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), so 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 waters viscosity.
Polymer Injection :
When polymers, are mixed in together with injection water for enhanced oil recovery, the polymer-water injection solution becomes more viscous. As a consequence, more oil will be pushed out and recovered from the reservoir compared to only water injection flooding alone. An illustration of a typical polymer flood operation is given in Figure 1.
Figure 1: Typical North Sea bulk separation and produced water treatment system
Inefficient oil recovery was recognized in the early 1900’s. Normally, reservoir oil is ten times more viscous than the water injection stream alone. This makes it hard for the injected water stream to sweep trapped oil from the reservoir to the production wells. By adding polymers to the injection water, the injection stream gets more viscous than the oil, reducing breakthrough (fingering) of the water to the production wells, and hence sweeps out more oil from the reservoir. Fingering of water is illustrated in Figure 2. The improved sweep efficiency when polymers are added is often called the “piston” effect, and can enhance oil recovery by 5 – 15 % compared to conventional methods.
Figure 2: Fingering of water. Kilde: Typhonix AS
Oil recovery from a reservoir is dependent on mobility ratio, and polymer injection improves the mobility ratio. Mobility is 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, water containing polymer is the displacing phase, and oil is the displaced phase, as can be seen in Equation 1.
K Effective permeability
Typically, polymer flooding involves injection and mixing of polymer over an extended period of time, until 1/3 - 1/2 of the reservoir pore volume has been injected. Then a continued long term water flooding follows. The water flooding drives the polymer slug and the oil bank towards the production wells, as illustrated in Figure 1.