(55d) Effect of Initial Water Saturation on Carbonate Rock Wettability: A Contact Angle Study

Carbonate reservoirs hold more than half of the world’s remaining oil. Wettability has been proven to have a significant influence on oil recovery from carbonate reservoirs. In most cases, carbonate rock tends to be intermediate-wet or oil-wet due to positive surface charge that induces the adsorption of organic materials. Besides factors including oil composition, temperature, rock mineral composition and so on, the resulting wettability of rock is also affected by initial water saturation. The main objective of this work is to find out how initial water saturation affects carbonate rock wettability.

Indiana limestone core plugs were cut into slices for the purpose of contact angle measurement. Filtered crude oil from the field was used for the oil-aging process. An oven was used to raise oil temperature to accelerate the oil-aging process. A vacuum pump and a desiccator were used to saturate the slices with water or oil. A pressure-plate apparatus was used to establish different initial water saturations. Ambient contact angle was measured using the device Attension Tensiometer.

Two similar curves describing the relation between initial water saturation and the applied air pressure were obtained. Both show a rapid saturation reduction from 100% to 80% in the beginning and a minor reduction as air pressure went above 40 psi. The lowest water saturation value was around 41%, while the air pressure was 66 psi. Different initial water saturation values resulted in different wettability, especially when the oil-aging process was limited to a few days. Generally, samples with lower initial water saturations had a stronger tendency to become oil-wet. After 12 hours of oil-aging, samples whose initial water saturation was lower than 60% became strongly oil-wet. Samples whose initial water saturations were between 60% and 80% became oil-wet. However, samples fully saturated took more than 4 days to become oil-wet. For samples that had high water saturation values, the time required to obtain strong oil-wetness was longer. Samples’ oil-wetness was found to be positively correlated to oil-aging time.

This study shows the effect of initial water saturation and oil-aging time on carbonate rock wettability. It also provides a method to have some control on rock slice wettability. This method can be helpful in studies related to wettability.