Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND SYSTEM FOR PRECISE DRILLING
GUIDANCE OF TWIN WELLS
BACKGROUND OF THE INVENTION
The present invention relates to the field of well drilling guidance and, in
particular, to guidance systems that use electromagnetic fields associated
with an existing well casing to steer the drilling of a second well proximate
to
the first well casing.
There is often a need to drill a second well adjacent an existing well. For
example, a pair of horizontal wells may be drilled to extract oil from a
deposit
of heavy oil or tar. A horizontal well includes well having a section that is
truly
horizontal through the earth and wells in which the "horizontal" section is
slanted up or downhill to track the interface of an oil (or other resource)
the
producing formation in the earth. Thus, the horizontal portion of the well may
not be geometrically horizontal and rather may follow a path that tracks a
formation in the earth. Of the pair of wells, an upper well may inject steam
into a subterranean deposit of heavy oil or tar while the lower well collects
liquefied oil from the deposit. The pair of wells are to be positioned within
a
few meters of each other along their lengths, especially the lateral portions
of
the wells that typically extend horizontally. The wells are positioned
proximate
to each other so that, for example, the oil liquefied by the steam from the
first
well can be collected by the second well.
There is a long-felt need for methods to drill wells, e.g., a pair of wells,
in
juxtaposition. Aligning a second well with respect to a first well is
difficult. The
drilling path of the second well may be specified to be within a few meters,
e.g., 4 to 10 meters, of the first well, and held to within a tolerance, for
example, of plus or minus 1 meter, of the desired drilling path. Drilling
guidance methods and system are needed to ensure that the drilling path of
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the second well remains properly aligned with the first well along the entire
drilling path of the second well.
Surveying the drilling path at successive points along the path is a
conventional drilling guidance method. A difficulty with typical surveying is
that a cumulative error arises in the surveyed well path because small errors
made at each successive survey point along the well path are introduced into
the survey calculation made at subsequent survey points. The cumulative
effect of these small errors may eventually cause the drilling path of the
second well to drift outside the specified desired ranges of distance or
direction relative to the first well.
U.S. Patents 6,530,154; 5,435,069; 5,230,387; 5,512,830 and 3,725,777, and
Published US Patent Application 2002/0112,856 disclose various drilling
guidance methods and systems to provide drilling path guidance and to
compensate for the cumulative effect of conventional survey errors. These
known techniques include sensing a magnetic field generated by the magnetic
properties of a well casing or a magnetic probe introduced into the well.
These methods and systems may require the use a second rig or other device
in the first well to push or pump down a magnetic signal source device. The
magnetic fields from such a source are subject to magnetic attenuation and
distortion by the first well casing, and may also generate a relatively weak
magnetic field that is difficult to sense from the desired second well
drilling
path. In view of these difficulties, there remains a long felt need for a
method
and system to guide the trajectory of a second well such that it is aligned
with
an existing well.
BRIEF DESCRIPTION OF THE INVENTION
A system and method have been developed to precisely guide the drilling
trajectory of a second well in a manner that ensures that the second well is
properly aligned with a first well. In one embodiment, a metallic casing in
the
first well conducts an alternating current that generates an alternating
magnetic field in the earth surrounding the first well. This magnetic field is
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substantially more predictable in magnitude than would be a magnetic field
due solely to the static magnetic properties of the first well. The intended
drilling trajectory of the second well is within the measurable magnetic field
generated by the current in the first well. A magnetic detector is included
within the drilling assembly used for guiding the boring of the second hole.
The magnetic detector senses the magnetic field generated by the alternating
current in the first well. Values measured of strength and direction of the
magnetic field are used to align the trajectory of the drilling assembly
drilling
the hole for the second well.
The system may be used to guide a second horizontal well being drilled near
a first horizontal well to enhance oil production from subterranean reservoirs
of heavy oil or tar sands. The two parallel wells may be positioned one above
the other and separated by a certain distance, e.g., within the range of 4 to
10
meters, through a horizontal section of a heavy oil or tar deposit. In one
embodiment, the method guides a drilling path so that the second horizontal
well is a consistent and short distance from the first well by: (1) causing a
known electrical current to flow in the metallic casing or liner (collectively
"casing") of the first well to produce a continuous magnetic field in the
region
about the first well, and (2) using magnetic field sensing instruments in the
second well while drilling to measure and calculate accurate distance and
direction information relative to the first well so that the driller can
correct the
trajectory of the second well and position the second well in the desired
relationship to the first well.
In another embodiment the invention is a method to guide a drilling path of a
second well in proximity to a first well including: applying a time-varying
electrical current to a conductor placed inside the casing of the first well;
from
the drilling path of the second well, sensing an electromagnetic field
generated by the current in the conductor, and guiding the drilling path
trajectory of the second well using the sensed electromagnetic field.
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The inventive method may be a method to guide the drilling path of a second
well in proximity to a first well comprising: drilling a third well towards a
distal
section of the first well and establishing a conductive path along the third
well
to the distal section of the first well; forming an electrical circuit
comprising an
electrical generator, a conductive casing of the first well and the conductive
path along the third well, wherein said generator applies a time-varying
electrical current to the circuit; from the drilling path of the second well,
sensing an electromagnetic field generated by the current in the first well,
and
guiding the drilling path of the second well using the sensed electromagnetic
field.
The invention may also be embodied as a drilling guidance system for guiding
a drilling path of a second well in proximity to a first well, said system
comprising: a first conductive path extending a length of the first well; a
generator of electrical current connected to opposite ends of the first well
to
apply current to the first conductive path, and a magnetic field sensor placed
within the drilling assembly of the second well and arranged to detect a field
strength and direction of an electromagnetic field generated by the current
applied to the first conductive path.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic illustration of an elevation of a well plan for
drilling
twin horizontal wells.
FIGURE 2 is a schematic map of locations for twin horizontal boreholes and
an acceptable region for the trajectory of the second well.
FIGURE 3 is a schematic diagram of an exemplary magnetic sensor array.
FIGURE 4 is a schematic diagram of an exemplary electrode assembly for
placement in a third well.
FIGURE 5 is a side view of an exemplary drilling guidance system forming an
electrical path through earth between an earth ground surface electrode and
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an electrode extending beyond the end of an existing underground well
casing.
FIGURE 6 is a side view of an exemplary drilling guidance system in which
current flows along a conductor inside the entire length of a casing of a
first
well, through earth between an electrode extending from the end of the casing
and a ground electrode.
FIGURE 7 is a side view of an exemplary drilling guidance system in which
current flows along the entire length of a casing of a first well, through
earth
between the distal end of the casing of a first well and an electrode lowered
into a third well extending near to but not intersecting with the casing of
the
first well.
DETAILED DESCRIPTION OF THE INVENTION
FIGURE 1 schematically illustrates a typical well plan for drilling twin
horizontal wells 10, 12. From the earth's surface 14, the wells may be drilled
from a single drilling platform 16, where the second well is drilled from a
second position of the drilling rig, located a short distance from the
position
from which the first well was drilled. After initially being drilled
substantially
vertically, the inclination angles of wells are built until they are
horizontal,
drilling into a desired deposit of, for example, heavy oil or tar. The first
well 12
is typically drilled and cased before drilling commences on the second
horizontal well 10. The casing or slotted liner for a well is metallic and
will
conduct electric current. The horizontal portion of the first well may be
below
the second well by several meters, e.g., 4 to 10 meters.
A directional survey is made of the first well to locate the trajectory of the
well
and facilitate planning a surface location for a small, vertical borehole 20
which is a third well. This small borehole will preferably nearly intersect 21
the
first well at the distal termination end of the first well. The small hole,
with a
temporary casing installed, preferably of a non-conductive material such as
PVC installed, need only to be large enough to accommodate a special
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electrode 22 to be lowered to a position near the bottom and near to the first
casing. The small vertical hole of the third well may be similar in size to a
water well and may extend a few meters deeper than the first well.
In the embodiment shown in Figure 1, a conductive path between the casing
18 in the first well 10 and the electrode in the third well may be enhanced if
needed by pumping a suitable conductive fluid into the third well 20. The
electrode 22 is lowered into the vertical hole to provide a current path
through
the small well. The electrode 22 electrically connects the casing or liner 18
(collectively "casing") of the first well to a conductive path, e.g. a wire,
in the
small bore hole 20. The conductive path may include earth between the
electrode 22 extending from the third well and the distal end of the casing 18
of the first well. By pumping a conductive fluid into the earth between the
distal end of the first well and the distal end of the third well, the
conductivity
of that region of earth is increased to facilitate the flow of current between
the
electrode 22 and the casing 18 of the first well.
An above ground conductive path, e.g., wires 24, connects the surface ends
of the third well 20 and the casing or liner 18 of the first well 10 to an
alternating-current (AC) electrical generator 26, or other source of time
varying current. A hoist 27, with a depth measurement instrument, may lower
and raise the wire and the electrode 22 in the third well. The hoist is
connected to the insulated surface wire 24 and includes a spool of insulated
wire to which the electrode 22 is attached. The hoist lowers the electrode 22
is
preferably lowered to the depth of the first well casing. The electrical power
from the generator drives a current 28 that flows through the wire 24, the
third
well 20, electrode 22, casing or liner of the first well 18 and is returned to
the
generator.
The alternating-current 28 produces an electromagnetic field 30 in the earth
surrounding the casing 18 of the first well. The characteristics of an
electromagnetic field from an AC conductive path are well-known. The
strength of the electromagnetic field 30 is proportional to the alternating
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current applied by the generator. The magnitude of current in the casing may
be measured with precision by an amp meter, for example. Because the
strength of the magnetic fields is proportional to the current, there is a
well-
defined relationship between the current, measured magnetic field strength at
the new well and the distance between the new well and casing of the first
well. The strength and direction of the magnetic field are indicative of the
distance and direction to the casing of the first well.
FIGURE 2 is a schematic view of the first and second wells at a cross-
sectional plane along the vertical sections through the wells. The
electromagnetic field 30 emanates from the casing 18 of the first well 10 and
into the surrounding earth. The second well 12 is shown as the upper well
however the position of the first and second well may be reversed depending
on the drilling application. A sensor assembly 40 in the second well senses
the earth's magnetic and gravity fields, and the electromagnetic field
emanating from the first well.
The acceptable drilling path of the second well is defined by a typical
acceptable zone 32 that is shown in cross-section in Figure 2. The acceptable
zone 32 may be a region that is usually centered in the range of 4 to 10
meters from the first well. The zone 32 may have a short axis along a radius
drawn from the upper well and a long axis perpendicular to a vertical plane
through the upper well. The dimensions of the acceptable zone may be plus
or minus one meter along the short axis and plus or minus two meters along
the long axis of the zone. The shape and dimensions of the acceptable zone
are known for each drilling application, but may differ depending on the
application.
The drilling trajectory for the second well should remain within the
acceptable
zone 32 for the entire length of the horizontal portion of the two wells. The
drilling guidance system, which includes the sensor assembly 40, is used to
maintain the drilling trajectory of the second well within the acceptable
zone.
Whether the drilling trajectory of the second well 12 is within the acceptable
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zone 32 is determined based on the direction and strength of the
electromagnetic field 30 along the second well path as sensed by the sensor
assembly 40.
Measurements of the field intensity and field direction by the sensor assembly
40, in the second well provide information sufficient to determine the
direction
to the first well and the distance between the two wells. This information is
provided to the driller in a convenient form so that he can take appropriate
action to maintain the trajectories of the two wells in the proper
relationship.
The sensor assembly 40 is incorporated into the down hole probe of a wireline
steering tool or MWD system for drilling the second well 12. The sensor
assembly thus guides the drilling of the second well for directional control
of
the drill path trajectory.
As alternating current flows in the conductive casing 18 of the first well,
the
alternating electromagnetic fields produced in the region surrounding the
conductor are predictable in terms of their field strength, distribution and
polarity. The magnetic field (B) produced by a long straight conductor, such
as the well casing, is proportional to the current (I) in the conductor and
inversely proportional to the perpendicular distance (r) from the conductor.
The relationship between magnetic field, current and distance is set forth in
Biot-Savart's Law which states:
B= pl /(2 II r)
Where p is the magnetic permeability of the region surrounding the conductor
and is constant. The distance (r) of the second bore hole from the casing of
the first well can thus be determined based on the measurement of the current
(I) in the casing and the magnetic field strength (B) at the second bore hole.
FIGURE 3 is a schematic diagram of a component-type sensor assembly 40
(shown in a cut-away view) having the ability to discriminate field direction.
Component-type magnetic sensors, e.g., magnetometers, and
accelerometers, are directional and survey sensors conventionally used in
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measurement-while-drilling (MWD) measurements. The sensor assembly 40
moves through the second bore hole typically a few meters behind the drill bit
and associated drilling equipment. The sensor assembly 40 collects data used
to determine the location of the second bore hole. This information issues to
guide the drill bit along a desired drilling trajectory of the second well.
The sensor assembly 40 includes both standard orientation sensors, such as
three orthogonal magnetometers 48 (to measure the magnetic field of the
earth), three orthogonal accelerometers 51 (to measure the gravity field of
the
earth), and three highly-sensitive orthogonal alternating-field magnetic
sensors 44, 46, 52 for detection of the electro magnetic field about the first
(reference or producer) well. The magnetic sensors, have a component
response pattern and are most sensitive to alternating magnetic field
intensity
corresponding to the frequency of the alternating current source. These
sensors are mounted in a fixed relative orientation in the housing for the
sensor assembly.
A pair of radial component-magnetic sensors 44 46 and 52 (typically three
sensors) are arranged in the sensor assembly 40 such that their magnetically
sensitive axes are mutually orthogonal. Each component sensor 44, 46 and
52 measures the relative magnetic field (B) strengths at the second well. The
sensors will each detect different field strengths due to their orthogonal
orientations. The direction on the field (B) may be determined by the inverse
tangent (tan-1) of the ratio of the field strength sensed by the radial
sensors
44, 46. The frame of reference for the radial sensors 44, 46 is the earth's
gravity and magnetic north, determined by the conventional magnetic sensors
48 and the gravity sensors 51. The direction to the conductor of current is
calculated by adding 90 degrees to the direction of the field at the point of
measurement. The direction from the sensors to the first well and the
perpendicular distance between the sensors and the first well provides
sufficient information to guide the trajectory of the second well in the
acceptable zone 32.
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FIGURE 4 is a schematic illustration of an exemplary electrode 22 lowered
into the small vertical hole 20 to the zone where conductive fluid has been
introduced. The electrode 22 includes metallic bow springs 50 e.g., an
expandable mesh, that expand to contact the walls of the open borehole of
the well 20. The spring elements 50 also retract to a size which slides
through
the temporary casing 53 of the vertical well 20. The temporary casing insures
that the material around the borehole does not slough into the hole. The
electrode 22 is positioned near the first casing 18 at the nearest to a point
of
intersection 21 of the two wells. A conductive fluid in the third well 20
seeps
into the earth 56 surrounding the intersection 21 between wells. The
conductive fluid enhances the electrical connectivity of the earth between the
first casing and the electrode in the third well. The electrode is connected
to
the insulated conductor wire 54 that extends through the well 20 and to the
surface. The wire 54 is connected via wire 24 to the return side of the
generator.
FIGURE 5 is a side view of an exemplary drilling guidance system 60 forming
an electrical path 62 through a region of earth 63 between a return ground
electrode 64 and an electrode 66 extending beyond the end of an existing
underground well casing 68.
The electrode 66 extends a few meters, e.g., ten or more, beyond the distal
end of the well casing 68. The distance between the electrode 66 and the end
of the well casing should be sufficient to avoid current flowing from the
electrode 66, up through the casing of the first well and to the surface
electrode.
Well casings are conventionally metallic and have slots to allow steam and
other gases to vent to the earth. Electromagnetic fields generated by the low
frequency of the AC current source, e.g., preferably below 10 Hertz and most
preferably at 5 Hertz, are not significantly attenuated by the slotted
metallic
casings in conventional wells. The electromagnetic fields generated by the
current in the insulated wire passes through the slots in the casing and into
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the earth. Eddy currents on the casing that could interfere with the
electromagnetic field are not significant due to the low frequency of the AC
source.
An alternating current (AC) source 70 applies an AC current to the return
ground electrode 64 and to the underground electrode 66 to form an electrical
current path including 62, e.g., producing a diffuse electrical field, through
the
earth 63 between the return ground electrode 64 at or near the surface and
the underground electrode 66. A wire 74 with an insulated covering extends
from the AC power source 70, through the entire length (S) of the well casing
68 and through the extended borehole a distance past the distal end of the
well casing to the electrode 66, contacting the earth. The current path 62
through the earth and to the return ground electrode 64 completes an
electrical circuit that includes the AC source 70, wire 74 and electrode 66.
The current path 62 through the earth and to the return ground electrode 64
completes an electrical circuit that includes the AC source 70, wire 74 and
electrode 66. Preferably, the wire 74 extending down through the first well
casing to the underground electrode 66 is insulated and has steel armor to
provide mechanical strength to the wire. Electromagnetic fields from the wire
74 pass through insulation, armor and the well casing 68 and into the earth.
The steel armor provides mechanical strength to the wire.
The surface wire 75 to the wire 74 and the surface wire 24 and wire 112
extending down the third well may have shielding to prevent electromagnetic
fields from these wires from generating spurious electromagnetic fields that
enter the earth. Further, the connections between the current source and the
wire 74 and the current source and surface wire 78 are established to avoid
current leakage to ground. Care is taken in setting up the electrical circuit
for
the drilling guidance system to ensure that current does not unintentionally
leak to ground and that unwanted electromagnetic fields are not created that
may affect the data collected by the sensors 88.
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The alternating current in the wire 74 generates an electromagnetic field that
extends around and beyond the casing 68 of the first well. A known current
value is applied to the wire 74 and electrode 66. Knowing the current in the
wire 74, a calculation, e.g. an application of Ampere's Law, can be made to
estimate the electromagnetic field at any given distance from the wire 74 and
the well casing 68. This calculated distance can be used to guide the drilling
of a second well.
FIGURE 6 is a side view of the drilling guidance system 60 in which a second
well 80 is being drilled parallel to the first well 68. A drilling rig 82,
which may
be the same rig used for the first well, guides a drill head 84 forming the
second well along a trajectory 86 that is parallel to the first well casing
68.
Electromagnetic sensors 88 in the second well and behind the drill head
detect the electromagnetic field from the first well 68 and wire 80 in the
well. A
current path 90 extends from the AC current source 70, along the wire 74
extending the length of the first well casing 68 and out from the distal end
of
that casing to the electrode 66, through the diffuse electrical path 62 in the
earth 63 between the electrode 66 and return ground electrode 64, and from
the return ground electrode along the return wire 92 to the source 70.
The AC sensors 88 are positioned approximately 18 or 20 meters behind the
bit, thus will not be affected by the more concentrated current in the region
where the current leaves the electrode and becomes more and more diffused
as it moves away from the electrode. In practice, the AC sensors in the
Injector well will be located some 40 or more meters behind the electrode at
the closest point, which will be near the termination of drilling of the
(lower)
Injector well.
The calculation of the estimated electromagnetic field strength at a distance
from the first well casing is used to estimate the distance from the first
well
casing of a second well trajectory 86 being drilled parallel to the first well
casing 68. Because the strength of the magnetic field at any distance from
first well casing can be calculated, the measured field strength from the
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sensors 88 can be used to determine the distance between the second well
and the first well. This information regarding the distance between the
positions of the electromagnetic sensors 88 in the second well will be used to
guide the trajectory of the drilling head 84 along a path parallel to the
first well
casing.
The calculation of the electromagnetic field around the first well casing may
also account for other elements of the AC circuit that contribute to the
magnetic field detected by the sensors 88 in the second well. For example,
electromagnetic fields that extend into the ground may be produced by the
surface mounted return wire 92 carrying current between the AC power
source 70 and the return ground electrode 64, e.g., a rod. Similarly, the
current-conducting wire 74 in the vertical section 94 of the first well casing
68
produces an electromagnetic field in the earth. These additional
electromagnetic fields should preferably be taken into account in calculating
an expected field intensity in the region of the earth near the horizontal
portion
of the first well. Calculations of expected electrical field strength from a
variety of current sources, e.g., wire 92, the vertical portion 84 of wire 74
and
the diffuse electrical current 62 in the earth region 63, can be accomplished
with known computational techniques for calculating electrical field
strengths.
Preferably, the calculations of the expected field intensity and the
measurement of the field intensity by sensors in the second well are
conducted in real time and substantially simultaneously.
The current 62 in the region of earth 63 between the electrode and the ground
rod is so thoroughly diffused that the field resulting from this current will
not be
detected at by the AC sensors 88 at their positions in the second well. Thus,
the current 62 can be ignored for purposes of calculating the electromagnetic
field around the casing of the first well. The electromagnetic field strength
of
the current 62 in the earth 63 may relatively strong in the vicinity of the
distal
end of the first well. However, it is not needed to measure the field at the
distal end of the first well because this point is at or near the end of the
second drilling path 86. At the end of the path there is likely to much less
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need, if any, to monitor the field because the drilling path is nearly
complete
and the trajectory will not significantly change further.
Deployment of the electrode outside the first well (the Producer well) 68
casing into open hole may be done in a variety of ways. The electrode may be
pumped down through whatever tubular is used to run it into the hole, pushed
into position with an extension of the tubing or drill pipe used to lower it
into
the hole, or it may be pushed into place with an extended well tractor. Yet
another possibility is the use of coiled tubing to push it into place.
Assuming that a suitable method of deployment is developed, this method
may well be more accurate than the three-well method because of the
lossless current conduction by the wire inside the pipe, with no loss of
accuracy due to poor information about the conductivity of formations
surrounding the casing.
FIGURE 7 is a side view of another exemplary drilling guidance system 100 in
which current flows along the entire length of a conductive casing 102 of a
first
well, through a region of earth 104 between a distal end 106 of the casing and
a return ground electrode 108 lowered into a third well casing 110 extending
near to but not intersecting with the casing 102 of the first well. A current
source 70 applies current directly to the conductive casing 102 of the first
well
and to a conductive return wire 112 extending along the surface from the
source 70 to and down the third well 110 to the return ground electrode 108.
The return ground electrode 108 extends beyond the distal end of the casing
of the third well into open borehole in the earth and is connected to the
return
wire that extends through the casing, which is preferably non-conductive, of
the third well.
A diffuse electrical current path 115 is formed in the earth between the
return
electrode 108 and the casing of the first well. This electrical path is
included in
the current path 114 extending from the source 70, casing 102 of the first
well,
return electrode 108 and return wire 112. The return electrode is positioned
close to the first well casing (and preferably in contact with the casing) to
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reduce the electrical path through earth between the casing and the return
electrode.
The current path 114 includes the current in a horizontal portion of the
casing
102 of the first well which generates an electromagnetic field around the
casing that is detected by sensors 88 in a second well 80 being drilled by a
drill head 84 following a desired drilling trajectory 86. By measuring the
electromagnetic field at the sensors 88 and knowing the current in the casing
of the first well, the distance between these sensors in the second well 80
can
be used to calculate the distance between the first well and the second well,
from the location of the sensors.
While there have been described herein what are considered to be preferred
and exemplary embodiments of the present invention, other modifications of
these embodiments falling within the scope of the invention described herein
shall be apparent to those skilled in the art.
