Returning to Renewable Energy

One promising renewable energy source is geothermal energy, which taps the energy of water heated within the Earth's crust. Geothermal power has tremendous potential, considering the massive heat reservoir inside the Earth. According to the California Energy Commission, geothermal resources generated over three times more electricity in California in 2001 than wind and solar resources combined, totaling five percent of the state's total electrical power generation.

To estimate the recoverable energy from a geothermal reservoir, scientists predict the quantity and thermodynamic state of the water that may be produced by a geothermal well. Because geothermal wells contain a highly turbulent mixture of liquid water and steam, no strictly analytical method exists to calculate well behavior. Scientists instead simulate fluid flow in geothermal systems with computer models that are based on correlations for oil wells. Not surprisingly, the differences between oil and water make petroleum-based simulations unreliable for geothermal systems.

Actual well data for simulations

For the first time in the industry, SAIC's Sabodh Garg, John Pritchett, and James Alexander have utilized actual geothermal well data to accurately simulate this complex fluid dynamics problem. "With the advent of modern well measurement tools, we now have dependable data on which to base our models," states Garg. Using high-quality data from over 40 flowing wells, the researchers explored what is known as the liquid hold-up correlation. "In order to model the fluid flow in a geothermal well, one of the critical tasks is to develop a relationship for the dramatic difference between the liquid and gas velocities," explains Garg. "This relationship is called liquid hold-up correlation."

Applying SAIC's proprietary well-simulation program, WELBOR, the team initially forced simulation results and actual data into agreement by adjusting the liquid hold-up correlation for each well. Then using the liquid hold-up correlation values for all the wells, the team created a generalized liquid hold-up correlation that can be applied to any geothermal well.

After integrating the new liquid hold-up correlation into WELBOR, the 40 wells were simulated with excellent agreement between the computed results and measured data. The new liquid hold-up correlation accurately predicts fluid flow in geothermal well bores and will aid in the search for economically recoverable geothermal energy sources.

"A new liquid hold-up correlation for geothermal wells" was published in Volume 33 of Geothermics. The research was sponsored by the U.S. Department of Energy, utilizing well data from Unocal Corporation and Caithness Energy.

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