METHOD FOR PRODUCTION OF INJECTION WATER AND/OR PROCESS WATER FROM SEAWATER

The present invention relates to a low-cost and an energy efficient method for in-situ production of injection water for offshore hydrocarbon extraction and/or process water.

BACKGROUND

Oil in an oil reservoir is usually pressurized from nature and will flow by its own pressure to the surface through a production well made to contact the reservoir in the first stage of an oil extraction process. This stage of an oil extraction is known as primary oil extraction . Typically, the primary oil extraction may extract up to about 1 /5 of the original oil content. As the oil is being extracted , the underground oil pressure in the reservoir decreases such that after a period the underground oil pressure has decreased sufficiently for the well not to be self-producing any longer. From this moment it becomes necessary to actively increase the underground oil pressure to force the oil up to the surface. This is typically obtained by injecting water and/or a gas into the oil carrying rock structure. This stage of an oil extraction is known as secondary oil extraction, and may typically extract another 1 /5 of the original oil content of the reservoir.

Enhanced , or tertiary, oil production may increase the extraction rate further by changing the mobility (flow resistance) of the remaining oil in the reservoir by heating by injection of steam, adding surfactants etc.

For offshore oil extraction (from reservoirs beneath the seabed) it is usually necessary to carry most of the processing equipment on offshore platforms due to the distance to the nearest land location. A typical offshore oil and gas platform is designed for a life-span of around 50 years in rather harsh weather conditions. As a consequence, the structure carrying parts of an offshore platform needs to be dimensioned so robust that every kg of pro- cess equipment the platform is intended to carry, results in a weight increase of the load carrying structure of the platform of about 3 - 5 kg. The weight of the process equipment to be mounted on an offshore oil and gas platform is therefore an important cost-factor. It is important to reduce both the foot-print (required instalment area), the weight of the process equipment and its energy demand.

The typical distance from offshore platforms to the nearest land location and the relatively large volumes of injection water required for secondary oil recovery makes it desirable and economical to produce injection water on-site at the platform. The transport costs associated with producing the injection water at land based locations becomes prohibitive.

The injection water needs to be almost completely depleted in oxygen to prevent in-hole problems with corrosion and/or bacteria growth. The oil industry has set a maximum limit for the oxygen content of 20 ppb. As a comparison, water in thermodynamic equilibrium with air at one atmosphere pressure contains about 8.5 - 10 ppm. It is i.e. necessary to reduce the oxygen content by a factor of 1000 to satisfy the requirements for being used as injection water in offshore secondary oil extraction .

The term "process water" is used herein as a definition of water almost depleted in oxygen, which can be used in many different applications, such as for example dissolving chemicals, or washing pumps, pipes etc. , to inter alia avoid corrosion of the equipment. Further, the crude oil extracted from the oil reservoirs may comprise seawater, which needs to be separated from the oil. A process for the removal of seawater from crude oil may utilize process water and take place on an offshore platform. The process water comprises a maximum and average oxygen content corresponding to the injection water de- scribed above and is mixed together with the crude oil, which comprises seawater. Thereafter a separation process separates the crude oil from the salts and water. The weight of the process equipment to be mounted on the offshore oil and gas platform is an important cost-factor. It is therefore important to reduce both the foot-print (required instalment area), the weight of the process equipment and its energy demand.

That the seawater is almost depleted in oxygen is to be understood as seawater having a maximum oxygen content of 30 ppb or less, and an average oxygen content of less than 18 ppb.

PRIOR ART

From WO 2001 /85622 it is known that membrane separators may be utilised for on-site production of injection water from seawater. The apparatus (1 ) comprises a substantially porous membrane (12) being provided with a catalyst means such as palladium and at least one inlet (16, 18) and one outlet (20, 22) for input of the oxygenated water and recovery of the de-oxygenated water. The sea water and hydrogen are introduced into the apparatus (1 ), the hydrogen typically being dispersed by the porous membrane (12). Ex- cess oxygen in the water reacts with the hydrogen , thus de-oxygenating the water.

From Pabby et al. (ed.), "Handbook of Membrane Separations, Chemical, Pharmaceutical, Food, and Biotechnological Applications", CRC Press, ed. 2009, pages 7-27, it is known that a type of membrane separator, which is often denoted contact membrane, is an effective type of membrane able to reduce the oxygen content of a water phase to significantly less than 20 ppb.

Document US 2010/0230366 teaches producing injection water on an offshore oil platform from seawater by using a battery involving serially connected membrane contactors using nitrogen gas as sweep gas.

A membrane contactor is usually composed of one elongated hollow cylinder housing hav- ing a liquid inlet at a first end of the cylinder housing and a liquid outlet at a second end opposite the first end of the cylinder housing. Inside the hollow room of the cylinder housing, there is a bundle of hydrophobic microporous hollow-fibre membranes extending from a gas distribution manifold at the first end of the cylinder body and to a gas colleting manifold at the second end of the cylinder body. The gas distribution manifold is connected to an external gas supply and makes the supplied gas to enter the inside (also denoted as the lumen side in the literature) of the hydrophobic microporous hollow-fibre membranes, flow through them and be collected by the gas collecting manifold which is connected to a gas outlet, usually equipped with a downstream vacuum pump to create a suction force which lowers the gas pressure inside the hydrophobic microporous hollow-fibre membranes. A liquid , e.g. water, is made to flow on the outside (also denoted as the shell side in the literature) of the hydrophobic microporous hollow-fibre membranes in the opposite direction of the gas flowing on the lumen side. Due to the hydrophobic nature of the microporous hollow-fibre membrane and the resulting capillary force on a water phase flowing at the , the water phase at the shell side will be prevented from flowing through the micro-pores and enter the lumen side, but will instead protrude partially into the pores. The water phase forms thus a meniscus inside the micro-pores which comes in direct contact with the flowing gas phase in the lumen side, thus the name contact membrane. The gas transport across the gas-liquid interface (the meniscus) becomes governed by Henry's law and transport equations for mass diffusion over an interface. See e.g. Wiesler [2] for further details.

At present, there is a major commercial producer of contact membranes for deoxidation of water. These membrane contactors are sold under the trademark name Liqui-Cel® Membrane Contactor. These membrane contactors are known to be tested for formation of in- jection water, as may be seen from e.g . a TechBrief [1 ]. This membrane contactor has been tested for production of injection water from seawater. The present invention is based on the use of commercially available contactors sold under the trademark Liqui-Cel® Membrane Contactor.

Under operation of a contact membrane in the practical life, there may be several factors which may have an effect on the degassing efficiency, such as e.g. inlet water temperature, inlet water composition , atmospheric air pressure, etc. Thus, to ensure that a membrane contactor operates within the specified maximum limit for oxygen content of the degassed water, it is conventionally considered necessary to set the operation parameters (seawater temperature, sweep gas purity and flow volume rate, water flow volume rate and vacuum level) to obtain a theoretically obtainable oxygen removal level well below the maximum limit for oxygen level of injection water and/or process water. It is customary to run the membrane contactors such that they obtain an average performance well below the set limit for maximum oxygen content to ensure that the natural variation does not result in producing injection water and/or process water with too much oxygen .

OBJECTIVE OF THE INVENTION

The main objective of the invention is to provide a method for production of injection water and/or process water from seawater in an energy efficient manner.

DESCRIPTION OF THE INVENTION

The present invention is the reduction to practice of the discovery that in the real life there is no need for using such a large safety margin when applying membrane contactors for producing injection water and/or process water from seawater. Thus, it becomes possible to save considerable amounts of energy and footprint/weight by allowing the membrane contactor apparatus to be run closer to the set limit for maximum oxygen content than which is presently considered safe. There is i.e. no need for having such large safety mar- gin as presently thought necessary in this field .

Thus, in a first aspect the present invention relates to a method for production of injection water and/or process water from seawater, wherein the method comprises:

- applying a set of at least one membrane contactor arranged in parallel,

where each membrane contactor in the set comprises:

- an elongated hollow housing having an inner elongated space,

a seawater inlet located at a first end of the housing and adapted to inject seawater into the inner elongated space, a seawater outlet located at a second end opposite the first end of the housing and adapted to extract seawater from the inner elongated space,

a sweep gas inlet connected to a sweep gas distribution manifold located in the inner elongated space at the second end of the housing,

a sweep gas outlet connected to a sweep gas collection manifold located in the inner elongated space at the first end of the housing ,

a sweep gas supply connected to the sweep gas inlet,

a vacuum pump located downstream of and connected to the sweep gas outlet, and

at least one bundle of hydrophobic microporous hollow-fibre membranes of polypropylene and having a total surface area in the range of 200 - 250 m ² located in the inner elongated space and connected in one end to the sweep gas distribution manifold and in another end to the gas collection manifold such that sweep gas may be passed through a lumen side of each of the hydrophobic microporous hollow-fibre membranes in the bundle,

- arranging the membrane contactor(s) in the set in parallel by connecting the seawater inlet of each of the membrane contactor(s) in the set to a seawater supply manifold, connecting the seawater outlet of each of the membrane contactor(s) in the set to an injection water and/or process water collector manifold, connecting the sweep gas inlet of each of the membrane contactor(s) in the set to a sweep gas supply manifold , and connecting the sweep gas outlet of each of the membrane contactor(s) in the set to a sweep gas collecting manifold , and

- producing the injection water and/or process water by supplying seawater which is to be degassed to the seawater supply manifold and simultaneously supplying sweep gas to the sweep gas supply manifold, engaging the vacuum pump, and collecting the degassed seawater exiting the injection water and/or process water collector manifold ,

and wherein

- the seawater to be degassed is having a temperature in the range of 1 5-36 °C and is supplied to the seawater supply manifold at a volume flow rate in the range between 12-18 m ³ /hour per membrane contactor being applied in the set of parallel arranged membrane contactor(s),

- the sweep gas contains less than 0.1 weight% oxygen and is supplied at a flow rate in the range between 0.75-1 .6 Nm ³ /hour per membrane contactor being applied in the set of parallel arranged membrane contactor(s) , and

- the vacuum pump is regulated to give a sweep gas pressure at the lumen side of the hydrophobic microporous hollow-fibre membranes is in the range of 6.67-16 kPa.

The method according to the invention may advantageously be carried out by an apparatus for production of injection water and/or process water from seawater, wherein the apparatus comprises:

- a set of at least one membrane contactor arranged in parallel,

where each membrane contactor in the set comprises:

- an elongated hollow housing having an inner elongated space,

- a seawater inlet located at a first end of the housing and adapted to inject seawater into the inner elongated space,

- a seawater outlet located at a second end opposite the first end of the housing and adapted to extract seawater from the inner elongated space, - a sweep gas inlet connected to a sweep gas distribution manifold located in the inner elongated space at the first end of the housing,

- a sweep gas outlet connected to a sweep gas collecting manifold located in the inner elongated space at the second end of the housing ,

- a sweep gas supply connected to the sweep gas inlet,

- a vacuum pump located downstream of and connected to the sweep gas outlet, and

- a bundle of hydrophobic microporous hollow-fibre membranes located in the inner elongated space and connected in one end to the sweep gas distribution manifold and in an other end to the sweep gas collecting manifold such that sweep gas may be passed through each of the hydrophobic microporous hollow-fibre membranes in the bundle at their lumen side,

- a seawater supply manifold connected to the inlet of each of the membrane contactor(s) in the set,

- an injection water and/or process water collector manifold connected to the seawater outlet of each of the membrane contactor(s) in the set,

- a sweep gas supply manifold connected to the sweep gas inlet of each of the membrane contactor(s) in the set,

- a sweep gas collecting manifold connected to connecting the sweep gas outlet of each of the membrane contactor(s) in the set,

- a sweep gas supply connected to the sweep gas supply manifold,

- a sweep gas discharge connected to the sweep gas collecting manifold,

- a vacuum pump located on the sweep gas discharge downstream of the sweep gas collecting manifold ,

- a seawater supply connected to the seawater supply manifold, and

- an injection water and/or process water discharge line connected to the injection water and/or process water collector.

The term "arranging the membrane contactor(s) in a set in parallel" as used herein, means that the membrane contactors being applied in the set of membrane contactors are mutual- ly arranged such that the seawater which is to be degassed only can pass through one membrane contactor in the arrangement. That is, the invention according to one aspect degasses the seawater in just one step by passing through one membrane contactor, as opposed to a series arrangement where the seawater is degassed in two or more steps by being successively passed through two or more membrane contactors.

The term "lumen side" as used herein, means the inside of a hydrophobic microporous hollow-fibre membrane, while the term "shell side" means at the outside of a hydrophobic microporous hollow-fibre membrane.

The term "sweep gas" as used herein , means any gas having an oxygen content of less than 0.1 weight% and which does not react chemically with the water, the hydrophobic mi- croporous hollow-fibre membranes and/or the walls of the membrane collector. Examples of suitable sweep gases include, but are not limited to, nitrogen, argon, hydrogen , gaseous hydrocarbons, and CO ₂ .

The term "vacuum level" is used herein as a measurement of the sweep gas pressure at the lumen side of the hydrophobic microporous hollow-fibre membranes. The term "seawater" is used herein in a broad sense and includes any water taken from the sea. The seawater may be pristine (i.e. used as is from the sea) or treated in any known manner before being deoxidised by the process according to the present invention . The upstream treatments of the seawater may be one or more of; sulphate reduction , salt re- duction, filtration, and combinations of these. The seawater may advantageously be subject to a filtration step upstream to remove particulates and other entrained objects having a characteristic diameter above 5 microns.

The invention provides a surprisingly good performance enabling using only one degassing step without being "punished" by having to use excessive amounts of the purging gas and/or excessive energy to create low vacuums in the lumen side of the membranes. This is obtained by allowing the process to be run with parameters providing an average oxygen level in the degassed seawater so close to the maximum allowable oxygen content that it is conventionally considered to be unsafe operation conditions. However, experience by the inventor has found that this is not a problem in the practical life.

A common safety precaution applied by the oil industry related to membrane based deoxi- dation of injection water and/or process water is to assume that the oxygen-removal efficiency of the membranes typically varies at a level requiring a safety factor of 1 .7. That is, if the maximum allowable oxygen content is 20 ppb, the average oxygen content of the water exiting the membrane based deoxidation must be no more than 1 1 .8 ppb to avoid the natural variation resulting in producing water having more than 20 ppb oxygen. The present inventor has discovered that this safety precaution is not properly founded in the real life. A membrane contactor may be run at a higher average oxygen content even though the natural variation in the membrane based deoxidation process may then, in theory, provide water having more than 20 ppb oxygen . In the practical life, this is no problem because if the maximum content is surpassed, it will only be the case for sufficiently small quanta of treated water to represent a problem for its use as injection water and/or process water. Thus, the membrane process according to the present invention is tuned to run at parameter levels, which , in theory, may produce injection water and/or process water having more than 20 ppb oxygen, but not in an amount that represent a problem for its use as injection water and/or process water. The technical effect of this is a considerable saving in material and energy use for deoxidation of seawater for use as injection water and/or process water.

The above effect of the invention is obtained in practice when the volume flow rate of the sweep gas is in one of the following ranges; from 0.75 to 1 .6 Nm ³ /hour, combined with any of the following ranges of the vacuum level at the lumen side of the hydrophobic micro- porous hollow-fibre membranes of; from 50 to 120 Torr (6.67 to 16 kPa), when the seawater is having a temperature from 15 to 36 °C.

When the seawater temperature is between 1 5 to 20 °C, the above effect of the invention is obtained in practice when the volume flow rate of the sweep gas is in one of the following ranges; from 0.75 to 1 .6 Nm ³ /hour, from 0.8 to 1 .1 Nm ³ /hour, from 0.8 to 1 .0 Nm ³ /hour, or from 0.85 to 0.95 Nm ³ /hour, combined with any of the following ranges of the vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes of; from 50 to 100 Torr (6.67 to 13.33 kPa) , preferably from 60 to 85 Torr (8 to 1 1 .33 kPa) , or from 70 to 80 Torr (9.33 to 1 0.67 kPa). An especially preferred combination of sweep gas consumption and vacuum level is a volume flow rate of the sweep gas of 0.8 Nm ³ /hour and a vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes of 80 Torr (10.67 kPa) when the seawater temperature is between 1 5 to 20 °C.

An especially preferred combination of sweep gas consumption and vacuum level is a volume flow rate of the sweep gas of 0.8 Nm ³ /hour and a vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes of 80 Torr (10.67 kPa) when the seawater temperature is 20 °C.

When the seawater temperature is between 20-27.5 °C, the above effect of the invention is obtained in practice when the volume flow rate of the sweep gas is in one of the following ranges; from 0.75 to 1 .6 Nm ³ /hour, more preferably from 0.75 to 1 .4 Nm ³ /hour, or even more preferably from 0.75 to 1 .1 Nm ³ /hour, combined with any of the following ranges of the vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes of; from 50 to 120 Torr (6.67 to 16 kPa) , more preferably from 60 to 100 Torr (8 to 13.33 kPa), or even more preferably from 70 to 90 Torr (9.33 to 12 kPa).

An especially preferred combination of sweep gas consumption and vacuum level is a volume flow rate of the sweep gas of 0.8 Nm ³ /hour and a vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes from 70 Torr (9.33 kPa) to 90 Torr (12 kPa) when the seawater temperature is from 20 to 27.5 °C.

An especially preferred combination of sweep gas consumption and vacuum level is a volume flow rate of the sweep gas of 0.8 Nm ³ /hour and a vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes of 90 Torr (12 kPa) when the seawater temperature is 27.5 °C.

When the seawater temperature is between 27.5-36 °C, the above effect of the invention is obtained in practice when the volume flow rate of the sweep gas is in one of the following ranges; from 0.75 to 1 .6 Nm ³ /hour, more preferably from 0.75 to 1 .4 Nm ³ /hour, or even more preferably from 0.75 to 1 .1 Nm ³ /hour, combined with any of the following ranges of the vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes of; from 50 to 120 Torr (6.67 to 16 kPa) , preferably from 60 to 1 10 Torr (8 to 14.67 kPa) , or even more preferably from 80 to 100 Torr (10.67 to 13.33 kPa) .

An especially preferred combination of sweep gas consumption and vacuum level is a volume flow rate of the sweep gas of 0.8 Nm ³ /hour and a vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes from 80 Torr (10.67 kPa) to 1 00 Torr (13.33 kPa) when the seawater temperature is from 27.5 to 36 °C.

An especially preferred combination of sweep gas consumption and vacuum level is a volume flow rate of the sweep gas of 0.8 Nm ³ /hour and a vacuum level at the lumen side of the hydrophobic microporous hollow-fibre membranes of 100 Torr (13.33 kPa) when the seawater temperature is 36 °C.

By degassing the seawater to an acceptable level of oxygen content to be used as injection water and/or process water by only one degassing step, the invention according to one aspect enables a considerable cost saving for offshore hydrocarbon extraction by using a relatively small number of membrane contactors (the present invention manages with less contact membranes as the system disclosed in e.g. US 2010/0230366 which teaches using three contactors assembled in series. Thus, the present apparatus has a significantly reduced footprint (required installation area) and weight, both very important aspects when designing offshore platform. The sweep gas consumption and the required vacuum formation capacity also becomes significantly less.

VERIFICATION OF THE INVENTION

A membrane contactor sold under the trademark name Liqui-Cel® 8x80 Extra-Flow by the 3M Company has been tested. This membrane contactor has a bundle of 2210 mm long micro-porous polypropylene membranes with a total surface area of 242 m ² . The seawater was filtered to remove particulates and small animals etc. before being passed into the membrane contactor.

Table 1 represents the obtained test results with a seawater temperature of 20 °C, sweep gas at 99.9 % purity and seawater flow volume rate of 15 m ³ /hour. As seen by Table 1 , the vacuum load (i.e. the suction capacity of the vacuum pump) decreases considerably if the membrane contactor is run with a sweep gas pressure/vacuum level on the lumen side of about 10 kPa while the average oxygen content of the degassed seawater is about 14 ppb, and the maximum oxygen content due to the normal variation in contact membranes may become as high as about 24 ppb. However, if the standard safety-precaution which demands that the maximum oxygen content due to the normal variation is to be kept below 20 ppb is applied , i.e. that the average oxygen content needs to be lowered to less than about 12 ppb, we see from Table 1 that the vacuum load increases by a factor of two or more.

Table 2 represents the obtained test results with a seawater temperature of 27.5 and 36 °C, sweep gas at 99.9 % purity and seawater flow volume rate of 12 m ³ /hour. As seen by Table 2, for seawater temperature of 27.5 °C, the vacuum load (i.e. the suction capacity of the vacuum pump) decreases considerably when the membrane contactor is run with a sweep gas flow rate of 0.8 Nm ³ /hour and a sweep gas pressure on the lumen side of above 12 kPa. The resulting average oxygen content of degassed seawater is about 14 ppb, and the maximum oxygen content due to the normal variation in contact membranes is as high as about 24 ppb. However, if the standard safety-precaution which demands that the maximum oxygen content due to the normal variation is to be kept below 20 ppb is applied, i.e. that the average oxygen content needs to be lowered to less than about 12 ppb, we see from Table 2 that the vacuum load increases by a factor of about 1 .3 or more.

For seawater temperature of 36 °C, the vacuum load (i.e. the suction capacity of the vacuum pump) decreases considerably if the membrane contactor is run with a sweep gas pressure on the lumen side of 13.33 kPa. The resulting average oxygen content of the degassed seawater is about 14 ppb, and the maximum oxygen content due to the normal variation in contact membranes may become as high as about 24 ppb. However, if the standard safety-precaution which demands that the maximum oxygen content due to the normal variation is to be kept below 20 ppb is applied , i.e. that the average oxygen content needs to be lowered to less than about 12 ppb, we see from Table 2 that the vacuum load increases by a factor 1 .3 or more.

Merely the same effect is shown when the sweep gas flow rate is 1 .6 Nm ³ /hour as shown in table 3.

Table 1 -3 show the results when running with a sweep gas having an oxygen level of less than 0.1 % . By increasing the purity of the sweep gas i.e. to an oxygen level of less than 0.01 %, a decrease in the maximum and average 0 ₂ content in the degassed seawater is obtained due to the increased gas transfer efficiency.

COMPARISON TEST

Recommended specifications by the supplier of the Liqui-Cel® 8x80 Extra-Flow membrane contactor, indicates that the membrane contactor should be applied with a sweep gas volume flow rate of 2.7 Nm ³ /hour and a vacuum at 50 Torr when the water flow volume rate is 15 m ³ /hour and the sweep gas contains less than 0.1 weight% oxygen.

With these parameters combined with the standard safety-precaution of a factor of 1 .7 times the average oxygen content is applied, i.e. the average oxygen content should be no higher than about 12 ppb to ensure that the maximum oxygen content does not exceed 20 ppb, it is obtained the following results summarised in Table 4 for a similar seawater flow of 15 m ³ /hour, a sweep gas volume flow of 2.7 Nm ³ /hour and vacuum of 6,67 kPa (50 torr) at the lumen side for seawater tem eratures ran in from 4 to 1 3 °C:

The results of Table 4 compared with the results of Tables 1 , 2 and 3 show that the vacuum load when running the membrane contactor as recommended by the supplier gives a vacuum load of about a factor of four times higher than obtained by the present invention and about three times as high nitrogen consumption . This gives a significant increase in the energy consumption for running the vacuum pump and producing the nitrogen gas, which is calculated to result in about 20 % less cost-efficient deoxidation facility than the present invention . That is, the reduced footprint and energy consumption of a membrane contactor facility for producing injection water and/or process water from seawater running with the process parameters of the present invention , may produce a similar amount of injection water and/process water at about 20 % less operation costs than a similar facility run as recommended by the supplier.