Aging Effect on Rock Wettability Alteration

One of the factors controlling fluid distribution in porous media is wettability, which is ranging from water-wet to oil wet. Wettability could be changed due to the long-time contact with hydrocarbon liquids. Wettability alteration resulting from the aging of core samples in hydrocarbon liquids becomes a widely used technique. The aging of Core samples from different locations in different types of liquids had been studied. The aging periods used in these studies range from hours to 110 days. In this work, synthetic core samples were aged for a period ranging from 205 to 824 days into two different hydrocarbon liquids (heavy crude from the East Baghdad oil field and gas oil). Results show that long time aging periods dramatically alter wettability; also, heavy crude oil alters wettability higher than that of light hydrocarbon.

Use simple mica substrates to investigate the impact of aging conditions on the macroscopic contact angle and the microscopic properties of the surface. Mica samples were pre-aged in the brine of variable salt contents for 24 hours before aging in crude oil at elevated temperature for 4 days [23]. Results declared a wettability alteration. Researchers in [24] studied the wettability alteration of an oil-wet Iranian carbonate reservoir undergoing seawater injection. The indirect contact angle measurement using the rise core technique was used to determine rock wettability.
The results show that neutral wetting is recognized after the aging process. The wettability of higher permeability samples remains unchanged after the aging process in seawater, while the wettability of low permeable samples changes to be slightly water wet. A comparison between the weighing technique and the direct volume measurement technique to measure the spontaneous imbibition. Samples of different sizes taken from the same rock source were used.
Results show that the weighing method gives more accurate results compared with the direct measurement. [24] This work is conducted to overcome these drawbacks by using strongly water-wet samples taken from synthetic porous media. So, there is no need for a cleaning process and completely saturating the core samples with the hydrocarbon under test. Five samples were saturated with heavy hydrocarbon and the other five samples were saturated with light hydrocarbon. All samples then aged for periods of 0, 205,304, 504, and 824 days.
The paper is classified into four main sections; the materials and methods are discussed in Section II. Section III represents the results and discussion. Finally, the conclusion is presented in Section IV.  Core Plugs: To accomplish the tests, cylindrical core plugs were taken from synthetic blocks composed of a mixture of clay and sand fired at a temperature ranging from 750 to 800 o C. These blocks have high porosity and are strongly water wet. Ten core plugs were prepared then they were divided into two groups; each group consist of five core samples.

Materials and Method
The first group was saturated with the EB heavy crude while the second group was saturated with gas oil.

Method
The spontaneous imbibition test is considered an indicator of core wettability. The tests were performed on one core sample from each group namely at zero aging period. The rest four cores from the first group were aged in EB crude while the other four cores from the second group were aged in gas oil. The aging was performed at ambient temperature for 205, 304, 504, and 824 days at ambient conditions.
At the end of each aging period, the spontaneous imbibition test was carried out to evaluate the wettability alteration. The tests were carried out for each sample as follows; 1. The saturated sample hanged by the metallic wire and completely immersed into the water container (the sample must not touch the wall or the bottom of the container), then its weight was recorded as the weight at zero-time.
2. While leaving the sample completely immersed into water, its weight recorded at different periods.
3. The test continued until there was no appreciable change in the weight readings between two successive steps.

Results and Discussion.
The readings resulting from the spontaneous imbibition tests were recorded and plotted as water saturation against time for the two groups of aged samples into two different hydrocarbons. Figure (3) is for cores aged into EB crude while the plot in Figure (4) is for those cores aged in gas oil. From Table (2), we can conclude the first-day measurement of water saturation could give an initial indication for wettability alteration since there is a rapid increase of water saturation on the first day of the imbibition test in a strongly water-wet system while the readings of water saturation are much lower in age's core samples.
In addition, the reduction of the water saturation after 25 days of imbibition test for the aged samples compared with that of non-aged samples shows that wettability tendency moving from strongly water-wet system to medium and oil-wet one especially if aging is in heavy crude. A comparison between water saturation readings after 25 days of test for samples aged in EB heavy crude and that aged in gas oil indicates that heavy crude has a higher effect on wettability. The results of the spontaneous imbibition test for every two core samples having the same aging period but aged in two different hydrocarbons were shown in Figures    These figures depicted that the longer the aging period the higher is wettability alteration and EB heavy crude has a higher effect on wettability than that of gas oil.

Conclusions
1. Generally, the aging of porous media in hydrocarbon liquids alters its wettability from strongly water-wet toward intermediate and oil-wet one.
2. Wettability alteration resulting from aging in heavier hydrocarbon is higher than that in lighter hydrocarbon.
3. Wettability alteration resulting from aging into the crude oil of East Baghdad oil field is higher than that resulted from aging into gas oil could be attributed to adhering of the heavy hydrocarbon component to the grains surface of the porous media.
4. Longer aging periods means enough time for the hydrocarbon molecules to adhere to the grain surfaces resulted in higher wettability alteration.
5. The first day reading of water saturation in an imbibition test could give a good idea about wettability alteration also the readings of water saturation at the end of the test shows the wettability tendency.