WO2008058298A1 - Method and apparatus for the delivery of under-saturated sour water into a geological formation - Google Patents

Abstract

A well completion and method for delivering an aqueous phase containing dissolved gas into a geological formation, wherein the well comprises: a cased borehole having injection tubing providing for the injection of fluid down the well; at least one downhole sensor situated inside the well and above an aperture in the casing through which fluid is injected into the geological formation; a control system enabled to receive and interpret signals emitted by the sensor to detect a gas-liquid contact in the well and adjust the rate of injection of fluid therein to maintain the gas-liquid contact at a level above the downhole sensor.

Description

METHOD AND APPARATUS FOR THE DELIVERY OF UNDER- SATURATED SOUR WATER INTO A GEOLOGICAL FORMSTION

E21B, B65G

FIELD OF THE INVENTION

The present invention relates to the fields of enhanced oil recovery by using carbonated water and the field of geological storage of greenhouse gas to mitigate climate change, in particular, the disposal of gaseous waste in geological formations. Specifically, the invention is related to the injection of gas-saturated water into the geological formation and the use of fluid levels inside the injection borehole.

BACKGROUND TO THE INVENTION In the specification, the terms water, aqueous phase and brine are used interchangeably. The term carbon dioxide is used to describe either pure carbon dioxide, a mixture of gases consisting mostly of carbon dioxide or any water soluble waste gas. The mixture may contain hydrogen sulphide, sulphur dioxide, nitrogen, hydrocarbon gases, water vapour and other impurities typically found in the application of the invention.

A critical parameter in a carbonated waterflood is the degree of saturation with carbon dioxide of the aqueous phase, which is defined as the amount of carbon dioxide dissolved as a fraction of the maximum amount that can be dissolved without free gas evolving.

There are various methods and apparatus utilizing carbonated water injection in the art. These include pure carbon dioxide injection or Water Alternating Gas (WAG) floods, which have been hybridized with carbonated waterflooding by saturating some of the intermediate or chase water slugs. In other cases, non- carbonated water has been used to chase a slug of carbonated water.

In all cases the water and carbon dioxide rates are metered separately before mixing and rates adjusted by valves to meet target ratios of carbon dioxide to water. Both manual and automatic control of rates on a continuous basis have been practiced, but adjustments are always made to correct metered rates to meet a pre-calculated target, rather than monitoring and verifying the phase of the injected fluid.

The degree of saturation of the water with the gas is a parameter of primary relevance in determining the behavior of the injectant on leaving the injection well and its subsequent interaction with the oil, water and gas in the reservoir.

In the current art, the degree of saturation is controlled by the ratio of carbon dioxide to water injected. The solubility of the carbon dioxide in water is strongly dependent on the pressure, temperature and the chemistry of the water under consideration. As a consequence, the degree of saturation can only be determined from the injection ratio if these parameters are all known to good accuracy at the bottom of the injection well. Downhole pressure and temperature gauges can be combined with either the results of solubility experiments on the injected water or published correlations to determine the degree of saturation. However, uncertainties in these measurements mean that confidence in the degree of saturation is often poor. In certain cases, the injected water may already contain some unknown or poorly known quantity of carbon dioxide, greatly complicating the calculation of how much extra carbon dioxide can be added before full saturation of the water occurs.

In US Patent 6,325,147 the use of state detectors in the injection zone was proposed to measure the thermodynamic parameters and thus control the ratio of aqueous phase and gas injected. No enabling disclosure is provided as to which thermodynamic parameters are to be measured or how these parameters would be translated into control signals. Reference is only made to the preparation of completely saturated solutions or over-saturated solutions in which excess gas is present. It is not intended for aqueous solutions that are under-saturated to any degree.

Incomplete mixing or insufficient time for dissolution causes free gas to enter the geological formation even if the ratio of gas to water is at, or slightly less than, the solubility ratio. The current art, however, does not provide for any monitoring and verification of the phase of fluids entering the geological formation. If a carbonated water flood (rather than free gas injection) is intended, the current art does not provide the operator with the means of checking the performance of the injection well.

Detection of fluid levels in a borehole is performed in the oil industry, typically for establishing the liquid level in a production well equipped with a downhole pump. Fluids are pumped up a production tubing to surface. This reduces the bottomhole pressure in the annulus between the casing and production tubing, which enhances flow into the well. The liquid level in the annulus is lowered below surface. Generally, this level needs to be maintained some optimum distance above the pump, high enough to cause flow into the pump, but low enough to significantly reduce bottomhole pressures. Such methods and apparatus cannot be applied in the context of an injection well and where the casing-tubing annulus has been sealed a short distance above the apertures in the well casing leading to the geological formation. The current art does not include the use of liquid levels and systems to monitor them in an injection well.

The storage of carbon dioxide in aquifers has been proposed as one of the few ways of significantly reducing global emissions of greenhouse gases. The current art for the. storage of carbon dioxide in aquifers is to inject essentially pure, supercritical (dense phase) carbon dioxide directly into an aquifer. An alternative storage technique for carbon dioxide in an aquifer is to inject carbonated water.

Carbonated water injection is not currently used for geological storage of carbon dioxide. In addition methods and apparatus to determine and monitor the particular degree of saturation of carbonated water injection are unknown.

Any waste gas can be disposed of by injection into buried geological formations. In particular, so called acid gas, which is a mixture of hydrogen sulphide and carbon dioxide, has been disposed of in this way. Well known examples are found in the processing of sour natural gas in Alberta, Canada. In some cases, and of particular relevance to the invention, the gas is first dissolved in water. Currently, there is no downhole (subsurface) monitoring of the absence of undissolved (free) gas, so the degree of confidence in the complete dissolution of the waste gas is limited.

To avoid any buoyant rise of injectant, no free gas should leave the well bore. It would therefore be advantageous to employ methods utilizing more direct monitoring and verification of carbon dioxide saturation.

It is the intention of the invention to at least partially ameliorate the abovementioned difficulties. SUMMARY OF THE INVENTION

According to the invention there is provided a well completion for delivering an aqueous phase containing dissolved gas into a geological formation, wherein the well comprises:

a) a cased borehole having injection tubing providing for the injection of fluid down the well; b) at least one downhole sensor situated inside the well and above an aperture in the casing through which fluid is injected into the geological formation; c) a control system enabled to receive and interpret signals emitted by the sensor to detect a gas-liquid contact in the well and adjust the rate of injection of fluid therein to maintain the gas-liquid contact at a level above the downhole sensor.

Preferably the well casing has a plurality of apertures along a portion of its length. In use, the well casing extends to below the geological formation targeted for injection and the apertures border at least a section of the target geological formation through which the well extends. The well casing may however extend only until the uppermost section of the target geological formation.

Conveniently the well may be deviated from vertical and comprise a casing with apertures on its underside.

Preferably the injection tubing comprises at least one aperture above the lowest downhole sensor. In use, water is injected through the apertures, which are located above the gas-liquid contact level. Accumulated carbon dioxide above the gas-liquid contact is thereby washed and at least partially dissolved into the aqueous solution. Preferably the control system and valves controlling the rate of fluid injected into the well are situated on the surface, at the wellhead or even remote from the well. Alternatively some or all of the control system elements may be placed downhole.

Preferably at least one downhole sensor is used to measure a response of bottomhole pressure to changes in the surface liquid or gas well injection rates. The sensors are thereby enabled to detect the system response time of fluid injection into the well.

Conveniently there may be provided a plurality of sensor types, including a combination thereof, enabled to measure at least one of the following: a) the electrical properties of the surrounding fluid; b) the acoustic properties of the surrounding fluid; c) the density of the surrounding fluid; or d) the pressure gradient in the surrounding fluid.

Conveniently at least two sensors are utilized to detect the gas-liquid contact level of which at least one sensor is a pressure sensor. This sensor configuration optimizes the calibration of the gas-liquid contact level.

Conveniently, at least two downhole pressure sensors are located inside the well at different levels and above the highest target entry point through which fluid is injected into the geological formation.

Conveniently at least one sensor may be placed inside a length of ventilated tubing situated in the downhole completion of the well so as to avoid zones of higher flow and turbulence, in use. Multiple sensors may be used to flexibly set the depth of the liquid-gas contact. Conveniently the well may contain packing material to enhance the mixing of gas and water. The packing material may be comprised of glass spheres or other destruction resistant and inert material that is denser than water and allows an easy flow of fluid through the packing material.

According to a further aspect of the invention, there is provided an apparatus for mixing water and gas on the surface at controlled rates and injecting the mixed fluid to a downhole delivery point in the well at controlled rates.

According to a further aspect of the invention there is provided a method of delivering an aqueous phase containing dissolved gas into a geological formation, comprising the steps of using a delivery apparatus as described above to deliver the aqueous phase.

Preferably the method comprises the step of establishing the gas-liquid contact above the top aperture of the casing, by adjusting the fluid injection rates. The fluid which comprises an aqueous phase, containing injected carbon dioxide, is close to fully saturated therewith. The under-saturation of and absence of free gas in the injected aqueous phase is ensured by the gravitational segregation of liquid and gaseous phases in the well.

Preferably the method of delivery of the aqueous phase comprises the steps of first adjusting the gas rate, followed by an adjustment of the liquid rate by means of an automated control system, thereby optimizing delivery.

According to another aspect of the invention there is provided a method of storing carbon dioxide in a geological formation, wherein the method and delivery apparatus as described above is employed to deliver an under saturated carbonated aqueous phase into the geological formation. In this patent specification reference to the word tubing implies at least one section of tubing and reference to the word fluid implies at least one type of fluid. The summary of invention forms an integral part of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic diagram of an apparatus of the invention for injecting carbonated water into a geological formation;

Figure 2 shows a longitudinal section through a vertical well completion of the invention as shown in Figure 1 ;

Figure 3 shows a longitudinal section through another embodiment of a vertical well completion of the invention as shown in Figure 1 ;

Figure 4 shows a longitudinal section through another embodiment of a vertical well completion of the invention as shown in Figure 1;

Figure 5 shows a longitudinal section through another embodiment of a vertical well completion of the invention as shown in Figure 1;

Figure 6 shows a longitudinal section through another embodiment of a vertical well completion of the invention as shown in Figure 1; and

Figure 7 shows a longitudinal section through a deviated well completion of the invention as shown in Figure 1.

DETAILED DESCRIPTION OF THE MAIN EMBODIMENTS In the drawings, figure 1 shows a schematic diagram of a method and apparatus for the delivery of near-saturated carbonated water to a geological formation utilizing carbonated water injection. The apparatus comprises an injection system 10 configured to deliver carbon dioxide and water separately. The method comprises the steps of connecting a source of water 12 to a pump 14, which is connected to a larger diameter injection tubing which extends from the pump through a flow meter 16, a control valve 18 and a wellhead 20 into the well 22. A high pressure source of soluble gas 24 is connected to a smaller diameter injection tubing 26 which extends from the source through a flow meter 28, a control valve 30 and the wellhead into the well. The water and gas tubing are integrated at the wellhead. The gas injection tubing is run inside the water injection tubing and both run to a downhole completion 32. Data from sensors 34 and surface flow meters 16, 28 are transmitted to a control computer 36 which adjusts the injection rates of gas and water by the control valves so as to maintain a gas-liquid contact 38 inside the well at a level above the apertures 40 in the casing abutting the target geological formation. The delivery rate of carbon dioxide and water injected is optimized by first adjusting the gas rate, followed by an adjustment of the water rate.

A control system is used to maintain the level of the contact just above the lowermost sensor. If less carbon dioxide is injected than can be dissolved in the water, the accumulation of free gas at the top of the space inside the well will diminish and the gas-liquid contact will move upwards. If more carbon dioxide is injected than can be dissolved in the water, the contact would move down until it reaches the top well casing aperture and free gas enters the formation. Before the contact reaches the top aperture, however, it is detected by sensors and the ratio of carbon dioxide to water is reduced.

In the preferred embodiment, a stable water injection rate is established. The flowing bottomhole pressure is then monitored continuously as small step changes are made to the surface water rate. These adjustments would be made manually but at the same point on the water injection system as will be used by an automated control system to adjust water rates during continuous injection. This experiment defines the impulse response of the water injection system and in particular, the delay between an adjustment on surface and its downhole expression.

The maximum gas rate that could be dissolved in the injected water is then estimated, using the flowing bottomhole pressure, as measured by the pressure sensor in the completion, the temperature of the injected water, which can be measured at surface, and the salinity of the injected water. Gas injection is then initiated at this calculated maximum rate reduced by some margin to take account of uncertainties in the calculation.

After allowing some time for stabilization of the system, the downhole sensor, an inductive resistivity sensor in the preferred embodiment, should confirm that the fluid immediately above the top perforation is still a liquid. Small decreases in resistivity may be observed as the water is now carbonated. If, however, gas is detected, the gas rate should be reduced and the system allowed to stabilize again. Gas rates should be reduced until the fluid detected by the lowermost sensor is indeed a liquid. Rate adjustments until this stage have been manual.

The water has a lower compressibility than gas. Therefore the water injection rate downhole will respond more quickly to a distant surface adjustment than would the gas system. The quicker response makes the control problem easier and enables surface adjustment rather than requiring more expensive downhole rate adjustment equipment.

If only one downhole sensor apart from the pressure sensor is employed, the control system will need to reduce the water rate if, after a certain period of time, a "gas at sensor" signal has not been generated. This is to avoid the gas-liquid contact level rising progressively further above the sensor. Alternatively, a second, higher sensor is employed and the water rate adjusted to maintain the gas-water contact at a level between the two sensors by having the control system seek simultaneous "liquid at sensor" and "gas at sensor" signals from the lower and upper sensors respectively. Multiple sensors may also be advantageous if there is a strong requirement to avoid any free gas entering the geological formation, even for short periods of time.

Once the control system has established a stable water rate corresponding to the manually set gas rate, the ratio of carbon dioxide to water can be monitored. If perfect mixing is occurring, this ratio will be the solubility. If mixing is imperfect, the ratio of carbon dioxide to water will be lower since the system avoids free gas injection. Anomalously low ratios may suggest changing the completion to one of the alternative embodiments for better mixing or changing the operating conditions to achieve better mixing. An injection rate to maximize the carbon dioxide to water ratio while not exceeding maximum permissible injection pressures can be found by experiment in the field. For each gas rate, the system finds the corresponding maximum water rate that still avoids free gas injection into the geological formation.

The sensors measure the electrical resistivity of the fluid adjacent to the sensor to give a "liquid at sensor" or "gas at sensor" signal. The resistivity of free carbon dioxide is greater than the resistivity of a brine or even fresh waters encountered in the subsurface. Sensors measuring acoustic propagation velocity or the density of the adjacent fluid would also distinguish gas from liquid. The choice of an electrical, sonic or density sensor would be made on the basis of the cost and reliability of such continuous readout, downhole gauges that were available at the time of project execution.

Figure 2 shows a vertical well completion 32 in which the water is introduced to the annulus 102 inside the casing 122 by means of tubing 104 immediately beneath a top packer 106 and above a second packer 108 through apertures in the tubing 110. The soluble gas is introduced by a different tubing 112 run inside the water tubing and through a nozzle 114 immediately above the top well casing aperture 116 establishing a gas-water contact 118, in use. A sensor 120 is located above the top well casing aperture to measure fluid properties inside the casing 122. In addition, a pressure sensor 124 is included in the string. Ventilation holes 126 in the water tubing beneath the second packer ensure fluid levels in the two annuli are the similar.

The packer 106 is set between the casing and the water injection tubing at a distance of 50m above the top perforation. The distance may be reduced to 10m. The packer spacing creates space for the optimum mixing of water and gas. The second packer 108 is set inside the water tubing a short distance below the top packer. Apertures 110 in the water tubing between the levels of the two packers allow injected water to enter the annulus 102 between the casing and tubing. The apertures are sufficiently small to cause the water to spray into the annulus, and large enough to avoid causing excessive pressure drops. The water tubing string 104 is extended some distance beneath the second packer to a level beneath the desired gas-liquid contact and shortly above the top well casing aperture 116. This extension of the water tubing carries the downhole sensors, which are inserted into the string to measure the fluids in the annulus between the carbon dioxide tubing and the water tubing. Apertures in the tubing beneath the second packer put the annulus between the carbon dioxide tubing and water tubing into contact with the annulus between the water tubing and the casing. The apertures should be sufficiently large and well distributed along this interval so as to ventilate the inner annulus and allow for the establishment of a gas-water contact corresponding to that in the outer annulus. In use, the less turbulent conditions inside the inner annulus provide a more suitable measuring environment for the downhole sensors.

The carbon dioxide injection tubing 112 extends a short distance beneath this lowermost packer, but ends above the top well casing aperture. The distance between the top aperture and end of the carbon dioxide injection tubing is sufficient for the carbon dioxide jet emerging from the tubing to lose its momentum and to prevent it from entering the casing apertures and the geological formation directly. A nozzle to direct the jet outwards toward the casing or upwards, rather than downwards, is used to minimize the distance needed for this purpose.

The water immediately beneath the gas-water contact should be a close as possible to fully saturated with carbon dioxide. The well configuration enables mixing for water arriving at the level of the contact. Downhole mixing of carbon dioxide is enhanced by partially injecting water above the gas-liquid contact level. Such water washes and dissolves accumulated carbon dioxide into the aqueous solution.

Mixing within the completion causes the carbon dioxide to dissolve in the water. Excess carbon dioxide remains as a distinct phase which segregates from the water under the influence of gravity to form a zone in the top of the space within the completion. In use, pressures and temperatures which cause the carbon dioxide to form a liquid are avoided, since the density of carbon dioxide is then not much less than that of water. This reduces the rate of gravity segregation in the well. The invention avoids such problems by forming a gas-liquid contact inside the well and the level of this contact is used to adjust the ratio of injected carbon dioxide to water.

The downhole configuration of the invention enhances the mixing of water and gas inside the borehole. In addition, the invention allows the establishment of a gravitationally induced gas-liquid contact inside the borehole. Alternative embodiments as illustrated in Figures 3 to 7 contain different mixing configurations, which may provide better mixing in some instances. Figure 3 shows a vertical well completion 32 in which water is introduced both through small apertures 110 immediately beneath the top packer 106 and also through the water injection tubing 104 extending below the perforations 116 in the casing. The carbon dioxide is injected through a different tubing 112, also extending to below the perforations. Two pressure sensors 140 are fixed to measure fluids in the casing annulus above the top perforation.

Figure 4 shows a vertical well completion, similar to that in Figure 2 except that the carbon dioxide injection tubing 112 has been extended to inject the carbon dioxide beneath packing material 142. The packing material is located inside the casing below the lowermost well casing aperture 144 so that gravity effects dominate viscous forces as the free gas moves past the apertures, thereby minimizing the amount of free gas pulled through the perforations by the flow of water. Packing material obstructs the flow of fluids and creates turbulence.

Also, since the packing material occupies part of the space inside the well bore, the fluid must flow through the remainder, causing fluid velocities to increase for a given flow rate. Both of the two factors above promote mixing and hence the dissolution of any undissolved gas in under-saturated water, but additional pressure is needed to maintain the same injection rates through the space. Dense packing of uniform spheres would occupy approximately 75% of the void space and increase fluid velocities four fold, creating significant pressure drops if the initial well diameter is small. The use of packing should thus be minimized, since large pressure drops will reduce the injection pressure at the entrance to the geological formation or demand more pumping, compression and higher pressure ratings for equipment upstream of the packing. The packing material is comprised of glass spheres or other destruction resistant and inert materials that are denser than water and allow an easy flow of fluid through the packing. Figure 5 shows a vertical well completion for co-injected water and gas delivered downhole by a single tubing 150. Fluids enter through apertures in the tubing 110 immediately beneath the top packer 106 and above a plug or end 152 in the tubing. Downhole sensors 154 are situated in the lowermost part of the tubing string to monitor the level of the gas-water contact 118 inside the tubing. Ventilation holes 126 situated along the length of the tubing connect the inside of the tubing with the casing annulus to allow the gas-liquid contact levels in the tubing and annulus to equalize.

Figure 6 shows a vertical well completion for co-injected water and gas where the mixture is injected both at the top of the completion 160 and in the rathole 162 below the lowest well casing aperture. Although there is some risk of free gas leaving through the well casing apertures, the pressure drop across the completion is minimized by having flow in two paths converging on the perforations. The downhole gauges are set to measure in the annulus between the casing and tubing. Currents and turbulence past these gauges is reduced because only a part of the fluid passes this way.

Figure 7 shows a modification to the completion for a deviated well 170.

Gas is injected from the tubing 112, whose outlet is located below the well casing apertures 116. Only the underside of the well casing extending across the reservoir interval 172 has apertures. The upper side 174 of the well situated across the reservoir is not perforated.

In this embodiment, the gas bubbles will tend to move along the top side of the casing, rather than being more evenly distributed throughout the water column in the well. The non perforated top casing contains the rising gas within the well, which bypasses the lower well casing apertures and continues into the top of the completion. It will be appreciated that the invention is not limited to the above described embodiments. The phrase "well casing" and "cased borehole" are used interchangeably and make reference to the same aspect of the well completion.

The claims form an integral part of the description of the invention.

Claims

CLAIMS:

1. A well completion for delivering an aqueous phase containing dissolved gas into a geological formation, wherein the well comprises:

- a cased borehole having injection tubing providing for the injection of fluid down the well;

- at least one downhole sensor situated inside the well and above an aperture in the casing through which fluid is injected into the geological formation;

- a control system enabled to receive and interpret signals emitted by the sensor to detect a gas-liquid contact in the well and adjust the rate of injection of fluid therein to maintain the gas-liquid contact at a level above the downhole sensor.

2. The well completion as claimed in claim 1 , wherein the well casing has a plurality of apertures along a portion of its length.

3. The well completion as claimed in claim 1 or 2, wherein at least one injection tubing comprises at least one aperture above the lowest downhole sensor.

4. The well completion as claimed in any_one of claims 1 to 3, wherein at least one downhole sensor is enabled to measure a response of bottomhole pressure to changes in the surface liquid or gas well injection rates.

5. The well completion as claimed in any one of claims 1 to 4, wherein there is provided a plurality of downhole sensors, each sensor, enabled to measure at least one of the following:

- the electrical properties of the surrounding fluid;

- the acoustic properties of the surrounding fluid;

- the density of the surrounding fluid; or - the pressure gradient in the surrounding fluid.

6. The well completion as claimed in any one of claims 1 to 5, wherein at least two downhole pressure sensors are located inside the well at different levels and above the highest target entry point through which fluid is injected into the geological formation.

7. The well completion as claimed in any one of claims 1 to 6, wherein at least one sensor may be placed inside a length of ventilated tubing situated in the downhole completion of the well.

8. The well completion as claimed in any one of claims 1 to 7, wherein the well contains packing material.

9. The well completion as claimed in any one of claims 1 to 8, wherein the control system which includes valves for controlling the rate of fluid injected into the well is at least partially situated at any one of the following: - at the wellhead;

- remote from the well;

- downhole

10. The well completion as claimed in any one of claims 1 to 9, wherein the well is deviated from vertical and comprises a casing with apertures on its underside.

11. A method of delivering an aqueous phase containing dissolved gas into a geological formation, comprising the steps of delivering the aqueous phase through a well completion, wherein the well comprises: - a cased borehole having injection tubing providing for the injection of fluid down the well;

- at least one downhole sensor situated inside the well and above an aperture in the casing through which fluid is injected into the geological formation; - a control system enabled to receive and interpret signals emitted by the sensor to detect a gas-liquid contact in the well and adjust the rate of injection of fluid therein to maintain the gas-liquid contact at a level above the downhole sensor.

12. The method of delivering an aqueous phase as claimed in claim 11 , wherein the well casing has a plurality of apertures along a portion of its length.

13. The method of delivering an aqueous phase as claimed in claim 11 or 12, wherein at least one injection tubing comprises at least one aperture above the lowest downhole sensor.

14. The method of delivering an aqueous phase as claimed in any one of claims 11 to 13, wherein at least one downhole sensor is used to measure a response of bottomhole pressure to changes in the surface liquid or gas well injection rates.

15. The method of delivering an aqueous phase as claimed in any one of claims 11 to 14, wherein there is provided a plurality of downhole sensors, each sensor, enabled to measure at least one of the following; - the electrical properties of the surrounding fluid;

- the acoustic properties of the surrounding fluid;

- the density of the surrounding fluid; or

- the pressure gradient in the surrounding fluid.

16. The method of delivering an aqueous phase as claimed in any one of claims 11 to 15, wherein at least two downhole pressure sensors are located inside the well at different levels and above the highest target entry point through which fluid is injected into the geological formation.

17. The method of delivering an aqueous phase as claimed in any one of claims 11 to 16, wherein at least one sensor may be placed inside a length of ventilated tubing situated in the downhole completion of the well.

18. The method of delivering an aqueous phase as claimed in any one of claims 11 to 17, wherein the well contains packing material.

19. The method of delivering an aqueous phase as claimed in any one of claims 11 to 18, wherein the control system which includes valves for controlling the rate of fluid injected into the well is at least partially situated at any one of the following:

- at the wellhead;

- remote from the well;

- downhole _*

20. The method of delivering an aqueous phase as claimed in any one of claims 11 to 19, wherein the well is deviated from vertical and comprises a casing with apertures on its underside.

21. The method of delivering an aqueous phase as claimed in any one of claims 11 to 20, wherein the well casing extends to below the geological formation targeted for injection and the apertures border at least a section of the target geological formation through which the well extends.

22. The method of delivering an aqueous phase as claimed in any one of claims 11 to 19, wherein the well casing extends only until the uppermost section of the target geological formation.

23. The method of delivering an aqueous phase as claimed in any one of claims 11 to 22 comprising the step of establishing the gas-liquid contact above the top aperture of the casing, by adjusting the fluid injection rates.

24. The method of delivering an aqueous phase as claimed in claim 23 comprising the step of first adjusting the gas rate, followed by an adjustment of the liquid rate.

25. The method of delivering an aqueous phase as claimed in claim 23 or 24, wherein water is injected through at least one aperture in the injection tubing which are located above the gas-liquid contact level.

26. The method of delivering an aqueous phase as claimed in claim 23 to 25, wherein multiple sensors are used to flexibly set the depth of the liquid-gas contact.

27.A method of storing aqueous phase containing dissolved gas in a geological formation, comprising the steps of delivering the aqueous phase into the geological formation as described in any one of claims 11 to 26.

28. A well completion substantially as described herein with reference to figures 1 to 7.