Sketch of an initially oil-wet fractured carbonate. Surfactant water can go into the matrix by compression of initial gas and change wettability. Then, water can be imbibed from the bottom, and oil can be produced from the top. (Images courtesy of University of Houston)

About half the oil reservoirs worldwide are carbonates, and many of these carbonate reservoirs are naturally fractured. The surface charge on carbonate minerals is positive, and often the reservoir oil has negatively charged organic molecules like naphthenic acids. These molecules adsorb on the mineral surface and make it oil-wet (Figure 1). Primary production is effective in these reservoirs, but waterflooding is not. Injected water runs through the fractures in such reservoirs without imbibing into the matrix because of oil-wettability, which leads to very low waterflood recovery.

Solution to the problem

Surfactant treatment techniques are being developed to improve oil recovery from oil-wet/mixed-wet, fractured carbonate formations. Studies have identified alkaline-anionic surfactant solutions that can change the wettability of initially oil-wet calcite plates to water-wet and reduce the interfacial tension to about 0.01 mN/m at very low (<0.1 wt%) surfactant concentrations.

Figure 2 shows oil recovery from initially oil-wet cores by different alkali surfactant solutions at two different temperatures. Although oil recovery is very low with just NaCl brine, recovery is significant with alkaline-surfactant solutions. As the temperature goes up, the oil recovery rate increases.

Surfactants alter the wettability by solubilizing adsorbed hydrophobic components. More than 60% of the original oil can be recovered from initially oil-wet cores by dilute (< 0.1 wt%) alkaline surfactant solution imbibition. The adsorption of surfactant on calcite minerals can be suppressed by adding alkali.

Modeling the surfactant solution imbibition process resulted in simulator results that matched experimental results at the laboratory scale. The simulations show that surfactant solution reduces capillary pressure and makes the pores preferentially water-wet.

Increased water-wettability increases oil relative permeability, which enhances the rate of oil drainage by gravity. Surfactant solution imbibes from the sides (and the bottom), and oil is recovered from the top.

Surfactants that alter the wettability to a water-wet regime give higher recovery rates for higher interfacial tension (IFT) systems. Surfactants that cannot alter wettability give higher recovery for lower IFT systems. As the wettability alteration increases, the rate of oil recovery increases. The recovery rate decreases with permeability significantly for a low IFT system, but only mildly for high IFT systems. Increasing the fracture block dimensions while increasing oil viscosity decreases the rate of oil recovery and is in accordance with the scaling group for a gravity driven process.

This alkaline-surfactant imbibition process slows down in the presence of non-zero gas saturations in the formation because the presence of gas blocks the flow of oil. This reduces the oil relative permeability.

Many abandoned reservoirs with successful primary production are expected to have non-zero gas saturation. An effective process comprising the following steps has been developed for such reservoirs.

First, a surfactant solution is injected into the reservoir with the production wells shut to increase the reservoir pressure. This step shrinks the gas phase, lowering the gas saturation. The surfactant solution invades the matrix to replace the volume created by gas shrinkage. The presence of surfactant solution changes the wettability to a preferentially water-wet condition, and interfacial tension is lowered, which reduces the capillary forces. This allows the oil to detach from the pore surface and move up in the matrix block via gravity drainage.

In the second step, surfactant solution injection resumes, and production wells are opened. The reservoir pressure must be kept at the high
pressure achieved by the compression in step one. Step two allows further imbibition of surfactant solution into matrix blocks, which in turn allows
oil production from matrix blocks into fractures and into the production wells. Injection and production wells should be drilled such that they connect to all the fracture networks. Adding the pressurization step leads to a significant increase in oil recovery rates as surfactant diffusion — the rate-determining step in conventional surfactant treatment processes that do not use pressurization — is replaced by forced imbibition. Figure 3 shows the increase in the oil recovery rate.

This process can also be applied in the “huff-n-puff” mode. It includes injecting a surfactant solution while keeping production wells closed, and then allowing the high pressure to force the surfactant solution deep inside the carbonate fracture blocks as a result of gas compression. The reservoir is left in this high-pressure state for a period of time, during which the surfactant changes wettability and lowers interfacial tension. Gravity can drain the oil up the matrix blocks into the fractures. When this has taken place, wells can be opened for production, allowing oil to be produced from fractured reservoirs.

The dilute surfactant injection process is a relatively inexpensive augmentation of the waterflooding process. Chemical cost is less than $1/bbl because the surfactant concentration is below 0.1 wt%.

The role of surfactants

Surfactants that change the wettability of initially oil-wet carbonate rocks at low concentrations can be identified. Pressurization of depleted fractured, carbonate reservoirs by surfactant-alkali solutions leads to lowering of gas saturation and increase in water-wettability. This drives oil out of the matrix and into the fractures, from which it can be produced to production wells.

The rate of oil recovery depends on fracture spacing and the degree of water-wettability attained. Regardless of the increase in production numbers, at a cost of less than $1/bbl, it’s well worth the investment.

Acknowledgements

This research was supported partly by US Department of Energy, ACS Petroleum Research Fund, and the Institute for Improved Oil Recovery.

Editor’s Note

This article is based on a patent submitted by University of Houston titled, Process for Enhancing the Production of Oil from Depleted, Fractured Reservoirs Using Surfactants and Gas Pressurization and SPE 110204, presented at the SPE Annual Technical Conference and Exhibition, 11-14 November, Anaheim, CA, 2007.