Patent Application
20180154282
The present invention relates generally to the field of separation of water, oil and gas from a mixture of water, oil and gas flowing in a straight pipe or straight channel.
To be able to remove water from a mixture of water and one or more other fluids, for example oil and gas, is of great significance in various technical processes. These processes relate to different types of dean chemical industries where clean phases are required, as well as to offshore fields where it is desirous to remove the water content in the produced oil and gas—a problem that increases during the lifetime of the field and limits the field's economical lifetime.
Separation by use of dynamical processes, i.e. based on centrifugal force, easily creates large shear stresses and problems by producing emulsions. Separation by use of gravitation only presupposes the velocity in the pipe/separator to be low enough for the flowing mixture to flow in separate layers (i.e. stratified flow), which puts limitations on velocity, and may cause significant volumes/dimensions.
Examples of references in the field include the following. Norwegian Patent No. NO 321386 to Norsk Hydro/Statoil, filing date: May 28, 1997, discloses separation preferably of e well fluid from a horizontal producer. Stratified water/oil, not gas, is employed. Level measurement with multilevel gamma radiation or water in oil and oil in reinjected water. No methodology is investigated for inlet or stratified flow. Residence time for densely packed layers in liquid which gives separator length: only data from normal large diameter separator tests. The outlet was not tested and tilting of separator is not mentioned in the patent. The main conclusion is that necessary technical details are not included in the patent at the filing date (May 28, 1997) or by patent granted date (Feb. 5, 2006).
US Patent Application US 2002 059866 A1, filing date Sep. 13, 2000 refers to patent NO 321386 to Norsk Hydro/Statoil. The specification discloses separation and possible reinjection of gas and water from a producer which tilts negatively 1-10 degrees. One step (gas and water are separated by gravitation) or two steps (gas in first step and oil/water in the next step) are employed.
Features include stratified water/oil and gas; removal of droplets in gas with swirl separator before compression and reinjection in reservoir; (droplet erosion, deposit of asphaltenes and the like); level measurements in gas/liquid- and oil/water separators.
Technical status: No methodology investigated for either inlet or stratified flow. Effect of gas with regards to tear-off of droplets from the gas/liquid interface was investigated at low and high pressures, but with small liquid volumes. Residence time for densely packed layers in liquid which gives the separator length: refers to patent 321386 to Norsk Hydro/Statoil. Outlet: refers to patent 321386 to Norsk Hydro/Statoil. Tilting of separator: air/water atmospheric tests for gas/water.
Main conclusion is that the necessary technical details were not included in the patent application by the date of filing (Sep. 13, 2000) nor when filed patent application was published (Sep. 13, 2001).
Norwegian Patent NO 20001954 to Kvaerner Oilfield Products As, filing date Apr. 13, 2000. Discloses methods and devices with separators. The patent relates to outlets from separator for gas/oil and water.
Technical status: Gas not included as fluid in the basic investigations for the patent. Calculations (CFD) give basis for detailed geometry as described in the patent. The outlet geometry was as such described in the patent without tests. Later in year 2000 tests with this geometry were performed. The results were not satisfactory.
The present invention is directed to both a method for ensuring optimal conditions for separation, and a separator device for extraction of water, oil and gas from a mixture of water-oil-gas flowing in a straight pipe or straight channel. A separation system consists of an inlet pipe followed by a transition to the separator.
The separator consists of a certain length of pipe with diameter larger than or equal to the inlet pipe and several inclined and profiled holes along the separator pipe wall. Water and oil/gas are separated such that dispersed flow (2-3 m/s) in the inlet is reduced to velocity equal or less than separated flow (0.7-1 m/s) at the outlet of the separator.
The method is based on the following phenomena. The velocity of the layered flow pattern is obtained at the outlet of the separator. Activation of surfactants in oil, which may create stable emulsions, is avoided. Significant flow of droplets in the gas is also avoided. Transition between inlet pipe and separator must have such a small angle that new droplets are not induced. Dense bed of droplets must coalesce before the outlet of the separator. Finally, gas/liquid slug flow must not occur.
The foregoing phenomena have been studied mainly with high-pressure tests. Allowed values for fluid parameters and geometry have been found. The limiting values have been included as part of the method.
The device utilizes the separating ability of inclined and optimally rounded and profiled holes in a pipe wall discovered and proven by the inventors for separating water and oil being extracted through the inclined holes from a water-oil mixture flowing in the pipe.
The device consists of a horizontal pipe with one or more inclined holes, or groups of holes, located in axial lines in the pipe wall at the bottom of the pipe for extraction of water, and one or more groups of inclined holes in the pipe wall at the top of the pipe for extraction of oil and possible gas. Diameter of inclined hole: 3-10 mm, inclination angle: 10-15 degrees, radius of curvature of wall at the inlet to hole: 2-5 x hole diameter.
Testing of the device has been performed by using a 20 mm ID pipe where an oil-water mixture was flowing. For separation of water, 10 inclined holes were provided in the pipe wall lying on an axial line at the bottom of the pipe. Hole diameter 3 mm, inclination angle 15 degrees and axial distances between holes 100 mm. For oil separation, the same pipe section was used by turning it 180 degrees around so that extraction of oil could be performed at the top of the pipe.
Limits for the separation ability of the inclined holes have been found. The separation capability for the individual holes in a group (an axial line) and located relatively close to each other is the same as when the single holes are acting alone; i.e. no negative effect of neighbourly has been observed. This is the case both for water separation, when the holes are located in a line at the bottom of the pipe, and for oil separation with the line of inclined holes located at the top of the pipe.
Limits for water cut WC for a certain velocity U.mix yielding good separation with the inclined holes have been found. For water continuous flow the separation efficiency for water=>1. Oil continuous flow yields separation efficiency for oil=>1. The efficiency is independent on the extraction velocity.
Maximum water separation (1.0) is obtained when WC is lying above a certain lower limit which is dependent on U.mix; and maximum oil separation (1.0) is obtained when WC is below an upper limit. Between them, water separation and oil separation=>1 is simultaneously achieved. For U.mix=1.0 m/s both water separation and oil separation=>1, when 0.32<WC<0.68 and for U.mix=1.5 m/s when 0.42<WC<0.58. For U.mix=2.0 m/s the limits are: 0.48<WC<0.54.
The range within a WC-U.mix diagram providing both maximal water separation maximal and oil separation, has the shape of a triangle. If the starting point for the separation is lying outside the triangle, the separation process is divided into two periods. In the first period, the dominant phase is extracted until the internal of the triangle is reached, or until the flow has become stratified. From that point on water and oil are sucked off in parallel until the velocity becomes 0. In that way the variation of the water cut and the mixture velocity along the pipe axis, can be calculated.
In many technical processes, it is of utmost importance to remove water from a liquid mixture, for example form a water-oil mixture. This can be operated with two liquids. Provided that the water is the heaviest of the phases, the water can be removed using the profiled inclined holes when they are provided in the bottom, even when the phases are well mixed.
The advantage of this separator compared to a pure gravitation separator is that the separation ability of inclined and optimally rounded holes, even for well mixed phases, makes separation possible for significantly higher velocities than stratified flow (2-3 times larger). This leads to reduced dimensions of pipes and other equipment.
Use of the separator downhole or subsea means that produced water in the well can be removed by re-injection to the reservoir by a deviated well. This removal of water can be performed on an early stage of the process, which leads to extended operation of an oil- or oil-gas well before shutdown.
To be able to remove water from a mixture of water and one or more other fluids, for example oil and gas, is of great significance in various technical processes.
The present method and equipment for separation, use of the specific separation capability of the inclined holes, can be applied even when the mixture velocity is significantly high. Even at a velocity of 2-3 times the velocity at stratified flow, the separation principle according to the invention has shown to give very close to clean water and clean oil from the mixture. Separation under considerable flow velocities (2-3 m/s) will result in that the dimensions of the separator is reduced. The separator is expected to be applicable for removing water on an early stage of the process, and thereby contribute to extending the profitable operation of an oil- or oil-gas well.
The foregoing and other features and advantages of the present invention will become apparent from the following detailed description of the invention, taken in conjunction with the accompanying drawings, which are briefly described as follows.
The present invention relates a separation system for separation of water, oil and gas from a flowing mixture of these phases. With reference to
Water and oil/gas is sucked off through these inclined holes so that the mixture flow velocity in the pipe reduces from dispersed flow (velocity 2-3 m/s) at the inlet of the separator—to a velocity less than or equal to the velocity for stratified flow (0.7-1.0 m/s) at the outlet/end of the separator.
The present invention includes a first embodiment directed to a process for providing suitable flow in the separator pipe and a device (i.e. the geometry and the location of the inclined holes) for removing water, oil and gas along the separator.
Tests (de Boer plot) demonstrate that density of oil and pressure at reservoir conditions determine if the important surfactant asphalts are activated. Without active surfactants, the separation of water from oil by continuous flow is rapid (30-60 seconds). The tested oils are so-called “live oils” that never have been pressure reduced, i.e. having the pressure reduced to lower than reservoir pressure. The oils have been collected in pressure bottles from the reservoirs. The separation time with pressure released oils (atmospheric pressure) increased from 30 seconds to 120 minutes. The reason is that surfactants in the oil prevent coalescence of the water droplets.
Tests demonstrate that droplets in the gas are avoided, see the graph of
To avoid generation of new drops in the connection (conical tube/diffusor) the outlet angle between the inlet pipe and the separator must be less than 10-20 degrees.
With water, continuous flow oil droplets may concentrate below the oil phase and produce a densely-packed layer. This layer will not coalesce against the oil until after a certain residence time.
The required residence time has been recorded in full scale tests for condensate. The length of the separator has to be increased to satisfy the required residence time for a dense packed layer of droplets. A too short separator will fill the entire system with droplets and separation will not be satisfactory.
The residence time is given by the following expression:
t=K*Cp*(ID/0.254) (seconds)
where:
The results shown in Table 1 for the oils investigated confirm that the expression above gives a satisfactory estimate for a new oil downhole when the diameter is equal for the separators.
We get the following limit value for superficial liquid velocity VLS-critical which gives a limit against stratified flow:
For light oil/water/gas with the above limiting value, the water phase velocity VW (with dense droplet layer) will be as shown in
VW varies mainly with VGS and not much with the water cut.
For oil/water flow it has been observed that the critical superficial liquid velocity VLS (m/s) is mainly a function of ID of the tube when the physical parameters for water and oil and water cut has been decided.
This parameter shows when a separated flow is going over to a dispersed flow as a function of diameter in
From the data in
V
L
S=1,8162*ID0.237 (m/s)
Tests demonstrate that slug flow in the inlet will continue in the separator and prevent stratified flow.
The critical superficial liquid velocity VLS (m/s) to avoid slug formation is mainly a function of gas density (ρ kg/m3)—see
VLS−critical−1.12*Cp−0.16*(−2E05* ρ2+0.01*ρ+0.051) (m/s)
Superficial liquid flow equals:
The example has different ID for inlet and separator. Resulting possible production rate to experience stratified gas and liquid in the separator is shown in Table 2.
The example has equal ID for inlet and separator. The results are given in Table 3. Depending on ID and gas volume, the produced volume of liquid is sufficient for the most oil-rich fields.
Critical superficial liquid velocity VLS (m/s) is substantially a function of tube diameter when physical parameters for water and oil and water cut are decided. For Crude oil 1 the flow with ID 8-12″ will be stratified in the inlet and the separator. Substantial rates can be obtained—see Table 4.
The device utilizes a phenomenon discovered by the inventors and experimentally proven that liquid being extracted from a flowing mixture of water and oil through inclined holes in the pipe wall undergoes a separation that takes place in the inlet to the inclined holes itself. The holes have a significant separating ability being due to their inclination and the specific profile/rounding of the pipe wall area in front of the entrance to the hole—an effect that is lacking for holes being normal to the pipe wall.
The inclination, typically 15 degrees to the pipe wall, in addition to the pipe wall being rounded and optimally profiled at front of the hole entrance—yields for extraction from a highly mixed flow of oil and water (velocity 2-3 times higher than by stratified flow) pure water when the holes are located at the bottom of the pipe and pure oil when the holes are located at the top of the pipe—supposed the combination of water cut and mixture velocity is lying within limits that have been found experimentally.
By applying the specific ability of the inclined holes for separation, the separation process can start, and proceed, at significant higher velocity in the pipe (2-3 m/s) than the velocity for stratified flow (0.7-1.0 m/s). That means separation can proceed at reduced dimensions of pipe and equipment.
The pipe is surrounded by a collection jacket 5 for collection of the extracted water (
This annulus is equipped with a horizontally partition wall 5.3 which coincides with the main pipe axis 1 and divides the annulus in a lower chamber 5.1 for collection of water being extracted through the lower inclined holes 3.1 and an upper chamber 5.2 for collection of oil, and possibly gas, being extracted through the inclined holes 3.2 at the upper pipe wall.
Upper and lower chambers 5.1 and 5.2 may in addition be divided into sub-chambers 5.5 at the bottom and 5.6 at the top which are made up by vertically mounted partition walls 5.4 as shown in
By use of regulation valves at the outlets from the various sub-chambers, the corresponding group of inclined holes may be switched on or switched off.
The separation principle, both the separation capability of the individual inclined holes and the separation capability for a group of inclined holes, has been verified by applying a test section as shown in
The test section consisted of a 20 mm. ID horizontal pipe 1 with 10 inclined holes located on an axial line at the bottom of the pipe. Hole diameter was 3 mm, hole inclination angle 15 degrees, internal distance between holes 100 mm and the holes were covering 1 meter of the pipe length. Through the test section a mixture of water and Shellsol D70 was flowing.
By regulating a valve connected to each inclined hole, extraction from each individual hole was performed, such that the extraction velocity Uh from each hole could be varied from hole to hole. Both extraction velocity Uh and the separation quality WC.sep of the separated liquid were measured for each hole and experiments were run for different velocities U.mix and water cuts WC.pipe in the pipe.
For the inclined holes at the bottom of the pipe both water cut WC.sep for the separated liquid and the extraction velocity Uh from each hole was measured for a variety of velocities U.mix and water cuts, WC.pipe in the pipe.
By turning the pipe 180 degrees around, so that the inclined holes were located at the top of the pipe, the oil cut OC.sep of the extracted liquid and the corresponding extraction velocity Uh were measured for each inclined hole for a large set of velocities U.mix and inlet water cuts, WC.pipe, in the separator pipe. Extraction through inclined holes yields pure phases when water cut and mixture velocities are lying within limits that have been found.
The separation quality WC.sep for water extraction at the bottom of the pipe is shown in
The separation quality or oil extraction at top of pipe is shown in
In
In
The range of WC yielding both WC.sep=>1 and OC.sep=>1 is shown for 4 values of U.mix in Table 5.
In
In
In
In
The effect of rounding the transition area 4 at the edge C in
In
To obtain separation quality=>1 for both oil and water it is necessary to regulate the extraction process along the pipe axis in a way that separation proceeds within the triangle in
Each sub chamber then includes and covers a selected group of inclined holes. Extraction from the various hole groups then are controlled hydraulically in a way that the actual hole group either is joined in or switched off. The total development of the separation along pipe axis then can be regulated so that the separation proceeds internally in the triangle in
Separation cases may occur where the process starts with e.g. a high water cut and significant mixture velocity U.mix (for example>1.5 m/s). The starting point is then lying outside the triangle in
The first group of holes for water extraction then are kept closed, until the internal of the triangle is reached (to point B). From that point on oil and water are extracted simultaneously until U.mix=0.
Selected geometry of separator: Separator pipe: ID=20 mm. The separator pipe is equipped with inclined holes at the top of the pipe and at the bottom of the pipe, in accordance to
Both of these are again divided by vertical partition walls 5.4 in 8 sub chambers at the upper part of the pipe and 8 sub chambers at the lower part of the pipe, all of them being of 0.5 m length. Total length of the separator then becomes 4 meters.
Each sub chamber at the top is covering 12 inclined holes at the top of pipe and every sub chamber at the bottom is covering 12 inclined holes at the bottom of pipe. From each of these sub chambers liquid is extracted through a valve which is either open or closed.
The starting point A in
Period 1: From A to B: Only oil separation is performed. The 6 groups of holes at the bottom of the pipe are closed. Only the first 6 groups of holes for oil extraction at the top of the pipe are functioning. The water cut then increases while the velocity in the pipe U.mix decreases. Point B in the middle of the triangle then is reached. Here WC has increased to=0.50 and U.mix has decreased to 0.60 m/s. (If sand is present the minimum value of U.mix will be that corresponding to stratified flow: 0.72 m/s).
Period 2: From B to C: Here both oil and water are extracted in parallel until U.mix=0 Totally 8 inclined hole groups for extraction of oil and 4 hole groups for extraction of water are required.
The separations performance along the pipe axis is shown in
The starting point A in
Only water separation extends well—i.e. only water has to be extracted until the triangle is entered. The water cut then decreases together with the mixture velocity U.mix. In the first 4 hole groups, only water is extracted (the corresponding 4 hole groups for oil are closed). The point B in the diagram then is reached. Here WC has been reduced to=0.50 and U.mix has been reduced to 0.30 m/s.
Here oil and water are separated in parallel until U.mix=0 m/s (If sand is present, U.mix minimum should be equal to stratified flow: 0.72 m/s).
Totally 9 groups of inclined holes are applied for removal of water and 4 hole groups for removal of oil.
The proceeding of the separation along the pipe axis has been shown in
Here too the total separator length becomes 4 meter: That means 48 inclined holes at top of pipe and 108 inclined holes at bottom.
Here the starting point A is lying internally in the triangle in
Point B, where simultaneous extraction of the two phases starts, may not lie exactly at the center of the triangle (i.e. at WC=0.50)—which has been chosen in the selected calculation examples. It is sufficient to extract the dominating phase until the triangle is entered—and from that point on both phases can be separated simultaneously until the velocity has become 0.
For several separation cases it is possible to reach the velocity for stratified flow (0.72 m/s for a 20 mm. ID pipe) immediately when the internally of the triangle is reached. When the flow has become stratified both phases are clean—such that water removal and oil removal then can proceed parallel simply and fast.
In order to keep water in oil and oil in water below acceptable levels samples of the separated liquids are taken. The water content in the oil coming from the seabed or platform is measured (by down-hole application). Measurement of the oil in water to be re-injected is performed by letting water from the separator be conducted with driving liquid returning from the pump to a dry place (for example an unmanned platform) where the concentration of oil is measured. The quantity of re-injected water is regulated by the pump so that the separation of water satisfies the required purity of the produced oil and the re-injected water. This regulation system has been tested and is applied for hydraulic managed down hole pumps for re-injection of water.
The level of water is regulated so that it corresponds to the required purity of oil and water.
Purity as high as possible (25 ppm) for both oil and water outlet from separator can be obtained by letting the separation process described above be repeated in two or more subsequent stages for the water and the oil outlet from Stage 1.
‘Stage 1’ then consists of the separator pipe as described in
The ‘Stage 2-water’ cleaning proceeds in a pipe similar to
The oil is treated in a similar way. That oil, after being extracted from the top of ‘Stage 1’, is lead through a ‘Stage 2-oil’—having inclined holes only at the bottom. From these holes water is extracted which may be somewhat polluted by oil. This water is re-circled—i.e. it is lead back to the inlet of ‘Stage 1’.
For cases of very strict requirements as for purity two third stages—two ‘Stage 3’-ones—being similar to these two ‘Stage 2’-ones—are applied in a similar way.
For three phase water-oil-gas flow the gas is separated from the liquid flow by extracting gas through the inclined holes at top of pipe, either alone or together with the oil. If the amount of gas is significant (e.g. UGS>2.5 m/s for a 250 mm pipe, 90 bar) gas only is extracted throughout the first inclined holes at top of pipe (same arrangement as in
If the amount of gas is low or moderate (UGS<2.5 m/s for a 250 mm pipe, 90 bar) the oil and the gas are extracted simultaneously and thereafter the procedure becomes similar to that of oil and water flow alone. Avoiding steady sand bed in the separator system
Sand filters enclosing the producer will ensure maximal sand particles to be approximately ⅓ of the opening in the sand filter. Maximum size of particles passing through standard filters will have a diameter of approximately 80 micrometers. In general the sand/particles are moving as a bed (or distributed in the liquid for larger diameter of the separator) when the velocity corresponds to stratified flow—i.e. the minimum velocity applied in the separator. Steady state sand bed is avoided by the velocity of stratified flow—which is the lower velocity for the separator—being higher than the velocity of a moving sand bed for different pipe diameters—as shown in Table 6.
The inclined holes at bottom of separator are located in a way that sand/particles for normal production passes through the inclined holes near bottom of separator and out in vertically part of annulus containing separated water.
The velocity of the fluids is sufficient low, 0.7-1.5 m/s, for sand erosion not to be significant. Removal of possible stationary sand particles is carried out by applying a flexible tube (e.g. applying the Schlumberger system).
The separator can be applied for cleaning of an oil-water mixture for velocities 2-3 times that of stratified flow—supposed no emulsion is present.
5% Excel D70 oil was mixed in a water tank and the mixture was left for 2-3 days. After passing through a centrifugal pump and separator, a stabile emulsion was produced (lifetime: several days). In order to avoid emulsion in the experiments presented, a height difference for driving the mixture was applied.
