Patent Number: 050227874
Section: summary

FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to a method for returning geothermal gases discharged from geothermal plants t the underground. At present, large amounts of noncondensing gases and toxic gases such as H.sub.2 S and SO.sub.2 that are contained in geothermal steam are extracted at condensers or the like and simply released into the atmosphere or released after being desulfurized at very high costs. OBJECT AND SUMMARY OF THE INVENTION The method of the present invention is an alternative to the conventional release of these noncondensing or toxic gases into the atmosphere, treated or untreated. According to the present invention, the noncondensing or toxic gases present in geothermal steam for geothermal turbines are returned to the underground by way of an underground return well for waste water together with the waste water. When returning to the underground in this way, however, the flow inside the return well becomes a two-phase vertical downward flow of the gas and liquid phases. Therefore, when the apparent gas velocity V.sub.go, in relation to the apparent velocity of the waste fluid V.sub.eo, becomes greater than a certain value for the ratio .alpha.cr=V.sub.go /V.sub.eo, the waste water does not carry (accompany) all the gas phase, and it becomes impossible to return the gas to the underground. Taking advantage of the flow characteristics of a two-phase vertical downward flow of the gas and the liquid phases in a tube, the present invention resolves the above problem by controlling the range of apparent velocities, V.sub.eo and V.sub.go, of the waste water and the gas with respect to the return well so that the size (diameter) of the return well can be selected to suit the amount of waste water Q.sub.e and the amount of waste gas Q.sub.g. The present invention is characterized in that, when the geothermal gas is returned to the underground together with the waste water through an underground return well, the range of the apparent velocities, V.sub.eo and V.sub.go, of the waste water and the gas, respectively, is set by an equation, V.sub.go &lt;1.33V.sub.eo -0.41, so that the geothermal gas is accompanied downward by the waste water and the hydrostatic pressure in the depth direction becomes effective at the same time. This is achieved in the present invention by studying the flow characteristics of a two-phase tubular flow of the gas and the liquid phases in the vertical and downward direction. According to the present invention, the geothermal gas which goes downward accompanied by the waste water in the return well is compressed by the water pressure and is also dissolved into the waste water so that the volume of the geothermal gas is reduced with respect to that of the waste water, and at the bottom of the well, the geothermal gas becomes completely dissolved or is turned into fine bubbles so as to flow into the earth crust together with the waste water.