CA2929935A1 - Use of independent measurements in magnetic ranging - Google Patents

Abstract

Methods for obtaining multiple independent magnetic ranging measurements during a well twinning operation are disclosed. The methods may include using a magnetic ranging tool having first and second axially spaced solenoids. The methods may alternatively and/or additionally include using a magnetic ranging tool having an electromagnet to calibrate magnetized casing. The methods may further include measuring the axial component of a magnetic field internal to a magnetized casing string after deployment of the string in a target well.

Description

USE OF INDEPENDENT MEASUREMENTS IN MAGNETIC RANGING
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent Application Serial No.
61/903,205, filed November 12, 2013. The entirety of this provisional patent application is incorporated by reference herein.
FIELD OF THE INVENTION

[0002] Disclosed embodiments relate generally to drilling and surveying subterranean boreholes such as for use in oil and natural gas exploration and more particularly to methods for improving the accuracy of the relative location of two wells via making multiple independent magnetic ranging measurements between a twin well and a target well in a well twinning operation.
BACKGROUND INFORMATION

[0003] Magnetic ranging techniques are commonly utilized in twin well drilling applications. For example, in steam assisted gravity drainage (SAGD) applications twin horizontal wells having a predetermined, consistent vertical separation distance, commonly in the range from about 4 to about 10 meters, are drilled. During production steam is injected into the upper well to heat the tar sand. The heated heavy oil contained in the tar sand and condensed steam is then recovered from the lower well. The success of such heavy oil recovery techniques is often dependent upon successfully drilling precisely positioned twin wells having a predetermined relative spacing in the horizontal injection/production zone.
Positioning the wells either too close or too far apart may severely limit production, or even result in no production, from the lower well.

[0004] Several magnetic ranging techniques have been used in SAGD well twinning operations. In general these techniques involve deploying a magnetic target (source) in one well and sensing the magnetic field emanating from the target in the other well. The intensity and direction of the measured field is then used to compute a distance and direction to the target. SAGD ranging operations have made use of various magnetic sources including active sources such as an AC solenoid or a DC electromagnet, and passive sources such as permanently magnetized casing. Active source methodology is designed to implicitly remove the effects of the earth field while explicit techniques are used for passive ranging methods.
While these techniques have been used with varying degrees of success, there is room for further improvement.

[0005] For example, when using an AC solenoid, the magnetic field is attenuated by the casing in the target well. The extent of the attenuation is not generally known with precision and can be influenced by many factors including the electrical and magnetic properties of the casing, the casing geometry including the thickness and the slot configuration, and the relative position of the AC solenoid in the casing. As such the calculated distance to the target is prone to error.

[0006] When using a DC electromagnet, multiple measurements are made at different source excitation states. Errors may arise if the magnetic sensors or the electromagnet move between acquisitions corresponding to different excitation states or if the data acquisition times are not correctly synchronized with respect to the excitation states.
Errors can also result when using either an AC solenoid or a DC electromagnet from undetected hardware failure such as current leakage due to worn or damaged insulation.

[0007] When using magnetized casing, the distance between the wells may be in error if the intensity of the casing magnetization differs from expectation. Moreover, all known ranging techniques are subject to the risk of errors caused by human failure.

[0008] Therefore a need remains for a method to reduce errors during well twinning operations and to verify well placement during drilling such that the well trajectory may be corrected in real time.
SUMMARY

[0009] A method for magnetic ranging includes deploying a drill string having a magnetic field sensor in a drilling well and a magnetic ranging tool having at least first and second axially spaced electromagnets in a target well. A first magnetic field measurement is made when the first and second electromagnets are de-energized. A second magnetic field measurement is made when the first electromagnet is energized and the second electromagnet is de-energized. A third magnetic field measurement is made when the first electromagnet is de-energized and the second electromagnet is energized. The magnetic field measurements are processed to compute (i) a distance and/or a direction from the drilling well to the target well and (ii) a quality parameter.

[0010] A method for magnetic ranging and calibrating a premagnetized casing string includes deploying a drill string having a magnetic field sensor in a drilling well and a magnetic ranging tool having at least one electromagnet in a target well. The target well further includes a premagnetized casing string deployed therein. A first magnetic field measurement is made when the electromagnet is de-energized and a second magnetic field measurement is made when the electromagnet is energized. The first and second magnetic field measurements are then processed to compute a calibration factor for the magnetized casing string.

[0011] A method for computing magnetic pole strength for magnetized casing includes magnetizing a casing string and deploying the magnetized casing string in a target well. An axial component of a magnetic field internal to the magnetized casing string is measured and processed to compute the magnetic pole strength of the magnetized casing string.

[0012] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.
BRIEF DESCRIPTION OF THE DRAWINGS

[0013] For a more complete understanding of the disclosed subject matter, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0014] FIGS. lA and 1B depict prior art arrangements for a well twinning operation.

[0015] FIG. 2 depicts a magnetic ranging tool having first and second electromagnets (solenoids).

[0016] FIGS. 3A and 3B depict flow charts of related methods of magnetic ranging with DC magnetic source.

[0017] FIG. 4 depicts a flow chart of a method for calibrating pre-magnetized casing.

[0018] FIG. 5 depicts a flow chart of a method for computing a pole strength of pre-magnetized casing.

[0019] FIG. 6 depicts a running tool for running casing into a wellbore.
DETAILED DESCRIPTION

[0020] FIG. lA schematically depicts one example of a well twinning operation such as a SAGD twinning operation. Common SAGD twinning operations include drilling a horizontal twin well 20 a substantially fixed distance substantially directly above a horizontal portion of the target well 30 (e.g., not deviating more than about 1-2 meters up or down or to the left or right of the lower well). In the depicted embodiment, the lower (target) well 30 is drilled first, e.g., near the bottom of the oil-bearing formation, using conventional directional drilling and MWD techniques. However, the disclosed embodiments are not limited in regard to which of the wells is drilled first. In the method shown on FIG. 1A, a high strength electromagnet 34 is positioned in the cased target well 30 while drilling the upper (twin) well 20. The electromagnet 34 may be positioned in the cased target well 30 via, for example, a tractor 32, coiled tubing (not shown), or on the end of a drill string (not shown). An MWD
tool 26 (including a tri-axial magnetic field sensor) deployed in the drill string 24 near drill bit 22 measures the magnitude and direction of the magnetic field. The solenoid may be energized with direct current (DC) or alternating current (AC). If the excitation is DC, a minimum of two measurements are required, the measurements corresponding to different excitation states. The difference between these two measurements is independent of any constant background magnetic field. The magnetic field measurements are processed to estimate the separation distance between the two wells and a direction from the target well 30 to the twin well 20 as described in U.S. Patent 4,710,708 and U.S. Reissue Patent RE36,569 (each of which is fully incorporated by reference herein).

[0021] FIG. 1B schematically depicts another example of a well twinning operation. As in the example depicted on FIG. 1A, the lower (target) well 30 is drilled first using conventional directional drilling and MWD techniques. The lower wellbore 30 is then cased using a plurality of premagnetized tubulars to form a magnetized casing string 35. In the embodiment shown, drill string 24 includes at least one tri-axial magnetic field measurement sensor 28 deployed in close proximity to the drill bit 22. Sensor 28 is used to measure the magnetic field about target well 30 as the twin well is drilled. Such measurements of the passive magnetic field are then utilized to compute the distance and direction to the target well 30 and to guide continued drilling of the twin well 20 along a predetermined path relative to the target well 30 (e.g., as described in U.S. Patents 7,617,049, 7,656,161, and 8,026,722, each of which is fully incorporated by reference herein).

[0022] FIG. 2 depicts a magnetic ranging tool 60 for use in a magnetic ranging operation such as depicted on FIGS. lA and 1B. Magnetic ranging tool 60 includes first and second axially spaced solenoids (electromagnets) 64 and 66 deployed in a tool body 62. Each solenoid 64 and 66 includes a solenoid winding wound about a substantially cylindrical magnetic core. The solenoids 64 and 66 may be configured to be energized in such a manner as to distinguish the magnetic field from each solenoid (e.g., the first solenoid 64 may be energized while the second solenoid 66 is de-energized and visa a versa or the solenoids may be energized with AC at different frequencies). The solenoids 64 and 66 are aligned with the tool axis 65 (and are therefore intended to be aligned with the borehole axis when deployed in a target well) and have an axial separation distance d.

[0023] The distance and direction between the twin well and target well may be computed independent of the measured magnetic field intensity. U.S. Patent 8,063,641 (which is incorporated by reference herein in its entirety) discloses a method by which the direction of the measured magnetic field induced by a first solenoid, the direction of the measured magnetic field induced by a second solenoid, and the axial distance between the first and second solenoids may be used to compute the distance and direction between a twin well and a target well without relying on magnetic field strengths.

[0024] FIG. 3A depicts a flow chart of a possible sequence of measurements demonstrating one method 100 of magnetic ranging using magnetic ranging tool 60. Method 100 includes making a first magnetic field measurement when both solenoids 64 and 66 are de-energized (off) at 102. A second magnetic field measurement is made at 104 when the first solenoid 64 is energized (on) and the second solenoid 66 is de-energized. A third magnetic field measurement is made at 106 when the first solenoid 64 is de-energized and the second solenoid 66 is energized. The magnetic field measurements are processed at 108 to compute a distance and/or a direction to the target well and a quality parameter.
Alternatively an AC
source may be used, in which case the need for a de-energized solenoid measurement is eliminated. Applicants note that numbering of the steps has been added for the purpose of annotating that a magnetic field measurement corresponds to a set of conditions to which the first and second solenoid are subjected and such numbering is not intended to suggest the order in which such magnetic field measurements must be taken. Indeed, such magnetic field measurements may be taken in any order.

[0025] FIG. 3B depicts a flow chart of another possible sequence of measurements demonstrating a second method 120 of magnetic ranging using magnetic ranging tool 60. A
first magnetic field measurement is made at 124 when the first solenoid 64 is energized with a first polarity (e.g., positive) and the second solenoid 66 is de-energized. A
second magnetic field measurement is made at 126 when the first solenoid 64 is energized with a second polarity (e.g., negative) and the second solenoid 66 is de-energized. A third magnetic field measurement is made at 128 when the first solenoid 64 is de-energized and the second solenoid 66 is energized with a first polarity (e.g., positive). A fourth magnetic field measurement is made at 130 when the first solenoid 64 is de-energized and the second solenoid 66 is energized with a second polarity (e.g., negative). Method 120 may include another, fifth, magnetic field measurement when both solenoids 64 and 66 are de-energized (off) at 122. The magnetic field measurements are processed at 132 to compute a distance and/or a direction to the target well and a quality parameter. Applicants note that numbering of the steps has been added for the purpose of annotating that a magnetic field measurement corresponds to a set of conditions to which the first and second solenoid are subjected and such numbering is not intended to suggest the order in which such magnetic field measurements must be taken. Indeed, such magnetic field measurements may be taken in any order.

[0026]
Methods 100 and 120 permit the distance and direction from the twin well to the target well to be computed by at least three independent ranging measurements.
For example, the intensity and direction of the measured magnetic field when the first solenoid is energized and the second solenoid is de-energized may be processed to provide a first estimate of the distance and direction to the target well. The intensity and direction of the measured magnetic field when the second solenoid is energized and the first solenoid is de-energized may be processed to provide a second independent estimate of the distance and direction to the target well. In addition, the method of U.S. Patent 8,063,641 provides a third independent estimate of the distance and direction to the target well. Since the magnetic sources are independent (two distinct solenoids), they therefore provide a quality check on errors which may be present in only one of the sources, such as source placement under a casing coupling or insulation failure causing a partial short-circuit the solenoid. 1

[0027] All the independent measurements described above have an associated level of uncertainty. These uncertainties may be estimated, for example, from the noise levels of the various magnetic field measurements as well as from other systematic factors associated with each of the measurements. When multiple methods are utilized (as described above), the discrepancies between computed distances and directions may be compared with the estimates of the corresponding uncertainties. Such comparisons may be used to further compute a joint quality parameter, a single maximum-likelihood distance and direction, and/or an estimate of an improved uncertainty associated with the maximum-likelihood distance and direction.

[0028] In one embodiment, two independent measurements of distance D1 and D2 have corresponding uncertainties described by variances 4 and I. A maximum-likelihood distance Dm', may be estimated, for example as follows:
D cr2 +D cr2 Dmi, = 1 22 22 1 Equation 1 0-1+0-2

[0029] A corresponding joint variance o-LL may be estimated, for example as follows:

,_ 2 _ al 62 'JAIL ¨ 2 2 Equation 2 al +62

[0030]
Moreover, differences between two or more calculated ranging distances may be used as a quality check. For example, if one or more of the differences exceeds a predetermined threshold such as two or three standard deviations, it may be concluded that the accuracy of at least one of the measurements is compromised. A numerical estimate of confidence in the result may be derived using a statistical test of significance, such as a two-sample t-test. If the results fail to meet the required confidence, corrective action may be taken immediately, before the well is placed incorrectly.

[0031] The following example of a positional uncertainty computation is based on an approximation by a dipole with moment m under the assumption that the two wells are parallel. Radial and axial components Br and Bz of the measured magnetic field may be expressed, for example, as follows:
3 mrz Br = ____________________________________________________________________ Equation 3 (r2 +z2)25 r2-2z2 B = _____________________________________________________________________ Equation 4 z (7.2 +z2 )2.5

[0032]
When Br and Bz are measured during a ranging operation, the position vector (r, z) may be found by inverting Equations 3 and 4. It may also be noted that in practice the problem is three-dimensional, and that a third magnetometer measurement is obtained by which the circumferential direction to the source (tool face to target) may be ascertained.

[0033]
Sensitivity to position or source strength may be found by differentiating the dipole equations given in Equations 3 and 4, for example, as follows:
SBr 3mz(z2-4r2) ¨ = ____________________________________ Equation 5 Sr (r2+z2)3 5 SBr 3mr(r2-4z2) ¨ = ____________________________________ Equation 6 Sz (r2+z2)3 5 SBr-3rz ¨ ______________________________________________________________________ Equation 7 sm(7.2+ z2)2.5 SBz-3mr(4z2-r2) Equation 8 Sr (õ2+z2)3.5 SBz-3mz(2z2-3r2) ¨Sz Equation 9 (r2+z2)3.5 SBz¨(r2-2z2) ¨ ______________________________________________________________________ Equation 10 sm (õ2+z2)2.5

[0034] By assuming small errors and using linearized equations:
SBr SBr SBr eBr = er ¨sr + ez em¨sm Equation 11 SBz SBz SBz eBz = er ¨sr + e-- +
Equation 12 where eB, and eBz represent the errors in the 13, and Bz measurements. The position errors er and ez may also be expressed, for example, as follows:
SBr SBz _, (8Br 8Bz 8Bz 8Br) eBzo-eBro-erno'n2.6-er = 8Br.8Bz_8Br.8Bz Equation 13 8z Sr Sr 8z SBr 813z 18Br SBz SBz 8Br) eBz or-eBr _ em or) ez = 8Br.8Bz_8Br.8Bz Equation 14 Sr 8z 8z Sr

[0035] The sensitivities of the calculated radial distance er to various measurement errors, may be expressed, for example, as follows:
SBz er(Br) 8z ¨ =
Equation 15 eB, 8z Sr Sr 8z SBr er(Bz) 8z ¨ =
eBz Equation 16 T, 2_ T, z 8z Sr Sr 8z SBr SBz 8Bz8Br er (m)8m 8z 8m 8z em = Equation 17 8,3 8,3 8,3 8,3 8z Sr Sr 8z

[0036] Equations 15-17 provide a means by which the measurement errors eB, and eBz and a source strength error (or a source strength modelling error) em may be transformed into corresponding position errors er and ez. The position errors may be used to compute a covariance matrix for each independent error source with the overall uncertainty being expressed in terms of the sum of these covariance matrices.

[0037]
When ranging by using the directions of fields (U.S. Patent 8,063,641) received from two transmitters, the response may be estimate in terms of tangents of the flux angles, for example using dipole approximations as follows:
Br 2 t1 = ¨= _________________________________________________________________ Equation 18 Br, 3rzi Bz2 r2-24 t2 = ¨= _________________________________________________________________ Equation 19 /3,2 3rz2 where z1 and z2 represent the axial positions of the magnetic field sensor with respect to the first and second solenoids.

[0038] In this configuration, the dipole equations may be inverted explicitly, for example, as follows:
4dz r = _____________________________________________________________________ Equation 20 3[ti-sign(mBr1)lq_ t2+sign(ni3,2),1q+1 dz[t2+sign(mBrj\lq +
Z1 = ____________________________________________________________________ Equation 21 t1+sign(mBr1),Iq+&-t2-sign(mBr2)\1q+&
where dz is the known spacing between the solenoids (i.e., z2 ¨ z1). In this example, the radial position uncertainty may be found via differentiation, for example, as follows:
er(ti) -3r2 1-sign(mBr1)'ti 1 =
Equation 22 4dz er(t2) 3r2 1-sign(mBr2)'t2 ¨ = ____________________________________________________________________ Equation 23 t2 4dzt22 +-98 er(dz) 4 -dz = 8 ____________ 8 Equation 24 t1-Sigil(MBri) \I ti -F-t2 +Sigil(MBr2 ) \It2 -F,

[0039]
FIG. 4 depicts a flow chart of a method 150 for magnetic ranging and calibrating a magnetized casing string such as depicted on FIG. 1B. While the magnetic intensity of the casing string may be measured at the surface, there is generally no verification of the intensity after the casing is deployed in the target well. Method 150 is intended to verify the magnetic intensity and/or provide a calibration factor for the magnetic intensity of the casing string. At 152 a magnetic ranging tool including at least one electromagnet (e.g., ranging tool 60 depicted on FIG. 2) is deployed in a target wellbore having a pre-magnetized casing string. A
first magnetic field measurement is made at 154 when the electromagnet(s) in the ranging tool are de-energized (off). This is used in to estimate the target location using a passive ranging with magnetized casing methodology. One or more active ranging field measurements are made at 156 using the methodologies described above. The active and passive ranging distances are compared and are then processed at 158 to compute a calibration factor for the passive ranging method. The calibration factor may include, for example, a correction factor for the magnetic pole strength of the magnetized casing such that the calibrated pole strength yields the same distance and direction as that obtained using the electromagnetic.

[0040] It will be understood that method 150 may be performed at multiple locations in the target well so as to obtain multiple correction factors. After obtaining a calibration factor (or factors) the remainder of the twin well may be drilled ranging only to the premagnetized casing. Moreover, if similar calibration factors are obtained for several wells in a given project (e.g., on a given pad) it may be possible to omit the verification/calibration procedure on subsequent wells.

[0041]
FIG. 5 depicts a flow chart of a method 180 for calibrating a magnetized casing string for use in subsequent magnetic ranging operations. The casing string may be magnetized at 182, for example, as described in U.S. Patent 7,538,650, which is incorporated by reference in its entirety herein. The external magnetic field is characterized along with at least one axial magnetic field measurement internal to the magnetized casing.
The casing string may be deployed in the target well (or a portion of the target well) at 184. After deployment in the target well, at least one magnetic field measurement is made internal to the magnetized casing string at 186. The magnetic field measurement may include multiple magnetic field measurements made at multiple axial positions in the magnetized casing string.
An axial component of the internal magnetic field at 182 and 186 is processed at 188 to compute a calibrated magnetic pole strength of the magnetized casing. The computed magnetic pole strength may then be used in subsequent magnetic ranging measurements to compute at least a distance between a twin well and the magnetized target well.

[0042] FIG. 6 depicts a running tool 200 (such as the Schlumberger Long Reach Running Tool) configured for running wellbore casing into a previously drilled wellbore. The running tool may be fitted with an extension tube 210 including an axial magnetometer 215. Running tool 200 may be used to run magnetized casing into a target wellbore using conventional means and to measure the axial component of the magnetic field internal to the magnetized casing, for example, when drawing the running tool back out of the target well. Such measurements may be made with minimum disturbance to rig activities. The running tool 200 may be configured to record the axial component of the magnetic field with time while drawing the tool back out of the target well. The recorded times may then be converted to depths and the magnetic field versus depth profile may be used to compute one or more magnetic pole strengths of the magnetized casing string. The use of running tool 200 and method 180 obviates the need to access the twin and target wells simultaneously. Moreover, no wireline tools are required.

[0043] With reference again to FIGS 2, 3A, and 3B, it will be understood that the described ranging measurements are not limited to the use of multiple solenoid transmitters and a single magnetometer receiver to provide independent estimates of distance and direction.
Independent distance and direction measurements may also be obtained using multiple axially spaced magnetometer receivers in combination with a magnetic ranging tool having a single solenoid transmitter. Likewise, independent distance and direction measurements may also be obtained using a combination including multiple solenoid transmitters and multiple magnetometer receivers. The analysis provided above is substantially independent of whether multiple solenoids and/or multiple axially spaced magnetometers are utilized.
Hence, the disclosed embodiments are not limited in this regard.

[0044] Although the use of independent measurements in magnetic ranging certain advantages thereof 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 disclosure as defined by the appended claims.

Claims (15)

What is claimed is:

1. A method for magnetic ranging comprising:
(a) deploying a drill string in a drilling well, the drill string including at least one magnetic field sensor;
(b) deploying a magnetic ranging tool in a target well, the magnetic ranging tool including first and second axially spaced electromagnets;
(c) making a first magnetic field measurement with the magnetic field sensor when the first and second electromagnets are de-energized;
(d) making a second magnetic field measurement with the magnetic field sensor when the first electromagnet is energized and the second electromagnet is de-energized;
(e) making a third magnetic field measurement with the magnetic field sensor when the first electromagnet is de-energized and the second electromagnet is energized; and (f) processing the first, second, and third magnetic field measurements to compute (i) a distance and/or a direction from the drilling well to the target well and (ii) a quality parameter.

2. The method of claim 1, wherein processing the second magnetic field measurement comprises calculating a first estimate of the distance and/or direction from the drilling well to the target well.

3. The method of claim 2, wherein processing the third magnetic field measurement comprises calculating a second estimate of the distance and/or direction from the drilling well to the target well.

4. The method of claim 1 further comprising:
estimating an uncertainty for each magnetic field measurement.

5. The method of claim 4, further comprising:
comparing each of the computed distances and/or directions to determine discrepancies between computed distances and directions;

comparing the determined discrepancies with the estimates of corresponding uncertainties.

6. The method of claim 5 further comprising:
computing at least one item from the list consisting of: a joint quality parameter, a single maximum-likelihood distance and direction, and an estimate of an improved uncertainty associated with the maximum-likelihood distance and direction.

7. A method for magnetic ranging and calibrating a premagnetized casing string comprises :
(a) deploying a drill string in a drilling well, the drill string including at least one magnetic field sensor;
(b) deploying a magnetic ranging tool in a target well, the magnetic ranging tool having at least one electromagnet, the target well being cased with a premagnetized casing string;
(c) making a first magnetic field measurement with the magnetic field sensor when the electromagnet is de-energized.
(d) making a second magnetic field measurement with the magnetic field sensor when the electromagnet is energized; and (e) processing the first and second magnetic field measurements to compute a calibration factor for the magnetized casing string.

8. The method of claim 7, wherein the calibration factor computed includes a correction factor for a magnetic pole strength of the magnetized casing.

9. The method of claim 7 further comprising repeating (c), (d), and (e) to obtain a plurality of calibration factors for the magnetic casing string.

10. A method for calibrating a magnetized casing string comprising:
(a) magnetizing a casing string;
(b) deploying the magnetized casing in a target well;
(c) measuring an axial component of a magnetic field internal to the magnetized casing string as deployed in (b); and (d) processing the axial component of the magnetic field measured in (c) to compute a magnetic pole strength of the magnetized casing string.

11. A method for magnetic ranging comprising:
(a) deploying a drill string in a drilling well, the drill string including at least one magnetic field sensor;
(b) deploying a magnetic ranging tool in a target well, the magnetic ranging tool including first and second axially spaced electromagnets;
(c) making a first magnetic field measurement with the magnetic field sensor when the first electromagnet is energized with a first polarity and the second electromagnet is de-energized;
(d) making a second magnetic field measurement with the magnetic field sensor when the first electromagnet is energized with a second polarity and the second electromagnet is de-energized;
(e) making a third magnetic field measurement with the magnetic field sensor when the first electromagnet is de-energized and the second electromagnet is energized with a first polarity;
(f) making a fourth magnetic field measurement with the magnetic field sensor when the first electromagnet is de-energized and the second electromagnet is energized with a second polarity; and (g) processing the first, second, third, and fourth magnetic field measurements to compute (i) a distance and/or a direction from the drilling well to the target well and (ii) a quality parameter.

12. The method of claim 11, further comprising:
making a fifth magnetic field measurement with the magnetic field sensor when the first electromagnet is de-energized and the second electromagnet is de-energized.

13. The method of claim 11, further comprising:
estimating an uncertainty for each magnetic field measurement.

14. The method of claim 13, further comprising:

comparing each of the computed distances and/or directions to determine discrepancies between computed distances and directions;
comparing the determined discrepancies with the estimates of corresponding uncertainties.

15. The method of claim 14 further comprising:
computing at least one item from the list consisting of: a joint quality parameter, a single maximum-likelihood distance and direction, and an estimate of an improved uncertainty associated with the maximum-likelihood distance and direction.