CA2460788A1

CA2460788A1 - Magnetic field enhancement for use in passive ranging

Info

Publication number: CA2460788A1
Application number: CA002460788A
Authority: CA
Inventors: Graham A. Mcelhinney
Original assignee: PathFinder Energy Services Inc
Current assignee: Schlumberger Canada Ltd
Priority date: 2004-03-12
Filing date: 2004-03-12
Publication date: 2005-09-12
Anticipated expiration: 2024-03-12
Also published as: CA2460788C

Abstract

A method for surveying a borehole is provided. The method includes providing a casing string having a plurality of opposing magnetic poles in a target borehole. The method also includes providing a tool having a magnetic field measurement device disposed thereon and positioning the tool in another borehole. Magnetic interference vectors are determined in the other borehole by comparing the measured magnetic field at that position with a known magnetic field of the earth. The magnetic interference vector indicates a direction to a casing string. Various embodiments of the invention may be utilized to drill one borehole along a predetermined course relative to another borehole.
The surveying methodology of this invention may be advantageously utilized to drill twin wells for steam assisted gravity drainage applications.

Description

MAGNETIC FIELD ENHANCEMENT
FOR USE IN PASSIVE RANGING
Inventor: Graham A. McElhinney 44 High Street, Inverurie Aberdeenshire, Scotland United Kingdom Citizenship: U,K.
FIELD OF THE INVENTION
[OOO1J The present invention relates generally to surveying a subterranean borehole to determine, for example, the path of the borehole. More particularly this invention relates to a method of passive ranging to determine directional and/or locational parameters of a borehole using sensors including one or more magnetic field measurement devices.

s BACKGROUND OF THE INVENTION
[0002] The use of magnetic field measurement devices (e.g., magnetometers) in prior art subterranean surveying techniques for determining the direction of the earth's magnetic field at a particular point is well known. The use of accelerometers or gyroscopes ixz combination with one or more magnetometers to determine direction is also known. Deployments of such sensor sets are well known to determine borehole characteristics such as inclination, azimuth, positions in space, tool face rotation, magnetic tool face, and magnetic azimuth (i.e., an azimuth value determined from magnetic field measurements). While magnetometers are known to provide valuable information to the surveyor, their use in borehole surveying, and in particular measurement while drilling (MWD) applications, tends to be limited by various factors.
For example, magnetic interference, such as from the magnetic steel components (e.g., liners, casings, etc.) of an adjacent borehole (also referred to as a target well herein) tends to interfere with the earth's magnetic field and thus may cause a deflection in the azimuth values obtained from a magnetometer set.

[0003] Passive ranging techniques may utilize such magnetic interference fields, for example, to help determine the location of an adjacent well (target well) to reduce the risk of collision and/or to place the well into a kill zone (e.g., near a well blow out where formation fluid is escaping to an adjacent well). U.S. Patent 5,675,488 and U.S. Patent Applications 10/368,257, 10/368,742, and 10/369,353 to McElhinney (herein referred to as the McElhinney patents) describe methods for determining the position of a target well with respect to a measured well (e.g., the well being drilled) in close proximity thereto.
Such methods utilize three-dimensional magnetic interference vectors determined at a k number of points in the measured well to determine azimuth and/or inclination of the target well and/or the distance from the measured well to the target well.

[0004] The methods described in the McElhinney patents have been shown to work well in a number of borehoie surveying applications, such as, for example;
well avoidance and or well kill applications. However, there remain certain other applications for which improved passive ranging techniques may advantageously be utilized. For example, well twinning applications (in particular in near horizontal well sections), in which a measured well is drilled essentially parallel to a target well, may benefit from such improved passive ranging techniques.

[0005] Moreover, well twinning in steam assisted gravity drainage applications may particularly benefit from improved passive ranging techniques. Currently, active ranging techniques, for example, in which high strength electromagnets are snaked through a target well during drilling of the twin well, are utilized in such applications. However simultaneous operation of the electromagnetic assembly in the target well and the drilling assembly in the twin well tends to be both expensive and awkward.
Additionally, active ranging techniques typically do not account for magnetic interference and are therefore further susceptible to error.

[0006] Therefore, there exists a need for improved borehole surveying methods utilizing various passive ranging techniques.

x (0007] Exemplary aspects of the present invention are intended to address the above described need for improved surveying methods utilizing various passive ranging techniques. Referring briefly to the accompanying figures, aspects of this invention include methods for surveying a borehole. Such methods make use of magnetic flux emanating from nearby magnetized subterranean structures (typically referred to herein as target wells), such as cased boreholes. Such magnetic flux may be passively measured to determine a direction and distance from the borehole being surveyed (also referred to herein as the measured well) to the target well. In various exemplary embodiments, the orientation of the measured well relative to the target well, the distance between the two wells, and the absolute coordinates, and the azimuth of the measured well may also be determined.

[0008] Exemplary embodiments of the present invention advantageously provide several technical advantages. For example, the direction and distance from a measured well to a target well may advantageously be determined without having to reposition the downhole tool in the measured well. Further, embodiments of this invention may be utilized to determine an azimuth value of the measured well. Such azimuth determination may be advantageous in certain drilling applications, such as in regions of magnetic interference where magnetic azimuth readings are often unreliable. Aspects of this invention may also advantageously be utilized in certain drilling applications, such as well twinning and/or relief well applications, to guide continued drilling of the measur~l well, for example, in a direction substantially parallel wii:h the target well. Exemplary embodiments of this invention may be advantageously utilized for drilling twin wells for steam assisted gravity drainage applications.

In one exemplary aspect the present invention includes a method for surveying a borehole. The method includes providing a casing string in a target borehole, the casing string including a plurality of opposing magnetic poles. The method further includes providing a downhole tool including first and second magnetic field measurement devices disposed at corresponding first and second positions in the borehole. The first and second positions are selected to be within sensory range of magnetic flux from the casing string.
The method further includes measuring total local magnetic fields at the first and second positions using the corresponding first and second magnetic field measurement devices, processing the total local magnetic fields at the first and second positions and a reference magnetic field to determine a portion of the total local magnetic fields attributable to the casing string, and generating interference magnetic field vectors at the first and second positions from the portion of the total local magnetic field attributable to the casing string.
The method further includes processing the interference magnetic field vectors to determine tool face to target angles at each of the first and second positions.

[0009] The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.
It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should be also be realize by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS
[4010] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
[0011] FIGURE 1 is a schematic representation of an exemplary embodiment of a MWD tool according to the present invention including both upper and lower sensor sets.
[0012] FIGURE 2 is a diagrammatic representation of a portion of the MWD tool of FIGURE 1 showing unit magnetic field and gravity vectors.
[0013] FIGURES 3A and 3B are schematic representations of an exemplary application of this invention.
[0014] FIGURES 4A through 4D depict exemplary target well casing configurations.
[0015] FIGURES SA and SB depict contour plots of a theoretical magnetic flux density about hypothetical casing strings.
[0016] FIGURE 6 depicts the theoretical magnetic field strength versus measured well depth along portions of the casing strings shown in FIGURES SA and SB.
[001'1] FIGURE 7 is a schematic representation of a cross sectional view along section 4-4 of FIGURE 3B.
[0018] FIGURE 8 is a schematic representation of a hypothetical plot of tool face to target versus well depth as an illustrative example of one embodiment of this invention.
[00191 FIGURE 9 depicts a cross sectional view similar to that of FIGURE 4 as an illustrative example of various embodiments ofthis invention.
[0020) FIGURES l0A and lOB depict cross sectional views similar to those of FIGURES 4 and 6 as illustrative examples of other embodiments of this invention.

[0021] FIGURE 11 depicts a cross sectional view similar to that of FIGURES 5, 7, 8A
and 8B as an illustrative example of still other exemplary embodiments of this invention.
[0022} FIGURE 12 is a display of a drilling plan for a hypothetical well twinning operation.
[0023] FIGURE 13 is another diagrammatic representation of a portion of the tool of FIGURE 1 showing the change in azimuth between the upper and lower sensor sets.

g DETAILED DESCRIPTION
[0024] Referring now to FIGURE 1, one exemplary embodiment of a downhole tool 100 useful in conjunction with the method of the present invention is illustrated. In FIGURE 1, downhole tool 100 is illustrated as a measurement while drilling (MWD) tool including upper 110 and lower 120 sensor sets coupled to a bottom hole assembly (BHA) 150 including, for example, a steering tool 154 and a drill bit assembly 158.
The upper 110 and lower 120 sensor sets are disposed at a known spacing, for example, on the order of from about 2 to about 20 meters (i.e., about 6 to about 60 feet). Each sensor set (114 and 120) includes at least two (and preferably three) mutually orthogonal magnetic field sensors, with at least one magnetic field sensor in each set having a known orientation with respect to the borehole, and three mutually orthogonal gravity sensors.
It will be appreciated that the method of this invention may also be practiced with a downhole tool including only a single sensor set having at least two magnetic field sensors.
[0025] Referring now to FIGURE 2, a diagrammatic representation of a portion of the MWD tool of FIGURE 1 is illustrated. In the embodiment shown on FIGURES 1 and 2, each sensor set includes three mutually perpendicular magnetic field sensors, one of which is oriented substantially parallel with the borehole and measures magnetic field vectors denoted as Bzl and Bz2 for the upper 110 and lower 120 sensor sets, respectively.
The upper 110 and lower 120 sensor sets are linked by a structure 140 (e.g., a semi-rigid tube such as a portion of a drill string) that permits bending along its longitudinal axis 50, but substantially resists rotation between the upper 110 and lower 120 sensor sets along the longitudinal axis 50. Each set of magnetic field sensors thus may be considered as determining a plane (Bx and By) and pole (Bz) as shown. As described in more detail below, embodiments of this invention typically only require magnetic field measurements in the plane of the tool face (Bx and By as shown in FIGURE 2 which corresponds with plane 121, for example, in sensor set 120}. T'he structure 140 between the upper 110 and lower 120 sensor sets may advantageously be part of, for example, a MWD tool as shown above in FIGURE 1. Alternatively, structure 140 may be a part of substantially any other logging and/or surveying apparatuses, such as a wireline surveying tool.
[0026] As described above, embodiments of this invention may be particularly useful, for example, in well twinning applications (e.g., relief well drilling), such as that shown in FIGURES 3A and 3B. Generally speaking twinning refers to applications in which one well is drilled in close proximity (e.g., parallel) to another well for various purposes.
Relief well drilling generally refers to an operation in which one will is drilled to intercept another well (e.g., to prevent a blow). Nevertheless, the terms twinning and relief well will be used synonymously and interchangeably in this disclosure.
In FIGURE
3A, a bottom hole assembly 150 is kicked off out of a casing window 178 in a pre-existing borehole 175. "Kicking off' refers to a quick change in the angle of a borehole, and may be associated, for example with drilling a new hole from the bottom or the side of an existing borehole. A relief well 177, for example, is then drilled substantially parallel with the pre-existing borehole 175, as shown in FIGURE 3B. In such applications there tends to be significant magnetic interference emanating from the pre existing borehole 175, e.g., from the well casing, owing, for example, to residual magnetization from magnetic particle inspection procedures. Normally, such magnetic interference fades (decreases) quickly as the distance to the pre-existing borehole increases. However, in relief well applications, for example, in which the distance between the relief well 177 and the pre-existing borehole 175 typically remains small (e.g., from about 1 to about IO feet); such magnetic interference tends to significantly interfere with the determination of borehole azimuth using conventional magnetic surveying techniques. Further, such relief well drilling applications are often carried out in near horizontal wells (e.g., to divert around a portion of a pre-existing borehole that is blocked or has collapsed). Thus conventional gyroscope and gravity azimuth surveying methods may be less than optimal for borehole surveying in such applications.
As described in more detail below, this invention looks to the magnetic interference from a target well (e.g., pre-existing borehole 175) to determine the azimuth of the measured well (e.g., relief well 177). Surveying according to the present invention may thus be useful in such relief well and/or well twinning applications. Other exemplary applications may include, but are not limited to, river crossings in which an existing well is followed around various obstacles, re-entry and/or well kill applications, well avoidance applications, and substantially any application in which multiple substantially parallel wells are desirable (such as also useful in mineral extraction and ground freeze applications).
[0027] Certain exemplary embodiments of this invexition may also be useful in twinning wells for heavy oil recovery applications, such as steam assisted gravity drainage applications. In such applications, twin horizontal wells having a vertical separation distance ranging from about 4 to about 20 meters are typically utilized. Steam is injected into the upper well to heat the heavy oil. The heated heavy oil and condensed steam are then produced from the lower well. The success of such heavy oil recovery techniques is often dependent upon maintaining a predetermined relative positioning of the wells in he injection/production zone (which may be up to several thousand feet in length). As such, exemplary embodiments of this invention, as described herein, may be particularly advantageous for the above described heavy oil recovery applications.

[0028] It should be noted that the magnetic interference may emanate from substantially any point or points on the target well. It may also have substantially any field strength and be oriented at substantially any angle to the target well, with the field strength at a particular location generally decreasing with distance from the target borehole. Further, the magnetic interference tends to be caused by the tubular elements in the target well, e.g., the casing, drill string, collars, and the like. The magnetic interference surrounding these elements is determined by the magnetism (both induced and permanent) in the metal. The shape of the interference pattern is particularly influenced by the homogeneity of the magnetism and the shape of the metal element.
Typically, the magnetism is substantially homogeneous and the shape rotationally symmetrical and tubular. Objects in a borehole, such as pipe sections and the like, are often threadably coupled to form a substantially continuous cylinder. Thus, the origin of any magnetic interference emanating from a borehole may generally be considered to originate in cylinders therefrom. The magnetic field ern. anating from such a borehole (target well) is typically caused by such cylinders in a manner typically displayed by cylindrical magnets. Such is the basis for the passive ranging techniques disclosed in the McElhinney patents.
[0029] It will be appreciated that the terms magnetic flux density and magnetic field are used interchangeably herein with the understanding that they are substantially proportional to one another and that the measurement of either may be converted to the other by known mathematical calculations.
[0030] As described in more detail herein, this invention looks to the magnetic interference emanating from a target well; caused, for example, by residual magnetization from magnetic particle inspection procedures. In certain applications, such as the above described heavy oil recovery applications, it may be advantageous to enhance the flux density of the magnetic field emanating radially outward from the target well and therefore to increase the effective distance of the passive ranging techniques described herein. It has been found that the flux density may be essentially focused radially outward from the target well, for example, by configuring the casing string to include a plurality of opposing magnetic poles (i.e., magnetic poles on certain individual casing joints oppose magnetic poles on certain other adjacent casing joints).
Exemplary target well casing configurations are shown on FIGURES 4A through 4D, which illustrate exemplary casing strings 190, 190', 190" and 190"', each including a plurality of magnetized casing joints 192 joined end to end. As shown, the casing strings may be configured to include opposing north (N) and south (S) poles at various intervals. In casing string 190 (FIGURE 4A), each seem 193 between individual casing joints includes opposing NN or SS poles. In casing string 190' (FIGURE 4B), every other seem 193 includes opposing NN or SS poles, while in casing sizing 190" (FIGURE 4C) every third seem 193 includes opposing NN or SS poles. Casing string 190"' (FIGURE
4D) does not include any opposing NN or SS poles and is shown for comparison purposes. It will be appreciated that substantially any casing string configuration including a pluratilh of opposing magnetic poles may be utilized in the target well, for example, to maximize the flux density at certain longitudinal positions or intervals along the target well.
[0031] The magnitude of the magnetic flux enhancement may be modeled using conventional finite element analysis methods. FIGURES SA and SB show gray-scale contours of the flux density (black being highest and while being lowest) about exemplary casing string configurations and thus may be utilized to compare and contrast the magnetic field strength about those casing strings. In FIGURE SA, the casing string includes opposing magnetic poles at every third seam as shown in FIGURE 4C. In FIGURE SB, the casing string includes no opposing magnetic poles. Rather the magnetic poles on the individual casing joints are aligned as shown in FIGURE 4D (e.g., with the magnetic north poles on the uphole side of each casing joint and the magnetic south poles on the downhole side of each casing joint). In each model, a casing string including 27 casing joints was utilized. The magnetic pole strengths were configured to vary randomly from one joint to the next in a manner consistent with known magnetic field strengths for such casing joints. Further, in this exemplary model, each casing joint was 13 meters in length by 0.6 meters in diameter, which is consistent with upper well dimensions in some steam assisted gravity dxainage applications. It will be appreciated that this invention is not limited by such model assumptions.
[0032] As shown by contrasting FIGURES SA and SB, including regular opposing magnetic poles in the casing string surprisingly significantly enhances the magnetic field strength (which is proportional to the magnetic flux density) about target well. It will be appreciated that in this exemplary model the magnetic flux enhancement is due only to the opposing magnetic poles. As expected, the magnetic field strength includes maxima near the opposing poles (also shown below in FIGURE 6). However, contrary to conventional wisdom, the magnetic field strength is significantly enhanced along the entire casing string as compared to a casing string including no opposing poles. Tt will also be appreciated that such significant enhancements are not limited to the examples shown in FIGURES 4A through 5B. Rather, it has been faund that that the magnetic field about substantially any casing string having a plurality of opposing magnetic poles is significantly enhanced as compared to a casing string that does not include opposing magnetic poles.

[0033] With reference now to FIGURE 6, the magnetic field strength versus measured depth (longitudinal position along the casing string) is shown at a radial distance of 6 meters. The magnetic field strength of the casing having opposing poles (e.g., casing 190" shown on FIGURE 4C) is shown at 196 while the magnetic field strength of the casing having no opposing poles (e.g., casing 190"' on FIGURE 4D) is shown at 198. It can be seen that configuring the casing string to include opposing poles every third seam increases the magnetic field strength by up to about five times. Such a pronounced increase in magnetic field strength advantageously increases the distance over which the passive ranging techniques described herein may be successfully utilized (e.g., up to about 20 meters in some applications). Increased magnetic field strength also tends to advantageously reduce errors associated with such passive ranging measurements, for example, via increasing the signal to noise ratio of the measured magnetic fields values (magnetometer readings).
[0034] In practice, casing strings having a plurality of opposing poles may be made up using substantially any procedure. For example, individual casing joints may be "cherry-picked" at the casing yard or at the rig site based on known magnetic properties of the individual casing joints (e.g., obtained via on-site magnetic measurements).
For example, two joints having magnetic north poles near their respective pin ends may be chosen first, followed by two joints having magnetic south poles near their respective pin ends, and so on to make up a casing string as shown in FIGURE 4B (casing string 190').
Alternatively, the individual casing joints may be magnetized (Gaussed up) with predetermined magnetic pole strengths and locations prior to being made up into the casing string. Such magnetization may be independent of or in cooperation with conventional particle inspection techniques.

[0035] It may also be advantageous on occasion to attach various magnetic sources, such as ring, spherical, cylindrical, and/or rod magnets to the casing string.
Such magnetic sources may enhance the local magnetic field about the target well and assist a drilling operator in orienting andlor aligning the measured well with respect to the target well.
(0036] The magnetic interference may be measured as a vector whose orientation depends on the location of the measurement point within the magnetic field. In order to determine the magnetic interference vector at any point downhole, the magnetic field of the earth must be subtracted from the measured magnetic field vector. The magnetic field of the earth (including both magnitude and direction components) is typically known, for example, from previous geological survey data. However, for some applications it may be advantageous to measure the magnetic field in real time on site at a location substantially free from magnetic interference, e.g., at the surface of the well or in a previously drilled well. Measurement of the magnetic field in real time is generally advantageous in that in that it accounts for time dependent variations in the earth's magnetic field, e.g., as caused by solar winds. However, at certain sites, such as on an offshore drilling rig, measurement of the earth's magnetic field in real time may not be possible. In such instances, it may be preferable to utilize previous geological survey data in combination with suitable interpolation and/or mathematical modeling (i.e., computer modeling) routines.
[0037] The earth's magnetic field at the tool may be expressed as follows:
M~ =HE(cosDsinAzcosR+cosDcosAzcoslhcsinR-sinDsinlncsinR) M~, =HE(cosDcosAzcoslnccosR+sinDsinlnccosR-cosDsinAzsinR) MEZ = HE (sin D cos Inc - co s D cos Az sin Inc) Equation 1 where Mex, Mey, and Mez represent the x, y, and z components, respectively, of the earth's magnetic field as measured at the downhole tool, where the z component is aligned with the borehole axis, He is known (or measured as described above) and represents the magnitude of the earth's magnetic field, and D, which is also known (or measured), represents the local magnetic dip. Inc, Az, and R, represent the Inclination, Azimuth and Rotation (also known as the gravity tool face), respectively, of the tool, which may be obtained, for example, from conventional gravity surveying techniques.
However, as described above, in various relief well applications, such as in near horizontal wells, azimuth determination from conventional surveying techniques tends to be unreliable. In such applications, since the measured borehole and the target borehole are essentially parallel (i.e., within a five or ten degrees of being parallel), Az values .from the target well, as determined, for example in a historical survey, may be utilized.
The magnetic interference vectors may then be represented as follows:
M~ = BX - M~
MIY = BY - MEY
M~ = BZ -MEZ Equation 2 where Mix, Miy, and Miz represent the x, y, and z components, respectively, of the magnetic interference vector and Bx, By, and Bz, as described above, represent the measured magnetic field vectors in the x, y, and z directions, respectively.
[0038] The artisan of ordinary skill will readily recognize that in determining the magnetic interference vectors it may also be necessary to subtract other magnetic field components, such as drill string and/or motor interference from the borehole being drilled, from the measured magnetic field vectors. Techniques for accounting for such other magnetic field components are well known in the art.
[0039] Referring now to FIGURES 7 through 12, embodiments of the method of this invention are described in further detail. With reference to FIGURE 7, a cross section as shown on FIGURE 3B is depicted looking down the target borehole 175. Since the measured borehole and the target borehole are approximately parallel, the view of FIGURE 7 is also essentially looking down the measured borehole. The magnetic flux lines 202 emanating from the target borehole 175 are shown to substantially intersect the target borehole 175 at a point T. Thus a magnetic field vector 205 determined at the measured borehole 1~7, for example, as determined by Equations 1 and 2 above, provides a direction from the measured borehole to the target borehole 175. Since the measured borehole and the target borehole are typically essentially parallel, determination of a two-dimensional magnetic field vector (e.g., in the planes of the tool faces 111 and 121 shown in FIGURE 2) and a two-dimensional interference vector is advantageously sufficient for determining the direction from the measured well to the target well. Two-dimensional magnetic field and interference vectors may be determined according to Equations 1 and 2 by solving for Mex, Mey, Mix, and Miy. As such measurement of the magnetic fzeld in two dimensions (e.g., Bx and By) may likewise be sufficient for determining the direction from the measured well to the target well. Nevertheless, for certain applications it may be preferable to measure the magnetic field in three dimensions.
[0040} A tool face to target (TFT) value (also referred to herein as a tool face to target angle) may be determined from the magnetic interference vectors given in Equation 2 as follows:

TFT = arctan(~'~ ) + arctan( ~'x ) Equation 3 err GY
[0041] where TFT represents a tool face to target direction (angular orientation), Mix and Miy represent the x and y components, respectively, of the magnetic interference vector, and Gx and Gy represent x and y components of the measured gravitational field (e.g., gravity vectors measured at at least one of the first and second sensor sets 110, 120 in FIGURE 2). As shown in FIGURE 5, the TFT indicates the direction from the measured well 177 to the target well 175. For example, a TFT of 90 degrees, as shown in FIGURE 7, indicates that the target well 175 is directly to the right of the measured well 177. A TFT of 270 degrees, on the other hand, indicates that the target well is directly to the left of the measured well. Further, at TFT values of 0 and 180 degrees the target well 175 is directly above and directly below, respectively, the measured well 177.
It will be appreciated that in certain applications, Equation 3 does not fully define the direction from the measured well 177 to the target well 175. Thus in such applications, prior knowledge regarding the general direction from the measured well to the target well (e.g., upwards, downwards, left, or right) may be utilized in combination with the TFT values determined in Equation 3. Alternatively, changes in the TFT values between adjacent survey points may be utilized to provide further indication of the direction from the measured well 177 to the target well 175.
[0042] In certain applications, determination of the TFT at two or more points along the measured well bore may be sufficient to guide continued drilling of the measured well, for example, in a direction substantially parallel with the target well.
This is shown schematically in FIGURE 8, which plots 250 TFT 252 versus Well Depth 254. Data sets 262, 264, 266, and 268 represent TFT values determined at various well depths.
Each Y
data set, e.g., data set 262, includes two data points, A and B, determined at a single survey location (station). In data set 262, for example, data point A is the TFT value determined from the magnetic interference vector measured at an upper sensor set (e.g., sensor set 110 in FIGURES 1 through 3B) and data point B is the TFT value determined from the magnetic interference vector measured at a lower sensor set (e.g., sensor set 120 in FIGURES 1 through 3B), which resides some fixed distance (e.g., from about 6 to about 60 feet) further down the borehole than the upper sensor set. Thus at each survey station (data sets 262, 264, 266, and 268) two magnetic interference vectors may be determined. The TFT at each data point indicates the direction to the target borehole from that point on the measured borehole. Additionally, and advantageously for MWD
embodiments including two sensor sets, comparison of the A and B data points at a given survey station (e.g., set 262) indicates the relative direction of drilling with respect to the target well at the location of that survey station. Further, since a drill bit is typically a known distance below the lower sensor set, the TFT at the drill bit may be determined by extrapolating the TFT values from the upper and lower sensor sets (points A
and B on FIGURE 6).
[0043] With continued reference to FIGURE 8, data sets 262, 264, 266, and 268 are described in more detail. In this hypothetical example, data sets 262, 264, 266, and 268 represent sequential survey stations (locations) during an MWD drilling operation and thus may be spaced at a known interval (e.g., about 50 feet) in the measured well. At data set 262, the target well is down and to the right of the measured well as indicated by the TFT values. Since the TFT at point B is closer to 90 degrees than that of point A, data set 262 indicates that the measured well is pointing downward relative to the target well. For a drilling operation in which it is intended to drill the measured well parallel and at the same vertical depth as the target well (e.g., at a TFT of 90 degrees), data set 262 would indicate that drilling should continue for a time in approximately the same direction. At data set 264, the measured well has moved below the target well as indicated by TFT
values below 90 degrees. Similar TFT values for points A and B indicate that the measured MWD tool (and therefore the measured well) is pointed horizontally relative to the target well. At data set 266, the measured well remains below the target well, but is pointing upward relative thereto. And at data set 268, the measured well is at about the same vertical depth as the target well and substantially aligned therewith vertically.
[0044] While tool face to target values determined from the magnetic interference vectors provide potentially valuable directional information relating to the position of a measured well relative to a target well, they do not, alone, provide an indication of the distance from the measured well to the target well. According to one aspect of this invention, the TFT values m~,y be utilized, along with survey data from the measured well (e.g., inclination values) and historical survey data from the target well, to determine a distance from the measured well to the target well. In one variation of this aspect, the direction and distance from the measured well to the target well may then be utilized to determine absolute coordinates and azimuth values for the measured well at various points along the length thereof.
[0045] With reference now to FIGURE 9, a view down the target borehole, similar to that of FIGURE 7, is shown. It will be appreciated that for near horizontal wells, the x and y directions in FIGURE 9 correspond essentially to horizontal and vertical directions relative to the target well 175. At first and second survey points 177, 177' (e.g., as measured at sensor sets 110 and 120, respectively, as shown in FIGURES 1 through 3B) the measured borehole is generally downward and to the left of target borehole 175, as shown. As described above, this is indicated by the TFT values TFT1, TFT2 at the two survey points being less than 90 degrees. In the general case illustrated in FIGURE 9, the measured well 177, 177' is not precisely parallel with the target well 175. As such, the relative position of the measbred well with respect to the target well 175 (in the view of FIGURE 9) is a function of the measured depth of the measured well (as shown by the relative change in position between the two wells at the first and second survey points 177, 177'). Such a change in the relative position at the first and second survey points 177, 177' is represented by ax and 0y in FIGURE 9, where Ox represents the relative change in horizontal position between the first and second survey points 177, 177' of the measured well and corresponding points on the target well 175 (e.g., substantially orthogonal to the longitudinal axis of the measured well at the first and second survey points), and Dy represents the relative change in vertical position between the first and second survey points 177, 177' of the measured well and corresponding points on the target well 175. As described above, in many instances the relative change in positions between the two wells (as defined by ~x and ~y) results in a change in the measured tool face to target value, ~TFT, between the first and second survey points 177, 177'. As described in greater detail below, for certain applications, the distances dl and d2 from the first and second survey points 177, 177' on the measured well to the target well 175 are approximately inversely proportional to OTFT.
[0046] It will be appreciated that based on FIGURE 9 and known trigonometric principles, the distances dl and d2 may be determined mathematically, for example, from fix, ~y, TFT1 and TFT2. With continued reference to FIGURE 9, and according to the Pythagorean Theorem, distances dl and d2 may be expressed mathematically as follows:

dl= x2 +(y-Dy)2 d 2 = (x - t1x) 2 + y 2 Equation 4 [0047] where x and y represent the horizontal distance from the first survey point 177 to the target well 175 and the vertical distance from the second survey point 177' to the target well 175, respectively. x and y may be expressed mathematically as follows:
- ~x tan(TFTI) - 4y tan(TFTI) tan(TFT 2) x=
tan(TFT 2) - tan(TFTl) _ - Dy tan(TFT I) - 0x y tan(TFT2) - tan(TFTl) Equation 5 [0048] where, as described above, ~x represents the relative change in horizontal position between the first and second survey points 177, 177' of the measured well and corresponding points on the target well 175, ~y represents the relative change in the vertical position between the first and second survey points 177, 177' on the measured well and corresponding points on the target well 175, and TFT l and TFT2 represent the tool face to target values at the first and second survey points 177, 177', respectively. As described in greater detail below, Ox and 6y may be determined, for example, from azimuth and inclination measurements of the measured and target wells.
[0049] Distances dl and d2 may alternatively be expressed mathematically as follows:
d 1= - ~x - °y tan(TFT2) cos(TFTl)[tan(TFT2) - tan(TFTl)]
d 2 = - ~ - ~y t~(TFT 1) Equation 6 cos(TFT 2)[tan(TFT 2) - tan(TFT1)]
[0050] where dl, d2, fix, TFT1; and TFT2 are defined above.

[0051] As shown below in more detail, ~x and Dy may be determined from azimuth and inclination values, respectively, of the measured and target wells: For some drilling applications in which embodiments of this invention are suitable, magnetic interference tends to interfere with the determination of azimuth values of the measured well using conventional magnetic surveying techniques. In such applications determination of ~x may be problematic. Thus, in certain applications, it may be advantageous to determine the distances dl and d2 independent from Ox (and therefore independent of the azimuth values of the measured and target wells).
[0052] In various applications, such as common well twinning and relief well drilling applications, the intent of the drilling operation is to position the measured well substantially parallel and side by side with the target well 175. As described above, the measured TFT values for such applications are approximately 90 or 270 degrees (e.g., within about 45 degrees thereof). It will be appreciated that in such applications relative changes in the horizontal position between the measured and target wells, fix, typically has a minimal effect on the measured TFT values (i.e., results in a relatively small ~TFT
value for a given fix). As such, for many applications, determination of the distances dl and d2 from survey points 177, 177' of the measured well to corresponding points on the target well 175 may be derived considering only relative changes in the vertical position, ~y, between the measured and target wells.
[0053] With reference now to FIGURE 10A, distances dl and d2 may be expressed mathematically with respect to ay, TFT1, and TFT2 as follows:
d 1= - DY tan(TFT 2) cos(TFTl)[tan(TFT2) - tan(TFTI)]

d 2 = - ~Y ~(TFT l) Equation 7 cos(TFT2)[tan(TFT2) - tan(TFTl)]
[0054] where, as described above, dl and d2 represent the distances from the measured well to the target well at the first and second survey points 177, 177', respectively, TFT1 and TFT2 represent the tool face to target values at the first and second survey points 177, 177', respectively, and ~y represents the relative change in vertical position between the first and second survey points 177, 177' of the measured well and corresponding points on the target well.
[0055] Turning now to FIGURE IOB, for certain applications (e.g., when a measured well is drilled substantially side by side with a target well), the tool face to target value may be assumed to be approximately equal to 90 or 270 degrees. Based on such an assumption, the distances dl and d2 may alternatively be expressed mathematically as follows:
d l = ~Y
tan(aTFT) d 2 = ~Y Equation 8 sin(OTFT) [0056] where, as stated above, OTFT is the difference between the tool face to target values at the first and second survey points 177, 177'. A,t relatively small ~TFT values (e.g., when OTFT is less than about 30 degrees), the distances dl and d2 may alternatively be expressed mathematically as follows:
d 1= d 2 = ~Y Equation 9 ~TFT
(0057] where OTFT is in units of radians.

[0058] Equation 9 advantageously describes distance (dl and d2) from the measured well to the target well 175 as being substantially proportional to Dy and as substantially inversely proportional to the change in tool face to target value aTFT. While not generally applicable to all well drilling applications (or even to all twinning applications), Equation 9 may be valuable for many applications in that it provides relatively simple operational guidance regarding the distance from the measured well to the target well.
For example, in certain applications, if the change in tool face to target value ~TFT
between two survey points is relatively small (e.g., less than about 5 degrees or 0.1 radians) then the distance to the taxget well is at least an order of magnitude greater than Dy (e.g., dl arid d2 are about a factor of 10 greater than Dy when ~TFT is about 5 degrees or 0.1 radians). Conversely, if OTFT is relatively large (e.g., about 30 degrees or 0.5 radians) then the distance to the target well is only marginally greater than ~y (e.g., dl and d2 are about a factor of 2 greater than 4y when OTFT is about 30 degrees ar 0.5 radians).
[0059] With continued reference to FIGURES 1OA and 10B, and Equations 7 through 9, it can be seen that the distances from the first and second survey points 177, 177' of the measured well to corresponding points on the target well are expressed mathematically as functions of ~y, TFT1 and TFT2. As described above, TFTl and TFT2 may be determined from magnetic interference emanating from the target well: Dy may typically be determined from conventional survey .data obtained for the measured well and/or from historical survey data for the target well. In one exernplan,~ embodiment of this invention, ~y may be determined from inclination values at the first and second survey points 177, 177' of the measured well and corresponding points on the target well. The inclination values for the measured well may be determined via substantially any known method, such as, for example, via local gravity measurements, as described in more detail below and in the McElhinney patents. The inclination values of the target well are typically known from a historical survey obtained, for example, via gyroscope or other conventional surveying methodologies in combination with known interpolation techniques as required. Such inclination values may be utilized in conjunction. with substantially any known approach, such as minimum curvature, radius of curvature, average angle, and balanced tangential techniques, to determine the relative change in vertical position between the two wells, 0y. Using one such technique, ~y may be expressed mathematically as follows:
O = ~MD sin IncMl + IncM2 _ IncTl + IhcT2 .Y ( ( 2 2 )) Equation 10 [0060] where OMD represents the change in measured depth between the first and second survey points, IncMl and IncM2 represent inclination values for the measured well at the first and second survey points 177, 177', and IncTl and IncT2 represent inclination values for the target well at corresponding first and second points.
[0061] As described above, for many drilling applications in which embodiments of this invention are suitable, magnetic interference from the target well tends to significantly interfere with the determination of the azimuth of the measured well using conventional magnetic surveying techniques. Further, such drilling applications are often carried out in near horizontal wells (e.g., to divert around a portion of a pre-existing borehole that has collapsed). Thus conventional gyroscope and gravity azimuth surveying methods may be less than optimal for borehole surveying in such applications.
As shown above, in Equations 7 through 10, the distances dl and d2 from the measured well to the target well may be determined from TFTI, TFT2, and the inclination values at corresponding points along the measured and target wells. It will be appreciated that Equations 7 through 10 are advantageously independent of the azimuth values of either the measured or target wells. Thus a determination of the azimuth values (or the relative change in azimuth values) is not necessary in the determination of distances dl and d2.
Further, as described in more detail below, the distances dl and d2, along with a historical survey of the target well, may be utilized to determine the coordinates of the first and second survey points 177, 177' and the local azimuth of the measured well.
[0062] It will be appreciated that according to Equations 4 through 9, determination of the distances dl and d2 requires a relative change in the position of the measured well with respect to the target well (e.g., ~x and/or ~y) that results in a measurable change in the tool face to target angle (4TFT) between the first and second survey points 177, 177'.
For certain applications in which the measured well closely parallels the target v~ell it may be desirable to occasionally deviate the path of the measured well with respect to the target well in order to achieve significant changes in tool face to target angles (e.g., OTFT
on the order of a few degrees or more). Such occasional deviation of the path of the measured well may advantageously improve the accuracy of a distance determination between the two wells. For example, in an application in which the measured well is essentially parallel with the target well at a tool face to target angle of about 90 degrees (i.e., the measured well lies to the right of the target well), it may be desirable to occasionally deviate the measured well path upwards and then back downwards with respect to the target well. Such upward and downward deviation of the measured well path may result in measurable ~y and OTFT values that may be advantageously utilized to calculate distance values as described above.

[0063] The artisan of ordinary skill will readily recognize that Equations 4 through 10 may be written in numerous equivalent or similar forms. For example, the definitions of TFT 1 and TFT2 or the signs of 0x and ~y may be modified depending the quadrant in which survey points 177 and 177' reside. In addition, the origin in FIGURES 9 through l0B may be located at one of survey points 177 or 177' rather than at the target well 175.
All such modifications will be understood to be within the scope of this invention.
[0064] With the determination of the direction (i.e., TFT or ~TFT) and the distance, dl or d2, from the measured borehole to the target borehole at various points along the measured borehole it is possible to determine the location (i.e., the absolute coordinates) of those points on the measured borehole based on historical survey data for the target well. The location at survey points 177 and 177' may be given as follows:
PMxl = PTx - d 1 sin(TFT l) PMyl = PTy - dl cos(TF7'1) PMx2 = PTy - d 2 sin(TFT 2) Equation 11 PlLIy2 = PTy - d2 cos(TFT2) [0065] where PMxI and PMyI, represent x and y coordinates at survey point 177, PMx2 and PMy2 represent x and y coordinates at survey point 177', PTx and PTy represent x and y coordinates of the target well 175, dl and d2 represent distances from survey points 177, 177' to the target well 175, and TFTI and TFT2 represent tool face to target values between the survey points 177 and 177' and the target well 175.
It will be appreciated that the coordinates determined in Equation 11 are in a coordinate system looking down the longitudinal axis of the target well. The artisan of ordinary skill will readily be able to convert such coordinates into one or more conventional coordinate systems, including, for example, true north, magnetic north, UTM, and other custom coordinates systems.

[0066] Once the coordinates have been determined at the survey points 177 and 177' in a conventional coordinates system, deterniination of azimuth values for the measured borehole may be derived as follows:
AzM=arctan(Cy2-Cyl) Equation 12 Cx2 - Cxl where AzM represents a local azimuth between survey points 177 and 177' and Cxl, Cx2, Cyl, and Cy2 represent x and y coordinates in a conventional coordinates system at survey points 177 and 177', respectively. Inclination values may be determined, for example, from conventional surveying methodologies, such as via gravity sensor measurements (as described in more detail below).
[0067] It can be seen that embodiments of this invention include a method for drilling a relief well (or a method for twinning a well) that includes utilizing the surveying techniques described herein to guide the drill string (the measured well) along a predetermined course substantially parallel with a target well. For example, as described above, an operator may utilize plots of tool face to target values versus well depth to adjust the vertical component of the drilling direction. Likewise a comparison of the azimuth values for the measured and target wells may be utilized to adjust the azimuthal (lateral) component of the drilling direction. Such a procedure enables the position of a measured well to be determined relative to the target well in substantially real time, thereby enabling the drilling direction to be adjusted to more closely parallel the target well.
[0068] In determining the magnetic interference vectors, tool face to target values, the distance between the measured and target wells, and the azimuth of the measured well, it may be advantageous in certain applications to employ one or more techniques to minimize or eliminate the effect of erroneous data. Several options are available. For example, it may be advantageous to apply statistical methods to eliminate outlying points, for example, removing points that are greater than some predetermined deviation away from a previously measured point. Thus for example, if the distance between two wells is 3 feet at a first survey point, a distance of 23 feet may be rejected at a second survey point. In certain instances it may also be desirable to remove individual interference vectors from the above analysis. For example, an interference vector may be removed when the magnitude of the interference magnetic field vector is less than some minimum threshold (e.g., 0.001 Gauss).
(0069] An alternative, and also optional, technique for minimizing error and reducing the effect of erroneous data is to make multiple magnetic field measurements at each survey station. For example, magnetic field measurements may be made at multiple tool face settings (e.g., at 0, 90, 180, and 270 degrees) at each survey station in the measured well bore. Such rotation of the tool face, while effecting the individual magnetometer readings (i.e.; Bx and By), does not effect the interference magnetic field, the tool face to target, the distance between the two wells, or the azimuth of the measured well.
[0070] With reference again to FIGURE 9 and Equations 4 and S, it was shown above that the distances dl and d2 between the first and second survey points 177, 177' on the measured well and corresponding points on the target well I7S may be expressed mathematically as a function of the tool face to target values TFT l and TFT2 and the relative changes in the horizontal dx and vertical dy positions between the first and second survey points 177, 177' on the measured well and corresponding points on the target well 17S. With reference again to FIGURES 1.0A and 10B and Equations 6 through 8, it was shown that for certain applications in which TFT1 and TFT2 are about m ___ . _. ..._.. ___._...e ~".~.,.~ ..~.. ~.

90 or 270 degrees (e.g., within about 45 degrees thereof) distances dl and d2 may alternatively be expressed mathematically as a function of ~y, TFT1, and TFT2 (i.e., substantially independent of fix). As described above, such an alternative approach advantageously enables dl and d2 to be determined based on the measured TFT
values (TFT1 and TFT2) and inclination values for the measured and target wells (i.e., independent of azimuth values which are sometimes unreliable in regions of magnetic interference). However, it should be noted that this alternative approach is not necessarily suitable for all drilling applications. Rather, for some applications determination of the distances dl and d2 may require knowledge of ~x as described in Equations 4 and 5 and shown in FIGURE 9.
(0071] As described above, both Ox and 8y may be determined from conventional survey data obtained for the measured well and historical survey data for the target well.
While Dy may be determined from inclination values, as shown in Equation 10, ~x may be determined from azimuth values at the first and second survey points 177, 177' of the measured well and corresponding points on the target well. The azimuth values for the measured well may be determined via substantially any known method, such as, for example, via gravity MWD measurements, as described in more detail below and in the McElhinney patents. Azimuth values of the target well are typically known fibm a historical survey obtained, for example, via gyroscope or other conventional surveying methodologies in combination with known interpolation techniques as required.
Such azimuth values may be utilized in conjunction with substantially any known approach, such as minimum curvature, radius of curvature, average angle, and balanced tangential techniques, to determine the relative change in horizontal position between the two wells, fix. Using one such technique, ~x may be expressed mathematically as follows:

~=~(s~{~4ziM12AziM2-AziTl2AziT2)) Eq~honl3 [0072] where ~MD represents the change in measured depth between the first and second survey points, AziMl and AziM2 represent azimuth values for the measured well at the first and second survey points 177, 177', and AziTl and AziT2 represent azimuth values for the target well at corresponding first and second points.
[0073] In certain of the above applications, the intent of the drilling operation may be to position the measured well substantially above or below the target well 175 (FIGURE 9) or to pass over or under the target well 175. As described above, the measured TFT
values for such applications are approximately 0 or 180 degrees (e.g., within about 45 degrees thereof]. It will be appreciated that in such applications relative changes in the vertical position, ~y, between the measured and target wells typically has a minimal effect on the measured TFT values (i.e., results in a relatively small ~TFT
value for a given t1y). As such, for these applications, determination of the distances dl and d2 from survey points 177, 177' of the measured well to corresponding points on the target well 175 may be derived considering only relative changes in the horizontal position, fix, between the measured and target wells.
[0074] With reference now to FIGURE lI, distances dl and d2 may be expressed mathematically with respect to Ox, TFT1, and TFT2 as follows:
dl =
cos(TFTl)[tan(TFT2) - tan(TFTl)]
d2 =- -- ~ Equation 14 cos{TFT 2)[tan(TFT 2) - tan(TFT 1)]
(0075] As described above with respect to Equations 6 through 8, Equation 14 may be expressed alternatively for applications in which the measured well is substantially .._....___._..~.~""~ ~p,._. ~~ .~. m_.,~,~"..-....~~, ~~.-M--- -».e~ ~,...
._~~_..._.__~.~___ parallel with and above or below the target well 175. In such instances, dl and d2 may be approximated as follows:
dl = d2 = ~ Equation 15 OTFT
[0076] where, as described above, ~x represents the relative change in horizontal position between first and second survey points 177, 177' of the measured well and corresponding points on the target well 175 and ~TFT represents the change in TFT value between the first and second survey points 177, 177'. Similar to Equation 9, described above, Equation 1 S advantageously describes the distance (dl and d2) from the measured well to the target well 175 as being substantially proportional to ax and as substantially inversely proportional to the change in tool face to target value aTFT. While, not generally applicable to all well drilling applications (or even to all twinning applications), Equation 15 may be valuable for certain exemplary applications in that it provides relatively simple operational guidance regarding the distance from the measured well to the target well.
(0077] The principles of exemplary embodiments of this invention advantageously provide for planning various well drilling applications, such as well twinning and/or relief well applications, in which a measured well passes within sensory range of magnetic flux of a target well. Such planning may, for example, advantageously provide expected tool face to target values (also referred to as bearing) and distances (also referred to as range) between the measured and target wells as a function of measured depth. With reference to FIGURE 12, one exemplary embodiment of a drilling plan 400 is shown for a hypothetical well twinning operation. The display may include, for example, conventional plan 403 and sectional405 views of the measured 277 and target well 275.

The display may also include, for example, a traveling cylinder view 401 looking down the target well, which is similar to that shown in FIGURES 7, 9, 10A, lOB, and 11, and plots of the tool face to target values 407 and distances 409 from the measured well to the target well.
[0078] At the beginning of the hypothetical operation shown, the measured well is essentially parallel with and to the right of the target well (having a tool face to target angle of about 260 degrees and a distance to the target well of about ten feet at a measured depth of about 15900 feet). The intent of the drilling operation is to remain essentially parallel with the target well for several hundred feet before crossing over and descending down and to the left of the target well. In the exemplary plan shown, the tool face to target value remains essentially unchanged to a measured depth of about 16200 feet. The measured well then builds slightly and crosses over the target well as shown in the traveling cylinder 401. At a measured depth of about 16600 feet the drilling plan has the measured well descending down and to the left away from the target well as shown making a closest approach to the target well at a range (distance) of about three feet at a bearing (TFT) of about 120 degrees. It will be appreciated that the drilling plan and the display shown in FIGURE 12 are merely exemplary and that numerous variations thereof are available within the full scope of the invention. Fox example, displays including inclination, azimuth, and relative changes in the horizontal and vertical position of the measured well relative to the target well may alternatively and/or additionally be shown.
[0079] As described above, exemplary embodiments of this invention may be utilized in drilling twin wells for use in steam assisted gravity drainage applications. In such applications either the upper or lower well may be drilled first using conventional directional drilling techniques. In certain embodiments it may be advantageous to drill the larger diameter upper well first, since the casing string in the larger well tends to produce greater magnetic flux. However, the invention is not limited in this regard, as either the upper or the lower well may be drilled first. After drilling of the first well is complete, it may be cased (lined) using conventional casing joints. For steam assisted gravity drainage applications it is typically desirable to configure at least the horizontal section of the casing string such that it includes a plurality of opposing magnetic poles, for example as shown on FIGURES 4A through 4C. As described above, the use of such casing strings increases the magnetic flux density about the target well and thus increases the range of suitable use of the passive ranging techniques described herein (e.g., up to about 20 meters in some applications).
[0080] In typical steam assisted gravity drainage applications, the well profile is often J-shaped, having a relatively straight/vertical section (e.g., with an inclination of about 30 degrees), followed by a build (e.g., at about 5 degrees per 100 feet) to an essentially straight/horizontal section (having an inclination of about 90 degrees) that may extend up to several thousand feet. After completion of the first well (e.g., drilling and casing a J-shaped upper well), a drilling plan for the lower well is typically refined (e.g., as described above with respect to FIGURE 12). The lower well may then be twinned for example, using conventional directional drilling techniques along with a suitable combination of the passive ranging techniques described herein.
[0081] In one useful twinning operation, the upper portion of the second well (e.g., the lower well) is twinned substantially parallel to and to the left of (or to the right of) the first well at a separation distance of about 2 meters. It will be appreciated that the invention is not limited in this regard: The TFT and distance between the two wells may be determined as described above with respect to Equations 3 through 10. In such an exemplary embodiment the direction of drilling is controlled to maintain a TFT
of about 90 degrees (e.g., as described above in more detail with respect to FIGURE 5) and a separation distance of about 2 meters. Surveys may be conducted at substantially any interval along the length of the well (e.g., 30 meters). Passive ranging and Gravity MWD
techniques may also be utilized to determine azimuth values of the second well as described in more detail herein.
[0082] As the well builds towards horizontal, the second well may be gradually moved below the first well. This may be accomplished by steering the second well with respect to the first well such that the TFT changes from about 90 degrees to about 0 degrees. It will be understood that gradually moving the first well above the second well would change the TFT from about 90 to about 180 degrees. Meanwhile the separation distance between the two wells may be gradually increased from about 2 meters to the desired separation distance (typically in the range of from about 4 to about 10 meters depending upon various reservoir characteristics). In this section of the well, the TFT
and distance may also be determined and controlled as described above with respect to Equations 3 through 10 and 13 through 15.
[0083] In typical steam assisted gravity drainage applications, it is desirable to maintain the lower well at a substantially fixed distance below the upper well in the straight/horizontal section. The distance between the two wells may be determined and controlled as described above with respect to Equations 4 through 6 and 13 through 15.
In certain embodiments, (e.g., those in which the first well includes a preconfigured casing string as described above with respect to FIGURES 4 through 6) the distance between the two wells may also be determined via the magnetic field strength measured at points on the second well. For example, in such embodiments, the magnetic properties _ _~~.~. ~, ..~~~.. x.-~...~. ~.....__ ____.__-._~.

of each casing joint (e.g., magnetic pole location arid strength) may be measured prior to insertion in the casing string. The magnetic properties of each casing joint may then be included in a mathematical model of the casing string to determine theoretical magnetic properties of the casing string (e.g., as shown above in FIGURES SA through 6 which were derived from a finite element model). Measurements of both the direction and strength of the magnetic field in the second well may then be compared to the theoretical magnetic field about the first well in order to determine a separation distance therebetween. For example, as shown in FIGURES SA through 6, a measured magnetic field strength of about 0.008 Gauss at a measured depth of about 2175 meters would indicate a separation distance between the two wells of about 6 meters.
[0084] In certain applications it may be advantageous to survey the second well at positions that are substantially longitudinally aligned with the opposing poles in the casing string of the first well (i.e., at substantially the maximum magnetic field values).
The measured depths of such opposing poles are typically known to a close approximation based on the characteristics of the preconfigured casing string.
Thus, positioning the drill bit (or a particular MWD survey tool such as sensor set 120 shown on FIGURE 1) in close proximity to a particular opposing pole may be straightforward for certain applications. Furthermore, the longitudinal position of the sensor sets relative to the opposing poles on the target well may be determined via measuring the component of the magnetic flux density parallel to the longitudinal axis of the second well (the z direction as shown on FIGURE 2). It will be appreciated that the longitudinal component of the magnetic flux density is substantially zero at the opposing poles and increases to a maximum between two adjacent opposing poles. In this manner any mismatch between the measured depths of the two wells may be accounted. In one advantageous embodiment, the longitudinal component of the magnetic :field may be transmitted uphole in substantially real time during drilling (e.g., via mud pulse telemetry).
Such dynamic surveying enables the relative longitudinal and/or radial position between the two wells to be monitored in real time and thus may enable the sensor sets) to be more accurately positioned adjacent opposing poles on the target well.
[0085] In a typical steam assisted gravity drainage application, it is also desirable that the lower well resided substantially directly below the upper well (i.e., not deviating more than about 1-2 meters to the left or right of the upper well). As described above, the tool face to target angle (TFT) indicates the relative position of the second well with respect to the first well: At a TFT angle of 0 degrees, the second well resides directly below the first well. This orientation may be maintained by controlling the TFT angle within certain predetermined tolerances. Table 1 summarizes exemplary TFT tolerances for separation distances of 4, 6, 8, 10, and 12 meters and left right tolerances of l, 2, and 3 meters, respectively. For example, to maintain a left right tolerance of ~ 1 meter at a separation distance of 6 meters, requires that the TFT angle be controlled within ~ 9 degrees. Likewise, to maintain a left right tolerance of f 2 meters at a separation distance of 6 meters requires that the T'FT angle be controlled within t 19 degrees.

4 meters 6 meters 8 meters 10 meters 12 meters +/-1 meterst 14 degrees~ 9 degrees~ 7 degrees~ 5 degreest 4 degrees +/-2 meters~ 30 degrees+ 19 degrees~ 14 degrees~ 11 degreest 9 degrees +/-3 meters~ 48 degrees+ 30 degrees~ 22 degrees+ 17 degrees~ 14 degrees [0086] Embodiments of this invention may also be utilized in combination with other surveying techniques. For example, in applications in which the inclination of the target well is less than about 80 degrees, gravity azimuth methods (also referred to as gravity MWD), such as those described in the McElhinney patents, may be advantageously used to determine borehole azimuth values in the presence of magnetic interference.
Such gravity MWD techniques are well suited for use with exemplary embodiments of this invention and may be advantageously utilized to determine ~x as described above.
Alternatively andlor additionally, the magnetic field measurements may be utilized to determine magnetic azimuth values via known methods. Such magnetic azimuth values may be advantageously utilized at points along the measured well at which the magnetic interference is low, e.g., near a target well that has been sufficiently demagnetized.
[0087] In a previous commonly-assigned application (U.S. Patent Application Ser. No.
IO/369,353) the applicant discloses methods for determining azimuth via gravity and magnetic field measurements using, for example, MWD tools such as that disclosed in FIGURE 1. Refernng now to FIGURES 2 and 13 (FIGURE 13 is abstracted from U.S.
Patent Application Ser. No. 101369,353), the lower sensor set 120 has been moved with respect to upper sensor set 110 (by bending structure 140) resulting in a change in azimuth (denoted 'delta-azimuth' in FIGURE 13). The following equations show how the foregoing methodology nay be achieved. Note that this analysis assumes that the upper 110 and lower 120 sensor sets are rotationally fixed relative to one another.
(0088] The borehole inclination (Incl and Inc2) may be described at the upper 110 and lower 120 sensor sets, respectively, as follows:
Gxl2 + Gyl2 ~ncl = arctan{ Gzl ) Equation 16 m Inc2 = axctan( Gx22 + Gy22 ) Equation 17 Gz2 where G represents a gravity sensor measurement (such as, for example, a gravity vector measurement), x, y, and z refer to alignment along the x, y, and z axes, respectively, and 1 and 2 refer to the upper 110 and lower 120 sensor sets, respectively.
Thus, for example, Gxl is a gravity sensor measurement aligned along the x-axis taken with the upper sensor set 110. The artisan of ordinary skill will readily recognize that the gravity measurements may be represented in unit vector form, and hence, Gxl, Gyl, etc., represent directional components thereof.
[0089] The borehole azimuth at the lower sensor set 120 may be described as follows:
BoreholeAzimuth= Ref'erenceAzimuth+DeltaAzimuth Equation 18 where the reference azimuth is the azimuth value at the upper sensor set 110 and where:
DeltaAzimuth = Betca Equation 19 1- Sin((Incl + Inc2) l 2) and:
{Gx2 * Gyl - Gy 2 * Gxl) * Gxl * Gyl * Gzl Beta = arctan( ) Equation 20 Gz2*(Gxla+Gyl2)+Gzl*(Gx2*Gxl+Gy2*Gyl) [0090) In other embodiments, Equation 19 may alternatively be expressed as follows:
DeltaAzimuth = -Beta * Cl + Incl ~ Equation 19A
Inc2 [0091] Using the above relationships, a surveying methodology may be established, in which first and second gravity sensor sets (e.g., accelerometer sets) are disposed, for example, in a drill string. As noted above, surveying in this way is known to be serviceable and has been disclosed in U.S. Patent 6,480,119 (the '119 patent).
In order to P

utilize this methodology, however, a directional tie-in, i.e., an azimuthal reference, is required at the start of a survey. The subsequent surveys are then chain referenced to the tie-in reference. For example, if a new survey point (also referred to herein as a survey station) has a delta azimuth of 2.51 degrees, it is conventionally added to the previous survey point (e.g., 183.40 degrees) to give a new azimuth (i.e., borehole azimuth) of 185.91 degrees. A subsequent survey point having a delta azimuth of 1.17 degrees is again added to the previous survey point giving a new azimuth of 187.08 degrees.
[0092] If a new survey point is not exactly the separation distance between the two sensor packages plus the depth of the previous survey point, the prior art recognizes that extrapolation or interpolation may be used to determine the reference azimuth.
However, extrapolation and interpolation techniques risk the introduction of error to the surveying results. These errors may become significant when long reference chains are required.
Thus it is generally preferred to survey at intervals equal to the separation distance between the sensor sets, which tends to increase the time and expense required to perform a reliable survey, especially when the separation distance is relatively small (e.g., about 30 feet). It is therefore desirable to enhance the downhole surveying technique described above with supplemental referencing, thereby reducing (potentially eliminating for some applications) the need for tie-in referencing.
[0093] U.S. Patent Application 10/369,353 discloses method for utilizing supplemental reference data in borehole surveying applications. The supplemental reference data may be in substantially any suitable form, e.g., as provided by one or more magnetometers and/or gyroscopes. With continued reference to FIGURES 2 and 13, in one embodiment, the supplemental reference data are in the form of supplemental magnetometer measurements obtained at the upper sensor set 110. The reference azimuth value at the i upper sensor set 110, may be represented mathematically, utilizing the supplemental magnetometer data, as follows:
(Gxl * Byl - Gy1 * Bxl) * .~Gxla + Gyl2 + Gzl2 Referencel4zimuth = arctan(Bz1 * (Gxl2 + Gyla ) - Gzl * (Gxl * Bxl - Gyl *
Byl) ) Equation2l where Bxl, Byl, and Bzl represent the measured magnetic field readings in the x, y, and z directions, respectively, at the upper sensor set 110 (e.g., via magnetometer readings).
The borehole azimuth at the lower sensor set 120 may thus be represented as follows:
(Gxl * Byl - Gyl * Bxl) * Gxl2 + Gyl2 + Gzl2 BoreholeAzimuth = arctan( ) + ...
Bzl*(Gxl2+Gyl2)-Gz1*(Gxl*Bxl-Gyl*Byl) Beta Eq~,hon 22 1- Sin((Incl + Inc2) l 2) where Beta is given by Equation 20 and Incl and Inc2 are given by Equations 16 and 17, respectively, as described previously.
[0094] It will be appreciated that the above arrangement in which the upper sensor set 110 (FIGURES 1 through 3B) includes a set of magnetometers is merely exemplary.
Magnetometer sets may likewise be disposed at the lower sensor set 120. For some applications, as described in more detail below, it may be advantageous to utilize magnetometer measurements at both the upper 110 and lower 120 sensor sets.
Gyroscopes, or other direction sensing devices, may also be utilized to obtain supplemental reference data at either the upper 110 or lower 120 sensor sets.
[0095] It will also be appreciated that the above discussion relates to the generalized case in which each sensor set provides three gravity vector measurements, i.e., in the x, y, and z directions. However, it will also be appreciated that it is possible to take only two gravity vector measurements, such as, for example, in the x and y directions only, and to solve for the third vector using existing knowledge of the total gravitational field in the area. Likewise, in the absence of magnetic interference, it is possible to take only two magnetic field measurements and to solve for the third using existing knowledge of the total magnetic field in the area.
[0096] While the passive ranging techniques described herein require only a single magnetometer set (e.g., located at the upper sensor set as ixi the above example), it will be appreciated that passive ranging may be further enhanced via the use of a second set of magnetometers (i.e., a first set of magnetometers at the upper sensor set and a second set of magnetometers at the lower sensor set). The use of two sets of magnetometers, along with the associated accelerometers, typically improves data density (i.e., more survey points per unit length of the measured well), as shown in the examples described above, reduces the time required to gather passive ranging vector data, increases the quality assurance of the generated data, and builds in redundancy.
[0097] It will be understood that the aspects and features of the present invention may be embodied as logic that may be represented as instructions processed by, for example, a computer, a microprocessor, hardware, firmware, programmable circuitry, or any other processing device well known in the art. Similarly the logic may be embodied on software suitable to be executed by a processor, as is also well known in the art. The invention is not limited in this regard. The software, firmware, and/or processing device may be included, for example, on a down hole assembly in. the form of a circuit board, on board a sensor sub, or MWDfLWD sub. Alternatively the processing system may be at the surface and configured to process data sent to the surface by sensor sets via a telemetry or data link system also well known in the art. Electronic information such as logic, software, or measured or processed data may be stored in memory (volatile or non-volatile), or on conventional electronic data storage devices such as are well known in the art.
[0098] The sensors and sensor sets referred to herein, such as accelerometers and magnetometers, are presently preferred to be chosen from among commercially available sensor devices that are well known in the art. Suitable accelerometer packages for use in service as disclosed herein include, for example, Part Number 979-0273-041 commercially available from Honeywell, and Part Number JA-SH175-1 commercially available from Japan Aviation Electronics Industry, Ltd. (JAE). Suitable magnetometer packages are commercially available called out by name from MicroTesla, Ltd., or under the brand name Tensor (TM) by Reuter Stokes, Inc. It will be understood that the foregoing commercial sensor packages are identified by way of example only, and that the invention is not limited to any particular deployment of commercially available sensors.
[0099] Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A method for surveying a measured borehole, the method comprising:
(a) providing a casing string in a target borehole, the casing string including a plurality of opposing magnetic poles;

(b) providing a downhole tool including first and second magnetic field measurement devices disposed at corresponding first and second positions in the measured borehole, the first and second positions selected to be within sensory range of magnetic flux from the casing string;

(c) measuring local magnetic fields at the first and second positions using the corresponding first and second magnetic field measurement devices;
(d) processing (1) the local magnetic fields at the first and second positions, and (2) a reference magnetic field, to determine a portion of the local magnetic fields attributable to the casing string;

(e) generating interference magnetic field vectors at the first and second positions from the portion of the local magnetic fields attributable to the casing string;
and (f) processing the interference magnetic field vectors to determine a tool face to target angle at each of the first and second positions, the tool face to target angles representing a corresponding direction from each of the first and second positions to the target subterranean structure.

2. A method for drilling a first borehole along a predetermined course relative to a second borehole, the method comprising:
(a) providing a casing string having a plurality of opposing magnetic poles in the second borehole;
(b) providing a downhole tool including a magnetic field measurement device disposed at a first position in the first borehole, the first position selected to be within sensory range of magnetic flux from the casing string;
(c) measuring a local magnetic field at the first position using the magnetic field measurement device;
(d) processing (1) the local magnetic field at the first position, and (2) a reference magnetic field, to determine a portion of the local magnetic field attributable to the casing string;
(e) generating an interference magnetic field vector at the first position from the portion of the local magnetic field attributable to the casing string;
(f) processing the interference magnetic field vector to determine a tool face to target angle at the first position;
(g) processing the tool face to target angle at the first position to determine a direction for subsequent drilling of the first borehole; and (h) drilling the first borehole along the direction for subsequent drilling determined in (g) such that the downhole tool is repositioned at a second position in the first borehole, the second position remaining within sensory range of magnetic flux from the second borehole; and (i) repeating (c), (d), (e), (f), (g), and (h).