Mineral maps are not currently applied in wettability studies but demonstrate that it can be a valuable tool for more reproducible measurements and a better understanding of the reservoir.