Patent Description:
When drilling a new borehole or a sidetrack in an existing well, the drilling head may drill into a low pressure zone, resulting in a loss of pressure. Thus, the mud entered into the hole while drilling to prevent blowout is lost in the low pressure zone, and there will be a substantial risk of a blowout if the drilling is continued. Cementing and thus sealing part of the annulus above the low pressure zone are also impossible, since the injected cement is lost as it disappears into the low pressure zone, and then this partly drilled borehole is abandoned and plugged from above and a new well is drilled.

It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art. More specifically, it is an object to provide an improved completion or drilling system which renders it possible to continue drilling past the above-mentioned low pressure zone.

The above objects, together with numerous other objects, advantages and features, which will become evident from the below description, are accomplished by a solution in accordance with the present invention by a downhole string for drilling through a low pressure zone in a formation in a well, comprising:.

wherein the downhole string further comprises an annular barrier having an expandable metal sleeve surrounding the drawdown casing, each end of the expandable metal sleeve being connected with the drawdown casing, and the expandable metal sleeve being adapted to contact a wall of a borehole or another casing so that the drawdown casing can rotate and slide in relation to the annular barrier after expansion of the expandable sleeve, and the downhole string further comprises a swivel mounted as part of the drawdown casing, dividing the drawdown casing into a first casing part and a second casing part for rotating the first casing part in relation to the second casing part, the second casing part being connected with the operational tool and the annular barrier.

In an embodiment, the annular barrier may be connected with the drawdown casing after expansion of the expandable sleeve.

Furthermore, the annular barrier may be slidably and/or rotationally connected with the drawdown casing after expansion of the expandable sleeve.

In an embodiment, the swivel may comprise a first swivel part connected to the first casing part and a second swivel part connected to the second casing part.

Furthermore, a ball bearing may be arranged between the first swivel part and the second swivel part.

Also, a sealing element may be arranged between the first swivel part and the second swivel part.

In addition, the drawdown casing may comprise openings arranged above the annular barrier.

An annular space may be arranged between the expandable metal sleeve and the drawdown casing.

Moreover, the annular space may comprise a compound adapted to expand the annular space.

Also, the compound may comprise at least one thermally decomposable compound adapted to generate gas or super-critical fluid upon decomposition.

Further, the compound may comprise nitrogen.

In addition, the compound may be selected from a group consisting of: ammonium dichromate, ammonium nitrate, ammonium nitrite, barium azide, sodium nitrate, or a combination thereof.

Furthermore, the compound may be present in the form of a powder, a powder dispersed in a liquid or a powder dissolved in a liquid.

An opening may be arranged in the drawdown casing opposite the expandable metal sleeve for letting pressurised fluid into the annular space to expand the expandable metal sleeve.

Moreover, a valve may be arranged in the opening.

Also, the valve may comprise an activatable closing element so that when an end of the expandable metal sleeve passes the activatable closing element, the valve is closed.

One or both ends of the expandable metal sleeve may be connected with the drawdown casing by means of connection parts.

Furthermore, a sealing means may be arranged between the connection part or end of the expandable metal sleeve and the drawdown casing.

Additionally, the operational tool may be a reamer, a drill head or a cement shoe.

Further, the drawdown casing may be mounted from tubular casing sections by means of casing collars.

Moreover, the drawdown casing may be capable of sliding between two adjacent casing collars.

Also, a sliding sleeve or a frac port may be arranged in the drawdown casing closer to the first end in relation to the annular barrier.

The present invention also relates to a downhole system for drilling through a low pressure zone in a formation in a well, comprising:.

Said operational unit may also be used for sliding and/or rotating the drawdown casing in relation to the expanded expandable metal sleeve.

The downhole system as described above may further comprise a pressurising unit for pressurising a fluid in the drawdown casing for expanding the expandable metal sleeve.

Furthermore, the downhole system as described above may further comprise a downhole tool, such as a cementing tool.

Moreover, the downhole system may further comprise a ball configured to be dropped into the drawdown casing for seating in a seat and closing part of the casing.

Additionally, the downhole system may comprise a drilling head connected in an end of a drill pipe for drilling from within the drawdown casing out into the formation.

The present invention furthermore relates to a downhole method for drilling past a low pressure zone in a formation in a well, comprising the steps of:.

Moreover, the step of providing cement may be performed after a cementing tool has been arranged opposite an opening in the casing above the annular barrier.

In addition, the cementing tool may be removed from the casing before the drilling head is introduced.

The invention and its many advantages will be described in more detail below with reference to the accompanying schematic drawings, which for the purpose of illustration show some non-limiting embodiments and in which:.

<FIG> shows a downhole string <NUM> for drilling through a low pressure zone Zlow in a formation in a well <NUM>. The downhole string <NUM> comprises a drawdown casing <NUM> having a first end <NUM> closest to a top <NUM> of the well <NUM> and a second end <NUM> closer to the bottom <NUM> of the well. The downhole string <NUM> further comprises an operational tool <NUM> connected to the second end <NUM> of the drawdown casing <NUM> for performing part of the drilling operation.

When drilling a new borehole or a sidetrack in an existing well, the drilling head 8a, <NUM> may drill into a low pressure zone Zlow, and the mud entered into the hole while drilling to prevent blowout may consequently be lost in the low pressure zone, and thus, there will be a substantial risk of a blowout if the drilling is continued. In order to prevent a loss of pressure, the downhole string <NUM> comprises an annular barrier <NUM> having an expandable metal sleeve <NUM> surrounding the drawdown casing <NUM>. Each end <NUM>, <NUM> of the expandable metal sleeve <NUM> is connected on the outside <NUM> of the drawdown casing <NUM>. The annular barrier <NUM> has an unexpanded condition, as shown in <FIG>, and an expanded condition, as shown in <FIG>. When entering the low pressure zone Zlow, the expandable metal sleeve <NUM> is expanded to contact a wall <NUM> of a borehole <NUM>, as shown in <FIG>, or another casing (not shown). In this way, the well is secured in that the annular barrier <NUM> together with the drawdown casing <NUM> prevent a formation fluid from creating a blowout.

Subsequently, another operation is performed, such as cementing the annulus <NUM> above the annular barrier <NUM>. While performing this subsequent job, the drawdown casing <NUM> can rotate and slide in relation to the annular barrier <NUM> which is securely fastened to the borehole wall <NUM>. This renders it possible to carry out the subsequent operation, e.g. cementing the annulus <NUM> above the annular barrier <NUM>, as shown in <FIG>, or continue the drilling operation. This is due to the fact that the expandable metal sleeve <NUM> is made of metal providing the rigidity necessary for allowing the drawdown casing <NUM> to move relative to the annular barrier <NUM>.

In order to cement the annulus <NUM> above the annular barrier <NUM>, a downhole tool <NUM> in the form of a cementing tool is submerged into the drawdown casing <NUM>. The cementing tool <NUM> is arranged opposite the zone which is to be cemented, and a first packer <NUM> or bottom packer of the cementing tool <NUM> is set to close off the bottom part of the drawdown casing <NUM>. Cement is then pumped down through the pipe string <NUM> and into the space <NUM> in the drawdown casing <NUM> between the first packer and a second packer <NUM> and into the annulus <NUM> above the annular barrier <NUM>. The second packer <NUM> may be a cup seal movable towards the first packer to squeeze the cement out through openings <NUM> in the drawdown casing above the annular barrier <NUM>. While cementing, the drawdown casing <NUM> oscillates up and down, as illustrated by the double arrow, to ensure that bubbles are not formed in the cement and that a proper cementing job is executed. This oscillating movement of the drawdown casing <NUM> in relation to the annular barrier <NUM> is thus important to the subsequent cementing job.

The annular barrier <NUM> comprises an expandable metal sleeve <NUM> which is connected with the drawdown casing <NUM> to form an annular space <NUM>, as shown in <FIG>. The annular barrier <NUM> is expanded by pressurising an inside <NUM> of the drawdown casing <NUM> and letting this pressurised fluid into the annular space <NUM> through an opening <NUM> in the drawdown casing <NUM> opposite the annular barrier, thus expanding the sleeve to contact the wall <NUM> of the borehole <NUM> and isolate a top first part <NUM> from a bottom second part <NUM> of the drawdown casing <NUM> and thus prevent a loss of pressure or blowout.

In another solution shown in <FIG>, the annular barrier <NUM> is expanded by activating a compound <NUM> present in the annular barrier <NUM> when submerging the drawdown casing <NUM>. When activating the compound <NUM>, the compound reacts chemically or the compound decomposes to generate gas or super-critical fluid upon decomposition. The compound <NUM> may comprise nitrogen and may be selected from a group consisting of: ammonium dichromate, ammonium nitrate, ammonium nitrite, barium azide, sodium nitrate, or a combination thereof. The compound may be present in the form of a powder, a powder dispersed in a liquid or a powder dissolved in a liquid.

As shown in <FIG>, the expandable metal sleeve <NUM> is connected directly to the outer face <NUM> of the drawdown casing <NUM> in that the expandable metal sleeve <NUM> has ends <NUM>, <NUM> having an increased thickness so that the ends <NUM>, <NUM> stay unexpanded during the expansion process. In <FIG> and <FIG>, the expandable metal sleeve <NUM> is connected to the outer face <NUM> of the drawdown casing by means of first and second connection parts <NUM>, <NUM> in the form of ring-shaped elements. In order to increase the sealing between the drawdown casing <NUM> and the expandable metal sleeve ends or the connection parts, sealing elements <NUM> may be arranged, as shown in <FIG>.

To prevent fluid from leaving the annular space <NUM> in the annular barrier <NUM>, a valve <NUM> is arranged in the opening <NUM>, as shown in <FIG>. The valve <NUM> may be a check valve so that fluid may enter the valve in order to expand the expandable metal sleeve <NUM>, but is prevented from returning into the drawdown casing <NUM>. The valve <NUM> comprises an activatable closing element <NUM> so that when an end of the expandable metal sleeve <NUM> passes the activatable closing element <NUM>, the valve <NUM> is closed to close off the drawdown casing <NUM> as the annular barrier <NUM> is no longer closing off the opening <NUM> in relation to the formation fluid, as the annular barrier has slid past the opening <NUM>. As can be seen in <FIG>, the drawdown casing <NUM> is capable of sliding between two adjacent casing collars <NUM> connecting two casing sections <NUM> from which the drawdown casing is mounted.

In <FIG>, the annular barrier <NUM> is connected to a drawdown casing <NUM> and thereby to a first drilling head 8a, <NUM>. When drilling, the drilling head 8a, <NUM> may be replaced by a reamer 8b, <NUM> (shown in <FIG>), and the annular barrier <NUM> may thus be connected to the drawdown casing <NUM> having the reamer, as shown in <FIG>. When the reamer 8b meets the low pressure zone Zlow, the annular barrier <NUM> is expanded and the annulus <NUM> above the annular barrier <NUM> is cemented. Subsequently, the drilling process is continued by inserting a second drilling head <NUM> (shown in <FIG>), e.g. on the drill pipe, having a smaller outer diameter than an inner diameter of the drawdown casing <NUM>. Then, the second drilling head <NUM> drills through the reamer 8b and through the low pressure zone Zlow and further out into the formation, thus prolonging the borehole. While the second drilling head <NUM> drills, mud matching the challenge of drilling through low pressure zones is ejected to seal off the low pressure zone.

As shown in <FIG>, the operational tool <NUM> may also be a shoe 8c, such as a cement shoe, a guide shoe or a float shoe. When the cementing tool <NUM> has cemented the annulus <NUM> above the annular barrier <NUM>, a second drilling head is inserted and the mud is likewise replaced with a suitable mud for drilling through the shoe and further into the formation <NUM>.

In <FIG>, the openings <NUM> in the drawdown casing <NUM> above the annular barrier <NUM> is a port, such as a frac port, where a sliding sleeve <NUM> is slidably arranged for opening or closing the opening <NUM>.

In <FIG>, the downhole string <NUM> further comprises a swivel <NUM> mounted as part of the drawdown casing, dividing the drawdown casing <NUM> into a first casing part <NUM> and a second casing part <NUM>. The second casing part <NUM> is connected with the operational tool <NUM> and the annular barrier <NUM>, and once the expandable metal sleeve <NUM> of the annular barrier is expanded, the second part of the drawdown casing <NUM> is fixedly fastened to the wall <NUM> of a borehole <NUM>, as shown in <FIG>. During a cement job, it is important to be able to rotate the casing in order to distribute the cement all around the annulus between the casing and the wall <NUM> of the borehole <NUM>. By having a swivel <NUM>, the first casing part <NUM> is able to rotate during the cement job without rotating the second casing part <NUM>. The seal provided by the annular barrier <NUM> is thus maintained and not jeopardised by also rotating the second casing part <NUM>. The openings <NUM> of the drawdown casing <NUM> are arranged above the annular barrier <NUM> and above the swivel <NUM>.

<FIG> shows a cross-sectional view of another drawdown casing <NUM> in which the swivel <NUM> comprises a first swivel part <NUM> connected to the first casing part <NUM> and a second swivel part <NUM> connected to a second casing part <NUM>. A ball bearing <NUM> is arranged between the first swivel part <NUM> and the second swivel part <NUM> to reduce the friction between the first swivel part and the second swivel part when the first swivel part rotates in relation to the second swivel part. Furthermore, a sealing element <NUM> is arranged between the first swivel part <NUM> and the second swivel part <NUM>, and this sealing element is thus part of a dynamic seal allowing the first casing part <NUM> to be rotated in relation to the second casing part <NUM> without causing a leak therebetween.

A downhole system <NUM> is also disclosed, shown in <FIG>, for drilling through a low pressure zone in a formation in a well, comprising the downhole string and an operating unit <NUM> for sliding and/or rotating the drawdown casing in relation to the expanded expandable metal sleeve of the annular barrier and thus the borehole. As can be seen, the downhole system <NUM> further comprises a pressurising unit <NUM> for pressurising a fluid in the drawdown casing <NUM> for expanding the expandable metal sleeve <NUM>. In <FIG>, the downhole system <NUM> further comprises a cementing tool <NUM>. If no opening is present in the drawdown casing <NUM> above the annular barrier <NUM>, openings may be made by means of a perforating gun.

In <FIG>, the downhole system further comprises a ball <NUM> which has been dropped into the drawdown casing <NUM>, abutting and seating in a seat <NUM> and closing part of the casing above the annular barrier <NUM>. The seat <NUM> is arranged in the swivel <NUM> but may in another embodiment be arranged further down or up the casing. The seat <NUM> is always arranged below the openings <NUM> to allow cement to enter into the annulus. The first casing part <NUM> can thus be rotated in relation to the second casing part <NUM>, e.g. during a cement job.

In <FIG>, the cementing tool <NUM> comprises cup seals <NUM> and an opening <NUM> arranged between the cup seals, providing a space <NUM> between the seals and the casing so that cement fed down the tool enters the space before entering the annulus between the wall <NUM> of the borehole <NUM> and the casing <NUM>. The cementing tool <NUM> is arranged opposite the openings <NUM> in the drawdown casing <NUM> and above the swivel so that the first casing part <NUM> is able to rotate in relation to the second casing part <NUM>, e.g. during a cement job.

The invention further relates to a downhole method for drilling past a low pressure zone in a formation in a well. First, a borehole is drilled and the presence of a low pressure zone is determined, and then the expandable metal sleeve of the annular barrier is expanded above the low pressure zone in relation to a top of the borehole. Subsequently, cement is provided above the annular barrier in an annulus between the casing and a wall of the borehole through an opening of the drawdown casing, e.g. a frac port or a perforated opening. When performing the cementing job, the casing is oscillated in relation to the annular barrier, and then the drilling process is continued, drilling past the low pressure zone, e.g. while rotating the drawdown casing. In order to continue the drilling operation, a drilling head and a drill pipe may be inserted into the drawdown casing.

By fluid or well fluid is meant any kind of fluid that may be present in oil or gas wells downhole, such as natural gas, oil, oil mud, crude oil, water, etc. By gas is meant any kind of gas composition present in a well, completion, or open hole, and by oil is meant any kind of oil composition, such as crude oil, an oil-containing fluid, etc. Gas, oil, and water fluids may thus all comprise other elements or substances than gas, oil, and/or water, respectively.

By a drawndown casing is meant any kind of pipe, tubing, tubular, liner, string etc. used downhole in relation to oil or natural gas production.

In the event that the tool is not submergible all the way into the casing, a downhole tractor can be used to push the tool all the way into position in the well. The downhole tractor may have projectable arms having wheels, wherein the wheels contact the inner surface of the casing for propelling the tractor and the tool forward in the casing. A downhole tractor is any kind of driving tool capable of pushing or pulling tools in a well downhole, such as a Well Tractor®.

Claim 1:
A downhole string (<NUM>) for drilling through a low pressure zone (Zlow) in a formation (<NUM>) in a well (<NUM>), comprising:
- a drawdown casing (<NUM>) having a first end (<NUM>) closest to a top (<NUM>) of the well and a second end (<NUM>), and
- an operational tool (<NUM>) connected to the second end of the drawdown casing,
wherein the downhole string further comprises an annular barrier (<NUM>) having an expandable metal sleeve (<NUM>) surrounding the drawdown casing, each end of the expandable metal sleeve being connected with the drawdown casing, and the expandable metal sleeve being adapted to contact a wall (<NUM>) of a borehole (<NUM>) or another casing so that the drawdown casing can rotate and slide in relation to the annular barrier after expansion of the expandable sleeve, characterized in that the downhole string further comprises a swivel (<NUM>) mounted as part of the drawdown casing, dividing the drawdown casing into a first casing part (<NUM>) and a second casing part (<NUM>) for rotating the first casing part in relation to the second casing part, the second casing part being connected with the operational tool (<NUM>) and the annular barrier (<NUM>).