In oil reservoirs about 60% of the original oil in place remains as residual oil after primary and secondary oil recovery because of geological and physical factors.
By means of improved oil recovery methods, additional oil can be mobilized. However, there is no universal IOR method to be implemented in any reservoir. Therefore, efforts are made to develop alternative tertiary methods with lower application risk. One of these alternatives is the application of sound/ultrasound waves in the reservoirs to overcome the interfacial tension between oil and water, resulted in reduction of capillary pressure in the pores.
In this study, laboratory experiments on core samples were conducted to investigate the effectivity of ultrasound waves on mobilizing additional oil. The core flooding was performed horizontally and vertically and the wave stimulation was applied at original oil in place and at residual oil saturation after initial waterflooding. Oil/water relative permeability were calculated to evaluate the flooding performance in presence and absence of wave stimulation and the rate of oil recovery was determined. In addition, water fractional flow curves were considered to determine the average water saturation after breakthrough in presence and absence of ultrasound waves. Moreover, the effect of wave stimulation on unconsolidated core samples was investigated.
Results show that the rate of oil displacement increases due to various identified mechanisms, and the interaction of the generated waves with the fluids in porous media causes changes in relative permeability and in water breakthrough. Wave stimulation at residual oil saturation lead to higher oil recovery than that at original oil in place and therefore, this method is advised to be used in depleted reservoirs.
Moreover, wave stimulation on core sample with a compressive strength less than 150 psi (unconsolidated) is not recommended due to sand production.
The world oil resources are limited and the exploitation of oil fields to a higher degree is desirable, thus new methods are required to improve the recovery rates of oil fields and to recover most of the oil found in pores between rock particles.
Great efforts are being made to maximize the production of petroleum in place in each reservoir. The production history of a petroleum reservoir may be divided into different production stages. The first stage is the initial approach to produce oil and it includes primary recovery process, in which the reservoir pressure causes the fluid to flow into production wells and then to the surface. If the reservoir pressure is not significant to maintain fluid flow to the surface due to pressure decline or flow resistance, downhole pumps or gas lift are used to raise the oil to surface until the energy available to drive the reservoir fluid through rock pores is not adequate to maintain reservoir productivity. The average primary recovery rate is around 10–15% of the original oil in place, depending on the oil and rock properties as well as drive mechanism.
The second production stage known as secondary recovery methods includes fluid injection, such as gas reinjection or water flooding, into the reservoir to improve oil recovery. The injection of fluids is implemented to replace the produced reservoir fluid, thus, the reservoir pressure can be maintained, or to displace the oil directly into the production wells and then to the surface. The most common method involves flooding the reservoir with water. The ultimate recovery factor can be increased to about 40% by employing the secondary recovery method.1–4
The main causes of the poor recovery of the first two production stages are the existence of the interfacial tension between oil and water (capillary forces), high mobility ratio, and the heterogeneities in the reservoir rock. Therefore, the oil left in the reservoir after the primary and secondary methods is the potential target of the third production stage, namely the tertiary recovery methods. The tertiary recovery method is often termed as Enhanced Oil Recovery (EOR). In order to recover some of the oil left in the reservoir, EOR-methods have to be applied to overcome the physical and geological effects. The main goal of the EOR methods is one or more of the following.2,5–9:
Reduction of the interfacial tensions between oil and water, which in turn reduces capillary pressure.
Decrease of the mobility ratio between oil and water by decreasing oil viscosity or increasing water viscosity.
Injection of chemical solvents.