WO2004081341A1

WO2004081341A1 - Downhole reversible pump

Info

Publication number: WO2004081341A1
Application number: PCT/GB2004/001015
Authority: WO
Inventors: Graeme John Collie
Original assignee: Fmc Technologies Inc.
Priority date: 2003-03-10
Filing date: 2004-03-09
Publication date: 2004-09-23
Also published as: GB0305455D0; GB2399360B; GB2399360A

Abstract

A reversible submersible pump (15) connected to production tubing (16) may be used to lift production fluid from a well (10) and to pump injection fluid into the formation (11). The pumping pressure may be reduced or reversed to allow access to the well without substantial loss of production fluid to the environment.

Description

DOWNHOLE REVERSIBLE PUMP

Background of the Invention

The present invention relates to a method for use in a well comprising a submersible pump suspended in the well and having means for conveying pumped fluid to the surface.

When prospecting for oil or gas, a well is drilled into a formation containing the hydrocarbons, tubing is installed in the well suspended from a wellhead and a christmas tree is placed on the wellhead for controlling the flow of hydrocarbons from the well.

Normally, the fluids in the formation are under a pressure that is high enough for the well to flow spontaneously. However, many formations have a pressure too low to enable the fluids to flow freely from the well and therefore require the addition of mechanical lift systems for raising fluids to the surface. Traditionally this has been achieved by placing a mechanical pump in the well, the pump being operated by a rod extending to the surface and attached to a means of reciprocation. This technology is traditionally known as a "nodding donkey".

In recent years, hydraulic or electrical pumps have been employed. These pumps are located within the well and driven by either hydraulic fluid or electric power supplied from the surface. In a subsea environment, this has become the only practical solution, since a plunger pump is clearly impossible in a well located hundreds or even thousands of meters below the water surface.

Wells that require artificial lift have a relatively short production life, often as low as 5 years compared to 25 years or more for natural lift wells. As a consequence after a relatively brief period the subsea equipment is no longer required and the well is abandoned.

Several types of pumps are known, one example is shown in UK published application no. 2360302 which shows a pump suspended on coiled tubing and arranged to pump the hydrocarbons up through the tubing. Other examples are shown in US patents nos. 4266607, 4928771 and 5620048. Another method of producing fluids from low pressure formations is by injecting fluid into the formation at a distance from the producing well, thereby increasing the pressure in the formation enabling hydrocarbons to flow towards the producing wells. This entails drilling a second well into the formation, adding to the expense of developing a field.

Even a to pressure formation will usually produce hydrocarbons unaided for an initial period, but pressure will quickly drop and the well will require artificial lift and/or injection wells to continue producing.

The drilling and completion of subsea wells is a very costly and time consuming exercise. Because low pressure wells can have a short life, it is often deemed uneconomic to develop such fields.

Summary of the Invention According to the present invention a method for producing hydrocarbons from a well comprises installing in the well a submersible pump in fluid communication with a production conduit and whose pumping direction when so installed is reversible. During a production phase, the pump may thus be run in one direction to lift hydrocarbons from the well, for example to cause the hydrocarbons to flow from a production outlet at the wellhead, as is conventional. In a subsequent production phase, the production fluid outlet can be connected to a source of injection fluid and the pumping direction reversed, such that the injection fluid is forced into the formation. In this way a producing well can be turned into an injection well saving considerable amounts of money in drilling dedicated injection wells.

Correspondingly, the invention provides apparatus for producing hydrocarbons from a well, comprising a submersible pump installed in the well in fluid communication with a production conduit, the pumping direction of the pump when so installed being reversible.

Wells with artificial lift systems often require the use of external Christmas tree caps. In the event of a problem with the downhole completion the tree cap must be removed and the well worked over. If pressure is present below the tree cap then the situation becomes more complicated. Where an internal tree cap is employed it may be removed while maintaining full well control by pulling the tree cap through a riser and BOP system. With an external tree cap the riser/BOP cannot be attached to the tree until the tree cap has been removed, so exposing the environment to produced fluids and possibly requiring the well to be killed. This makes the removal of the tree cap problematic from a well control, environmental and safety point of view.

The situation would not be a considered as problem if the wellhead shut in pressure is below that of the ambient seabed pressure, although there may still be environmental issues surrounding the release of hydrocarbons to sea. However, for most wells requiring artificial lift the initial wellhead shut in pressure is significantly above ambient pressure, and may remain so for the first few years of production. If a workover is required there will be difficulties in controlling the well prior to removing the tree cap. Conventional well kill techniques may be impossible e.g. due to the presence of permanently installed coiled tubing, lack of DHS V etc.

According to an alternative preferred use of the method and apparatus of the invention the pump pressure may be reduced or reversed to create a pressure reduction in the production conduits installed in the well, permitting' well entry, e.g. by removal of a tree cap, without substantial loss of production fluid to the environment. Thus by operating the pump in reverse, access to the well may be safely achieved, e.g. by removing an external tree cap and connecting a BOP to the tree, even if a downhole safety valve has malfunctioned or is not present, and even if the formation pressure is such that the pressure contained beneath the tree cap in the absence of any pumping exceeds the ambient external hydrostatic pressure. Thus it will be possible to create a pumped pressure that is large enough in conjunction with the hydrostatic head of production fluid in the tubing to hold the well at bay long enough to remove the tree cap and secure the riser/BOP. Certain designs of positive displacement pumps, e.g. reciprocating or other pumps with inlet and/or outlet valves, may serve to contain any formation pressure merely by stopping the pump.

The change in pressure produced by stopping or reversing the pump may not be large in absolute terms (e.g. in comparison to the absolute pressure of the production fluid just below the pump), but the pressure change can be sufficient to convert a positive hydrostatic pressure difference (internal minus external) across a tree cap into a zero or negative difference, thereby serving to control the well in a completion employing a downhole pump for artificial lift. Preferably the pressure difference is accurately controlled by regulating the pump pressure to produce a zero or very slightly negative tree cap pressure difference, thereby to avoid release of well fluid into the environment on removal of the tree cap, whilst avoiding the adverse effects on subsequent production of injecting fluid back into the formation at substantial pressure ("bullheading")- Where the tree cap pressure difference (internal minus external) is naturally negative, it may not even be necessary or desirable to reverse the pump, but merely to reduce the upward pumping pressure so as to produce a zero or small negative tree cap pressure difference.

Brief Description of the Drawings

Illustrative embodiments of the invention will now be described, reference being made to the accompanying drawings, in which:

Fig. 1 is a schematic drawing of a well with a pump installed in the tubing. Fig. 2 shows the well of figure 1 converted for injection.

Description of the Preferred Embodiments

In Fig. 1 a well 10 is shown penetrating a producing formation 11. The well is isolated from the formation by casing 12. Production tubing 16, usually supported by tubing hangers (not shown) in a wellhead 13 extends down the well for conveying produced fluids to the surface through an outlet 20 and flowline 18. The wellhead 13 and Christmas tree 14 is of a standard design and familiar to those skilled in the art. A packer 17 isolates the lower part of the wellbore.

Attached to the lower end of the tubing is a reversible flow submersible pump installation 15. The pump can be driven by an electric motor fed by a power cable 25 extending from the surface. A control unit 26 controls the electric motor. This layout is only shown as an example and many other types of pump/motor assemblies can be used with the invention, for example hung from cable or from coiled tubing or a type where a retrievable rotor unit is located in the tubing as shown in US 5620048, or a pump driven by turbine, itself turned by hydraulic fluid provided from the surface.

Many types of pumps may be suitable for this purpose. A centrifugal pump can be equipped with movable vanes whereby moving the vanes changes the direction of the flow. In a screw pump it will only be necessary to_change the direction of the electric motor to obtain a reversal of the flow direction. Pumps which are reasonably efficient in either pumping direction are preferred. Reversible positive displacement pumps are therefore suitable also.

Produced fluids flow from the formation and into the lower end of the pump as indicated by the arrows in the drawing. The fluids are transported up the production tubing 16 and through the wellhead 13 to the flowline 18. At 31 there is indicated a possible processing unit and 32 indicates a possible booster pump. The processing unit can easily be converted to an injection unit by connecting a former production outlet with a means for supplying inj ection fluid(s) .

The well might flow by its own pressure for a short time. When the pressure drops below what is necessary to drive the fluids up the well column, the submersible pump unit will be deployed in the well. This drives out fluids from the well and into the flowline.

If after some time, the well can' no longer sustain production due to depletion of the reservoir, the well may now be turned into an injection well by connecting a production outlet 20 to a source of injection fluid, for example seawater, CO₂, etc, and reversing the pump 15. This will increase the pressure in the formation, forcing fluids to flow into a nearby well and thus enabling more comprehensive draining of the reservoir. This arrangement is shown in Fig. 2.

Another example of the use of the invention is in exploiting a low pressure reservoir. A number of wells are drilled and equipped with pumps to draw off the hydrocarbons. Then other wells are drilled in the same formation. As the new wells come into production, the older wells are turned into injection wells by simply reversing the pumps, enabling the new wells to produce without artificial lift. In another use of the invention, the pump pressure is reduced or reversed to lower the pressure in the Christmas tree and enable access to the well in a safe manner. It is particularly suitable for wells having tree caps (e.g. external tree caps 22) which cannot be retrieved through a BOP. Prior to re-entering the well the void below the tree cap is monitored for pressure. If a positive hydrostatic pressure difference (internal minus external) is found to be present then the pump is reversed, lowering the pressure in the tubing (and thereby in the tree). Otherwise the pumping pressure is reduced to provide a zero or preferably a slightly negative pressure difference e.g. of about lOOpsi (0.69MNm^" ) across the tree walls. The external tree cap can now be removed and a BOP placed on and locked to the tree. Now any necessary refurbishment, modifications or repairs can be carried out on the well or completion (e.g. a leaking tubing hanger or downhole safety valve can be removed through the BOP and riser while maintaining control of the well). After work is carried out, the BOP is removed. The tree cap is mounted to the tree and production can be resumed.

Claims

1. A method for producing hydrocarbons using a well, comprising the step of installing in the well a submersible pump in fluid communication with a production conduit; characterised in that the pumping direction of the pump when so installed is reversible.

2. The method of claim 1 comprising the further steps of: during a production phase, running the pump in one direction to lift hydrocarbons from the well, and during a subsequent production phase, connecting a production fluid outlet of the well to a source of injection fluid and reversing the direction of the pump, such that the injection fluid is forced into the formation.

3. Apparatus for producing hydrocarbons from a well, comprising a submersible pump installed in the well in fluid communication with a production conduit, characterised in that the pumping direction of the pump when so installed is reversible.

4. A method of entering a well without substantial loss of production fluids, characterised by the steps of: installing a submersible pump in the well in fluid communication with a production conduit to lift production fluid out of the well; and reducing or reversing the pump pressure to permit said well entry.

5. The method of claim 4 in which the production conduit is in fluid communication with a wellhead and, in the absence of pumping, the internal pressure minus the external pressure at the wellhead is positive, comprising the step of reversing the pump pressure to permit said well entry.

6. The method of claim 4 in which the production conduit is in fluid communication with a wellhead and, in the absence of pumping, the internal pressure minus the external pressure at the wellhead is negative, comprising the step of reducing the pump pressure to permit said well entry.