Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF DRILLING A STABLE BOREHOLE
BACKGROUND OF THE INVENTION
The present invention relates to a method of drilling
a stable borehole in a formation in a formation
containing a stress field having a direction of maximal
stress.
Generally the earth formation surrounding the
borehole is subjected to stresses including first, second
and third principal stresses. One of these principal
stresses is the largest of the three, thus the direction
of maximal stress is along one of the three principal
stress directions.
Often during drilling of a borehole, the formation
rock in which a borehole is drilled may be threatened to
collapse due to formation stress acting on the borehole
walls. Collapse can be avoided by mounting casing or by
choosing a sufficient mud weight of the mud that is =
circulated through the borehole. On the other hand, too
high a mud weight results in a risk of loosing mud to the
formation.
During production, reservoir rock can be loosely
consolidated, so that it tends to disintegrate and flow
into the wellbore under the influence of hydrocarbon
fluid flowing through the pore spaces.
The possibility of such a collapse of the borehole or
inflow of rock particles, the latter generally referred
to as sand production, is a frequently occurring problem
in the industry of hydrocarbon fluid production, as the
produced sand particles tend to erode production
equipment such as tubings and valves.
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Conventional methods of sand control in reservoirs
include the installation of supporting perforated liners
or screens, which allow the hydrocarbon fluid to pass but
exclude the sand particles. Also, gravel packs are
installed between the liners or screens and the wellbore
wall to control sand production. Although such liners,
screens and gravel packs have often been successfully
applied, there are potential drawbacks such as clogging
of the perforations, screens or gravel packs leading to
diminished fluid production.
US patent 6,283,214 discloses a method of improving
near wellbore stability and reduction of sand intrusion
by making elliptically shaped perforations of a
particular orientation into the well casing or formation.
US patent application US 2003/0168216 discloses a
method for reducing sand production by optimally
orienting perforations.
SUMMARY OF THE INVENTION
Some embodiments of the invention may provide an
improved method of drilling a borehole, which further
enhances the wellbore stability and reduces sand
intrusion and/or collapse of the borehole.
In accordance with an aspect of the invention there is provided
a method of drilling a borehole in a formation containing a
stress field having a direction of maximal stress,
whereby the borehole is drilled with an elongated non-
circular cross sectional contour along an axis of
elongation and whereby a directional component of the
axis of elongation is kept oriented substantially
parallel to the direction of maximal stress.
The presence of the borehole in the rock formation
leads to stress concentrations in the rock formation
= region near the wellbore, when compared to the
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undisturbed rock formation. Such stress concentrations
are believed to involve relatively high tangential
stresses where tangential direction in the cross section
of the borehole wall is approximately perpendicular to
the direction of maximal stress in the formation. This
can cause local compressive failure in some regions of
the borehole wall and fracturing in others.
By orienting the direction of elongation of the
borehole substantially parallel to the dirPction of
maximal stress in the formation, in some embodiments the
variation of tangential stress around the borehole
contour is reduced. Particularly, the tangential stress,
in those areas of the borehole where tangential direction
in the cross section of the borehole wall is
approximately parallel to the direction of maximal stress
in the formation, reduces relative to the value of the
maximal formation stress.
Thus the tendency of local rock formation failure and
corresponding collapse, fracturing or sand production, is
thereby reduced. Thereby a larger window becomes
available for selecting the mud weight during drilling.
It is to be understood that the direction of
elongation does not need to extend parallel to the
longitudinal axis of the borehole, but can, for example,
extend in the form of a helix along the wellbore wall
depending on the variation of the formation stress field.
Suitably an elongate borehole can have a cross
sectional contour with a circular section and an elongate
section extending in a direction substantially parallel
to the largest a selected one of said principal stresses.
In case the borehole extends substantially vertically
into the formation, it is preferred that said axis of
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elongation extends in a direction substantially parallel
to the largest horizontal principal stress.
In case the borehole extends substantially
horizontally, it is preferred that said axis of
elongation extends in a direction substantially parallel
to the vertical principal stress.
It is possible to create a plurality of perforations
in the wall of the borehole, said perforations forming a
row extending in axial direction of the borehole. The
perforations are closely spaced so as to form a pseudo-
slot.
More preferably rock material is removed from each
elongate section by creating a slot in the wall of the
borehole, the slot extending in axial direction of the
borehole. The slot can be wedge shaped in a cross-
sectional plane of the wellbore, whereby the width of the
slot decreases in radially outward direction.
However, preferably the elongate contour is made at
the same time as progressing the borehole into the
formation.
The borehole can be drilled for instance using a
drill bit wherein two or more cutting sections are
rotated instead of the entire drill bit.
Alternatively, the borehole can be drilled for
instance using a down hole motor to rotate the drill bit
or cutting sections thereof about an axis perpendicular
to the longitudinal direction along which the borehole
extends.
Alternatively, the borehole can be drilled for
instance using a steerable drill bit that is brought to
pendule reciprocably in a plane parallel to the
longitudinal direction of the borehole.
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Alternatively, jetting excavation or laser excavating may be employed to
drill the borehole or to modify a circular borehole after it is drilled.
According to another aspect of the present invention, there is provided
a borehole in a formation containing a stress field having a direction of
maximal
stress, whereby the borehole has an elongated non-circular cross sectional
contour,
along an axis of elongation, and whereby a directional component of the axis
of
elongation is oriented substantially parallel to the direction of maximal
stress.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described hereinafter in more detail and by way of
example, with reference to the accompanying drawings in which:
Figs. 1A to 1C schematically show cross sectional views of various
elongate borehole cross sectional contours;
Fig. 2 shows a circular cross-section of a wellbore section which may
be extending horizontally;
Fig. 3A schematically shows a wellbore in which an embodiment of the
method of the invention is applied, at an initial stage of the method;
Fig. 3B shows the wellbore of Fig. 3A at a final stage of the method;
and
Fig. 4 schematically shows a wellbore in which another embodiment of
the method of the invention is applied.
DETAILED DESCRIPTION OF EMBODIMENTS SHOWN IN THE DRAWINGS
In the Figures, like reference signs relate to like components.
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Referring to Figs. 1A to C, there are shown examples of boreholes
having various non-circular cross sectional contours 3 formed in a formation
under
stress.
Figure 1A shows a borehole 40 having an elliptical or oval contour,
Figure 1B shows a borehole 50 having a lobed contour, and Figure 1C
corresponds
to borehole 60 having a cross sectional contour with a circular section and an
elongate section for instance in the form of slots. In each of these figures,
one of the
principal stress axes of the stress field in the formation extends
perpendicular to the
plane of the paper, and the other two are indicated by G2 whereby 02
corresponds to
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the direction of maximal stress. The shown borehole
contours are elongate in the direction along an axis of
elongation E. A directional component of the axis of
elongation E is oriented substantially parallel to the
direction of maximal stress o-2.
In Fig. 2 is shown a circular cross-section of a
wellbore section 30 which may be extending horizonally.
The formation is subjected to in-situ stresses of which
the principal stress al has the largest magnitude. This
may be the vertical principal stress. The presence of the
wellbore 30 in the formation 2 causes stress
concentrations whereby the highest shear stresses occur
near the wellbore wall, in point B about halfway the top
and the bottom of the horizontal wellbore section 30.
When comparing the contours of Figs. 1A to C to a
circular contour as depicted in Fig. 2, the tangential
stress concentrations in point B can be significantly
reduced. In the case of a circular contour, the
tangential stress in point B is approximately 3 times al
due to an arching effect of the borehole wall whereas for
an elliptical contour such as depicted in Fig. 1A the
tangential stress in point B is approximately 1.2 to 2.0
times ol where al indicates the maximum stress direction.
Rock failure in point B is therefore less likely in the
case of an elliptical contour than a circular contour.
For instance, in a horizontally extending borehole,
the vertical stress corresponding to al could be
1 psi/ft, and the horizontal stress corresponding to 02
could be 0.9 x al. In the case of the circular contour of
Fig. 2, such conditions could lead to a tangential stress
concentration in point A of 1.7 psi/ft, and in point B of
2.1 psi/ft. The elliptical hole of Fig. 1A, taking a
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ratio of short axis over long axis of the ellipse of 0.5
could lead to a tangential stress concentration of 1.5
psi/ft in point A and of 1.1 in point B. Taking a ratio
of short axis over long axis of 0.4, the tangential
stress concentration in point A could be 1.07 and in
point B 0.9 psi/ft. Hence, the stress concentration
magnitudes are reduced and the stress concentration is
more evenly distributed in the case of the elongate
borehole contour.
Referring to Fig. 3A there is shown a borehole in the
form of a wellbore 1 for the production of hydrocarbon
fluid, the wellbore 1 extending into in an earth
formation 2. An upper part of the bore wellbore 1 is
provided with a casing 4 suspended from a wellhead 5 at
the earth surface 6. The casing 4 is fixed in the
wellbore by a layer of cement 7 located between the
wellbore wall and the casing 4. The wellbore 1 has
subsequently been drilled beyond the length of the casing
4 forming an open hole section of the wellbore 1. An
injection string 8 for injecting cutting fluid extends
from a drill rig 10 at surface, into the wellbore 1. The
injection string 8 is at the lower end thereof provided
with a fluid jet cutter 12 having a pair of jetting
nozzles 14 oppositely arranged each other. The fluid jet
cutter 12 is located near the lower end of the formation
zone 3. Fluid jets 16 are ejected from the nozzles 14
against the wall of the wellbore 1 thereby creating
slots 16 oppositely arranged in the wellbore wall.
In Fig. 3B is shown the wellbore 1 after the
injection string 8 has been raised to a position whereby
the fluid jet cutter 12 is located near the upper end of
the formation zone 3. The slots 16 extend in axial
direction 17 of the wellbore 1 and along substantially
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the whole length of the open hole section of the
wellbore 1.
It is remarked that the open hole section of the
wellbore may reach into a formation zone containing a
hydrocarbon fluid to be produced.
During normal use the wellbore 1 is drilled to a
certain depth, the casing 4 is installed, and cement is
pumped between the casing 4 and the wellbore wall to form
the layer of cement 7. Subsequently the wellbore 1 is
further drilled to form a so-called open hole section.
The injection string 8 is lowered into the wellbore 1
such that the jet cutter 12 is located near the bottom of
the wellbore 1 (Fig. 3A). Cutting fluid (e.g. water or an
abrasive particle containing mixture) is then pumped
through the string 8, so as to induce the fluid jet
cutter to jet two opposite jet streams against the
wellbore wall. As a result slots 16 are created in the
wellbore wall. Simultaneously with pumping cutting fluid
through the string 8, the string is gradually raised in
the wellbore 1 until the jet cutter 12 is located near
the top of the open hole section near the casing 4. Thus
the slots 16 are formed along substantially the whole
length of the open hole section of the wellbore 1 below
the casing 4.
In the embodiment shown in Fig. 3, the jet cutter 12
is kept oriented in the wellbore 1 such that the
nozzles 14 are positioned in along the direction of the
maximal stress in the formation.
Instead of creating slots or rows of perforations, in
the open-hole section of a wellbore, such slots or rows
of perforations suitably can be formed in the rock
formation behind a perforated liner or casing.
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Instead of creating the slots using the jet cutter
described hereinbefore, the slots can be created by a
mechanical device such as a chain saw, or by an explosive
charge.
A preferred embodiment of the method is illustrated
with reference to Fig. 4. In this case, a specially
adapted drill bit 22 is employed on a lower end of a
drill string 28. The drill bit is provided with two
cutting sections 22A and 22B, which can each be rotated
instead of the entire drill bit 22. In order to allow
drill string rotation of the drill string 28, a clutch
unit 25 can be employed that uncouples drill string 28
rotation from rotation of the drill bit 22. A downhole
motor unit 24 can be employed to drive the cutting
sections 22A and 22B.
By drilling the hole with the drill bit 22 as
depicted, it is expected that a lobed borehole cross-
section can be drilled such as is shown in Fig. 1B,
whereby reference numeral 26 corresponds to the
elongation in the form of lobes.
An advantage of this embodiment over the embodiment
of Fig. 3 is that the elongate contour is made at the
same time as progressing the borehole into the formation.
This advantage would also be achieved by using a
steerable drill bit that is brought to pendule
reciprocably in a plane parallel to the longitudinal
direction 17 of the borehole. Such a method would result
in an oval borehole contour such as is depicted in
Fig. 1A.
The method as illustrated in Fig. 3 can be used to
form any of the contours of Fig. 1, including the slotted
contour of Fig. 10.
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