Setting cement plugs in a borehole is a common oilfield operation. A cement plug involves a relatively small volume of cement slurry placed in a borehole for various purposes: - to sidetrack above a fish (a piece of equipment stuck in a borehole that cannot be removed) or to initiate directional drilling; - to plug back a zone or plug back a well; - to attempt to solve lost circulation problems during the drilling phase (see above); and - to provide an anchor for openhole tests.

One problem in setting cement plugs is that it can be difficult to retain the cement slurry in position in the borehole, especially when the plug is being set above the lowest point of the borehole ("off bottom"). Since cement slurries are often denser than borehole or drilling fluids, there is a natural tendency for the slurry to sink to the bottom of the well. US 5,667, 015 proposes an apparatus to stop this happening.

There have been previous proposals that attempt to address this problem by using a downhole sleeve to confine the cement to a specific zone of the well. Examples of this can be seen in US 2,796, 134 and US 3,032, 115. In both cases, a drillable delivery pipe is placed in the well with openings to allow cement to pass into the annulus. A sleeve is located around the openings and cement is pumped through the pipe into the sleeve to inflate it and seal against the formation. Suitable materials proposed for these uses are plastic or rubber. Once the cement has set, the delivery pipe is disconnected above the sleeve and the portion in the sleeve drilled out leaving the cement sheath in place. In these schemes, an impermeable sleeve is used to ensure that the cement is confined to the area of interest. None of these proposals demonstrate particularly effective cement properties in the region of particular interest: the borehole wall.

It is an object of the present invention to provide an apparatus which obviates or mitigates these problems.

A first aspect of the invention provides apparatus for setting a plug in a borehole, comprising: a delivery pipe for delivering a plugging fluid and a sleeve section comprising a flexible, expandable sleeve secured to delivery pipe and into which plugging fluid is delivered; wherein the apparatus further includes operable ports to allow fluid to flow into the borehole, and a disconnect mechanism to allow the delivery pipe to be disconnected from the sleeve, wherein the sleeve is formed from a permeable material.

The operable ports can be located in the sleeve and/or in the delivery pipe. The ports allow fluid to be circulated directly into the borehole once the sleeve has been filled to a desired degree. These ports can comprise shear membranes or other one-time operating devices, or pressure operated valves or the like. It is particularly preferred to operate the ports by fluid pressure in the delivery pipe.

When located in the delivery pipe, it is preferred that the ports are situated downstream of the disconnect mechanism such that further pumping of fluid can be used to deliver an operating device such as a ball or dart to the disconnect mechanism.

One particularly convenient form of disconnect mechanism includes the use of shear pins. Other mechanisms can also be used.

Preferably, there is a non-return valve in the delivery pipe so as to prevent fluid flowing back from the sleeve through the delivery pipe. The valve is preferably a float valve, although other forms of non-return valves may also be appropriate. The valve typically operates when the sleeve is filled with fluid and is unable to expand further such that further flow of fluid is low. Consequently, when flow of fluid is stopped or diverted elsewhere, the sleeve is maintained in its expanded state.

The delivery pipe preferably comprises a drillable stinger that is left in the borehole after the plugging fluid has filled the sleeve. This stinger can comprise a tube formed

from aluminium or fibre reinforced plastics material or other such material. The delivery pipe typically has a diameter that is smaller than the diameter of the borehole in the zone to be stabilised and smaller than the diameter of any casing above this zone.

It is particularly preferred that the drillable stinger is connected to the remaining part of the delivery pipe by means of a releasable connector which is operable such that the remaining part of the delivery pipe can be disconnected from the stinger and withdrawn from the borehole after the treatment fluid has been placed.

Openings can be provided in the sidewall of the delivery pipe, the sleeve being connected to the outside of the pipe above and below the openings. Diverters can be positioned on the delivery pipe so as to cause plugging fluid flowing through the openings to flow in an axial direction.

The sleeve preferably has a mesh-like structure that can be formed, for example, by weaving or knitting fibres. Suitable fibre materials are steel, glass fibre, carbon fibre, Kevlar and other such materials, and combinations thereof. The mesh is typically sufficiently loose to allow expansion of the sleeve when filled with treatment fluid without losing its ability to retain at least part of the fluid. This expansion can be up to 50% of the unexpanded diameter of the sleeve, although even greater expansion can be achieved according to the design of the mesh and the degree of filtering required.

The unexpanded diameter of the sleeve can typically be 3-4 times the diameter of the delivery pipe.

Where the plugging fluid has both solid and liquid components, the portion of the treatment fluid passing through the sleeve can be primarily a liquid component of the fluid so as to form an enriched solids layer of fluid near the sleeve. Materials can be included in the fluid to encourage formation of the enriched solids layer by filtering at the sleeve. Typical materials are fibres such as polypropylene, novoloid resin or other such materials. Also, reinforcing fibres such as metal fibres can be included to achieve appropriate mechanical properties of the cement when set.

In another embodiment of the invention, a whipstock is located on the delivery pipe above the sleeve. In this case, it is preferred that the disconnect mechanism is located above the whipstock.

A second aspect of the invention provides a method of installing a plug in a borehole, comprising: positioning an apparatus according to the first aspect of the invention in the borehole at the position at which the plug is to be installed, pumping fluid into the sleeve via the delivery pipe so as to inflate the sleeve, operating the ports to allow fluid to flow into the borehole, operating the disconnect mechanism and withdrawing the delivery pipe from the borehole.

It is preferred that, where the fluid is a cement slurry comprising solid and liquid components, the predetermined degree of inflation is sufficient to cause a solids enriched layer (cake) to build up inside the sleeve. The inflation can continue until cement passes through the sleeve so as to form a layer on its upper surface.

Operation of the ports is typically achieved by pumping fluid in the delivery pipe at a predetermined pressure. Where the ports comprise shear membranes, this is the pressure required to shear the membrane.

The disconnect mechanism can be supplemented-or replaced-by a piece of fragile pipe that can be sheared by applying a pull force, once the fluid is placed and the cement is set. The operating member, typically a ball or dart, is used to operate the disconnect mechanism, for example by using fluid pressure to shear pins in the mechanism.

Following the step of flowing the treatment fluid into the sleeve, it is preferred to leave at least the portion of the delivery pipe in the zone to be stabilised. In the case in which the method is performed before the borehole drilling is complete, the portion of the delivery pipe in the well is drilled out. It is particularly preferred to disconnect the portion of the delivery pipe extending from the zone to the surface from the portion remaining in the zone prior to recommencing drilling.

Typically, treatment fluid is placed in the sleeve so as to expand the sleeve until it fills substantially all of the annulus in the zone to be stabilised. When the annulus is very large or irregular, it may be desirable to fill the sleeve until a predetermined pressure of fluid is reached.

Once the sleeve is filled and/or the predetermined pressure reached, further fluid can be flowed directly into the annulus above the sleeve.

The treatment fluid is preferably a cement slurry comprising liquid and solid components. The effect of the sleeve is to concentrate the solid component of the slurry near the borehole wall leading to improved physical properties in this region.

To avoid excessive loss of solids through the sleeve, it is preferred to include suitable plugging materials, such as fibres, in the slurry.

It is particularly preferred to use a cement slurry with an optimised particle size distribution of solid materials to obtain a high packing volume fraction.

Other treatment fluids may also be useful in the present invention, for example dispersed gels or polymers that can concentrate at the borehole wall.

A method of setting a whipstock according to the invention comprises - positioning an apparatus according to the first aspect of the invention, including a whipstock, in the well borehole at a determined position; - inflating the sleeve by pumping cement through the delivery pipe; - operating the ports so as to allow cement to fill the borehole around the whipstock ; - operating the disconnect mechanism; and - withdrawing the delivery pipe from the borehole.

Following setting of the cement, drilling can resume, in a direction determined according to the orientation of the whipstock.

Examples of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows an embodiment of an apparatus according to the invention; Figure 2 shows the embodiment of Figure 1 in use; Figure 3 shows a later phase of the use of the embodiment of Figure l ; and Figure 4 shows a borehole that has been stabilised after under-reaming.

The present invention involves the use of a flexible and permeable sleeve made from a woven material to permit placement of a cement plug in a borehole under naturally unstable conditions while forcing the cement slurry to remain in the borehole at the desired position. The unstable conditions in which the apparatus and method of the invention can be used include lost circulation conditions such as can occur in massively fractured formations, or off-bottom positioning of the plug with a layer of borehole or drilling fluid being present in the borehole below the plug.

A setting tool according to one aspect of the invention is shown in Figure 1 and comprises a drillable tube or stinger 10 with a flexible permeable sleeve 12 wrapped around it. The stinger 10 may be made of aluminum or glass fiber composite, for instance, or any other suitable, drillable material. Orifices 14 are drilled in the stinger 10 to allow a cement slurry to flow into the sleeve 12 to inflate it. A diverter 16 is installed around the orifices 14 to direct the flow in an axial direction. A float valve 18 is placed above the sleeve 12 to prevent flow back of any cement through the drillable stinger 10. Above the float valve 18, ports 20 are provided that are normally closed by shear membranes 22. These shear membranes 22 are designed to open when enough differential pressure is applied across them. Above these ports 20, a disconnect mechanism 24 is installed that can be activated using a ball or a dart in a known manner. The stinger is connected to a delivery pipe 26 (drill pipe or coiled tubing) by the disconnect mechanism 24.

In use (see Figure 2), the tool is lowered in the borehole 30 to the depth of interest. A cement slurry is pumped through the delivery pipe 26 and stinger 10 so as to inflate the sleeve 12 until it comes in contact with the borehole walls 32. Pumping is continued so that a cement cake 34 is formed inside the sleeve 12, and the pressure inside the sleeve 12 increases. This phase can be considered as a short squeeze step.

The cake 34 will provide higher mechanical strength due to its increased solids

content. As a pre-defined pressure is reached, the shear membranes 22 closing the ports 20 are broken and the cement slurry flows through these ports 20 into the borehole 36 above the sleeve 12. Since there is no further flow into the sleeve 12, the float valve 18 closes. When the whole cement slurry is pumped, a ball or a dart (not shown) is launched that allows disconnection of the tool from the delivery pipe 26 by activation of the disconnect mechanism 24. The delivery pipe 26 (drill string) is then pulled out using the well known balanced plug rules to prevent mixing the cement with the displacement fluid and the cement slurry is allowed to set.

The apparatus and method described above has the advantages of : prevention of fluid swapping-the cement slurry is not mixed with the fluid left underneath the tool; reduced loss of fluid to the formation; and strong mechanical properties of the cement, allowing for instance side-tracking (this is made possible by either the squeeze step, or the use of metallic fibers or both together).

The cement slurry used in this process typically includes fibers or mixtures of fibers.

These fibers act in various ways, first by helping building a cake on the internal surface of the sleeve 12, then by preventing loss of cement from the borehole 36 above the sleeve 12 and finally by increasing the mechanical properties of the set cement to a point such that it will withstand subsequent drilling operations. When only a single type of fiber is used, flexible fibers are preferred: the use of such fibers has previously been proposed for use in lost circulation situations and they prevent the cement sheath from disintegrating after being drilled. When a mixture of fibers is used, a first type of fiber can provide the cement slurry with strong mechanical properties, which are beneficial for instance for kick-off cement plugs. These fibers are for instance the metallic fibers described in WO 99/58467. The second type of fiber can be similar to the flexible fibers described above. The fibers do not need to be added homogeneously to the whole slurry. For example, the flexible fibers can be used for the part of the slurry that inflates the sleeve 12, while metallic fibers can be used in the second part (filling the borehole 36 above the sleeve 12), which needs strong mechanical properties.

In order for the cement slurry to build a cake inside the sleeve 12, it is preferable that the slurry contains a large volume fraction of solids and does not possess too large fluid loss control properties. A composition that provides such properties can utilise an optimised particle size distribution for the solid components of the slurry such as is described in EP 0 621 247.

Where a low density cement slurry is required, the approach proposed in WO 01/09056 is preferred. An example of such a base low density slurry is given in Table 1 below: Table 1 BVOB = by volume of total solids in slurry gal/sk = gallons per sack of cement ppg = pounds per gallon porosity % = (volume of water/volume of slurry) * 100 Class G Cement (20-25 micron) 35% BVOB Crystalline Silica (1-10 micron) 10% BVOB Aluminium Silicate Microspheres SG 55% BVOB 0.65-0. 85 (100-400 micron) Polypropylene Glycol antifoam agent 0.025 gal/sk Water 5. 029 gal/sk Density 12. 13 ppg Porosity 43% A suitable base higher density slurry is given in Table 2 below (same abbreviations as Table 1) : Table 2 Class G Cement (20-25 micron) 40% BVOB Crystalline Silica (1-10 micron) 10% BVOB Iron Oxide weighting agent, SG 4.8-6. 0 10% BVOB (100-600 micron) Silicon Dioxide weighting agent, SG 2.5 40% BVOB - 2.8 (100-600 micron) Polymeric Aliphatic Amide fluid loss 0. 3% BVOB control additive Polypropylene Glycol antifoam agent 0.025 gal/sk Water 2. 625 gal/sk Density 18. 7 ppg Porosity 40. 5%

Fibre material is mixed with the base slurry to provide structure to the mass. Such fibres can be metallic (see, for example, WO 99/58467) or polymeric (see, for example, PCT/EP02/07899). Two suitable fibre materials and a proposed level of use in the cement slurries are given in Table 3 below: Table 3 Fibre Material Concentration Novoloid polymer fibres (18-22 mm) 3 g/l of slurry Amorphous cast metal fibres (5-10 mm) 100 g/1 of slurry The sleeve 12 can be formed from a woven carbon fibre or Kevlar material such as the preformed tubular materials available from A&P Technology under reference RA3827SPAR and RF1345 (it will be appreciated that other materials can also be used). For a nominal 8 inch borehole, an 8 inch sleeve (unexpanded) is proposed. The sleeve 12 is attached to the stinger 10 by means of clamps 38 that are made from a drillable material such as epoxy resin materials, aluminium, etc..

Variations in the apparatus and method described above are possible within the scope of the invention. For example, where there is no need to drill through the plug once it has been set, the volume of cement might be lower and the layer of cement on top of the sleeve can be relatively small or non-existent.

A further embodiment of the invention is shown in Figures 3 and 4 and comprises a tool and method for setting a whipstock in a borehole. The tool comprises a two-part circulating mandrel 40 (delivery pipe) defining the backbone of the tool. A landing

shoe 42 is connected at the bottom of the lower part of the circulating mandrel 40.

The sleeve 44 is wrapped around the lower part of the circulating mandrel 40 just above the landing shoe 42. The sleeve 44 is similar to that described above but includes ports 45 (covered by the shear membranes 46 may also be placed above the sleeve, as on Figure 1) installed at the upper part thereof. Orifices 48 are provided in the mandrel 40 that allow cement to flow into the sleeve 44. A diverter is installed around these orifices to direct the flow in an axial direction. These orifices 48 are normally close by a sliding sleeve 50 connected to the landing shoe 42. A wiper dart or a ball is used to actuate the sliding sleeve 50 to open the orifices 48 and prevent flow back of any fluid through the landing shoe 42.

A milling whipstock 52 is set on the upper part of the circulating mandrel 40 well above the upper clamp 54 of the sleeve 44. The whipstock 52 may be made with a steel shell 56 permanently connected to the circulating mandrel 40 by the means of welded ribs (not shown). Partially enclosed in the whipstock 52, a disconnect mechanism 58 is provided that can be activated for instance using a ball or a dart as described previously.

The tool is connected to a drill string 60 and lowered in the borehole 62 at the depth of interest. The tool is then oriented using conventional directional drilling techniques. A dart or ball is released at the interface between circulating mud 64 and cement slurry 66. The dart or ball 68 closes the landing shoe 42 and the cement slurry is pumped through the circulating mandrel 40, which inflates the sleeve 44 until it comes in contact with the borehole walls. Pumping is continued so that a cement cake is formed inside the sleeve 42 and the pressure inside the sleeve 42 increases. As a pre-defined pressure is reached, the shear membrane 46 placed at the top of the sleeve 42 is broken and the cement slurry flows through the ports 45 into the borehole. A defined volume of cement slurry is pumped to embed the whipstock 52 in the borehole. When the whole cement slurry is pumped, a ball or a dart 70 is launched that allows disconnecting the tool from the drill string 60. The drill string is then pulled out using balanced plug rules to prevent mixing the cement with the displacement fluid and the cement slurry is allowed to set.

A steerable BHA 72 is run into the borehole, the cement above the whipstock 52 drilled away and the kick off initiated and new bore drilled using directional drilling techniques.

The use of the plug setting tool and methods of the invention allow accurate placement of the whipstock even when off-bottom. Also, the ability to accurately place a plug of very strong cement allows more accurate control of the kick-off process, even if a whipstock is not used. The problem of the cement slurry sinking to the bottom of the well is avoided as before.