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Laboratory Measurements of Properties for Steam/Water Flow in Geothermal Rock (2003-2004)

Figure 1: Comparison of the experimental relative permeability with the channel-

 

Reporting Period: FY 2003 (October 1, 2002 to September 30, 2003)
DOE Grant / Contract #: DE-FG36-02ID14418
Performing Organization: Stanford Geothermal Program, Department of Petroleum Engineering, 367 Panama Street, Stanford University, CA 94305-2220
Principal Investigato : Roland N. Horne 1
Phone: 1(650)723-9595
E-Mail: horne@stanford.edu
Collaborating Researchers: none
DOE HQ Program Manager: Allan Jelacic
Phone: (202) 586-6054
E-mail: allan.jelacic@hq.doe.gov
DOE Funding Allocation
: 175,000
Cost Share Funding: 0

Project Objective

Steam/water relative permeability and capillary pressure are important properties for geothermal reservoir engineering, in that they have a major influence on the performance of geothermal reservoirs under development. All numerical simulations of geothermal reservoir performance require the input of relative permeability and capillary pressure values, yet actual data on these parameters has not been available. The Stanford Geothermal Program (SGP) has succeeded in making fundamental measurements of steam/water flow in porous media and thereby made significant contribution to the industry by providing both understanding of the phenomena as well as actual parameter value measurements. Two of the important problems left to undertake is the measurement of steam/water relative permeability and capillary pressure in geothermal rock (most of the previous study was conducted in high permeability sandstone as well-controlled test material), as well as the understanding of how steam-water boiling mixtures flow in fractures.

Background / Approach

The main objective is to improve the ability of engineers and scientists to forecast the future performance of geothermal reservoirs. By understanding the production characteristics, development decisions can be made sooner and with greater certainty. This will result in more efficient utilization of the geothermal energy resource. Another objective is to provide engineers and scientists direct methods to estimate the energy production rate of geothermal reservoirs and practical models of steam-water flow properties, including steam-water relative permeability and capillary pressure models.

The Stanford Geothermal Program uses both theoretical and experimental approaches to conduct the research. We use numerical simulation for modeling work and we use an X-ray CT scanner as one of our main experimental tools to measure in-situ water saturation and its distribution. We also design and construct purpose-built apparatus to conduct the experiments needed.

Status / Accomplishments
In this research, an experimental apparatus has been built to capture the unstable nature of the two-phase flow in a smooth-walled fracture and display the flow patterns in different flow configurations real time. Air-water relative permeabilities were obtained from experiments at both room temperature and high temperature. These results showed deviation from the X-curve suggested by earlier studies. Through this work the relationship between phase-channel morphology and relative permeability in fractures was determined. A physical channel-oriented model was proposed which could replicate experimental results in both room- and high-temperature cases. Other relative permeability models (viscous-coupling model, X-curve model and Corey-curve model) were also investigated. These models could not, however, represent the experimental relative permeabilities as well as the proposed channel-oriented model, as shown in Figure 1. Hence, we concluded that the two-phase relative permeability in fractures depends not only on liquid type and fracture geometry but also on the two-phase flow patterns.

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Figure 1: Comparison of the experimental relative permeability with the channel-oriented model, viscous-coupling model and X-curve for the nitrogen-water experiment at 90ºC.

Capillary pressure is an important parameter in reservoir engineering. It is essential to represent capillary pressure curves mathematically in an appropriate way. The Brooks-Corey capillary pressure model has been accepted widely, however it has been found that the Brooks-Corey model can not represent capillary pressure curves of The Geysers rock samples, as shown in Figure 2. In fact, few existing models work for these rock samples. To this end, we modeled porous media using fractal geometry and derived a universal capillary pressure model theoretically, as shown in Figure 3. It was found that the universal capillary pressure model could be reduced to the frequently-used Brooks-Corey capillary pressure model when the fractal dimension of the porous media takes a limiting value. We also developed a relative permeability model from the universal capillary pressure model.

In another facet of the project, we calculated the steam and water relative permeabilities at The Geysers and Salton Sea geothermal reservoirs from available production data. A method was developed to estimate the relative permeability curves using Darcy’s Law from mass production rates of steam and water that is available from the California Division of Oil, Gas and Geothermal Resources (DOGGR) database. Results show The Geysers behavior approaches the X-curve behavior and Salton Sea behavior approaches Corey curve behavior, as seen in Figure 4.

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Figure 2: Capillary pressure curves of The Geysers rock and Berea sandstone.

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Figure 3: Relationships between N(r) and r of The Geysers rock and Berea sandstone.

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Figure 4: Plot of relative permeability curves against water saturation for The Geysers and Salton Sea Geothermal Fields.

Reports & Articles Published in FY 2003

Reyes, J.L.P., and Horne, R.N.: "Analysis of The Geysers Well Field Performance Data to Infer In-Situ Water Saturation", Geothermal Resources Council Transactions 26 (2002).

Li, K., and Horne, R.N.: "A Capillary Pressure Model for Geothermal Reservoirs", Geothermal Resources Council Transactions 26 (2002).

Chen, C.Y., Diomampo, G.P., Li, K., and Horne, R.N.: "Steam-Water Relative Permeability in Fractures", Geothermal Resources Council Transactions 26 (2002).

Li, K., and Horne, R.N.: "Fractal Characterization of The Geysers Rock", Geothermal Resources Council Transactions 27 (2003).

Powell, T. and Li, K.: "A Depletion Mechanism for the Behavior of Noncondensible Gases at The Geysers", Geothermal Resources Council Transactions 27 (2003).

Chen, C.Y., Li, K., and Horne, R.N.: " Difference Between Steam-Water And Air-Water Relative Permeabilities In Fractures", Geothermal Resources Council Transactions 27 (2003).

Reyes, J.L.P., Li, K., and Horne, R.N.: "Estimating Water Saturation at The Geysers Based on Historical Pressure and Temperature Production Data and By Direct Measurement ", Geothermal Resources Council Transactions 27 (2003).

"Estimating Water Saturation at The Geysers Based on Historical Pressure and Temperature Production Data", Jericho L.P. Reyes, June 2003

"Relative Permeability of Fractured Rock", Mark D. Habana, June 2002

Presentations Made in FY 2003

Li, K. and Horne, R.N.: “Wettability of Steam-Water-Rock Systems,” Proceedings of the 7th International Symposium on Reservoir Wettability, Freycinet, Tasmania, Australia, March 12-15, 2002.

Li, K. and Horne, R.N.: “Scaling of Spontaneous Imbibition in Gas-Liquid Systems,” SPE 75167, Proceedings of the SPE/DOE Thirteenth Symposium on Improved Oil Recovery held in Tulsa, Oklahoma, April 13–17, 2002.

Li, K. and Horne, R.N.: “A Scaling Method of Spontaneous Imbibition in Systems with Different Wettability,” Proceedings of the 2002 International Symposium of the Society of Core Analysts, California, USA, September 22-25, 2002.

Li, K. and Horne, R.N.: “A General Scaling Method for Spontaneous Imbibition,” SPE 77544, Proceedings of the 2002 SPE Annual Technical Conference and Exhibition, San Antonio, TX, USA, September 29 to October 02, 2002.

Li, K. and Horne, R.N.: “Direct Measurement of In-Situ Water Saturation in The Geysers Rock”, Proeceedings, Twenty-Eighth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, January 27-29, 2003

Reyes, J.L.P., and Horne, R.N.: “Inferred Water Saturation in The Geysers Based on Well Performance Data” Proeceedings, Twenty-Eighth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, January 27-29, 2003

Planned FY 2004 Milestones

1.Completion of steam/water relative permeability experiments in fractures. Sept 04
2. Completion of steam/water relative permeability experiments in geothermal rock. Sept 04