Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF DRILLING AND JET DRILLING SYSTEM
The invention relates to a method of drilling into an
object, in particular by jet drilling, and to a jet
drilling system. The object can in particular be a
subsurface earth formation.
In the course of making a borehole, it is often
desirable to control the drilling direction so as to
provide a borehole along a predetermined trajectory. In
conventional mechanical drilling, a common technology is
to use equipment like bent subs, mud motors and rotating
seals, to set only the lower part of the drill string
with the drill bit to drill in a particular direction.
Conventional mechanical drilling uses drilling bits with
mechanical cutters such as roller-cones or
polycrystalline diamonds, that produce cuttings by
scraping at the borehole bottom and at the sides. More
recently, rotary steerable systems (RSS) have been
developed, which can operate with the entire drill string
rotating. Known RSS methods point the mechanical drill
bit into a desired direction using a complex bending
mechanism, or push the drill bit to a particular side
using expandable thrust pads. The side-cutting ability of
the mechanical drill bits used for directional drilling
then allows to deviate the borehole in the desired
direction. For example, PDC bits have cutters not only on
the front end but also at the sides. Known mechanical
directional drilling systems are for example described in
US5168941, US5520255, US5857531, US5875859, US6092610 and
US2007/0163810.
Another type of drilling methods uses fluid jetting,
instead of mechanical interaction between the drill bit
or drill head and the object. However, directional
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drilling methods known from mechanical drilling cannot
generally be used with jet drill systems. For example,
equipment like bent subs, mod motors and rotary seals
cannot be used at the high fluid pressures applied in jet
drilling, which can be 100 bar or more, in particular 250
bar or more, more in particular 350 bar or more. Moreover
known jet drilling heads bit do not have the side-cutting
action that is needed for known mechanical directional
drilling to work. Furthermore, abrasive jet drilling
requires only minimal or even no weight on bit, which
makes such drilling system react totally differently than
conventional mechanical directional drilling methods and
devices.
A jet drill system and method of making a hole in an
object is disclosed in WO-A-2005/005767. The known system
comprises an excavating tool, herein also referred to as
abrasive jet drill head, mounted on a lower end of a
drill string that is inserted from the surface into a
hole in a subterranean earth formation. The drill string
is provided with a longitudinal passage for transporting
a drilling fluid mixture comprising abrasive particles to
the drill head. The drill head comprises jet means
arranged to generate an abrasive jet in a jetting
direction into impingement with the earth formation in an
impingement area. The abrasive jet contains magnetic
abrasive particles (steel shot). A recirculation system
is provided, which captures abrasive particles from the
return stream to surface, after erosive impingement, by
means of a magnet, and re-mixes the abrasive particles at
a mixing location with the mixture received via the drill
string. The magnet is arranged as a rotatable conveyor,
attracting particles to be recycled and conveying them
towards a mixing location with fresh fluid from surface.
In the known system directional drilling is achieved by a
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modulation means in form of a controllable drive means
for the conveyor, which is arranged so as to modulate the
recirculation rate, and in this way the quantity of
particles in the abrasive jet at the jet means is
modulated. When the abrasive jet is moved along a
trajectory in the hole, in particular in a rotating
motion, the amount of erosion in each impingement area
along the trajectory can be selectively varied, and
directional control is achieved. Reference is also made
in this regard to WO 2005/05766.
The control of directional effect via the
recirculation system is relatively complex, and in fact
requires the presence of abrasive particles in the fluid
jet as well as a recirculation system.
There is a need for a new method of directional
drilling for fluid jet drilling, which can be used
independent of the presence and/or operation of a
recirculation system for abrasive particles in the fluid
jet.
In accordance with the present invention there is
provided a method of drilling into an object, the method
comprising
- providing a drill string in a borehole in the object,
the borehole having at its bottom a borehole axis, the
drill string comprising a borehole-centralized jet drill
head at its lower end, the jet drill head having a drill
axis, and comprising a jet nozzle;
- providing a stabilizer means for the drill string, the
stabilizer means determining an inclination angle between
the borehole axis and the drill axis; and
- generating a fluid jet at the jet nozzle, so as to
blast with an erosive power on an impingement area of the
borehole, thereby deepening the borehole, wherein the
drill string is rotated while deepening the borehole.
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The invention is based on the insight that
directional tendency in a low weight-on-bit (WOB) jet
drill system without side-cutting action is mainly
determined by the angle between the drill axis and the
borehole axis. Setting this angle to a non-zero value by
a suitable stabilizer means is relatively easy in view of
the low or virtually zero weight in bit. A stabilizer
arranged on the drill string above the jet drill head can
deviate the drill string slightly and sufficiently to
form an inclination angle with the borehole, so that the
fluid jetting builds a curved borehole. The rate at which
the inclination angle of the drill bit increases depends
on said angle. The better the drill string is
centralized, the smaller this tendency is. The expression
'stabilizer means' is used to refer to a structure or
device that protrudes radially with respect to adjoining
drill string parts so as to provide at least one point of
contact with the borehole wall during drilling. The point
of contact together with the borehole-centralizing
property of the jet drill bit mainly define a direction
of progression of drilling. The object into which is
being drilled can in particular be a subsurface earth
formation containing a rock layer, such as a layer of
sandstone, limestone, granit, basalt, or combinations of
such layers. In such a formation, drilling system as for
example known from BE 1001450A3 is not usable, as it is
designed for providing a tunnel in the soil, wherein soil
is compacted during progression. This is not possible in
a rock formation. The known drilling system is moreover
inadequate in that is does not rotate, and in that there
is no centralizing means for a jet drill head.
In a preferred embodiment the fluid jet is an
abrasive fluid jet and the jet drill head is an abrasive
jet drill head. An abrasive fluid jet is a jet of a fluid
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mixture comprising a concentration of abrasive particles,
e.g. steel shot in an aqueous liquid such as water.
The borehole centralized jet drill head can have a
centralization means such as an annular centralizer,
5 suitably at or around the drill head, and in any event
downhole from the stabilizer means. In one preferred
embodiment, the jet drill head is a self-centralizing jet
drill head, which by its design remains centralized in
the borehole during operation and does not require
additional centralizing means such as an annular
centralizer. The jet drill head can be self-centralizing
as well as suitable for jetting abrasive particles.
In a preferred embodiment the stabilizer means is
fixed with respect to the drill string while deepening
the borehole, i.e. when the drill string is rotated
during operation for deepening the borehole, the
stabilizer means rotates together with the drill string
and does not need to be fixed against the borehole wall,
and released as the drill string progresses.
In a particular embodiment the stabilizer means is an
eccentric stabilizer means, preferably the eccentric
stabilizer means is rotated along with the drill string
while deepening the borehole, more preferably the outer
dimensions of the eccentric stabilizer means
substantially conform to the borehole.
An eccentric stabilizer means has a non-uniform
radial extension around the drill string. When the drill
string rotates with the eccentric stabilizer means, and
the eccentricity is fixed, the inclination angle between
drill axis and borehole axis changes periodically. When
the eccentric stabilizer conforms to the borehole wall,
the inclination angle circles about the borehole axis.
Depending on the relative direction of the jetting nozzle
and the eccentricity, this way a straight borehole of
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increased ("overgauged borehole") or in fact smaller
diameter can be drilled when compared with a centralized
drill string.
In certain embodiments the stabilizer means has a
diameter smaller than the borehole diameter, preferably
the largest diameter of the stabilizer means is smaller
than the borehole diameter. When a borehole is somewhat
deviated and no stabilizer is provided, an abrasive jet
drill string has a natural tendency to build inclination
angle, because the drill string due to the low weight on
bit will be pulled to the lower side of the borehole
under the influence of gravity. A stabilizer of smaller
size than the borehole makes use of this effect, but
defines the point of contact and therefore the
inclination angle better.
In certain embodiments at least one radial dimension
of the stabilizer means is adjusted in the course of a
drilling operation, so as to deepen the borehole along a
predetermined trajectory. A radial dimension is a
dimension or direction perpendicular to the drill axis of
the drill string. By changing a radial dimension the
inclination angle can be influenced. When the stabilizer
means is for example concentric with the drill string and
smaller than the borehole, the inclination angle is set
by setting the diameter of the stabilizer means arranged
at a certain distance above the centralized jet drill
head. When the stabilizer means is an eccentric
stabilizer, the eccentricity can be changed, for example
changing the borehole diameter that is being drilled.
In certain embodiments the stabilizer means has a
cross-section that is adjusted while rotating the drill
string, preferably wherein the stabilizer cross section
is adjusted to maintain the drill axis geostationary
during at least one rotation, more in particular wherein
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the geostationary drill string axis has a smaller angle
with the vertical than the borehole axis. Such a
stabilizer means can for example be an actively
controlled means that maintains an eccentricity that is
geostationary during rotation. The inclination angle can
then be fixed, and in particular a negative inclination,
i.e. an arrangement wherein the geostationary drill
string axis has a smaller angle with the vertical than
the borehole axis can be provided.
The invention moreover provides a jet drilling system
for drilling a borehole into an object, comprising a
drill string with an jet drill head at its lower end, the
drill head being provided with a jet nozzle, a stabilizer
means for the drill string at a position above the jet
drill head, the stabilizer means protruding radially with
respect to adjoining drill string parts so as to provide
a point of contact with the borehole wall, and a rotation
means for rotating the drill string.
The jet drilling system can further comprise centralizing
means for the jet drill head downhole of the stabilizer
means.
The jet drill had can have a front end for engaging
the borehole bottom and an axis, and wherein the front
end has a recess at least in an axial region. During
operation of such a jet drill head, the undrilled
material is present in the recess in the axial region,
e.g. substantially in the form of a cone, and centralizes
the jet drill head during rotation. Such a jet drill head
is therefore self-centralizing. The front end engaging
the borehole bottom can be a substantially
circumferential front end. In the recess the jet nozzle
can be arranged, preferably directed obliquely with
respect to the drill axis, such as to form a cone in the
recess during rotation.
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The stabilizer means can be adjustable in at least
one radial dimension, and to this end preferably further
comprises an actuator for manipulating the at least one
radial dimension. Such stabilizer means can comprise a
plurality of pads the radial extension of which can be
set by the actuator, e.g. hydraulically or with an
electrical motor such as a stepper motor. An adjustable
stabilizer is for example known from US4572305.
An adjustable jet drilling system preferably further
comprises a control means for controlling the at least
one radial stabilizer dimension, in particular downhole
control means. Preferably the control means comprise a
control signal receiver, such as a weight-on-bit sensor,
a fluid pressure sensor, a rotational speed sensor.
In one preferred embodiment the jet drill head is an
abrasive jet drill head.
In the following the invention will be described in
more detail and by way of example also with reference to
the accompanying drawings, wherein
Figure 1 shows schematically a first embodiment of
the invention; and
Figures 2-5 show schematically several further
embodiments of the invention.
In the Figures, like reference numerals are used to refer
to the same or similar objects.
In one aspect the invention is related to a jet drill
method and system, in particular an abrasive jet drill
system, comprising a rotatable drill string, a rotatable
abrasive jet drill head connected to the lower end of the
drillstring, the drill head being provided with at least
one jet nozzle for discharging a mixture of drilling
fluid and abrasive particles, drive means for rotating
the drill bit and drill string and pump means for
generating a flow of said mixture comprising drilling
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fluid and abrasive particles. Abrasive jet drill systems
are known for example from WO 00/66872, WO 2002/034653,
WO 2005/005766, W02008/119821, WO 2008/113843, WO
2008/113844.
By the combined action of the high pressure mixture
jet which contains hard particles, and the rotational
movement of the drill string and the drill bit, a
borehole is formed in a rock formation as a result of the
gradual erosion of the rock material. This erosion is
solely obtained by the abrasive jet, without any
mechanical cutting action of a cutter or drill bit, due
to the high pressure drop over the jet nozzle of e.g. 250
bar or more, preferably 350 bar or more of. The abrasive
particle concentration in the drilling fluid may be in
the range of 2-10 vol%, such as 3 vol% or more. However,
in case use is made of a downhole abrasive particle
recycling device, the abrasive particle concentration
passed from surface through the drill string may be as
lower, such as 1.5 vol% or less while still giving a good
penetration rate.
An aim of the present invention is to provide an
abrasive jet drilling system wherein use is made of more
simple devices and method steps for obtaining directional
control. This aim can be achieved by means of at least
one stabilizer at a position above the drill head, said
stabilizer protruding radially with respect to the
adjoining drill string parts. The influence of such
stabilizer on the direction of the drilling process
relies on the angle between the drill bit axis and the
borehole axis. The stabilizer may rotate together with
the drill string, in which case the stabilizer may extend
around the drill string. For a fixed directional
tendency, a stabilizer with a specific fixed radial
dimension may be used. When a stronger tendency is
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required, a stabilizer with a reduced radial dimension or
diameter should be used. As an example, a fixed spiral
melon shaped drill collar stabilizer can be used. Such
stabilizer can be positioned directly behind the abrasive
jet drill head, possibly with a downhole abrasive
particle recirculating device in between or integrated in
the drill head.
Alternatively, the stabilizer may have an adjustable
radial dimension. In that case, the directional tendency
of the drilling system may be influenced by changing the
radial dimension of the stabilizer. Such adjustable
stabilizers can be embodied in several ways. For
instance, in case the stabilizer comprises segments which
are arranged next to each other in circumferential
direction, these segments can be adjustable in radial
direction. Preferably, said segments are adjustable
independently from each other, e.g. by steering means
such as stepper motors or hydraulic piston/cylinder
devices.
In the latter embodiment, in case one of the segments
is adjusted to a relatively large radial dimension and
the opposite segment to a relatively small radial
dimension, the axis of the drill head will show a non-
zero angle with respect to the borehole axis. This
adjustment may for instance be carried out at the earth
surface and be kept during the ensuing drilling cycle.
Alternatively however, downhole control means may be
provided for controlling the stabilizer dimension. In the
latter case, the direction of the borehole may be changed
during the course of the drilling process, whereby a very
flexible control of the drilling trajectory is obtained.
The direction of the drilling trajectory may be
controlled from the earth surface. In that case, the
drilling system may comprise downhole control means with
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at least one sensor, such as a weight-on-bit sensor, a
fluid pressure sensor, a rotational speed sensor and the
like, as well as a control unit for controlling the
steering means on the basis of data detected by said
sensor. Preferably, the downhole control means may
comprise a memory containing stored trajectory data as
well as control means containing preprogrammed control
data for controlling the stabilizer radial dimension
dependant on the trajectory.
In one aspect the invention relates to a method for
operating an abrasive jet drilling system as described
before, comprising the steps of:
- making the drill string and abrasive jet drill head
rotate,
- applying a specific data sequence, such as a weight-on-
bit sequence, as a code,
- detecting said code by the sensor,
- controlling the radial dimension of the stabilizer on
the basis of said detected code.
Alternatively, the method may comprise the steps of:
- making the drill string and abrasive jet drill head
rotate,
- applying a specific data sequence, such as a weight-on-
bit sequence, as a code,
- detecting said code by the sensor,
- controlling the directional action of the stabilizer on
the basis of said detected code.
According to yet another possibility, the method may
comprise the steps of:
- making the drill string and abrasive jet drill head
rotate,
- applying a specific data sequence, such as a weight-on-
bit sequence, as a code,
- detecting said code by the sensor,
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- controlling the eccentricity of the stabilizer on the
basis of said detected code.
The radial dimension of the stabilizer may be adapted
while carrying out a drilling operation. According to
another possibility, the stabilizer settings may be
controlled as a function of a pre-programmed borehole
trajectory in combination with downhole measurements
while drilling. Such method may thus comprise the steps
of:
- making the drill string and abrasive jet drill head
rotate,
- detecting a downhole condition by the sensor,
- adjusting the stabilizer size on the basis of the
stored trajectory data and the detected downhole
condition.
The invention will now be described further by way of
example with reference to the embodiments shown in the
drawings.
As shown in Figure 1, an abrasive jet drilling system
including an abrasive jet drilling assembly according to
the invention comprises a drill string 1 in a borehole 2
in an object. This object is here a subterranean earth
formation 5, in particular to provide a borehole for the
manufacture of a well for production of mineral
hydrocarbons. The drill string 1 is which at its upper
end at surface 8 connected to a rotational drive device
(not shown, but indicated by arrow 10) and at the other,
lower, end to an abrasive jet drill head 16 with jet
nozzle 18. The drill string 1 has a passageway 20 for
fluid, which is in fluid communication with the jet
nozzle, via a passage through the stabilizer means and
passage 24 of abrasive jet drill head 16. Furthermore,
pump means (not shown) are provided at surface for
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circulating the drilling fluid from the surface through
the drill string 2 to the drill head 16.
The nozzle 18 is obliquely oriented in a recess 17 in
an axial area so that the impingement area is located
eccentric with respect to the drill axis or rotary axis
21, and in this case rotating the abrasive jet in the
hole results in the jet 19 and the impingement area
moving along an essentially circular trajectory in the
hole. Preferably, the eccentric impingement area overlaps
with the centre of rotation, so that also the middle of
the bore hole is subject to the erosive power of the
abrasive jet.
The jet nozzle 18 is arranged above an optional foot
part 29, and the nozzle is inclined relative to the
longitudinal direction of the system (drill axis 20) at a
nozzle angle of 15-30 relative to the drill axis, but
other angles can be used. Preferably the nozzle angle is
about 21 which is optimal for abrasively eroding the
bottom of the bore hole by axially rotating the complete
tool inside the bore hole.
The abrasive jetting drill head in this embodiment
moreover comprises a recirculation system for abrasive
particles, which is generally indicated as 30, with an
inlet 32 in fluid communication with the annulus 33
between abrasive jet drill head 16 and the borehole 2,
and an outlet 34 to a mixing chamber 36 arranged at a
mixing location 37 of the passageway 24.
The optional foot part 29 forms a front end of the
drill head and engages the borehole bottom providing for
a distance from the borehole bottom and suitably contains
slots for drilling fluid and cuttings to flow via the
annulus 33 upwardly. The abrasive jet drill head 18 can
for example be a head as described in e.g. W02008/113843,
WO 2008/113844.
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In operation, the system works as follows. A stream
of drilling fluid including abrasive particles such as
steel shot, is pumped from the object's surface (e.g.
earth's surface) by a suitable pump (not shown) through
the longitudinal passage 20 of the drill string 2. Part
or all of the drilling fluid is led to the jet nozzle 18
where an abrasive jet 19 is generated. The abrasive jet
is blasted into impingement with the formation. The
formation is eroded in the impingement area as a result
of the abrasive jet 19 impinging the formation 5, thereby
deepening the borehole 2. The positioning and oblique
orientation of the nozzle in the axial area provides that
a substantially cone-shaped central area of the borehole
bottom is created, which centralizes the jet drill head
16, i.e. the head is self-centralizing.
Simultaneously, the abrasive jet is rotated about the
rotary axis 20. Thus, the impingement area is moved along
a circular trajectory in the hole so that the formation
can be eroded at all azimuths at the borehole bottom. By
keeping the erosive power of the abrasive jet constant,
the formation is eroded evenly on all sides of the hole
and consequently the hole is excavated straight.
Nevertheless, distortions in the rotating of the
excavation tool, or variations in rock formation
properties in the hole region, or other causes may result
in uneven erosion in the hole. A directional correction
may be required by modulating the erosive power to
compensating for the unintentional uneven erosion.
As shown in Figure 1, the trajectory of the borehole
2 is curved. This curvature is obtained by the action of
the stabilizer means 40 which is connected to or provided
around the drill string 1 immediately above or at a small
distance above the drill head 2. Said stabilizer 40 is in
contact with the wall of the borehole at 42 under the
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influence of gravity. The distance between the contact
point 42 and the point where the drill head is
centralized (in this example at the front end of the foot
part 29) can be between 0.1 m and 50 m, such as between
0.5 m and 10 m.
The stabilizer 40 of this embodiment is coaxial with
the drill string 1, defining a circular cross-section at
the widest radial dimension. It is fixedly arranged
around the drill string so that it rotates together with
the drill string 1, and it has a diameter smaller than
the borehole diameter. The diameter together with the
distance from the centralized drill head determines the
inclination angle 45 between the drill axis 20 and the
borehole axis 48. The better the jetting assembly is
centralized at the first point of contact with the
borehole above the drill head, the smaller the
directional tendency is. Thus, if the first point of
contact is at a stabilizer, the outer diameter of this
stabilizer determines the building tendency. If the outer
diameter of the near-bit stabilizer can be changed while
drilling, the building tendency can be controlled while
drilling.
For directional drilling operation the inclination
angle is normally non-zero, and can for example be
between 0.01 and 20 degrees, such as between 0.1 and 5
degrees. The inclination angle in the Figure is positive,
i.e. the borehole is deviated upwardly. The invention can
however also provide negative inclination angles
(deviation towards the vertical), and lateral
inclinations, so that directional drilling into any
desired spatial direction can be provided.
It shall be clear that the schematic drawing
illustrates the various components but does not
necessarily represent their relative size correctly. For
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relatively large inclination angles the diameter of the
drill pipe or collar can be limiting, so that the drill
pipe or collar 3 between drill head 16 and stabilizer 40
and/or the drill string or tubular above stabilizer 40
has to be sufficiently thin for a desired inclination
angle, to ensure that the stabiliser above the bit is
touching the borehole wall.
The expressions upper, above, upstream, uphole,
lower, below, downstream, downhole, and the like, are
used herein with reference to a drill string with jet
drill head in a borehole, wherein upper or above is
closer to surface than lower or below; and upstream and
downstream are with respect to drilling fluid flowing
generally downwards through the drill string, and upwards
to surface though the annulus with the borehole wall.
The mechanical forces on the drilling system that is
based on abrasive jetting are much smaller than is the
case for systems based on mechanical rock removal. This
has the advantage that the sensors can be located very
close to the excavating tool, making early and accurate
signal communication possible to the modulation control
means. The sensors can for instance be provided in the
same chamber as the modulation control means. The
control means can comprise a memory for storing
trajectory data.
Alternatively, the position and and/or the direction
of progress through the formation of the abrasive jet can
be determined on the basis of parameters available on the
surface 8, including torque on the drill string 2 and
azimuthal position of the drill string 2, and axial
position and velocity of the drill string 2.
A decision to change or correct drilling direction
may also be taken via the operator of the directional
system at surface. In case of the signal originating from
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a down-hole measurement while drilling sensor, a mud-
pulse telemetry system or any other suitable data
transfer system can be employed to transfer the data to
the surface. Via similar means of data transfer a control
signal can be sent to the down hole control means
triggering a series of control actions required for the
desired direction drilling correction.
A thruster (not shown) is advantageously provided for
pressing the abrasive jetting system upon the bottom 39
of the hole 2. Best results are obtained when the
pressing force is not much higher than what is required
to keep the abrasive jet drill head 16 at the bottom, in
order to avoid unnecessary wear on the abrasive jet drill
head 6, bending of the system, and loss of directional
control. Thus, the pressing force is preferably just
sufficient to counteract the axial recoil force of the
abrasive jet and the friction forces in the thruster and
between the abrasive jet system and the hole wall.
Typically, the pressing force is well below 10 kN.
A suitable abrasive jet comprises a mixture
containing a fluid, such as the drilling fluid, and a
certain controlled concentration of abrasive particles.
The erosive power of the jet correlates with the total
power vested in the abrasive particles entrained in the
mixture. This depends on the mass flow rate of abrasive
particles and on the square of the velocity of the
abrasive particles.
Still referring to Fig. 1, the abrasive particles
will be entrained in a return stream of drilling fluid
through the excavated hole, running for instance through
an annular space 33 between the hole 1 and the drilling
system (2,16, 40).
In order to reduce the concentration of abrasive
particles to be transported all the way back to the
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surface, the drilling system, in particular the abrasive
jet drill head 16, can be provided with recirculation
means 30 arranged to recirculate at least a part of the
abrasive particles from the return stream downstream from
impingement with the formation, back into the abrasive
jet 10 again. The abrasive particles to be recirculated
can be mixed with the fresh stream of drilling fluid
containing a supply concentration of abrasive particles,
for instance in a mixing chamber to which both the fresh
stream of drilling fluid and the recirculated abrasive
particles are admitted, to obtain a jetting fluid mixture
comprising a jetting concentration of abrasive particles.
The abrasive particles referred to herein preferably
comprise or consist of magnetisable material, i.e.
paramagnetic or ferromagnetic material, such as for
instance steel shot or steel grit. This will herein also
be referred to as "magnetic material" although it does
not need to have a permanent magnetization. The
recirculation system can comprise a magnet attracting
magnetic particles from the drilling fluid flowing
upwardly in annulus 33, and conveying the particles via
outlet 34 to the mixing chamber 36. Generally suitable
recirculation systems are for example described in WO
2002/034653, WO 2005/005766, W02008/119821, WO
2008/113844. A recirculation system is however optional
and not required for the present invention to function.
In an illustrative example, the rotation of the
string can typically take 1 sec. In the case a downhole
recirculation device is used, the supply concentration of
particles pumped through the drill string is typically in
the range of 0.1 to 4% by volume, such as 0.4 to 2 vol%,
considering steel shot in an aqueous fluid, e.g. water.
When a recirculation system is used, the drilling fluid
in the abrasive jet may contain a jetting concentration
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of up to 10 % by volume, typically up to 5 vol% of
magnetic abrasive particles, and is on average higher
than the supply concentration. When there is no
recirculation system, the supply concentration via the
drill string is typically the same as the jetting
concentration, apart from a possible time lag of changes,
and can e.g. be in the range of 0.5 to 10 vol%, such as
2-5 vol%, e.g. 3 vol%. Recycle frequency preferably
exceeds the rotational frequency of the drill string. The
recycle frequency can for example be between between 10
and 40 Hz. The rotation of the drill string, or at least
the abrasive jet drill head excavating tool, is typically
between 0.3 and 3 Hz.
Referring to Figure 2, there is shown a drill string
1,3 with jet drill bit at its lower end, and a stabilizer
40 that is concentric and fixed with the drill string,
and circumferentially engaging the borehole wall. The
stabilizer acts as a centralizer, the drill axis 20 is
collinear with the borehole axis 48, i.e. there is no
inclination angle. If the stabilizer is of a type that
can change its diameter, by lowering the diameter a
configuration similar to Figure 1 in accordance with the
invention would be obtained.
Figure 3 shows an embodiment of the invention similar
to Figure 1, but in which the drill string above the
stabilizer is of a narrower diameter than the drill
string part 3, so that the stabilizer 40 provides the
first point of contact 42. The upper drill string part
can also be of a flexible tube. The drill string part 3
can be thicker or also thin drill pipe, or could be a
collar containing e.g. a control means and/or a sensor
means and/or a communication means.
Figure 4 shows an embodiment with an eccentric
stabilizer 50. It has a non-uniform radial extension
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around the drill string. The extension in the radial
dimension 52 is smaller than in radial dimension 54. The
stabilizer conforms to the borehole wall, so that the
contact 42 is around the circumference. Here it rotates
with the drill string, with the long and short radial
dimensions also turning, and therefore the drill axis 48
turns around the borehole axis at the inclination angle.
When a nozzle is arranged like in Figure 1, obliquely in
the axial area of the recess 17, and with the
eccentricity as drawn, the angle between nozzle and
borehole axis is increased by the inclination angle, and
a straight, but wider borehole is created. If the nozzle
would be turned by 180 degrees to jet against the
opposite side of the borehole bottom, a narrower straight
borehole would be created. If the eccentric stabilizer
would not conform to the borehole, a wider or narrower
curved borehole would be drilled.
For larger overgauge hole sizes the bottom profile
and the internal conical profile of the distance holder
(foot part 29) should be modified to allow for the angle
between the drilling assembly and the borehole axis 48.
Figure 5 is similar to Figure 4, but the eccentric
stabilizer 60 has a cross-section that is adjusted while
rotating the drill string to maintain the drill axis 20
geostationary during at rotation. The drill axis 20 has a
negative inclination with regard to the borehole axis 48.
The stabilizer means can actively controlled. An
adjustable eccentric stabilizer can be made as follows:
If the stabilizer has four pads that can be moved
radially independent of each other, of which one pad is
moved outward and one pad moved inward, the axis of the
connected drilling assembly is not parallel to the bore
hole axis anymore. The pads of an adjustable eccentric
stabilizer are adjustable down hole with, for instance,
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controlled stepper motors or hydraulically operated
pistons that are controlled by a down hole control unit,
e.g. in collar 3, so that the radial dimensions 62 and 64
remain constant as shown during rotation. Varying the pad
positions synchronized with the rotation of the drilling
assembly provides directional control in all drilling
directions, i.e. both azimuth and inclination control.
In order to establish the current drilling direction
through the formation, the system can be provided with a
navigational sensor, for instance a measurement while
drilling sensor, for providing a signal indicative of the
direction under which the making of the hole in the earth
formation progresses.
Such a navigational sensor can be provided in the form of
one of or a combination of a directional sensor providing
a signal indicative of the direction of the device
relative to a reference vector; a positional sensor
providing a signal indicative of one or more positional
coordinates relative to a reference point; a formation
density sensor providing information on a distance to a
change of formation type or formation content nearby; or
any other suitable sensor.
Over longer time scales than a rotation period, a
downhole control unit can moreover adjust the adjustable
stabiliser settings as a function of a preprogrammed
trajectory and down hole measurements while drilling.
Also, the instructions can be sent from surface to the
control unit, e.g. by data sequences such as a weight on
bit sequence, a rotational speed (RPM) sequence, or a
hydraulic pressure sequence, e.g. to adjust the magnitude
of the inclination or the direction. Based on such
transmission of data, one or more of an outer diameter of
the stabilizer, and eccentricity, or a directional
drilling mode can be set.
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It is attractive to combine such options with jet
drilling because of the minimal mechanical loading of the
drilling assembly compared to mechanical drilling. The
mechanical forces on the drilling system that is based on
fluid jetting are much smaller than is the case for
systems based on mechanical rock removal. This has the
advantage that the sensors can be located very close to
the jet drill head, making early and accurate signal
communication possible to the modulation control means.
The sensors can for instance be provided in the same
chamber as the modulation control means.
