WO2001066910A1

WO2001066910A1 - Device and method for separating oil and water

Info

Publication number: WO2001066910A1
Application number: PCT/NO2001/000100
Authority: WO
Inventors: Torleif Holt; Ole Torsaeter
Original assignee: Torleif Holt; Ole Torsaeter
Priority date: 2000-03-08
Filing date: 2001-03-08
Publication date: 2001-09-13
Also published as: AU3959901A; NO20001184D0

Abstract

A device for separating oil and water, positioned in a production zone (7) in a bore hole (6) in a reservoir, wherein a selective membrane (3) is utilized. The device is connected at an upper end to a production pipe (8) transporting oil to a discharge area, and if necessary means are available for regulating the pressure in the bore hole so that the inflow of reservoir fluids to the bore hole (6) is secured. In the production zone (7) the device includes hydrophobic membrane (3) being selective for oil, as the membrane (3) is designed to withstand the radial pressure that will appear during oil production in the production zone. A method for separation of oil and water down in a production zone in a bore hole, is also described, by applying the described arrangement.

Description

Device and method for separating oil and water

The invention relates to a device for separation of oil and water during oil production and a method for production of oil, according to the independent claims.

Background.

A formation containing petroleum (petroleum reservoir) comprises a porous rock with a pore volume filled with hydrocarbon gas, oil and water, or oil and water, or gas and water. During production from the reservoir a pressure field will be created around the bore hole causing an influx of oil. In some cases the pressure can be so large that water and/or gas are also forced into the bore hole. During the production period of the reservoir the amount of oil will decrease, and eventually a gas/oil interface will move downwards towards the production zone, while an oil/water interface will move upwards. The result is that the amount of water and gas increases during the production period of the reservoir. The increase of water and/or gas may also be caused by the fact that the production phase is in the gas or water zone, that the reservoir has inhomogeneities making it particularly favourable for gas and/or water to flow towards the production zone, or other factors. The result will be that water and/or gas are produced together with the oil, and this is undesirable because the oil production rate decreases and the mixture must be separated. Oil, gas and water are separated in a process plant, and when there is a large amount of water and/or gas a correspondingly high capacity separation plant will be necessary, requiring much space and expense. By utilizing a device that separates water from gas and oil in the production zone, the processing of produced fluids will be substantially simplified. In an installation at the sea bed, separating water at the well head is especially favourable, since transport of water, oil and gas in the same pipeline can cause problems due to the risks of hydrate formation.

Several attempts have been made to develop these kinds of separation devices using selective membranes. US 5,673,752 describes production of gas from a reservoir containing gas and water. The method includes use of a hydrophobic membrane letting the gas through, but not water. The membrane is arranged as a spiral module down in the bore hole. No system for production/reinjection of water or cleaning of the membrane is described. US 4,296,810 describes separation of oil / water down in the bore hole by using water wetted (hydrophilic) membranes packed as spiral modules. Pumps are used for separation and reinjection of water, the pumps being placed down in the bore hole.

WO Al 95/09970 shows a system for separation of oil / gas and water with cyclones and separation of undesired gases (CO₂, H₂S, H₂O) from the hydrocarbon gases by use of membranes.

US patent 6,015,011 describes a device for separation of hydrocarbons and water down in the bore hole, based on the use of one or more filters allowing transport of gas and oil at given differential pressure across the filter, but retaining the water. Selective functionality is related to permeability effects, relative size and dissociation properties of water molecules. This results in a flow of gas through the filters at a lower differential pressure than oil, which again will flow through at a lower differential pressure than water. It is also stated in the patent that the filter can be designed so that only fractions of the oil are produced. This can be possible if the oil has components as particles/aggregates, but these conditions will clog the filters, within short time.

Object

The main object of the invention is to provide a simple device for separation of oil and water in the production zone in a petroleum reservoir. Additional objects are that the device should have a long lifetime, and be simple to maintain without the need for bringing the equipment to the surface. The device should not require much space and should not result in significant reduction of the production rate from the reservoir.

The invention.

The object is met with a device according to the characterizing part of claim 1, and a method according to claim 8. Further advantageous features are stated in the related dependent claims. With a device according to the present invention, water/oil separation will be obtained by allowing oil to flow through a hydrophobic membrane. There are significant differences between separation of oil from water, and separation of gas from water. It is essential to the invention that the oil and not the water is allowed to flow through the membrane. Membrane is in this context defined as any material which is oil wet, or by treatment can be made oil wet, and which has small enough pores that the entry pressure for an undesirable phase (in this case water and possibly gas) will be so large that it will not flow into the pores at the pressure conditions occurring in the production zone during oil production. The membrane may either have such a construction, that it will require mechanical support upon the use in question, or the membrane can be constructed with several reinforced layers, so that the membrane itself will be strong enough to keep its shape at the pressure conditions arising in the production zone during oil production. In most cases water will also flow towards the production zone during oil production. If the water is not produced/removed from the production zone, it will accumulate and the oil transport towards the production zone will be reduced, and may in some cases stop completely. In order to remove water around the production zone, the water can be separated from the oil with a device according to claim 1, and transported away. Whether the water should be removed continuously, in intervals or not at all, will depend on the amount of water flowing into the production zone at any time.

In order to maintain a water/oil separation process with an acceptable oil flux for a long period of time, most reservoir types will require cleaning of the membrane at regular intervals. The device according to the present invention has a simple construction for separating oil and water, which makes simple cleaning of the membrane possible. The device is part of a total system for oil production/reinjection of produced water, and cleaning of the membrane. The system has the further advantage that there are no moving parts in the bore hole.

The pressure in the bore hole is regulated with well known techniques. In cases where these techniques require separate devices, these can be placed on the surface, for instance on a production platform, or a subsea installation. When the pressure is lower in the production zone than the surrounding formation in the reservoir, the reservoir fluids will flow into the production zone.

In cases where water is contacting the membrane, it will not flow through the membrane in the separation device, unless the pressure in the water phase is exceeding the entry pressure given by equation (1), below. Examples of entry pressures are given in Table 1. The calculations are performed for a completely oil wet material, with two different pore radii.

Table 1 Calculation of entry pressure for water in an oil wet material.

Pore radius (μm) friterfacial tension ^a Entry pressure (bar) 0,1 lO mN/m 2

0,02 lO mN/m 10 ^a The interfacial tension used in the table is representative for oil/water systems but will vary To avoid the membrane being pressed flat when oil is flowing through the membrane and into the production pipe, the membrane must be reinforced. The membrane can either be constructed of a material that is sufficiently stiff to keep its shape during the whole or at least part of the lifetime of the reservoir, or the membrane can cover a stiff supporting pipe with perforations. In the latter case there will be no requirements of stiffness of the membrane material itself. Another possible embodiment is a combination of the above mentioned solutions, wherein the membrane has an inner reinforcement, providing sufficient strength.

Current membrane materials can be for instance, polytetrafluorethylene (PTFE), polyvinylidenefluoride (PNDF), polypropylene and polyethylene. It is possible to produce oil wet membranes in several ways, for instance by coating a micro grid system with e.g. teflon, or to expose a film of synthetic material to radiation to obtain micro holes.

A membrane often consists of several layers. The selective layer is often very thin compared to the total thickness of the membrane, and is therefore placed on one of several supporting layers of material with gradually increasing pore size. The inner supporting layer may be a perforated layer of e.g. metal or synthetic material.

The radial pressure in various wells can differ, and the pressure may change during the production period. The radial pressure acting on the membrane will, however, in most horizontal wells be lower than the corresponding pressure in vertical wells. The requirements for the membrane or the inner supporting pipe material, are therefore larger regarding the capacity to maintain a given shape, in cases where the device according to present invention is used in a vertical bore hole, as compared to a horizontal bore hole.

In this context radial pressure is defined as the pressure difference between the pressure at the bore hole wall and the pressure in the production pipe, in a direction approximately perpendicular to the length axis of the production pipe. By using a separation device according to the present invention in horizontal wells, the separation device for oil and water can cover the whole length of the well, which can be a kilometer or more. The whole or parts of the membrane will in most cases be in direct contact with the oil phase, and oil may therefore pass through the membrane without being in contact with water.

Example

The device according to the present invention will be described in the following, with reference to figures, where Figure 1 shows a drop of water on the surface of an almost completely hydrophobic material,

Figure 2 shows an embodiment of the device according to the present invention,

Figure 3 shows a sketch of a system for production of oil by applying a separation device for oil and water, according to the present invention, and Figure 4 shows a sketch of a system for reinjection of water, preferably added a cleaning agent for cleaning the membrane in the device, according to the present invention.

Referring firstly to Figure 1, water 1 will enter into an oil filled pore 2 in a porous hydrophobic material 3 if the excessive pressure in the water phase 1 compared to the oil 2 is larger than Δp given by

Δp = -2γcosφ/r (1)

where γ is interfacial tension between oil and water and φ is the contact angle between oil 2, water 1 and the porous material 3, and r is the pore radius of the porous material 3. For a completely oil wetted material 3 the contact angle will be approximately 180°, i.e. cosφ will be close to minus one. This principle can be used to separate oil and water, provided that the oil is produced through a porous material with small enough pores to obtain a necessary high entry pressure for water. Separation of oil and water in a bore hole is obtained according to the above, with a device according to the present invention. In the embodiment shown in Figure 2, the device comprises an inner mechanical support 4, which is hollow and provided with perforations (not shown), e.g. a perforated pipe. The material and the thickness 5 of the wall of the inner support 4, must be chosen so that the support 4 will be sufficiently stiff to keep its shape in the conditions prevailing in the production zone 7.

The outer side of the mechanical support 4 is covered by a hydrophobic membrane 3 or another selective wetted material. The inner support 4, with the membrane 3 on the outside, is positioned in a bore hole 6, in the production zone. In the upper part of the production zone 7 the inner support 4 with the membrane 3 is mounted on a conventional production pipe 8 which transports oil to the surface. The production zone 7 is separated from the rest of the bore hole 6 in the upper part, by means of packers 16 to avoid the reservoir fluid flowing to the surface in the annulus between the bore hole 6 and the production pipe 8. Completely or almost completely water free oil flows through the membrane 3 into the mechanical support 4, and is then transported to the surface. In an alternative device, not shown, the membrane itself is produced of a material that is so strong that an inner perforated support is not necessary. In cases where the rock in the reservoir is unstable, a casing 9 can be positioned against the formation. The pipe 8 is perforated in the production zone 7, so that reservoir fluid can flow into the pipe and into a deposit zone 10 where water can flow out of the bore hole 6. A space is created, hereinafter denoted annulus 11, either between casing 9 and membrane 3, or membrane 3 and a wall of the bore hole 6. Other conditions may also be advantageous for the positioning of an outer pipe, for instance a casing that is perforated at least in the production zone and the deposit zone in the bore hole.

In some cases it may be relevant to fill the annulus 11 with granular oil wet material. This is to ensure capillary continuity between the reservoir and the membrane 3 in the device according to the present invention. This will ease the oil flow towards the membrane 3, even in the case where the annulus 11 is mainly filled with water.

A protecting cover 12 of metal or some other appropriate material can be placed around the hydrophobic membrane 3 to avoid the membrane being damaged. This cover material 12 will also give membrane 3 physical support during an eventual cleansing process and during mounting of the separation device. The production pipe 8 is held in a desired position, preferably in the centre of the bore hole

6 by supporting rings (not shown) placed between the bore hole and the production pipe. In cases where the bore hole 6 is supplied with a casing 9, the support rings will be positioned between the casing 9 and the production pipe 8. In the production zone the rings will be positioned between the cover material 11 and the casing 9, or in cases where the membrane 3 is not supplied with a cover material, the rings are situated between the casing 9 and the membrane 3. Such rings can be positioned at desired length-intervals.

Water flowing towards the production zone can be pumped into a water bearing geological layer (an aquifer), e.g. deposit zone 10, shown in Figure 3. In order to transport the water, a pump may be placed down into the bore hole, transporting water out of the production zone 7, and into for example deposit zone 10. This is, however, not an ideal solution, as it requires that movable - and relatively voluminous parts in positions in the bore hole 6. A better solution is to place a pump 17 adjacent the opening of the bore hole, i.e. on the sea bed or at the earth surface. This pump can occupy a larger area and will be relatively simple to maintain and repair, compared to pumps which are positioned in the bore hole 6.

In the last mentioned case, an additional pipe to the production pipe must be positioned in the bore hole. This pipe, hereinafter denoted deposit pipe 13, can either transport water from the production zone, or to the deposit zone. In the case shown in Figure 3, the annulus 11 in the production zone 7 ends up in the deposit pipe 13, and water is transported up through the pipe to the pump 17. From pump 17 water is transported, via a pipe 14, down into the upper part of the bore hole, between the casing 9 and the production pipe 8 and the deposition pipe 13, and then into the deposit zone 10. Contaminants, including solid particles like sand, clay and similar material may adhere to the surface of the membrane 3, and will with time reduce the oil flux. This phenomena evolves in most membrane processes and requires maintenance. The problem can be solved with well known techniques. According to the present invention, there is provided a combination of a simple system for periodic cleaning of the membrane, and the pump 17 which transports the water from the production zone 7 to a deposition zone 10 during normal production, a sketch of the system is shown in the Figure 4.

When the membrane has to be cleaned, oil production must be stopped and water (sea water) is pumped into the production pipe 8, by pump 17. If pressure in the water in production pipe 8 get sufficiently high, the differential pressure between the water phase 1 and the oil 2 will be larger than Δp in formula (1) and therefore the water will escape from production pipe 8 through membrane 3 ( see Figure 1). In this way, the water will clean the pores and the oil flux will increase to an acceptable level.

If a cleansing additive 15 is added to the water, which will be injected to the production pipe 8, the interfacial tension, corresponding to γ in the formulae (1), will decrease and the necessary pressure to force the water out through the membrane 3 will be lower. The membrane will, corresponding to the above mentioned case, be flushed clean so that the oil flux increases to an acceptable level.

Cleansing additive is in this context defined as any additive giving a sufficient decrease in interfacial tension, for water to flow through and clean the hydrophobic membrane. When a sufficient amount of water, alternatively with cleansing additive 15, for instance surfactants like etoxylated alkylsulphonates, is injected into the production pipe 8, the fluid in the annulus 11 of the well, can be pumped into the deposit zone 10, the aquifer, by using the same system as was used for transporting the water away from the production zone 7. After the cleansing process some water will be left in the production pipe 8 and this water has to be removed before restarting normal oil production.

The production pipe 8 can be provided with a valve operated branch 16 in the upper end. When normal oil production is restarted the oil that is transported from the reservoir will push ahead water left in the production pipe after the cleaning process, and water is removed through the valve operated branch 16, by removing the first part of the produced fluids. When the flow in the production pipe 8 is approximately pure oil, the valve is closed and the oil is transported to the production site, as normal.

Water left in the production pipe 8 after the cleansing process, can also be displaced into the annulus 11 by means of oil, preferably from a buffer tank (not shown in the figure). This oil will then displace the water into the annulus 11 and when normal production resumes, the oil will flow through the membrane 3 while water remains in the annulus 11 and is finally transported to the deposition zone 10.

The cleansing cycle as described above may be performed by means of a process pump 17 and a set of regulation valves, as shown in Figure 4. The specific detailed embodiment of the cleaning system is a question of optimization and several pumps, sensors and process regulation systems may be applied. The whole cleaning system is placed on the surface or on the sea bottom. For maintenance, single components or the whole system can be changed.

Hereinafter the invention will be illustrated with reference to a laboratory experiment. A hydrophobic, oil wet membrane, a PTFE membrane (polytetrafluoretylen) from Cole-

Parmer, 0,2 micro meter (pore size), was mounted in a standard filter apparatus. The apparatus was connected to an Erlenmeyer- beaker which was connected to a water beam pump creating approximately vacuum conditions in the beaker. Both water and lamp oil with a coloring additive were exposed to the membrane, as the water and oil were separated into two layers, the lower layer was water.

In the case where only water was in contact with the membrane no liquid flowed through the membrane.

The filter apparatus was then tilted, or otherwise rearranged, so that both oil and water came in direct contact with the membrane. Oil flowed through the membrane while the water was retained, and the flow continued as long as the oil was in contact with the membrane.

Insignificant amounts of water broke through some big pores where the entry pressure was less or equal to the differential pressure across the membrane which was approximately one bar in the experiment. It will be understood by persons skilled in the art that the present invention is not limited to what is shown and described above. The invention also comprises combinations and sub-combinations of the described features and modifications and variations of these which are obvious for a person knowing the state of the art within the following claims.

Claims

1. Separation device for oil and water positioned in a production zone (7) in a bore hole (6) in a reservoir, using a selective membrane (3), wherein an upper end of the device is connected to a production pipe (8) transporting oil to a discharge area, whereat it may be provided means for regulation of the pressure in the bore hole so that the inflow of reservoir fluids to the bore hole (6) is secured; characterized in that the device in the production zone (7), comprises a hydrophobic membrane (3), which is selective for oil and is designed to withstand the radial pressures occurring in the production zone, during oil production.

2. Device according to claim 1, characterized in that the membrane (3) encloses an inner perforated support pipe (4), wherein the support pipe (4) is designed to withstand the radial pressure that occurs during oil production in the production zone (7), and the membrane (3) bears against this.

3. Device according to claim 1, characterized in that the membrane (3) is made of a material having sufficient strength to withstand radial pressure occurring during oil production in the oil zone.

4. Device according to anyone of the preceding claims, characterized in that the device further comprises an outer pipe (9) close to the reservoir wall so that an annulus (11) between the outer pipe (9) and the membrane (3) is created, the pipe (9) is perforated at least in the production zone (7) and in a the deposit zone (10).

5. Device according to claim 4, characterized in that the outer pipe (9) is a casing.

6. Device according to anyone of the preceding claims, characterized in that the membrane (3) is enclosed by a perforated layer (12) for protection, preferably a grid structure.

7. Device according to anyone of the preceding claims, characterized in that a space (11) between the reservoir wall and the membrane (3), or between the casing (9) and the membrane (3), or between the casing (9) and the protecting layer (12) or between the reservoir wall and the protecting layer (12), is filled with a granular oil wet material.

8. Method for production of substantially water free oil from a reservoir which at least contains oil and water, comprising the mounting of a production pipe (8) extending along a bore hole (6), with possible regulation of the pressure in the bore hole so that the reservoir fluids are drawn into the bore hole (6), through the perforations in the outer pipe (9), characterized in that the production pipe (8) in the production zone (7) is connected to a separation device according to claims 1 - 7.

9. Method according to claim 8, characterized in that water is pumped, with a pump (17) down the production pipe (8) to clean the membrane (3).

10. Method according to claim 9, characterized in that cleaning additives (15) are being added to the water.

11. Method according to anyone of the claims 8 - 10, characterized in that water in the annulus (11) is transported to a deposit zone (10) via a pump, said pump preferably placed on the surface adjacent the bore hole (6).