Wettability and Oil Recovery by Imbibition and Viscous Displacement from Fractured and Heterogeneous Carbonates

DE-FC26-02NT15344

This project was selected in response to DOE's Oil Exploration and Production solicitation DE-PS26-01NT41048, Focus Area No. 13, Critical Upstream Advanced Diagnostics and Imaging Technologies.

Project Goal
The project goals are to:

• Relate wettability alteration of carbonate surfaces to methods of wettability control of carbonate rocks by adsorption from selected crude oils.
• Measure oil recovery and characterize wettability by spontaneous imbibition measurements for strongly water-wet carbonate rocks and for the same rocks after systematic changes in wettability induced by adsorption from crude oil.
• Investigate the sensitivity of oil recovery to displacement rate for carbonate rocks.

Performers
University of Wyoming
Laramie, WY

New Mexico Institute of Mining and Technology
Socorro, NM

Project Results
Researchers measured wettability alteration at carbonate surfaces for more than 16 crude oils. Correlations were reported for imbibition into carbonate rocks for very strongly water-wet and crude oil-induced wettability variations. The project also demonstrated the sensitivity of oil recovery to flow rate for heterogeneous outcrop limestone for very strongly water-wet conditions and for mixed-wettability states established by adsorption from crude oil.

Researchers identified sources of outcrop carbonate rock and made unique comparisons for oil recovery by spontaneous imbibition and waterflooding for very strongly water-wet conditions versus a range of mixed-wettability states for different rock types and crude oils. They also demonstrated the rate sensitivity of waterflood residual oil originally observed for reservoir carbonate rocks.

Benefits
Major advances have been made in methods of wettability control in carbonates that serve as model rocks. The completed parametric studies of oil recovery by spontaneous imbibition and viscous displacement will find many other applications in improved oil recovery and reservoir diagnostics. These results confirm and explain the unexpected but repeatedly observed sensitivity of oil recovery to injection rate reported by industry for reservoir carbonate cores. Companies (mainly independents) operating in states such as Wyoming-in which a substantial fraction of oil-in-place is held in heterogeneous and fractured carbonates-have a direct economic interest in this project.

Background
About half of U.S. oil reserves is held in carbonate formations. The remaining oil in low-permeability fractured carbonate reservoirs is a major domestic target for improved oil recovery. This study addresses the quantification of crude oil/brine/rock interactions and the impact of heterogeneity on oil recovery by spontaneous imbibition and viscous displacement.

The problem of wettability and oil recovery from carbonate reservoirs was recognized throughout the oil industry as being largely unexplored. Both laboratory and field evidence of the sensitivity of waterflood recovery to flow rate-even down to field rates-revealed phenomena that could be turned to advantage with respect to improved recovery. For example, increased injection pressure, increased injectivity, decreased well spacing, or lowering of interfacial tension would lead to additional oil recovery.

Project Summary
In this project, researchers have:

• Identified sources of outcrop carbonate rock.
• Identified crude oils for oil recovery studies from contact angle measurements.
• Made unique comparisons for oil recovery by spontaneous imbibition and waterflooding for very strongly water-wet conditions versus a range of mixed wettability states.
• Demonstrated the rate sensitivity of waterflood residual oil at below and above field flow rates for heterogeneous outcrop carbonate rock.

Wettability-alteration characteristics of crude oils measured at carbonate surfaces were related to the properties of the crude oils. The effect of wettability on oil recovery was investigated for limestones that cover more than three orders of magnitude in permeability and a factor of four variations in porosity.

The rate sensitivity of oil recovery to waterflooding has been demonstrated for very strongly water-wet and mixed-wet heterogeneous carbonate rocks. Homogeneous samples did not exhibit rate sensitivity at any wetting condition. These results are consistent with reservoir condition tests reported by industry researchers.

Current Status
Nearly all aspects of the project are now complete. A 6-month, no-cost extension has been requested, mainly to allow completion of tracer test work that will utilize equipment at the University of Bergen's Department of Physics. (Two PhD students from the University of Wyoming are currently at Norway's University of Bergen with Prof. Arne Graue.)

Industry has shown strong interest in this project. Operators continue to observe the rate sensitivity of oil recovery to waterflooding for many carbonate reservoirs and are likely to provide matching support for further work on rate effects and relative permeabilities for carbonate rocks. The advances made in wettability control for a variety of rock types (including sandstones) provide the opportunity for the study of variation in relative permeabilities resulting from systematic change in wettability.

Publications
Tie, H.G., Tong, Z.X., and Morrow, N.R., The Effect of Different Crude Oil/Brine/Rock Combinations on Wettability through Spontaneous Imbibition, Proceedings of the International Society of Core Analysts Annual Meeting, Pau, France, September 2003.

Fischer, H., Morrow, N.R., and Mason, G., Application of water desorption/adsorption isotherms data to characterization of micro- and meso-porosity in sandstone and carbonate rocks, presented at the 7th International Symposium on the Characterization of Porous Solids, COPS VII, Aix-en-Provence, France, May 26-28, 2005.

Tie, H., and Morrow, N.R., Oil Recovery by Spontaneous Imbibition Before and After Wettability Alteration of Three Carbonate Rocks by a Moderately Asphaltic Crude Oil, to be presented at the International Society of Core Analysts Annual Meeting, Toronto, Canada, August 21-34, 2005.

Tie, H., and Morrow, N.R., Low Flood Rate Residual Saturations in Carbonate Rocks, to be presented at the International Technology Conference, Doha, Qatar, November 21-23, 2005.

Project Start: July 18, 2002
Project End: July 17, 2006

Anticipated DOE Contribution: $749,522
Performer Contribution: $333,315 (30% of total)

Contact Information
NETL - Chandra Nautiyal (chandra.nautiyal@netl.doe.gov or 918 699-2021)
U. of Wyoming - Norman Morrow (morrownr@uwyo.edu or 307-766-2838)

Wyoming Cottonwood carbonate.

Yates carbonate.