One of the fundamental realities of oil production is that simply drilling into a reservoir will not release all, or even most, of the oil within in. Primary and secondary oil recovery methods only release 20% to 40% of a reservoir’s oil. Enhanced oil recovery can extract more than 30% and sometimes more than 60% of the oil in a formation, depending on the wettability of the formation of the surfaces in contact with the oil.
The surface of a solid substance is said to be wet when a liquid adheres to it. For wetting to occur, the forces of adhesion between the solid surface and the liquid must be greater than the forces of cohesion that keep the liquid flowing.
Wettability is the tendency of a liquid to spread over a solid surface. In enhanced recovery of oil, wettability determines the interactions between the rock or minerals that surround the oil and the liquids in the underground reservoir, either oil or brine.
Rock is said to be water-wet when it is preferentially in contact with brine. It is said to be oil-wet when it is preferentially in contact with oil. It can be mixed-wet when it is contact with both.
The wettability of the rock walls of a formation is measured in terms of contact angles. A narrow angle of contact occurs when the oil or brine tends to flow off the rock containing it. A wide angle of contact occurs when oil or brine tends to flow across the rock or oil containing it.
Contact angles can be objectively measured. A relatively narrow (<75°) contact angle between oil and water means that water adheres to rock and oil flows off it. A relatively wide (>180°) contact angle means that the rock is oil-wet, and oil tends to cling to it. The objective of changing wettability is to ensure that the fluids in the reservoir become water-wet, so water clings to rock, and oil flows up the wellhead.
Why Is Wettability a Concern in Enhanced Recovery?
Wettability is one of the most important concerns in enhanced recovery. It’s particularly a concern when oil is extracted from carbonate, usually limestone, rock. Between 50% and 60% of all the world’s oil occurs in limestone formations.
Recovery is easier when oil is held with a non-carbonaceous rock formation, usually sandstone. It is more challenging to recover oil from carbonate rocks because they tend to be riddled with fracture corridors. These cracks in the oil reservoir provide a path for pressurized water to displace oil, forcing it to the surface, but they also provide paths for oil in the reservoir to become more widely dispersed, outside the scope of the recovery process.
Reducing rock wettability is key to boosting recovery rates from reservoirs in limestone. Three methods exist for changing wettability to increase recovery.
Carbon dioxide injection is the most common method for changing the wetting property of carbonate rocks to improve the results of enhanced recovery. CO2 dissolves in both oil and water, carrying it throughout the formation. One of the advantages of CO2 injection is that it reacts with injection water to form carbonic acid. The carbonic acid reacts with rock and makes it less oil wettable. Yet another advantage of CO2 injection is that the gas can be recovered along with the oil coming to the surface and reinjected to continue the process without finding a new supply.
The CO2 injected into the well is subjected to pressures that liquify it as it flows deeper into the well. This poses the downside to CO2 injection. CO2 is less viscous than either water or oil, so it tends to travel through the cracks and fractures of the limestone formation outside the reservoir. The pressure front for the CO2 becomes unstable, so some parts of the reservoir may not be reached by the gas while some of the gas travels entirely outside the reservoir. Volumetric sweep efficiencies can be improved by addition of foams and surfactants to make CO2 itself more wettable.
Chemical recovery methods encompass different kinds of polymers, surfactants, and injections of water with low salt content. Chemicals and fresh water reduce the IFT (interfacial tension) between oil and rock to make it more water-wettable and less oil-wettable.
Of these three methods, only polymer injection produces reliable results. However, surfactant in nanoparticles are being studied as a method of enhanced recovery. The theory is that nanoparticles can reach the smallest pores in the rock so less surfactant is needed for the same amount of recovered oil. However, the optimal size and composition of surfactant nanoparticles has not yet been established.
Heat has intuitive appeal s a method of enhanced oil recovery, but it has not proven to be the most productive approach to this issue. Thermal recovery methods include steam stimulation, steam flooding, and in situ combustion.
All of the thermal methods make oil less viscous, flowing more freely, and increase the water-wettability of the carbonate reservoir. They dissolve materials that may have been adsorbed to the surface of the rock, and decrease tension at the interface of water and oil. But because carbonate reservoirs tend to be highly fractures, the steam injected into the well tends to disperse.
The currently most promising method of thermal recovery involves superheated air, but this is also a technology still under development.
The Importance of Enhanced Recovery
Since the largest percentage of the world’s proven oil reserves are held in carbonaceous rock, enhanced recovery methods will continue to be key to sustained production. Changing wettability of rock will continue to be key to getting the greatest possible production of the majority of the world’s oil fields.