Enhanced oil recovery (EOR) is a tertiary oil recovery method used to increase the amount of oil that can be extracted from a reservoir. Primary pressure depletion, and secondary oil recovery, like water injection, only recovers 20 – 50 % of original oil in place (OOIP). Therefore, a large amount of oil is still trapped in the reservoir after conventional oil recovery processes. There are many different tertiary EOR methods, however, polymer injection and polymer flooding is the most important chemical EOR method used in sandstone reservoirs.
Polymers are long chain molecules composed of many repeated subunits with high molecular weight. When added to water, either as a solution or as powder, they increase the water viscosity. The more viscous polymer-water solution is injected into the reservoir for enhanced oil recovery. As a consequence, more oil will be pushed out and recovered from the reservoir compared to only waterflooding alone. An illustration of a typical polymer flood operation is given in Fig. 1.
Inefficient oil recovery was recognized in the early 1900’s. Reservoir oil can be several orders more viscous than water. This makes it hard for the injected water to sweep trapped oil from the reservoir to the production wells. By adding polymers the injection stream gets more viscous than the oil. Alteration in the viscosity reduces breakthrough (fingering) of the water to the production wells, and hence increase efficiency of oil sweeping from the reservoir. Fingering of water is illustrated in Fig. 2. The improved sweep efficiency by polymer flooding is often called the “piston” effect, and can enhance oil recovery by 5 – 15 % compared to the conventional waterflooding method.
Oil recovery by flooding a reservoir is dependent on mobility ratio, and polymer injection improves the mobility ratio. Mobility is parameter determining how permeable a porous media is to a given phase, divided by the viscosity of the phase. Mobility ratio is the mobility of the displacing phase divided by the mobility of the displaced face. In polymer flooding polymer-water solution is the displacing phase and oil is the displaced phase, as can be seen in Eq. 1.
, - Dynamic viscosity of oil and water respectively, cp
, - Relative permeability of oil and water respectively, -.
Typically, polymer flooding involves injection of polymer solution over an extended period of time, until 1/3 - 1/2 of the reservoir pore volume has been injected. Then a continued long term waterflooding follows. The waterflooding drives the polymer slug and the oil bank towards the production wells, as illustrated in Fig. 1 and Fig. 2.
The benefits of polymer waterflooding are:
Mechanical degradation of injected polymers
One of the main challenges with polymer injection is that polymers easily degrade. When a polymer solution degrades, the long chain polymer molecules are broken down. Consequently, the molecular weight of the polymers decreases and the solution loses much of its viscosity. There are many things that contribute to the degradation of the polymer solution. The biggest source of the degradation is mechanical shear in fluid flow devices used for polymer injection operations. Other important factors are salt concentration, oxygen concentration and high temperatures. With today’s technologies it is expected to get at least 50% loss in viscosity of injected polymer solution due to the degradation. Until now, the well-known problems with polymer degradation have been coped with adding more polymer and using higher concentrations. This is expensive, may have a negative impact on the environment, and a more complex separation process for the produced well stream is needed. The answer to the mechanical degradation challenges may be solved with new technologies, especially low shear technologies.
Low shear polymer flow control valve
To reduce mechanical degradation of polymer solution it is important to control the level of shear forces present in the process equipment. Low shear equipment controls the turbulence in the fluid flow by reducing or increasing the pressure in a greater volume. This measure reduces the level of energy dissipation rate in the fluids, thus reducing shear forces and allowing gentler handling of the fluids.
During polymer injection, a control valve on the wellhead of the injection well regulates the injection pressure and the flow rate of polymer solution. Using a conventional choke valve similar to ones installed on production wells, would cause significant shear. The polymer solution would lose its properties even before it reaches the reservoir. To cope with this challenge the development of a Low shear polymer flow control valve is reported (www.typhonix.com). Different locations where polymer degradation occurs have been investigated, and new geometries and solutions have been implemented in the new polymer injection valve. The new valve keeps the strain rate under its critical value, which means that the polymer chains are not broken down, and polymer degradation is minimized. Simultaneously, the new injection valve can maintain control of the fluid flow in terms of pressure and flow rate. The main objective of the valve development is to reduce mechanical degradation of a polymer solution down to values of 10-15 %. Less degraded polymer can increase the oil recovery with at least 12 % compared to conventional control valves. Recent testing has shown that this objective is well within reason. The new polymer flow control valve can be designed for a given amount of degradation based on flow rate and pressure drop. This will not only increase the oil recovery for a given reservoir, but it will also reduce costs of used polymer tremendously.
Alvarado, V.; Manrique, E. Enhanced Oil Recovery: An Update Review. Energies 2010, 3, 1529-1575.
Zaitoun, A., Makakou, P., Blin, N. et al. 2011. Shear Stability of EOR Polymers. Paper SPE 141113 presented at the SPE International Symposium on Oilfield Chemistry, The Woodlands, Texas, 11-13 April.