Wettability and Prediction of Oil Recovery from Reservoirs Developed with Modern Drilling and Completion Fluids

DE-FC26-01BC15164

Project Goal
The objectives of this project are to:

• Improve understanding of the wettability alteration of mixed-wet rocks that results from contact with the components of synthetic oil-based mud (SBM) formulated to meet the needs of arctic drilling.
• Investigate cleaning methods to reverse the wettability alteration of mixed-wet cores caused by contact with these SBM components.
• Develop new approaches to restoration of wetting that will permit the use of cores drilled with SBM formulations for valid studies of reservoir properties.

Performers
New Mexico Institute of Mining and Technology
Socorro, NM

University of Wyoming
Laramie, WY

Project Results
In the course of this project, performers have studied the effects of SBM surfactants on wetting in mixed-wet cores, identified a previously unrecognized mechanism for wetting alteration due to asphaltene destabilization that can occur when crude oil mixes with synthetic base oils, and developed a cleaning procedure for use with contaminated cores.

Benefits
In the Arctic, severe surface conditions combined with a fragile ecosystem make advance planning of production facilities especially critical while limiting the choices of fluids that can be used for drilling. SBMs muds now can be formulated with low viscosities so that they can be used in arctic conditions, but all oil-based muds adversely affect the wetting state of recovered cores. Using outcrop rock and selected stock tank crude oils researchers have been able to assess the affects of synthetic base oils and surface active additives on wetting and to design a cleaning system that adequately reverses the effects of oil-based drilling mud contamination, allowing wettability assessment of the limited samples that would be available from an arctic drilling project to proceed.

Background
Wettability controls the rate and extent of fluid flow when two or more phases are flowing in a reservoir. Correct assessment of reservoir wetting is an important part of planning production facilities and strategies. However, the uncontaminated core and oil that are required to perform wettability assessments correctly are not available, especially when wells are drilled with oil-based fluids. Nevertheless, traditional and synthetic oil-based drilling fluids are increasingly chosen, necessitating improved methods of identifying contamination, assessing its effects, and, if possible, cleaning samples for further study. In this project, researchers have addressed the issues associated with the various components of SBM of the sort most likely to be selected for arctic drilling projects.

Project Summary
Project researchers have:

• Developed interfacial tension, contact angle, and atomic force microscopy (AFM) methods for studying complex interactions among crude oils, SBM base oils, SBM surfactants, and mineral surfaces.
• Investigated effects of crude oils, SBM base oils, and SBM surfactants on wetting of mixed-wet sandstone cores.
• Demonstrated that SBM base oils can destabilize asphaltenes and alter wetting by surface precipitation.
• Developed a cleaning procedure that adequately removes contaminants from cores acquired during drilling/coring projects that use SBM drilling fluids, allowing wettability testing of contaminated samples.

Two main approaches to wettability testing were combined in this work: surface tests of mineral wetting and core wettability tests of the rate of imbibition. Parallel tests of similarly treated surfaces and cores can help to differentiate between effects that are correctly ascribed to wettability alteration and effects that may have other explanations. The project performers also chose to examine the effects of some individual SBM components in order to demonstrate the effects of surface active additives, such as emulsifiers, as well as the effects of synthetic base oils that are not themselves surface-active but affect wetting by changing the stability of asphaltenes.

Mixed wetting-with corners and smallest pores remaining water-wet and larger pores that are exposed to oil becoming less water-wet or more oil-wet-is widely regarded as a likely wetting scenario in oil reservoirs. The project performers have focused attention on the effects of SBM components on the wetting of surfaces after exposure to various crude oils and on the wetting of mixed-wet cores. Initial experiments were designed to select a range of crude oils for subsequent SBM tests. In most cases, treatments with emulsifiers made cores less water-wet. Cores could also be made less water-wet by displacement of asphaltic crude oils by paraffinic or olefinic synthetic base oils.

Finally, the researchers have designed and tested a cleaning regimen for removal of SBM-related wetting damage that should allow the use of SBM-contaminated cores in reservoir wettability tests.

Current Status (July 2006)
This project has been completed.

Project Start: October 1, 2001
Project End: September 30, 2005

Anticipated DOE Contribution: $850,000
Performer Contribution: $420,000 (33% of Total)

Contact Information:
NETL - Rhonda Jacobs (rhonda.jacobs@netl.doe.gov or 918-699-2037)
New Mexico - Jill Buckley (jill@prrc.nmt.edu, or 505-835-5405)

Publications
Final Report [PDF-1.36MB]

Seven semi-annual reports to DOE.

Bryant, E., Bowman, R.S., and Buckley, J.S., Wetting Alteration of Mica Surfaces with Polyethoxylated Amine Surfactants, presented at the 8th International Reservoir Wettability Symposium, May 16-18, 2004, Houston, TX (accepted for publication in J. Pet. Sci. Eng., 2005).

Buckley, J.S., and Fan, T., Crude Oil/Brine Interfacial Tensions, paper SCA 2005-P017, presented at the 2005 SCA Symposium, Toronto, Canada, August 21-25.

Creek, J.L., Gonzalez, D., Wang, J.X., Muhammad, M., Chapman, W., Hirasaki, G.J., and Buckley, J.S., Effect of Synthetic Oil-Based Drilling Fluid Contamination on Asphaltene Stability, presented at the 6th International Conf. on Petroleum Phase Behavior and Fouling, Amsterdam, Netherlands, June 19-23, 2005 (submitted to J. Disp. Sci. Tech.).

Skalli, L., Buckley, J.S., Zhang, Y., and Morrow, N.R., Surface and Core Wetting Effects of Surfactants in Oil-Based Drilling Fluids, presented at the 8th International Reservoir Wettability Symposium, May 16-18, 2004, Houston, TX (accepted for publication in J. Pet. Sci. Eng., 2005).

Tong, Z. and Morrow, N.R., Variations in Wetting Behavior of Mixed-Wet Cores Resulting from Probe Oil Solvency and Exposure to Synthetic Oil-Based Mud Emulsifiers, presented at the 8th International Reservoir Wettability Symposium, May 16-18, 2004, Houston, TX (accepted for publication in J. Pet. Sci. Eng., 2005).

Zhang, Y., Wang, J.X., Morrow, N.R., and Buckley, J.S.: "Effect of Synthetic Drilling Fluid Base Oils on Asphaltene Stability and Wetting in Sandstone Cores," presented at the 5th International Conference on Petroleum Phase Behaviour and Fouling, Banff, Alberta, Canada, June 13-17, 2004 (accepted for publication in Energy&Fuels, 2005)

AFM image of a mica surface after exposure to an SBM emulsifier. The surfactant rearranges but cannot be removed by scraping (as shown in the center).