Engineers from the University of Houston and the University of Texas report that waterflooding with a surfactant—thereby altering the wettability of oil reservoir rocks—can increase oil recovery from 62 to 85%. In their paper in the ACS journal Energy & Fuels, they say the increase is “significant, and needs to be evaluated at the field scale.”
About 60% of the world’s remaining oil resides in carbonate rock reservoirs; the rock can be fractured or nonfractured, they note. Carbonate rocks tend to be “oil-wet”—i.e., in contact with an oil phase rather than a water or gas phase. Oil-wet rocks preferentially imbibe oil.
Current recovery techniques, such as waterflooding, recover only 40–50% of the original oil in place (OOIP) in nonfractured carbonates; the recovery is much smaller in fractured oil-wet carbonate reservoirs. Considering the large amount of oil in place, the poor waterflood recovery, and the increasing world energy demand, it is very important to improve oil recovery from carbonate reservoirs. In this work, we are interested in improving oil recovery from a low-permeability (2–5 md) nonfractured carbonate reservoir with a relatively low reservoir temperature (52 °C).
Enhanced oil recovery (EOR) techniques, such as CO2 flooding and alkaline/surfactant/polymer (ASP) flooding, have been developed to recover the remaining oil after waterflood. CO2 is miscible with oil at pressures higher than its minimum miscibility pressure and displaces oil miscibly. The sweep efficiency can be improved by water-alternating gas or foam injection. The key problem in applying CO2 flooding is the availability of a large quantity of CO2
Surfactants can be used to lower the interfacial tension (IFT) between oil and water and increase the capillary number to mobilize residual oil blobs. Polymers are used to improve the sweep efficiency by providing mobility control. Alkali can be used to generate in situ surfactants with acidic oils and increase pH to lower surfactant adsorption. ASP techniques were initially developed for sandstone reservoirs.
In the last 10 years, development of ASP techniques for carbonate reservoirs has received considerable attention. Even then, applications are limited to reservoirs with permeabilities of 20 md and higher because it is difficult to find polymers that can pass through lower permeability rocks and provide mobility control.
If EOR techniques cannot be applied, then the efficiency of the waterflood should be increased.—Mohan et al.
Mohan et al. added a surfactant to a high-salinity injection brine. To make the process economically favorable, they focused on low surfactant concentrations, about 0.2 wt %. They chose a surfactant that changed the wettability to an intermediate wettability.
Using just brine (without surfactant), they found that secondary waterflooding recovers about 62% of the oil in about 3 PV (pore volume) injection and 80% in about 16 PV. Secondary flooding with surfactant recovered about 85% in about 3 PV and about 90% in 4 PV. Injection of 1 PV of surfactant solution followed by waterflood also recovers about 83% of the oil in a total injection of 3 PV.
In typical field waterfloods, 2–3 PV of water is injected. Thus, this increase in oil recovery from 62 to 85% is very significant. Future studies will be directed at optimizing the surfactant slug size.—Mohan et al.
K. Mohan, R. Gupta, K. K. Mohanty (2011)Wettability Altering Secondary Oil Recovery in Carbonate Rocks. Energy & Fuels Article ASAP doi: 10.1021/ef200449y