METHOD AND APPARATUS FOR EXTRACTING GEOTHERMAL FLUID

Abstract: A method and an apparatus for extracting geothermal fluid are disclosed. Pressurized and superheated steam is injected (9) into a hot water region below the flashing point (7) of the hot water in the well so as to lower the level of said flashing point (7) in the well, while the well head pressure of a production well drilled into a water-dominated geothermal reservoir is held constant. As a result of this injection, the pressure of water head in the well relative to the pressure of the geothermal reservoir fluid is significantly reduced and both the extraction of connate water and the production of steam at the well head are remarkably enhanced.

Description: The present invention relates to a method and appara­ tus for extracting geothermal fluid. In the wake of the recent growing interest in the utilization of geothermal energy in Japan, new technology has been established for exploring and exploiting water-­ dominated geothermal fields and this has caused a rapid increase in the rate of development of geothermal energy in Japan which is abundant with water-dominated geothermal fields. However, the water-dominated geothermal fields in Japan have a fatal inherent defect in that the production capacity of production wells diminishes year by year. If difficulty is expected in maintaining the desired output of electric power generation from water-dominated geothermal reservoirs as a result of decrease in the steam production at individual production wells, a practice is currently undertaken to compensate for the decrease in steam production from the existing production wells by drilling additional production wells at seemingly promising sites. However, this method is not capable of increasing the ini­ tially estimated quantity of hot water or energy that can be extracted from geothermal reservoirs. Therefore, drilling additional production wells inevitably shortens the lifetime over which steam can be produced from reservoirs. If the quantity of hot water or energy that can be extracted from geothermal reservoirs could be increased, it would become possible to produce more steam for a longer period of time without increasing the number of production wells. The present invention relates to a method for increas­ ing both the capacity of a steam production well and the amount of steam that can be extracted from geothermal reser­ voirs. This object can be attained by injecting a heated gas into a hot water region which is located below the flashing point of connate geothermal water in a production well so as to lower the position of said flashing point within the well to thereby reduce the hydrokinetic bottom hole pressure relative to the hydrostatic bottom hole pressure. Fig. l (a) and (b) shows the concept and operating principle of the method of the present invention for extracting geother­ mal fluid, respectiveIy ; Fig. 2 is a schematic showing the concept of a steam enhanced circulation system which is a preferred embodiment of the present invention; Fig. 3 shows the specifications of a well (a) and the temperature distribution of a reservoir around the well (b), both of which were inputted for calculations by simulation; Fig. 4 shows the pressure and temperature distribu­ tions in the well (well head pressure preset at 24.7 KSCA) that were outputted from calculations by simulation; and Fig. 5 shows the relationship between the amount of steam injected into a well and the amount of steam produced from the well (well head pressure preset at 24.7 KSCA). It is generally held that the production capacity of production wells decreases as a result of the following effects: l) pressure drop accompanying the steam production from geothermal reservoirs; 2) temperature drop of the reservoir fluid; and 3) reduction of the productivity index (PI) of the well. Among these factors, l) the pressure drop in geo­ thermal reservoirs and 2) the temperature drop of the reservoir fluid have been found predominant by the present investigations. The total production rate Q that can be extracted from a geothermal field can be expressed basically by the following equation: where P s is the hydrostatic bottom hole pressure or the hydrostatic reservoir pressure and P f is the hydrokinetic bottom hole pressure. The mass specific gravity of a fluid in a production well is a function of the temperature and pressure in the well. The reduction of the productivity index of a produc­ tion well is usually small, so it may safely be concluded that the production capacity of production wells decreases chiefly because of a diminution of the bottom hole pressure draw down due to the decrease in the reservoir pressure and of a drop in the vapor/liquid ratio in the well owing to the decrease in the temperature of the reservoir fluid. Based on this observation, the present invention increases the steam production capacity of a well by inject­ ing a pressurized gas, more specifically, by circulating superheated steam, so as to enhance both the pressure draw down at the bottom of the well and the vapor/liquid ratio in the well. The operating principle of the method of the present invention is hereunder described with reference to Fig. l. In Fig. l(a), l is a casing; 2 is a tubing (pressurized steam injection pipe); 3 is a reservoir; 4 is hot water; 5 is superheated steam; 6 is saturated steam; 7 is the flashing point of connate geothermal water maintained prior to injection of superheated steam; 8 is a flashing point achieved after injection of superheated steam; and 9 is the point at which the steam is injected. The tubing 2 is inserted into the well so that super­ heated steam can be injected into hot water in accordance with the present invention. This tubing is not employed in the prior art method wherein hot water 4 which flows up from the reservoir 3 to ascend through the casing l is simply recovered from the well head and guided to a collector. In this operation, a flashing point (the surface at which hot water boils to be vaporized) forms at a certain depth which is determined by such factors as the hydrostatic bottom hole pressure of the reservoir and the temperature of the connate hot water. In the long run, the position of the flashing point will become lower as the hydrokinetic bottom hole pressure decreases but, in the actual process of extracting hot water, the position of the flashing point will not experience any discernible change even if the extraction of hot water is continued for a considerable period of time. This substantially constant position of the flashing point is indicated at 7 in Fig. l. For example, the depth of the flashing point will remain virtually constant until the quantity of hot water being extracted is decreased from 300 tons/hr to about l00 tons/hr. The flashing point starts to sink when the quantity of hot water being extracted has dropped to about l00 tons/hr but the degree of its subsi­ dence is no more than a few meters below the initial level. From an economic viewpoint, it is particularly effective to apply the method of the present invention after the position of the flashing point has started to decline from the ini­ tial level but it should of course be understood that in order to attain the very object of increasing the steam production rate, the method of the present invention may be applied from the start of the extracting operation. In accordance with the present invention, the tubing 2 is inserted into the hot water in the casing l in such a manner that the tip of the tubing is located typically several hundred meters below the initial depth of the flash­ ing point. Superheated steam (ca. 300 - 400°C) is then injected through the tubing. The hot pressurized steam gushing out of the tip of the tubing is pushed back by the pressure of hot water flowing up from below and immediately changes its direction to ascend together with the hot water in the casing. In Fig. l(b), the static bottom hole pres­ sure, or the pressure of geothermal fluid that prevails before drilling a well, is indicated by point C. As soon as a well is drilled, the pressure of geothermal fluid drops to the level indicated by D₁, creating a fluid pressure distri­ bution in the well as indicated by solid line A. Point A7 on line A denotes a fluid pressure being exerted at the position corresponding to point 7 in Fig. l(a) which indi­ cates the flashing point maintained prior to the injection of super heated steam. Point A9 denotes a fluid pressure prevailing at the depth corresponding to the position 9 at which the tip of the tubing 2 is located. In order for the superheated steam to be successfully injected into connate geothermal water, the steam must have a pressure greater than A9. This is indeed an extreme pressure and usually the steam will not readily gush out of the tip of the tubing unless its pressure is increased in steps with the aid of some suitable means such as kickoff valves. However, once the steam has started to gush out of the tubing, its sus­ tained injection is fairly easy to achieve. As the pressure distribution in the well is as shown by curve A, the fluid pressure becomes higher at depths below A9 and the injected steam will change its direction to ascend through the casing together with hot water. This cases accelerated vaporiza­ tion of the hot water in the well since the large thermal energy of the superheated (300 - 400°C) steam is absorbed chiefly by the hot water present at a position above the level where the steam gushes out of the tubing. Since the well head pressure is substantially constant, the increase in the thermal energy of the hot water causes a buildup in saturation pressure of the nearby area and the flashing point is consequently displaced to a lower position. The new flashing point which forms as a result of steam injec­ tion is indicated at 8 in Fig. l(a). The pressure distribu­ tion which is attained by steam injection is shown in Fig. l(b) by dashed line B. The fluid pressure which corresponds to the flashing point 8 attained by steam injection is indicated by B8. The bottom hole pressure lowers to the position indicated by D₂. The pressures at points D₁ and D₂ are generally referred to as hydrokinetic bottom hole pres­ sures. The temperature distribution of the fluid in the well that prevails before steam injection is shown in Fig. l(b) by solid line X. Dashed line Y in Fig. l(b) indicates the temperature distribution of the fluid that prevails at depths above the gushing level 9 after steam injection. As equation (l) indicates, the production rate of steam from a geothermal field is proportional to the differ­ ence between hydrostatic and hydrokinetic bottom hole pres­ sures, which may be translated to the difference between C and D in Fig. l(b). Obviously, the hydrokinetic bottom hole pressure D has dropped from D₁ to D₂ as a result of steam injection and the production of steam is increased because (C - D₂) > (C - D₁). Theoretically, the method of the present invention may be implemented even if injected steam is replaced by any other appropriate pressurized hot gas but, in practical applications, part of the steam produced is preferably circulated for injection because it will preclude the entrance of any undesirable gas into the product steam and enable efficient energy utilization. Therefore, the method of the present invention, when taken in a broad sense, may be described as a steam enhanced circulation (SEC) lifting system. The SEC system is intended for achieving a higher steam production rate by injecting an appropriate pres­ surized superheated steam into the hot water region in a production well (Fig. l). In accordance with this system, an appropriate pressurized steam is injected, while the well head pressure of a production well drilled into a water-­ dominated geothermal reservoir is held constant, into a hot water region below the flashing point of the hot water in the well so as to lower the level of said flashing point in the well, with the result that the pressure of water head in the well (i.e., the hydrokinetic bottom hole pressure) relative to the pressure of the geothermal reservoir fluid (i.e., the hydrostatic bottom hole pressure) is sufficiently reduced to enhance both the extraction of connate water and the production of steam at the well head. If a heat insulating material is placed between the tubing (pipe inserted into a well) and the annulus of the casing fitted in the well bore, the circulative utilization of energy can be achieved in the practice of the method of the present invention by injecting part of the saturated product steam after it has been pressurized to an adequately high level. A preferable embodiment of this circulation system is shown in Fig. 2. While a steam turbine compressor is used to pressurize the saturated product steam, the turbine itself may be driven with part of the saturated steam produced at the well head. In geothermal power gener­ ating systems currently in operation, the pressure at the head of a production well is higher than the pressure of the steam that is fed to a turbine generator, so that the product steam must be expanded by depressurization through a separator before it is employed for power generation. Therefore, if the pressure at the exhaust side of the steam turbine used for pressurizing the steam to be injected is set to the value that is attainable by the separator, the product steam that has passed through said steam turbine can be directly used for the purpose of electric power genera­ tion. In other words, the amount of energy that will be lost to the outside of the system operating based on this approach is virtually negligible except for the portion regarding the efficiency of the steam turbine compressor. Fig. 2 shows the concept of a steam enhanced circula­ tion system suitable for use in the practice of the present invention: l is a casing; 2 is a tubing through which superheated steam is injected; l0 is a valve at the well head connecting to the tubing 2; ll is a valve positioned at the well head for recovery of the product steam; l2 is a separator; l3 is a steam pipeline; l4 is a separator; l5 is a compressor; l6 is a turbine; l7 is a separator; a , b and c represent the pressures at respective positions; and KSC signifies kg/cm². A major portion of the product steam is transported through the pipeline l3 and expanded by depres­ surization to b kg/cm² in the separator l4 before it is fed to a steam turbine generator (not shown). Another part of the product steam is withdrawn at a pressure of a kg/cm² from the separator l2 through the other outlet; part of the withdrawn steam is used to drive the turbine l6 while the remainder is pressurized to c kg/cm² in the compressor l5 and sent to the well head from which it is injected into hot water region through the tubing 2. The steam which has been used to drive the turbine l6 is discharged at b kg/cm² and, after being passed through the separator l7, is combined with the product steam being sent to the turbine generator at a pressure of b kg/cm². By controlling the respective pressures in such a manner that c > a > b , the process of steam extraction can be operated with the least energy loss. The advantages that could be attained by applying the method of the present invention to geothermal steam extrac­ tion on a commercial scale were estimated by computer simu­ lation with a steam-water two-phase vertical flow calculat­ ing simulator GEOVER. Fig. 3 shows the specifications of a well and the temperature distribution of a reservoir around the well, both of which were inputted for calculation by simulation. Table l lists the other input conditions. The simulator was also programmed to calculate the amount of heat that would be lost from the well to the nearby reser­ voir but, in the five cases of simulation, the total produc­ tion rate at the well head was very high (≧300 tons/hr) and the resulting heat loss was negligible. In four of the five cases, steam was injected at rates of l0, 20, 30 and 40 tons/hr, respectively. The simulated distributions of pres­ sures and temperatures within the well are shown in Fig. 4, and details of the results of calculations by simulation are given in Table 2. According to Fig. 4, two-phase states were formed at the steam injected depth when the injection rates were 30 and 40 tons/hr. The data in Table 2 are shown graphically in Fig. 5 from which one can see that the total amount of steam produced increases with increasing steam injection. However, as column l5 in Table 2 shows, the rate of increase in the amount of energy generation decreases with increasing steam injection. This is particularly true when the amount of steam injection is increased from 30 tons/hr to 40 tons/hr. When a two-phase state is created at the steam injected depth, in deeper areas than this depth only hot water exists and the subsequent change in the pres­ sure of water head in the production well would be too small to achieve a marked increase in the total amount of steam production from the reservoir. The validity of this assump­ tion is supported by the fact that the pressure profiles for steam injection at rates of 30 tons/hr and 40 tons/hr are almost identical to each other. It can therefore be con­ cluded that the increase in the total amount of the energy of steam that is produced by injecting 40 tons/hr of steam is due only to the thermal energy provided by the injected steam. As Fig. 4 and Table 2 show, by injecting super heated steam into hot water in a production well in accordance with the method of the present invention, the flashing point of hot water is displaced to a deeper position and the produc­ tivity of the reservoir is significantly increased. It has also been demonstrated that by incorporating the SEC system into the method of the present invention, the amount of hot water or energy that can be extracted from a geothermal reservoir is increased and at the same time the amount of steam feed to a turbine generator can be increased with less energy consumption. It is of course imperative that before applying the SEC system to a real geothermal reservoir, its characteris­ tics should be fully investigated and selection be made for most economically feasible specifications of a production well, gas to be injected, depth at which the gas is injected, amount of gas injection, tubing through which the gas is injected, and heat insulating material.

Claims: 1. A method of extracting geothermal fluid wherein super­ heated steam is injected, while the well head pressure of a production well drilled into a water-dominated geothermal reservoir is held constant, into a hot water region below the depth of the flashing point of connate hot water in the well so as to displace said flashing point to a deeper posi­ tion and to lower the pressure of water head in the well (i.e., the hydrokinetic bottom hole pressure) relative to the pressure of the geothermal reservoir fluid (i.e., the hydrostatic bottom hole pressure), so that both the amount of hot water extracted per unit time and the rate of steam production at the well head are increased. 2. A method according to Claim l wherein said super­ heated steam is part of the product steam that has been separated and adiabatically compressed in a compressor. 3. A method according to Claim 2 wherein a separated part of the product steam is used as a power source for a turbine that is intended to drive said compressor. 4. A method according to Claim l wherein the product steam recovered from the well head is divided into a first portion having a pressure of a kg/cm² and a second portion having a pressure of b kg/cm², said first portion being further divided into third and fourth portions each having a pressure of a kg/cm², said third portion being adiabatically compressed in the compressor to create a fifth portion having a pressure of c kg/cm², said fourth portion being used as a power source for the compressor driving turbine, said turbine discharging a sixth portion having a pressure of b kg/cm², the respective pressures being so adjusted that c > a > b, to thereby allow the fifth portion having a pre sure of c kg/cm² to be continuously injected into connate geothermal water below the initially formed flashing point while the second and sixth portions each having a pressure of b kg/cm² are combined to be continuously fed into a steam turbine generator. 5. An apparatus for extracting geothermal fluid compris­ ing the combination of: (I) a superheated steam injecting mechanism which comprises (i) a double-walled tubing for injecting heated steam that is inserted into the casing of a steam production well, the top of which is fixed to a casing head installed at the well head while the bottom of which is located at a predetermined depth, with an appropriate liquid heat insulating material being injected into the annulus of said tubing; (ii) kickoff valves that are installed at appropriately selected depths and which may optionally be actuated to lower the pressure at which the injection of superheated steam is started; and (iii) the casing head by which said double-walled tubing is fixed to the well head; (II) a production mechanism for circulating steam which comprises (i)steam separating means that separates part of the product steam for supplying both the steam to be injected and the steam to be used as a power source; and (ii) steam pressurizing means that pressurizes the sepa­ rated steam to produce the steam to be injected; and (III) a processing mechanism by which all of the available product steam except on which is subjected to enhanced circulation through the well is processed to the state where it has a sufficient degree of dryness to be used industrially.

Inventors: WADA, SABRO
Applicants: NIPPON OIL ENG KK
Application year: 1986
Main classification: F24J3
IPC codes: F24J3, E21B43, F03G7, F03G4