GB2474559A

GB2474559A - Deaeration of water

Info

Publication number: GB2474559A
Application number: GB201017075A
Authority: GB
Inventors: Patrick Lee Mcguire
Original assignee: BP Corp North America Inc
Current assignee: BP Corp North America Inc
Priority date: 2009-10-13
Filing date: 2010-10-11
Publication date: 2011-04-20
Also published as: GB201017075D0

Abstract

A process for removing dissolved oxygen from an injection water in a gas stripping system that comprises at least one stripping column 2, the process comprises:(a) introducing water to be deoxygenated to the bottom of a stripping column;(b) introducing 7 an oxygen-free stripping gas into the stripping column via one or more injection points wherein at least one injection point 8 is provided at or near the bottom of the stripping column;(c) maintaining a pressure at the top of the stripping column such that the two phase mixture of water and stripping gas adopts an annular flow regime as the mixture passes up the stripping column and the dissolved oxygen content of the water phase at the top of the stripping column is reduced;(d) withdrawing the mixture of water and stripping gas from the top of the column and passing the mixture to a separator that separates the stripping gas from the water, and removing the stripping gas and water from the separator through separate outlets; and(e) passing the separated water to an injection system for injection into a hydrocarbon bearing formation via an injection well.

Description

DEAERATION OF WATER

This invention relates to the deaeration of water that is to be injected into an oil reservoir for enhanced recovery of oil therefrom, and, in particular, to the deaeration of water by contacting the water with a striping gas that is substantially free of oxygen.

In the production of hydrocarbons from a subterranean formation, water may be injected into a producing formation as the hydrocarbons are withdrawn in order to maintain the pressure in the formation. At offshore locations, seawater is routinely used for this purpose. It is also known to use estuarine water, river water, or lake water as injection water. It is important to remove oxygen from injection water in order to reduce corrosion rates in water injection wells and to prevent the growth of aerobic bacteria. Accordingly, the water has to be treated, i.e. deaerated, to remove dissolved oxygen before the water is injected into the producing formation.

It is also known to use aquifer water or produced water as injection water. These waters are generally free of dissolved oxygen. However, it is also known that the dissolved solids content (salinity) of an injection water may be manipulated in order to achieve enhanced production of hydrocarbons from a hydrocarbon bearing formation and/or to reduce the risk of scale formation. For example, the total dissolved solids content of the injection water may be reduced to a value in the range of 200 to 10,000 ppm and the multivalent cation content of the injection water may be maintained below the multivalent cation content of the formation water (water found associated with the hydrocarbons in the producing formation) in order to achieve enhanced recovery of hydrocarbons from the producing formation. The injection water may also be softened by removing scale precursor ions, such as sulfate ions, thereby avoiding the risk of mineral scale formation in the producing formation or the producing wellbore. Where the dissolved solids content of an aquifer water or produced water is manipulated prior to injection into the producing formation, this may result in the water coming into contact with air such that the water contains dissolved oxygen. Again, it would be necessary to deaerate the injection water prior to injection into the formation.

A variety of methods are known for removing dissolved oxygen from water. For example, reducing agents may be added to the water to chemically reduce the dissolved oxygen. However, owing to the large volume of water that must be treated, the cost of such chemical deaeration methods is prohibitive except to remove trace amounts of dissolved oxygen.

Several mechanical deaeration techniques have been proposed. These include vacuum stripping, thermal stripping and gas stripping. In gas stripping, the partial pressure of the dissolved oxygen in the injection water is reduced by contacting the water with an oxygen-free gas such as methane (natural gas), nitrogen, or other inert gas thereby allowing the oxygen to come out of solution and to be carried away by the stripping gas.

Gas stripping systems generally require smaller and less expensive equipment than either thermal or vacuum deaeration. Also, given a ready supply of stripping gas (i.e. natural gas), the operating costs are less than with other known deaeration systems.

In the known gas stripping deaeration systems, the stripping gas is generally passed countercurrent to the flow of the water being treated. This countercurrent flow normally occurs within a large column which contains trays, packing materials, and sprayers which adds substantially to the construction and installation costs. A typical column used for an offshore deaeration operation may be up to 21 metres (70 feet) in height and more than 4.3 metres (14 feet) in diameter. The column required for countercurrent flow deaeration takes up considerable space on a production platform. The operation of countercurrent deaeration columns is described in "Recommended Practice -Design and Operation of Stripping Columns for Removal of Oxygen from Water", NACE Standard RP-01-78, National Association of Corrosion Engineer, Houston, Texas 1978; and "Desorption of Oxygen from Water using Natural Gas for Countercurrent Stripping", Journal of Petroleum Technology, May, 1963, pp 51.

Although known gas stripping deaeration systems are generally smaller and less expensive than other known deaeration systems, they are still quite large and expensive to construct and assemble, particularly at offshore sites.

It has also been proposed to carry out the pumping and degassing of water simultaneously by a process known as "gas-lift", consisting of injecting an inert gas under pressure at the bottom of an upright column immersed in a body of water and open at its bottom end, so that this inert gas draws the water present in this column upwards and at the same time releases most of the oxygen dissolved in the water which it entrains. Thus, according to the Oil and Gas Journal, December 23, 1968, pages 74-76, gas lift has been used to artificially lift water out of a lake and up into a surge tank. Gas lifting is said to serve two useful purposes: (a) it provides a more flexible method to obtain the variable volumes of water required, and (b) it performs the deoxygenation of the water. It is stated that natural gas, in fact, acts as a stripping agent. By the time lake water reaches the surge tank, most of the entrained oxygen has been freed. However, details of how to implement gas lift deaeration of the lake water are not provided.

US 4,613,347 relates to a process for deaerating water in an upright column immersed in a body of water, or an underground water-bearing stratum, said column being open at its lower end and provided in the region of said open end with an electrical pump which lifts the water in the column, comprising injecting into said upright column, at at least one injection point situated above said electrical pump, an inert gas under a pressure which is at most equal to the hydrostatic pressure of the column full of water at the level of the injection point. It is said that the electric pump supplies at least 80% of the energy absorbed by the water lift in the upright column. Preferably, the injection takes place in the form of fine bubbles of gas with a diameter of the order of one millimetre. According to US 4,613,347, when the intention is to deaerate the water, the volume of injected gas does not need to be great. However, contrary to the teaching of US 4,613,347, it has now been found that gas lift deaeration is particularly efficient when operating a gas lift deaeration system under conditions of high water flow rate and high gas to water ratio so as to achieve an annular flow regime.

Thus, the present invention relates to a process for removing dissolved oxygen from an injection water in a gas stripping system that comprises at least one stripping column having a cross sectional area in the range of 10 to 300 inches2 (64.51 to 1935.3 cm2) and a length in the range of 200 to 6000 feet (60.96 to 1828.8 metres), preferably in the range of 500 to 3000 feet (152.4 to 914.4 metres), the process comprising: (a) introducing the water that is to be deoxygenated to the bottom of the stripping column at a rate in the range of 5,000 to 100,000 barrels (bbls) of water per day; (b) introducing the oxygen-free stripping gas into the stripping column via one or more injection points wherein at least one injection point is provided at or near the bottom of the stripping column and wherein the ratio of stripping gas to water that is introduced into the stripping colunm is in the range of 150 standard cubic feet (scfs) stripping gas/bbl water to 100,000 scfs stripping gas/bbl water; (c) maintaining the pressure at the top of the stripping column in the range of atmospheric pressure to 500 psia (3.447 MPa) such that the two phase mixture of water and stripping gas adopts an annular flow regime as the mixture passes up the stripping column and the dissolved oxygen content of the water phase at the top of the stripping column is reduced to less than 500 ppb, preferably less than 100 ppb; (d) withdrawing the mixture of water and stripping gas from the top of the column and passing the mixture to a separator that separates the stripping gas from the water, and removing the stripping gas and water from the separator through separate outlets; and (e) passing the separated water to an injection system for injection into a hydrocarbon bearing formation via an injection well.

The oxygen-free stripping gas is inert to water and may be selected from natural gas, nitrogen or other inert gas. The oxygen-free stripping gas is supplied to an injection point at or near the bottom of the stripping column and optionally to one or more additional injection points, for example, two or three additional injection points further up the column. Where the stripping gas is supplied to more than injection point it is preferred that the injection points are spaced apart along the length of the stripping column. The ratio of stripping gas to water that is injected into the stripping column is sufficiently high that the water becomes entrained in the stripping gas and the two phase mixture of water and stripping gas adopts an annular flow regime as it passes up the stripping column. The ratio of stripping gas to water that is injected into the stripping column is in the range of 150 scfs stripping gasfbbl water to 100,000 scfs stripping gaslbbl water (where methane is used as the stripping gas, this range corresponds to 0.002 kg stripping gas/kg water to 12.5 kg stripping gas/kg water), preferably, 300 scfs stripping gas/bbl water to 50,000 scfs stripping gas/bbl water (where methane is used as stripping gas, this preferred range corresponds to 0.004 kg stripping gas/kg water to 6.25 kg stripping gas/kg water).

Typically, the water is injected into the stripping column at a rate of 5,000 to 100,000 bbls of water per day, preferably, 10,000 to 75,000 bbls of water per day, in particular, 20,000 to 60,000 bbls of water per day, for example, about 50,000 bbls of water per day. The gas to water ratio dictates the amount of stripping gas that is injected into the stripping column.

For example, where the water injection rate into the stripping column is 50,000 bbls of water per day and the desired stripping gas to water ratio is 500 scfs per bbl of water, the amount of stripping gas that is injected into the stripping column per day is 25,000,000 scfs per day. The number of stripping columns of the gas stripping system is dependent upon the injection water requirements for the producing formation. For example, if each stripping column has a capacity of 50,000 bbls of water per day and the water injection requirement for the producing formation is 150,000 bbls per day, three stripping columns will be required.

Where natural gas is used as the stripping gas it may be obtained from a production facility. Typically, the natural gas that is separated in the production facility is at high pressure, for example, a pressure in the range of 20 to 70 barg. Accordingly, there may be no requirement to pressurise the natural gas prior to passing the gas to the gas lift deaeration system of the present invention. However, if desired, a compressor may be provided to boost the pressure of the stripping gas. Typically, the natural gas that is obtained from the production facility has an oxygen content below the limits of detection (for example, below 1 ppb). The natural gas that contains the released oxygen that is withdrawn from the separator may be returned to the production facility where it is diluted into the produced natural gas. Alternatively, the natural gas that contains the released oxygen may be used as fuel gas in the production facility or as a fuel gas in the gas stripping system, for example, to heat the water prior to injection into the stripping column (see below). It is also envisaged that the natural gas that contains the released oxygen may be passed to a gas lift system for dilution into the gas that is used to lift produced hydrocarbons in a production well.

Where the stripping gas is nitrogen or another inert gas, it is preferred to recycle the gas that is withdrawn from the separator to the stripping column. In order to avoid the build up of oxygen in the stripping gas, the oxygen is removed from the stripping gas prior to it being recycled to the stripping colunm. Typically, the released oxygen may be removed from the stripping gas by reacting the oxygen with a stoichiometric amount of a fuel such as hydrogen, methanol or ethanol over a catalyst comprising palladium or platinum precipitated on a granulated inorganic support material such as alumina, thereby forming water vapour. Where hydrogen is used as fuel for this catalytic process, the hydrogen may be generated electrochemically. It is also envisaged that the oxygen may be removed from the stripping gas by using a cryogenic separation process that lowers the temperature of the recycle stream to below the liquefaction temperature for oxygen or by using a membrane that is capable of separating oxygen from the stripping gas.

The two phase mixture of water and stripping gas may initially adopt a churn flow regime as it passes up the stripping column but owing to the high gas to liquid ratio, the two phase mixture will settle down into an annular flow regime or a wispy annular flow regime, preferably, an annular flow regime. In an annular flow regime, the liquid phase (water) flows on the wall of the stripping column as a film and the gaseous phase (stripping gas) flows in the centre of the column (referred to herein as "gaseous core"). Droplets of aqueous phase are entrained in the gaseous core such that the aqueous droplets are in equilibrium with the gaseous phase. During annular flow, water droplets are continuously being stripped from the liquid film while the water droplets in the gaseous core are continuously undergoing coalescence and being returned to the liquid film. Accordingly, the aqueous phase and gas phase are in equilibrium. The water droplets that are entrained in the gaseous core during annular flow typically have a size range of 1 to 100 pPm. As the liquid flow rate is increased relative to the gas flow rate, the concentration of droplets in the gas core increases and ultimately, droplet coalescence in the core leads to large lumps or streaks (wisps) of liquid in the gas core. This is referred to as wispy annular flow and is characteristic of flows with high mass flux.

As is well known in the art, the stripping gas reduces the partial pressure of the oxygen in the water thereby allowing a substantial portion of the dissolved oxygen to come out of solution and partition into the gaseous phase (stripping gas). The oxygen dissolved in the water tends to be released to the stripping gas as long as the oxygen partial pressure in the gas phase remains below a certain value P's, the equilibrium being defined by Henry's Law: CH. P'3 where C is the concentration of oxygen dissolved in the water, in kg/m3; H is Henry's constant for oxygen and expressed in kg/rn3 Pa; and P'3 is the partial pressure of oxygen in the gas phase in equilibrium with the liquid phase, expressed in Pa. In the stripping system of the present invention, the volume of the stripping gas is sufficiently high that the partial pressure of oxygen in the gas phase is kept close to zero so that oxygen is released from the aqueous phase along the entire length of the stripping column.

There is a pressure difference over the stripping column with the pressure at the top of the column being lower than the pressure at the bottom of the stripping column. The pressure at the bottom of the stripping column will vary depending on parameters such as the water injection rate, the cross-sectional area of the stripping column, the gas to water ratio, and the height of the stripping colunm (hydrostatic head pressure). Typically, the pressure at the bottom of the stripping colunm is in the range of 500 to 2000 psia.

It is desirable to minimise the pressure at the top of the colunm as the dissolved oxygen content of the water phase is proportional to the partial pressure of oxygen in the gaseous phase. Accordingly, the lower the pressure at the top of the colunrn, the lower the partial pressure of oxygen in the gaseous phase and hence the lower the dissolved oxygen content in the water phase. The pressure at the top of the stripping column may be adjusted using, for example, a choke valve. Typically, the pressure at the top of the stripping column is maintained in the range of from atmospheric pressure to 500 psia.

Where the stripping gas is natural gas it is preferred that the pressure at the top of the stripping column is in the range of 150 to 500 psia, for example, 200 to 300 psia, thereby allowing the separated natural gas to be returned to the production facility or to be delivered to a gas lift system without any requirement for repressurisation of the natural gas. Where the stripping gas is nitrogen or another inert gas, it is preferred that the pressure at the top of the colunm is in the range of 150 to 500 psia, for example, 200 to 300 psia, so that the separated stripping gas is at the pressure required for catalytic reaction of oxygen with a fuel such as hydrogen, methanol or ethanol to form water vapour (see above). However, if a compressor is provided to compress the separated stripping gas, the pressure at the top of the stripping column may be less than 150 psia, for example, in the range of from atmospheric pressure to 100 psia, preferably, from atmospheric pressure to psia.

The stripping column may be directly immersed in a body of water in which case it is closed at the bottom and is provided with an inlet for the water that is to be deaerated at or near the bottom thereof. The gas injection line may be strapped to the outside of the stripping column and has an outlet (injection point) at or near the bottom of the stripping column. Thus, the gas injection line penetrates through the wall of the stripping column.

Alternatively, the gas injection line may be arranged within the stripping column and has an outlet (injection point) at or near the bottom of the stripping column. It is envisaged that the gas injection line may have additional outlets (injection points) at positions higher up the stripping column in order to ensure that the two phase mixture of water and gas is maintained in an annular flow regime as it passes up the column. Typically, the additional gas outlets are spaced apart along the length of the stripping column. As an alternative to directly immersing the stripping column in the body of water, the stripping column may be open at its bottom and may be arranged within an outer column that is closed at its bottom such that the open end of the stripping column is at least 5 inches above the bottom of the closed outer column and an annular space is formed between the stripping column and the outer column. Typically, this annular space has a width in the range of 5 to 20 inches, preferably 7.5 to 15 inches. The outer column is in fluid communication with the body of water via one or more inlets such that the water level within the annular space is the same as the level of the body of water.

Alternatively, the stripping column may be arranged within a well in which case the stripping column is open at its bottom. Typically, the well is a cased wellbore with a tubing that is open at the bottom arranged therein such that an annular space is formed between the casing and the tubing. A gas injection line may either be arranged within the tubing, or within the annular space if it is of sufficient width to accommodate the gas injection line. The gas injection line has an outlet at or immediately above the bottom of the tubing, for example, at a height of 2 metres from the bottom of the tubing. It is envisaged that the water that is to be deaerated may be injected into the wellbore through the annular space and the two phase mixture of water and stripping gas passes up through the tubing which therefore acts as the stripping column. In this configuration, the gas injection line is arranged within the tubing. Alternatively, the water that is to be deaerated may be injected into the welibore through the tubing, and the two phase mixture of water and stripping gas passes up through the annular space formed between the casing and tubing such that the annular space forms the stripping column. In this configuration, the gas injection line is arranged within the annular space.

Preferably, the length of the stripping column is in the range of to 200 to 6,000 feet (60.96 to 1828.8 metres), more preferably, 500 to 3000 feet (152.4 to 914.4 metres), in particular, 750 to 1500 feet (228.6 to 457.2 metres), for example, 1000 to 1200 feet (304.8 to 365.76 metres).

Where the stripping column comprises a tubing, the internal (inner) diameter of the stripping column is preferably in the range of 4 to 20 inches, in particular, 8 to 15 inches while the external (outer) diameter of the stripping column is preferably in the range of 4.5 to 21.5 inches, in particular, 8.75 to 16.5 inches, for example, 9-5/8 inches. Where the tubular stripping column is arranged in a weilbore or is arranged within an outer column that is immersed in a body of water, the casing of the welibore or the outer column preferably has an inner diameter in the range of 8 to 25 inches and an outer diameter in the range of 8.75 to 26.5 inches, for example, 13-3/8 inches. Preferably, the gas injection line has an inner diameter in the range of ito 3.5 inches, in particular, 1.5 to 2.5 inches and an outer diameter in the range of 1.5 to 4 inches, in particular, 2 to 3 inches, for example, 2- 7/8 inches.

Where the stripping column comprises the annular space formed between the casing of a weilbore and a water injection tubing, the preferred inner and outer diameters for the casing and inner and outer diameters of gas injection lines are as described above.

Preferably, the outer diameter of the water injection tubing is selected such that the annular space that acts as the stripping column is capable of accommodating the gas injection line.

Typically, the width of the annular space that acts as the stripping column is in the range of to 20 inches, preferably 7.5 to 15 inches.

Preferably, the cross-sectional area of the stripping column is in the range of 10 to 300 in2 (64.62 to 1935.3 cm2), preferably, 25 to 250 in2 (161.275 to 1612.75 cm2), in particular, 50 to 175 in2 (322.55 to 1128.93 cm2).

The two phase mixture of water and gas that is removed from the top of the stripping colun-in is passed to a separator where a gaseous phase comprising stripping gas and released (free) oxygen separates from an aqueous phase of reduced dissolved oxygen content. The gaseous phase and aqueous phase are removed from the separator via separate outlets.

It is envisaged that the two phase mixture of water and gas that is removed from the top of the stripping column may be separated under gravity in a separator tank with the gaseous phase being removed from at or near the top of the tank via a gas outlet and the aqueous phase being removed from at or near the bottom of the tank via a water outlet. If natural gas is used as the stripping gas, the separated gas may be used as fuel gas in the production facility or may be returned to the production facility, for example, to a gas separator. A chemical oxygen scavenger agent, for example, ammonium bisulfite or sodium sulfite, may be added to the water in the tank to further reduce the dissolved oxygen content of the water before the water is withdrawn from the water outlet.

Alternatively, the oxygen scavenger agent may be added to the water stream that is withdrawn from the separator tank via the water outlet. Typically, the water is then passed via a flow line to an injection system comprising at least one high pressure pump, which pumps the deaerated water into a producing formation via at least one injection well.

Alternatively, the deaerated water may be passed to an ionic species removal plant before being injected into the producing formation.

The two phase mixture of water and gas that is removed from the top of stripping column may also be separated using at least one gas-liquid cylindrical cyclone (GLCC) separator, preferably, two GLCC separators arranged in series. The mixture of water and gas is introduced tangentially into the first GLCC separator and a water stream having a reduced oxygen content compared with the water that is injected into the stripping column is removed from an outlet at or near the bottom of the GLCC separator and stripping gas containing released oxygen is removed from at or near the top of the GLCC separator. The water that is withdrawn from the outlet of the first GLCC separator may then be reduced in pressure across a valve before being introduced tangentially into the second GLCC separator. If desired the water stream is treated with an oxygen scavenger, for example, ammonium bisulfite or sodium sulfite, before being introduced into the second GLCC separator. Accordingly, a water stream that is further reduced in dissolved oxygen content is removed from at or near the bottom of the GLCC separator and may be passed to an injection system comprising at least one high pressure pump for injection into a producing formation via at least one injection well. Alternatively, the deaerated water may be passed to an ionic species removal plant, as described above for the tank separator.

Provision may be made in the separator for an inlet for antifoam agents, and downstream of the separator for an inlet to the deaerated water flowline for scale inhibitors and other additives. Suitably, the deaerated water may also be chlorinated to control bacteria by injecting a chlorinating chemical or by passing the deaerated water through an electrochemical chlorinator.

The water that is to be deaerated may be seawater, estuarine water, lake water, or a river water. Where the water is seawater, estuarine or lake water, it is preferably obtained at a depth below which solids and animal and plant matter (e.g. plankton and algae) are present in substantial amount It is also envisaged that the water that is to be deaerated may be an aquifer water or a produced water. Typically, such waters are naturally oxygen free, but deaeration becomes necessary if the aquifer water or produced water has lost containment i.e. come into contact with air, prior to injection into the producing formation.

Typically, the water that is deaerated using the process of the present invention comprises 5 to 15 parts per million (ppm), for example, 8 to 10 ppm of dissolved oxygen.

Typically, the stripping column removes at least 98%, preferably, at least 99%, in particular, at least 99.5% of the dissolved oxygen from the water, with the amount that is removed increasing with increasing temperature of the water that is injected into the stripping column. Preferably, the dissolved oxygen content of the water phase at the top of the stripping column, and hence the dissolved oxygen content of the water that is withdrawn from the separator, is less than 500 ppb, preferably, less than 250 ppb, in particular, less than 100 ppb. The oxygen content of the water that is withdrawn from the separator may be reduced to a level of 20 ppb or less by treatment with an oxygen scavenger agent. As discussed above, the oxygen scavenger may be added to the water within the separator or to the water downstream of the separator.

The water that is deaerated using the process of the present invention may be used to maintain pressure in the producing formation or may be a low salinity water used for enhanced recovery of hydrocarbons from the producing formation or may be a water that has been treated to remove scale precursor ions (e.g. sulfate anions). Where the dissolved solids content of the injection water has been manipulated using an ionic species removal plant, it is envisaged that the water may be deaerated prior to being passed to an ionic species removal plant or the treated water from the ionic species removal plant may be deaerated prior to being injected into the hydrocarbon producing formation. However, where the water is deaerated prior to being passed to the ionic species removal plant it is essential that the water does not come into contact with air within the ionic species removal plant. The ionic species removal plant may be, for example, a reverse osmosis (RO) plant, a nanofiltration (NF) plant or a water softening plant where the water is passed through an ion exchange resin column that exchanges, for example, sodium ions for multivalent cations contained in the injection water.

It is envisaged that filters may be provided downstream of the gas lift deaeration system of the present invention for removal of particulates from the deaerated injection water. However, as an alternative or in addition, filters may be provided for removal of particulates from the water prior to it being injected into the stripping column.

If necessary the water that is to be deaerated is heated prior to being introduced into the gas stripping system in order to avoid gas hydrate formation. For example, the water may be heated by heat exchange with a hot process stream of the production facility or may be heated using a heater that employs natural gas that contains released oxygen (obtained from the separator) as fuel gas. Heating of the water to avoid gas hydrate formation is of particular importance where the stripping gas is natural gas as gas hydrates can form under pressure at temperatures of up to 18°C. If desired, gas hydrate inhibitors may be added to the water prior to injection of the water into the stripping column.

The present invention will now be exemplified by reference to the following Figures and Examples.

Figure 1 is a schematic view of a first embodiment of the gas lift deaeration system of the present invention that comprises a surface production facility 1 (for example, a platform or a floating production storage and offloading facility, FPSO) and a stripping column 2 having an external diameter of 9-5/8 inches and a length of about 1000 feet where the stripping column 2 is immersed in a body of water 3. The water that is to be treated is typically obtained at a depth below which solids and animal and plant matter (e.g. plankton and algae) are present in substantial amounts. The water is introduced to the bottom of the stripping column 2 through an inlet 5 at a rate of 50,000 bbls/day using a pump 6. Typically, the water may be passed through one or more filters (not shown) to reduce the solids content of the water. The stripping gas is taken as a side stream from the natural gas that is separated at a surface production facility 1 and is introduced into a gas line 7 that is strapped to the stripping column 2. The gas injection line has an external diameter of 2-7/8 inches. The natural gas is injected into the stripping column 2 through an inlet 8 located at or near the bottom thereof at a rate that achieves the desired gas to water ratio, for example, a ratio in the range of 150 scfs gas/bbl water to 8000 scfs gas/bbl water (0.002 kg gas/kg water to 1 kg gas! kg water). If desired, one or more additional gas inlets may be provided further up the stripping column. The pressure at the bottom of the stripping column 2 will be dependent on the height of the stripping column, its internal diameter (cross sectional area), and the gas and water injection rates. The water injection rate will be dependent on the desired capacity of the stripping column (bbls of deaerated water produced per day) while the gas injection rate will be dependent on the desired gas to water ratio. As an example, the pressures at the bottom and top of the stripping column are 500 psia and 200 psia respectively.

The ratio of gas to water in the stripping column 2 is sufficient for the water to be entrained into the stripping gas and for the two phase mixture of stripping gas and water passes to adopt an annular flow regime as it passed up the stripping column. The aqueous and gaseous phases are intimately mixed in the stripping column 2 owing to droplets of water being continuously stripped from the liquid film that is formed on the column walls and droplets of water contained in the gaseous core continuously recoalescing with the liquid film. Accordingly, dissolved oxygen in the water comes out of solution. The two phase mixture of water and stripping gas (now containing free oxygen) is removed from the top of the stripping column 2 and is passed to a separator tank 9 through a water-gas inlet 10. The stripping gas containing free oxygen is vented via gas line 11 while the deaerated water is withdrawn via line 13. Where natural gas is used as stripping gas, the vented gas may be used as fuel gas aboard the production facility 1 or can be returned to the production facility, for example, to a gas-liquid separator (not shown) where the stripping gas that contains free oxygen is diluted into the produced natural gas.

Alternatively, the natural gas may be introduced into a gas lift system. A portion of the vented gas may be used to heat the water that is to be deaerated.

If desired, a chemical oxygen scavenger agent such as ammonium bisulfite or sodium sulfite is added to the water in the separator tank 9 through line 12 andlor is added to the water that is withdrawn via line 13 to further reduce the amount of dissolved oxygen in the water before the water is passed to an injection system (not shown) comprising at least one high pressure injection pump which, in turn, pipes the deaerated water into a producing formation (not shown).

As an alternative to separating the two phase mixture of water and stripping gas in a settling tank, the separation may be achieved using a gas-liquid cylindrical cyclone (GLCC) separator, preferably, two GLCC separators arranged in series, as shown in Figure 2. The mixture of water and gas that is removed from the top of the stripping column 2 is introduced tangentially into the first GLCC separator 20 via inlet 21 and an aqueous phase containing, for example, less than 100 ppb oxygen, is removed from an outlet 22 at or near the bottom of the first GLCC separator and the gaseous phase (stripping gas containing free oxygen) is removed from an outlet 26 at or near the top of the first GLCC separator.

The aqueous phase that is withdrawn from outlet 22 may then be reduced in pressure across a valve 23 before being introduced tangentially into the second GLCC separator 24.

If desired the water stream is treated with an oxygen scavenger, for example, ammonium bisulfite or sodium sulfite, before being introduced into the second GLCC separator 24.

Accordingly, a water stream that is further reduced in dissolved oxygen content, for example, containing less than 20 ppb 02, is removed from an outlet 25 at or near the bottom of the second GLCC separator 24 and is passed to an injection water system comprising at least one high pressure pump for injection into a hydrocarbon bearing formation via at least one injection well (not shown).

The stripping gas (natural gas) that is removed from the top of the first GLCC separator 20 (49 MMscfs/day) is passed to a flowline 27 at a pressure of about 300 psia and may be returned to the production facility where it is diluted into the produced natural gas. The stripping gas that is removed from the top of the second GLCC separator (1.2 MMscfs/day) is at a pressure of 50 psia and is passed to a flowline 28. This stripping gas may be used as fuel gas for the production facility or to heat the water that is to be deaerated. Alternatively, the stripping gas may be passed to a flare stack or could be boosted in pressure and returned to the production facility where it is diluted into the produced natural gas.

Figure 3 is a schematic illustration of a further embodiment of the gas lift deaeration system of the present invention that comprises a production facility 30 and a well 31. The weEl 31 is provided with a casing 32 and has a stripping column 33 and a gas injection line 34 arrange therein. The stripping column 33 is open at the bottom. The gas injection line 34 is shown arranged within the stripping column 33. As an example, the casing 32 has an external diameter of 13-3/8 inch, the stripping column 33 has an external diameter of 9-5/8 inch and the gas injection line 34 an external diameter of 2-7/8 inch. The well has a depth of about 1000 feet. The water that is to be treated is passed via a water supply line 36 and a low pressure pump 37 to an inlet 38 at the wellhead 39 and down the annulus 35 between the casing 32 and stripping column 33. Typically, the water may be passed through one or more filters (not shown) to reduce the solids in the water before being passed into the well 31. The stripping gas is taken as a side stream from the natural gas that is separated at a production facility 1 and is passed via line 41 to an inlet 40 at the wellhead 39 and into the gas injection line 34 that has an outlet 42 at or near the bottom of the stripping column. One or more additional outlets may be provided along the gas injection line 34 so that additional stripping gas may introduced higher up the column.

Where the gas injection line 34 is provided with a plurality of additional outlets, it is preferred that these are spaced apart along the length of the gas injection line 34. The natural gas is injected into the stripping column at a rate that achieves the desired gas to water ratio of, for example, 500 scfs gas/bbl water to 8000 scfs gasfbbl water (for example, 0.01 kg gas!kg water to 1 kg gas! kg water). As discussed above, the pressure at the bottom of the stripping column will be dependent on the height of the stripping column, its internal diameter, and the gas and water injection rates. The water injection rate will be dependent on the desired capacity of the stripping column (bbls of deaerated water produced per day) while the gas injection rate will be dependent on the desired gas to water ratio. As an example, the pressure at the bottom and top of the column may be 1500 psia and 200 psia respectively.

Owing to the high gas to liquid ratio, the two phase mixture of stripping gas and water that passes up the column adopts an annular flow regime with a water film on the internal wall of the stripping column and on the external wall of the gas injection line (if positioned in the stripping column) and a gas core comprising droplets of water. The stripping column has an outlet 43 at the wellhead 39 and the two phase mixture of stripping gas and water is passed to a separator vessel as described in Figure 1 or a GLCC separator, as described in Figure 2.

Figure 4 is a schematic illustration of a further embodiment of the gas lift deaeration system of the present invention that comprises a production facility 50 and a well 51. The well 51 is provided with a casing 52 and has a water injection tubing 53 and a gas injection line 54 arranged therein wherein the water injection tubing and gas injection line are each open at the bottom. Accordingly, the annular space 55 formed between the casing 52 of the welibore and the water injection tubing 53 comprises the stripping column. The water that is to be treated is passed via a water supply line 56 and a low pressure pump 57 to an inlet 58 at the wellhead 59 and down the water injection tubing 53.

Typically, the water that is to be deaerated may be passed through one or more filters (not shown) to reduce the solids content of the water before being passed into the water supply line 56. The stripping gas is taken as a side stream from the natural gas that is separated at the production facility 50 and is passed via line 54 to an inlet 60 at the welihead 59 and into the gas injection line 54. As discussed above, the gas injection line 54 is open at its bottom. One or more additional outlets may be provided along the gas injection line so that additional stripping gas may introduced higher up the annular space 55. Where the gas injection line is provided with a plurality of additional outlets, it is preferred that these are spaced apart along the length of the gas injection line. The natural gas is injected into the well at a rate that achieves the desired gas to water ratio of, for example, 500 scfs gas/bbl water to 8000 scfs gas/bbl water (0.01 kg gas/kg water to 1 kg gas! kg water). The pressure at the bottom of the annular space 55 will be dependent on the depth of the water injection tubular in the weilbore, the available cross sectional area for gas lift (cross-sectional area of the weilbore -the cross-sectional area taken up by the water injection tubing and gas injection line) and the gas and water injection rates. The water injection rate will be dependent on the desired capacity of the stripping column (for example, 500 bbls of deaerated water produced per day) while the gas injection rate will be dependent on the desired gas to water ratio. The pressures at the bottom and top of the colunm may be, for example, 1500 and 200 psia respectively.

The two phase mixture of stripping gas and water that passes up the annular space (stripping column) will adopt an annular flow regime with a water film on the internal wall of the casing, on the external wall of the water injection tubing and the external wall of the gas injection line and a gas core comprising droplets of water. The annular space (stripping column) has an outlet at the wellhead 59 and the two phase mixture of stripping gas and water is passed to a separator vessel as described in Figure 1 or a GLCC separator as described in Figure 2.

Although the process of the present invention has been exemplified in Figure 1 using a vertical stripping column arranged in a body of water and in Figures 3 and 4 using vertical wells, it is envisaged that the stripping column or well could be inclined or deviated without any significant impact on the deaeration process, for example, the stripping column or well could be deviated by an angle of up to 25° from vertical.

Example

Table 1 shows simulated operating conditions for stripping columns of varying cross sectional area. The simulation is based on the stripping column treating 50,000 bbl of fresh water (water having a salinity of less than 500 ppm) per day with a pressure at the bottom and top of the column of 1500 psia and 200 psia respectively. The gas flow rates at the bottom and top of the column are given for a gas to water ratio of 500 standard cubic feet (scfs) gas per barrel (bbl) of water and for a gas to water ratio of 1000 scfs gas per bbl water. The simulation achieves 99.2% removal of dissolved oxygen (for a gas to water ratio of 500 scfs gas per bbl of water) when the water is at a temperature of 0°C and 99.5% removal of dissolved oxygen when the water is heated to a temperature of 25°C. The simulation achieves 99.6% removal of dissolved oxygen (for a gas to water ratio of 1000 scfs gas per bbl of water) when the water is at a temperature of 0°C and 99.8% removal of dissolved oxygen when the water is heated to a temperature of 25°C.

Table 1

It.fIkr 1 9.625 8.525 57.1 2.50 2.18 16.35 4.36 32.71 10.750 9.894 76.9 1.85 1.62 12.14 3.24 24.28 10.750 9.875 76.6 1.86 1.63 12.19 3.25 24.38 10.750 9.694 73.8 1.93 1.69 12.65 3.37 25.29 10.750 9.625 72.8 1.96 1.71 12.83 3.42 25.66 10.750 9.504 70.9 2.01 1.75 13.16 3.51 26.32 10.750 9.5 70.9 2.01 1.76 13.17 3.51 26.34 11.750 10.844 92.4 1.54 1.35 10.11 2.70 20.21 11.750 10.724 90.3 1.58 1.38 10.33 2.76 20.67 11.750 10.625 88.7 1.61 1.40 10.53 2.81 21.06 11.750 10.625 88.7 1.61 1.40 10.53 2.81 21.06 11.750 10.43 85.4 1.67 1.46 10.93 2.91 21.85 11.750 10.376 84.6 1.69 1.47 11.04 2.94 22.08 11.750 10.25 82,5 1.73 1.51 11.31 3.02 22.62 11.875 10.625 88.7 1.61 1.40 10.53 2.81 21.06 13.375 12.459 121.9 1.17 1.02 7.66 2.04 15.31 13.375 12.359 120.0 1.19 1.04 7.78 2.07 15.56 13.375 12.259 118.0 1.21 1.05 7.91 2.11 15.82 13.375 12.25 117.9 1.21 1.06 7.92 2.11 15.84 13.375 12.119 115.4 1.24 1.08 8.09 2.16 16.18 13.375 12.059 114.2 1.25 1.09 8.17 2.18 16.35 13.375 12.003 113.2 1.26 1.10 8.25 2.20 16.50 13.375 12 113.1 1.26 1.10 8.25 2.20 16.51 13.500 12.25 117.9 1.21 1.06 7.92 2.11 15.84 13.625 12.25 117.9 1.21 1.06 7.92 2.11 15.84 15.000 13.812 149.8 0.95 0.83 6.23 1.66 12.46 16.000 14.936 175.2 0.81 0.71 5.33 1.42 10.66 16.000 14.822 172.5 0.83 0.72 5.41 1.44 10.82 16.000 14.75 170.9 0.83 0.73 5.46 1.46 10.93 16.000 14.75 170.9 0.83 0.73 5.46 1.46 10.93 16.000 14.5 165.1 0.86 0.75 5.65 1.51 11.31 16.000 14.382 162.5 0.88 0.77 5.75 1.53 11.49 16.125 14.805 172.2 0.83 0.72 5.42 1.45 10.84 17.000 16 201.1 0.71 0.62 4.64 1.24 9.29 17.000 15.812 196.4 0.73 0.63 4.75 1.27 9.51 17.875 16.687 218.7 0.65 0.57 4.27 1.14 8.54 17.875 16.5 213.8 0.67 0.58 4.37 1.16 8.73 18.000 16.812 222.0 0.64 0.56 4.20 1.12 8.41 18.000 16.688 218.7 0.65 0.57 4.27 1.14 8.54 * 18.625 17.567 242.4 0.59 0.51 3.85 1.03 7.70 18.625 17.517 241.0 0.59 0.52 3.87 1.03 7.75 18.625 17.5 240.5 0.59 0.52 3.88 1.03 7.76 18.625 17.311 235.4 0.61 0.53 3.97 1.06 7.93 18.625_ 17.279 234.5 0.61 0.53 3.98 1.06 7.96

Claims

Claims 1. A process for removing dissolved oxygen from an injection water in a gas stripping system that comprises at least one stripping column having a cross sectional area in the range of 10 to 300 inches2 (64.51 to 1935.3 cm2) and a length in the range of 200 to 6000 feet (60.96 to 1828.8 metres), preferably in the range of 500 to 3000 feet (152.4 to 914.4 metres), the process comprising: (a) introducing the water that is to be deoxygenated to the bottom of the stripping column at a rate in the range of 5,000 to 100,000 barrels (bbls) of water per day; (b) introducing the oxygen-free stripping gas into the stripping column via one or more injection points wherein at least one injection point is provided at or near the bottom of the stripping column and wherein the ratio of stripping gas to water that is introduced into the stripping column is in the range of 150 standard cubic feet (scfs) stripping gas/bbl water to 100,000 scfs stripping gas/bbl water; (c) maintaining the pressure at the top of the stripping column in the range of atmospheric pressure to 500 psia (3.447 MPa) such that the two phase mixture of water and stripping gas adopts an annular flow regime as the mixture passes up the stripping column and the dissolved oxygen content of the water phase at the top of the stripping column is reduced to less than 500 ppb, preferably less than 100 ppb; (d) withdrawing the mixture of water and stripping gas from the top of the column and passing the mixture to a separator that separates the stripping gas from the water, and removing the stripping gas and water from the separator through separate outlets; and (e) passing the separated water to an injection system for injection into a hydrocarbon bearing formation via an injection well.
2. A process as claimed in claim 1 wherein the oxygen-free stripping gas is natural gas obtained from a production facility at a pressure in the range of 20 to 70 barg and wherein the natural gas that is separated from the water in step (d) is returned to the production facility or is passed to a gas lift system that delivers gas to a production well.
3. A process as claimed in claim 1 wherein the stripping gas is nitrogen or another inert gas and the stripping gas that is separated from the water in step (d) is recycled to the stripping column after removing the released oxygen from the stripping gas.
4. A process as claimed in any one of the preceding claims wherein the stripping column is closed at its bottom and is immersed in a body of water and the water that is to be deaerated is taken from the body of water and is introduced into the stripping column via an inlet provided at or near the bottom of the stripping column and the oxygen free stripping gas is delivered to the one or more injection points of the stripping column via a gas injection line that is either strapped to the outside of the stripping column or is arranged within the stripping column.
5. A process as claimed in any one of claims 1 to 3 wherein the stripping column is open at its bottom and is arranged within an outer column that is closed at its bottom such that the open end of the stripping column is at least 5 inches above the bottom of the outer column and an annular space having a width in the range of 5 to 20 inches is formed between the stripping column and outer column and wherein the outer column is immersed in a body of water and the water that is to be deaerated is taken from the body of water and is introduced to the outer column via at least one inlet and the oxygen free stripping gas is delivered to the one or more injection points of the stripping column via a gas injection line that is either arranged within the annular space or within the stripping column.
6. A process as claimed in claim 5 wherein the outer column has an internal (inner) diameter in the range of 8 to 25 inches and an external (outer) diameter in the range of 8.75 to 26.5 inches, stripping column has an internal diameter in the range of 4 to 20 inches and an external diameter in the range of 4.5 to 21.5 inches, and the gas injection line has an internal diameter in the range of I to 3.5 inches and an external diameter in the range of 1.5 to 4 inches
7. A process as claimed in any one of claims 1 to 3 wherein the stripping column is a tubing that is open at the bottom and is arranged within a cased welibore such that an annular space is formed between the casing and the tubing and a gas injection line is arranged either within the tubing or within the annular space and has an outlet at or immediately above the bottom of the tubing and the water that is to be deaerated is injected into the weilbore through the annular space and the two phase mixture of water and stripping gas passes up through the tubing that acts as the stripping column.
8. A process as claimed in claim 7 wherein the tubing has an internal (inner) diameter in the range of 4 to 20 inches and an external (outer) diameter in the range of 4.5 to 21.5, the casing of the weilbore has an internal diameter in the range of 8 to 25 inches and an external diameter in the range of 8.75 to 26.5 inches, and the gas injection line has an internal diameter in the range of I to 3.5 inches and an external diameter in the range of 1.5 to 4 inches.
9. A process as claimed in any one of claims 1 to 3 wherein a water injection tubing that is open at the bottom is arranged within a cased welibore such that an annular space is formed between the casing and the tubing and a gas injection line is arranged within the annular space having an outlet at or immediately above the bottom of the tubing and the water that is to be deaerated is injected into the wellbore through the water injection tubing and the two phase mixture of water and stripping gas passes up through the annular space such that the annular space acts as the stripping column.
10. A process as claimed in claim 9 wherein the casing of the wellbore has an internal (inner) diameter in the range of 8 to 25 inches and an external (outer) diameter in the range of 8.75 to 26.5 inches, the gas injection line has an internal diameter in the range of 1 to 3.5 inches and an external diameter in the range of 1.5 to 4 inches, and the external diameter of the water injection tubing is selected such that the width of the annular space that acts as the stripping column is in the range of 5 to 20 inches.
11. A process as claimed in any one of the preceding claims wherein the two phase mixture of water and gas that is removed from the top of the stripping column is separated under gravity in a separator tank with the gaseous phase being removed from at or near the top of the tank via a gas outlet and the aqueous phase being removed from at or near the bottom of the tank via a water outlet.
12. A process as claimed in any one of claims 1 to 10 wherein the two phase mixture of water and gas that is removed from the top of stripping column is separated using a gas-liquid cylindrical cyclone (GLCC) separator wherein the mixture of water and gas is introduced tangentially into the GLCC separator and a water stream having a reduced oxygen content compared with the water that is injected into the stripping column is removed from an outlet at or near the bottom of the GLCC separator and stripping gas containing released oxygen is removed from at or near the top of the GLCC separator.
13. A process as claimed in claims 11 or 12 wherein a chemical oxygen scavenger agent is added to the water in the tank or to the water in the GLCC separator to further reduce the dissolved oxygen content of the water andlor is added to the water stream that is withdrawn from the separator tank or from the GLCC separator.
14. A process as claimed in any one of the preceding claims wherein the deaerated water is passed through an ionic species removal plant before being injected into the producing formation.
15. A process as claimed in any one of the preceding claims wherein the injection water is heated to a temperature of at least 18°C, preferably, in the range of 20 to 30°C prior to being introduced into the gas stripping system.