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Stream-water Relative Permeability in Geothermal Rocks (1996)

Figure 1: Steam saturation distribution obtained by using X-ray CT scanner durin

 

Investigators:
Cengiz Satik and Roland N. Horne, Stanford University

Keywords:
Relative permeability, steam-water, phase change, X-ray CT scanner

Project Objective:
The reliable measurement of relative permeability functions for steam-water flows in porous media is of great importance for reservoir simulation in order to forecast production performance of geothermal reservoirs. Despite their importance, these functions are poorly known due to the lack of understanding of steam-water flows, and to the difficulty of making measurements. Traditionally, these functions are taken directly from isothermal, immiscible gas-liquid displacement processes. However, the use of such functions is not appropriate because these processes do not involve the two important phenomena pertinent to steam- water flows: heat transfer and phase change. Therefore, the objective of this research project is to determine the appropriate relative permeability functions for steam-water flows in geothermal reservoirs.

Approach:
A review of previous experimental and theoretical studies suggests the use of the steady-state experimental method to determine relative permeabilities. In this method, a known ratio of steam and water at a given temperature is injected into a core at a constant rate. A steady-state condition is achieved when injection and production rates become equal for each fluid and also when fluid pressures and temperatures have stabilized. At the onset of a steady state, pressures and temperatures are measured using pressure taps and thermocouples placed at several locations along the core. From these readings, the relative permeability at a given ratio of steam and water is calculated using Darcy’s law modified for multiphase flow. The analysis requires the knowledge of the saturation corresponding to the calculated relative permeability value and relies on the existence of a flat saturation profile along the portion of the core where the measurements are taken. Next, the injection rate is changed and the previous procedure is repeated until the relative permeability values corresponding to the whole saturation range (from 0 to 1) are obtained.

Numerical simulations have been utilized in the design of the experimental apparatus. An important advantage over previous experimental attempts is that our high resolution X-ray CT (Computer Tomography) scanner is used to obtain accurate saturation profiles while the experiment is actually being conducted (Figure 1). The effects of heat transfer and phase change are determined by comparing results obtained from the experiments with steam and water with those obtained from nitrogen and water. Finally, experiments will be conducted at various temperatures and with cores of various permeabilities to investigate the effect of temperature and flow rate on steam-water relative permeability.

Project Status:
The first phase of this project involved carrying out numerical simulations in order to choose the optimal experimental parameters, such as fluid injection rates, length of the core and locations of pressure taps and thermocouples. The experimental conditions were simulated using a commercial thermal simulator and the optimum operating conditions and the requirements of a typical laboratory experiment (nitrogen-water and steam-water) were obtained. This stage has been completed.

With the information obtained from the numerical simulations, the second phase was to design and construct an experimental apparatus with a core holder that could be used for experiments at elevated temperatures (140 oC). The second phase also included several preliminary experiments to verify our numerical results. The construction of the core holder, made of high temperature epoxy and plastic materials (a restriction imposed by the X-ray CT scanner), has also been completed. Several preliminary experiments with both nitrogen-water and steam-water mixtures have been conducted. In the new core holder design, several pressure taps, thermocouples and heat flux sensors were placed at various locations along the core to measure pressure, temperature and heat losses, respectively. Saturations as a function of position along the core were measured with the X-ray CT scanner (Figure 2) so that the flat saturation profile could be identified and used for the relative permeability calculation. Also, an extensive data acquisition system for the purposes of data collection and computer-automation of experiments was implemented. Preliminary experiments were conducted for steam and water at 115 oC. As a result, a set of steam-water relative permeability curves was obtained (Figure 3). The results indicate a linear relationship for steam-water relative permeability. The next stage of the project will include conducting experiments with nitrogen-water and steam-water on the same cores and comparing the results with our preliminary experimental findings. Finally, our objective of measuring relative permeabilities will be accomplished after conducting experiments at different temperatures and flow rates and with cores of different permeability.

Research Results:
Our research over the past year has yielded the following very important preliminary results:

  1. An experimental apparatus with a nonmetallic core holder that could be used for steam-water flow experiments at elevated temperatures was constructed.
  2. Preliminary flow experiments with both steam-water and nitrogen-water were conducted.
  3. We implemented an extensive data acquisition system for the purposes of collecting data, controlling the apparatus and analyzing the X-ray CT data during and after an experiment.
  4. A preliminary steam-water relative permeability experiment conducted at 115 oC indicated the need for further modifications and improvements to the apparatus.
  5. Preliminary curves obtained from an experiment indicate a linear relationship for steam-water relative permeabilities.

Plans:
This research project is being conducted by Research Assistant Raul Tovar, Dr. Cengiz Satik and Prof. Roland N. Horne. We have also engaged the services of two undergraduate students to assist in the experimental work. In addition we are collaborating with scientists from the other research groups within Stanford University. Experimental results and discussion are posted regularly to the Stanford Geothermal Program web page.

References:
Ambusso, W.J. 1996. Experimental determination of steam-water relative permeability relations. MS Thesis, Stanford University, Stanford, CA .

Ambusso, W.J., Satik, C. and Horne, R.N. 1996. A study of relative permeability for steam-water flow in porous media. Proc. of 21st Stanford Workshop on Geothermal Reservoir Engineering.

Ambusso, W.J., Satik , C. and Horne, R.N. 1996. Steam-water relative permeability. 1996 GRC Annual Meeting, Portland, Oregon.

Ambusso, W.J., Satik , C. and Horne, R.N. 1996. Determination of relative permeability for steam-water flow in porous media. SPE 36682, 1996 SPE Annual Technical Conference, Denver, Colorado, 6-9 Oct.

Steam saturation distribution
Figure 1: Steam saturation distribution obtained by using X-ray CT scanner during an experiment.


Figure 2: Saturation profiles for all of the steps conducted during an experiment.


Figure 3: Relative permeability for steam and water, obtained from an experiment.