Capillary (spontaneous) imbibition recovery in naturally fractured reservoirs can be enhanced by adding surfactant into injected water. Selection of proper surfactant type and concentration is essential in this process as the reduction in IFT by addition of surfactant does not always yield an incremental recovery over waterflooding. Capillary imbibition experiments conducted on sandstone, chalk, and limestone samples using different oils (crude and processed with different viscosities) and different surfactant solutions were evaluated for surfactant screening. In this evaluation IFT, surfactant type and concentration were considered. In addition to these, a new technique has been adopted to facilitate the surfactant screening process. This technique is based on assigning inorganic and organic property values and plotting Organic Conception Diagrams (OCD) for surfactants. OCD defines the property of a compound in terms of physical chemistry in such a way that the property that depends much on Van der Waals force is called "organic" and the one that depends much on electric affinity is called "inorganic". The OCD has been widely applied especially in the field of environmental and pharmaceutical chemistry as means that express the property of organic materials that feature relatively complex interactions. By using the OCD, hydrophilic and lipophilic characters of surfactants have been established to evaluate their potential to minimize oil/water interfacial tension.
Correlations between the capillary imbibition recovery performance and the properties of surfactant and oil (organic value (OV), inorganic value (IV), and IFT of surfactant solutions, oil viscosity, and surfactant type) were sought. These correlations are expected to be useful in selecting the proper surfactant for improved oil recovery as well as identifying the effects of surfactant properties on the capillary imbibition performance.
Using surfactant solutions as a displacement fluid to recover matrix oil in naturally fractured reservoirs (NFR) has gained a great deal of attention in recent years. Capillary imbibition recovery can be enhanced by the addition of surfactant into water to reduce the IFT.1 Depending on the size and wettability characteristics of the reservoir matrix, the gravitational forces might play a role in the matrix recovery when the IFT is lowered using surfactant solutions as an aqueous phase.2–5
A group of studies tested the capillary imbibition recovery performance of sandstone matrix when surfactant solutions are used1–10. Typically an increase in the ultimate recovery with lowered IFT was observed. The imbibition rate also changes with reduced IFT. Another group of study used carbonate rocks.1,8,11,12 Due to less water-wet character of this type of rocks, in many cases, very low imbibition recovery was obtained with brine. Reduction of IFT by addition of surfactant yielded significant increase in the recovery due to enhanced capillary imbibition and additional contribution due to gravity effect.1,7,8 More attention was devoted to chalky carbonates as they are found more responsive to the imbibition recovery compared to the dolomitic carbonates or strongly water-wet sandstones.13–21
It was observed that the response to the surfactant capillary imbibition recovery could be very different depending on the rock type and fluid properties even if the same surfactant type and concentration are used.