Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND SYSTEM FOR FORMING A NON-CIRCULAR
BOREHOLE
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent Application
No.
60/786,456, filed March 27, 2006 whose teachings are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
100021 The present invention relates in general to the formation of
intentionally non-
circular boreholes. These non-circular boreholes may be shaped for optimized
use in a
specific system or application, such as underground heat exchange systems, or
they may be
formed as a means to stabilize the borehole during the drilling, casing, and
completion or in
operation.
[0003] Most conventional and non-conventional drilling techniques are designed
to
produce boreholes that are substantially circular. In some formations, such as
hard rocks with
primary stresses oriented vertically, circular boreholes are inherently
unstable which might be
caused by the non-uniform stress conditions in the rock or from a general
weakness in the
rock. Once a circular hole is produced, these stresses may cause portions of
the formation to
break from the wall, often referred to by those skilled in the art, and herein
referred to, as
"break-out". This uncontrolled break-out often occurs during or sometime after
the
extraction of the drill string, including during the insertion of the casing,
and can cause
significant disruptions in the drilling process and completion process.
Uncontrolled break-
out can be a particular problem in bore sections which have a horizontal or
non-vertical
orientation. Break-out often creates a substantially non-circular or
elliptical cross-section,
with the longer axis of the ellipse often in a substantially horizontal plane,
or parallel to the
earth's surface. To prevent the uncontrolled break-out from occurring, it
would be
advantageous to be able to drill or create a non-circular bore, or modify a
bore so as to be
non-circular, in an orientation that partially relieved the stresses in the
formation (e.g. see
Bjorn Lund, 2000; Crustal stress studies using micro earthquakes and
boreholes,
Comprehensive sunimaries of Uppsala dissertations from the faculty of science
and
technology, 517, 75 pp, Uppsala University, Sweden)
[0004] There are also specific applications that would benefit from non-
circular holes with
reduced tendency towards uncontrolled break-out. These applications may
include
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geothermal power generation, such as enhanced or engineered geothermal systems
(herein
referred to as EGS) and hot dry rock (herein referred to as HDR), or
applications where the
bore hole will be left unsupported for extended periods (minutes, hours, or
days), such as in
oil and gas exploration and production (herein referred to as oil and gas E&P)
operations, or
in situations where the wellbore will be left unsupported indefinitely, such
as in an uncased
wellbore. An uncased wellbore may have an inner surface that comprises the
formation, or
one that is substantially comprised of fused rock, ice, a layer of a non-
metallic material, such
as a thermoplastic, thennoset, composite or ceramic, or a layer of fused
metallic material. In
addition to EGS-HDR and oil and gas E&P, other conventional applications could
benefit by
the drilling of non-circular boreholes with reduced tendency towards break-
out, including,
but not limited to, water well drilling, trenchless pipe installation, sewer
and municipal
system construction, resource mining, chemical disposal wells, CO2 or nuclear
storage wells,
downhole chemical reactions (such as, but not limited to, municipal waste
oxidation or
biofermentation), bores in ice, or wells for scientific or geologic study,
including test holes or
secondary holes used for measurements in the above or other operations and
applications.
[0005] Another application for non-circular boreholes is the installation of
underground
heat exchange systems for geothermal or ground source heat pumps (herein
referred to
interchangeably as GHP or GSHP). GHP's are used throughout the world as a
means to
effectively heat and cool houses and businesses through a heat exchange loop
system located
in the earth. A typical heat exchange loop may be comprised of tubing
installed in holes 150
feet to 500 feet deep which circulates fluids and extracts or disposes of heat
into the ground.
The number of holes used depends on the heat exchange requirements for the
complete
system. In some newer configurations, the drill holes start in the same
general location but
then divert at approximately a 30 degree angle from the vertical, forming a
cone like array, as
shown in Fig 1 a. This cone shape creates a compact connection point for all
of the heat
exchange tubes. Many locations, however, use numerous vertical holes, usually
aligned in
rows, with separation of about 20 feet, as shown in Fig lb.
[0006] Typically, 4 to 6 inch diameter holes are required in GHP applications
when
reasonably large heat flow lines need to be installed. Inside the holes, flow
lines are inserted
which typically range from 0.75 inch to 1.25 inches in inner diameter. If too
small a hole is
drilled, it is difficult to insert the tubes into the hole as well as get a
good placement of grout
into the hole and around the tube. A less than optimum grout placement may
create a
condition where bubbles can get entrained in the grout reducing the
effectiveness of the heat
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exchange system. The in-flowing and out-flowing lines must be adequately
separated to
prevent the out-going line from transferring heat back into the in-coming
line; in a sense,
short circuiting the desired heat transfer to the earth.
100071 GHP's are well known, and there are a number of standard techniques of
creating
the holes for these systems. Conventional drilling technologies have
traditionally been
employed for the drilling of holes for geothermal systems including, but not
limited to, auger,
rotary bit, rotary impact (hammer), percussion, and sonic drilling methods.
However, no
single system has yet been found to be ideal for drilling in all rock types.
Furthermore, any
of these technologies using a single bit cannot create a substantially non-
circular hole because
the required rotation and contact of the drill bit with the rock during the
drilling process
inherently produces a substantially circular profile.
[0008] Drilling large holes for purposes such as installing heat exchange
loops can be
especially problematic in hard rock. As used herein, "rock" may loosely refer
to any material
in the well bore, including loose and unconsolidated soils, consolidated
soils, clays, sands,
conglomerates, soft or hard rock, or any formation of any naturally occurring
composition.
As used herein, a hard rock refers to a rock that is well consolidated and
typically has a
number of hard minerals such as feldspars and quartz and lacks significant
amount of clays.
Such a hard rock can have a compressive strength of about or greater than 10
ksi. Also as
used herein, a large diameter hole refers to a hole having a diameter that is
larger than about 5
inches. Drilling holes in hard rock is a time consuming and costly process. In
fact, if a large
number of holes in the hard rocks are required, the drilling costs can exceed
the costs of the
rest of the heat pump and flow lines combined. In addition, hard rocks
typically have lower
fluid contents so that heat exchange from the tubing to the rock is reduced
and more flow
lines may be needed compared to soft rock for the same amount of heat
exchange.
100091 Hard rock drilling has traditionally been accomplished by several
different methods,
including impact hammer drill or rotary drilling. Drill rates for these
techniques in hard rock
can be as slow as 10 feet per hour or less for larger diameter holes. Non-
contact drilling
technologies such as flame jet spallation, hydrothermal spallation, particle
impact drilling, or
water jet drilling (with or without abrasive particulates) have the advantage
of being able to
drill faster in hard rocks. Non-contact drilling is herein defined as
technologies which do not
require contact between the bottom hole assembly and rock in order to remove
the rock by
the intended means, but may use the rock wall for secondary purposes, such as
orientation,
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stabilization, propulsion, or the like. For example, tests conducted in the
field have shown
drilling rates of more than about 30 feet/hr rates for 8 inch holes using air-
fuel flame jet
combustion drills. A summary of some of the known non-contact drilling
techniques is
provided below.
100101 Several flame jet drilling techniques are known. For example, U.S.
Patent No.
3,045,766 discloses a suspension type rotary piercing process and apparatus
and describes a
blowpipe type rotary flame jet system suspended from a cable to allow for
vertical jet
drilling. U.S. Patent No. 3,103,251 is directed to a flame cutting method and
describes a
flame cutting process improved by the addition of air or inert gas to the jet
to increase drill
rates. U.S. Patent No. 3,182,734 is directed to a fusion piercing or drilling
machine and
discloses a system that uses a rotating combustion chamber in conjunction with
outside
scrapers to spall and melt rock to create a clean, consistent, circular bore.
U.S. Patent No.
3,322,213 is directed to thermal mechanical mineral piercing and discloses a
rotating
combination grinding and flame jet drill system for creating consistent, round
bores through a
process of thermal spallation and wear. U.S. Patent No. 3,476,194 is directed
to flame jet
drilling and discloses a method for creating smaller holes by adding coolant
closer to the
flame outlet to diminish the thermal process away from the flame jet tool.
U.S. Patent No.
4,066,137 is directed to a flame jet tool for drilling cross holes and
discloses a method for
creating a side bore using thermal flame jet technology. U.S. Patent No.
4,073,351 is
directed to burners for a flame jet drill and discloses a technique for mixing
both flame jet
and water jets into the same drill head with different options for the shape
and orientation of
the nozzles and jets. A rotating head is used in this design which creates
circular shaped
holes. However, none of the known flam,e jet drilling techniques suggests
shaped or non-
circular cross-section holes.
[00111 It is known that superheated steam or a high pressure fluid may also be
used for
drilling. However, such known techniques have only been used for the drilling
of round
holes. Several water jet drilling techniques are known. For example, U.S.
Patent No.
5,402,855 directed to coiled tubing tools for jet drilling of deviated wells
describes a jet
nozzle drill for creating deviated (e.g., off vertical) wellbores from a cased
vertical well. This
patent provides a discussion of how to shift the high pressure nozzle to
create a tilt in the jet
system. This patent also provides a discussion of how the nozzle is shifted
using a series of
plungers tied to a control system and a flexible rubber boot assembly. U.S.
Patent No.
4,369,850 uses a rotating fluid jet assembly with multiple nozzles to create
circular holes.
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U.S. Patent No. 3,576,222 discloses a drill bit with hydraulic action using a
number of
nozzles and a rotating head for circular shaped hole generation. U.S. Patent
No. 5,111,891
describes a technology for creating a biasing in a wellbore for changing
direction using a
water jet for soil erosion. When rotated, the nozzle creates a circular hole.
When the nozzle
is stopped from rotating, it cuts a side bore path to allow the jet to relieve
the adjacent area
and enable the drill to change directions. U.S. Patent No. 4,930,586 discloses
a hydraulic
drilling system with a single outlet jet and a series of four side jets which
enable the control
of the drilling direction. U.S. Patent No. 5,944,123 discloses a system for
creating holes
using a water jet technique with capabilities for controlling orientation and
rotation of the
head. U.S. Patent No. 4,871,037 discloses a combination rotating mechanical
grinding and
jet nozzle drilling system. However, none of these water jet drilling
techniques suggest the
creation of a non-circular shaped borehole or a shaped nozzle for the same.
100121 While it is known that particle impact drilling may also be used to
form boreholes
using either water or mud suited to well drilling, and while such a known
technique has been
used with a rotating head and for creating circular holes, there is no
suggestion in this area for
creating shaped or non-circular holes. For example, U.S. Patent No. 6,386,300
describes the
use of a particle impact process using small metal spheres to increase
drilling rates in
traditional rotating drilling systems as well as a system to remove the
spheres from the
drilling mud. U.S. Patent No. 4,768,709 describes a system for using
individual fluid
particulate jets to create channels and notches into existing boreholes to
improve blasting
characteristics.
[0013] Another known technology uses a supercritical fluid for the drilling
task. Such
known techniques create circular holes. For example, U.S. Patent No. 6,347,675
uses COZ as
the drilling fluid in a conventional coil tube drill system and as a jet fluid
for enhanced
drilling rates. The '675 patent describes a system for providing and
separating CO2 from the
drilling fluid. There is no disclosure or suggestion for the generation of
shaped or non-
circular holes.
[0014] Another known drilling technology involves chemical drilling systems.
Such
known chemical drilling systems also create circular holes and do not suggest
techniques for
creating shaped or non-circular holes. In particular, the known chemical
drilling references
are related to using chemical drills to create holes in rocks and removing the
reaction
products from the hole. For example, U.S. Patent No. 6,742,603 discloses using
a high
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temperature sodium hydroxide (NaOH) fluid stream to etch a hole in rock. U.S.
Patent No.
4,431,069 describes a system that uses acidic or basic slurries to drill
boreholes; the'069
patent also describes some aspect of directing the flow jet to change
direction in order to
create a directional, yet circular, borehole. U.S. Patent No. 6,772,847
discusses using acids
or other chemicals to create bore holes and suggests creating a cloverleaf
shaped hole by
using multiple nozzles. A cloverleaf-shaped borehole is not a suitable
borehole profile for
applications such as heat exchanger tube installations, where a minimal amount
of rock
removal is desired, nor does it help stabilize the borehole. It may also cause
greater head
losses for flowing fluids.
[0015] In connection with drilling, tube installation and grout placement for
geothermal
heat pump applications, a few patents are summarized below. For example, U.S.
Patent No.
5,590,715 discloses a system for placing heat loops in place and then grouting
them using a
separate grout filling line. Published Application No. 2005/0139353 describes
a system for
installing heat loops in conjunction with sonic drilling techniques. U.S.
Patent No. 4,286,651
discusses a technique for driving a pipe into the ground to install shallow
geothermal heat
loops with a circular design drive pipe. All references discussed above are
incorporated by
reference herein.
[0016] However, there still exists a need for drilling boreholes where the
preferred cross-
sectional geometry is non-circular. There are also exist a need for forming
boreholes having
a non-circular hole geometry that can prevent or reduce break-outs from the
wellbore.
BRIEF SUMMARY OF THE INVENTION
[0017] The present invention describes the benefits, methods and systems for
creating non-
circular, generally elliptical, oval or eccentric shaped holes. The
embodiments of the present
invention can be applied to the creation of non-circular boreholes or
modification of circular
boreholes to a more non-circular shape. Certain aspects of the present
invention are
particularly well suited for making non-circular holes in hard rock.
[0018] A substantially non-circular hole may help stabilize the well bore and
prevent break
outs. A substantially non-circular hole may also facilitate the installation
or operation of a
system, such as in the installation of piping for ground source heat pumps. A
substantially
elliptical hole provides an improved geometry for heat exchange in the ground
source heat
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pump piping loops while also enabling a much faster and more efficient
drilling that does not
suffer from the shortcomings of the prior techniques.
100191 In one aspect the present invention provides a system and method for
creating
shaped drilling holes in rocks especially for the intention of stabilizing the
wellbore. The
system can create an elliptical shaped hole that intentionally reduces the
stresses in the
formation around the bore, limiting uncontrolled break-out. Break-out is a
process in which
rock breaks from the wall of the wellbore, creating a non-circular and
substantially elliptical
cross-section, often with the longer axis in a substantially horizontal
orientation, in order to
relieve the stresses in the formation around the well-bore. Break-out may be
particularly
severe in significantly horizontal or non-vertical portions of the wellbore.
Breakout can be
caused by significant non-uniform stress concentration in the unsupported
borehole resulting
in localized shear failure at two opposite nodes in a direction normal to the
main stress
direction. Once the break-out occurs, the borehole has a more stable geometry
but the
material generated by the small scale collapse can cause major complications,
delays, and
expenses in the drilling, casing and completion of wells. By intentionally
reshaping or
ovalizing the hole under controlled conditions or circumstances, the problems
caused by
uncontrolled break-out can be reduced or mitigated. lt is particularly
attractive to
intentionally create an oval hole during the actual drilling or just after the
drilling operation
(such as by a selective milling procedure or spallation or erosion) so that
the rock which is
cut from the wellbore to form the non-circular cross-section can be removed as
part of the
cuttings.
[0020] Alternatively, the non-circular hole may be used for optimized
drilling, placing and
grouting of tubing such as, but not limited to, heat exchange loops. The
significantly reduced
cross-sectional area of the elliptical borehole increases the overall drilling
rate by reducing
the amount of material that must be removed by as much as 30-40 percent for
the same tube
to tube separation of heat exchange loops. The shape of the hole is optimum
for heat transfer
from the ground and for minimizing heat transfer from the inlet to outlet
tubes. The shape of
the hole also requires the least amount of grout to be used in the completion
of the system.
However, it should be realized that non-circular boreholes may find use in
various other
applications such as, but not limited to, the installation of parallel piping,
tubing, conduit,
cables or the like.
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100211 One system for drilling non-circular holes uses a non-contacting
drilling system
which in one embodiment uses a supersonic flame jet drilling system with a
movable nozzle.
In one embodiment the non-circular shaped hole is created by an abrasive fluid
or particle
bearing-fluid jet drill. A defined herein, a fluid is any substance that is
capable of flow,
including liquids, gases, and supercritical fluids. The fluid used in the
fluid jet drill can be
water, drilling mud, or other fluids such as supercritical carbon dioxide
(C02) and fluids that
erode the rock chemically using basic or acidic chemicals (such as sodium
hydroxide, or
other bases, or hydrofluoric or other acids) in solution. In another
embodiment, a non-
contacting drill can be used that uses a high velocity air stream with
suspended or entrained
particulates as the abrasive drilling means. A shaped jet outlet nozzle may
also be used to
create the novel asymmetric erosion shape. The non-circular shaped hole is
created by either
the high temperature flame, high temperature steam or water, water-particle
jet or chemically
active fluid jet as it removes rock material by erosion, dissolution and/or
thermal spalling.
[0022] For a further understanding of the nature and advantages of the
invention, reference
should be made to the following description taken in conjunction with the
accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
100231 Fig. I a is an exemplary schematic diagram of a geothermal heat pump
setup
showing multiple shallow drill holes in an off-vertical configuration.
100241 Fig. 1 b is an exemplary schematic diagram of a geothermal heat pump
setup
showing multiple vertical boreholes in a line.
[0025] Fig. 2 is an exemplary diagram showing a heat pump flow line as well as
grout in
the ground.
[0026] Fig. 3a is an exemplary cross-sectional view of a circular hole.
[0027] Fig. 3b is an exemplary cross-sectional view of a long width elliptical
hole produced
in accordance with the embodiments of the present invention.
[0028] Fig. 3c is an exemplary cross-sectional view of a short width non-
circular hole
produced in accordance with the embodiments of the present invention.
100291 Fig. 3d is an exemplary cross-section view of a shaped hole with an
even longer
aspect ratio that can accommodate several of the flow tubes in parallel or in
a parallel plane.
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[0030] Fig. 3e is a transverse view corresponding to Fig. 3d.
[0031] Fig. 4 is an exemplary schematic diagram of a flame jet drilling system
for shallow
non-circular holes, in accordance with one embodiment of the present
invention.
100321 Fig. 5a is an exemplary diagram of a jet drill head swivel drilling
system in
accordance with one embodiment of the present invention for drilling a non-
circular
borehole. Fig. 5b is a top view of Fig. 5a.
[0033] Fig. 6a is an exemplary diagram of a shaped nozzle drilling system for
generating
elliptical shaped boreholes without swiveling using flame or fluid jets, in
accordance with
one embodiment of the present invention. Fig. 6b is a top view of Fig. 6a.
[0034] Fig. 7a is an exemplary diagram of a dual nutating (e.g., wobbling or
rotating)
nozzle particle impact drilling system with a detailed view shown in Fig. 7b
and the generally
elongate overlapping circular borehole shape that results from this type of
drilling method
shown in Fig. 7c.
[0035] Fig. 8a is an exemplary diagram of a horizontal circular borehole with
the principal
stress in the vertical direction and horizontal regions where breakout can
occur.
100361 Fig. 8b is an exemplary diagram of a generally elliptical shape created
from the
circular bore using a borehole milling technique.
[0037] Fig. 8c is an exemplary diagram of a version of an elliptical shape
with more
pronounced milled or eroded lobes.
[0038] Fig. 8d is an exemplary diagram of a shaped borehole using a borehole
milling
technique.
DETAILED DESCRIPTION OF THE INVENTION
100391 Figs. la and lb show the general layout of a geothermal heat pump
system
which is one such system where oval (e.g., non-circular) boreholes may be
beneficial. It
should be noted that there exist a number of different ways for locating the
boreholes in such
a system, and that the general layout of the hole orientation is not critical
to the disclosed
embodiments of the present invention. In the particular version shown, the
heat exchange
drill holes are oriented outward from a more or less central point in an
excavated trench 2
where the heat exchanger tubes are connected together into a manifold system
3. The out-
flowing 4 and in-flowing 5 tubes or pipes are connected to the manifold system
3. The holes
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are directed from substantially central location to simplify the attachment of
the flow tubes to
a manifold and to minimize the damage to the terrain around the drill site. A
trench is
typically used to hide the connection holes and tubing. The flow tubes are
typically
connected in parallel to the manifold system. The flow tubes are installed
after drilling and
thereafter a typically mineral-based grout is pumped into the wellbore to
insulate the tubes
from one another and to provide for improved contact between the tubes and the
rock, as well
as to prevent the thermal short circuiting by fluid around the heat flow
tubes. The wellbore
detail is described in conjunction with Fig. 2 which shows a detail view of
the heat tube in the
ground showing the in-flow 6 and out-flow tubes 7, the grout 8 filling the
region between the
tubes with the wellbore 9 including the surrounding earth 10. The rounded
connection tube
1 I between in-flow and out-flow tubes is attached by a union connection 12.
Hole Shape
[0040] Fig. 3a shows a cross-sectional drawing of a circular hole showing the
borehole 13,
earth 14 and heat exchange tube 15. Figs. 3b-c show two elliptical holes of
different length
to width ratios (L/W or L:W) showing the elliptical holes 16 and 17. Figs. 3b-
c also shows
the ability of varying the length and width of the hole to suit the tube
geometry. Depending
on the size of the flow tubes and the proximity of the connections of the
tubes, a smaller or
larger hole width or length to width ratio can be drilled. In a typical
installation, this novel
non-circular cross-sectional hole will require about 40 percent less rock to
be drilled out to
make sufficient room to place the heat exchange tubes, as shown in Figs 3a-c.
The reduction
in the drilled hole size considerably increases the rate at which the hole can
be drilled,
possibly decreasing cost, time on site, and amount of cuttings for disposal. A
shaped hole
with an even longer L:W aspect ratio could accommodate several of the flow
tubes in parallel
as is shown in Figure 3d and 3e. This allows for drilling half the number of
holes in order to
obtain a similar heat transfer capability. This shape maximizes the contact
between the tubes
and the rock while reducing the tube to tube heat exchange. Further, thermal
insulation can
be placed between the in-flowing and out-flowing tubes to reduce thermal
transfer between
them.
[0041] The shaped, non-circular hole can have any non-circular cross-section.
The non-
circular, cross-section holes in accordance with the embodiments of the
present inventions
are holes that have aspect ratios that are larger than one (1.00), where an
aspect ratio is
defined as the ratio of a length to a width of the cross-section shape. So,
for example, as
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defined herein, a circle has an aspect ratio of one, and an ellipse has an
aspect ratio greater
than one since its major axis (length) is larger than its minor axis (width).
In one
embodiment, an elliptical cross-section hole is formed. The elliptical cross-
section hole can
be formed by any means, including, but not limited to, a swiveling drill head,
or with a
shaped nozzle drilling system, as described below. The length or angle of
throw of the nozzle
as it swivels, or the width of the jet outlet in a shaped nozzle system,
determines the overall
width of the elliptical hole. The shaped borehole in accordance with the
embodiments of the
present invention is not limited to an elliptical-shaped hole. Other holes
with aspect ratios
greater than 1.0 include, but are not limited to, substantially diamond,
slotted, eye-shaped,
vesica pisces shaped, egg, dog bone, dumbbell, rattle, crescent, "C"-shaped,
"T"-shaped, "L"-
shaped, "I"-shaped, triangle, square, tetrahedral, rectangular, or
parallelogram-shaped
boreholes, or variations thereof, including but not limited to "pinched" or
distorted versions
of each, are also considered as shaped non-circular boreholes within the scope
of the present
invention. The cross-section may be symmetric or asymmetric. The profile may
change
along the length of the borehole, may include some circular portions along the
length of the
borehole, and the orientation of the non-circular shape may rotate along the
length of the
borehole the x, y, or z planes relative to the surface. Such shaped boreholes
may allow for
the installation of heat exchange tubes having an optimum separation between
them. From a
thermodynamic perspective, there is an optimum separation between the tubes.
The optimum
distance between the tubes is dimensioned to insure that there is a minimum
amount of heat
transfer from the in-coming to the out-going liquid. The tubes can be made of
a rigid, semi-
rigid, elastic, or a flexible material.
100421 In one specific embodiment, an elliptical hole with a cross-section
that is about 2
inches wide by about 5-6 inches long will enable the placement of in-flow and
out-flow tubes
which would have required a 5 inch or 6 inch diameter circular hole. This
novel non-circular
cross-section hole will require about 40 percent less rock to be drilled out
to make sufficient
room to place the heat exchange tubes, as shown in Figs 3a-c. This reduction
in the drilled
hole size considerably increases the rate at which the hole can be drilled.
The hole will also
reduce the amount of grout that needs to be replaced, as well as potentially
improve the heat
transfer properties of the hole.
[0043] The shaped or non-circular cross-section holes of Figs. 3b-c can be
formed in a
compositions, materials, soils, rocks or formations, including but not limited
to soil,
consolidated soils and unconsolidated soils, sands, clays, rocks of all
geological types, as well
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as cements and concretes and the embodiments of the present invention are not
limited to any
one matenal type.
[0044] The shaped or non-circular cross-section holes of Figs. 3b-c can be
drilled with a
number of different technologies, and the embodiments of the present invention
are not
limited to any one exemplary drilling technology. For example, conventional
rotary,
hammer, or coiled tubing drilling techniques which produce a circular hole can
be combined
with auxiliary processes to produce a substantially non-circular cross-
section. The drilling
technologies may include hard rock or non-contact drilling technologies.
Exemplary and
effective technologies for drilling hard rock efficiently include, but are not
limited to, flame
jet spallation, hydrothermal spallation, and water and particle jet
techniques,
[0045] The substantially non-circular hole can be formed by a single process
or
mechanism, such as, but not limited to, hydrothermal spallation using a shaped
nozzle or
nozzles array, or by rotary drilling using a pulsating cutting too] which can
vary in distance
from the center point of the hole in a regular pattern to produce a non-
circular shape, such as
that in 8b. Alternatively, the non-circular hole can be formed by two or more
different
processes or mechanisms. In one example, a rotary drill head can drill a
circular hole and an
abrasive fluid jet can cut triangular indentations to produce a shape such as
that shown in 8d.
The substantially non-circular bore hole can be fonned in one step such as,
but not limited to,
air-particle drilling with shaped nozzles, or multiple steps, such as but not
limited to coiled
tubing drilling with a mud motor followed by insertion of a secondary abrasive
milling
mechanism with two counter rotating heads driven by an electric or mud motor
to produce a
shape such as that in 8c. The drilling process or processes can occur
concurrently at the
bottom of the drilling assembly, such as, but not limited to, a single
pulsating cutting tool, or
they may be staggered along the drilling assembly such that a "primary"
drilling process
forms the primary, circular hole while a following "secondary" process higher
up the bottom
hole assembly (BHA) shapes the hole into a non-circular cross-section.
Alternatively, the
drilling process or processes can be performed in two separate or independent
operations. As
one example, a hole is drilled using rotary bit drilling; in a secondary
process, an erosive or
spallation jet is used to increase the length of the hole without
significantly changing the
width. The secondary operation or process can take place when the drill string
from the
primary operation or process is still in place, is being removed, or after the
drill string from
the primary operation or process has been completely removed.
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[0046] The primary drilling process may include, but is not limited to,
conventional drilling
processes such as, but not limited to, rotary bit, auger, rotary impact,
percussion or sonic
drilling, or coiled tubing drilling to form a substantially circular hole. The
secondary process
may include contact or non-contact drilling processes, such as, but not
limited to rotary bit,
grinding, milling, abrasion, particle abrasion, spallation, sonication,
scraping, cutting,
melting, or fusing or combinations of these processes. Power to supply the
secondary process
may be derived from the rotation of the primary or secondary process drill
stem, hydraulic
flow of fluids, including but not limited to water, circulating fluids, or
drilling muds, or from
another source, such as, but not limited to, compressed air flow, or by
electrical, thermal,
mechanical, or chemical means.
[0047] The formation of the non-circular hole can include technologies that
use a rotating
drill stem, such as, but not limited to, rotary abrasive drilling or auger
drilling through the use
of multiple drill heads, off-set drill heads, pulsed jets, pulsed cutters, or
other mechanisms.
There may be one bit or multiple drill bits, nozzles or drilling surfaces.
Bits may be
vertically or horizontally offset from each other_ Bits, nozzles, or drilling
surfaces may be
oriented vertically, horizontally, or in other directions. The bits, nozzles
or drilling surfaces
may be opposing or counter-rotating. Nozzle or jets may be oriented to reduce
"hold-down"
of rock or to aid in cuttings lift and returns. In addition, there may be
secondary or operations
to reduce the size of the cuttings or particles in the return fluids including
grinding,
pulverizing, chemical degradation or dissolution, thermal treatments, or the
like.
[0048] The formation of the non-circular hole can include technologies which
do not
require a rotating drill stem, such as, but not limited to, coiled tubing
drilling, water jet, air
jet, air spallation, particle impact, hydrothermal spallation, fusion, laser,
chemical, plasma,
sonication, or percussion through the use of multiple or shaped jets, or
through the use of a
secondary rotating cutting mechanism in a vertical, horizontal or otherwise
offset position.
There may be one drill head or multiple drill heads or nozzles. Drill heads
may be offset
from each other or oriented in different directions.
[0049] In another embodiment, a wellbore milling system that cuts an existing
circular
wellbore into the preferred profile can be used. These drilling techniques can
include both
drilling techniques that erode away the rock such as fluid and fluid particle
jets, high
temperature spallation flame jet, or hot fluid or steam drills. In another
embodiment, a
chemical drill that uses fluids that are either alkaline or acidic can be
used. These jets of
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fluids or gases are either directed in the bore hole to create the non-
circular shape, or a shaped
nozzle is used to create the desired hole geometry, as described above. There
are also ways
to create non-circular drills using rotating contacting systems with multiple
cutters or by
reaming or shaping the hole after drilling to create the optimum profile.
Again, this reaming,
or milling, spalling or other process can occur slightly behind the drilling
head or further up
the drill string or in multiple milling tools placed at different point on the
drill string.
[0050] It may be necessary or helpful to determine maximum principal stress in
borehole,
the orientation of the BHA, the orientation of the non-circular borehole, or
that orientation of
the BHA or borehole relative to the stresses in the borehole during the course
of creating the
borehole. Either can be achieved by known processes. Information gathered by
downhole
instrumentation may be communicated to the surface, in real time or with some
delay, where
it is processed and used to guide the drilling mechanisms in the formation of
the borehole
shape or orientation. Alternatively, information gathered by the
instrumentation may be
processed by down-hole equipment or "smarts" and fed directly back to the
drilIing
mechanism, with or without storage or the additional relay of the information
to the surface.
Information of the principal stresses may be gathered by current or future
technologies in
advance of drilling, in "real-time," periodically during the drilling
operation with the drilling
mechanism is functioning or stopped, or after the primary drilling has been
completed.
[0051] In order to disclose a method for drilling the novel shaped or non-
circular cross-
section boreholes in accordance with the embodiments of the present invention,
one such
system (e.g. flame jet drilling) which is improved to enable the formation of
the novel shaped
holes is described below.
Improved Flame Jet Drilling System
[0052] Fig. 4 shows an exemplary schematic diagram of a flame jet drilling
system for
shallow shaped holes, in accordance with one embodiment of the present
invention. As used
herein a shallow hole is a hole having a depth that is no more than about 500
feet. However,
the embodiments of the present invention are not limited to the forming of
such shallow
holes, and deeper holes can be drilled via the techniques disclosed herein.
Flame jet drilling
has been shown to be most effective when a high velocity, high temperature
combustion
stream (e.g., burning hydrogen or hydrocarbon fuels with air or oxygen) is
forced against a
rock surface. The rock may fail by rapid heating and flaking of the mineral
structure,
sometimes referred to as "spalling." The rapidly moving combustion gases strip
the "spalls"
14
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from the surface exposing a new surface to be heated, spalled and removed. A
flame jet
system in accordance with the embodiments of the present invention can include
the
underground components including the burner head 18 with swivel mechanism 19,
centralizers 20, all of which are connected to the surface through a tube
connection system 21
which conveys the oxidant, fuel, supplemental air, coolant, and water along
with the
electronic control signals to a coupler 22. At the surface a rigid frame 23
supports the drill
system and a coiled tube spool 24 holds the nested or bundled tubing which is
connected with
the air compressor and fuel source 25. During operation the burner is
continuously fed or
translated into the ground at a fixed rate of penetration. The swivel
mechanism 19 oscillates
the nozzle and burner head 18 back and forth using an air pressure drive or
motor driven head
to create the elliptical shaped hole.
[0053] Certain details of the drilling head for the system of Fig. 4 are shown
in Figs. 5a-b.
Fig. 5a shows a simplified exemplary diagram of a jet drill head swivel
drilling system in
accordance with one embodiment of the present invention for drilling a non-
circular
borehole; and Fig. 5b shows an orthogonal cross-sectional view corresponding
to Fig. 5a. As
set forth above, a flame jet burner head is used to create a supersonic
combustion flame
burning propane or diesel with compressed air as the oxidant in its spallation
mode of
operation. Fuel which can be either propane or diesel is mixed using a
distributor with air
which has been preheated via the cooling of the flame jet chamber. The mixing
of the fuel
and air creates a rapid combustion process that creates a high velocity stream
in the
combustion chamber. The diameter and shape of the chamber determines the
extent of the
combustion as well as heat transfer to the in-coming air stream. A flame
holder maintains the
combustion during the high flow rate conditions. Further details of the
combustion chamber
design are described in references related to flame jet drilling, discussed
above. The high
velocity gas stream is forced out through the nozzle 26 where the gas expands
into a
supersonic flame 27. Exit velocities of the high pressure, high temperature
combustion
product gases in the range of 1.5 to 2 times the speed of sound are typical
depending on the
inlet pressure and flow rate of the air and fuel. The rock material that
spalls off the surface
28 moves upward in the air stream where it is mixed with coolant water 29 or
air and
transported up the wellbore annulus 30. The nozzle swivel 31 is a mechanism
actuated by air
or electronic means to swivel the jet back and forth over a predefined arc 32.
One or more
centralizers 33 are attached to the drill head 34 and keep the drill head
centered in the
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wellbore as the nozzle swivels back and forth creating the slot, the
elliptical or the non-
circular-shaped hole.
[0054] Alternatively, the elliptical, slot-shaped or non-circular hole be made
by using a
water jet or particulate flow system. A number of water jet designs are
already used in rock
drilling and typically include a high pressure pump at the surface with a flow
line down to the
small bore nozzle where a very hard material such as silicon carbide is used
to focus the fluid
flow into a tight jet. Particulates can be added to the stream to increase
formation erosion and
drilling rates. In some methods a drilling mud is used for suspending solids
and these solids
which can be non-metallic or metallic particles which at high velocities can
impact and
remove the rock. A conical jet method has also been described that creates an
erosive cone of
fluid that cuts into rock. Such systems can be modified to include a swiveling
mechanism as
described above to enable the formation of the slot, the elliptical or the non-
circular-shaped
hole, in accordance with the present invention. Further details of such jet
drilling systems are
disclosed in the references above, which are incorporated by reference herein.
100551 In accordance with the embodiments of the present invention, in order
to create the
non-circular hole design, the drill head may either be shaped to create the
non-circular hole or
alternatively the head is enabled to swivel between to endpoints at a rate and
total movement
that is optimized for the drilling process. This process requires that there
be flexibility of the
head and the flow components. For example, a ball type swivel mechanism 31 is
shown in
Fig. 5a. The swivel system can be actuated using several different mechanisms
including
pneumatic, hydraulic and electrical actuation. The sweeping process of the
swivel also helps
remove the spalled material from the rock face when a flame jet technique is
used.
[0056] As an alternative to the drill head enhanced with a swiveling
mechanism, a shaped
jet design for drilling non-circular or elliptical cross-section holes is
shown in Fig. 6a-b. In
this system the non-circular geometry is created by using a slot-forming
shaped or multiple
outlet jets that erode away rock using either flame jet or other water jet
drilling methods
described above to create the substantially non-circular and/or elliptical-
shaped hole. A
nozzle head using three directed nozzles is shown in Fig. 6a and a projected
view thereof is
shown in Fig. 6b. The drill head of Figs. 6a-b includes the nozzle body 35 and
the multiple
openings 36 designed to force fluid or hot gases out at an angle from the
drill head, thus
creating the elliptical shaped hole. A plurality of smaller openings can be
used is place of
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each jet with the same effect to create an elliptical hole when the smaller
openings are
oriented in a manner to form the non-circular cross-sectioned hole.
[0057] Another alternative technique for drilling non-circular cross-sectional
boreholes is
shown in Fig. 7a. This system and the resultant hole it drills are shown in
Figs. 7a-c. As
shown in Fig. 7b the system can use particulate flow in a high pressure stream
of either air or
fluid such as water or drilling mud to create overlapping circular bores that
form a more or
less elliptical bore hole shape. Fig. 7a shows an overall view of the system
while Fig. 7b is a
more detailed drawing of the wobbler-nutating section of the system. The
drawing is shown
without the cover box installed which protects the components inside from the
fluids, air and
particulates. The system has two reinforced rubber, thenmoplastic, thermoset,
or composite
flow tubes 37 that provide a mixture of fluid or air with particulates 39 into
the dual nozzles
38 which are manufactured from a hard material such as tungsten or silicon
carbide. The
nozzles 38 have a convergent inlet 41 and a long straight or slightly
divergent outlet section
42 where the mixture of fluid or air and particles 39 are accelerated to high
velocity and then
impacted against the rock surface 51. To create a wobble motion as is shown in
Fig. 7b, the
nozzles 38 are connected through a spherical ball 43 which is attached to a
bearing surface 44
that slides against the inclined surface of the wobble plate 47. The wobble
plate 47 is
centered to the main mount plates 42 on the same axis as the spherical ball 43
using several
removable bearing plates 46. The wobble plate 47 and integral gear assembly is
then rotated
by applying a rotary motion using either an air, hydraulic or electric motor
through a belt 48
or by direct drive through a central hub gear 49 on the motor 40 to the side
gears 47 on the
nutating assemblies. The motor is attached to the block using a motor mount
plate 50. The
low spots on the two wobbler plates 47 are oriented at 180 out of phase from
one another so
that the sideway force of the nozzles counteracts each other helping to keep
the drill assembly
centered in the wellbore. The nozzles and tubes are kept from rotating by
fixing the tubes 37
up at a point above the end of the air motor. The entire assembly is enclosed
in a metal cover
box (not shown in the drawing) and the nozzles sealed using rubber bellows.
The resultant
hole shape 52 and the superposition of the heat exchange flow tubes 53 is
shown in the cross
section Fig. 7c. The same general concept can work with rotation of the
nozzles instead of
wobbling if the outlet holes are offset from the center or have jets that are
directed towards
the sides of the bore as well.
[0058] It should be realized that the shaped boreholes in accordance with the
embodiments
of the present invention are not limited to vertically extending holes. The
techniques and
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systems in accordance with the embodiments of the present invention can also
be used to
form horizontal boreholes. The non-circular boreholes may also improve the
drilling and
borehole stability. In accordance with this aspect of the present invention,
the drilling system
can create an elliptical, eye, or slot-shaped hole with the long direction
perpendicular to the
principal or maximum stress direction. In many cases, this maximum stress
direction is
vertical, in which can long direction would be in the horizontal direction.
This orientation
and geometry is desirable for the wellbore to survive in the high vertical
stresses found
especially in deep subterranean formations by preventing or minimizing
uncontrolled well-
bore breakouts or cave-ins. In addition, increased stability of the borehole
can allow the
driller to use lower mud pressures in the borehole possibly increasing the
drilling speed by
reducing cuttings "hold-down", creating a more underbalanced drilling
environment, and
other issues. The non-circular hole may also provide conduits for pumping
cement in the
annulus between the wellbore and outside of the casing in traditional or novel
cementing and
completion operations.
[0059] The optimum shape of the non-circular, slot-shaped or elliptical hole
can be
determined by an estimate of the reservoir stresses present and by applying
finite element
analysis techniques. A system to monitor the bit or BHA position relative to
up/down
direction in the wellbore can be useful as a part of the system design. Prior
drilling
experience in the reservoir can help determine the best orientation for the
non circular
borehole shape. Use of this non-circular approach allows for horizontal bores
that can be left
uncased (open hole) for more extended periods of time. In one embodiment, the
formation of
such holes requires the use of a non-contacting flame jet drilling system with
a movable
nozzle that swings between pivot points. In a second embodiment, the non-
circular hole can
be created by an abrasive fluid or particle-bearing fluid jet drill that moves
between pivot
points. In another embodiment a non-contacting drill can be used that uses
superheated
steam or water to drill by means of abrasion, erosion or spallation. The fluid
used in the fluid
jet drill can also be water, drilling mud, or other fluids such as
supercritical carbon dioxide
(C02) and fluids that erode the rock chemically using basic or acidic
chemicals (such as
sodium hydroxide or hydrofluoric acid in solution). A shaped multiple port
nozzle may also
be used to create the non-circular, slot-shaped, or elliptical hole. The non-
circular shaped
hole is created by either the high temperature flame or water-particle jet or
chemically active
fluid jet as it removes rock material by erosion, abrasion, dissolution and or
thermal spalling
18
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WO 2007/112387 PCT/US2007/065014
or in some cases melting of the minerals. In one aspect, monitoring of the bit
position using a
remote position sensor is preferred to control the orientation of the
elliptical hole.
[0060] Horizontal or deviated wellbores have become a major part of oil and
gas
production and stimulation processes. In many areas these horizontal wellbore
sections may
be hundreds to thousands of feet in length and may produce oil or gas from a
large part of the
horizontal section. Horizontal wells are also being considered for other
applications
including production of geothermal fluids. Large vertical stresses may be
present in these
environments especially at great depths. These stresses can cause the wellbore
to collapse
where the rocks are of limited strength and/or the pore pressure in the
wellbore drops with
through production, drilling, or post drilling operations. Horizontal drilling
has traditionally
been done by rotary drilling either from a coiled tube rig or by conventional
drilling systems.
The conventional technologies will typically drill a more or less circular
wellbore in these
horizontal sections. The use of downhole motors and wellbore tractors allows
for extended
reach wells where the significantly horizontal sections can be over 20,000
feet.
100611 Fig. 8a shows a circular generally horizontal borehole created during
conventional
drilling for oil and gas exploration. The borehole 54 is typically subjected
to high principal
stresses in the primarily vertical direction shown by the arrows 55. Breakouts
56 caused by
rock failure are shown as the scooped out regions (e.g., lobes) in the
horizontal direction
relative to the original circular borehole shape 57. Tensional fractures 58
created during
drilling can also be found parallel to the direction of the maximum principal
stress 55. Fig. 8b
shows the same circular borehole 54 extended into a more elliptical shape by
two lobes 59
created by a secondary drilling process, as described previously. An alternate
version of the
generally elliptical shape is shown in Fig. 8c where the shaped regions 60 are
more
pronounced and look much like the breakout themselves. The secondary drilling
process
removes the rock materials that would eventually collapse into the borehole
when the drill is
removed from the borehole. As a bonus effect, by strengthening the borehole in
this manner
the driller may be able to use lower mud pressures during drilling and which
can lead to
increases in the drilling rates. Reaming the entire borehole to a larger
circular shape as is
commonly done will only lead to breakouts again because the circular geometry
in not
inherently stable under these stress conditions. Fig. 8d shows the borehole 54
extended to
include regions 61 that have a more pronounced shape that can be a stable hole
geometry.
Figs. 8b-d show in general the shapes of boreholes that can be possible for
achieving a more
stable borehole.
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[0062] In addition to the so-called non-contact techniques for forming shaped
boreholes
described above, shaped boreholes in accordance with the embodiments of the
present
invention may also be formed using conventional systems that have been
modified to enable
the formation of the novel shaped boreholes of the present invention. Using
these
conventional drilling technologies, a way for creating a shaped borehole
(e.g., elliptical
shape, oval, eye, or slot-shaped) involves using multiple heads that rotate
and that are driven
individually. Such multiple head bits can be configured to form the shaped
boreholes in
accordance with the embodiments of the present invention. Exemplary geometries
for such
multiple head bits are shown in U.S. Patent No. 4,185,703, which discloses an
apparatus for
producing deep boreholes, the disclosure of which is herein incorporated by
reference.
[00631 In summary, drilling non-circular boreholes has the advantage of
improving the
efficiency of the drilling and/or completion operation, providing a wellbore
with a shape
more optimized for the application, and may produce holes that are inherently
more stable
and resistant to collapse or break-out. These non-circular holes through the
use of traditional
contact drilling technologies complimented by a secondary operation. In
addition, non-
contact drilling systems may be even better suited for the task of combination
of higher drill
rates possible with the spallation or non-contacting systems and the reduced
area of cutting
provided by the non-circular, shaped, elliptical, or slot-shaped hole concept
in accordance
with the embodiments of the present invention enable extremely fast drilling
rates to be
obtained. Drilling a non-circular hole may be more economic in applications
such as, but not
limited to, GHP's where the non-circular hole requires less time to drill and
less grout to
secure the heat transfer tube in place for an equivalent outer diameter hole.
This shape may
also have more optimized heat transfer properties compared to a circular bore.
These will
dramatically affect the economics of certain drilling projects and make them
more feasible in
many areas around the world.
[00641 Several embodiments of the present invention have several advantages
over prior art
methods and systems by being inherently more suitable for forming a non-
circular hole. For
example, using a swiveling or a shaped jet drilling head, the present system
is able to forrn
non-circular cross-sectioned bore holes by using a non contacting drill
mechanism. Such
holes can be drilled to much greater depths at much faster rates and at a
reduced rate of
material excavation, leading to significant cost savings. This may also
produce boreholes
which are inherently more stable, thereby reducing the time and expense of
uncontrolled
CA 02647079 2008-09-22
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break-out. The non-circular shape may also allow for certain wellbores to be
left
unsupported or uncased for longer periods of time, including indefinitely.
[0065] The applications for such non-circular shaped boreholes may include
geothermal
power generation, such as enhanced geothermal systems (herein referred to as
EGS) and hot
dry rock (herein referred to as HDR), or applications where the bore hole will
be left
unsupported for extended periods (minutes, hours, or days), such as in oil and
gas exploration
and production (herein referred to as oil and gas E&P) operations, or in
situations where the
wellbore will be left unsupported indefinitely, such as in an uncased
wellbore. An uncased
wellbore may have an inner surface that comprises the formation, or one that
is substantially
comprised of fused rock, ice, a layer of a non-metallic material, such as a
thermoplastic,
thermoset, composite or ceramic, or a layer of fused metallic material. In
addition to EGS-
HDR and oil and gas E&P, other conventional applications could benefit by the
drilling of
non-circular boreholes with reduced tendency towards break-out, including, but
not limited
to, water well drilling, trenchless pipe installation, sewer and municipal
system construction,
resource mining, chemical disposal wells, COZ or nuclear storage wells,
downhole chemical
reactions (such as, but not limited to, municipal waste oxidation or
biofermentation), bores in
ice, or wells for scientific or geologic study, including test holes or
secondary holes used for
measurements in the above or other operations and applications.
[00661 All patents, patent applications, publications, and descriptions
mentioned above are
herein incorporated by reference. None is admitted to be prior art.
[0067] As will be understood by those skilled in the art, other equivalent or
alternative
systems and methods for forming shaped boreholes according to the embodiments
of the
present invention can be envisioned without departing from the essential
characteristics
thereof. Accordingly, the foregoing disclosure is intended to be illustrative,
but not limiting,
of the scope of the invention which is set forth in the following claims.
21
