GB2482703A - Drilling apparatus for enlarging or maintaining a borehole - Google Patents

Abstract

A drill bit (24, figure 1) for maintaining or enlarging the bore of a borehole. The apparatus comprises a hollow tube 42 which is insertable into the borehole and which is advanced along it in use. The apparatus further comprises a blade means (54, figure 2) and an aperture 64 which communicates with the interior of the tube and is located axially forward of the blade means. The removed material may be directed into the tube and the aperture may be a slit and has an edge that forms the blade.

Description

ENLARGING BOREHOLES

This invention relates to an apparatus and method for enlarging boreholes. Boreholes are used in drilling operations to enable pipes or other drilling equipment etc. to communicate with underground cavities from which fluids (e.g. gas and oil) are to be extracted. In a typical drilling operation, a borehole is sunk into the ground from the surface using a drill string whose tip proceeds along a desired direction. In the simplest drilling operations, the borehole is sunk vertically and straight, although it is possible, and nowadays quite common, for the drill string to follow a serpentine path, controlled using computers and directional drilling heads, to avoid underground obstacles and/or to intersect a number of cavities to be mined.

In many situations, the borehole is relatively stable inasmuch as its dimensions remain substantially unchanged over extended periods of time.

However, this very much depends on the geology of the drill site, soil chemistry, weather conditions, water table levels etc. In other situations, however, the borehole is susceptible to collapsing under the weight of the earth above it, and due to swelling and shrinkage effects that can occur naturally, or as a result of the drilling process itself.

One situation where boreholes tend to be relatively unstable is where the borehole is sunk through clay. In may situations, clay can become destabilised so that it behaves like a plastic material that flows under expansion and formation stress. This can cause the clay to flow or swell to partially block the borehole, especially where the borehole is non-vertical and/or where the clay is relatively unstable. Thus, a freshly drilled borehole in clay can rapidly become obstructed, thereby impeding the subsequent insertion of pipes and/or other drilling equipment.

A known solution to this problem is to line the borehole using a tubular inset to stabilise its sidewalls. Tubular stabilising inserts can be continuous, although for reasons of economy, flexibility and ease of handling/transport, reticulated (net-like) liners are often used. However, a reticulated liner can be largely ineffective in stabilising a borehole drilled through a viscous liquid, such as clay, because the clay is able to flow through the holes in the mesh and thus continue to obstruct/close-off the borehole.

This invention is particularly directed to the pre-completion phase of borehole construction. A typical well-drilling operation includes the initial drilling stage whereby the borehole is drilled into the ground. This is followed by a test and measurement phase in which the dimensions and continuity of the borehole are checked and modified using test and enlargement strings until the borehole complies with the desired specifications. Once the borehole has been passed as being compliant with the design specifications, it is handed over to a pre-completion team, whose job it is to line the borehole and prepare it for the production strings to be inserted.

The test and measurement phase often includes testing the flow characteristics of the borehole using brine, which is pumped into and out of the borehole and its flow characteristics measured. However, it is often not fully appreciated, by the drilling team, that if the specific gravity of the brine (also known as the "pre-completion fluid") is lower than that of the mud through which the borehole has been drilled, then the brine can cause the mud/clay to swell, thereby damaging the well.

It is also believed that various other factors can influence the stability of the clay, including the specific gravity or salinity of the mud or pre-completion fluid used, having a pH in of range 9-10 (in which case there may be active hydrogen ions that can destabilise the mud/clay), prolonged jetting (especially around dog-legs), by the bit and/or under-reamer during the actual drilling operation, and lost circulation.

As such, a measurement string could determine that the borehole is of an acceptable diameter along its entire length, and the pressure/flow testing using the brine could confirm this. However, the brine itself could have caused time-delayed damage to the borehole, which is not detected until the pre-completion stage begins, by which time it is too late to re-drill the borehole. Such a situation has the potential to render the borehole uneconomic to use, which can result in expensive re-drilling of a side well or abandonment of the well altogether.

Moreover, where the borehole is sunk through clay or other materials that are susceptible to movement or swelling, its dimensions can change, in a matter of hours or days, and in the worst cases, can become blocked completely.

Thus, it is critical that the pre-completion team is able to begin work as soon as possible after the initial drilling phase ends so as to minimise the amount of collapse/blockage that can occur. Nevertheless, a certain amount of movement is almost inevitable, but it is uneconomic to re-drill the borehole where the well is only partially obstructed. As such, a need therefore arises for the pre-completion team to be able to affect repairs (i.e. post-drilling, but pre-screening) to a partially collapsed/blocked borehole to avoid the need for re-drilling and to bring forward well completion as much as possible. A need also arises more generally to preserve the internal dimensions of a borehole, and/or for clearing a partially obstructed borehole to enable further drill strings, pipes or other equipment to be inserted therethrough.

According to a first aspect of the invention, there is provided an apparatus for enlarging or maintaining the bore of a borehole comprising: a hollow tube axially receivable in the borehole; means for moving the tube axially within the borehole in a first direction; blade means located radially outwardly of the hollow tube; and an aperture communicating with the interior of the tube and located axially forward of the blade means.

In accordance with the invention, axial movement of the tube within a borehole whose bore diameter is greater than that of the blades will be substantially uninhibited, i.e. there will be little or no interference between the blades and the sidewalls of the borehole. However, where the bore diameter of the borehole is less than that of the blades, the blades will cut into, scoop or skim the sidewalls of the borehole thereby enlarging it to a predetermined dimeter as determined by the amount that the tips of the blade or blades project radially outwardly from the tube.

One problem with known bore enlargers of a similar type is that the removed material remains within the borehole and can be pushed along to create localised blockages and/or to inhibit the movement of the enlarger.

However, in the present invention, through holes are provided in the tube, and the blades are configured to direct removed material through the apertures.

Thus, as the borehole enlarger advances within the borehole, material cut by the blades is scooped/channelled/passes into the interior of the tube.

Moreover, since the tube is constantly moving along/within the borehole and being extended from the surface, its capacity for removing material from the borehole (in other words "absorbing" the cut material) is potentially very large.

According to a second aspect of the invention, there is provided an apparatus for maintaining or enlarging the inner diameter of a borehole, the apparatus comprising: a tube axially receivable within the borehole, a closure means for closing-off a leading end of the receiving tube, a central ising means for substantially centralising the tube within the borehole, radially-extending blade means located on the exterior of the tube and an aperture in a sidewall of the tube, the aperture communicating with the interior of the receiving tube and being located axially forward of the blade means.

In a preferred embodiment of the invention, a plurality of blades are provided at different, and preferably overlapping, radial positions. Such a configuration enables the each blade to cut a smaller amount of material from the sidewall of the borehole, thereby reducing the likelihood of blockages occurring. Moreover, since each blade cuts a smaller amount of material, the collected material within the tube is made up of a number of slivers/pieces, thereby enabling it to be pushed more easily along the interior of the tube.

The or each blade may be associated with a guide means or centraliser, which helps to maintain the tube centrally within the borehole. An additional centraliser means may additionally be provided for this purpose. In a preferred embodiment of the invention, the blades are formed as slits in inclined plates that extend outwardly from the tube. In such a configuration, the borehole enlarger operates along a similar principle to that of a cheese grater, albeit on a much larger scale.

A third aspect of the invention provides a method of enlarging or maintaining the internal diameter of a borehole comprising the steps of inserting a borehole enlarger as herein described into the borehole and causing the borehole enlarger to move axially through the borehole.

The method may further comprise the step of recovering material cut by the borehole enlarger.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a typical drilling operation to which the present invention is applicable; Figure 2 is a schematic longitudinal section through a first borehole enlarger in accordance with the invention; and Figure 3 is a schematic side view of a second embodiment of a borehole enlarger in accordance with the invention.

In Figure 1, a typical drilling operation is shown in which a borehole 10 has been sunk from the surface 12 to intersect two underground cavities 14, 16 containing oil to be mined. The borehole 10 is created using a drilling rig 18 located at the surface 12, which feeds and directs a drill string 20. The borehole's path is serpentine and has been chosen to avoid an underground obstruction 22, e.g. a sheet of rock, and to intersect the two cavities 14, 16 containing oil to be mined. The borehole can range from a few hundred metres to several kilometres in length, and typically has a diameter of between 200mm and 400mm.

The borehole 10 is drilled using a drill string 20 having a computer-controlled, directional drilling head 24, which guides the tip of the drill along a predetermined path. Certain sections 26 of the borehole 10 are generally vertical, whereas other sections 28 are non-vertical. In non-vertical sections 28 of the borehole 10 that pass through clay strata 30, there is a tendency for the borehole's diameter to reduce 32 as the weight of the clay above presses down. In addition, a high water table can cause the clay to swell, thereby partially obstructing the borehole 10.

After the initial drilling operation has been completed, a reticulated liner 34 is fed into the borehole 10 to help to stabilise it. Such a liner typically comprises a reticulated screen having through apertures of approximately 180 microns in size. The liner 34 is of a slightly smaller external diameter than the diameter of the borehole 10, thereby leaving an annular gap around it which is filled using a gravel pack.

The gravel pack (not shown) is formed by pumping a sand slurry into the annular space between the liner 34 and the borehole 10. The sand precipitates downwardly thereby tightly filling the annulus and forming a barrier between the exterior of the liner 34 and the interior sidewall of the borehole 10.

The sand layer cannot penetrate the holes in the liner 34 and also prevents clay/mud from coming into contact with the liner 34.

However, where the liner 34 is non-concentric with the borehole 10, or where the clay has swelled thereby partially closing the borehole 10, is possible that the liner 34 will contact the clay. In such a situation, it is possible that silica precipitates in the clay, whose diameter is frequently less than 180 microns, penetrate the mesh of the liner to form aggregates within it 34.

These aggregates, when combined with oil, are very abrasive and can quickly damage the liner and the valves, pumps etc. of the subsequently fitted drilling apparatus.

The invention therefore provides a borehole enlarger that can be run into the borehole 10 ahead of the liner 34. By doing so, the dimensions of the borehole can be restored to within acceptable parameters for the short period of time that is necessary to enable the liner to be inserted and back-filled using the gravel pack. In Figure 2, a borehole enlarger 40 in accordance with the invention comprises a central tube 42 whose leading end 44 (i.e. the end that faces forwards during insertion of the enlarger 40 into the borehole 10) is closed off using a nose cap 46. The sidewalls 48 of the tube 42 taper outwardly to facilitate centralising the tube 42 within the borehole 10.

The trailing edge 50 of the tapered portions 48 terminate with a through hole 52 that communicates with the interior of the tube 42. Behind each through hole 52 there is provided a radially outwardly spaced blade 54 whose outer diameter is equal to the nominal, i.e. the desired, diameter of the borehole, e.g. -9 inches I -20cm.

As the borehole enlarger 40 is advanced axially along the borehole 10, the blades 54 cut away any material 53 that has penetrated the liner 34 and guide it into the interior of the tube 42 via the through hole 52. Cut material 53 continues to be scooped into, and move rearwardly along, the interior of the tube 42 until the obstruction has been cleared, whereupon the blades 54 will disengage the sidewall of the borehole 10 enabling the borehole enlarger 40 to move therealong substantially uninhibited.

In Figure 3, a second embodiment of a borehole enlarger 40 in accordance with the invention is shown having a slightly different configuration, although identical reference signs have been used to identify identical features for the sake of clarity.

In the second embodiment, the borehole enlarger 40 comprises a number of interchangeable sections that can be connected to one another end-to-end. The leading end 44 of the borehole enlarger 40 comprises a hemielliposoidal nose cap 46 that is screwthreadedly receivable in the end of a cylindrical tube 42 of a first enlarger section 56.

In the first enlarger section 56, the central tube 42 is substantially cylindrical and comprises a first plurality of tapered blade assemblies 58, and a second plurality of non-tapered blade assemblies 60, affixed to the external sidewall thereof.

Each of the tapered blade assemblies 58 comprises a hollow tapered body portion 62 whose narrow end 63 faces towards the forward direction, in use, of the borehole enlarger 40. The tapered blade assemblies 58 serve to guide and centralise the borehole enlarger substantially along the longitudinal axis of the borehole 10. Towards the wider end 65 of each tapered blade assembly 58, a slit 64 is provided that communicates with the interior of the body portion 62 and the interior of the central tube 42. The edges of the slit 64 are chamfered to form a blade and are preferably hardened or coated with an abrasive or hardening material to facilitate cutting of the soil, clay or rock.

The tapered blade assemblies 58 are located at equally-spaced radial positions around the central tube 42.

Each of the non-tapered blade assemblies 60 is similar in construction to the tapered blade assemblies 58, except that the body portions 66 thereof do not need to be (as) tapered. It will be noted that the non-tapered blade assemblies 60 are also located at equally-spaced radial positions around the central tube 42, whose positions correspond to the radial gaps between the tapered blade assemblies 58.

The trailing end 59 of the first enlarger section 56 is screwthreadedly engageable with the leading end of a hollow connector tube 61.

The leading end of a second enlarger section 68, similar or identical to the first enlarger section 56 is screwthreadedly connected to the trailing end of the connector tube 61. Further enlarger sections 68 and/or connectors 61 can be added to obtain a desired overall borehole enlarger 40 configuration.

It will also be seen that the borehole enlarger 40 can be affixed to the leading end of a centraliser 70 and/or to a connector 72 for feeding a liner 34 into the borehole 10.

The invention is not necessarily restricted to the details of the foregoing embodiments, which are merely exemplary. For example, the shape, size and configuration of the enlarger may be varied, and various materials and/or manufacturing methods may be used to construct the enlarger. In addition, the configuration of the borehole itself and the method of forming it may be varied considerably depending on the situation. For example, the invention may be adapted for use in offshore drilling operations as well as the onshore drilling operation described herein.

Claims (12)

1. CLAIMS1. An apparatus for enlarging or maintaining the bore of a borehole comprising: a hollow tube which is axially receivable in the borehole and is movable axially along the borehole in a forward direction; blade means located radially outwardly of the hollow tube; and an aperture communicating with the interior of the tube and located axially forward of the blade means.
2. 2. An apparatus as claimed in claim 1 in which the blades are configured to direct removed material through the apertures.
3. 3. An apparatus as claimed in claim I or claim 2 further comprising a N. closure means for closing-off a leading end of the tube. Co
4. 4. An apparatus as claimed in any of claims 1 to 3 further comprising a centralising means for substantially centralising the tube within the borehole.
5. 5. An apparatus as claimed in any preceding claim in which a plurality of blades are provided at different radial positions.
6. 6. An apparatus as claimed in any preceding claim in which the blade(s) is(are) formed upon a borehole enlarger body.
7. 7. An apparatus as claimed in claim 6 in which the borehole enlarger body is screw threadedly connected to the tube.
8. 8. An apparatus as claimed in any preceding claim comprising at least one set of tapered blade assemblies comprising a hollow tapered body portion having a narrow end facing toward the forward direction.
9. 9. An apparatus as claimed in claim 8 further comprising a set of non-tapered blade assemblies, the tapered blade assemblies being forward of the non-tapered blade assembles.
10. 10. An apparatus as claimed in claim 8 or claim 9 in which the blade assemblies each comprise a slit that communicates with the interior of the tube and has an edge that forms the aforesaid blade. Co c'J
11. 11. A method of enlarging or maintaining the internal diameter of a borehole comprising the steps of inserting a borehole enlarger as claimed in any preceding claim into the borehole and causing the borehole enlarger to move axially through the borehole.
12. 12. An apparatus for enlarging or maintaining the bore of a borehole substantially as herein described with reference to, and as illustrated in, accompanying Figure 2 or accompanying Figure 3.