Influence of oil aging time, pressure and temperature on contact angle measurements of reservoir mineral surfaces

Abstract

Reservoir wettability plays a critical role in determining the fluid interactions, distributions, and flow paths in porous mediums, which is directly reflected in oil production and recovery strategies during a field’s production life. Many factors can influence reservoir wettability characterisation on the laboratory scale such as: sample origin, mineralogical composition, cleaning procedures for further analysis, pressure, temperature, and the oil aging process. The best practice to restore the sample's original wettability by aging processes is unclear, impacting the assessment of contact angle (CA) measurements. This work addresses the oil aging time and experimental conditions associated with the pressure and temperature in CA measurements on rock surfaces from a Brazilian pre-salt reservoir with a predominance of calcite, dolomite, and quartz, respectively. For this purpose, all surfaces used were cleaned with solvents and saturated with synthetic formation water under vacuum conditions. To evaluate the influence of oil aging time on the samples' wettability, the CA measurements, by captive bubble method, were performed on unaged surfaces. The CA measurements were then taken again on the same surfaces after 14 and 42 days of oil aging. To evaluate the pressure and temperature effects on the CA values, the experiments were run at 14.7 psi and 22 °C (room conditions), 14.7 psi and 60 °C and 4000 psi and 60 °C. The results showed a mischaracterization of all surfaces' original wettability due to the cleaning process, showing the higher hydrophilic degree and wettability character of neutral and water-wet surfaces. The oil aging procedure was confirmed to be a fundamental process in restoring the wettability of rock surfaces, requiring more than 14 days to be achieved. Pressure and temperature conditions also matter on CA evaluations. The surface's hydrophobic degree increased as the pressure increased from 14.7 psi to 4000 psi, and the temperature from 22 °C to 60 °C. Furthermore, a higher influence of temperature than pressure was observed to impact CA values. Oil aging and experimental conditions presented a larger effect on increasing the hydrophobic degree of samples with calcite and dolomite predominance than those with quartz predominance.