EP3510233B1

EP3510233B1 - Extended directional drilling

Info

Publication number: EP3510233B1
Application number: EP17780629.6A
Authority: EP
Inventors: Rüdiger KÖGLER
Original assignee: Perforator GmbH
Current assignee: Perforator GmbH
Priority date: 2016-09-12
Filing date: 2017-09-12
Publication date: 2022-03-16
Anticipated expiration: 2037-09-12
Also published as: EP3510233A2; DK3510233T3; WO2018046757A3; WO2018046757A2

Description

The present invention relates to a drillpipe module for drilling a borehole in ground, to a system for drilling such a borehole in ground including drillpipe modules, to a control unit and an operation unit for use in drilling such a borehole in ground, to a method for drilling such a borehole in ground to a corresponding computer program.
A pipe structure comprising inner and outer pipe sections that are coaxial with each other is disclosed in US 2012/0031616 A1 . A cylindrical truss structure may be disposed between the inner and outer pipe sections to provide support to the pipe structure while reducing weight. Such a pipe structure may be used to form lightweight drill pipe that may be used in oil, gas, geothermal, and horizontal drilling.
According to GB 2 163 465 A , a drill rod for use in making up a drill string for drilling holes in the Earth's crust in the presence of drilling mud is formed with a buoyancy space which can be filled with liquid, such as water, less dense than the drilling mud to reduce the weight of the rod when immersed in the mud. A vent hole is provided at the bottom of the buoyancy space to equalise the pressures in the buoyancy space and in the surrounding mud. A further closable vent is provided at the top of the buoyancy space to permit air to vent from the buoyancy space when it is primed with the buoyancy liquid.
WO 86/04950 A1 discloses a drilling pipe for making of a drill string, in particular for deviation drilling, comprising an outer tube and an inner tube connected to heads on each end, in such a manner that the inner tube communicates with apertures through each head, and in such a manner that a closed cavity is constituted between the tubes, said cavity contributing to give the drilling pipe and the drill string a buoyancy in the bore hole.
There are known several methods for trenchless provisions of underground pipes, conduits, cables or the like including horizontal directional drilling (HDD), microtunneling, pipe ramming or jacking or horizontal auger boring, while larger diameters may require tunneling and smaller diameters (with short distances) may be provided by moling or the like.
As the trenchless approach provides minimal disruption as to the surface, this approach is of major importance in particular in areas where stopping traffic or business on the surface may not be feasible or where a nature protection area may allow no or only a very limited footprint in terms of trenching or excavating.
In particular horizontal directional drilling or boring sees an extensive use, while there is a desire for extending the range (in terms of possible distance or reach) and capabilities (e.g. in terms of diameter of borehole). An extended reach allows for coverage of greater distances without the need for additional entry and exit points. Similarly, a larger diameter may allow for providing just one borehole instead of multiple parallel boreholes.
In WO 2015/197828 A1 , an approach for extending the reach in creating a borehole in ground is disclosed, where there are provided an inner pipe and an outer pipe having an annular region therebetween, while the inner pipe is used for exerting (axial) force on just a drilling head and not on the outer pipe, save for a tensional force on the outer pipe provided by a clamp and a bearing, while the outer pipe is advanced separately therefrom by means of an axial force exerted from the outside. As the annular region between the inner and the outer pipe is filled with air or the like, the effective weight of the drilling arrangement in the borehole is significantly reduced, thus reducing a friction between the wall of the borehole and the outer pipe. The reduced friction allows for a longer range. The separate application of force to the inner pipe and the outer pipe makes it necessary to provide more complex equipment in comparison to previous conventional techniques, A further limiting factor to the range of the system discussed in WO 2015/197828 A1 is the amount of force which might be applied for advancement unless there would be buckling or the like.
In DE 20 2012 004 882 U1 an approach is provided where the advancement is done by pushing only an outer pipe, in which a separate supply line may be provided for bringing drilling fluid to the work face.
An aim underlying the present invention is to allow for an extension in particular in term of reach of a drilling in ground in comparison to conventional trenchless approaches.
According to a first aspect of the invention, a buoyancy enhanced drillpipe module for drilling a borehole in ground and being immersed in debris laden drilling fluid is proposed as defined in claim 1, in particular comprising a pipe, a first and a second connection unit fixed to the respective ends of the pipe, the connection units each allowing a connection of the drillpipe module respectively with another drillpipe module, and a passage for drilling fluid, wherein at least in a part of the space between the first and second connection unit a buoyancy unit is provided, wherein, for a drilling fluid having a density in the range of 1.0 to 1.1 kg/l and/or a debris laden drilling fluid having a density in the range of 1.1 to 1.5 kg/l, the buoyancy unit has a lower density than the drilling fluid and wherein the drillpipe module is dimensioned such that a ratio between an absolute value of an effective weight force per unit length of the drillpipe module in debris laden drilling fluid, with drilling fluid in the passage, and a total cross sectional area of those elements of the drillpipe module that are due to transfer axial force is < 10,000 N/m³.
According to a second aspect of the invention, a system for drilling a borehole in ground is proposed as defined in claim 8, in particular comprising a plurality of buoyancy enhanced drillpipe modules according to the invention and a drilling head arranged for being attached to a drillpipe formed by the plurality of drillpipe modules.
According to a third aspect of the invention, a control unit for use in drilling a borehole in ground with a drillpipe including a plurality of buoyancy enhanced drillpipe modules is proposed as defined in claim 9, wherein drilling fluid is provided through the passage of the drillpipe to a drilling head and the drillpipe is immersed in debris laden drilling fluid returning from a work face of the drilling head, wherein the control unit is arranged for outputting control signals for controlling the density of the debris laden drilling fluid for adjusting the effective weight of the drillpipe by adjusting a density of the drilling fluid provided through the passage, adjusting a pump rate of the drilling fluid through the passage and/or adjusting a rate of advancement of the drillpipe.
According to a fourth aspect of the invention, an operation unit for use in drilling a borehole in ground with a drillpipe including a plurality of buoyancy enhanced drillpipe modules is proposed as further defined in claim 9, wherein drilling fluid is provided through the passage of the drillpipe to a drilling head and the drillpipe is immersed in debris laden drilling fluid returning from a work face of the drilling head, wherein the operation unit includes means for adjusting a density of the drilling fluid provided through the passage, adjusting a pump rate of the drilling fluid through the passage and/or adjusting a rate of advancement of the drillpipe, so to control the density of the debris laden drilling fluid for adjusting the effective weight of the drillpipe, wherein the operation unit is arranged to operate the means in response to control signals.
According to a fifth aspect of the invention, a method for drilling a borehole in ground is proposed as defined in claim 10, in particular comprising the steps of providing a plurality of buoyancy enhanced drillpipe modules according to the invention forming a drillpipe and a drilling head attached to the drillpipe, advancing the drillpipe with the drilling head being pushed and/or turned, wherein drilling fluid is provided through the combined passages the plurality of drillpipe modules to the drilling head, the drilling fluid returning as debris laden drilling fluid immersing the drillpipe.
It was realized that, while an improvement in terms of range may be attempted by reducing the friction of the drillpipe inside the borehole and such reduction may be provided by reducing the normal force of the drillpipe on the wall of the borehole, the conventional approaches for reducing the normal force have shortcomings in terms of the amount of applicable force.
The arrangement of the buoyancy enhanced drillpipe module of the present invention addresses the stability of the overall drillpipe and thus the applicable force by taking into consideration a ratio between the absolute value of the effective weight of the drillpipe (module) and its ability to withstand and conduct an axial force.
While, from a theoretical point of view, a small ratio between the effective weight of a drillpipe per length unit and the cross section of the elements transferring axial force may be achieved by using a light weight material, such replacement would only make sense if the resulting arrangement were still able to be used indeed for drilling the borehole, i.e. for transferring a force comparable to conventional drillpipes made of, for example, steel. When contemplating a very sophisticated light-weight material, a practical implementation - if at all possible - becomes infeasible as the costs for such material will be prohibitively high, when sufficient strength in terms of torsion (in case of rotating the drillpipe), buckling resistances (in particular in view of the force needed for advancing), and pressure resistance (the pressure of the drilling fluid inside the drillpipe may be in the range of about 100 bar or more; while furthermore the pressure of the debris laden drilling fluid outside (i.e. immersing) the drillpipe may be in the range of about 10 bar or more) is needed in a context of drilling a borehole in the length of several kilometers. Another difficulty in terms of practical implementations of a use of light-weight material is the connectability of modules thereof for forming the drillpipe, as, for example, threaded connections thereof may not withstand repeated connecting and disconnecting, i.e. are not sufficiently reusable.
The provision of the buoyancy unit allows for a reduction of the absolute value of the effective weight force of the drillpipe when immersed in (debris laden) drilling fluid. The modular design allows not only for a simple assembling when in use but also give further advantages. The provision of the connection units improve the overall strength of a drillpipe formed by the drillpipe modules against buckling, similar to the nodes in a bamboo stem. Furthermore, in comparison to an arrangement of just two pipes with an annular space between them, the modular design limits a failure in the outer pipe in which debris laden drilling fluid enters though the outer pipe to just one module, allowing for the drilling to be continued.
Even though it is preferable to completely compensate the weight of the drillpipe, i.e. to have a neutral lift, a significant effect on the friction reduction may be provided by just approximating the density of the debris laden drilling fluid and the drillpipe. Such approximation may also include the case that the density of the debris laden drilling fluid is higher than that of the overall drillpipe, so that there might be a certain amount of positive lift (resulting in a friction between the drillpipe and the ceiling of the borehole).
It is indeed particularly preferred that, provided a range or variety of density of the debris laden drilling fluid is expected during the drilling of the borehole (as it normally will be, in case the density of the debris laden drilling fluid is not controlled or regulated during the drilling), the drillpipe modules are designed such that a neutral lift occurs within such range, rather than at one of the extremes thereof. By such arrangement, the reachable drilling range may be optimized, the (average of the) absolute value of the effective weight force being as small as possible throughout the different conditions resulting in differing density of the debris laden drilling fluid.
The present invention may be realized by using drilling fluid having a conventional density in the range of 1.0 to 1.1 kg/I. Nevertheless, it is also possible to provide fresh drilling fluid already having a comparatively higher density. Drilling fluids with a density higher than that conventionally used for horizontal drilling are known, for example, in drilling for an artesian well or in case of drilling for oil or gas with an increased reservoir pressure. The density of the debris laden drilling fluid is higher than that of the fresh drilling fluid due to the inclusion of debris generated at the working face of the drilling head during operation (unless the debris would have a density lower than that of the drilling fluid).
In addition to the positive effect on the buoyancy of the drillpipe, there is an additional benefit of an increased density of the drilling fluid in increasing the stability of the borehole against caving in. Depending on the circumstances, an impact on the dynamic and static pressure in the borehole may increase the risk of a break out or break through of the drilling fluid to the surface, while this can be countered by increasing the depths of the level at which the horizontal drilling takes place. In addition or as alternative other technical means may be employed, e.g. adding pump capacity for influencing the actual dynamic and static pressure in the bore hole, e.g. by providing pumping from vertical pressure release holes.
In an advantageous embodiment of an aspect of the invention, in the area of the buoyancy unit, the elements of the drillpipe module that are due to transfer axial force have a compressive strength of 50 MPa or more, averaged over the total cross section.
With such a compressive strength, the ratio of between the absolute value of the effective weight force per meter of the drillpipe in operation (i.e. with fresh drilling fluid in the passage and immersed in debris laden drilling fluid) and the transferrable axial force is < 0.0002 1/m.
In another advantageous embodiment of an aspect of the invention, the first and second connection unit are arranged such that, when the drillpipe module is connected with another drillpipe module, the axial force transferrable by the elements of the drillpipe that are due to transfer axial force at each point along the length of the drillpipe module, except a shoulder portion of the connection units having a higher strength, does not vary by more than 20 %, preferably by no more than 5%.
The overall ability of the drillpipe module to transfer and withstand an axial force largely depends on its weakest portion, while the connection units - due to their design in defining the front and back enclosure of the buoyancy unit - will typically have a geometrical moment of inertia higher than the average moment of other portions along the length of the module.
In other advantageous embodiments of an aspect of the invention (i) the pipe is an inner pipe and encloses the passage, wherein the drillpipe module further includes an outer pipe fixed to the first and second connection unit, wherein the buoyancy unit is provided in the space between the outer pipe and the inner pipe, wherein the buoyancy unit is preferably formed by gaseous material, (ii) the pipe encloses the passage and the buoyancy unit at least partially encloses the pipe, wherein the buoyancy unit is formed by solid material, (iii) the pipe encloses the buoyancy unit formed by solid material, wherein the buoyancy unit encloses the passage, or (iv) the buoyancy unit is formed by solid material and includes a first unit and a second unit, wherein the first unit encloses the passage and the second unit encloses the pipe, which encloses the first unit.
Such combinations of pipe and buoyancy unit allow for a high pressure inside the passage and a high pressure outside the drillpipe, wherein the (in comparison to just the inner pipe) increased cross sectional area provides a large area for receiving forwardly directed (axial) push force and the geometrical moment of inertia provides resistance against folding or buckling.
The arrangement where there is a further (outer) pipe, which, in turn, surrounds the buoyancy unit, is particularly beneficial insofar as it allows for the buoyancy unit being made of a material (e.g. polyurethane foam or another material like polyethylene with gas or air bubbles therein) which by itself may not be able to withstand the conditions inside the borehole, e.g. due to abrasion at the walls of the borehole or due to pressure of the debris laden drilling fluid return from the working face. In such case, the outer pipe may have a smaller wall thickness, so just to withstand the pressure from the outside. The arrangement of an inner pipe and an outer pipe also allows that the buoyancy unit is formed by gas enclosed between the pipes.
It is also possible, provided suitable (a) material(s) for the buoyancy unit are used, that the drilling fluid is provided inside the buoyancy unit, which is enclosed by the pipe, wherein the pipe may also be sandwiched between two portions of the buoyancy unit.
In a variation of one of the above embodiments, between the inner pipe and the outer pipe one or more stiffeners are provided, wherein the one or more stiffeners preferably extend along the whole length of the buoyancy unit and in radial direction from the inner pipe to the outer pipe.
With the stiffeners extending along the whole length of the buoyancy unit, the stiffeners by themselves may contribute to the transfer of axial force. However, independently from receiving and transferring any axial force, the stiffeners contribute to the buckling or folding resistance of the drillpipe module in linking the inner and the outer pipe against a radial deforming of either pipe. In addition, the stiffeners provide further strengthening of the outer pipe in terms of hoop stress or collapse resistance, i.e. assist in preventing that the outer pipe collapses under the pressure of the debris laden drilling fluid.
The stiffeners and the inner and outer pipe are not necessarily made of the same material (or material class). In particular, the inner and outer pipe may be made of steel, as with conventional drillpipes, while the stiffeners may be made of, for example, fiber-reinforced plastic. It is, however, also possible to provide other mixes of materials, including, for example, an inner pipe made of steel, stiffeners made of steel or aluminum and an outer pipe made of fiber reinforced plastic, possibly provided with a further coating.
In a modification of the above variation, at least one of the one or more stiffeners is arranged to fix the inner pipe and the outer pipe together and/or the outer pipe is releasably fixed to the connection units.
A fixing of the inner and outer pipe by means of the stiffeners allows for a further improvement in terms of stability, as such fixing not only prevents, for example, a radial deforming of the outer pipe in the direction to the inner pipe but also a deforming in the opposite direction.
On the other hand, it is also possible that the outer pipe only abuts the stiffeners and is furthermore connected to the connection units is a releasable manner, so that the outer pipe may be removed and replaced easily in order to address wear and tear.
In another advantageous embodiment of an aspect of the invention, the connection units are arranged for allowing a releasable connection with the other drillpipe module, wherein the releasable connection is preferably a screwed connection, wherein the connection units most preferably include a pin connection unit and a box connection unit.
Preferably, the connection between drillpipe modules of the present invention is comparable or even compatible with connections conventionally used for, for example, HDD arrangements.
A particularly preferred example of such embodiment includes that there are provided an API box connection and/or an API pin connection, while also other connections may be provided, preferably including trapezoidal threading and a sealing shoulder.
In a further advantageous variation of such embodiment, the pin connection unit includes a projection portion provided with a sleeve arranged for being attached over the projection portion.
In case of modules, it is known from conventional drilling, that such module may be damaged, e.g. during handling in the course of assembling the drillpipe modules or of disassembling, wherein, in particular, a damage to the ends of the module are of concern where adjacent modules are linked. As, in particular relative minor, variations of the respective length of a drillpipe module of a series of drillpipe modules are of no concern, rather than discarding a damaged module, the damaged end may be cut off, while a new connection is provided, e.g. by cutting a fresh thread in case of threaded connections. This may be done rather easily if at least one of the connection units projects from the buoyancy unit. Once the drillpipe modules are connected, the sleeve may be provided so to provide a rather smooth and flush outer surface of the connection area. The sleeve, if needed, may also be cut accordingly for fitting. A preferred variation of such sleeve includes two halves, which are fit on the connection area and fixed together along secants of the cross section, e.g. by bolts.
The pump provided for pumping the drilling fluid through the drillpipe may be a centrifugal pump and/or is preferably arranged to pump drilling fluid with a solids content of 5% or more.
It may be noted that the drilling fluid provided to the drilling head does not necessarily has to be freshly composed drilling fluid, as it is indeed possible to recycle the debris laden drilling fluid by at least partially removing debris therefrom. Insofar as the present application includes the term "fresh drilling fluid" this is to be understood as being in contrast to the debris laden drilling fluid returning from the working face.
The drilling head or some other suitable element of the arrangement inside the borehole may be provided with steering elements, wherein furthermore the drilling head is preferably provided with a position, orientation and/or attitude detecting element.
A preferable implementation of the invention includes the provision of a motor for driving the drilling head, arranged between the drillpipe and the drilling head, wherein most preferably the motor is a mud motor driven, in turn, by the drilling fluid.
In order to increase the torque and reduce the rotational speed of a known and commercially available mud motor, a transmission may be provided between the mud motor and the drilling head.
While any additional equipment (e.g. a sensor or the like) in the area of the drilling head may be provided with batteries or the like as power sources, it is also foreseen that power may be derived from the mud motor or by other means from the flow of the drilling fluid and/or power is provided from above ground by means of, for example, wiring provided inside or along the drillpipe.
In an advantageous embodiment of an aspect of the invention, the drilling method includes adjusting a density of the drilling fluid provided through the passage, adjusting a pump rate of the drilling fluid through the passage, and/or adjusting a rate of advancement of the drillpipe, so to control the density of the debris laden drilling fluid for adjusting the effective weight of the drillpipe.
It may be sufficient in certain cases to just set a particular density (i.e. composition) of the drilling fluid provided to the drill head, to provide for a certain advancing of the drilling and to provide for a certain pump rate of the drilling fluid. Controlling of these parameters, which influence the density of the debris laden drilling fluid, however, allows for adjusting in view of different drilling conditions. If, for example, due to the situation at the working face, the rate of advancement needs to be reduced, so that there is less debris in the returning drilling fluid, the density of the provided drilling fluid may be increased for compensation. Depending on the ground, the composition (and thus the density) of the debris at the working face may also change, which might similarly be addressed by appropriately controlling the density of the drilling fluid, the pump rate and/or the rate of advancement.
The method may also comprises detecting the density of the debris laden drilling fluid at the drill head and/or above ground, wherein the detected density is used in controlling the density. Measuring, sensing or detecting the density of the debris laden drilling fluid and modifying the control of the density of the (fresh) drilling fluid, the pump rate and/or the rate of advancement allows for a feedback control or regulation loop, while, however, the density of the debris laden drilling fluid and the ratio thereof in comparison to the density of the drillpipe may also be inferred otherwise. For example, the amount of friction between the drillpipe and the borehole may also be used, derived from the force needed for advancing and possibly from knowledge about the ground, as a basis for controlling the parameters influencing the uplift or downlift of the drillpipe in the borehole.
The method for drilling preferably further includes a steering of the drill head (indirectly via the drillpipe and/or directly) during the drilling. It is particularly preferred that the steering is based on positional information obtained in situ, e.g. by means of a sensor or probe attached to the drill head providing position information of the drill head or allowing a detection of the drill head's position from above surface. The positional information may preferably be supplemented with information on the attitude and/or orientation of the drill head. Conventional approaches on steering and determination of the position and the like may be used in the context of the invention, as appreciated by the skilled person.
The present invention allows for, depending on the particular embodiment, advancing the drillpipe by pushing the drillpipe (with the drill head being driven independently from the drillpipe) or by rotating or turning the drillpipe itself so to turn the drill head. This might be combined with the steering.
In particular in an embodiment in which the drill head is driven by means of a drill motor (or mud motor) included in the overall drilling arrangement (specifically between the drillpipe and the drill head), the drillpipe may be pressed at its end above the surface in a known way. However, it is also possible to provide to pre-assemble a length of drillpipe of 100 m or more from comparatively short drillpipe modules (e.g. 10 m long), while the axial force on the drillpipe is provided not (only) from its end but also from outside (e.g. by clamping) in an area preferably close to the entrance of the borehole. The pre-assembly is, of course, only possible if there is sufficient space, while the benefit lies in the reduced amount of time needed for inserting the numerous drillpipe modules directly at the entrance of the borehole.
Features of preferred embodiments of the invention are defined, in particular, in the dependent claims, while further advantageous features, embodiments and implementations are apparent to the skilled person from the above explanation and the following discussion.
In the following, the present invention is further elucidated and exemplified under reference to embodiments illustrated in the attached drawings, in which

Fig. 1: shows a schematic representation for illustrating a first exemplary embodiment of the present invention,
Fig. 2: shows a schematic representation of a buoyancy enhanced drillpipe module according to an embodiment of the invention,
Fig. 3: shows a schematic representation of a buoyancy enhanced drillpipe module according to another embodiment of the invention,
Fig. 4: shows a schematic representation of buoyancy enhanced drillpipe modules according to another embodiment of the invention,
Fig. 5: shows a schematic representation of buoyancy enhanced drillpipe modules according to another embodiment of the invention,
Fig. 6: shows a schematic representation of buoyancy enhanced drillpipe modules according to another embodiment of the invention,
Fig. 7: shows a schematic representation of buoyancy enhanced drillpipe modules according to another embodiment of the invention,
Fig. 8: shows a schematic representation of a system for drilling a borehole in ground according to an embodiment of the invention, and
Fig. 9: shows a schematic flow diagram of an exemplary embodiment of a method for drilling a borehole in ground according to the invention.

In the attached drawings and the explanations on these drawings elements, which are in relation or in correspondence, are indicated - where expedient - by corresponding or similar reference signs, regardless of whether or not the elements are part of the same embodiment.
Fig. 1 shows a schematic representation for illustrating a first exemplary embodiment of the present invention.
Similar to a known HDD arrangement, there is provided a pump 10 and a drill rig 12, which are coupled to a drillpipe 16, which extends inside a borehole in ground 14. Forward to the drillpipe 16, a mud motor 18 is provided, which is coupled to a transmission 20, which in turn is coupled to a drilling head 22. The drilling head 22 includes a drive 24, which drives the drilling bit 26 at the working face of the borehole.
From the pump 10, fresh drilling fluid 28 is provided through the drillpipe 16 to the mud motor 18 and further to the drilling bit 26, where the drilling fluid takes up debris from the drilling operation and returns in the space between the drillpipe 16 and the walls of the borehole as debris laden drilling fluid 30. The illustrated path of the drilling fluid 28 is to be understood as schematically.
The drillpipe 16 is formed by buoyancy enhanced drillpipe modules, which includes a pipe 32 and a buoyancy unit 34, wherein the buoyancy body has a lower density than the debris laden drilling fluid 30 and the dimensions of the buoyancy body 34 and the passage inside the pipe 32 in which the fresh drilling fluid 28 flows are set such that a density ratio between a combined density of the drillpipe 16 including the fresh drilling fluid 28 and a density of the debris laden drilling fluid 30 is such that, the effective weight of the drillpipe is thus reduced significantly.
In the schematic overall illustration provided by Fig. 1, the details of the drillpipe modules including the arrangement of the connection units thereof and not shown and reference is made insofar to Fig. 2 to 7.
Fig. 2 shows a schematic representation of a buoyancy enhanced drillpipe module 40 according to an embodiment of the invention. Specifically, Fig. 2a) shows a partial view of the drillpipe module, Fig. 2b) shows a view of the left face of the drillpipe module shown in Fig 2a) and Fig. 2c) and 2d) show cross sectional views at corresponding positions.
The drillpipe module 40 is immersed in debris laden drilling fluid 30 and includes a passage 46 in which fresh drilling fluid 28 is provided.
The drillpipe module 40 comprises an inner pipe 41 made of steel, which is surrounded by an outer pipe 41' also made of steel. At their ends (only one which is shown in Fig. 2) the pipes 41, 41' are connected to a respective connection unit. In the case of Fig. 2, the connection unit is formed by a steel shoulder 45 together with a connection portion of the inner pipe 41. The inner pipe 41, in its form, corresponds to a conventional drillpipe module having API box and pin connections.
The inner pipe 41, the steel shoulder 45 and the outer pipe 41' enclose and define a hollow portion filled with air, which forms the buoyancy unit 42 of the drillpipe module 40.
As further shown in Figs. 2c) and 2d), the area between the inner pipe 41 and the outer pipe 41' is provided with stiffeners or stabilizers 48, which extend radially from the inner pipe 41 to the outer pipe 41', adding to the stability and strength of the combination of inner and outer pipe 41, 41'.
Providing a borehole having a diameter of 19.1 inch (486 mm), the inner pipe has a diameter of 110 mm and a wall thickness of 12.5 mm, while the outer pipe is provided with a diameter of 324 mm and a wall thickness of 5 mm. There are provided six stiffeners, having a length (in radial direction) of 101.5 mm and a thickness of 5 mm.
Such arrangement as shown in Fig. 2 with the values above gives a cross sectional area for transferring axial force of 11.885 mm², while in debris laden drilling fluid having a density of 1.1 kg/I, the effective weight force per meter is -84 N/m (the negative sign indicating here a force directed downwards) (+78 N/m with a debris laden drilling fluid having a density of 1.3 kg/I; the positive sign indicating a force directed upward, i.e. an uplift with the drillpipe pressing against the ceiling of the borehole). The ratio between the absolute value of the effective weight force per meter and the total cross section of those elements of the drillpipe module that are due to transfer axial force is thus is the range of 6.500 to 7.500 N/m³.
It was found that with a weight on bit of 5 kN/in (i.e. a total force of 96 kN), a theoretical range of approx. 10 km may be expected, while under similar circumstances a range of only 2 km may be expected for a conventional drillpipe.
It is to be understood that the above values are exemplary and that other dimensions are of course also possible.
Fig. 3 shows a schematic representation of a buoyancy enhanced drillpipe module 50 according to another embodiment of the invention.
The module 50 includes a steel pipe 51 and a buoyancy unit 52 made of high density polyethylene (having a density of 0.985 kg/I), wherein the HPDE surrounds the steel pipe 51 and is fixed thereto.
The dimensions of the steel pipe 51 (which forms a passage 56) and the buoyancy unit 52 are set such that, with drilling fluid (not shown) flowing inside the passage 56 and debris laden drilling fluid outside the drillpipe module 50, the effective weight of the module 50 is basically compensated by the buoyancy of the module 50 in the drilling fluid.
The module 50 is furthermore provided, at each end thereof, with a steel shoulder 55, which serves as protection and break-out area for an API pin connection 53 and an API box connection 54 provided with the steel pipe 51, which allow combination of the module 50 with other such modules. A typical length of such module 50 is about 10 m.
The steel shoulder 55 and the connection portions 53, 54 of the steel pipe 51 form the connection units, wherein these connection units have the buoyancy unit 52 provided therebetween.
Fig. 4 shows a schematic representation of buoyancy enhanced drillpipe modules 60 according to another embodiment of the invention.
Similar to the drillpipe modules 40, 50 shown in Fig. 2 and Fig. 3, the buoyancy enhanced drillpipe modules 60 include a steel pipe 61 and a buoyancy unit 62, wherein the steel pipe 61 is provided with API pin and box connections 63, 64 and defines a passage 66 for drilling fluid.
In the illustration of Fig. 4, the details of the drillpipe modules 60 in terms of the connection units are not shown, while, however, the connection 63, 64 extend beyond a shoulder form by or in the respective connection unit, while the modules 60 are provided with a sleeve 67.
As shown in Fig. 4

a), the steel pipe 61 projects from the shoulders of the drillpipe module on both sides in lengthwise direction. In Fig. 4
b), the sleeve 67 is provided, such that also in the area of projection of the steel pipe 61 in comparison to the remaining portion of the drillpipe module 60 there is a rather smooth surface, substantially without a recess or a projection beyond the periphery of the module 60 otherwise. Fig. 4
c) shows a simplified cross section of the arrangement shown in Fig. 4b).

The sleeve 67 is provided in the form of two halves, which are bolted in place in the recess formed between the shoulders of the neighboring modules 60.
Such arrangement allows, for example, a re-cutting of the pin and box connections 63, 64 in case these may be damaged. Furthermore, it is contemplated that that the shoulders or disc-shaped elements bridging the area between the inner pipe and the outer pipe on both sides of the buoyancy unit are provided with standardized pin and box connections, which may thus be used despite their outer diameter being smaller than the outer diameter of the drillpipe module. In other words, a connection unit may be formed by combining (e.g. welding together) a conventional (and standardized) drillpipe of comparative small diameter with a flange of larger diameter.
Figs. 5 to 7 show schematic representations of buoyancy enhanced drillpipe modules 70, 80, 90 according to further embodiments of the invention.
Again similar to the drillpipe modules 40, 50 and 60 shown in Fig. 2, Fig. 3 and Fig. 4, the buoyancy enhanced drillpipe modules 70, 80, 90 each include a steel pipe 71, 81, 91 and a buoyancy unit 72, 82, 92, wherein there is also provided a passage 76, 86, 96 for drilling fluid.
In the case of Fig. 4, the steel pipe 71 also projects beyond the buoyancy unit 72, while here the pipe 71 is provided, at each end respectively, with a pin connection 73 and a box connection 74 (including, as illustrated, trapezoid threads and sealing shoulders), which form the connection units between which the buoyancy unit 72 is provided.
As it is the case with the embodiments shown in Fig. 2, 3 and 4, the buoyancy unit 72 surrounds the pipe 71.
In comparison to the case of Fig. 5, in the embodiment shown in Fig. 6, there is additionally provided a further pipe 81', which surrounds the buoyancy unit 82 and partially the connection units 83, 84.
If the contacts between the connection units 83, 84 and the pipes 81, 81' are sufficiently tight, instead of a buoyancy body 82 is the form of a plastic or foam material, there might also be provided just gas, e.g. air, in the compartment thus formed. While evacuating the compartment (e.g. providing a reduced pressure therein) might be possible, the resulting gain in weight reduction is insignificant. The provision of gas instead of a solid material allows for a reduced overall weight.
Fig. 7 shows an embodiment where the buoyancy unit 92 is provided inside the pipe 91, i.e. the relative positions are exchanged in comparison to the embodiment shown in Fig. 6.
Fig. 8 shows a schematic representation of a system 1 for drilling a borehole in ground according to an embodiment of the invention.
The pump 10, the drill rig 12, the drillpipe 16 (made of drillpipe modules as discussed above), the mud motor 18, the transmission 20 and the drilling head 22 discussed above with reference to Fig. 1 are part of the system 1, wherein the pump 10 and the drill rig 12 are part of an operation unit 37, which further includes a drilling fluid conditioning and recycling unit 38.
The drilling head 22 is additionally provided with a density sensor 35 for detecting the density of the debris laden drilling fluid returning to the surface from the working face.
The system furthermore includes a control unit 36.
During the drilling, the control unit 36 receives data from the density sensor 35 and uses this data for determining whether the rate of advancement of the drillpipe 16 or the composition (and thus density) of the drilling fluid provided to the drillpipe 16 are to be changed in order to provide for a desired ratio between the density of the debris laden drilling fluid and the drillpipe (including fresh drilling fluid). In accordance therewith, the drill rig 12 and/or the drilling fluid conditioning and recycling unit 38 are controlled by the control unit 36.
Fig. 9 shows a schematic flow diagram of an exemplary embodiment of a method for drilling a borehole in ground according to the invention.
In a provision step 101, a system as illustrated in Fig. 8 is provided, including in particular a buoyancy enhanced drillpipe made of a plurality of drillpipe module as discussed above and a drilling head. For the drilling, drilling fluid is used, which - when laden with debris and returning from the working face - in operation immerses the drillpipe. The buoyancy unit has a lower density than the drilling fluid and the dimensions of the buoyancy unit and the passage are set such that the overall drillpipe module is dimensioned such a ratio between an absolute value of an effective weight force per meter of the drillpipe module in debris laden drilling fluid, with drilling fluid in the passage, and a total cross section of those elements of the drillpipe module that are due to transfer axial force is < 10,000 N/m³.
In an advancing step 102, the drillpipe is advanced upon pushing and/or turning the drilling head. In the course of this, i.e. in parallel, the rate of advancement or drilling is controlled in a control step 103 and, in an adjustment step 104, the density of the fresh drilling fluid is adjusted.
In a further detecting step 105, the density of the debris laden drilling fluid is detected, wherein this data is then used in the control step 103 and/or the adjustment step 104.
The proportions shown in the figures are merely for illustrative purposes and the illustrations are not to scale.
Even if in the drawings different aspects or features of the invention are shown in combination, the skilled person will appreciate - unless indicated otherwise - that the combinations shown and discussed are not exhaustive and variations thereof are possible. In particular, corresponding elements or feature complexes may be mutually exchanges between different embodiments. The scope of the invention is defined by the appended claims.

List of reference signs

1: system
10: pump
12: drill rig
14: ground
16: drillpipe
18: mud motor
20: transmission
22: drilling head
24: drive shaft
26: drilling bit
28: fresh drilling fluid
30: debris laden drilling fluid
32: pipe
34: buoyancy body
35: density sensor
36: control unit
37: operation unit
38: drilling fluid conditioning and recycling unit
40, 50, 60, 70, 80, 90: buoyancy enhanced drillpipe module
41, 41', 51, 61, 71, 81, 81' 91: steel pipe
42, 52, 62, 72, 82, 92: buoyancy unit
53, 63, 73, 83, 93: pin connection
54, 65, 74, 84, 94: box connection
45, 55: steel shoulder
46, 56, 66, 76, 86, 96: passage
67: sleeve
101: provision step
102: advancing step
103: control step
104: adjustment step
105: detecting step

Claims

A buoyancy enhanced drillpipe module (40, 50, 60, 70, 80, 90) for drilling a borehole in ground (14) and being immersed in debris laden drilling fluid (30), comprising:
a pipe (41, 41', 51, 61, 71, 81, 81', 91),

a first and a second connection unit (45, 55, 73, 74, 83, 84, 93, 94) fixed to the respective ends of the pipe, the connection units (45, 55, 73, 74, 83, 84, 93, 94) each allowing a connection of the drillpipe module (40, 50, 60, 70, 80, 90) respectively with another drillpipe module (40, 50, 60, 70, 80, 90), and

a passage (46, 56, 66, 76, 86, 96) for drilling fluid (28),

wherein at least in a part of the space between the first and second connection unit (45, 55, 73, 74, 83, 84, 93, 94) a buoyancy unit (42, 52, 62, 72, 82, 92) is provided, and

wherein, for a drilling fluid (28) having a density in the range of 1.0 to 1.1 kg/l and/or a debris laden drilling fluid (30) having a density in the range of 1.1 to 1.5 kg/l, the buoyancy unit (42, 52, 62, 72, 82, 92) has a lower density than the drilling fluid (28, 30)

characterized in that the drillpipe module (40, 50, 60, 70, 80, 90) is dimensioned such that a ratio between an absolute value of an effective weight force per unit length of the drillpipe module (40, 50, 60, 70, 80, 90) in debris laden drilling fluid (30), with drilling fluid (28) in the passage (46, 56, 66, 76, 86, 96), and a total cross sectional area of those elements of the drillpipe module (40, 50, 60, 70, 80, 90) that are due to transfer axial force is < 10,000 N/m³.
The buoyancy enhanced drillpipe module (40, 50, 60, 70, 80, 90) according to claim 1, wherein, in the area of the buoyancy unit (42, 52, 62, 72, 82, 92), the elements of the drillpipe module (40, 50, 60, 70, 80, 90) that are due to transfer axial force have a compressive strength of 50 MPa or more, averaged over the total cross sectional area.
The buoyancy enhanced drillpipe module (40, 50, 60, 70, 80, 90) according to any one of the preceding claims, wherein the first and second connection unit (45, 55, 73, 74, 83, 84, 93, 94) are arranged such that, when the drillpipe module (40, 50, 60, 70, 80, 90) is connected with another drillpipe module (40, 50, 60, 70, 80, 90), the axial force transferrable by the elements of the drillpipe that are due to transfer axial force at each point along the length of the drillpipe module (40, 50, 60, 70, 80, 90), except a shoulder portion of the connection units (45, 55, 73, 74, 83, 84, 93, 94) having a higher strength, does not vary by more than 20 %, preferably by no more than 5%.
The buoyancy enhanced drillpipe module (40, 50, 60, 70, 80, 90) according to any one of the preceding claims, wherein
(i) the pipe is an inner pipe (41, 81) and encloses the passage (46, 86), wherein the drillpipe module (40, 80) further includes an outer pipe (41', 81') fixed to the first and second connection unit (45, 83, 84), wherein the buoyancy unit (42, 82) is provided in the space between the outer pipe (41', 81') and the inner pipe (41, 81), wherein the buoyancy unit (42, 82) is preferably formed by gaseous material,

(ii) the pipe (71) encloses the passage (76) and the buoyancy unit (72) at least partially encloses the pipe (71), wherein the buoyancy unit (72) is formed by solid material,

(iii) the pipe (91) encloses the buoyancy unit (92) formed by solid material, wherein the buoyancy unit (92) encloses the passage (96), or

(iv) the buoyancy unit is formed by solid material and includes a first unit and a second unit, wherein the first unit encloses the passage and the second unit encloses the pipe, which encloses the first unit.
The buoyancy enhanced drillpipe module (40, 80) according to aspect (i) of claim 4, wherein between the inner pipe (41, 81) and the outer pipe (41', 81') one or more stiffeners (48) are provided, wherein the one or more stiffeners (48) preferably extend along the whole length of the buoyancy unit (42, 82) and in radial direction from the inner pipe (41, 81) to the outer pipe (41', 81').
The buoyancy enhanced drillpipe module (40, 80) according to claim 5, wherein at least one of the one or more stiffeners (48) is arranged to fix the inner pipe (41, 81) and the outer pipe (41', 81') together and/or the outer pipe (41, 81) is releasably fixed to the connection units (45, 83, 84).
The buoyancy enhanced drillpipe module (40, 50, 60, 70, 80, 90) according to any one of the preceding claims, wherein the connection units (45, 55, 73, 74, 83, 84, 93, 94) are arranged for allowing a releasable connection with the other drillpipe module (40, 50, 60, 70, 80, 90), wherein the releasable connection is preferably a screwed connection, wherein the connection units (45, 55, 73, 74, 83, 84, 93, 94) most preferably include a pin connection unit and a box connection unit.
A system (1) for drilling a borehole in ground (14), comprising:
a plurality of buoyancy enhanced drillpipe modules (40, 50, 60, 70, 80, 90) according to any one of the preceding claims, and

a drilling head (22) arranged for being attached to a drillpipe (16) formed by the plurality of drillpipe modules (40, 50, 60, 70, 80, 90).
The system (1) according to claim 8, further comprising
a control unit (36) for use in drilling a borehole in ground (14) with a drillpipe (16) including a plurality of buoyancy enhanced drillpipe modules (40, 50, 60, 70, 80, 90), wherein drilling fluid (28) is provided through the passage (46, 56, 66, 76, 86, 96) of the drillpipe (16) to a drilling head (22) and the drillpipe (16) is immersed in debris laden drilling fluid (30) returning from a work face of the drilling head (22),

wherein the control unit (36) is arranged for outputting control signals for controlling the density of the debris laden drilling fluid (30) for adjusting the effective weight of the drillpipe (16) by adjusting a density of the drilling fluid (28) provided through the passage (46, 56, 66, 76, 86, 96), adjusting a pump rate of the drilling fluid (28) through the passage (46, 56, 66, 76, 86, 96) and/or adjusting a rate of advancement of the drillpipe (16), and

an operation unit (37) for use in drilling a borehole in ground (14) with a drillpipe (16) including a plurality of buoyancy enhanced drillpipe modules (40, 50, 60, 70, 80, 90), wherein drilling fluid (28) is provided through the passage (46, 56, 66, 76, 86, 96) of the drillpipe (16) to a drilling head (22) and the drillpipe (16) is immersed in debris laden drilling fluid (30) returning from a work face of the drilling head (22),

wherein the operation unit (37) includes means for adjusting a density of the drilling fluid (28) provided through the passage (46, 56, 66, 76, 86, 96), adjusting a pump rate of the drilling fluid (28) through the passage (46, 56, 66, 76, 86, 96) and/or adjusting a rate of advancement of the drillpipe (16), so to control the density of the debris laden drilling fluid (30) for adjusting the effective weight of the drillpipe (16),

wherein the operation unit (37) is arranged to operate the means in response to control signals.
A method for drilling a borehole in ground, comprising the steps of:
providing (101) a plurality of buoyancy enhanced drillpipe modules forming a drillpipe and a drilling head attached to the drillpipe,

advancing (102) the drillpipe with the drilling head being pushed and/or turned,

wherein drilling fluid is provided through the combined passages of the plurality of drillpipe modules to the drilling head, the drilling fluid returning as debris laden drilling fluid immersing the drillpipe,

characterized in that the buoyancy enhanced drillpipe modules are buoyancy enhanced drillpipe modules according to any one of claims 1 to 7.
The method according to claim 10, further comprising:
adjusting (104) a density of the drilling fluid provided through the passage,

adjusting (104) a pump rate of the drilling fluid through the passage, and/or

adjusting (104) a rate of advancement of the drillpipe,

so to control the density of the debris laden drilling fluid for adjusting the effective weight of the drillpipe.
A computer program with computer program code means for causing the system (1) according to claim 9 to carry out the steps of the method according to claim 11, when the computer program is run on the system (1).