Patent Description:
In recognition of these potentially enormous expenses, added emphasis has been placed on well monitoring and maintenance throughout the life of an oilfield. Maintaining production from a host of wells at a subsea oilfield often requires the use of pumping to aid in recovery of production fluids. Along these lines, a host of multiphase pumps are generally incorporated into the layout of the field.

Pumps may be used to enhance production by reducing wellhead pressure to allow a more rapid depletion and to lift weak wells in concert with production flow from stronger wells. Multiphase pumps are also used in the field layout due to the often inconsistent or changing nature of the production fluids. That is, produced fluids may be a mixture of liquid and gas. Often such a fluid mixture is referenced in terms of its gas volume fraction (GVF). So, for example, a production fluid that is <NUM>% gas may be noted as having a <NUM>% GVF. Regardless, a multiphase pump may be configured to effectively pump such fluid mixtures. In many cases produced fluids from subsea fields are substantially liquid at the outset with the GVF rising over time to reach <NUM>%, <NUM>% or higher. Of course, this is not universally the case and there may be periods of high GVF at the outset of production or for intermittent periods over the life of any well.

Regardless of when high GVF is presented, recovery of production fluids will be more of a challenge as GVF rises. This is because in order to attain effective pumping assistance, even with a multiphase pump, the production fluid should consist of a sufficient liquid fraction in order to support a substantial differential pressure. By way of example, a conventional multiphase pump presented with production fluids having a negligible GVF might attain a <NUM> bar differential and pump at <NUM>,<NUM> rpm for substantial production assistance. However, as the GVF rises, the differential pressure that the pump is able to generate diminishes. More specifically, as a practical matter, once the GVF reaches <NUM>-<NUM>%, the assistance provided by the pump is largely inefficient. By the time the GVF reaches <NUM>% or more, no real pumping assistance is available.

Alternative forms of production assistance may be available. For example, rather than attempting to inefficiently continue pumping when a GVF of <NUM>% emerges, artificial gas lift may be utilized. This technique involves introducing pressured gas down through the well annulus to reach the bottom of the well and thereby ultimately effecting production out of the well.

Unfortunately, utilizing gas lift as described, requires dedicating a host of other new resources to the site. A gas source is required as well as the equipment necessary to supply the gas and at sufficient pressure. Once more, not only is a new gas fluid introduced but it will also need to be collected and processed at a later point in time along with all other production fluids. Further, this entirely new circulation system of artificial gas lift may be utilized in the face of a high GVF that might turn out to be only temporary. That is, as noted above, while GVF often increases over the life of a field, this is not always so. Once more, predicting GVF can be more of an art. This means that the economic burden of gas lift measures are often unnecessarily, or at least prematurely, resorted to when conventional lower cost pumping assistance would have turned out to be sufficient.

Of course, the alternative of delaying the introduction of gas lift or other less cost effective assistance may also have a downside. If gas lift hardware is provided to the field and available, how long should the operator continue to delay such assistance when the GVF has rendered multiphase pumping assistance inefficient? Even if this could be ascertained with a degree of certainty, what of the cost incurred in making sure that the gas lift hardware is incorporated into the field and a ready supply of gas and other equipment made available? At present, with no guarantee of continued pumping assistance being available once GVF reaches a certain point, these unknowns continue to remain a substantial burden for operators.

<CIT> describes a flow splitter that forms part of a multiphase pumping station, the flow splitter comprising an outer tank having an upper inlet and an actuatable inner vessel disposed within the outer vessel, wherein multiphase fluid can pass from the outer vessel into the inner vessel though large upper openings. The inner vessel is configured to be actuatable such that the inner vessel moves in a vertical direction, thereby altering the size of an annular opening between the bottom of the inner vessel and the outer vessel. <CIT> describes a multiphase pumping block device comprising a pump, a recycle circuit and a splitter tank connected to an area downstream of the pump and configured to separate liquid and gaseous phases using pumping fluid. The recycle circuit is connected to the splitter tank and is configured to enable flow of the liquid phase from the splitter tank to an area upstream of the pump. The recycle circuit includes a controllable opening valve that is pre-opened in order to allow partial flow of the liquid phase in the recycle circuit. <CIT> describes a fluid processing system containing a pump and a fluid reservoir. The pump includes a casing, one or more pump stages, a pump inlet, and a pump outlet. The casing includes one or more slots, with at least one slot configured to extract at least a portion of a multiphase fluid flowing within the pump. The fluid reservoir encompasses at least a portion of the casing and is configured to receive and separate the portion of the multiphase fluid into an extracted liquid phase and an extracted gaseous phase. The fluid reservoir includes a re-circulation conduit disposed proximate to the pump inlet and a discharge device coupled to the re-circulation conduit. The discharge device regulates re-circulation of at least a portion of the extracted liquid phase to the pump via the pump inlet for reducing a gas volume fraction of the multiphase fluid being fed to the pump. <CIT> describes a multiphase pumping system for subsea operation comprising a pump, a flow inlet for accepting incoming multiphase flow and directing incoming multiphase flow generally towards the pump, a flow outlet for directing outgoing multiphase flow generally away from the pump, a flow management apparatus in fluid communication with the pump and at least one of the flow inlet and flow outlet. The flow management apparatus acts to ensure a minimum liquid content in multiphase flow entering the pump and comprises a gas liquid cylindrical cyclone in communication with the flow outlet, a recirculation port disposed in the gas liquid cylindrical cyclone and a recirculation line in communication with the recirculation port, the recirculation line acting to direct flow generally towards the pump.

The present invention resides in a pump system for use at an oilfield as defined in claim <NUM> and in a method as defined in claim <NUM>.

In the following description, numerous details are set forth to provide an understanding of the present disclosure. However, it will be understood by those skilled in the art that the embodiments described may be practiced without these particular details. Further, numerous variations or modifications may be employed which remain contemplated by the embodiments as specifically described.

Embodiments are described with reference to certain types of subsea oilfield layouts utilizing permanently installed subsea pumps at the seabed to facilitate continuous production from wells of the oilfield. However, no particular layout is required. For example, the system and techniques described herein may be directed at a single well or even utilized in a surface environment. So long as a splitter assembly is available to recirculate liquid fluid back to the pump during pumping operations for reducing the GVF within the pump itself to ensure continued pumping function, appreciable benefit may be realized.

Referring now to <FIG>, a perspective sectional view of an embodiment of a splitter assembly <NUM> is shown. With added reference to <FIG> and <FIG>, the assembly <NUM> is for use with a subsea pump system <NUM>. Specifically, an inlet <NUM> is fluidly coupled to a multiphase pump <NUM>, which may be of a type often utilized at a subsea oilfield <NUM>. However, as suggested, to help ensure continuous pumping aid to production even in the face of high GVF, production fluids are routed through the splitter assembly <NUM>, initially via the inlet <NUM> as indicated.

Once reaching the interior of the assembly <NUM>, production fluids are faced with a multi-tiered flow path. That is, given that the production fluid is often a mixture of liquid and gas, sometimes with a high GVF, the splitter assembly <NUM> is configured to "split" away the gas of the fluid and recirculate a portion of the liquid fraction back to the pump <NUM> (see <FIG>). This is achieved by way of the noted multi-tiered flow path which allows for liquid production fluid to return to the pump <NUM> of <FIG> by way of a recirculation outlet <NUM>.

Continuing with reference to <FIG> and <FIG>, production fluid enters the splitter assembly <NUM> via the inlet <NUM> at a location above the noted outlet <NUM>. Thus, the fluid is presented with a chamber that effectively allows the fluid types to split with the liquid fraction <NUM> falling below the gas fraction <NUM>. This is readily illustrated in the schematic of <FIG>. With specific reference to <FIG>, this initial chamber is defined by the assembly housing <NUM>. An outer chamber or tube <NUM> is open at the top but secured by a circumferential support mechanism <NUM> to the inner side of the housing <NUM>.

Note that the liquid <NUM> of the production fluid which falls to the lower portion of the assembly <NUM> is allowed to escape either through continued production flow (arrow <NUM>) or through the outlet <NUM> as indicated above. Of course, with operations focused on ultimately obtaining production fluids, allowing the liquid <NUM> to continue along the production flow path is understandable. However, keeping the pump <NUM> of <FIG> running may be key in this regard. Thus, to ensure a sufficient priming liquid supply to the pump <NUM> for continued pump assistance, a portion of the liquid fraction <NUM> is also recirculated through the outlet <NUM> and back to the pump <NUM> as described. In certain embodiments, additional liquids may be introduced with the priming such as methanol, monoethylene glycol or other conventional chemical injection liquids to reduce startup time, for cooling purposes and/or to add to the liquid level at the pump.

As illustrated, the lower portion of the assembly <NUM> includes a deflector <NUM>. The deflector <NUM> is a shield plate that deflects sand and debris of the production fluid such that the liquid directed through the outlet <NUM> and back over to the pump <NUM> is more free of unhelpful particulates. In this way, priming liquid support for continued pump function may be further enhanced (see <FIG>). That is, while the production fluid on the whole may be of a GVF that is too high to support a sufficient differential for effective pumping, the pump <NUM> is not pumping production fluid on the whole. Rather, the pump <NUM> is pumping production fluid mixed with recirculated liquid of the production fluid, thereby reducing the GVF and allowing for continuous priming for continuous pump function.

With specific reference to <FIG>, incoming production fluid faces a low pressure drop with exposure to the comparatively large volume of the housing <NUM>. Thus, liquid collects at the bottom of the assembly <NUM> where it pools until a level between the tubes <NUM>, <NUM> exceeds the height of the inner tube <NUM>. At this point, this portion of the liquid begins to spill over <NUM> as described here. This result is what is often referred to as a "Weir" effect. That is, an accumulation of liquid at the base of one or more barriers is presented without halting fluid flow. This Weir effect and splitting of the multiphase fluid may occur to the benefit of continued pump function as detailed herein.

In the embodiment shown, the inner tube <NUM> governs the Weir effect as noted which aids in re-mixing of gas <NUM> and liquid <NUM>. That is, the production fluid is to be collected and not merely recirculated. Thus, the inner tube <NUM> is also configured to allow liquid production to continue along a production flow path (see arrow <NUM>). However, the inner tube <NUM> serving as a Weir-type barrier also helps to ensure sufficient pooling of the liquid production <NUM> for recirculation as noted above and illustrated in <FIG>. So, for example, unlike the outer tube <NUM>, the inner tube <NUM> is fully secured and sealed at the base <NUM> of the assembly <NUM>. Alternatively, the outer tube <NUM> includes an opening <NUM> at the bottom that allows for fluid communication with the inner tube <NUM>. The opening <NUM> is restricted in size and positioned below the vertical position of the recirculation outlet <NUM>. Thus, as the production fluid enters the assembly <NUM> and the pooling liquid <NUM> develops, it is afforded ample opportunity to exit through either the outlet <NUM> or the opening <NUM>.

As illustrated, the inner tube <NUM> is shorter than the outer tube <NUM> to ultimately facilitate liquid spillover <NUM> in the direction of production flow toward the production outlet <NUM> of the assembly <NUM>. Similarly, the inner tube <NUM> avoids presenting any barrier to gas flow (see arrow <NUM>). Thus, with the exception of the portion of the pooled liquid that is diverted through the recirculation outlet <NUM>, all of the production fluid that advances into the assembly <NUM> further advances in the noted direct of production flow toward the production outlet <NUM>.

As noted above, the deflector <NUM> may encourage unhelpful particulate toward a base <NUM> and away from recirculation. The base <NUM> may be cup shaped to encourage collection of particulate thereat as illustrated in <FIG>. As production continues via the production outlet <NUM>, this particulate may be produced with other produced fluids.

Referring specifically now to <FIG>, a larger schematic representation of the subsea pump system <NUM> that utilizes the splitter assembly <NUM> of <FIG> is shown. The assembly <NUM> is coupled to the pump <NUM> as discussed above. However, in the embodiment shown, recirculated liquid production is initially directed toward a mixer <NUM> and combined with production fluids drawn from the oilfield before reaching the multiphase pump <NUM>. Thus, the GVF of the production fluid is beneficially altered before reaching the pump <NUM> as described above. As with conventional circulation, use of a mixer <NUM> may also dampen sever slugging and help ensure an equitable split of flow among pumps where multiple pumps are utilized. Note that the flow of production fluid <NUM> proceeds along a production line with a portion of the fluid diverted to the mixer <NUM> and/or splitter <NUM> as described above before being returned to the line for continued advancement and eventual collection. In this way, the subsea pump system <NUM> is effectively a system that has been coupled to a standard production line to facilitate continuous production at an oilfield <NUM> even when faced with an undesirably high GVF for a substantial portion of the wells (e.g. see <NUM>, <NUM>, <NUM> and <NUM> of <FIG>).

Referring now to <FIG>, an overview depiction of a subsea oilfield <NUM> is shown taking advantage of subsea pump systems <NUM> as illustrated in <FIG>. In this particular layout, multiple well clusters <NUM>, <NUM> are coupled to manifolds <NUM>, <NUM>. This oilfield <NUM> includes a conventional offshore platform <NUM> from which subsea operations may be directed. In this particular example, bundled water and production lines <NUM> and bundled electrical/hydraulic lines <NUM> may run along the seabed between the platform <NUM> and the cluster locations.

The oilfield <NUM> accommodates embodiments of the subsea pump systems <NUM> described hereinabove to help facilitate and promote production of fluids from the clusters <NUM>, <NUM> of wells <NUM>, <NUM>, <NUM>, <NUM> (see arrow <NUM>). In spite of the potential for elevated GVF from the well clusters <NUM>, <NUM> on the whole, as described hereinabove, the GVF that is encountered by the pump <NUM> of each system <NUM> remains below about <NUM>% (see <FIG>). Indeed, the GVF exposed to the pump <NUM> may remain at such low percentages even where the GVF of production exceeds <NUM>% at an individual well <NUM>, <NUM>, <NUM>, <NUM>, cluster <NUM>, <NUM> or the overall field <NUM>. Thus, gas lock from a gas bubble may be avoided and a sufficient pressure differential maintained for continuous pumping aid for circulating production fluids (e.g. to the platform <NUM>).

Referring now to <FIG>, a cross-sectional side view of the splitter assembly <NUM> of <FIG> is shown at a start-up of pumping operations. Notice that as production fluid enters through the inlet <NUM>, the comparatively large volume of the assembly <NUM> and overall housing <NUM>, immediately allows for the falling of the liquid fraction <NUM>. Similarly, the gas fraction <NUM> is at the top of the assembly interior in the form of a gas cap.

Continuing with reference to <FIG>, recall that the depiction is of a period following start up of a dead, non-producing production line. Therefore, jumping ahead to the circulatory exit at the production outlet <NUM> reveals only gas fraction, consistent with the non-production initially at hand. However, following start-up of the pump <NUM> of <FIG>, for example via external priming if necessary, the influx of production fluid occurs as indicated with the liquid fraction <NUM> falling to the bottom of the assembly <NUM>. By the same token, a gas compressor may be coupled to the piping in advance of the inlet <NUM> to increase the liquid fraction <NUM> entering the assembly <NUM>. This may be by way of a separate discrete compressor between the splitter assembly <NUM> and the pump <NUM> or the pump <NUM> may be a liquid tolerant compressor with pump functionality.

Recall that the liquid fraction <NUM> is allowed to pass below the outer tube <NUM> to reach a Weir barrier in the form of an inner tube <NUM> where the level rises until reaching the top of the inner tube <NUM>. With added reference to <FIG>, this top level may be reached and the liquid begin to spill over and into the inner tube <NUM> to reach the production outlet <NUM>. Notice at this spill over location (e.g. <NUM> of <FIG>), the gas <NUM> and liquid <NUM> fractions begin to remix together as the production fluid heads toward the outlet <NUM>.

Recall also that the deflector <NUM> has encouraged sand and other debris to remain with this portion of the circulating liquid fraction <NUM>. Thus, as the liquid is produced through the production outlet <NUM> sand and other debris may be produced as well. This is in contrast to the portion of the liquid fraction <NUM> that alternatively leaves the recirculation outlet <NUM> for benefit of decreasing GVF at the pump <NUM> of <FIG>.

Referring now to <FIG>, a schematic representation of an alternate embodiment, not forming part of the present invention, of a splitter assembly <NUM> for a subsea pump system (e.g. <NUM>) is illustrated (see also <FIG>). In this embodiment, a Weir type of configuration is attained through the unique arrangement of conventional piping components. For example, the inlet <NUM> delivers production fluid to a conventional large volume chamber which serves as the outer tube <NUM>. Liquid fraction in this outer chamber <NUM> may be allowed to flow out through an exit line <NUM> and over to another chamber <NUM> which serves the inner tube function detailed hereinabove. Specifically, this chamber <NUM> may serve as a Weir type of barrier against which liquid fraction may rise until spilling over into the exit line (e.g. the production outlet <NUM>). As with the embodiments described above, this is where the gas and liquid production fractions will recombine. Meanwhile, the liquid fraction exiting the outer chamber <NUM> is also presented the option of exiting through the recirculation outlet <NUM> for ultimately routing to a pump <NUM> to promote continued function (see <FIG>).

Referring now to <FIG>, with added reference to <FIG>, a schematic representation of another alternate embodiment, not forming part of the present invention, of a splitter assembly <NUM> for a subsea pump system <NUM> is shown. This embodiment is largely the same as that illustrated in <FIG>. However, in this embodiment, the gas fraction exits the outer tube/chamber <NUM> through a pipe at the top and the liquid fraction for production is allowed to similarly exit from below the outer tube/chamber <NUM>. This more restricted or choked manner of circulation may help avoid sand circulation through the gas fraction and increase pressure in the liquid fraction below to encourage sand production ultimately toward the outlet <NUM>. Additionally, in this embodiment, the architecture of the inner chamber <NUM> directs the liquid fraction for production to recombine with the gas fraction at a higher level, near a terminal end of the chamber <NUM> where the production outlet <NUM> is now located.

Referring now to <FIG>, a flow-chart summarizing an embodiment of utilizing a splitter assembly of a subsea pump system to startup <NUM> and maintain <NUM> production flow of higher GVF fluids is illustrated. As indicated at <NUM> production is routed from the multiphase pump to a splitter assembly utilizing unique architecture. Due to this architecture, the gas fraction of the production fluid may be split from the liquid fraction as noted at <NUM> with a portion of the liquid fraction being made available for circulation back to the pump (see <NUM>). Note that from startup at <NUM>, to gas separation at <NUM> and liquid fraction routing at <NUM>, a dead well may be started by effectively producing a gas cap at the splitter assembly as a means of reducing pressure at the wellhead to begin flowing. Regardless, throughout, the GVF of the production fluid that is actually pumped by the pump may be kept to a minimum to enhance pump function and avoid gas locking. As indicated at <NUM>, the remainder of the liquid fraction may then be combined with the gas fraction and produced.

Embodiments described hereinabove include a system and techniques for cost effective production assistance when faced with higher GVF fluids. These embodiments allow for continuous pumping to aid production from subsea oilfield wells whether the production fluid is predominantly liquid or has transitioned to higher GVF production. Thus, more costly gas lift equipment and techniques may be avoided. Further, in circumstances where higher GVF has lead to gas lock and dead wells, the equipment and techniques detailed herein may be retrofitted onto such systems to restart pumping and attain effective production.

Claim 1:
A pump system for use at an oilfield accommodating a well containing multiphase production fluid, the pump system including a multiphase pump (<NUM>) and a splitter assembly (<NUM>), the splitter assembly comprising:
an inlet (<NUM>) in fluid communication with an outlet of the multiphase pump (<NUM>) at the oilfield;
an outer chamber (<NUM>, <NUM>) coupled to the inlet (<NUM>) for receiving multiphase fluid of the well from the multiphase pump (<NUM>) with a gas fraction (<NUM>) of the fluid over a liquid fraction (<NUM>) of the fluid;
a recirculation outlet (<NUM>) at a lower portion of the outer chamber (<NUM>; <NUM>) to direct a first portion of the liquid fraction (<NUM>) to the multiphase pump (<NUM>) to enhance pumping thereof;
an inner chamber (<NUM>; <NUM>) in fluid communication with the outer chamber (<NUM>; <NUM>) and configured to receive a second portion of the liquid fraction (<NUM>), wherein the second portion of the liquid fraction (<NUM>) pools in the inner chamber (<NUM>; <NUM>) until the liquid level reaches a spillover location and flows out of the inner chamber (<NUM>; <NUM>); and
a production outlet (<NUM>) in fluid communication with the spillover location in the inner chamber (<NUM>, <NUM>), the production outlet configured to receive the gas fraction (<NUM>) and the second portion of the liquid fraction (<NUM>) exiting the inner chamber (<NUM>; <NUM>) for production;
characterized in that the outer chamber is an outer tube (<NUM>; <NUM>) and the inner chamber is an inner tube (<NUM>; <NUM>), the inner tube (<NUM>; <NUM>) being disposed within the outer tube (<NUM>; <NUM>).