Patent Number: 050227874
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention. Waste water Q.sub.e flows into a return well 3 by way of a water pipe 2 above the return well by the action of a pump 1. A geothermal gas Q.sub.g is forced into a gas pipe 5 by a compressor 4 and into the waste water in the return well 3 through a top opening 6. 7 indicates the ground level. For the geothermal gas sent into the waste gas to be accompanied (carried) by the waste water all the way down to the bottom of the return well, the apparent velocity of the waste water V.sub.eo (=Q.sub.e /A, where A=.pi.D.sub.3.sup.2 /4) relative to the well 3 is set to a value equal to or more than 1.0m/s. Furthermore, in order to put the gas flowing downward under a hydrostatic pressure of the waste water, the apparent velocity of the gas V.sub.go (=Q.sub.g /A, where A=.pi.D.sub.3.sup.2 /4) is controlled in the range satisfying an equation: V.sub.go &lt;1.33V.sub.eo -0.41. In the above equation Q.sub.e is the volume flow rate of the waste water, G.sub.g the volume flow rate of the gas, V.sub.eo the apparent flow velocity of the waste water in the return well, V.sub.go the apparent flow velocity of the gas in the well, and D.sub.3 the diameter of the return well. Thus, as shown in FIG. 2, the manner of flow below the mixing point of gas and liquid in the return well turns from a froth flow to a slug flow and then to a bubble flow as going down in the well. The volume of the gas becomes reduced while the gas is carried to the bottom of the return well, accompanied by the waste water. In FIG. 2, as indicated the regions designated (x) represent bubbles, the regions designated (y) represent water (water flow), and the regions designated (z) represent water droplets. In the upper pipe section, froth flow is represented in the middle pipe section, slug flow is represented; in the lower pipe section, bubble flow is represented. FIG. 3 shows the waste water velocity condition, V.sub.eo &lt;1.0m/s, as a limit for accompanying (carrying) the geothermal gas and for the downward flow of the gas. In FIG. 3, the vertical dash-dot line separates the perfectly accompanying region as in the present invention on the right-hand side, from the imperfectly accompanying region on the left-hand side. The dashed curve with the circled points is the experimental confirmation. The straight line curve (near point B) represented by the dashed-double-dotted line defines below it the region of flow parameters in the return well as set in the present invention, namely, where V.sub.go &lt;1.33V.sub.eo -0.41. FIG. 3 also shows the experimental relationship between the waste water velocity V.sub.eo and the gas velocity V.sub.go for making the hydrostatic pressure effective in the return well. Also, FIG. 4 shows an example of the pressure distribution in the return well. In FIG. 4, H is the corresponding head of water between points (1) and (2) of the return well; H' is the experimentally observed head of water; the circled points on the dashed curve were taken under flow conditions corresponding to point (A) in FIG. 3 (annular flow); the triangular points on the solid line curve were taken under flow conditions corresponding to point (B) in FIG. 3 (froth flow); the diamond points on the dash-dot curve were taken under flow conditions corresponding to point (C) in FIG. 3 (bubble flow). In this figure, in the flow pattern region (A) where the waste water velocity V.sub.eo is large compared to the gas velocity V.sub.go and a large annular spray flow or an annular flow is observed, the pressure in the return well is almost constant and the hydrostatic pressure does not play any role. Thus, changes in the gas volume in the return well are small, and most of the gas is carried to the bottom of the well as it is, remaining in the gaseous state. This makes it more difficult to return the gas to the earth crust. When the gas velocity becomes relatively small and the manner of the flow below the mixing point becomes a froth flow, however, the hydrostatic pressure becomes effective in the depth direction and sloped as shown by the pressure distribution (B). As the gas flows down deep in the well, the gas volume is reduced, and the change of the flow pattern is supported by these data. The third curve for flow conditions at point (C) demonstrates the increase in hydrostatic pressure as the gas velocity is further reduced. The divisional line in the V.sub.go -V.sub.eo plane in FIG. 3 is obtained by plotting the experimental bordering points at which the pressure distribution becomes sloped immediately after the mixing point of gas and liquid and at which the hydrostatic pressure begins to be effective.