|Publisher||Society of Petroleum Engineers||Language||English|
|Content Type||Conference Paper|
|Title||RADON TRANSECT ANALYSIS IN GEOTHERMAL RESERVOIRS|
|Authors||Semprini, Lewis, Kruger, Paul, Stanford University|
SPE California Regional Meeting, 9-11 April 1980, Los Angeles, California
Radon transect experiments have been conduct to study flow and emanation characteristics of geothermal reservoirs. Transect experiments have carried out at three geothermal reservoirs: 1) vapor-dominated field at The Geysers, California; 2) the liquid-dominated field at Wairakei, New Zealand; and 3) the liquid-dominated field at Cerro Prieto, Mexico. The radon transect technique involves an examination of the concentration gradients of radon and other natural components in the geofluid along a line of geothermal wells having structural significance in the reservoir. To obtain information on the flow regime in the reservoir, stable gas components in the geothermal fluid are measured in addition to radon. If the transport time of the fluid from the radon source to the wellhead is within a range commensurate with the 3.83-day half life of radon, variations in radon concentration along the transect relative to stable component concentrations provide temporal information on the flow regime. provide temporal information on the flow regime. Data from the transect experiments are being evaluated with mass transport models for vapor and liquid-dominated reservoirs. Results from The Geysers transect experiments show correlation between radon and stable gas components concentration gradients along structural features, indicating physical processes other than radioactive decay are governing processes other than radioactive decay are governing radon concentration at the wellhead. Transect tests in the liquid-dominated reservoirs show correlation between radon concentration and fluid enthalpy, indicating the influence of two phase flow conditions in the reservoir. These initial results suggest that radon transect analysis may be useful in studying geothermal reservoirs. They should also help in the selection of wells for pressure transient and mass transient analysis, since it appears that wells in different areas of a large geothermal reservoir may be operating under different thermodynamic conditions.
The use of radon as an internal tracer in geothermal fluids has been discussed by several investigators. Kruger, Stoker and Umana (1977) reported large differences in radon concentration between vapor-and liquid-dominated reservoirs as well as significant variations within each type. The use of radon to study mass transient phenomena in geothermal reservoirs was demonstrated by Kruger and Warren (1979). They also suggested that transect measurements in geothermal fields may be useful in studying flow regime or changes in reservoir properties. Transect experiments have been performed at The Geysers vapor-dominated reservoir in California, and at the liquid-dominated reservoirs at Wairakei, New Zealand and Cerro Prieto, Mexico.
Radon with its 3.83 day half-life is an ideal radioactive trace for reservoir studies. It is produced from the decay of 1600-year 226Ra, which is produced from the decay of 1600-year 226Ra, which is assumed to be uniformly distributed in reservoir rock. Radon produced in the rock can emanate into the pore fluid. The emanation process as a function of temperature, pressure and rock size is currently being investigated by Macias and Kruger (1979).
The equilibrium concentration of radon in geothermal pore fluids is dependent on the emanating power of radon and several important reservoir power of radon and several important reservoir properties which include porosity, rock type, radium properties which include porosity, rock type, radium content, and pore fluid density. The equilibrium concentration in the pore fluid can be estimated by the relationship given by D'Amore et al (1978),
where Cs is expressed in units of radioactivity per unit mass of fluid (e.g., nCi/kg). Warren and Kruger (1979) discussed the importance of the fluid density, where the equilibrium radon concentration is inversely proportional to fluid density. Their calculations proportional to fluid density. Their calculations showed radon concentrations at a reservoir pressure of 460 psia would be approximately 50 times greater in steam-dominated reservoirs than liquid systems.
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