How Altering Salinity Can Increase Oil Production
Initially, petroleum reservoirs are filled with water that is distributed throughout the porosity and permeability architecture. As oil migrates into the reservoir, it fills as much as 80-90% of the porosity, displacing the water. Some of the oil is free-phase liquid, but a portion adheres to the rock surface. This oil is ‘wetting’ the rock. While the free-phase oil can be displaced by water injection, some of the oil and water is wetting or bound and cannot be displaced. This condition is attributed to the system’s wettability, which is the chemical equilibrium between rock surfaces, water and oil. The immobile water and oil are termed irreducible because they cannot be displaced (see Figure 1).
Current production practices recover 32% of oil in place (global average) after waterflood (secondary recovery) due to the relative permeability effect and geological heterogeneity. Relative permeability is the observation that mobility of both liquids is proportional to their saturation. If the rock is completely water saturated, the relative permeability to water is one. Adding an immiscible liquid such as oil to the pore system lowers the permeability of the rock to water in proportion to how much oil is present. The oil, in effect, lowers the area through which water can move until the oil is pushed out of the way.
Figure 1. Effect of saturation on relative permeabilites to water and oil in unconsolidated sands. Modified from Buckley and Leverett, SPE 942107 (1942). Fluid displacement cannot move more than 80% of the water or 10% of the oil in this system. Each reservoir system has different curves.
High oil saturations favor oil mobility while hindering water mobility. So when oil is most of the liquid (green field), oil is easily displaced through the higher permeability zones of the reservoir. This condition favors production of oil while hindering the production of water (low water cut). The permeability to water increases during a waterflood as the percentage of oil drops. Injected water is preferentially channeled along the high permeability zones, sweeping out more oil and further increasing permeability to water until oil saturation is low enough that most oil movement is effectively stopped. Water cuts increase and the field is considered a brown field. Further water injection is now channeled along the high permeability pathways leaving the lower permeability portions of the reservoir relatively untouched. The initial permeability contrasts within the reservoir have been further enhanced by the relative permeability effects, stranding the majority of the oil. This effect generates the production curves seen in Figure 2.
Figure 2. fw, fraction of water in flowing stream (blue curve) and change in fw with change in water saturation (black curve) versus water saturation in unconsolidated sands. Modified from Buckley and Leverett (1942). Green area is equivalent to oil recovery.
The remaining free-phase oil in the lower permeability zones and the bound oil distributed throughout the reservoir is trapped. Because this oil cannot be recovered by current technology, it is classified as a resource. World oil resources are more than double the current total production (1 trillion barrels). Recovery of some of the remaining 2 trillion barrels would yield very significant income. For instance, recovering an additional one percent globally equals an additional 20 billion barrels, worth about $1 trillion ($50/bbl).
We can achieve this increased production with engineered salinity. We alter the salinity of the injected water to alter the reservoir wettability. Changing wettability releases the bound oil. Referring to Figure 3, releasing the bound oil increases the local oil saturation(1) which lowers the permeability to water(2). As the higher permeability zones are now less permeable to water, injected water is now diverted into lower permeability zones displacing more free-phase oil and releasing more bound oil. This restarts the process of oil movement as seen in Figure 3. This process can recover up to an additional 30% of the original oil in place. ESal™ can design and apply the correct salinity to increase recovery.
Figure 3. fw, fraction of water in flowing stream (blue curve), and change in fw with change in water saturation (black curve) versus water saturation in unconsolidated sands. Release of bound oil increases the local oil saturation(1) lowering the permeability to water(2) and restarts oil flow.