Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
METAL EXTRACTION METHOD AND SYSTEM
Document Type and Number:
WIPO Patent Application WO/2019/081892
Kind Code:
A1
Abstract:
A method of extracting metal from a subterranean formation, the method comprising: injecting a stimulation fluid into an inlet of a first bore (B1), the first bore being in fluid communication with a brine lens (16) containing metal containing magmatic brine which has a temperature of at least 400°C; flowing brine from the brine lens into a second bore (B2) provided in fluid communication with the brine lens, the second bore having an outlet in fluid communication with the surface.

Inventors:
BLUNDY JON (GB)
AFANASYEV ANDREY (RU)
Application Number:
PCT/GB2018/053010
Publication Date:
May 02, 2019
Filing Date:
October 18, 2018
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
UNIV BRISTOL (GB)
International Classes:
E21B43/241; E21B43/28; E21B43/285; F24T10/20
Foreign References:
US4211613A1980-07-08
US5240687A1993-08-31
US3679264A1972-07-25
Attorney, Agent or Firm:
LATHAM, Stuart (GB)
Download PDF:
Claims:
Claims

1. A method of extracting metal from a subterranean formation, the method comprising :

injecting a stimulation fluid into an inlet of a first bore, the first bore being in fluid communication with a brine lens containing metal-bearing magmatic brine which has a temperature of at least 400 °C;

flowing brine from the brine lens into a second bore provided in fluid communication with the brine lens, the second bore having an outlet accessible from the surface.

2. A method according to claim 1, wherein the magmatic brine has a temperature of at least 450 °C.

3. A method according to any preceding claim, wherein at the point of injection into the first bore the stimulation fluid has a temperature of at least 20 °C.

4. A method according to any preceding claim, wherein the stimulation fluid comprises oxygenated water.

5. A method according to any preceding claim, wherein the stimulation fluid comprises one or more chemicals to enhance metal solubility in the brine.

6. A method according to any preceding claim, wherein a portion of the first bore within the brine lens is horizontal and a portion of the second bore within the brine lens is horizontal.

7. A method according to claim 6, wherein the portion of the first bore within the brine lens is parallel with respect to the portion of the second bore within the brine lens.

8. A method according to claim 7, wherein the portion of the first bore within the brine lens is located within 1000 m of the portion of the second bore within the brine lens.

9. A method according to claim 7 or claim 8, wherein the portion of the first bore within the brine lens is located above the portion of the second bore within the brine lens.

10. A method according to any preceding claim, whereby the second bore is provided with a liner formed from a relatively inert material.

11. A method according to any preceding claim, whereby brine from the second bore is passed through a heat exchanger to heat water to form steam that drives a geothermal turbine and generator unit to produce electricity.

12. A system for extracting metal from a subterranean formation below a surface, the system comprising :

a first bore provided in fluid communication with a brine lens containing magmatic brine, the first bore having an inlet for injecting a stimulation fluid into the first bore from the surface;

a second bore provided in fluid communication with the brine lens, the second bore having an outlet for flowing brine to the surface,

wherein, the brine lens contains magmatic brine which has a temperature of at least 400 °C.

13. A system according to claim 12, further comprising a fluid pumping station in fluid communication with the inlet of the first bore for injecting the stimulation fluid into the first bore from the surface and/or a storage station in fluid communication with the outlet of the second bore for collecting brine.

14. A method of forming a system according to claim 12, comprising :

drilling a first bore in fluid communication with a brine lens containing magmatic brine which has a temperature of at least 400 °C, the first bore having an inlet for injecting a stimulation fluid into the first bore from the surface; and

drilling a second bore provided in fluid communication with the brine lens, the second bore having an outlet for flowing the brine to the surface.

15. A method according to claim 14, further comprising : injecting pressurised fluid into the first bore to create fractures extending from the first bore to the second bore and introducing proppant into the fractures.

Description:
Metal Extraction Method and System

Background

It is known to extract or mine metal from a subterranean formation by excavating metal ore from the subterranean formation using extraction tools such as diggers or hand tools and then transporting the ore to the surface for processing.

The present inventor has devised an improved way of extracting metal from a subterranean formation.

Summary

In accordance with a first aspect of the invention, there is provided a method of extracting metal from a subterranean formation. The method can comprise injecting a stimulation fluid into an inlet of a bore, the bore being in fluid communication with a brine lens. The brine lens contains magmatic brine containing dissolved metal. The brine has a temperature of at least 400 °C. The method can comprise flowing the brine from the brine lens into a bore provided in fluid communication with the brine lens, the bore having an outlet in fluid communication with the surface.

Extraction of high temperature (at least 400 °C), metal-bearing brines represents a novel, alternative resource for mining, that obviates the need for excavating large pits and processing relatively low grade sulphide ore. The brines targeted in embodiments of the invention are much hotter than subterranean formations used for known geothermal wells. The present inventor has recognised that high temperature brine results in a level of metal solubility that enables the metal to be carried to the surface within the brine.

Significant heat is extracted from the system in the process, and this has potential as a geothermal source of electricity for in situ mining. Advantageously, the recovered "ore" is in solution form and requires no crushing or leaching.

The magmatic brine can have a temperature of up to 800 °C, although typical brine lenses beneath volcanoes can have temperatures of around 450 °C to 550 °C or 600 °C. Advantageously, higher temperature brines can dissolve more metal and reduce the likelihood of, and in some cases inhibit, precipitation in the producing bore. At the point of injection into the first bore, the stimulation fluid can have a temperature of at least 20 °C. Advantageously, this can reduce the likelihood of, and in some cases inhibit, precipitation in the producing bore.

The stimulation fluid can comprise oxygenated water. Advantageously, this can reduce the likelihood of, and in some cases inhibit, precipitation in the producing bore.

The stimulation fluid can comprise one or more chemicals to enhance metal such as Cu solubility in the brine, including, but not limited to, clay muds. Advantageously, this can reduce the likelihood of, and in some cases inhibit, precipitation in the producing bore.

A portion of the first bore within the brine lens can be horizontal and a portion of the second bore within the brine lens can be horizontal. Horizontal can mean more horizontal than vertical, or can for example be within 15% of being truly horizontal and in some cases generally horizontal.

The portion of the first bore within the brine lens can be parallel with respect to the portion of the second bore within the brine lens.

The portion of the first bore within the brine lens can be located within 1000 m, within 500m and in some cases within 100m or less of the portion of the second bore within the brine lens.

The portion of the first bore within the brine lens can be located above the portion of the second bore within the brine lens.

The second bore can be lined with a production liner formed from a relatively inert material such as titanium. This can reduce the level of corrosion in the producing well bore in comparison to a steel lined or open hole bore.

The first and or second bore can each have a generally uniform diameter of between about lm and 0.05m, optionally between about 0.5m and 0.1m and in some cases about 0.15 meters.

In accordance with a second aspect of the invention, there is provided a system for extracting metal from a subterranean formation below a surface. The system can comprise a first bore provided in fluid communication with a brine lens containing magmatic brine, the first bore having an inlet suitable for injecting a stimulation fluid into the first bore from the surface and optionally an outlet for flowing brine to the surface. The system can comprise a second bore provided in fluid communication with the brine lens, the second bore having an outlet for flowing brine to the surface. The brine lens can contain magmatic brine which has a temperature of at least 450 °C.

Optional features of the first aspect can be applied to the second aspect in an analogous manner.

The system can further comprise a fluid pumping station in fluid communication with the inlet of the first bore for injecting the stimulation fluid into the first bore from the surface and/or a storage station in fluid communication with the outlet of the second bore for collecting brine.

In accordance with a third aspect of the invention, there is provided a method of forming a system according to the second aspect. The method can comprise drilling a first bore in fluid communication with a brine lens containing magmatic brine which has a temperature of at least 450 °C. The first bore can have an inlet for injecting a stimulation fluid into the first bore from the surface and optionally an outlet for flowing brine to the surface. The method can comprise drilling a second bore provided in fluid communication with the brine lens, the second bore having an outlet for flowing the brine to the surface.

The method can further comprise injecting pressurised fluid into the first bore to create fractures extending from the first bore to the second bore and introducing proppant into the fractures.

Brief Description of the Drawings

Figure 1 is a diagram of a system according to an embodiment of the invention; Figure 2 is a plot showing brine lens growth;

Figure 3 is graph illustrating a desired temperature and pressure region for flowing brine to the surface;

Figure 4 is a modelled system of Figure 1; and Figures 5 to 10 each show three graphs showing properties of a modelled system. Detailed Description

By way of a non-limiting overview, methods and systems according to embodiments of the invention involve stimulating a sufficiently hot (at least 400 °C), metal containing brine lens with fluid and flowing the hot brine to the surface.

There are technical challenges regarding extraction of high temperature fluids from deep boreholes. A particular issue is regulating the pressure-temperature path followed by extracted brines to prevent precipitation in the producing bore. It is desirable to provide a system with high brine recovery, which can indicate a high Cu (and other metals) production, and zero or limited precipitation of NaCI and other solutes contained in the brine.

Figure 1 is a diagram of a system 10 for flushing metal containing brine from a subterranean formation at temperatures of several hundred degrees Celsius and flowing the brine to the surface.

Degassing magma bodies 12 beneath arc volcanoes 14 discharge fluids of moderate salinity (5-10 wt% NaCleq) into the overlying crust. The presence of a high permeability conduit system, composed of brecciated and damaged rocks, focuses upwards fluid flow.

As fluid decompresses it undergoes phase separation into hypersaline brine and low salinity vapour as it crosses the solvus in the NaCI-H20 system. The less dense vapour segregates from the brine, which becomes retained within pore space in and around the conduit. Halite precipitation from the ascending fluid forms a cap, which drives brines laterally to form a brine lens 16, with fluid salinity in excess of 40 wt% NaCleq at temperatures of 400-600 °C. Thus, the brine lens 16 contains magmatic brine.

The lens 16 has an annular form, with the highest NaCI concentrations outside the original conduit. The depth of lens formation D can for example be 1-2 km below the surface 18. The radius R grows with time from for example 1-3 km.

Brine lens 16 persists long after degassing ceases, migrating slowly downwards and being eroded from above by dissolution of the halite cap into convecting groundwater. Brine lenses 16 can be found under volcanoes that have been inactive for tens to hundreds of thousands of years.

A first bore Bl is formed into the subterranean formation 20 by any suitable drilling means such as those that may be employed in the oil & gas or geothermal industries. The first bore Bl extends so as to be in fluid communication with the brine lens 16. The first bore Bl has an inlet for injecting a fluid into the first bore from the surface 18 and an outlet for delivering fluid to the brine lens 16.

A second bore B2 is formed into the subterranean formation 20 by any suitable drilling means such as those that may be employed in the oil & gas or geothermal industries. The second bore B2 extends so as to be in fluid communication with the brine lens 16. The second bore B2 has an inlet for collecting magmatic brine from the lens 16 and an outlet for flowing magmatic brine to the surface 18.

Portions of the first and second bores Bl, B2 within the brine lens can be perforated to define producing zones, using conventional equipment as may be employed in a single or multi stage hydraulic fracturing process.

The first and/or second bores Bl, B2 can be formed using conventional directional drilling equipment so that one or each bore has a relatively vertical portion VI, V2 and a relatively horizontal portion H I, H2. A horizontal well is a more preferable option in comparison with a purely vertical well because hot regions close to the volcano conduit 22 can be avoided when constructing the horizontal well. Thus, the bores Bl, B2 can each be drilled initially in a vertical manner until a desired total depth is reached or approached and then the drill bit can be steered horizontally to access the brine lens 16.

In other embodiments the system 10 can include a plurality of first bores Bl and/or a plurality of second bores B2, and in other embodiments just one or more second bores B2 i.e. no stimulation bore.

The first and second bores Bl, B2 can be configured such that at least a portion HI of the first bore Bl within the brine lens 16 extends generally parallel with respect to a portion H2 of the second bore B2 within the brine lens 16. It is preferred that the parallel portion HI of the first bore Bl is within 1000 m and/or above the parallel portion H2 of the second bore B2 because this can result in an increased level of production. The, or each, second bore B2 can be provided with a protective liner (not shown) which extends along some or all of the length of the second bore B2 to define production tubing. The liner can be formed from a relatively inert material such as titanium. The purpose of the liner is to inhibit corrosion of the bore by the brine, which due to its high temperature and chemical composition can be more corrosive in nature than, say, an oil & gas production fluid. The liner can be introduced in the same way as an oil well is completed, with the liner tube being inserted into the well bore and then cemented in place to barrier between the outer surface of the liner tube and the inner surface of the well bore. Alternatively a liner can be used which utilises expandable 'packers' for isolating one or more zones in the annulus between the liner and drilled borehole.

A fluid pumping station 24 is provided in fluid communication with the wellhead inlet of the first bore Bl . The fluid pumping station 24 is arranged to supply pressurised stimulation fluid to the first bore Bl . The fluid pumping station 24 can comprise conventional equipment as may be used to stimulate a shale formation in a hydraulic fracturing process. In one example, the pumping station 24 can be arranged to supply up to 1000 tonnes of stimulation fluid per day. The fluid pumping station 24 can further be arranged to heat the stimulation fluid using conventional heating equipment. With the benefit of the disclosure of the present application, the skilled person would be able to identify suitable fluid heating and pumping equipment for a specific bore and/or lens configuration without undue burden or experimentation.

Porous but impermeable host rock can require stimulation to enhance brine recovery in much the same way as fracking enhances shale-gas recovery. For example, with the first bore completed, a first interval (usually the furthest interval from the inlet) is selected. The first interval can be perforated using explosive charges suspended from coil tubing, or can for example be perforated by using a frac ball or the like to open perforation holes in a conventional multi stage fracturing tooling which forms part of the liner. The perforations provide access between the brine lens and the inside of the production tubing. The pumping station then injects fracturing fluid such as water into the production liner of the first bore at an initial flow rate such as 27 Litres per second (L/s) / 10 barrels per minute (BPM). The pumping rate can then be increased in stages of 33 L/s / 20 BPM until a pressure of between 270 L/s / 100 BPM to 540 L/s / 200 BPM is reached. This can result in fractures which stem from the first bore Bl through the impermeable host rock to communicate with the second bore B2. Proppant such as sand can then be added to enter the fractures to keep them open and allow magmatic brine to flow through fractures. Chemicals can be added to the water to modify its viscosity to better carry the proppant. The first interval can then be isolated, such as by way of a frac ball, and the adjacent second interval can be fractured in the same manner. Once all of the intervals have been fractured, the pumping station can move to a production cycle by injecting stimulation fluid into the first bore Bl .

The stimulation fluid can comprise water at temperatures greater than 20 °C, or steam. Regardless of injection temperature the fluid quickly heats up as it moves down the first bore. The water used to form the stimulation fluid can be oxygenated because it limits the tendency for formation of insoluble sulphide precipitates in the brine lens. In some embodiments, the stimulation fluid can further comprise chemical additives such as clay-rich muds to enhance Cu (and other metal) solubility in the brine by reducing the pH in the brine lens.

A brine collection station 26 is provided in fluid communication with the outlet of the second bore B2 and arranged to collect high temperature, liquid brine which flows from the second bore B2. The brine collection station 26 can include storage tanks to store collected brine locally, and/or a transportation pipe 28 to deliver collected brine to a remote location for storage and/or processing. Storage tanks can for example be maintained at elevated temperature conditions and/or filled with chemical reagents such as sulphuric acid suitable for keeping metals in solution..

A simulated brine lens, and its temporal development, is shown in Figure 2. The scale at the bottom represents bulk salinity. The parameters used to generate the simulation are typical for relatively small arc magmatic system, with a total volume of 60 km3. This is at the lower end of estimated pluton volumes in volcanic arcs. Variations in the input parameters can produce larger and more saline lens.

The present inventor has identified that Cu (and other metals) is strongly partitioned from silicate melts into fluids according to the fluid salinity. Similarly, Cu partitioning between brine and fluid is linearly dependent on the salinity ratio between the two phases. Thus Cu follows CI, such that contours of bulk NaCI in Figure 2 can be mapped directly onto contours of Cu. Thus Figure 2 represents a map of Cu grade in a sub- volcanic brine body. The region in black is the halite cap.

Cu (and other metals) precipitation from the brine will occur if there is sufficient sulphur in the brine and/or the temperature falls below the relevant solubility product. Methods according to embodiments of the invention inhibit Cu (and other metals) precipitation within the second bore by a combination of the high temperature of the selected brine lens and/or the high temperature injection fluid and/or the injection fluid maintaining high pressure within the lens to maintain brine pressure as it flows to the surface via the second bore. Where chemicals are added to the injection fluid, this can further reduce precipitation as the brine is flowed to the surface.

For brine lenses such as that shown in Figure 2, the total brine mass can be 6xlO n kg. For an initial fluid composition similar to that of intermediate density ore fluids (e.g. from Bingham Canyon) the Cu content of the trapped brine is just over 5000 ppm. At this concentration the total contained Cu can be 3 Mt. The amount of contained Cu will be increased pro rata for a larger magmatic system, but reduced if the solubility of Cu sulphide is reduced due to brine lens cooling. For example, if the solubility is 50 ppm at the ambient brine temperature, so the contained Cu is reduced by a factor 100. Brine Cu contents in excess of 1 wt% are known from ore deposits. In situ mining thus requires high-temperature brines such as >450 °C, or chemical means to enhance Cu solubility in the brine lens. Low temperature brines at <250 °C, such as those sampled in conventional geothermal sites, have correspondingly lower Cu solubility and are thus unsuited to metal extraction.

Identifying and characterising hot brine lenses in dormant (or recently extinct) magmatic systems can be achieved using a program of geophysical (MT, seismic) and geochemical (fluid analysis) investigations.

It is known that magmatic brines from porphyry copper deposits (PCDs) can contain < 10 wt% Cu and other metals. For rocks with a porosity of >0.2, consistent with existing estimates for shallow (<2 km) arc crust based on reduced P-wave velocities, brines with 5 wt% Cu will confer a grade of > 1 wt%, comparable to economic PCDs.

The brine flowed to the surface via the second bore B2 can be passed through a heat exchanger to heat water to form steam that drives a conventional geothermal turbine and generator unit (not shown) at the surface to produce electricity, which can be used to power one or more components of the mining process.

EXAMPLE

Brine recovery was modelling using the MUFITS Reservoir Simulation Software (http ://www . mufits. imec. msu . ru/) . Referring to Figure 3, typical pressure-temperature conditions in the brine lens are around 20-25 MPa and around 450C. The physics behind the simulated process is that halite precipitation within the brine lens and within the well bore is an undesirable process. Both pressure P and temperature T decrease in the lens can result in precipitation of halite and other solutes dissolved in the brine. While the following description focused on halite (NaCI) precipitation because the simulated fluid is NaCI- H20, the term halite is intended to cover various solid precipitates that may be encountered when flowing hot magmatic brine to the surface.

Halite precipitation, denoted by arrow HP, occurs if pressure drops in the producing well bore due to fluid extraction, denoted by arrow B. Thus, systems and method according to embodiments of the invention aim to avoid parameters going into the HP region. Precipitation can be inhibited by drilling a second horizontal well through which water is injected, denoted by arrow I, to maintain high pressure within the second bore.

If the brine is cooled or if brine evaporates, denoted by arrow C, precipitation can also occur. Precipitation occurs because the solubility of NaCI and other solutes in water decrease with decreasing temperature. This effect can be mitigated by injecting hot water, denoted by arrow H, to maintain a higher temperature in the lens.

Figure 4 shows an axisymmetric domain system used in modelling. The domain, grid and bulk salinity distribution are shown. Bulk salinity is expressed as mass fraction of NaCI in liquid and vapour phases (excluding solid halite) multiplied by porosity. All physical parameters are as in the reference scenario discussed in Figure 3. Two wells are referred to as injector Bl and producer B2. Both wells are completed (perforated) only in the i=2,3,4 columns of grid blocks (that is, outside the volcano conduit which is located at i = l). Thus, producer B2 is completed in the grid blocks i=2,3,4 and k=7 below the halite cap. We consider different injector Bl locations. The well location shown in Figure 4 is the default location if not stated otherwise.

Over 300,000 years we simulate magma degassing, which results in a quasi-steady behaviour of the lens. Over the next 3000 days we simulate brine recovery from the lens, that is stopped when wells are switched to injection/production.

We consider different injection and production rates that are actually subject to constraints imposed by the bottom-hole pressures. The minimum bottom-hole pressure BHP of producer well B2 is 20 MPa, which is slightly less than the initial pressure in the lens (a lower minimum BHP pressure can result in intense precipitation of halite). If this constraint is reached due to fluid recovery with a target production rate then the production rate is reduced and the well operates under constant (minimum) bottom-hole pressure. The maximum bottom-hole pressure of injector Bl is 60 MPa. If this constraint is reached then the injection rate is reduced in order to not violate the constraint.

Note, because we consider the axisymmetric case (1/100 of the full circle is simulated) there are actually 100 well pairs, and injection and production rates for these groups of wells should be multiplied by 100 to get true values for the full circle.

Influence of the production rate

Referring to Figure 5, we consider a scenario when injector Bl is shut down and only producer B2 operates. We investigate how production rate of the brine influences the parameters of the model. The parameters on the left axes are shown with solid lines, whereas the parameters on the right axes are shown with dashed lines. The horizontal axis is time, in days.

With increasing target production rate the bottom-hole pressure BHP (solid lines) decreases reaching its limit of 20 MPa. If the rate is 250 ton/day then the well almost immediately switches to production at a constant BHP of 20 MPa. Any further increase of the production rate will not have any influence on the system as compared to the 250 ton/day rate. There is only a minor influence on the temperature of produced fluid or bottom-hole temperature BHT (dashed lines, °C).

The NaCI production rate NPR (solid lines, ton/day) and total production NTP (dashed lines, ton) also increase with target fluid production rate and at 250 ton/day it reaches its maximum limit for this case.

With increasing target rate the NaCI precipitation or halite volume fraction HVF (solid lines) increases due to pressure drop around the well. It starts at t~2500 days and it is not observed at smaller rates of 50 ton/day.

The total NaCI mass TMN (dashed lines, ton) in the fluid phases in the lens decreases when production starts and it decreases on average with rising target rate. Of course, this occurs as some amount of NaCI is recovered through the well but mostly due to NaCI precipitation in the lens. NaCI drops out of the fluid in the solid phase, thus its mass in the fluid phases decreases.

Influence of minimum Bottom-hole pressure

Figure 6 shows the influence of the minimum BHP in the no-water-injection scenario. In the previous results the minimum BHP was 20 MPa. The production rate is 250 ton/day of reservoir brine. The parameters on the left axes are shown with the solid line, whereas the parameters on the right axes are shown with dashed lines. The horizontal axis is time, in days.

A lower BHP results in a higher production rate and total production due to a higher pressure drawdown. A lower BHP results in a much higher precipitation around the wellbore and in a smaller bottom-hole temperature. The temperature decreases because of brine evaporation which extracts heat from the system.

Since halite precipitation should be avoided, we consider simulations at minimum BHP of 20 MPa which is only slightly less than the initial pressure in the lens.

Water injection - influence of the injection rate

Reference should now be made to Figure 7. The parameters on the left axes are shown with the solid line, whereas the parameters on the right axes are shown with dashed lines. The horizontal axis is time, in days.

The previous scenario shown in Figure 5 demonstrates that due to pressure drop and precipitation near the wellbore the total production of NaCI cannot exceed a maximum value (for a given BHP constraint). In order to increase the production we introduce an injection well BH1 as is shown in Figure 5. The injector BH1 is located above the producer BH2 in the grid blocks k=6, i=2,3,4. Using this scenario we consider several injection rates of pure water at T=20C when the target production rate is 250 ton/day of reservoir brine. Here, minimum BHP is equal to 20 MPa.

With increasing injection rate the NaCI production rate and total production rate increases. It is at least 10 times higher as compared to the no-injection scenario shown in Figure 5. Partly, this is because NaCI precipitation is significantly reduced and may not occur. Indeed, as shown in Figure 7, halite precipitation does not occur if water is injected. Influence of the injection temperature

Figure 8 shows how the temperature of injected water influences NaCI production. The parameters on the left axes are shown with the solid line, whereas the parameters on the right axes are shown with dashed lines. The horizontal axis is time, in days.

Here, the target injection rate is 1000 ton/day or 11.5 L/s. The production rate is 250 ton/day of reservoir brine, injection rate is 1000 ton/day of pure water. The parameters on the left axes are shown with the solid line, whereas the parameters on the right axes are shown with dashed lines

The injection temperature has a minor effect on production, but in general the production rate increases with increasing injection temperature. In this case the total NaCI mass in the lens in the fluid phases is higher at 3000 days than at 0 days because an amount of solid phase is dissolving in pure water.

Influence of the wells location

Referring to Figure 9, we investigate which injector location is preferable. The production rate is 250 ton/day of reservoir brine, injection rate is 1000 ton/day of pure water, and injection temperature is 20°C. The parameters on the left axes are shown with the solid line, whereas the parameters on the right axes are shown with dashed lines. The horizontal axis is time, in days.

We consider three options:

1. The injector BH1 is above producer BH2 at k=6. This case was considered above. This is denoted by trace A.

2. The injector BH1 is below producer BH2 at k=8. This is denoted by trace B.

3. The injector BH1 is located very close (within approximately 100 m) to the producer BH2 in the same grid blocks k=7. This is denoted by trace S.

We see that a much higher production rate is achieved when injector BH 1 is located closer to the producer BH2 (the same depth case S). In this case producer BH2 switches back to the target production rate of 250 ton/day at t~500 days. The bottom-hole temperature considerably decreases because colder water is injected close to the producer BH2. If comparing the above A and below B cases, the injector BH 1 location above the producer BH2 is preferable. In the above A case the production rate is higher.

Influence of well length

Figure 10 shows the influence of the length of the horizontal segments of the wells. Here, the injector is in the k=6 grid blocks. The production rate is 250 ton/day of reservoir brine, injection rate is 1000 ton/day of pure water, and injection temperature is 20°C. The parameters on the left axes are shown with the solid line, whereas the parameters on the right axes are shown with dashed lines. The horizontal axis is time, in days.

We consider 3 cases:

1. IX - both wells are completed in the i=2,3,4 grid blocks;

2. 0.66X - both wells are completed in the i=3,4 grid blocks;

3. 0.33X - both wells are completed only in the i=4 grid block.

As can be seen, a longer wellbore, which is drilled closer to the conduit at i = l, produces a greater amount of NaCI. The bottom-hole temperature decreases for a shorter well. Nevertheless, a short well (0.33X) produces a considerable amount of NaCI as compared to the longer well in the no-injection scenario. This can be advantageous due to technical difficulties of drilling into very hot regions.

Summary

• A higher NaCI production can be achieved when water is injected in order to maintain reservoir pressure and mitigate halite precipitation.

• It is preferable to inject water closer to and above the production well.

• A higher NaCI production can be achieved when injecting hotter water.

• A longer horizontal segment of the wells is preferable, but a considerable production can also be reached with short wells.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications can be made without departing from the scope of the invention as defined in the appended claims. The word "comprising" can mean "including" or "consisting of" and therefore does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.