Drilling apparatus

An apparatus for drilling a wellbore (10) is disclosed, the apparatus comprising a lowermost pilot drill bit (16) having a diameter less than that of the diameter of wellbore (12) drilled by the apparatus, above which is a reamer, having an adjustable drill diameter from a first diameter less than that of the pilot drill to a second diameter greater than that of the pilot drill, above which is a body portion (24) having a lower diameter less than that of the pilot drill and an upper diameter greater than that of the pilot' drill but less than the second diameter of the reamer.

TECHNICAL FIELD

The invention relates to an apparatus for drilling a wellbore and to a method of operating the apparatus.

BACKGROUND

When drilling a fresh wellbore in the vicinity of underground hydrocarbon reservoirs, it is often desirable to fracture the walls of the drilled wellbore following drilling. This stimulates the productivity and flow of hydrocarbons into the wellbore.

Known methods of fracturing involve withdrawing the drill string from the wellbore followed by pressurising the downhole environment to overcome the fracture pressure of the surrounding formation, to produce fracturing. This is typically followed by further drilling followed by again withdrawing the drill string and pressurising the wellbore to cause fracture. This sequence of drilling followed by fracturing can be repeated several times as a wellbore is drilled.

SUMMARY

The invention relates to an apparatus for drilling a wellbore comprising a lowermost pilot drill bit having a diameter less than that of the diameter of wellbore drilled by the apparatus, above which is a reamer, having an adjustable drill diameter from a first diameter less than that of the pilot drill to a second diameter greater than that of the pilot drill, above which is a body portion having a lower diameter less than that of the pilot drill and an upper diameter greater than that of the pilot drill but less than the second diameter of the reamer.

In this way, the wellbore can be drilled with the pilot drill bit creating a pilot hole which is then increased in diameter by the reamer when in its second diameter position. When it is desired to fracture a drilled region the reamer is retracted to its first diameter while drilling with the pilot bit continues, the reamer passing into the drilled pilot hole, until the body portion blocks off the pilot hole by virtue of its variable diameter. At this point fracturing fluid is passed into the pilot hole, the pressure of which rises due to the fact that the pilot hole is hydraulically sealed by the body portion, eventually causing fractures in the walls of the pilot hole. The apparatus can then be withdrawn until the reamer is above the pilot hole, the reamer is then extended to its second diameter and drilling resumes until the apparatus reaches another region where fracturing is desired and the above steps are repeated.

Thus, the apparatus of the invention allows fracturing to be carried out while drilling, preventing the need to fully withdraw the drill string and reducing the time and significant cost associated with drilling.

Thus, in a second aspect, the invention relates to a method of drilling and fracturing a wellbore employing an apparatus according to the invention, wherein the pilot drill creates a pilot hole which is increased in diameter by the reamer in its second diameter position, retracting the reamer to its first diameter, continuing to drill the pilot hole until the body portion hydraulically seals the pilot hole by virtue of its variable diameter, passing fracturing fluid into the pilot hole sufficient to cause fracture of the walls of the pilot hole, withdrawing the apparatus until the reamer is above the pilot hole, extending the reamer to its second diameter and resuming drilling.

Typically the pilot drill bit will be connected to the body portion via the reamer and a shaft member with a diameter less than that of the pilot drill. This provides an annular space in the pilot hole into which the fracturing fluid passes during fracturing.

In an embodiment of the present invention, the shaft member has a variable length, e.g. extendable between a short length and a long length and biased to the long length such as a thruster, an axial spring or a shock sub. Thus, when downwards force is applied to the apparatus with the pilot drill in contact with the bottom of the hole, the apparatus moves downwards as the shaft member shortens, increasing the contact force between the pilot drill and the bottom hole.

The body portion may be tubular and have a circular cross section along its length. Thus the diameter of the circular cross section at the bottom is less than the diameter of the cross section at the upper diameter.

The change in diameter of the body portion with respect to height may be sudden or gradual and may involve more than one sudden contraction in diameter. However, preferably the diameter alters gradually with respect to height, i.e. producing a conical or frustro-conical body shape which may have a diameter which increases linearly with height or in a non-linear manner.

A gradual change in diameter provides a good seal at the top of the pilot hole as the body portion hydraulically seals it. As the body portion just touches the top of the pilot hole there will be an approximately circular contact region between the body portion and the pilot hole surface. This will generally not be sufficient contact area to resist the downwards force being applied to the apparatus and further downwards movement of the apparatus can be expected. This will result in the body portion deforming the top surface of the pilot hole to accommodate the increasing diameter until the rock formation resists any applied down force and further downwards movement of the apparatus ceases. Thus, the body portion plugs the pilot hole, hydraulically sealing it for fracturing.

In one aspect of the present invention, the body portion has a diameter above the upper diameter which is less than the upper diameter. In this embodiment the body portion will have a maximum diameter in the middle (the upper diameter). This aids removal of the apparatus from the drilled wellbore.

The reamer typically comprises retractable drilling elements which provide the adjustable drill diameter. The elements can be extended and/or retracted, for example, according to a command from the surface or as an automated response to a detected change in environmental properties, such as pressure drop across the bit.

In a typical drilling operation, drilling mud will pass down through the centre of the drill string and flow out at the pilot drill bit. The mud passes upwards outside the drill string and is collected at the surface. Once the reamer has been retracted to its first diameter, the pilot drill continues to drill down.

As the body portion begins to close onto the top of the pilot hole surface, the pressure in the annulus in the pilot hole will begin to rise as the exit area for the drilling mud becomes less and less. Too great a rise in pressure might result in premature fracturing of the walls of the pilot hole. This is generally undesirable as drilling muds are expensive and are desirably recovered, and more importantly a bespoke fracturing fluid is desirably used to fracture the pilot hole. Therefore, the flow of drilling mud is stopped if the pressure rises too high. The rise in pressure is therefore a reliable indicator that the body portion is about to contact the top of the pilot hole, as discussed above. Alternatively, a knowledge of the length of hole drilled may be used to determine when the body portion is about to contact the top of the pilot hole.

Therefore, before the body portion contacts the top of the pilot hole the drilling mud is preferably circulated out and replaced by a fluid suitable for fracturing. Such a fracturing fluid may desirably comprise proppant to prevent any formed fractures from collapsing and other chemical agents known to the person skilled in the art to be useful in a fracturing fluid.

To assist the replacement of drilling mud with fracturing fluid it may be necessary to lift the apparatus a short distance off bottom.

Once the body portion has hydraulically sealed the pilot hole, as described above, the fracturing fluid is pumped until its pressure causes the walls to fracture and fracturing fluid to enter them. Known fracturing procedures can then be employed and as soon as the formed fractures are completed and propped the apparatus can be raised until the reamer is above the pilot hole, the diameter of the reamer extended to its second diameter, drilling mud is restarted and drilling resumes.

In some aspects of the present invention, the wellbore may be sealed above the apparatus and pressurise to reduce the upwards force on the apparatus due to pressure difference. Care must be taken that this balancing pressure does not rise so high as to cause fracturing.

As drilling resumes, the pilot drill will not be in contact with the formation. Once the pilot drill makes contact either the apparatus can continue drilling until a new fracture is desired or the method of the invention can begin again.

In an embodiment of the present invention, the apparatus comprises at least one flow channel for directing upwards flowing fluid. Such a flow channel has its lowermost entry port above the reamer but below the region of the body portion which contacts the top of the pilot hole. The uppermost exit port being above the region of the body portion which contacts the top of the pilot hole, preferably at a portion above the maximum diameter of the body portion.

Such flow channels allow fluid to flow out of the pilot hole even when the body portion has hydraulically sealed it, preventing sudden rises in pressure as the body portion connects with the top of the pilot hole. Clearly such flow channels must be closeable, in order for pressure in the pilot hole to rise sufficient to cause fracturing, however this may be achieved independently of the hydraulic sealing of the pilot hole, giving greater operational flexibility.

The flow channels may be closeable in a wide variety of ways. One preferred method is to introduce a slideably mounted body in the drill string above the exit port of the flow channels. Such a slideably mounted body could be biased to a withdrawn position, allowing fluid to flow out of the exit port. Once sufficient downforce is applied to the slideably mounted body it slides into a mating sleeve and closes off the exit port by physically gating it closed.

DESCRIPTION

Referring toFIGS. 1 to 4, a sequence of steps in the carrying out of the present invention is shown.FIG. 1shows a bottom hole apparatus10drilling a wellbore12initiated by the drilling of pilot hole14. Apparatus10comprises a pilot drill bit16connected to a variable length shaft member18of diameter less than that of the pilot drill bit16. Above that is reamer20comprising extendable/retractable drilling elements22. Above that is tubular body portion24having a circular cross section the diameter of which increases linearly with respect to height from initial diameter26until apex28, thereafter the diameter decreasing linearly with respect to height.

The diameter of the reamer20when elements22are fully retracted is less than that of pilot drill16. When elements22are fully extended the diameter of the reamer20is greater than that of the pilot drill16.FIG. 1shows elements22fully extended expanding the diameter of pilot hole14to that of the wellbore12.

Body portion24has a lower diameter26less than that of the pilot drill and upper diameter28greater than that of the pilot drill.

Also shown is the portion of the pilot hole yet to be drilled30in the subsequent figures.

From the position shown inFIG. 1the elements22are fully retracted and drilling continues in the pilot hole14only until body portion24approaches the top of pilot hole32and the apparatus10has the position shown inFIG. 2. As can be seen inFIG. 2, as the second diameter of the reamer20is less than that of the pilot drill, the reamer passes into the pilot hole14. As the lower diameter26of the body portion24is less than that of the pilot hole then that too passes into the pilot hole14.

As the upper diameter28is greater than that of the pilot hole14then the body portion blocks the top of the pilot hole32.

A short distance before hydraulically sealing the pilot hole, drilling mud will have been flushed out of the pilot hole with fracturing fluid, passing out of the pilot drill16.

Once sealed, fracturing fluid continues to enter the pilot hole14until the pressure rises to such an extent that fractures34occur in the walls of the pilot hole14. These fractures are propped and the apparatus10is withdrawn until reamer20is free of the pilot hole14, at which point elements22are fully extended and the apparatus is in the position as shown inFIG. 3.

Drilling mud is then pumped again through pilot drill16and drilling commences, initially only with reamer20expanding the pilot hole14to the size of the wellbore12as the pilot drill is raised above the bottom. Drilling continues until apparatus reaches the point where pilot drill16reaches the floor whereupon the procedure can be repeated from the position shown inFIG. 1or further drilling, can continue before the above procedure is repeated.

FIG. 5shows an alternative embodiment to that shown inFIGS. 1 to 4and features which are the same or analogous carry the same number but increased by 100.FIG. 5shows an apparatus100having broadly the same arrangement as shown inFIGS. 1 to 4.

Apparatus100differs in that it comprises flow channels having inlet ports140and outlet ports142. Additionally the apparatus100has a slideably mounted body150which is biased to the withdrawn position shown inFIG. 5, leaving outlet ports142open.

In use, once body portion124contacts and hydraulically seals the pilot hole, flow channel entry ports140are positioned in the sealed pilot hole, allowing fluid to flow out of the pilot hole and preventing too great an increase in fluid pressure.

As before, drilling mud will then stop being pumped and instead fracturing fluid will enter the pilot hole until the drilling mud has been flushed out. Then, additional force is applied to the apparatus until slideably mounted body150is forced downwards to slide into the body portion124and thereby close off exit ports142.

With exit ports142closed, the pressure of fracturing fluid in the pilot hole increases until fracturing occurs.