GB2243693A

GB2243693A - Guiding a tool along a subterranean path

Info

Publication number: GB2243693A
Application number: GB9109116A
Authority: GB
Inventors: Nicholas Peter Wallis; Anne Holmes
Original assignee: Baroid Technology Inc
Current assignee: Baroid Technology Inc
Priority date: 1990-05-04
Filing date: 1991-04-26
Publication date: 1991-11-06
Also published as: NO911740D0; GB9010096D0; GB9109116D0; FR2661712A1; ITMI911219A0; NO911740L; IT1248782B; ITMI911219A1; NL9100772A; DE4114784A1; CA2041622A1

Abstract

In order to guide a tool along a subterranean path 6 close to the surface, as is required to cross a river 7 for example, the tool includes a magnetic detector and a transmitter loop 10, 12 is positioned as a reference for the tool at the surface. In one measurement phase, a direct current is caused to flow in one direction in the loop 10, 12 and the resulting magnetic field in the vicinity of the tool is detected by means of the magnetic detector. In another measurement phase, a direct current is caused to flow in the opposite direction in the loop 10, 12 and the resulting magnetic field in the vicinity of the tool is detected by means of the magnetic detector. Processing techniques are used to determine positional information using the detected magnetic field values and tool orientation data, and steering of the tool may be effected as required on the basis of such positional information. The technique permits guiding of a tool along a subterranean path with high accuracy and without requiring non-standard detection and processing circuitry. <IMAGE>

Description

"Improvements in or Relating to Guiding of a Tool along a Subterranean Path" This invention relates to a method and apparatus for guiding a tool along a subterranean path close to the surface, for example for drilling of a subterranean passage beneath a river or other obstacle.

It is known to drill a passage under a river using a so-called magnetic steering tool which includes a drill head which is oriented so that the drilling direction is dependent on the tool face angle, that is on the highside angle or roll angle of the tool about its axis relative to a reference. The steering of the tool is controlled by varying the tool face angle, generally under operator control, in dependence on measurements made by magnetic or gravitational detectors on the tool indicative of the direction of drilling relative to the earth's magnetic field and the tool attitude. Examples of such magnetic steering tools are disclosed in U.K. Patent Specification No. 1342475 and U.S. Patent Specification No. 3791043.

However, in the vicinity of a typical river crossing, there are many sources of magnetic interference, such as underground pipes, which will tend to lead to misleading magnetic measurements resulting in inaccurate determination of the azimuth angle and inaccurate steering of the tool. Furthermore the magnetic and gravitational detectors must be close to the drilling bit to establish the required directional (azimuth and inclination) control, and the magnetic measurements will therefore be affected by the magnetic material of the bit. Furthermore experience has shown that, if the passage being drilled is to emerge close to the intended target, it is essential that it should be possible to accurately determine the current position of the tool and to accurately plan the required direction (azimuth and inclination) to target, in the course of the drilling run.

It is known from U.S. Patent Specification No.

4710708 to track the position of a probe incorporating a special receiver by using the receiver to detect the alternating magnetic field transmitted by a remotely located transmitter. However it is not possible to utilise such a technique using existing steering tools.

Furthermore it is known from US Patent Specification No. 4875014 to track the position of a magnetic steering tool by sensing by means of a magnetic detector of the tool a magnetic field generated by a current flowing in a conductive loop made up of straight segments placed on the surface of the ground above the proposed path of the borehole. However accurate positioning of such straight segments may prove difficult, particularly on uneven ground, and may involve complex surveying techniques.

It is an object of the invention to provide a technique for accurately guiding a tool along a subterranean path with high accuracy and without requiring non-standard detection and processing circuitry.

According to the present invention there is provided a method of guiding a tool along a subterranean path close to the surface, wherein the tool includes a magnetic detector and wherein a transmitter loop is positioned as a reference for the tool at the surface, which method comprises, in one measurement phase, causing a direct current to flow in one direction in the loop and detecting the magnetic field in the vicinity of the tool in said one measurement phase by means of the magnetic detector, and, in another measurement phase, causing a direct current to flow in the opposite direction in the loop and detecting the magnetic field in the vicinity of the tool in said other measurement phase by means of the magnetic detector, and steering the tool in dependence on the results of these measurements.

The flow of current in the loop in one direction will generate a magnetic field in one sense superimposed on the earth's magnetic field as sensed by the magnetic detector, and the flow of current in the loop in the other direction will generate a magnetic field in the opposite sense superimposed on the earth's magnetic field, so that the difference between the magnetic measurements made in the two measurement phases will provide an indication of the magnetic field due to the current in the loop (from which the earth's magnetic field has been eliminated). Straightforward processing techniques can be used to determine positional information using the detected magnetic field values.

In this method it is preferred that at least a portion of the transmitter loop is in the form of a regular curve. This renders it easier to lay the loop on uneven ground than would be the case if the loop consisted of a series of straight segments. Thus, for example, if the loop comprises at least one portion in the form of an arc of a circle, it will be appreciated that the or each arcuate portion can be positioned accurately by use of a guide element fixed at a point corresponding to the centre of the circle. Conveniently the loop comprises a semicircular portion and a further, rectilinear portion lying along the diameter of the semicircle.

Other layouts of the loop are possible within the scope of the invention. For example, the loop may be substantially in the form of an ellipse, a circle, a parabola or a hyperbola, or a part thereof. An ellipse may be laid out in known manner using a guide element of a predetermined length fixed at its two ends at the foci of the ellipse.

In a preferred method, in a further measurement phase, preferably carried out between the two aforementioned measurement phases, no current flows in the loop and the magnetic field in the vicinity of the tool in said further measurement phase is detected by means of the magnetic detector.

Furthermore it is preferred that the tool includes a gravitational detector which detects the earth's gravitational field in the vicinity of the tool, and that tool orientation data, such as the highside angle, the inclination angle and the azimuth angle, is determined from measurements made by the magnetic detector and the gravitational detector in said further measurement phase. These measurements will generally comprise magnetic and gravitational field components along three mutually orthogonal axes, one of which is the tool axis.

An indication of tool position relative to the loop may be obtained by a calculation method based on relating the magnetic measurements made in said one and said other measurement phases may be related to the expected magnetic field at the tool position due to the current in the loop by means of transformation equations utilising the highside angle, the inclination angle and the azimuth angle. Positional information relating to the current position of the tool and the target position of the tool may then be used to produce a tool orientation signal for steering the tool.

The invention also provides apparatus for guiding a tool along a subterranean path close to the surface, wherein the tool includes a magnetic detector, the apparatus comprising a transmitter loop to be positioned as a reference for the tool at the surface, current supply means for causing a direct current to flow in one direction in the loop in one measurement phase and for causing a direct current to flow in the other direction in the loop in another measurement phase, and control means for detecting the magnetic field in the vicinity of the tool by means of the magnetic detector in each of said measurement phases and for steering the tool in dependence on the results of these measurements.

In order that the invention may be more fully understood, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is an explanatory diagram illustrative of a technique in accordance with the invention; Figure 2 is a diagram showing the practical implementation of a development of the technique; and Figure 3 is a further explanatory diagram.

A technique for guiding a conventional steering tool, such as is described in U.S. Patent Specification No. 3791043, along a subterranean path 5 close to the surface will now be described with reference to the explanatory diagram of Figure 1. This technique utilizes a rectangular wire loop or grid 1 of known dimensions, namely of length 21 and width 2w, to be placed around or near the required exit point 2 as a target for the tool, and is described first by way of general explanation of the approach behind the present invention, as the use of a rectangular grid is simplest to describe.

The rectangular grid 1 is supported at at least its corners by support posts (not shown) so as to lie in a substantially horizontal plane above the surface of the ground. Furthermore a direct current electrical power source (not shown) is connected to the grid 1 by means of a change-over switch, and the following series of measurements is carried out at intervals of about 10m along the path 5 followed by the steering tool during drilling which extends from an entry point 3 to the exit point 2.

At each measurement station the tool is stopped so that measurements may be taken by means of three-axis fluxgates and three-axis accelerometers in three measurement phases, as follows: 1. The change-over switch is operated to cause a direct current of a predetermined magnitude to be passed through the grid 1 in a direction determined by the position of the change-over switch in a first measurement phase, and magnetic field measurements are taken along three mutually perpendicular tool-fixed axes by means of the fluxgates.

2. The change-over switch is then positioned so that no current flows through the grid 1 in a second measurement phase, and magnetic and gravitational field measurements are taken along the three mutually perpendicular tool-fixed axes by means of the fluxgates and the accelerometers.

3. The change-over switch is then positioned so that a direct current of the same predetermined magnitude is passed through the grid 1 in the opposite direction in a third measurement phase, and magnetic field measurements are taken along the three mutually perpendicular tool-fixed axes by means of the fluxgates.

Each of the three measurement phases occurs only once whilst the tool is maintained stationary at a measurement location, and each measurement phase is initiated by suitable positioning of the change-over switch so that there are three distinct current levels in the grid 1 in the three phases. Furthermore it will be appreciated that the background magnetic field, that is the earth's magnetic field, can be eliminated by subtracting the magnetic field measurements taken in the third measurement phase from the magnetic field measurements taken in the first measurement phase, so as to determine the components of the magnetic field in the vicinity of the tool due solely to the current in the loop. This has the additional advantage of smoothing some of the noise in the fluxgate measurements and of highlighting any obvious errors in the measurements.If there is any significant difference in the magnitude of the magnetic field measurements taken in the first measurement phase and in the third measurement phase, then additional fluxgate measurements may need to be taken.

The position of the tool at the measurement location may be determined by a technique which relates the magnetic measurements taken by the fluxgates, and particularly the readings due to the current in the loop (from which the effect of the earth's magnetic field has been eliminated), to the calculated values of the magnetic field at the measurement location due to the current in the loop determined from the grid dimensions, the current magnitude and the position of the measurement location relative to the grid. This is done by converting the magnetic field readings related to tool-fixed axes to values related to grid-fixed axes by resolution through the highside angle , the inclination angle e and the azimuth angle e, which are the angles by which the toolfixed axes rotate into the grid-fixed axes, as described, for example, in U.K.Patent Specification No. 1578053.

The highside, inclination and azimuth angles are determined from the magnetic field and gravitational field measurements taken in the second measurement phase in known manner, for example as described in U.S. Patent Specification No. 3791043. Any errors in these angles will result in a corresponding error in the final calculated tool position.

The final tool position in the measurement location is calculated from. the magnetic readings converted to grid-fixed axes by an iterative method which requires an initial estimate of tool position, as described more fully below with reference to the mathematical basis of the method.

Conversion of Magnetic Readings to Grid-Fixed Axes The magnetic readings due to the current in the loop in the tool-fixed axes (xt, yt, zt) can be transformed to the grid-fixed axes (xg, yg, zg), in which the zg axis is perpendicular to the grid xg, yg plane of Figure 1, using the following equations bxg = bXt.(sificosecosb+cos sins) byt. (sinsscosesin-cos cos#) + bzt.sinçsine ... (1) byg = bxt.(cos#cos#cos#-sin sin#) byt (cos#cos#sin#+sin cos#) + bzt.cos#sin# ... (2) bzg = - bxt.sin#cos# + byt.sin#sin# + bzt.cos# ... (3) where # = #t - #g where bxt, byt, bzt are the magnetic readings due to the effect of the current in the loop in the tool-fixed axes bxg, byg, bzg are the same magnetic readings transformed to the grid-fixed axes t is the tool azimuth g is the grid azimuth The equations assume that the loop is in the horizontal plane. Grid axes for an inclined plane loop are determined by additional resolutions through the loop inclination.

Calculation of Tool Position Furthermore the direction and magnitude of the magnetic field due to the current in the loop is a function of the tool position (x, y, z) in grid-fixed axes relative to the centre of the grid.

With reference to figure 3, using the Biot Savart law dB = dI x iQp (3a) 4# rQp where is the earth's relative permeability dI is a current element rQp is the distance from dI to the measurement location iQp is the unit vector in the direction QP If 2w is the width of the grid 21 is the length of the grid I is the current magnitude and the parameters a, b, c, d, m1, m2, m3, m4, m5, m6, m7, m8, p are defined by the following equalities: a=w+x b=w-x c=l+y d=l-y

= = 4g * 10-7 where all distances are expressed in metres the magnetic field (Bx, By, Bz) at the measurement location (x, y, z) in the grid axes is defined by the equations: Bx = - z (-m1-m2+m3+m4) * 106 (in units of T) (4) 4 By = 1 z (m5+m6-m7-m8) * i06 (in units of T) (5) 4 Bz = I {a(m1+m2)+b(m3+m4)+c(m5+m6)+d(m7+m8)} * 106 4 (in units of T) (6) In order to determine the tool position the above equations must be solved for x, y and z.

Since these equations are non-linear an iterative method must be used. One such method is based on the Taylor series extended to 3 variables, that is: f(x,y,z)= f(xo,yo,zo)+(x-xo)df+(y-yo)df+(z-zo)df+ ...

dx dy dz By this method bxg = Bx + #x dBx + #y dBx + #z dBx (7) dx dy dz byg = By + x dBy + by dBy + sz dBy (8) dx dy dz bzg = Bz + sx dBz + #y dBz + (z dBz (9) dx dy dz where Bx, By, Bz are obtained by substituting (x, y, z) in equations (4), (5) and (6).

These are solved iteratively for (dx,dy, sz) until the values of (x, y, z) converge (that is sx, sy,

using any standard method for the solution of linear equations.

Using this iterative technique the solution generally converges quickly, that is within 4 to 8 iterations.

The total differential matrix is obtained by differentiating equations (4), (5) and (6) with respect to x, y and z. This can be done either numerically or analytically. Numerical differentiation can be prone to inaccuracies since it generally involves division by a small number. Differentiating the equations analytically is not as complex as would first appear, since the equations are all basically of similar form, that is

However, an initial study into the two methods of differentiation produced no significant differences between the two techniques.

The values calculated for x, y, z are based on the centre of the grid. Distances from the entry point 3 can also be determined provided that the grid position relative to the entry point 3 is known.

Figure 2 shows a practical implementation of the technique for drilling a subterranean passage 6 under a river 7 from an entry point 8 to an exit point 9. In this technique a first horizontal semicircular grid 10 is positioned on the entry side of the river 7 between the entry point 8 and the river 7 with its longitudinal axis 11 directed towards the target. Furthermore a second horizontal semicircular grid 12 is placed on the exit side of the river 7 between the river 7 and the exit point 9 with its longitudinal axis 13 directed towards the exit point 9. If desired two or more grids may be provided on either side of the river at intervals along the intended path of the subterranean passage. Furthermore each grid 10 or 12 is provided with a respective direct current supply 14 or 15 and a respective change-over switch 16 or 17.

In this technique the magnetic steering tool is guided along the intended path of the subterranean passage from the entry point 8 towards a target in the vicinity of the intended exit point 9 by stopping the tool every 10 m and conducting the measurements described above at each measurement location. For each series of three measurement phases at a particular measurement location the appropriate currents are caused to flow in an appropriate one of the grids 10 and 12. Thus, when the passage is being drilled on the entry side of the river 7, the currents are caused to flow in the grid 10, whereas, when the passage is being drilled on the exit side of the river 7, the currents are caused to flow in the grid 12.

Each series of measurements may be used generally in the manner already described to determine an updated position for the tool and this may be used by the direction drilling engineer to control the tool face in accordance with the drilling direction determined by the current tool position relative to the intended target.

However, in each case, the relevant equations to determine the tool position (x, y, z) will differ to take account of the semicircular shape of the relevant grid, as opposed to the rectangular shape previously described.

Using equation (3a), the magnetic field at position P (x, y, z) can be calculated from

In another embodiment the or each grid has the shape of an ellipse, in which case the magnetic field at the position P can be calculated from

where E denotes the ellipse with semi axes a and b, and where dI = -a sine. do. 1x + b cosy. do. ;; rQp = A + R - 2AR cos# A = a cos# + b sin# R2 = X2 + y2 + z2 The above-described techniques provide accurate determination of tool position with consequent fine control of tool orientation and steering using conventional magnetic steering tools in which the detection circuitry is housed in a short non-magnetic drill collar close to the drill bit. Furthermore such determination of tool position can be made even when the tool lies outside the projection of the area of the loop in the plane of the tool.

It may also be advantageous in certain circumstances to modify the techniques described above with reference to the drawings. Thus, instead of a substantially constant direct current being supplied to the transmitter loop, a current may be supplied which varies with respect to time in a predetermined way, either cyclically or in a steadily increasing or decreasing manner. If an alternating current is supplied to the loop, there is no need to arrange for the current to be supplied in opposite directions in two separate measurement phases by operation of a change-over switch.

The method of the invention enables transmitter loops of a wide variety of shapes incorporating curves to be used provided that, by application of a series of simple linear measurements, it is possible to calculate an equation defining the curve. This can be readily implemented in a field system using computational techniques, and is more cost effective than lengthy traditional surveying methods. The technique allows rapid calculation of the x, y and z coordinates whilst enabling a cross-check to be made with the bit depth measured by conventional techniques.

In the technique of the invention the transmitter loop does not have to be located in a horizontal plane, since an initial set up procedure is used with the tool positioned at the surface within or above the loop in order to determine the roll and pitch of the loop, as well as the yaw with respect to magnetic North. With the tool initially positioned at the centre of the loop at a known distance from the plane of the loop, magnetic readings due to the current in the loop are taken in the manner already described, and the scale factor relating the measured magnetic field due to the loop current to the expected magnetic field is determined from the known tool position. The tool is then positioned at two or more further known positions within or above the loop and further magnetic readings are taken which, by use of the equations relating tool position to measured readings, may be used to determine the pitch and roll of the loop in an iterative process. The yaw of the loop may be determined by aligning the tool parallel to the plane of the loop and along the central axis of the loop and again relating the magnetic readings to the known tool position for four equiangularly spaced roll positions of the tool. No direct measurement of the current in the loop is necessary.

Claims

1. A method of guiding a tool along a subterranean path close to the surface, wherein the tool includes a magnetic detector and wherein a transmitter loop is positioned as a reference for the tool at the surface, which method comprises, in one measurement phase, causing a direct current to flow in one direction in the loop and detecting the magnetic field in the vicinity of the tool in said one measurement phase by means of the magnetic detector, and, in another measurement phase, causing a direct current to flow in the opposite direction in the loop and detecting the magnetic field in the vicinity of the tool in said other measurement phase by means of the magnetic detector, and steering the tool in dependence on the results of these measurements.

2. A method according to claim 1, wherein the transmitter loop comprises at least one portion in the form of a curve.

3. A method according to claim 2, wherein the transmitter loop comprises a portion substantially in the form of a semicircle, and a further, rectilinear portion lying along a diameter of said semicircle.

4. A method according to claim 2, wherein the transmitter loop comprises a portion substantially in the form of an ellipse.

5. A method according to any preceding claim, wherein, in a further measurement phase, no current flows in the loop and the magnetic field in the vicinity of the tool in said further measurement phase is detected by means of the magnetic detector.

6. A method according to claim 5, wherein said further measurement phase is intermediate said one measurement phase and said other measurement phase.

7. A method according to claim 5 or 6, wherein the tool includes a gravitational detector which detects the earth's gravitational field in the vicinity of the tool, and wherein tool orientation data is determined from measurements made by the magnetic detector and the gravitational detector in said further measurement phase.

8. A method according to claim 7, wherein an indication of tool position relative to the loop is obtained by a calculation based on relating the magnetic measurements made in said one and said other measurement phases to the expected magnetic field at the tool position due to the current in the loop by means of transformation equations utilising tool orientation data, such as the highside angle.

9. A method according to any preceding claim, wherein positional information relating to the current position of the tool and the target position of the tool is used to produce a tool orientation signal for steering the tool.

10. Apparatus for guiding a tool along a subterranean path close to the surface, wherein the tool includes a magnetic detector, the apparatus comprising a transmitter loop to be positioned as a reference for the tool at the surface, current supply means for causing a direct current to flow in one direction in the loop in one measurement phase and for causing a direct current to flow in the other direction in the loop in another measurement phase, and control means for detecting the magnetic field in the vicinity of the tool by means of the magnetic detector in each of said measurement phases and for steering the tool in dependence on the results of these measurements.

11. A method of guiding a tool along a subterranean path close to the surface, the method being substantially as hereinbefore described with reference to the accompanying drawings.

12. Apparatus for guiding a tool along a subterranean path close to the surface, the apparatus being substantially as hereinbefore described with reference to the accompanying drawings.