US3782468A - Geothermal hot water recovery process and system - Google Patents

Abstract

An improved process and system for recovering high temperature hot water from a geothermal supply. A well casing assembly is sunk to position an apertured end portion of a casing in proximity to an underground hot water supply with an apertured end portion of a centrally disposed conduit within the casing adjacent the apertured casing end. By reducing the pressure at the well head, flashing of the hot water to steam occurs to elevate a two phase mixture of steam and hot water through the casing assembly. Such flashing is accompanied by the evolution of substantially non-condensible gases, including carbon dioxide. Such gases are separated with the steam from the elevated hot water, the latter being withdrawn from the system for its intended use, such as the generation of electric power. The separated steam and gases are heat exchanged to condense the steam and to withdraw useful heat from the gases, following which the gases are compressed and recycled into contact with the underground hot water supply to assist in elevating additional hot water. The carbon dioxide containing gases are effective in countering the problems normally created when geothermal hot water is allowed to flash to steam, namely the formation of system clogging deposits, of which calcium carbonate and magnesium hydroxide are typical, within the well and related apparatus. The presence of carbon dioxide in the recycled gases during the steam flashing maintains chemical equilibrium in the system to prevent formation of such deposits. Thus, without utilizing mechanical or other pumping devices within the well, recovery of a substantially constant stream of geothermal hot water from a deep underground well may be effected without attendant precipitation of clogging chemical compounds commonly encountered when geothermal hot water is elevated utilizing a steam flashing procedure.

Description

United States Patent [191 Kuwada 14 1 Jan. 1, 1974 1 GEOTHERMAL HOT WATER RECOVERY PROCESS AND SYSTEM [75] Inventor: James T. Kuwada, Richmond, Calif.

[73] Assignee: Rogers Engineering Co., Inc., San Francisco, Calif.

22 Filed: Sept. 20, 1971 [21] App]. No.: 182,054

[52] US. Cl 166/267, 166/265, 166/279, 166/310 [51] Int. Cl E2lb 43/00 [58] Field of Search 55/39, 208; 165/45; 166/227, 265, 266, 267, 279, 302, 310, 314; 417/108 {56] References Cited UNITED STATES PATENTS 3,258,069 5/1966 Hottman 166/265 2,809,698 10/1957 Bond et al. 166/267 3,380,913 4/1968 Henderson 166/267 3,099,318 7/1963 Miller et al. 166/227 753,045 2/1904 Cooper 166/267 3,399,623 9/1968 Creed 166/302 Primary Examiner-Marvin A. Champion Assistant Examiner.lack E. Ebel Attorney-Flehr, Hohbach, Test, Albritton & Herbert [57] ABSTRACT An improved process and system for recovering high temperature hot water from a geothermal supply. A well casing assembly is sunk to position an apertured end portion of a casing in proximity to an underground hot water supply with an apertured end portion of a centrally disposed conduit within the casing adjacent the apertured casing end. By reducing the pressure at the well head, flashing of the hot water to steam occurs to elevate a two phase mixture of steam and hot water through the casing assembly. Such flashing is accompanied by the evolution of substantially non-condensible gases, including carbon dioxide. Such gases are separated with the steam from the elevated hot water, the latter being withdrawn from the system for its intended use, such as the generation of electric power. The separated steam and gases are heat exchanged to condense the steam and to withdraw useful heat from the gases, following which the gases are compressed and recycled into contact with the underground hot water supply to assist in elevating additional hot water.

The carbon dioxide containing gases are effective in countering the problems normally created when geothermal hot water is allowed. to flash to steam, namely the formation of system clogging deposits, of which calcium carbonate and magnesium hydroxide are typical, within the well and related apparatus. The presence of carbon dioxide in the recycled gases during the steam flashing maintains chemical equilibrium in the system to prevent formation of such deposits.

Thus, without utilizing mechanical or other pumping devices within the well, recovery of a substantially constant stream of geothermal hot water from a deep underground well may be effected without attendant precipitation of clogging chemical compounds commonly encountered when geothermal hot water is elevated utilizing a steam flashing procedure.

8 Claims, 8 Drawing Figures GEOTIIERMAL HOT WATER RECOVERY PROCESS AND SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to the field of improved processes and systems for recovering hot water from geothermal supplies thereof. More particularly, this invention relates to the field of removing geother' mal hot water from underground supplies thereof without utilizing moving mechanical or other pumping devices within the well and without attendant chemical compound deposits being formed which would normally clog the well and related equipment.

Still more particularly, this invention relates to the field of recovery of geothermal hot water by recycling substantially non-condensible gases into the system which have been obtained from the geothermal hot water supply itself. This invention further relates to the field of recovering and recycling carbon dioxide containing gases obtained from a geothermal hot water supply, which gases are evolved in conjunction with the flashing of the hot water to steam, the presence of such gases when recycled into the system controlling the chemical equilibrium of the system to prevent formation of system clogging deposits which would normally be formed in the absence of such recycled carbon dioxide containing gases.

This invention additionally relates to the field of an apparatus system for recovering evolved gases in conjunction with flashing of a two phase steam-hot water mixture from a geothermal hot water supply and the separation of such gases for compression and recycling thereof into the system to assist in elevating additional hot water and to counteract the formation of system clogging chemical precipitants.

2. Description of the Prior Art So far as is known, the particular geothermal hot water recovery system of this invention has been unknown heretofore. Prior attempts to recover geothermal hot water from deep underground wells, both successful and unsuccessful, heretofore have commonly utilized the introduction of a mechanical or like pumping device deeply into the well to affect mechanical lifting of the hot water therefrom. However, with particularly deep wells, such as those at depths of several thousand feet, such as 3,000 feet and deeper, it is extremely difficult, if not impossible, to properly position a pumping device at the depth which will permit withdrawal of geothermal hot water from a deep underground supply thereof. Additionally, such mechanical pumping devices require the utilization of substantial sized well casing assemblies, such as those having diameters as large as eighteen inches. Obviously, such large well casings are more difficult and expensive to place than smaller diameter casings.

Furthermore, in conjunction with geothermal hot water supplies at very high temperatures, such as 400F. or higher, for example, substantial pressure in the system is required to prevent the water from uncontrollably flashing to steam and such pressure requirements compound the problem of holding and positioning the pump at a substantial depth. such as one thousand feet or more. It has been determined that generally the maximum depth to which a pump may be located in conjunction with a geothermal well is approximately 1,000 feet, and where the water temperature encountered at such depths is very high, that is 400F. and above, increased difficulty is encountered in maintaining the pump at even such a depth.

In that latter regard, for utilization of geothermal hot water supplies in the generation of electric power, the higher the temperature of the recovered water, the better, because less heat exchange equipment and other equipment is required, and accordingly less costs are incurred. However, as noted, when moving mechanical pumping devices are utilized in recovering such hot water, very high temperatures compound the problems otherwise encountered in utilizing such devices.

The present invention has the distinct advantage over known geothermal hot water recovery procedures and systems in that no moving mechanical or other type of pumping device is necessary within the well and substantially all the apparatus, except the well casing assembly itself, is located above ground. Also, recovery depth limitations encountered with prior known geothermal recovery procedures are obviated with the subject system.

Additionally, with prior known procedures, the problems of high temperature geothermal hot water deposits flashing to steam results in the formation of equipment clogging chemical compound deposits, such as calcium carbonate and magnesium hydroxide. Such problems are obviated with the present invention in that the gases evolved in conjunction with such a flashing procedure are recovered and recycled to utilize the carbon dioxide content of such gases to counteract the formation of such system clogging chemical compounds. That is, contrary to the prior used concepts in which steam flashing created problems, the present invention utilizes such concepts advantageously to prevent the formation of well clogging and equipment clogging chemical compound deposits.

SUMMARY OF THE INVENTION The present invention relates to an improved process and system for recovering geothermal hot water from underground hot water supplies thereof. More particularly, this invention relates to an improved process and system for recovering geothermal hot water for various uses, such as electric power production, by recycling gases in the system which have evolved from the geothermal hot water supply itself.

Still more particularly, this invention relates to an improved process and system for effecting the flashing of geothermal hot water into a two phase mixture of steam and hot water and utilizing the same in conjunction with elevating the mixture through a well casing assembly, such steam flashing being accompanied by the evolution of substantially non-condensible gases which contain carbon dioxide. This invention further relates to the recovery of such carbon dioxide containing gases and the recycling of the same into the geothermal system to assist in elevating additional quantities of steamhot water while at the same time preventing the formation of chemical compound deposits which normally accompany such steam flashing.

Still more particularly, this invention relates to an improved process and system for recovering evolved carbon dioxide containing gases from a geothermal hot water supply during flashing of the hot water supply to a two phase mixture of steam and hot water and to separation of such gases for subsequent compression and reintroduction into the geothermal hot water recovery system.

This invention further relates to an improved and simplified process and system for recovering geothermal hot water without attendant chemical compound deposition problems heretofore encountered and without requiring moving mechanical pumping devices or like pumping devices within the well, thereby permitting recovery of geothermal hot water from well holes at depths heretofore not feasible. In that regard, mechanical pumping devices can be employed only with great difficulty, if at all, to recover geothermal hot water when the hot water being recovered is at very high temperatures and is found at very great depths below ground. Attempts to utilize mechanical or like pumping devices at such depths result in clogging of such pumping devices and related equipment which necessitate repeated shut-downs of the recovery system and the attendant electric power generation equipment supported thereby for cleaning and maintenance thereof.

From the foregoing, it should be understood that objects of this invention include: the provision of an improved geothermal hot water recovery process and system; the provision of an improved apparatus combination in a geothermal hot water recovery system which requires no moving mechanical or like pumping devices within the well; the provision of an improved process for obtaining and recycling substantially noncondensible gases evolved from a geothermal hot water supply itself; and the provision of an improved and simplified process and system for compressing and recycling gases obtained during flashing of geothermal hot water to a two phase mixture of steam and hot water while at the same time countering the formation of system clogging chemical compound deposits which nor mally attend such steam flashing.

These and other objects of this invention will become apparent from a study of the following detailed description in which reference is directed to the attached drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a generally schematic view of one preferred embodiment of the geothermal hot water recovery system of this invention;

FIG. 2 is a generally schematic view of a portion of another preferred embodiment of the subject system;

FIG. 3 is a partially cut away view of the lower end portion of the well casing assembly illustrating details of construction thereof;

FIG. 4 is an enlarged view of a portion of the conduit which forms part of the well casing assembly illustrating a preferred pattern of perforations therein which effectively permits recycled gases to exit therethrough;

FIGS. 5 and 6 are graphs which illustrate various illustrative operating conditions for the subject system;

FIG. 7 is a generally schematic view of a portion of another preferred embodiment of the subject system; and

FIG. 8 is a horizontal sectional view of the embodiment of FIG. 7 taken in the plane of line 8-8 thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As will become evident from a study of the following detail description ofthe various embodiments disclosed hereinafter, all of which are utilizable for the intended purpose of improving procedures for recovering geothermal hot water, the basis of the process and system in each of such embodiments involves the recycling of evolved gases into an underground well casing assembly for use in withdrawing underground deposits to the well head for subsequent use above ground. The recycling route for the gases may vary as illustrated by the three embodiments disclosed herein without departing from the inventive concepts hereof.

The general principles of utilizing gas to assist in lifting underground liquid deposits have been known for many years, for example, in conjunction with the air lifting of water from cold water wells and the inert gas lifting of crude oil from oil wells. However, those well known principles, if applied directly to the recovery of geothermal hot water, could not produce the desirable and beneficial results disclosed and emphasized herein because of the unique problems which are encountered in conjunction with the recovery of high temperature geothermal hot water from deep underground wells in those locations in the world where such wells are found.

Thus, it should be understood that the concepts disclosed herein are directed specifically to recognition of the unique problems encountered in conjunction with the recovery of high temperature geothermal hot water from very deep underground wells, and the satisfactory resolution of such problems without the attendant complications heretofore encountered in conjunction with such hot water recovery.

That is, this invention recognizes and solves the serious problems previously encountered in conjunction with geothermal hot water recovery, namely the formation of well clogging chemical deposits in conjunction with steam flashing and the counteracting of such chemical deposit formation in a fashion which insures long-term operation of a well without such clogging so that repeated shut-downs encounterable at short intervals with geothermal recovery procedures utilizing moving mechanical or like pumping devices within the well are obviated. As noted herein previously, and as amplified hereinafter, this invention relates specifically to the lifting or elevating of high temperature geothermal hot water out of deep underground supplies thereof by utilizing recycled carbon dioxide containing gases in a manner which prevents entirely, or minimizes within manageable limits, the precipitation of chemical compounds which normally cause fouling or plugging of the well casing assembly, associated conduit structure, and related processing equipment installed to utilize the geothermal hot water in conjunction with the generation of electric power or like procedures.

Because of the ever increasing demand for electric power and clean water in this country and in other countries of the world, much interest is currently being focused on the use of geothermal hot water energy in the form of steam or hot water for the generation of electric power and the recovery of desalinized water and the valuable minerals normally contained therein. From exploratory investigations of geothermal active areas throughout much of the world, it has been determined that the predominant form of geothermal energy exists as high temperature hot water, rather than as steam. This invention is directed primarily to the recovery of underground supplies of such hot water, rather than the recovery of underground steam.

Heretofore, methods commonly employed to recover hot water from geothermal wells involved either mechanical pumping with vertical deep well pumps inserted directly into the wells, or the utilization of pressure reduction techniques, the latter involving reducing pressure at the well head so that geothermal hot water flashes partly to steam to produce a column of two phase mixture of steam-hot water which rises in the well casing assembly sunk to the appropriate depth into the ground. Such a flashed column of steam-hot water has mixed densities which collectively are less than the head of clear water in the below-ground geothermal hot water deposit on the outside of the well casing assembly. As a result, the steam-hot water mixture flows upwardly and out of the well head in conjunction with a steam flashing reaction created by pressure reduction at the well head, the steam providing the primary motive force.

However, it has been determined that utilization of the steam flashing method commonly results in system clogging due to the precipitation of chemicals con tained in the hot water, particularly calcium carbonate and magnesium hydroxide. Such chemical precipitation occurs as the result of the evolution of carbon dioxide containing gases during the flashing of the geothermal hot water to steam. Such gas evolution shifts the chemical equilibrium of the system from the bicarbonate form to the carbonate form, and such precipitation is best expressed by the following reactions:

3. C0, Ca :2 CaCO (s) Reaction 2 establishes that the evolution of CO containing gas shifts the equilibrium of the geothermal system to the right, thereby promoting the precipitation of calcium carbonate (CaCO in the manner indicated by reaction 3.

The high temperature of the geothermal hot water further enhances the hydrolysis of carbonates according to reaction 4. Additionally, therefore, depending upon the temperature of the geothermal water supply, and the magnesium content of the water, magnesium hydroxide [Mg (Oi-1),] formed according to reaction 5.

The principal objective of this invention is based upon recognition that such chemical precipitants can be counteracted in conjunction with the withdrawal of large quantities of geothermal hot water utilizing a steam flashing procedure without permitting the shift in equilibrium which causes such precipitation.

Recovery procedures used with other geothermal hot water systems involving insertion of a mechanical or like pumping device directly into a geothermal well casingassembly to physically pump water out of the well necessitate keeping the water at a pressure above its saturation pressure, thereby preventing steam flashing and the evolution of carbon dioxide. However, as noted previously, there are practical mechanical limitations placed on pump shaft length and like considerations which limit the depths at which mechanical pumping devices may be utilized. Also, as noted previously, such mechanical pumping devices are difficult to maintain operable in conjunction with very high temperature geothermal hot water supplies, such as those at 400F. and above, because of the inherent tendency of such high temperature water to turn to high pressure steam when a supply thereof is tapped. Thus, it is necessary to maintain pressure on the water to be withdrawn to prevent the same from turning to steam. Such pressure maintenance problems make utilization of mechanical and like pumping devices at depths of one thousand feet or below extremely difficult, if not impossible, to effect, particuarly in conjunction with those. geothermal hot water deposits at temperatures at 400F. and above which are most desirable for use in conjunction with electrical power generation.

Thus, the present invention is based on turning a serious disadvantage heretofore encountered with geothermal well procedures into a workable advantage, namely utilizing effectively the carbon dioxide containing gases evolved in conjunction with steam flashing of geothermal hot water as a steam hot water column is elevated in a well casing. As a result, the present invention is not restricted by the mechanical problems encountered in conjunction with utilization of pumping devices inserted within a well, nor by the precipitation problems of chemical compounds which heretofore normally resulted from steam flashing.

The improved features of this invention are accomplished by utilizing an apparatus and process of the type schematically illustrated in the embodiment shown in FIG. 1.

in that regard, the geothermal hot water recovery system is generally designated 1 and comprises a well casing assembly generally designated 2 which is positioned a suitable depth in the ground 3 by any suitable drilling technique of any known type. In the embodiment illustrated, the well casing assembly comprises an outer pipe or well casing 4 and an inner pipe or conduit 6 positioned within the casing and surrounded thereby so that an open annular space 7 exists between the casing and the conduit.

The well casing assembly is sunk into a hole 5 drilled into the ground to a depth sufficient to position an apertured lower end 8 of the casing; 4 in proximity to an underground supply of geothermal hot water, generally designated 9, so that such hot water may enter the apertured end of the casing during the water recovery procedure. In that regard, the apertured casing end 8 preferably is defined by a socalled well screen" of the type commonly utilized in conjunction with underground fluid deposit recovery procedures. Such a well screen, as perhaps best seen in FIG. 3, is generally designated 11 and is formed by a series of adjacent convolutions of suitable metal wire or cable wound around a supporting longitudinal skeletal framework (not shown). Such convolutions may be formed from any suitable material, such as stainless steel, galvanized low carbon steel, Monel, galvanized iron, or the like in a manner and by procedures well known in the well drilling industry. Preferably, each of such convolutions is formed with a generally triangular cross-section so that the screen defines a series of narrow slots extending around the periphery of the well casing. Thus, such slots are open and unrestricted to an increasing degree internally of the well screen to facilitate entry of liquid therethrough and to make more difficult the clogging thereof, in known fashion. Generally, the lower end of the casing 4 is closed off by a suitable plug 12 in known fashion so that hot water entering the casing passes through the slots formed in the well screen section 11 thereof.

In that regard, the particular configuration of the easing 4 and the well screen 11 formed on the bottom thereof forms no direct part of the inventive concept of this invention. However, a full and thorough description of well screen production and utilization is contained in Ground Water and Wells, First Edition 1966, published by Edward E. Johnson, Inc., St. Paul, Minn., copyrighted in 1966. Specifically, pages l45l56 of such reference work for the well drilling industry illustrate and describe well screen formation and use and reference is directed thereto for a more thorough understanding of such well screen construction.

The conduit 6 contained within casing 4 is formed with a perforated section 13 for a predetermined portion of its length which, as will be described hereinafter more fully, is formed to extend longitudinally a predetermined distance along the length of the conduit starting at a predetermined point spaced upwardly from the lower end thereof. Thus, an imperforate section 14 separates the perforated section 13 from the open lower end .15 of the conduit, as perhaps best seen in FIG. 3.

As noted previously, in conjunction with recovery of very hot geothermal water deposits, the casing assembly may be sunk to substantial depths of 3,000 feet or more before a suitable geothermal hot water supply 9 is encountered. Because no pumping device is required within the well casing assembly, a well casing of much smaller diameter is usable than would be usable if a pumping device were to be inserted therein, thereby minimizing drilling and attendant costs.

The subject system is designed to recover carbon dioxide containing gases evolved from the hot water during steam flashing thereof and to recirculate the same into the well casing assembly. In the first embodiment shown in FIG. I, such recirculation is effected through the annular space 7 surrounding the conduit 6. However, that embodiment of the invention, although illustrated and discussed first herein, should not be considered of greater importance than the other embodimentshereinafter described.

With the system shown in FIG. 1, the two phase mixture of steam and geothermal hot water is forced to enter the central conduit 6 and to pass upwardly therethrough to permit recovery of hot water therefrom above ground. As noted, this invention utilizes the procedure of pressure reduction at the well head to effect steam flashing, and to utilize such steam flashing in a manner which turns the heretofore detrimental effects thereof into favorable operation conditions.

When the well casing assembly 2 has been positioned to a suitable depth in the drilled hole 5, the well is capped in any known fashion, such as by a conventional cap structure 14 positioned thereover. At one side of the casing 4 and below the cap 14 is provided a fitting for a conduit 16 used in recycling gases into the system through the well casing assembly as will be described. Another conduit 17, separated from the well head by a blocking valve I8, is provided for operatively connecting the casing assembly with a conventional flash tank 19 located adjacent the well head. The function of the flash tank is to create pressure reduction at the well head in known fashion to effect flashing of steam in the geothermal water supply which results inof recycled gases surrounding conduit 6 and passing downwardly therearound. Such gases will enter conduit 6 through the perforated section 13 thereof and will bubble upwardly through the conduit with the steamhot water mixture to assist in lifting thereof.

In the flash tank 19 the geothermal hot water-steam mixture is separated into steam plus the noncondensible carbon dioxide containing gases which are evolved in conjunction with the flashing procedure, and hot water. The separated hot water is removed from the flash tank in known fashion through a suitable conduit 21 and is transferred by a pumping device 22 into another conduit 23 for removal for use in the generation of electric power or other known uses in known fashion. A drain conduit 24 is provided in conjunction with the flash tank for the obvious purpose. Throughout the system, a series of blocking valves 18 corresponding to the previously mentioned valve 18 are provided at required locations for the well known purpose. Also, check valves 25 may be positioned within the system at required locations, as noted in FIG. 1. The construction of such valves is conventional.

After the geothermal hot water has been separated from the evolved gases and steam from the geothermal hot water supply, the steam and gases are transferred from the flash tank through conduit 26 into a conventional heat exchanger unit 27 in which the steam is condensed and the useful heat is extracted from the noncondensible evolved gases to extract the useful heat therefrom while effecting cooling of such gases. The steam condensate and cooled gases are then passed through another conduit 28 into a conventional compressor suction knock-out drum 29 of known construction. In drum 29, the steam condensate is separated from the evolved gases and is removed through a con duit 31 for disposition in any suitable fashion.

The separated evolved gases are then introduced through a conduit 32, provided with a pressure vent 33 for control purposes, into a conventional compressor unit 34 of known construction, In the compressor 34, which is operated by an electric motor 36 in known fashion, the gases are compressed prior to recycling the same back into the system. In that regard, the aforementioned conduit 16 is connected with the discharge end of the compressor and receives compressed gases therefrom and introduces the same back into the well casing assembly as seen in FIG. 1.

The recycled compressed gas flow rate is determined in accordance with maintaining partial pressure on the carbon dioxide containing gases in the flash tank 19 at the level which is required to suppress chemical precipitation in accordance with known pressure criteria. In conjunction therewith, the flash tank operating pressure is selected and adjusted to permit a predetermined amount of steam flashing to occur in the conduit 6 of the well casing assembly. This is done to create additional gas lift volume, thereby reducing the horsepower requirements of the gas compressor 34 for introducing recycled gases into the system. By so doing, steam flashing is utilized as the primary motive power of the hot water elevating system to thereby reduce horsepower requirements of the gas compressor which would obviously be higher if all the lifting force were provided by utilizing recycled gases, Thus, system operating costs are minimized. The total volume of lifting gas, which includes recycled carbon dioxide containing gases and flashed steam, is predicated upon the volume which is required to prevent unstable or sluggish flow of the two phase mixture of steam and hot water rising inconduit 6, as well as to maintain the desired temperature of geothermal hot water being recovered. The specific quantities and values for a given geothermal well will have to be determined on an individual basis, taking into consideration such factors as well depth, water temperature and composition, the relative diameters of the conduit 6 and casing 4, well productivity and draw down, submergence, and like factors, all of which are within the knowledge of a competent well engineer, and all of which can be evaluated by such an engineer to develop the necessary gas flow volume, pressure and related criteria necessary to effectively operate a given system.

For initial start-up of the system, the system can be initially gas-filled by allowing initial quantities of the hot water to be lifted by steam flashing only, pursuant to well head pressure reduction. Such steam flashing will result in the evolution of non-condensible gases including carbon dioxide in the manner noted previously. While some chemical precipitation within flash tank 19 will most likely occur during the initial start-up, such precipitation will be limited. Any precipitants formed can be removed from the flash tank 19 to prevent contamination thereby of the pump 22 and other equipment positioned downstream of the flash tank. However, after any such initial chemical precipitation, and removal thereof from the system, further precipitation is completely eliminated, or maintained within nondetrimental manageable limits which will not result in clogging of the system or rendering the same inoperative. Once the recycle gas system beyond flash tank 19 is filled with gases evolved from the geothermal hot water supply, and compressor 34 is started, the flash tank pressure can be gradually increased as the CO carrying gases evolve to replace steam in the system to the appropriate ratio which satisfies both the carbon dioxide partial pressure and gas lifting requirements of the system. That is, by recyclingthe carbon dioxide containing gases evolved and recovered from the geothermal hot water supply, carbon dioxide partial pressure can be maintained on the geothermal hot water supply so that it will suppress further evolution of carbon dioxide gas when such water flashes to steam during saturation of the recycled gases. Thus, equilibrium of the system can be maintained, or shifted to the left referring to the foregoing formula 3, to counteract any tendency of the system to precipitate calcium carbonate and/or magnesium hydroxide so that such precipitants will not be formed to create the clogging problems noted previously.

Once the equilibrium status desired for the system has been reached, flashing steam acts as the principal lifting gas for elevating thermal hot water, while the carbon dioxide containing recycled gases act primarily as a chemical anti-precipitant. With prior known cold water or oil well drilling operations utilizing gas introduction, liquid lifting is done entirely by the lifting gas introduced into the system which requires substantially greater volume of gas and more horsepower to compress and introduce the same into the system. Additionally, in such prior known arrangements, the lifting gases are not evolved directly from the product to be recovered from the ground as in the present invention.

If geothermal hot water were free of carbonates, gas lifting could be done almost entirely with flashed steam and the need for a gas compressor would not exist and compressor horsepower requirements would be zero. But, because in known available geothermal hot water deposits, substantial carbonate content is encountered, recycled gas flow has to be established in the manner described herein to maintain the required carbon dioxide partial pressure on the hot water to prevent chemical precipitation as noted.

The required amounts of recycled gases can be determined for a particular well by measuring the. noncondensible gas evolution at the well head flash drum at various operation drum pressures. This determination can be made at the time of well drilling, development and flow testing in known fashion. More than the minimum required gas flow may prove desirable, based on criteria such as the particular well casing diameter, depth and quantity of water available, and water recovery temperature desired.

In that regard, a graph showing water flow versus recycled gas flow is illustrated in FIG. 5 which shows variable combinations of gas flow rates and water flow rates for a well operated at'varying recycle gas pressures. Curves A, B, C and D shown therein represent illustrative wellhead pressures-at "the wellhead separator flash tank 19 of PSIA, PSIA, 78 PSIA and 80 PSIA, respectively.

Thus, for any given recycle gas flow from compressor 34, water flow into the well casing assembly at the apertured bottom end 8 of casing 4 may be determined in accordance with the operating pressure level at the wellhead. As a result, the best combination of gas flow pressure, and hot water flow may be determined for a given well. I i

In that connection, the curves of FIG. 5 were determined in accordance with operation of an exemplary well in conjunction with the following exemplary criteria: Water supply temperature at the bottom of the well casing assembly, 320F.; water supply pressure, l 10 PSIA; well flow string, 290 feet utilizing an 8 inch diameter casing. 6

Similar operating curves can be formulated for other operating conditions encountered at a given well in conjunction with curves E, F, G and H, shown in FIG. 6. Such latter curves set out the relationship between the temperature at the well head separator flash tank 19 and the recycled gas rate, taken under the same exemplary operating conditions described above with respect to FIG. 5, and showing the geothermal water supply beneath ground being at an exemplary temperature of 320F. In that regard the respective curves E,F, G and H designate flash tank pressures of 70 PSIA, 75 PSIA, 78 PSIA and 80 PSIA respectively.

Thus, with the first embodiment of the system illustrated in FIG. 1 operated under the exemplary conditions described, a substantially continuous flow of thermal hot water may be recovered from an underground supply thereof in conjunction with the recycling of evolved and subsequently compressed carbon dioxide containing gases from the thermal hot water supply itself.

FIG. 2 shows another highly effective system arrangement in which the recycled gases are introduced through the central conduit 6 with the thermal hot water being elevated through the annular space 7 between the casing 4 and the conduit 6 as seen. This alternate embodiment functions fully as effectively and in the same basic fashion as described previously and the principal distinction therebetween and the FIG. 1 embodiment resides in the direction of flow of the recycled gases and of the flashed steam-water mixture. That is, the flow paths of the recycled gases and hot water are reversed relative to each other in the embodiments of FIGS. 1' and 2.

Effective distribution of gases through the perforated section 13 of the conduit 6, in the FIG. 2 embodiment, insures effective disbursement of the recycled gas stream into the water supply entering the casing assembly to provide intimate mixing of the two phases without substantial gas pressure loss. The perforated gas distributor section 13 should be designed to insure unobstructed water flow into the well casing assembly. It also should be designed so that the system will not become clogged due to dirt or scale passing into contact with the perforations formed in conduit 6.

An example of a gas distributor conduit which satisfies the noted criteria is shown in FIGS. 3 and 4. In that regard, it will be noted that reference is directed to the perforate gas distributor section being formed in the conduit 6, as shown in FIG. 2. It should be understood that the reversal thereof may also be employed with the water being intended to enter the conduit 6 with the gases passing through the space 7 surrounding the conduit'in the manner described previously with respect to FIG. I. v

A particular perforated gas distributor section 13 is illustrated in FIGS. 3 and 4 and reference is directed thereto for an understanding of the principles discussed. It should be understood that such perforated section shown is intended to be illustrative of one embodiment thereof and that modifications to meet particular needs may be developed in line with the disclosures contained herein.

In that regard, referring to FIG. 4, the perforated conduit section 13 is formed with a series of generally circular holes or openings 41 arranged in a preferred pattern. Such openings preferably are arranged in a three row pattern having a triangular arrangement or pitch between adjacent openings as shown in dotted lines in FIG. 4. That is, the openings are oriented in parallel rows about the periphery of conduit 6 with the openings in one row being offset and centered between the openings of an adjacent row. Thus, the openings in alternate rows are longitudinally aligned with each other, as seen in FIG. 4.

The spacing (D--FIG. 4) between adjacent openings of adjacent rows is determined by the gas flow desired through an individual opening. In that regard, utilizing an exemplary opening size of 541 inch diameter (d) and based on a two psi pressure drop (AP) the gas flow per opening is determined by the following standard equation for gas flow through a given orifice:

Gas flow/opening, W 1891 Y dfc (AP/v,)"' pounds/hour/opening where Y expansion factor through opening for compressible flow, d diameter of opening in inches, 0 opening flow coefficient 0.61, AP pressure drop, psi and v, specific volume of recycled gases at upstream conditions in cu. feet/- pound. Referring to FIG. 3, in which Ah is the water supply level depression and with the assumed pressure drop (AP) of 2 psi, Ah is determined in accordance with the following formula:

Ah (AP) 2.31 feet/psi/S. G. at t F. feet where AP pressure drop, psi and S.G. specific gravity at fluid temperature, Fahrenheit degrees.

In that regard, as seen in FIG. 3, the liquid static seal preferred for a given installation is 2Ah.

In conjunction with a specific example, the above consideration may be utilized to determine center to center spacing of the openings 41 as follows, assuming a recycled gas flow of 2,000 pounds per hour utilizing ,4; inch diameter (d) round openings as noted previously.

With exemplary recycle CO gas density being determined as follows:

Density (mol.wt.) (pressure)/(gas constant,R) (T,R) (compressibility factor) (44) (l12)/(l0.73) (780) (1) 0.59 pounds/feet Thus,

W 1891(1) (0.125) (0.61) 2/(1.7) pounds/hour/opening.

Thus, the number of openings required per each three rows of openings at the exemplary flow rate noted is 2000 pounds/hour/l9.5/pounds/hour/opening 102 openings. Therefore, in each three row section of openings, 102/3 or 34 openings per row are required.

Assuming an exemplary four inch diameter conduit having a circumference [1r (41)] of 3.14 X 4 12.5 inches, the center to center spacing between openings in adjacent rows, (distance D in FIG. 4) is three-eighths inch.

It should be understood, of course, that for flow rates greater than the exemplary 2,000 pounds per hour described, additional rows of openings are required. For example, for flow rates of 6,000 pounds per hour, other operating criteria being the same as described, three segments of three rows each would be required.

A third embodiment of this system is shown in FIGS. 7 and 8. In such embodiment, the basic operational features and functions described herein previously are applicable and corresponding reference numerals for corresponding components of the system are utilized.

The principal distinction of the embodiment of FIGS. 7 and 8 from the two earlier described embodiments resides in the fact that conduit 6 positioned within cas ing 4 is imperforate for its length and is not axially arranged (note FIG. 8) as was true in the prior embodiments. That is, conduit 6 is laterally offset relative to the axis of casing 4 to provide room for a separate gas recycle pipe 42 which extends longitudinally through the well casing assembly to carry recycled gases from the compressor 34 through conduit 16 to the lower end of the well casing assembly for bubbling upwardly through the conduit 6 with steam-hot water mixture rising into such conduit through its open end 15.

In that regard, the lower end of recycle pipe 42 is curved to enter an opening provided in the wall of conduit 6 and to direct recycled gases upwardly through the conduit for the purpose explained previously. The end of recycle pipe 42 terminates in an orifice 43 which may or may not have a nozzle configuration or construction, as required. .Thus, the recycled gases pass through the annular space surrounding conduit 6 but are retained during such passage within the confines of pipe 42.

The operating conditions mentioned previously with respect to the two previously discussed embodiments are also applicable to the embodiment of FIGS. 7 and 8 and the improved results of this system are accomplished thereby in a specifically different although closely related physical structure.

While hot .water may enter the annular space between conduit 6 and casing 4, such water, and any flashed steam, is retained in the well casing assembly by cap structure14 at the well head. Thus, all geothermal hot water is recovered through conduit 6 as shown in FIG. 7.

From the foregoing, it should be understood that an improved process and system for recovering geothermal hot water has been disclosed in conjunction with specific examples of utilization thereof under exemplary operating criteria. For the scope of protection to be afforded in the invention disclosed herein, reference is directed to the appended claims.

I claim:

1. A process of obtaining high temperature geothermal hot water, such as hot water at a temperature of above 300F., as opposed to oil and like minerals, from a deep underground well without utilizing a moving mechanical or other pumping device within said well and without requiring gas-lift techniques, comprising A. positioning a well casing assembly, comprising a well casing and a conduit within said casing, in the ground to a depth sufficient to position an apertured end portion of said casing in proximity to an underground hot water supply and with an apertured end portion of said conduit adjacent said casing end portion,

B. inducing flashing to steam of at least some of said hot water entering said casing end portion,

C. recovering an essentially non-condensible gas, including carbon dioxide, from said flashed hot water, and

D. recycling said recovered gas through said well casing assembly to bring said gas into contact with additional hot water entering said casing assembly through said apertured end portion of said casing to counteract and obviate formation of deposits which tend to clog said well casing assembly by maintaining partial pressure on the recovered carbon dioxide bearing gas under all operating conditions.

2. The process of claim 1 in which said gas is recycled through said well casing assembly through said conduit so that said hot water passes into and is elevated through the space between said conduit and said surrounding well casing.

3. The process of claim 1 in which said gas is recycled through the space between said conduit and said surrounding well casing so that said hot water passes into and is elevated through said conduit within said casing.

4. The process of obtaining high temperature geothermal hot water, such as hot water at a temperature of above 300F., as opposed to oil and like minerals, from a deep underground well without attendant precipitation of chemical compounds which would clog the well system and associated equipment and without utilizing a mechanical or other pumping device within said well and without requiring gas-lift techniques comprising A. positioning a well casing assembly, comprising a well casing and a conduit within said casing and surrounded thereby, in the ground to a depth sufficient to position an apertured end portion of said casing in proximity to an underground hot water supply and with an apertured. end portion of said conduit adjacent said casing end portion,

B. reducing the pressure at the well head toinduce flashing to steam of at least some of said hot water entering said casing assembly,

1. such flashing resulting in a two phase mixture of steam-hot water rising in said casing assembly to said well head and the attendant evolution of substantially non-condensible gases including CO in conjunction with such flashing,

C. separating said evolved gases from said steam and hot water rising through said casing assembly, and

D. compressing said evolved gases and recycling the same down through said casing assembly as a substantially continuous stream to bring the same into contact with additional hot water entering said casing assembly through said apertured end portion of said casing to counteract formation of well system clogging deposits by maintaining partial pressure on a stream of two phase mixture of steam-hot water which rises continuously through said casing assembly,

1. the presence of CO in conjunction with said recycled gases precluding formation of system clogging chemical compounds such as calcium carbonate and magnesium hydroxide which would normally be formed in conjunction with said steam flashing in the absence of such recycled CO 5. The process of claim 4 which includes E. collecting and withdrawing any chemical compounds at the well head which may have formed during initial start-up of the process prior to recycling of said evolved CO carrying gases into said casing assembly, and

F. continuing compressing and recycling said evolved gases through said casing assembly in conjunction with separation of steam and said gases from hot water elevated with said flashed steam and recycled gases to effect a substantially continuous flow of hot water from said well head without formation and build-up of said clogging chemical compounds in conjunction therewith.

6. The process of claim 4 which further includes G. heat exchanging said evolved gases and steam following separation thereof from said hot water adjacent said well head to condense said steam and to extract useful heat while cooling said gases, and

H. separating said condensed steam from said cooled evolved gases prior to compressing the same for recycling through said casing assembly.

7. The process of claim 4 in which said gas is recycled through said well casing assembly through said conduit so that said hot water passes into and is elevated through the space between said conduit and said surrounding well casing.

8. The process of claim 4 in which said gas is recycled through the space between said conduit and said surrounding well casing so that said hot water passes into and is elevated through said conduit within said casing. l a: :r

Claims (8)

1. A process of obtaining high temperature geothermal hot water, such as hot water at a temperature of above 300*F., as opposed to oil and like minerals, from a deep underground well without utilizing a moving mechanical or other pumping device within said well and without requiring gas-lift techniques, comprising A. positioning a well casing assembly, comprising a well casing and a conduit within said casing, in the ground to a depth sufficient to position an apertured end portion of said casing in proximity to an underground hot water supply and with an apertured end portion of said conduit adjacent said casing end portion, B. inducing flashing to steam of at least some of said hot water entering said casing end portion, C. recovering an essentially non-condensible gas, including carbon dioxide, from said flashed hot water, and D. recycling said recovered gas through said well casing assembly to bring said gas into contact with additional hot water entering said casing assembly through said apertured end portion of said casing to counteract and obviate formation of deposits which tend to clog said well casing assembly by maintaining partial pressure on the recovered carbon dioxide bearing gas under all operating conditions.

2. The process of claim 1 in which said gas is recycled through said well casing assembly through said conduit so that said hot water passes into and is elevated through the space between said conduit and said surrounding well casing.

3. The process of claim 1 in which said gas is recycled through the space between said conduit and said surrounding well casing so that said hot water passes into and is elevated through said conduit within said casing.

4. The process of obtaining high temperature geothermal hot water, such as hot water at a temperature of above 300*F., as opposed to oil and like minerals, from a deep underground well without attendant precipitation of chemical compounds which would clog the well system and associated equipment and without utilizing a mechanical or other pumping device within said well and without requiring gas-lift techniques comprising A. positioning a well casing assembly, comprising a well casing and a conduit within said casing and surrounded thereby, in the ground to a depth sufficient to position an apertured end portion of said casing in proximity to an underground hot water supply and with an apertured end portion of said conduit adjacent said casing end portion, B. reducing the pressure at the well head to induce flashing to steam of at least some of said hot water entering said casing assembly,

5. The process of claim 4 which includes E. collecting and withdrawing any chemical compounds at the well head which may have formed during initial start-up of the process prior to recycling of said evolved CO2 carrying gases into said casing assembly, and F. continuing compressing and recycling said evolved gases through said casing assembly in conjunction with separation of steam and said gases from hot water elevated with said flashed steam and recycled gases to effect a substantially continuous flow of hot water from said well head without formation and build-up of said clogging chemical compounds in conjunction therewith.

6. The process of claim 4 which further includes G. heat exchanging said evolved gases and steam following separation thereof from said hot water adjacent said well head to condense said steam and to extract useful heat while cooling said gases, and H. separating said condensed steam from said cooled evolved gases prior to compressing the same for recycling through said casing assembly.

7. The process of claim 4 in which said gas is recycled through said well casing assembly through said conduit so that said hot water passes into and is elevated through the space between said conduit and said surrounding well casing.

8. The process of claim 4 in which said gas is recycled through the space between said conduit and said surrounding well casing so that said hot water passes into and is elevated through said conduit within said casing.

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