The invention relates to a wireline-operated drilling unit, especially for forming a second borehole sidetracked from an established first borehole in an underground structure and for reaming an established borehole, the drilling unit comprising a drilling section, a transport section and several propulsion actuators, which are arranged to be in releasable engagement with a borehole wall. The invention also relates to methods for forming a second borehole sidetracked from an established first borehole in an underground structure and for reaming an established borehole in an underground structure by the use of a wireline- or umbilical-operated drilling unit.

Drilling for oil and gas requires the use of a drilling rig with associated equipment, and the preparation, the actual drilling operation and the removal of the equipment used are costly and time-consuming . Often, it will thereby not be profitable to establish production wells in small reservoirs by drilling lateral bores from a main well, especially into small pockets that are discovered after a field has been put into production and the drilling rig has been removed and replaced by production equipment. This results in hydrocarbon deposits discovered being considered as non-exploitable as there are no technological solutions applicable for this type of deposits.

Correspondingly, if formation damage occurs during production, there will be a need for complementing drilling, wherein it is desirable to be able to carry out this without bringing in an expensive drilling rig. It is also desirable to be able to ream parts of a borehole, that is to say widen the cross section of the borehole without bringing in a conventional drilling rig.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art.

The object is achieved through the features, which are specified in the description below and in the claims that follow. A drilling unit is provided, which is connected to a surface installation via an umbilical, for example a wireline on which the drilling unit hangs when it is being run into and pulled out of a borehole. Handling wireline-operated equipment or autonomous equipment connected to the surface installation requires surface installations less complicated than those required for drilling operations that are carried out with an ordinary drill string rotated by a drilling machine placed on the surface.

When a lateral borehole is to be sidetracked from a cased existing borehole, a lateral window is established in a manner known per se in the casing of the existing borehole, for example by a whipstock, which has been set at a prescribed depth in the existing borehole, guiding a metal-cutting tool or the like in the desired direction towards the casing. The lateral borehole is formed in the underground structure of interest, for example where it is expected to find hydrocarbons or with a suitable distance to formation damage. The lateral borehole may be sidetracked in a direction away from the borehole, typically approximately horizontally and with a length normally limited to a few hundred metres. The lateral borehole is established by means of an automotive drilling unit driven by means of power supply through cables from the surface installation. In a preferred embodiment, the drilling unit is electrically operated .

Correspondingly, a portion of the existing borehole may be reamed, be given an expanded cross section that is, by said automotive drilling unit being carried into the portion of the borehole, which is to be worked, where the drilling unit is set into operation.

In the further description, the concepts of "drilling", "borehole" etc. are used without the things described being restricted to the ordinary establishing of a borehole, as the things described also cover reaming operations performed on a previously established borehole unless something else is explicitly expressed.

The drilling unit is provided with a propulsion device, which gives sufficient drill-bit pressure against the underground also in horizontal drilling. A preferred propulsion device includes at least first and second propulsion actuators formed as radially expandable packers or packer groups arranged to attach to the wall of the borehole. The propulsion actuators are characterized by the packers / packer groups encircling a tubular housing of the drilling unit, wherein at least one of the packers / packer groups are connected to a sliding sleeve which is axially displaceable on a portion of the housing, as described in the NO-patent 328189. By the packers / packer groups bearing against the wall of the borehole by turns, and the axially displaceable packers) / packer group(s) alternating between pushing the drilling unit in the axial direc- tion by means of an associated linear actuator, for example an annular piston, while the packer / packer group is firmly set against the borehole wall, and returning to an initial position while the respective packer / packer group is withdrawn from engagement with the borehole wall. Through their bearing against the borehole wall, the packers / packer groups are also arranged to prevent the drilling unit from rotating around its centre axis as a reaction to the rotation of the drill bit. In addition to functioning as a propulsion means, the propulsion actuators will also function as packers between the drilling unit and the borehole wall, and portions of the borehole may thus be isolated as well. The patent publication NO 328189 is included in its entirety in this description by reference.

The drilling unit is provided with a transport section including a cuttings container, the transport section being disconnectable from a drilling section, which is temporarily left behind in the borehole. At least one of the propulsion actuators of the drilling unit is arranged on the transport section, which can be moved out of the borehole for the cuttings container to be emptied, by the transport section either being moved by means of the propulsion actuators, being pulled by the wireline or being moved by a combined use of these means. The cuttings are usually brought up to the surface installation, but depositing in other places is not excluded, for example in the main borehole, if this is a well that has been depleted . An emptied transport section is returned to the drilling unit and connected to this so that the drilling operation may continue.

The drilling unit may be arranged to place sand-screen pipes in the drilled sidetrack. Sand-screen-pipe sections are preferably arranged to be joined together in the borehole by means of the drilling unit as the borehole is being established, for example by the sand-screen-pipe sections being carried into the borehole by means of the transport section when it is returning after the cuttings container has been emptied.

The drilling unit, possibly the transport section and the drilling sections separately, and any associated work and equipment sections that are to be connectable to the drilling unit or to be installed in the sidetrack, for example the sand-screen-pipe sections, or in the main borehole, for example the whipstock, have a length that allows them, alone or connected to another unit/section, to be passed through the lubricator unit of a wellhead. A typical maximum lubricator length may be up to approximately 20 metres.

For a drilling fluid, that is to say a liquid used for cooling the drilling unit and for transporting cuttings, the available formation fluid may be used. The invention is defined by the independent claims. The dependent claims define advantageous embodiments of the invention.

In a first aspect, the invention relates more specifically to a wireline- or umbilical- operated drilling unit, especially for forming a borehole sidetracked from an established first borehole in an underground structure and for reaming an established first or second borehole, the drilling unit comprising a drilling section, a transport section and at least one propulsion actuator which is arranged to be in releasable engagement with a borehole wall, characterized by the transport section being releasable from the drilling section and being provided with a cuttings container and being connected to a wireline or an umbilical which is connected to a surface installation and is arranged to transmit power between the surface installation and the transport section, and at least one of the at least one propulsion actuator and an associated hydraulic unit being arranged on the transport section and being arranged to move the transport section in the axial direction of the second borehole.

The transport section may be arranged to be pulled away from the drilling section by means of the wireline.

The transport section may comprise at least one axially displaceable and radially expandable packer element, which is arranged to be set against the borehole wall. Alternatively the transport section may comprise at least one axially displaceable propulsion actuator, which is provided with a radially expandable packer element which is arranged to be set against the borehole wall.

The transport section may form a portion of a transport system for drilling fluid including at least perforations arranged in the cuttings container and arranged to retain cuttings, and a pump filter arranged to filter a flow of drilling fluid in the direction from the cutting container towards a circulation pump.

At least some of the propulsion actuators may be arranged on the drilling section.

At least one drilling-fluid port forms a fluid-flow connection from an annulus, as formed between the drilling unit and a borehole wall, to the interior of the transport section.

At least one of the at least one radially expandable packer elements may form a flow barrier in an annulus which is formed between the drilling unit and the borehole wall, the packer element being arranged between a drilling-fluid port, which forms a fluid- flow connection from the annulus formed between the drilling unit and the borehole wall to the interior of the transport section, and a pump filter, which is arranged to filter a flow of drilling fluid in the direction from the cuttings container towards a circulation pump.

The overall length of the drilling section and the transport section, or the length of each of the drilling section and the transport section may at a maximum equal the capacity of the lubricator unit of a wellhead.

In a second aspect, the invention relates more specifically to a method of forming a second borehole sidetracked from an established first borehole in an underground structure, characterized by the method including the following steps:

providing a lateral opening in the first borehole;

providing a wireline- or umbilical-operated drilling unit;

sluicing the drilling unit through a lubricator unit of a wellhead;

making the second borehole by rotating the drill bit of a drilling section arranged in a front portion of the drilling unit;

filling drilled cuttings into a cuttings container arranged in a transport section arranged in a rear portion of the drilling unit;

disconnecting the transport section from the drilling section;

moving the transport section out of the second borehole;

returning an empty transport section to the second borehole; and

connecting the transport section and the drilling section to each other.

The method may include the additional step of:

moving the transport section out of the second borehole by pulling the transport section by means of a wireline or by moving it by activating propulsion actuators arranged on the transport section.

The method may include the additional step of:

before connecting the transport section and drilling section to each other, moving the transport section towards the drilling section by activating propulsion actuators arranged on the transport section.

The method may include the additional step of:

carrying the drilling unit into the first borehole by sluicing the drilling section and the transport section separately through the lubricator unit, and connecting the drilling section and transport section to each other in the first borehole or in the second borehole;

respectively separating the drilling section and transport section in the first borehole or the second borehole and carrying the drilling section and the transport section separately out of the first borehole through the lubricator unit.

In a third aspect, the invention relates more specifically to a method of reaming an established borehole in an underground structure, characterized by the method including the following steps:

providing a wireline- or umbilical-operated drilling unit;

sluicing the drilling unit through a lubricator unit of a wellhead;

reaming the borehole by expanding and rotating the drill bit of a drilling section arranged in a front portion of the drilling unit;

filling drilled cuttings into a cuttings container arranged in a transport section arranged in a rear portion of the drilling unit;

disconnecting the transport section from the drilling section;

moving the transport section out of the borehole;

returning an empty transport section to the borehole; and

connecting the transport section and the drilling section to each other.

In what follows, an example of a preferred embodiment is described, which is visualized in the accompanying drawings, in which :

Figure 1 shows a principle drawing of a borehole in which a drilling unit according to the invention is forming a sidetrack; and

Figure 2 shows a side view of the drilling unit, on a larger scale.

Reference is first made to figure 1, in which the reference numeral 1 indicates an underground structure in which, in an established first borehole 11, a whipstock 111 is arranged in order to guide a substantially cylindrical, elongated drilling unit 2 into the underground structure 1 when drilling a sidetracked second borehole 12, also called a lateral borehole. The lateral borehole 12 is defined against the underground structure 1 by a borehole wall 121. A surface installation 13 includes a wellhead 131 a lubricator unit 132 for running equipment into the first borehole 11, especially equipment of the type that is lowered into and pulled out of said borehole 11 by means of a wireline 141 connected to a wireline unit 14 arranged on or at the surface installation 13. Sidetracked boreholes of this kind typically have a diameter of 4-5 inches.

Alternatively, the wireline 141 and the wireline unit 14 may be replaced by an umbilical (not shown) which is not arranged to be able to carry the weight of said equipment, but is suitable for transmitting power, control signals and so on. Reference is now made to figure 2, in which the drilling unit 2 is shown in greater detail. A front drilling section 21 includes a rotatable drill bit 211, which is connected to a drive 213 including a motor 2132 and a first gear 2132 for adjusting the rotational speed of the drill bit 211 in relation to the number of revolutions of the motor 2131. The drive 213 is also provided with a circulation pump 2133 arranged for the circulation of drilling fluid. The circulation pump 2133 is connected to the motor 2131 via a second gear 2134. In an embodiment not shown, the circulation pump 2133, possibly together with the motor 2131 and said second gear 2134, is arranged in such a way that it may be released from the drilling section 21 and retrieved to the surface for maintenance, alone or together with the transport section 22. The drill bit 211 may be radially expandable, in a manner known per se, for use in reaming the existing main borehole 11 or a previously established lateral borehole 12.

The drilling section 21 is further provided with lateral-control means 212 for changing the direction of drilling. Finally, the drilling section 21 is provided with a unit 214 for recording relevant drilling data, typically a so-called MWD unit (MWD = measurement while drilling). A first coupling part 215 is arranged in the rear end portion of the drilling section 21 and includes means of mechanically, lockably connecting the drilling section 21 to a transport section 22 and means of connecting signal lines, fluid lines and so on extending between the drilling section 21 and the transport section 22.

The transport section 22, which, in a connected position, extends rearwards from the drilling section 21 is provided with a second coupling part 226 which is complementary to the first coupling part 215 on the drilling section. The transport section 22 is further provided with a hydraulic unit 221 provided with a control unit and is arranged at least to ensure a controlled activation of several associated propulsion actuators 224 arranged on the transport section 22. In addition to being able to displace the transport section 22 in the longitudinal direction of the borehole 12, the propulsion actuators 224 will be able to apply the necessary load to the drill bit 211 during drilling and prevent the drilling unit 2 from rotating as a reaction to the torque applied to the drill bit 211 during drilling.

In a preferred embodiment, the propulsion actuators 224 are of a kind including several annular packers 2241 encircling a cylinder-shaped housing, being expandable towards the adjacent borehole wall 121 and being axially displaceable in the longitudinal direction of the housing by means of linear actuators (not shown) . In this form, front and rear propulsion actuators 224, 224' are in engagement with the borehole wall 121 by turns. Reference is made to NO 328189 for a detailed description of the propulsion actuators 224 of a kind in which activatable packers are arranged externally on a cylinder-shaped body, and the packers, in an active state, are designed to seal the annu- lus between the body and a borehole wall, and in which at least one packer group of the activatable packers are connected to sliding sleeves, so that at least one group of packers are displaceable relative to the cylinder-shaped body by means of an associated linear actuator.

The transport section 22 is further provided with a cuttings container 225 arranged for collecting cuttings that are carried in the drilling fluid from the drill bit 211 by means of the circulation pump 2133 of the drilling section 21 through an annulus 3 formed between the drilling unit 2 and the borehole wall 121, in through one or more drilling- fluid ports 222 arranged in the wall of the transport section 22 and into the cuttings container 225, in which cuttings are retained while the drilling fluid is flowing out through perforations 2251 in the wall of the cuttings container 225 and returning to the circulation pump 2133 via a pump filter 223 and flow channels (not shown) arranged therefor in the transport section 22 and the drilling section 21. The preferred expandable propulsion actuators 224 separate the cuttings-containing drilling-fluid flow from the cuttings-free drilling-fluid flow as the drilling-fluid port(s) 222 and the pump filter 223 are arranged on opposite sides of the propulsion actuators 224 that function as a seal between them, or the packer function is possibly ensured by another packer element not shown. The preferred embodiment of the propulsion actuators 224 clearly shows to advantage by its combined function with the alternating expansion of the expandable packer element 2241.

The transport section 22 is connected at its rear end to the wireline 14 via a wireline connector 227 in which the load-carrying component of the wireline 141 is anchored and the other components of the wireline 141 are extended in power and signal lines (not shown) to the driving gear 213, signal lines to the MWD unit 214 and so on in passages (not shown) suitable therefor in the transport section 22, coupling parts 215, 226 and drilling section 21.

Alternatively, the drilling section 21 may also be provided with several propulsion actuators (not shown) arranged for activation coordinated with or independent of the activation of the propulsion actuators 224 of the transport section 22.

The drilling unit 2 is advantageously operated by electric current via the motor 2131 and hydraulic unit 221, but the invention is not limited thereto, as the object of the invention is also achieved by circulating a pressurized hydraulic medium, for example hydraulic oil, from the surface installation 13 through suitable lines (not shown) in association with the wireline 141 or said umbilical not shown.

The total length of the drilling unit 2 primarily lies within the maximum length that can be accommodated by the lubricator unit 132. Alternatively, the length of each of the drilling section 21 and the transport section 22 separately lies within the capacity of the lubricator unit 132, and the drilling section 21 and the transport section 22 may then be sluiced in or out separately, as they are connected to each other in the first borehole 11 or in the second borehole 12.

When a second borehole 12 is to be provided as a lateral from an existing first borehole 11 in the underground structure 1, for example in order to recover hydrocarbons from a presumedly small pocket or to inject water into the structure, a whipstock 111 is lowered first into the first borehole 11 and anchored in a desired position. Then a lateral opening 113 is established in the casing 112 of the first borehole 11 with a suitable tool (not shown). The drilling unit 2 according to the invention is then sluiced into the first borehole 11 as a unit, possibly separately as a drilling section 21 and transport section 22, which are connected to each other in the first borehole 11. Then the drilling unit 2 is put into operation, drilling the second borehole 12 as the drill bit 211 is rotated with directional control, and drilled cuttings are collected in the cuttings container 225.

When the cuttings container 225 is full, the drilling is stopped, the transport container 22 is disconnected from the drilling section 21 and the transport section 22 is pulled out of the second borehole 12 by means of the wireline unit 14 and the wireline 141, possibly out of the first borehole 11, and emptied of cuttings. Then the transport section 22 is carried into the second borehole 12 again and connected to the drilling section 21. When, by degrees, the second borehole 12 deviates a great deal from the direction of the first borehole 11 and has got so small a gradient that the transport section 22 does not by its own weight slide outwards in the second borehole 12, the propulsion actuators 224 of the transport section 22 may be activated for assisted propulsion of the transport section 22 by the very fact of the transport section 22 having power supply through the wireline 141 to the integrated hydraulic unit 221.

When the transport section 22 is connected to the drilling section 21, the power supply and signal connection between the drilling section 21 and the surface installation 13 have been restored, and the drilling operation may continue.

In an embodiment not shown, the cuttings container 225 is arranged to dump its contents of cuttings into the first borehole 11, typically in a situation in which the first borehole 11 is not producing any longer. This may be achieved, for example, by arranging the cuttings container 225 nearest to the drilling section 21, and by the whip- stock 111 being provided with passages for the cuttings that are being dumped.

In a further embodiment (not shown), the transport unit 22 is arranged to carry with it and set well components (not shown) in the second borehole 12, for example sand screens.

When a portion of an established borehole, for example said first borehole 11 or said second borehole 12, is to be reamed, its cross section is to be expanded, that is, the drilling unit 2 according to the invention is sluiced into the first borehole 11 as de- scribed above and moved up to the portion to be worked. Then the drilling unit 2 is set into operation, widening the borehole 11, or 12, as the drill bit 211 is expanded and rotated, and drilled cuttings are collected in the cuttings container 225.