OA12123A - System for producing de-watered oil. - Google Patents

Abstract

System for producing de-watered oil from an underground formation (2) to the surface (4), which system comprises a reception well (7) having a substantially horizontal or inclined section (10) for primary oil/water separation of the well fluid; a water discharge system (12) having an upstream end (13) that is capable of receiving during normal operation liquid from the lower region (14) of the downstream part (9) of the reception well (7); and a secondary underground oil/water separator (18) having an upstream end (19) that is capable of receiving during normal operation liquid from the upper region (20) of the downstream part (9) of the reception well (7), the secondary separator having an outlet (21) for de-watered oil that is in fluid communication with the inlet (5) of a production well and an outlet (22) for a water-enriched component that is in fluid communication with the water discharge system (12).

Description

1 12123

The présent invention relates to a System forproducing de-watered oil from an underground formation.

In the spécification and in the daims, theexpression 'well fluid' will be used to refer to a fluid 5 comprising hydrocarbon oil and water that is received by a system according to the présent invention from anunderground formation. Further, hydrocarbon oil will bereferred to as oil.

The présent invention relates in particular to a10 system, wherein a well fluid can be separated underground, such that oil is produced to the surfacethat has been de-watered below the surface. It will beunderstood, that the surface may also be the bottom ofthe sea. 15 International patent application publication

No. WO 98/41304 discloses a system for producing oil froman underground formation in accordance with the pre-ambleof claim 1, which system comprises a production well extending downwardly from the 20 surface and having an inlet below the surface; a réception well penetrating the underground formation and capable of receiving well fluid therefrom,wherein the downstream part of the réception wellcomprises a substantially horizontal section; and 25 - a water discharge System having an upstream end that is capable of receiving liquid from the lower région ofthe horizontal section, wherein the inlet of the production well is arrangedto receive liquid from the upper région of the horizontal 30 ‘ section. 2 12123

During normal operation of the known System, the flowof well fluid is selected such that the well fluid isseparated in the horizontal section. Liquid layers areformed in the upper and lower régions of the horizontalsection, and an interface is formed between the layers.Near the downstream end of the horizontal section theliquid flowing in the lower région is a water-richcomponent, and the liquid flowing in the upper région isan oil-rich component of the well fluid. The oil-richcomponent is produced to the surface, and the remainingwater-rich component is disposed. Optionally, the water-rich phase is subjected to a further séparation step.

The known system provides only bulk removal of water.In order to obtain a substantially water-free oil havinga water concentration that is sufficiently low to allowpipeline transport of the oil, the known System furthercomprises an oil-water separator at the surface.Furthermore it is disclosed in the publication, that forthis bulk removal of water the level of the interfaceshould be kept within narrow limits.

Not only is the known system directed to the bulkremoval of water, but it is also directed to getting alow oil concentration in the water-rich component, and ifnecessary this is done at the cost of a higher waterconcentration of the produced oil.

Applicant has reviewed the séparation behaviour of amixture of oil and water using a proprietary model. Themodel calculations, of which results will be discussedwith reference to Figures 1 and 2 below, hâve revealedthat for realistic operation conditions in horizontalwells (including flow rate of the well fluid, length anddiameter of the horizontal section), the concentration ofwater in the oil-rich component is considérable. Inpractice this will require de-watering of the producedoil before it can be transported from the wellhead, e.g. 3 12123 through a pipeline.

In this regard it is observed, that unrealisticoperating conditions were used to arrive at the resultsdepicted in Figures 2 and 3 of the above-mentionedInternational patent application. UK Patent application No. GB 2 326 895 A discloses anapparatus for producing fluid containing hydrocarbons andwater from an underground formation by using a singleunderground séparation step, in order to permit réductionof the séparation equipment at the surface. The apparatuscomprises an inclined well section wherein at least twoseparate flow paths are arranged, which flow paths aresplit by means of baffles, pipes and the like. Fluidreceived from a hydrocarbon enriched part in the wellsection is directly pumped to the surface, and fluidreceived from a water enriched part in the well sectioncan be injected back into the formation. At least onepump is operationally controlled by a detector which isplaced in the vicinity of the splitting means.

It is an object of the présent invention to provide aSystem for producing oil from an underground formation tothe surface, wherein the oil can be de-watered below thesurface, such that the water concentration of theproduced oil is sufficiently low that no further de-watering at the surface is needed before the oil can betransported away from the wellhead.

It is another object of the invention to provide sucha System which can be used under realistic operatingconditions.

It is yet another object of the invention to providea System for underground séparation of a well fluid,which System is easy to operate, robust and efficient.

To this end, in accordance with the présent inventionis provided a system for producing de-watered oil from anunderground formation to the surface, which system 4 12123 comprises a production well extending downwardly from thesurface and having an inlet below the surface; a réception well penetrating the undergroundformation and capable of receiving well fluid therefrom,wherein the downstream part of the réception wellcomprises a substantially horizontal or inclined sectionfor primary oil/water séparation of the well fluid; a water discharge system having an upstream end thatis capable of receiving during normal operation liquidfrom the lower région of the downstream part of theréception well; and a secondary underground oil/water separator,characterised in that the secondary separator has anupstream end that is capable of receiving during normaloperation liquid from the upper région of the downstreampart of the réception well, the secondary separatorhaving an outlet for de-watered oil that is in fluidcommunication with the inlet of the production well andan outlet for a water-enriched component that is in fluidcommunication with the water discharge system.

The présent invention is based on the insight gainedby Applicant by using a proprietary model, that wellfluid flowing in a substantially horizontal or inclinedwell section séparâtes under realistic operatingconditions such that near the downstream end of thehorizontal or inclined section the water concentration(vol%) in the upper, oil-rich component is significantlylarger than the oil concentration (vol%) in the lower,water-rich component. In particular it has been found,that the oil-rich component under realistic operatingconditions contains more than 10 vol% of water. Thewater-rich component can hâve an oil concentrationbetween 0.01 vol% and 0.1 vol%. In the spécification andin the daims the expressions 'upper région' and 'lower 5 12123 région' are used in connection with the horizontalsection to refer to the space above a horizontal planeintersecting the horizontal section, and the expressionsalso refer to a space of the same form when used inrelation to an inclined section. The expression" substantially horizontal" section is used in order toaccount for the fact that directional undergrounddrilling in practice may resuit in déviations from anintended horizontal direction. An inclined section is awell section that is not substantially horizontal, andcan hâve an inclination angle of up to 80 degrees from ahorizontal plane, wherein the well section is upwardlyinclined from its -upstream part where well fluid isreceived.

The présent invention will now be described by way ofexample in more detail with reference to the accompanyingdrawings, wherein

Figure 1 shows a first resuit of model calculationsof the séparation of a well fluid in a horizontal pipe,

Figure 2 shows a second resuit of model calculationsof the séparation of a well fluid in a horizontal pipe,

Figure 3 shows schematically a first embodiment ofthe présent invention,

Figure 4 shows schematically a second embodiment ofthe présent invention,

Figure 5 shows schematically a third embodiment ofthe présent invention,

Figure 6 shows schematically a fourth embodiment ofthe présent invention

Figure 7 shows schematically an embodiment of astatic separator suitable for use as secondary separatorin the présent invention, and

Figure 8 shows schematically a detail of theembodiment of the static separator shown in Figure 7.

Reference is now made to Figure 1, in which are 6 12123 displayed results of calculations that hâve been performed using the model developed by Applicant.

Figure 1 shows, for an oil/water mixture flowing in ahorizontal pipe, the calculated water concentration(vol%) of the oil-rich component in the upper région atthe end of the horizontal pipe (ordinate) as a functionof the length of the horizontal pipe in meters(abscissa) .

The calculations were performed by applying theproprietary model, which model allows to estimateparameters that characterize the séparation of a flowingoil/water mixture in horizontal pipes into an upper, oil-rich component and a lower, water-rich component. Themodel takes into account a number of input parameters,including viscosities and flow rates of oil and water,pipe diameter, initial droplet size. The model has beenexperimentally verified under field conditions inhorizontal pipes.

For the calculations input parameters hâve beenselected such that they are typical and fall within therange of realistic operating conditions for theapplication of the présent invention. The selected inputparameters include oil density 790 kg/m^, viscosity ofthe oil 1 mPa.s, flow rate 2000 m^/day, diameter of thepipe 0.23 m, overall water concentration of the mixture50 vol%, initial water droplet size 50 μπι.

As will be clear from Figure 1, the concentration ofwater in the oil-rich component decreases with increasinglength of the horizontal pipe. The model predicts, thatat a length of 1000 m the oil-rich component contains ca.12 vol% water.

For other results of the model calculations referenceis made to Figure 2. Figure 2 shows, for an oil/watermixture flowing in a horizontal pipe, the calculatedwater concentration (vol%) of the oil-rich component in 7 12123 the upper région at the end of a horizontal pipe having alength of 1000 m (ordinate), as a function of theviscosity of the oil in mPa.s (abscissa), for flow ratesof 1000 m3/day (curve 1), 1600 m3/day (curve 2) and2000 m3/day (curve 3). The other input parameters werethe same as used for the calculation of Figure 1.

Reference is now made to Figure 3. The System 1 forproducing de-watered oil from an underground formation 2comprises a production well 3 extending downwardly fromthe surface 4 and having an inlet 5 below the surface 4and an outlet 6 provided with a wellhead 6a at thesurface 4.

The System further comprises a réception well 7,penetrating the underground formation 2, and capable ofreceiving well fluid therefrom through inlet means 8,wherein the downstream part 9 of the réception well 7comprises a substantially horizontal section 10, whereinduring normal operation primary séparation of well fluidtakes place. The réception well 7 is arranged to connectat junction 11 to the production well 3 upstream of theinlet 5.

Furthermore, a water discharge System 12 is provided,having an upstream end 13 that is arranged to receiveduring normal operation liquid from the lower région 14of the downstream part 9 of the réception well 7.

Optionally, weirs, apertures, splitters, packers orthe like (not shown) may be arranged in or ,near theupstream end 13 and/or the junction 11, to guide and keepseparated the streams of fluid components.

The water discharge System 12 in this example isarranged in a downward extension of the production well 3below the junction 11, wherein the cross section of theextension can differ from that of the production well 3.

Further, the water discharge System 12 has a port 15for receiving a water-enriched component, and a pump 16, 12123 which is arranged to discharge liquid front the waterdischarge System into a well section 17 downstream of thepump 16. The well section 17 is suitably arranged toallow injection of the liquid from the water dischargeSystem into an underground formation (not shown), and thewell section 17 is further provided with means to preventwater from flowing back.

In addition there is provided a secondary undergroundoil/water separator 18 having at its upstream end aninlet 19 that is capable of receiving during normaloperation liquid from the upper région 20 of the.downstream part 9 of the réception well 7. Theseparator 18 has an outlet 21 for de-watered oil that isin fluid communication with the inlet 5 of the productionwell 3, and an outlet 22 for a water-enriched componentthat is connected via conduit 23 with the port 15 in thewater discharge System 12. The separator in this exampleis arranged in a section of the production well 3, whichsection is arranged above the junction 11 in such a waythat the separator can not be bypassed during normaloperation. The section of the production well 3 in whichthe separator 18 is arranged can be underreamed.

During normal operation of a system 1 according tothe embodiment shown in Figure 3, the well fluid receivedthrough the inlet means 8 of the réception well 7 flowsto the downstream part 9 including the horizontalsection 10, and séparâtes. Liquid layers are formed inthe upper and lower régions of the downstream part 9 ofthe réception wellbore 7, and an interface is formedbetween the layers (not shown). Near the downstream endof the réception wellbore 7 the liquid flowing in thelower région 14 is a water-rich component, and the liquidflowing in the upper région 20 is an oil-rich componentof the well fluid. The flow of the well fluid isseparated in this primary séparation step to the extent, 9 12123 that the water-rich component has sufficiently low oilconcentration.

The water-rich component enters the water dischargeSystem 12 at the upstream end 13 near the junction 11.

The oil-rich component enters the secondary separator18 through the inlet 19, and is separated into de-wateredoil, containing typically less than 10 vol% of water,preferably less than 2 vol%, more preferably less than0.5 vol% of water, and a water-enriched component, thatcan contain between 0.01 vol% and 0.1 vol% of oil. Theséparation efficiency dépends in part on the type ofseparator that is used.

The de-watered oil leaves the separator 18 via theoutlet 21 and flows on through inlet 5 into theproduction well 3 and further to the surface 4, where itis discharged from the System 1 through the wellhead 6aat the outlet 6. The water-enriched component leaves theseparator via the outlet 22 and conduit 23 and mixes atport 15 with the water-rich component to form de-oiledwater in the water discharge System 12. During normaloperation, the water discharge System 12 will be filledup to a certain water level (not shown) with de-oiledwater. The de-oiled water is discharged via wellsection 17 by means of pump 16.

As becomes clear from the foregoing description ofthe System depicted in Figure 3, a particular advantageof the présent invention is, that the well.fluid isseparated into de-watered oil and de-oiled water. In theevent, that the de-oiled water is discharged into anunderground formation, the System according to theprésent invention produces only de-watered oil to thesurface.

The curvature of the well section between thesubstantially horizontal section 10 and the junction 11is designed such that the quality of séparation does not 10 12123 substantially deteriorate.

Reference is now made to Figure 4, whichschematically shows another embodiment of the présentinvention. Parts that are similar to parts discussed withreference to Figure 3 are referred to with the samereference numerals. The System 100 is an extension of theSystem 1 shown in Figure 3 in that it further comprises aconnection well 101. The connection well 101 in thisembodiment is arranged such that it connects to theréception well 7 at a junction 102 near the downstreamend 103 of the substantially horizontal section 10, andto the water discharge System 12 at a junction 106 belowthe junction 11. The inlet of the connection well 101 isarranged at junction 102 so as to receive fluid from thelower région 14, and the outlet of the connection well101 at junction 106 is in fluid communication with thewater discharge System. USA patent publication No. 4,390,067 discloses a wellSystem comprising at least two wellbores extendingdownward from the surface, and connected by at least onegenerally horizontal wellbore.

During normal operation of the System 100, the water-rich component does not enter the water discharge systemfrom the junction 11. To this end, optionally apacker 108 can be arranged just below the junction 11,which packer suitably has an opening for a conduit 23connecting the outlet 22 to the port 15.

Reference is now made to Figure 5, which showsschematically a third embodiment of the présentinvention. Parts that are similar to parts discussed withreference to Figure 3 are referred to with the samereference numerals. The System 200 shown in Figure 5differs from the system 1 shown in Figure 3, in that thewater discharge system 202 comprises a water dischargewell 204 that is arranged as a branch of the réception ι 12123 well 7. The junction 206 of the wells 7 and 204 isarranged near the outlet of the substantially horizontalsection 10 of the downstream part 9 of the réceptionwell 7.

During normal operation, well fluid undergoes primaryséparation in the substantially horizontal section 10 andenters, when passing the junction 206 near the downstreamend of the horizontal section, a section 208 having alower région 209. The lower région 209 receives thewater-rich component from the lower région 14 of thedownstream part 9 of the réception well 7.

During normal operation, the oil-rich component ofthe well fluid flows in a layer in the upper région 210of the section 208 and then through an upwardly curvedwell section 212. From the well section 212 it enters thesecondary oil-water separator 18 through the inlet 19. Inthe oil-water separator 18 the oil-rich component isseparated into de-watered oil, and a water-enrichedcomponent. The de-watered oil leaves the separator 18 viathe outlet 21 to inlet 5 of the production well 3, andfurther to the surface 4, where it is discharged from thesystem 200 through the wellhead 6a at the outlet 6. Thewater-enriched component leaves the separator via theoutlet 22 and flows through conduit 23 to port 15 that isarranged in the lower région 209 of the section 208.There, the water-enriched component mixes with the water-rich component to form de-oiled water.

Via conduit 216 having an inlet 217 arranged in thelower région 209, the de-oiled water is received by thewater discharge system 202. By means of pump 16 the de-oiled water is pumped through the water dischargewell 204, and disposed through outlet means 218 into theunderground formation 220.

Reference is now made to Figure 6, which showsschematically a fourth embodiment of the présent 12123 12 invention. Parts that are similar to parts discussed withreference to Figure 3 are referred to with the samereference numerals. The System 300 shown in Figure 6differs from the System that has been discussed withreference to Figure 3 in the arrangement of the secondaryoil-water separator and of the water discharge System.

The secondary oil-water separator 18 of the System300 is arranged in an underreamed section at the lowerend of the production well 3. The separator 18 isarranged to receive, through its inlet 19, liquid fromthe upper région 302 of the horizontal section 10 of thedownstream part 9 of the réception well 7, wherein theseparator 18 is located near the downstream end 303 ofthe horizontal section 10. The oil-water separator 18further has an outlet 21 for de-watered oil that is influid communication with the inlet 5 of the productionwell 3, and an outlet 22 for a water-enriched component.Outlet 22 is connected via conduit 23 to port 15, whichport 15 is arranged in the lower région 304 of thehorizontal section 10, near the downstream end 303 of thehorizontal section 10.

The water discharge System 305 in this embodimentcomprises a water-discharge well 306 of which the slopedéclinés in the direction of fluid flow. The water-discharge well 306 has an inlet 310 at its upper end thatconnects to the downstream end 303 of the horizontalsection 10. The slope of the water-discharge well 306 isselected such that an incoming stratified flow is notsubstantially disturbed.

Downstream in the water-discharge well 306, at aposition below the lowest level of the horizontalsection 10, a pump 16 is arranged to discharge the de-oiled water through outlet 312 into the undergroundformation 315, and there is further provided means toprevent water from flowing back (not shown). 13 12123

During normal operation of a System 300 as shown inFigure 6, the well fluid received through the inlet means8 of the réception well 7 flows to the downstream part 9including the horizontal section 10, which acts asprimary separator for the well fluid. Liquid layers areformed in the upper and lower régions of the horizontalsection 10, and an interface is formed between the layers(not shown). Near the downstream end 303 of the horizontal section 10 the liquid flowing in the lowerrégion 304 is a water-rich component, and the liquidflowing in the upper région 302 is an oil-rich componentof the well fluid. The flow of the well fluid isseparated to the extent, that the water-rich componenthas sufficiently low oil concentration.

The oil-rich component enters the secondary separator18 through the inlet 19, and is separated into de-wateredoil and a water-enriched component, wherein the de-watered oil is passed to the surface 4 as described withreference to Figure 3. The water-enriched componentleaves the separator through the outlet 22 and conduit 23and mixes near port 15 in the lower région 304 with thewater-rich component to form, downstream of port 15, de-oiled water.

The de-oiled water is received by the water-dischargewell 306 through inlet 310. Below the lowest level of thesubstantially horizontals section the water-dischargewell will, during normal operation, be filled with de-oiled water. By means of pump 16 the de-oiled water ispumped through the water-discharge well 306, and disposedthrough outlet means 312 into the underground formation 315.

It may be désirable to produce oil from multipleréception wells by using a single production well and asingle oil/water separator. In this case, the Systemaccording to the invention comprises one or more 14 12123 additional réception wells, which penetrate the underground formation at different locations and receivewell fluid therefrom, wherein the downstream parts of theadditional réception wells are in fluid communicationwith the downstream part of the réception well. Theséparation of well fluid into water-rich and oil-richcomponents may occur in the multiple réception wellsindividually, or in a common downstream part after mixingail well fluid, or partly in both ways.

In the International Patent application withpublication No. WO 98/25005 is disclosed an undergroundwell System comprising a substantially vertical wellboreand one or more horizontal well sections extending fromthe vertical wellbore.

In the International Patent application withpublication No. WO 98/50679 is disclosed an undergroundwell System comprising a main well and one or moreadditional wells, wherein each well extends downwardlyfrom the surface and comprises a substantially horizontalsection arranged in a production formation. Thehorizontal sections of the additional wells are in fluidcommunication with the horizontal section of the mainwell through the production formation, but do notphysically intersect with the main well.

The underground oil/water separator for use in aSystem according to the présent invention can be ofvarious types known in the art, such as for example acyclone, a coalescer, or a static separator. Withadvantage the separator is a static one, which isarranged in a séparation chamber, wherein the height ofthe séparation chamber is larger than the thickness ofthe oil/water dispersion band that is formed thereinunder normal operation conditions. The séparation chambercan with advantage be arranged in an underreamed sectionof the production well. 15 12123

It has been recognised that in an undergroundséparation chamber one can take advantage of the physicalconditions in the well, e.g. elevated température andpressure, which influence the séparation behaviour of oiland water such that efficient séparation of the liquidreceived from the upper région of the downstream part ofthe réception well into relatively dry oil and relativelypure water can be achieved under practically andeconomically feasible conditions.

The liquid received during normal operation by astatic separator from the upper région of the downstreampart of the réception well is an oil-rich component ofthe well fluid in the form of an oil/water dispersion,containing more than 10 vol% of water. The séparation ofsuch an oil/water dispersion in a séparation chamberunder the influence of gravity can be described by meansof a model developed by Applicant. This so-calledDispersion Band Model, is published in H.G. Polderman etal., SPE paper No. 38816, 1997. The model can be used todescribe séparation in a séparation chamber. An importantmechanism of séparation is based on coalescence of smallwater droplets in the dispersion band, which sink to thelower layer once the drops hâve grown large enough.

During normal operation, three liquid layers are formed:a bottom layer of relatively pure water, a middle layercontaining an oil and water dispersion and an upper layerof relatively dry oil. The middle layer is, also referredto as the dispersion band.

Suitably, the inlet and the outlets of the separatorare arranged such that the feed and the separatedcomponents flow vertically or nearly vertical in and outof the séparation chamber.

In a first embodiment of such a static separator theseparator further comprises a flow distributor means,arranged to distribute at a predetermined vertical 16 12123 position the liquid over the cross-sectional area of theséparation chamber. Preferably, the liquid is admittedinto the séparation chamber at a predetermined verticalposition through one or more openings ata local flowvelocity below 1 m/s. In the séparation chamber theliquid is allowed to separate into a lower layer of awater-enriched component, a middle layer of an oil andwater dispersion component and an upper layer of an de-watered oil component. Liquid from the upper and lowerlayers can be withdrawn via the outlets for de-wateredoil and the water-enriched component, respectively. Theseparator can further comprise a level detector means formeasuring the vertical position of the interface betweentwo liquid layers and a flow control means in order tomaintain during normal operation an interface between twoliquid layers at a predetermined vertical level.

In a second embodiment, a static separator for use assecondary separator with the présent invention furthercomprises - a stack of vertically spaced apart inclined plates,wherein between each pair of neighbouring plates aséparation space is defined; - a substantially vertical inlet conduit communicatingwith the separator' s upstream end, which inlet conduittraverses the stack of plates and is arranged to receivethe liquid from the upper région of the downstream partof the réception well at its lower end, and is providedwith one or more fluid outlets each of which opens into aséparation space; - a substantially vertical oil collection channel havingan oil outlet at its upper end communicating with theseparator's outlet for the de-watered oil, which oilcollection channel has one or more oil inlets, each oilinlet being arranged to receive fluid from the uppermostrégion of a séparation space, wherein at least the plate 17 12123 immediately below each oil inlet is provided with avertically upward pointing baffle; and- a substantially vertical water collection channelhaving a water outlet at its lower end communicating withthe separator's outlet for the water-enriched component,which oil collection channel has one or more waterinlets, each water inlet being arranged to receive fluidfrom the lowermost région of a séparation space, whereinat least the plate immediately above each water inlet isprovided with a vertically downward pointing baffle.

Reference is now made to Figures 7 and 8. Figure 7shows an example of a static separator 410 which isarranged in a séparation -chamber 406 in an underreamedsection of the production well (not shown). Theséparation chamber 406 has a substantially circular crosssection. The vertical wall 408 of the séparationchamber 406 is formed by the surrounding formation 409,but it will be understood that the wall can also beprovided by a well tubular, such as a casing. The wall ofthe séparation chamber also forms the wall of theseparator. The static separator 410 comprises a stack ofinclined, substantially fiat plates 430, 431, 432 thatare arranged substantially parallel to each other andvertically spaced apart at an equal distance. The spacedelimited between two neighbouring plates is referred toas the séparation space. For example, plates 430 and 431define the séparation space 435, plates 431 and 432define the séparation space 436. Underneath the lowestplate 432 of the stack of plates a parallel baseplate 437 is arranged, wherein the outer rim of the baseplate sealingly engages the walls of the séparationchamber 406. Between the plate 432 and the base plate 437a further séparation space 438 is defined.

The stack of plates is traversed by the inletconduit 440, which extends vertically upwardly from an 18 12123 opening 442 through the stack of plates in the centre ofthe séparation chamber 406. The passage of the inletconduit through a plate, for example the passage 443through plate 431, is thereby arranged such that the wallof the inlet conduit 440 sealingly fits to the plate, forexample plate 431, thereby preventing fluid communicationbetween neighbouring séparation spaces, for exampleséparation spaces 435 and 436, along the inlet conduit.Further, the inlet conduit is provided with radial outletopenings 444, 445, 446, which open into the séparationspaces 435, 436, 438, respectively. It will be clear,that further outlet openings can be arranged opening intodifferent radial directions. An outlet opening is withadvantage arranged in the direction of the axis in thehorizontal plane around which the plates are inclined,i.e. in Figure 7 an axis perpendicular to the paperplane.

Further details about the inclined plates will now bediscussed with reference to Figure 8, whereinschematically the plates 431 and 432 of Figure 7 areshown. The rim 447 of plate 431 includes at the upperside 448 of the plate 431 a straight edge 449 to which anupward pointing baffle plate 450 is attached. At thelower side 452 the rim 447 includes a straight edge 454to which a downward pointing baffle plate 456 isattached.

Referring again to Figure 7, the other inclinedplates of the stack of plates are similarly provided withupward and downward pointing baffles 458, 459, 460, 461at the their upper and lower sides, respectively. Theremaining parts of the rim of each inclined plate towhich no baffle is attached are arranged to sealinglyengage the wall 408.

The static separator 410 further comprises an oilcollection channel 465, which is formed by the space 19 12123 segment delimited by the upward pointing baffles, 458,450, 459, and the wall 408. The oil collectionchannel 465 comprises oil inlets, for example oilinlet 470 arranged to receive fluid from the uppermostrégion 472 of the séparation space 436. Oil inlet 470 isdefined by the upper edge 449 of the plate 431 and theupward pointing baffle 459 of the plate 432 immediatelybelow the oil inlet 470. The oil collection channel 465further comprises an outlet 473 in communication with theoutlet 415 of the static separator 410.

Opposite to the oil collection channel 465 theseparator 410 comprises a water collection channel 475,which is formed by the space segment delimited by thedownward pointing baffles, 460, 456, 461, and thewall 408. The water collection channel 475 compriseswater inlets, for example water inlet 480 arranged to.receive fluid from the lowermost région 482 of theséparation space 435. Water inlet 480 is defined by thelower edge 454 of the plate 431 and the downward pointingbaffle of the plate 430 immediately above the waterinlet 480. The water collection channel 465 furthercomprises an outlet 483 in communication with theoutlet 418 of the separator 410.

The plates 430, 431 and 432 with the attached bafflesare arranged such that the shortest horizontal distancebetween an upward pointing baffle and the wall 408increases from bottom to top, and that the shortesthorizontal distance between a downward pointing baffleand the wall 408 increases from top to bottom. Γη thisway the cross-sectional areas of both the oil collectionchannel 465 and the water collection channel 475 increasein the direction towards their respective outlets 473 and483. Since the separator 410 does not contain parts thatare moving during normal operation it represents a staticoil-water separator. 12123 20

During normal operation fluid enters the staticseparator 410 its upstream end 412, enters the inletconduit 440 at the opening 442 and is admitted into theinterior of the séparation spaces 435, 436, 437 via theoutlet openings 444, 445 and 446. It has been found thatgood séparation results are obtained if ail openings hâvethe same cross-sectional area. Good results are obtainedif the diameter of the openings is of the order of thediameter of the inlet conduit, such that the pressuredrop over the opening is small.

The séparation will now be discussed. To this end wetake a doser look on the séparation space 436 betweenplates 431 and 432. In this séparation space 436, threeliquid layers are formed, an upper, de-watered oil layer,a middle dispersion band layer and a lower, water-enriched layer. The de-watered oil layer flows towardsthe uppermost région 472 of the séparation space 436,from where it leaves the séparation space to enter theoil collection channel through inlet 470. The water-enriched layer flows towards the lowermost région 485 ofthe séparation space 436, from where it enters the watercollection channel through inlet 486. Séparation in thespaces 435 and 435 is similar. The oil collectionchannel 465 receives a de-watered oil component from ailséparation spaces, and since the cross-section of thechannel widens towards the outlet 473, the verticallyupward flow velocity of the de-watered oil component inthe channel 465 can remain substantially constant. Fromthe outlet 473 the collected de-watered oil componentflows to the separator's outlet for de-watered oil 415.

The water-collection channel 475 receives a water-enriched component from ail séparation spaces, and sinceits cross-section widens from top to bottom towards theoutlet 483, the vertically downward flow velocity of thewater-enriched component in the channel 475 can remain 21 12123 substantially constant. From the outlet 483 the collectedwater-enriched component flows to the separator's outletfor a water-enriched component 418.

In a further embodiment the inclined plates can hâvesubstantially the form of funnels arranged substantiallyparallel to each other, wherein each funnel is providedwith a central opening.

By installing a stack of vertically spaced apartinclined plates the efficiency of a séparation chambercan be increased, i.e. a chamber of smaller height canhandle the same spécifie throughput as a largerséparation chamber without a plate pack. In practiceoften a réduction of the-required height of theséparation chamber by a factor in the range of from 1.5to 6 can be achieved. Sometimes, the height of theséparation chamber is not a limiting factor for the welldesign, and in this case a separator without a stack ofplates can be used.

Typical dimensions of the séparation chamber hâvebeen calculated using the Dispersion Band Model under thefollowing assumptions: gross flow rate through theseparator 1000 m^/day of well fluid containing 50 vol% ofwater, dry oil viscosity 0.001 Pa.s. In this case aséparation chamber of about 1 m diameter and 5 m heightis required. For comparison it is noted that by installing a stack of plates in the séparation chamberthe height requirement can be decreased to for example 2m. Suitably the height/diameter ratio of the séparationchamber is smaller than 6, wherein under diameter isunderstood the diameter of a circle having the samecross-sectional area as the volume of the séparationchamber divided by its height.

It will be appreciated, that in practicalapplications of the présent invention additionaltechnical measures may be implemented which are well 22 12123 known in the art and of which the expert is master. Byway of example some of those measures will briefly bedescribed hereinafter.

The wells of a System according to the présentinvention, or sections thereof, may be provided withcasing, tubing, packing, flow controllers, measurementequipment, data communication Unes, power transfer Unesto underground equipment or other means known in the artfor operating and controlling a well System.

In the event that the well fluid comprises inaddition to oil and water also gas, it is possible thatin the downstream part of the réception well a gas layeris formed on top of the layer in which the remainderliquid flows. Gas may decrease the separating efficiencyof the separator. It may therefore be advantageous toarrange an outlet for gas connected to a gas-dischargeSystem for gas at a suitable position in the System.

It may be désirable to perform measurements usingunderground equipment. This may be of advantage formonitoring and controlling the operation of the system.

As an example, measurement equipment may be installedto monitor the oil, gas or water content of fluids atcertain positions in the system. E.g., the water or oilcontent of the de-oiled water, the water-rich component,the water-enriched component, or of the de-watered oil,may be measured by suitable equipment.

Further, although the exact vertical level of aninterface between layers of different components at acertain position in the system is generally not criticalfor the function of the system, and may vary withinpredetermined limits, it may be désirable to measure thelevel by a detector.

The resuit of such a measurement may e.g. be used tocontrol the flow rate of a fluid at a certain position inthe system to stay within predetermined limits. It is 23 12123 well known in the art how to control a flow rate in aSystem according to the présent invention, e.g. the flowrate of inflowing well fluid, liquid from the upperrégion or from the lower région of the downstream part ofthe réception well, de-oiled water or de-watered oil. Tothis end, the System may comprise controllable valves,pumps, restrictions, movable sleeves, adjustableapertures or other suitable equipment.

It may be désirable to promote the séparation offluid components by physical or Chemical means, e.g. bythe injection of Chemicals that are known in the art.

In the event that an inclined well section isprovided for primary séparation of the well fluid, it canbe advantageous to arrange at the downstream end of theinclined section, in the area upstream of and around thesecondary separator, a substantially horizontal section,which can be for example up to 100 meters long.

It will be appreciated, that the de-oiled water canbe injected in the underground formation, from which wellfluid is removed. In this way, the injection of de-oiledwater can serve to maintain the pressure in the underground formation.

Thus, the présent invention provides a System forproducing oil from an underground formation to thesurface, wherein the oil can be de-watered below thesurface, such that the water concentration of theproduced oil is sufficiently low that no further de-watering at the surface is needed before the oil can betransported away from the wellhead.

Claims (20)

24 1212 3 C L A I M S

1. System for producing de-watered oil from anunderground formation to the surface, which Systemcomprises a production well extending downwardly from thesurface and having an inlet below the surface; •a réception well penetrating the undergroundformation and capable of receiving well fluid therefrom,wherein the. downstream part of the réception wellcomprises a substantially horizontal or inclined sectionfor primary oil/water séparation of the well fluid; a water discharge System having an upstream end thatis capable of receiving during normal operation liquidfrom the lower région of the downstream part of theréception well; and a secondary underground oil/water separator,characterised in that the secondary separator has anupstream end that is capable of receiving during normaloperation liquid from the upper région of the downstreampart of the réception well, the secondary separatorhaving an outlet for de-watered oil that is in fluidcommunication with the inlet of the production well andan outlet for a water-enriched component that is in fluidcommunication with the water discharge System.

2. System according to claim 1, wherein the waterdischarge system comprises means to inject the liquidfrom the lower région and the water-enriched componentinto an underground formation.

3. System according to claim 1 or 2, further comprisinga connection well, wherein the connection well has an ' inlet arranged to receive liquid from the lower région ofthe downstream part of the réception well, and an outlet 12123 25 in fluid communication with the water discharge System.

4. System according to claim 1 or 2, wherein the waterdischarge system comprises a water discharge well that isa branch of the réception well.

5. System according to claim 1 or 2, wherein the waterdischarge system comprises a water-discharge well ofwhich the slope déclinés in the direction of fluid flow.

6. System according to any one of the daims 1-5,further comprising an additional réception well arrangedto receive well fluid from the underground formation,wherein the downstream part of the additional réceptionwell is in fluid communication with the downstream partof the réception well.

7. System according to any one of the daims 1-6,further comprising underground measurement equipment tomeasure a characteristic of a fluid at a certain positionin the system.

8. System according to claim 7, wherein the characteristic is a concentration of a component in afluid.

9. System according to claim 7, wherein the characteristic is the vertical level of an interfacebetween layers of different components of the well fluidat a certain position in the system.

10. System according to any one of the daims 1-9,further comprising means to control the flow of a fluidat a certain position in the system.

11. System according to any one of the daims 7-9,wherein the system comprises means to control the flow ofa fluid at a certain position in the system, and whereindata obtained from the underground measurement equipmentis used as input for the means to control the flow of afluid.

12. System according to any one of daims 1-11, whereinthe secondary underground oil/water separator is selected 26 12123 from the group comprising a cyclone, a coalescer, or astatic separator.

13. System according to claim 12, wherein the secondaryseparator is a static separator which is arranged in aséparation chamber, and wherein the height of theséparation chamber is larger than the thickness of thedispersion band that is formed therein under normaloperation conditions.

14. System according to claim 13, wherein the staticseparator further comprises a flow distributor means,arranged to distribute at a predetermined verticalposition the well fluid received through the separator'sinlet over the cross-sectional area of the séparationchamber.

15. System according to claim 13 or 14, wherein thestatic separator further comprises a level detector meansand a flow control means in order to maintain duringnormal operation an interface between two liquid layersat a predetermined level.

16. System according to claim 13, wherein the staticseparator further comprises a stack of vertically spaced apart inclined plates,wherein between each pair of neighbouring plates aséparation space is defined; a substantially vertical inlet conduit communicatingwith the separator's upstream end, which inlet conduittraverses the stack of plates and is arranged to receivethe well fluid at its lower end, and is provided with oneor more outlets each of which opens into a séparationspace; a substantially vertical oil collection channelhaving an oil outlet at its upper end communicating withthe separator's outlet for de-watered oil, which oilcollection channel has one or more oil inlets, each oilinlet being arranged to receive fluid from the uppermost 12123 27 région of a séparation space, wherein at least the plateimmediately below each oil inlet is provided with avertically upward pointing baffle; and a substantially vertical water collection channelhaving a water outlet at its lower end communicating withthe separator's outlet for the water-enriched component,which oil collection channel has one or more waterinlets, each water inlet being arranged to receive fluidfrom the lowermost région of a séparation space, whereinat least the plate immediately above each water inlet isprovided with a vertically downward pointing baffle.

17. System according to claim 16, wherein the inclinedplates are substantially fiat and arranged substantiallyparallel to each other, wherein each inclined plate isprovided with a downward pointing baffle attached to therim at the lower side of the inclined plate and an upwardpointing baffle attached to the rim at the upper side ofthe inclined plate, wherein the remaining parts of therim fit sealingly to the wall of the séparation chamber,wherein the oil collection channel is formed by the spacedelimited by the upward pointing baffles and the wall,and wherein the water collection channel is formed by thespace delimited by the downward pointing baffles and thewall.

18. System according to claim 16, wherein the inclinedplates hâve substantially the form of funnels arrangedsubstantially parallel to each other, wherein each funnelis provided with a central opening.

19. System according to any one of daims 13-19, whereinthe séparation chamber has a height/diameter ratiosmaller than 6.

- 20. System according to any one of daims 1-19, whereinthe secondary underground oil/water separator is arrangedin an underreamed section of the production well.