The goal of this research is to evaluate dilute (hence relatively inexpensive) surfactant methods for carbonate formations and identify conditions under which they can be effective.

Program
This project was selected in response to DOE's Oil Exploration and Production solicitation DE-PS26-01NT41048 (July 2001).

University of Houston
Houston, TX

There are many carbonate reservoirs in the United States (and the world) with light oil and fracture pressure below its minimum miscibility pressure (or the reservoir may be naturally fractured). Many carbonate reservoirs are naturally fractured. Waterflooding is effective in fractured reservoirs, if the formation is water-wet. Many fractured carbonate reservoirs, however, are mixed-wet, and recoveries with conventional methods are low (less than 10%). Thermal and miscible tertiary recovery techniques are not effective in these reservoirs. Surfactant flooding is the only hope, yet it was developed for sandstone reservoirs in the past. This project is aimed at developing an inexpensive, surfactant-based process for improved oil recovery.

Project Results
Surfactants have been identified that alter wettability of calcite minerals aged with a crude oil and that lower interfacial tension. Surfactant adsorption can be minimized by the use of an alkali. Laboratory imbibition tests show about 61% oil recovery with an anionic surfactant and 37% in the case of a cationic surfactant. A numerical model has been developed that fits the rate of imbibition of the laboratory experiments. Field-scale fracture-block simulation shows that as the fracture spacing increases, so does the time of recovery.

Benefits
The waterflood recovery in fractured carbonate reservoirs is typically very low. This dilute surfactant method can be used to improve the oil recovery in high-permeability reservoirs by almost 60%.

Project Summary
The major achievements of this project are:

• Identification of anionic surfactants that alter wettability and reduce surface tension.
• Reduction of adsorption by the use of an alkali.
• Laboratory demonstration of high oil recovery by surfactant solution imbibition.
• Mechanistic simulation showing the scale-up of the process.

This project is aimed at developing a dilute surfactant flooding process for fractured carbonates. Researchers have conducted laboratory tests with a West Texas crude oil. They have matched the experiments with mechanistic simulations. More than 60% of the oil can be recovered by spontaneous imbibition of a 0.05 percent by weight surfactant solution.

(August 2005)
The five tasks for the project are 1) adsorption, 2) wettability alteration, 3) gravity and viscous mobilization, 4) imbibition, and 5) simulation. Researchers have finished the first three tasks and are currently working on the last two tasks, which will be completed in the next four months.

Publications
Seethepalli, A., Adibhatla, B. and Mohanty, K.K., Physicochemical Interactions During Surfactant Flooding of Carbonate Reservoirs, SPE J., 9 (4), 411-418, December 2004.

Kumar, K., Dao, E., and Mohanty, K.K.,Atomic Force Microscopy Study of Wettability Alteration, SPE 93009, Proceedings of SPE Intl. Symp. on Oil Field Chemistry, Woodlands, Feb. 2-5, 2005.

$592,661

$150,000 (20% of total)

NETL - Betty Felber (betty.felber@netl.doe.gov or 918-699-2031)
U. of Houston - K.K. Mohanty (mohanty@uh.edu or 713-743-4331)