Wettability in Oil and Gas Reservoirs
Wettability is a measure of how strongly a fluid sticks to (or wets) the surface of the grains compared to a second fluid in the same pore space. As an analogue you can look at water droplets on a non-stick ceramic frying pan versus water droplets on a regular glass. Generally the water droplets in the pan will form more sphere-like shapes, while the droplets on the glass will form lenses. Basically the water on the glass has stronger atomic bonds with the glass and therefore extends the surface area over which it contacts the glass. An similar example is shown below.
The forces pulling the water towards the glass are counteracted by surface tension (between water and air in this example; NB. the air is considered the second ‘fluid’ here) which try to make the surface as flat as possible. In the example with the frying pan these forces are the same but the force pulling the water towards the pans surface is smaller, hence the droplets seem to lie on the surface instead of sticking to it. This is also why the droplets move a lot easier when tilting the pan. Wettability in Reservoirs
The wettability of a formation determines the irreducible water saturation in the reservoir. Furthermore it determines how easily a fluid moves and has tremendous impact on what fluid is primarily produced.
In an oil or gas reservoir there is always some water (which you generally do not want to produce) in the pore space and therefore wettability is an important rock characteristic to understand. In general, wettability is determined by looking at a core sample through a microscope and applying a drop of oil or water to the rocks surface and observing it's shape.Measure of wettability
If the observed angle between the surface and the slope of the droplet is lower than 75 degrees the rock is considered water wet. If the slope is larger than 105 degrees the rock is considered oil wet. Any angle in between 75 and 105 degrees is considered intermediately wet. This is depicted in the following image. Wettability and reservoir performance
Even though wettability is not always determined it can be of tremendous importance for reservoir performance. If you consider an oil reservoir that is water wet the water will preferably stick to the grains while the oil will flow through the pore throat (eventhough somewhat hampered by the film of water) and generally you can recover a lot of oil before water sweeps in from the aquifer. If this same system was oil wet the oil would preferably stick to the grains and water would flow more readily. This would lead to much quicker water production that causes obvious problems for lift mechanisms in the well, surface treatment and the general recovery factor.
This is somewhat counteracted by oil wet systems having lower irreducible water saturation and associated higher average oil saturations, but this oil is generally less mobile and harder to extract. A water wet and an oil wet pore system is shown below:Gas wettability
Even though the example (with the non-stick frying pan) implicated that there are gas wet systems (the air had a preferential bond to stick to the pan over the water) this never occurs in subsurface reservoirs. There are no gas wet systems.
For gas reservoirs you can safely assume that the wetting phase is either the oil or the water. The preference for the oil or water sticking to the grains is so strong that as soon as there is influx of these fluids they can quickly form films around grains and can thus plug pore throats and restrict gas movement tremendously. Having a strong aquifer under a gas reservoir is therefore not very favorable. Also have a gas oil contact move upwards during production is also not considered good reservoir management. In both cases chances are that when the oil or water contacts pores that were previously mainly in contact with gas will now become coated in oil or water hampering the movement of gas to the well bore. What determines wettability
The geological processes or mineralogy governing wettability is not fully understood but in general there are some rules of thumb.
Usually clastic sandstones are prone to be water wet, while most carbonates are oil wet. There are a lot of exceptions to this rule though. Another common rule of thumb is that wettability develops over time. In some settings is believed that over time oil in a reservoir will start to chemically interact with the rock and start displacing water thereby converting the reservoir into an oil-wet system. The length over which (in geological time) a reservoir is exposed to a certain fluid can therefore determine the wettability of that reservoir.Other methods of Determining wettability
As mentioned before the most reliable way to determine wettability is by using a microscope in the lab. However due to core handling and effects of using certain drilling fluids can alter the original rock characteristics (at reservoir conditions) this method is not error prone or truly accurate.
Alternatively one can look at analogues, like nearby fields or similar sedimentary settings. In this case one should also take the reservoir conditions into account and the fluid compositions, plus the mineralogy, which is often not reported in detail for most reservoir studies.
Another method relies on the effects of wettability in affecting relative permeability’s
. In general oil wet rocks will have lower irreducible water saturations and higher irreducible oil saturations. Also the cross over between the oil and water rel. permeability curves usually lies further to the middle of the rel. perm plot and the plot is more symmetrical (maximum relative permeability of water closely matched that of the oil). In water wet systems the irreducible water saturation is somewhat higher and the irreducible water saturation is lower. The movement of water is much more restricted and the maximum rel. perm of water is not as high as that of the oil. Also the cross over point between both rel. perm curves lies more towards 100% water saturation. This is depicted in the image below.
Lastly, one can look at reservoir (water cut) performance or well tests. Oil wet systems generally produce water early on in field life as the water is more mobile. However, if a field is already fully developed and the wettability is only then determined, this is usually too late. Only solvents may then help change the wettability and help drive out more oil.
Remember, the methods described here are rules of thumb and should never be used in isolation, but should be ideally checked with other methods/measurements.