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.
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.
U.S. Pat. No. 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.
U.S. patent application US 2003/0168216 discloses a method for reducing sand production by optimally orienting perforations.
It is an object of the invention to 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 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 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 direction of maximal stress in the formation, it is achieved that 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 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.
Alternatively, jetting excavation or laser excavating may be employed to drill the borehole or to modify a circular borehole after it is drilled.
The invention will be described hereinafter in more detail and by way of example, with reference to the accompanying drawings in which:
In the Figures, like reference signs relate to like components.
Referring to
In
When comparing the contours of
For instance, in a horizontally extending borehole, the vertical stress corresponding to σ1 could be 1 psi/ft, and the horizontal stress corresponding to σ2 could be 0.9×σ1. In the case of the circular contour of
Referring to
In
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 (
In the embodiment shown in
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.
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
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
An advantage of this embodiment over the embodiment of
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
The method as illustrated in
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US2005/017082 | 5/17/2005 | WO | 00 | 2/19/2008 |