Artificial Lift System

Abstract

A method and system for artificially lifting enriched solvents in solution mining wells are disclosed. A liquid immiscible with and lighter in gravity than the solvent is injected in the well annulus while the solvent enriched with minerals is produced through the well tubing. The system can selectively be placed in continuous or intermittent operation.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to artificial lifting of liquid from a subsurface elevation and more particularly to artificial lift means for solution mining.

2. Description of the Prior Art

Solution mining is a mineral recovery process which utilizes a leaching solvent for dissolving the mineral in situ. In one form, it involves the injection of the leaching solvent into the mineral-bearing formation through selected input wells and the recovery of the enriched solvent through selected producing wells. The mineral can be present in the matrix of a host rock or as a massive deposit. In either situation, communication between the input and producing wells can be induced or improved by fracturing techniques developed in the petroleum industry. Owing largely to advanced metallurgical and formation treating technology, solution mining, of late, has been applied in the in situ recovery of uranium and has been proposed for the recovery of a host of other minerals such as copper, phosphate, and manganese. As the technology continues to advance and as the demand for minerals increases accompanied by the inexorable depletion of present reserves, it is reasonable to expect the continued expansion of solution mining techniques.

While solution mining offers significant advantages over excavation mining--particularly in the treatment of low-grade ores--it is beset by the serious and continuing problem of corrosion. Solvents having the capabilities of selectively dissolving the desired minerals generally are highly corrosive. For example, uranium is leached in situ by aqueous solutions of nitric or sulfuric acid, the corrosivities of which are well known. Even in the solution mining of salt, the corrosivity of the produced brine requires the use of special corrosive-resistant equipment.

In the recovery phase of the operation, the enriched solvent must be lifted from the subterranean formation. A prime consideration in selecting the type of artificial lift system is the system's ability to handle the highly corrosive solvents. Conventional artificial lift facilities include submersible pumps and air lifts. Both of these facilities require the exposure of substantial amounts of equipment to the corrosive liquids and therefore experience relatively short operating lives. The repair and replacement of subsurface pumps is a particularly expensive operation requiring the withdrawal of the entire assembly. While the air-lift system obviates some of the operational disadvantages of the subsurface pump, particularly in regards to the replacement of parts, it presents other problems. The commingled air and solvent resulting from the air lift, in fact, increases the rate of corrosion so that while replacement of parts is facilitated, the frequency of replacement is increased.

SUMMARY OF THE INVENTION

The present invention contemplates lifting the solvent by injecting a power liquid down the well annulus and producing the solvent through the well tubing. The subsurface equipment includes a packer and check valve assembly for permitting flow into the casing while preventing backflow during the lifting phase of the cycle.

Under certain conditions, the formation may have sufficient pressure to cause the static fluid level to stand high in the well. Under these conditions flow can be induced by reducing the pressure gradient in the fluid column such that the back pressure on the formation is less than the formation pressure. The present invention contemplates the continuous commingling of a light liquid and solvent in the producing string. The rate of production attainable by this continuous lift system depends upon the pressure draw-down imparted on the formation, which in turn depends upon the difference in liquid and solvent densities and the relative volumes commingled.

As the formation pressure is dissipated by the withdrawal of formation fluids, the differential pressure gradually declines to a point that a more positive lift may be required in order to maintain the desired production rate. The surface facilities may then be modified to provide an intermittent lift which operates on a displacement principle. In intermittent lift operation, the solvent is permitted to rise in the annulus while maintaining a column of light liquid above the solvent fluid level. When the solvent fluid level reaches a predetermined upper level in the annulus, a power liquid which can be the same composition as the light liquid is injected in the casing forcing the solvent down the annulus and up the tubing. The solvent fluid level is displaced downwardly within the annulus to a predetermined lower elevation whereupon injection is discontinued placing the system in a condition for repeating the lift cycle. Thus, the lift cycle comprises (1) a solvent entry phase where solvent from the formation enters the well annulus rising to a predetermined elevation therein and (2) a displacement phase where a power liquid displaces solvent in the annulus forcing it up the well tubing.

A particular advantageous feature of the lifting system contemplated by the present invention is the use of few moving parts exposed to the corrosive solvent. The intermittent lift system has the added advantage of permitting the accurate control of injection of the power liquid. Moreover the intermittent operation does not entail commingling of the power liquid and solvent and therefore requires no surface separation.

The injection of the power liquid when in intermittent operation can be made operatively responsive to the solvent fluid level in the annulus so that the operation thereof is a function of the producing rate of the well. The amount of power liquid injected in each displacement phase of the cycle is regulated by accurate volumetric control.

By way of summary then the present invention provides a novel method and system for lifting corrosive liquids, the system being adapted for continuous or intermittent operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating the artificial lift system according to the present invention under continuous operation; and

FIGS. 2 and 3 are diagrammatic views illustrating the artificial lift system under intermittent operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1 a well 10 is shown completed for draining a subterranean formation 11. The formation 11 is mineral-bearing having characteristics such that it is amenable to the solution mining process. The mineral deposits may be present in the matrix of a host rock or may be present as a massive continuous deposit. The mineral can be uranium, copper, phosphate, salt, or other mineral which can be extracted by in situ leaching according to presently known techniques. The formation 11 can have natural matrix permeability or can be fractured for improving the permeability. In either event there is sufficient permeability for conducting the leaching solvent from an adjacent input well (not shown) to the producing well 10. The usual solution mining operation involves injecting a mineral dissolving or leaching solvent via the input well into the mineral-bearing formation 11 and withdrawing the solvent enriched with the mineral at the producing well 10.

The well 10 is drilled and completed by techniques well known in the petroleum industry. In completing the well 10 as depicted, a borehole 12 is drilled to a depth sufficient to penetrate the formation 11. Casing 13 is run to the top of the formation 11 and cemented in place. A packer 14 carried at the lower end of the tubing string 15 is set at the base of the casing 13.

Owing to the high density of the enriched solvent, the formation pressure may not be sufficient to cause the well to flow at economic rates. In this event, an artificial lift system is required. Because of the highly corrosive nature of the solvent, it is desirable that the lift system employed have few moving subsurface parts exposed to the solvent.

The present invention contemplates lifting the enriched solvent through the tubing 15 by injecting a low-density liquid into the annulus 17 defined by the casing 13 and tubing 15. The surface facilities for injecting the light liquid are designed to permit either continuous or intermittent operation. The subsurface equipment can accommodate either type of operation without modification.

More specifically, the tubing string 15 comprises, from bottom up, a lower landing nipple 18 secured to the packer 14, a perforated nipple 19, an upper landing nipple 20 disposed immediately above the perforated nipple 19, and a continuous conduit 21 to the surface. The landing nipples 18 and 20 are adapted to receive wireline retrievable standing valves 22 and 23, respectively.

When the well 10 is placed on production the formation pressure forces solvent into the wellbore 12 and up the tubing string 15. If the formation pressure is greater than the back pressure imposed on the formation face by a fluid column in the tubing string 15, the well flows by natural process. The pressure in formation 11 can be due to the hydrostatic pressure imposed by the weight of water indigenous to the formation or overburden pressure resulting from the weight of upper formations. Or the formation 11 can be pressurized by injection of fluids in the input wells.

If the formation pressure is sufficiently high to provide a static fluid level close to the surface, flow can be induced by reducing the pressure gradient in the tubing string 15. The present invention, in one aspect, contemplates a continuous lift process involving the commingling of a light liquid with the produced solvent in the tubing string 15. Preferably the light liquid is immiscible with the solvent so as to facilitate the surface separation of the liquid and solvent. Accordingly, a light lift liquid is continuously injected down the annulus 17 at such a rate to cause liquid to enter perforated nipple 19 and intermix with the solvent stream in string 15. The rate of injection is controlled to provide the proper mixture of lift liquid and solvent to give the desired pressure gradient. This reduces the formation back pressure, imparting a pressure draw-down on the formation 11. The ratio of lift liquid to solvent to effect a given pressure gradient in the tubing column of course will depend upon their respective densities. If the densities of the solvent and lift liquid are such to separately provide a pressure gradient of 0.54 psi per foot (specific gravity of 1.25) and 0.35 psi per foot (specific gravity of 0.80), respectively, a mixture of two parts of the former to one part of the latter provides a pressure gradient of 0.48 psi per foot. Thus if the formation 11 has a static fluid column of 2,000 feet, a pressure draw-down of 120 psi can be imparted on the formation by the continuous injection of a light liquid at a rate equal to one-third the total fluid withdrawal rate.

The continuous lift process described above is applicable in wells completed in formations having relatively high pressures. Its primary disadvantage is the limitation on the pressure draw-down on the formation.

As the formation pressure declines, the flowing gradient must be decreased correspondingly to maintain the desired level of production. Lower pressure gradients are obtained by increasing the ratio of lift liquid to solvent. At a point where an excessive amount of liquid is required to maintain the desired pressure draw-down, it may be more economical to switch to the intermittent operation. While the intermittent operation reduces the flowing time of the well, it permits a greater pressure draw-down on the formation which should more than offset the losses resulting from the nonproducing time intervals of the lifting cycle.

The intermittent lift system according to the present invention operates on the principle of displacement. Under dynamic lift conditions, solvent from the formation 11 is permitted to rise in the annulus 17 while a column of power liquid is maintained to the surface above the solvent fluid level. The selected power liquid is immiscible with and lighter in gravity than the solvent so that the former floats on the latter, the contact being at the interface 25 shown in FIG. 2. At a predetermined upper elevation of the interface 25, the power liquid is injected into the annulus 17. This displaces the interface 25 downwardly within the annulus 17 forcing solvent through the perforated nipple 19 and up the tubing string 15 (see FIG. 3). At a predetermined lower elevation of the interface 25, liquid injection is discontinued and the system is placed in a condition for repeating the lift cycle. Thus the back pressure on the formation 11 with the interface 25 in the lower position, is principally the product of the pressure gradient of the power liquid and the depth of the well. In the well of the above example wherein the solvent and liquid have pressure gradients of 0.54 and 0.35 psi per foot, the pressure draw-down on the formation can be increased by about 370 psi. If the formation pressure balances a static fluid column 1,500 feet in the tubing string, a pressure draw-down of 100 psi is attainable by the intermittent lift system.

The subsurface equipment in the well 10 is identical for either the continuous or intermittent operation. It should be noted that the only moving parts are the two standing valves 22 and 23 which are easily retrieved by conventional wireline equipment. If the flow area of the annulus 17 is substantially greater than the flow area of the tubing 15, valve 23 is not required. However its presence provides for a more efficient displacement operation since it prevents backflow from the tubing 15 during the solvent entry phase of the lift cycle.

In either the continuous or intermittent operation, the pressure draw-down attainable is a function of the relative lift or power liquid and the solvent. The produced solvents laden with the extracted mineral generally will have a specific gravity in the order of 1.25, but could have a specific gravity as low as 1.1. The wider the separation of gravities, the greater the draw-down on the formation. Accordingly it is preferred that the differential of specific gravities be at least 0.2.

In solution mining operations where the solvent is an aqueous solution of a mineral-dissolving material such as that used in the solution mining of salt or uranium, a liquid satisfying the requirements of immiscibility and light gravity is found in the light petroleum fractions such as gasoline, kerosene, or diesel fuel. These hydrocarbon liquids have the property of being stable at the normal operating temperatures and pressures so that they can be recycled thereby providing a closed system.

The surface facilities are designed to permit either type of operation. Considering first the continuous lift operation (FIG. 1), the components include a separator 26, a reservoir 27, and a motor-driven pump 28, and the necessary piping for interconnecting the parts as depicted. Through suitable connections, the pump 28 receives the lift liquid from the reservoir 27 and delivers it to the annulus 17. The rate of injection can be calculated or can be regulated to achieve the desired solvent producing rate. The produced mixture of solvent and lift liquid is directed to the separator 26 where the lighter liquid is returned by an overhead line to the reservoir 27 while the heavier solvent is discharged to storage. An interface level controller 30 is operatively connected to the separator discharge valve 31 and serves to maintain the interface within the confines of separator 26.

The surface equipment for the intermittent operation includes the reservoir 27 and pump 28 plus a motor valve 29 and liquid level controllers 32 and 33 (see FIGS. 2 and 3). The level controllers 32 and 33, respectively, provide high-level and low-level control points within the reservoir 27. The pump 28 through suitable controls is operatively responsive to actuation of the high-level controller 32. The motor valve 29 is a three-way, two-position, directional valve having a first position which provides fluid communication between the pump 28 and annulus 17 and a second position which provides fluid communication between the reservoir 27 and the annulus 17. The valve 29 is normally in the first position and is energized to the second position by actuation of the low-level controller 33.

At the beginning of the lift cycle, occasioned by the actuation of the low-level controller 33, the energized motor valve 29, in the second position, permits the flow of power liquid from the annulus 17 to the reservoir via suitable conduits. The solvent entering the annulus 17 through the lower standing valve 18 and perforated nipple 19 displaces a like amount of power liquid into the reservoir 27. The solvent fluid level 26 rises in the annulus 17 until sufficient power liquid has been displaced in the reservoir 27 to actuate the high-level controller 32. This deenergizes the motor valve 29 and activates the pump 28, a suitable time delay being provided if necessary. Power liquid is pumped down the annulus 17 depressing the interface 25 therein until a predetermined liquid slug has been injected as determined by the reservoir volume between the upper and lower level controllers 32 and 33. The solvent, being restricted from backflowing into the formation 11 by the lower standing valve 18, is displaced up the tubing string 15 and discharged at the surface to storage, standing valve 23 preventing backflow from the tubing 15. The volume of each slug can vary within a wide range. In a well having 7-inch casing and 2-inch tubing, a slug of 150 gallons provides an annular displacement of 100 feet. Since the power liquid is not commingled with the solvent, the separator 26 can be bypassed as illustrated. It should be observed that the displacement medium (power liquid) being essentially an incompressible liquid permits the actuation of the displacement phase of the lift cycle in response to the producing ability of the formation 11. In other words the injection of the power liquid does not commence until a predetermined volume of solvent has entered the annulus 17. Moreover, by simply adjusting the space between the high-level and low-level controllers, 32 and 33, the volume of each slug of power liquid injected can be varied to meet a variety of producing conditions. The individual components of the surface facilities, reservoir 27, pump 28, valve 29, and associated controls are conventional and except for the combination claimed form no part of the present invention.

In describing the operation of the intermittent lift system, let it be assumed that the upper and lower pumping levels are those shown in FIGS. 2 and 3, respectively. As indicated above, these operating levels of the interface 25 are determined by the relative positions of the high- and low-level controllers within reservoir 27 and that by simple adjustment the operating levels can be changed. At the beginning of the solvent entry phase of the lift cycle, the tubing 15 is full of the solvent being retained by the standing valve 23, and the annulus 17 is filled principally with the lighter power liquid. Thus the static head imposed on the formation 11 is substantially less than the formation pressure. Accordingly solvent enters the wellbore 12, passes through the standing valve 22, through the perforated nipple 22 and up the annulus 17. The interface 25 rises displacing the power liquid ahead of it forcing it into the reservoir 27. Interface 25 rises a predetermined elevation in the annulus 17 determined by the location of the high-level controller 32. Actuation of the high-level controller places the system in condition for performing the displacement phase of the cycle. A slug of power liquid is injected in the annulus 17 depressing the interface 25 to the low-level elevation forcing the solvent up the tubing 15 and eventually to the storage facilities. The injection rate is considerably faster than the fluid entry rate so that the solvent entry phase of the lifting cycle is considerably longer in duration than the displacement phase.

Summarizing, the present invention provides for an artificial lift method and system particularly adapted to the solution mining operations and is characterized as having a minimum of moving parts disposed to the corrosive solution mining solvents. The lift system can be used in intermittent or continuous operation. In intermittent operation, the system offers the added feature of displacing the solvent in the annulus without the necessity of commingling the displacing medium and the produced solvent, and provides means for actuating the system responsive to the producing ability of the well.

Claims

We claim:

1. A method for lifting a corrosive aqueous solution in a cased well completed for draining a mineral-bearing formation, and having a tubing string disposed therein, said formation having sufficient pressure to provide a static fluid column which substantially fills said tubing string, said method comprising continuously introducing a hydrocarbon liquid into the lower end of said tubing string, said hydrocarbon liquid being immiscible with said aqueous solution and having a density substantially less than the density of said aqueous solution, the volume of said hydrocarbon liquid introduced into said tubing string being sufficient to substantially reduce the pressure gradient of said fluid column therein to permit the aqueous solution to be produced.

2. The method as recited in claim 1 wherein the aqueous solution has a specific gravity greater than about 1.1 and the liquid hydrocarbon has a specific gravity less than about 0.9 at operating conditions.

3. The method as recited in 1 and further comprising the steps of separating the produced aqueous solution and hydrocarbon liquid, and reintroducing the hydrocarbon liquid into the lower end of said tubing.

4. A method of lifting a mineral solvent in a cased well completed for draining a mineral-bearing formation and having a tubing string disposed therein, the lower end of said tubing string being in fluid communication with the casing-tubing annulus, said method comprising the repetitive steps of permitting the solvent to enter the annulus under formation pressure while maintaining said annulus above the solvent fluid level therein full of a power liquid immiscible with and lighter in gravity than said solvent; and thereafter injecting a power liquid into the annulus while preventing backflow of the solvent into the formation to displace solvent received in said annulus through said lower end and into said tubing string.

5. The method of claim 4 wherein the power liquid is a hydrocarbon liquid having a specific gravity less than about 0.9 and adapted for use in solution mining wells using an aqueous solution of a mineral solvent having a specific gravity greater than about 1.1.

6. The method of claim 5 wherein the volume of power liquid injected into said annulus is volumetrically controlled to prevent power liquid from entering said tubing string.

7. A system for lifting a mineral solvent in a cased well completed for draining a mineral-bearing formation, said system comprising a tubing string disposed in said well and having an inlet at its lower end; valve means connected to an upper portion of the annulus defined by the well casing and said tubing string for controlling fluid flow into and out of said upper portion of said annulus; pump means for injecting a power liquid through said valve means and into said annulus, said power liquid being immiscible with and lighter in gravity than said solvent; control means for operating said pump means and said valve means in a repeating pump cycle which comprises a solvent entry phase wherein pressure is relieved from said annulus through said valve means permitting solvent to flow from said formation into a lower portion of said annulus, and a lift phase wherein said pump means is operated to inject power liquid into said annulus to displace solvent from said lower portion of said annulus into said tubing string.

8. The system of claim 7 wherein said pump means includes a pump, a power liquid reservoir connected to said pump, piping connecting said pump and said reservoir to said valve means, said valve means includes a motor valve having a first position providing fluid communication between said pump and said annulus and a second position providing fluid communication between said annulus and said reservoir, and said control means includes first control means for operating said pump and for placing said motor valve in said first position attendant to said solvent reaching a predetermined upper elevation in said annulus and second control means for deactivating said pump and for placing said motor valve in said second position attendant to said solvent being displaced to a predetermined lower elevation in said annulus.

9. The system as recited in claim 8 wherein said first and second control means include high-level and low-level controllers disposed in said reservoir.