GB2254430A

GB2254430A - Drilling apparatus

Info

Publication number: GB2254430A
Application number: GB9107122A
Authority: GB
Inventors: Anthony William King; Andrew Malcolm Westwood Hay
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1991-04-05
Filing date: 1991-04-05
Publication date: 1992-10-07
Also published as: GB9107122D0

Abstract

Drilling apparatus for enabling a relief well to be drilled to intercept an existing well comprises various means used in succession to guide the drill head to the existing well. Initially guidance is by inertial navigation using a gyroscope connected for drift by a fluxgate magnetometer. A pair of fluxgate magnetometers, one sensitive to the field due to well casing 4 and the other correcting for geomagnetism, then guides the head to within a few meters of the target well. A system of search coils and, finally, a high frequency acoustic transducer take over for the close approach. <IMAGE>

Description

DRILLING APPARATUS This invention relates to drilling apparatus and in particular but not exclusively to apparatus for drilling a well to intercept an existing well.

It is sometimes required to drill a well, hereinafter referred to as "the relief well", to intercept an existing well, hereinafter referred to as "the target well". For example if the existing well is an oil well and the well head has been damaged or is on fire it may be that no access can be gained to that well head. In this case, the only means of controlling the flow from the well is by accessing the well below ground level by means of a relief well and then using any of various methods to stop the flow of oil through the target well, for example by pumping heavy mud into the target well via the relief well.

The major problem encountered in drilling a relief well is ensuring that it intercepts the target well. This is particularly difficult as no well is ever exactly vertical and so the exact position of the well at a point below ground is unlikely to correspond to the well head position. For many modern wells accurate charts of the position of the well are recorded when the well is drilled, but for older wells such charts, if they every existed, are often lost or inaccurate.

One solution proposed for intersecting a target well consists of locating a magnetometer in the target well and a magnet in the drill head in order to deduce in which direction it is required for the drill to be steered in order to intercept the target well. A variation of this is to produce an AC current in the steel casing of the target well and use a magnetometer in the relief well. Both these methods rely on gaining access to the target well and can only be used to guide the drill over a distance from about 70 metres from the target well, to within about 3 metres.

The relief well drill head has previously been guided towards the estimated position of the target well by means of an inertial guidance system comprising an arrangement of gyroscopes and accelerometers. However as drilling is a considerably time consuming process, the gyroscopes are subject to drift. To overcome this problem it is necessary to stop drilling and lower a gyroscope and accelerometer in the form of an inertial navigation device from the relief well head to the bottom of the drill string in order to obtain the position of the drill bit, and then to remove the device before continuing drilling. This slows down the drilling process and does not provide a continuous tracking of the drill head.

According to a first aspect of the present invention there is provided drilling apparatus comprising an inertial navigation system including a gyroscope, a fluxgate magnetometer and means for calculating drift in the gyroscope from values of the earths magnetic field received from the magnetometer. This enables the accuracies achievable with a gyroscope to be used to determine rates and duration of turn, while the fluxgate magnetometer enables drift in the gyroscope with time to be compensated for by providing an output dependant upon the direction of the earths magnetic field relative to the magnetometer.

According to a second aspect of the invention there is provided drilling apparatus comprising two fluxgate magnetometers adapted to be positioned in a drill string, one in close proximity to the lower end of the drill string, and the other remote from the said lower end, and means for determining the difference between the output of the two magnetometers. Both magnetometers will be influenced by the strong earths magnetic field but only the one at the lower end will be influenced by local magnetic field close to the drill head. The difference between the values obtained from each magnetometer enables a weak field relative to the earths magnetic field to be determined close to the drill head. This enables the drill to be guided by means of the weak magnetic field that is generated by any large metallic object, and in particular by a steel casing of the target well.

According to a third aspect of the invention there is provided drilling apparatus comprising a first core for inducing a magnetic field at the lower end of a drill string and means for detecting the strength of the induced field.

The inductance of the coil will be effected by any metal object in close proximity to the core and thereby provides a means of detecting metal objects in close proximity to the drill bit. Preferably the coil is elliptical and the axis of the coil is offset from the axis of the lower end of the drill string such that as the drill string is rotated, the axis through the coil will scan a cone in front of the drill string. By determining the position of the drill string in relation to any peak in signal strength the direction of the metal object relative to the drill head can be determined.

Advantageously there is provided a second coil remote from the first coil for determining the induced field strength and preferably two coils are provided which are electrically connected to each other and positioned on different sides of the axis of the first coil. Any difference between the fields induced in these further coils indicates the direction of a metallic object relative to the plane perpendicular to the two coils and parallel to the axis of the first coil. Advantageously four coils are arranged substantially equidistant to each other about the axis of the first coil each opposite pair of coils being electrically connected and arranged substantially perpendicular to the coils of the other pair, enabling the two dimensional direction relative to the drill head of any metal object to be determined.

According to a fourth aspect of the invention there is provided drilling apparatus comprising an acoustic transducer adapted to be positioned at the lower end of a drill string for transmitting and receiving high frequency signals. This enables the shape of a target well casing to be determined and any defects, for example corrosion or fractures, in the well casing to be detected. It may also be possible to determine the material on the other side of the casing. Preferably the axis of the acoustic transducer is offset from the axis of the drill string and therefore as the drill approaches the casing the transducer will scan the surface of the casing in a inwardly spiralling manner.

According to a fifth aspect of the invention there is provided drilling apparatus comprising a communication cord for communicating data from inside a drill sting to the surface, the communication cord being wound on a bobbin from which it may be dispensed. This enables the bobbin and the cord to be passed through pipes about to be connected to the drill string so that the drill string can be continually extended without the need for connecting lengths of communication cord together.

The invention will now be described by way of example only with reference to the accompanying Figures of which; Figure 1 illustrates the components of a drilling system in accordance with the present invention; Figure 2 illustrates the regions in which different navigation apparatus is used when drilling a relief well to intersect an existing well; Figure 2a is a table of the distances from an existing well over which the different navigation methods of Figure 2 can be applied; Figure 3 illustrates the principle of operation of a fluxgate magnetometer guidance method in accordance with the present invention; Figure 4 illustrates the principle of operation of a magnetic search coil in accordance with the present invention; Figure 5 schematically illustrates an alternative search coil arrangement;; Figure 6 schematically illustrates the principle of searching with an acoustic transducer; and Figure 7 is a cross section of apparatus for transmitting signals from a rotating drill string to the surface.

Referring to Figure 1 there is illustrated the components of a drilling system to drill a relief well to intercept an oil well 4, the well head of which is inaccessible due to fire. The system comprises a control truck 1 for controlling drilling equipment 2. A drill string 3 extends from drilling equipment 2 towards a target well 4 and comprises telemetry equipment 5, navigation sensors 6, and a direction control mechanism 7 for the cutting head 8. The drilling equipment 2 may either be a water jet system or conventional drilling system using a substantially horizontal derrick or a conventional derrick.

A conventional derrick has the disadvantage that a deeper well is required before the target well 4 can be intercepted.

Figures 2 and 2a illustrate the region over which an inertial navigation system is used to guide the cutting head to the approximate location of the well. This uses approximate known co-ordinates of the well. The inertial navigation system comprises a fluxgate magnetometer which detects the direction of the earths magnetic field and compensates for gyroscope drift during the drilling process.

Once the cutting head is in close proximity to the target well 4 the magnetic field which is created by the large steel casing of the well is detected by an arrangement of fluxgate magnetometers, guide the cutting head to within a few meters of the target well. This operation is more fully explained below with reference to Figure 3. Search coils are then employed which can guide the drill head to within 0.3 metres of the target well, the operation of which is explained below with reference to Figures 4 and 5.

The last stage of the cutting operation is guided by a high frequency acoustic transducer described below with reference to Figure 6.

Referring to Figure 3 there is seen two fluxgate magnetometers 10 and 11 in the drill string 3. Both are subject to the strong earths magnetic field, but in addition magnetometer 11 is subject to the magnetic field generated by the casing of the target well 4. By subtracting the values obtained from the magnetometer 10 from those received from the magnetometer 11 a value is obtained which is equivalent to the effect of the casing of the well 4 on magnetometer 11. Each magnetometer detects components of force in the X and Y directions as indicated. Once the X and Y components of the earths magnetic field have been subtracted from the values received from the magnetometer 11, the remaining components in the X and Y direction will indicate the direction in which the target well 4 lies relative to the magnetometer 11.As the drilling process continues, and the magnetometers are moved to the positions indicated by the broken lines the X and Y, components of the magnetic field due to the well change. This provides a second bearing to the well enabling the position of the well to be determined and the cutting head directed to that position.

When within a few meter of the well a search coil as illustrated in Figure 4 is deployed at the front of the drill string. This comprises an electromagnetic coil 20 which is energised by DC or AC electric field and generates a magnetic field as indicated. The inductance of the coil will vary as the well is approached as any metallic object within the region X will alter the inductance of the arrangement. Sensing coils 21 and 22 are connected in opposite. sense so that in the absence of any target object they will give a zero output. As a target object is approached the magnetic field will be deflected causing a potential difference to be generated by the two coils which can be detected.As the axis of the coil 20 is offset from the axis of the drill string 3, as the drill string is rotated the axis of the coil 20 will rotate to the position indicated by the broken line sweeping out a cone. Any metallic object located to one side of the axis of the drill string will cause the output from the coils 21, 22 to peak as the drill string is rotated, enabling the position of the metallic object or the well casing to be deduced.

Figure 5 illustrates an alternative arrangement of search coil. The axis of the search coil 30 is co-linear to the axis of the drill string 3. Two pairs of sensing coils 31 and 32, 33 and 34 are equidistantly spaced about the axis of the coil 30 each pair again being wound in opposite sense and electrically connected. Any metallic object in front of the drill string will cause the magnetic field to deviate resulting in a potential difference being generated between respective pairs of coils. This again enables the direction of the metallic object affecting the field to be determined.

Once the cutting head has been guided to the target well an acoustic transducer 40 is located at the lower end of the drill string 3 as illustrated in Figure 6. This produces a pencil beam 41 offset from the axis of the drill string. As the drill string is rotated this pencil beam scans the casing of the well 4 in a inwardly spiralling manner as the drill string proceeds towards the casing. The transducer comprises a peizoelectric device producing a signal having a frequency of 2 GHz. The signal is reflected by the casing of the well 4 and is detected by the transducer 40 and enables information to be ascertained as to the shape position and thickness of the casing, as well as indicating whether the casing is fractured or corroded.

In addition it may be possible to determine the material within the casing and therefore to determine whether the casing is an outer casing of the well or whether the casing is actually filled with oil.

The signals are conveyed from the various navigational apparatus to the telemetry unit 5 in Figure 1. The signal from the telemetry unit is transmitted via cable 50 depicted in Figure 7. This cable 50 is dispensed from the inside of a bobbin 51 which is passed through new sections of the pipe before they are connected to the drill string. Once the section of pipe 52 has been connected to the drill string 3 the bobbin 51 is connected to a mounting structure 53 attached to the top of the section of pipe 52. The bobbin comprises a coil 54 to which the cable is electrically connected.

During drilling it is necessary to pass water or water/mud mixtures to the drill head via the pipe 52. This is achieved by attaching a housing 55 over the pipe 52 to which it is sealed by means of seal 56. The housing 55 contains a support structure 57 comprising another coil 58.

When the housing 55 is connected to the pipe 52 the coil 58 is in close proximity to the coil 54 of the bobbin 51. The coil 58 is connected to the control truck of Figure 1 and the coils 54 and 58 provide an inductive coupling which permits signals to be transmitted between the truck and the rotating drill string without any fixed connection means. Although an inductive coupling has above been described, it will be realised that by modifying the circuitry and replacing the coils with conductive plates the signals can be transmitted by means of a capacitive coupling.

Claims

1. Drilling apparatus comprising an inertial navigation system including a gyroscope, a fluxgate magnetometer and means for calculating drift in the gyroscope from values of the earths magnetic field received from the magnetometer.

2. Drilling apparatus comprising two fluxgate magnetometers adapted to be positioned in a drill string, one in close proximity to the lower end of the drill sting, and the other remote from the said lower end, and means for determining the difference between the output of the two magnetometers.

3. Drilling apparatus as claimed in claim 1 comprising two fluxgate magnetometers adapted to be positioned in a drill string, one in close proximity to the lower end of the drill sting, and the other remote from the said lower end, and means for determining the difference between the output of the two magnetometers.

4. Drilling apparatus comprising a first coil for inducing a magnetic field at the lower end of a drill b. a fluxgate magnetometer, c. electromagnetic coils, d. a high frequency acoustic transducer.

20. A method of drilling a well substantially as hereinbefore described with reference to any one of the accompanying drawings.

surface, the communication cord being wound on a bobbin from which it may be dispensed.

15. Apparatus as claimed in claims 13 or 14 comprising a rotatable joint including an inductive coupling.

16. Drilling apparatus substantially as hereinbefore described with reference to any one of the accompanying drawings.

17. A method of drilling a relief well comprising sensing a magnetic field generated by the steel casing of a target well and guiding the relief well drill in dependence on the sensed magnetic field.

18. A method of drilling a relief well comprising the use of drilling apparatus in accordance with any preceding claim.

19. A method of drilling a relief well to intersect a target well comprising guiding the drill head by three of the following means used in succession: a. a inertial navigation system, electrically connected, and arranged substantially perpendicular to the coils of the other pair.

10. Drilling apparatus comprising an acoustic transducer adapted to be positioned at the lower end of a drill string for transmitting and receiving high frequency signals.

11. Drilling apparatus as claimed in any one of claims 1 to 9 comprising an acoustic transducer adapted to be positioned at the lower end of a drill string for transmitting and receiving high frequency signals.

12. Drilling apparatus as claimed in claims 10 or 11 wherein the axis of the acoustic transducer is offset from the axis of the drill string.

13. Drilling apparatus comprising a communication cord for communicating data from inside a drill string to the surface, the communication cord being wound on a bobbin from which it may be dispensed.

14. Drilling apparatus as claimed in any one of claims 1 to 12 comprising a communication cord for communicating data from inside a drill string to the string, and means for detecting the strength of the induced field.

5. Drilling apparatus as claimed in claims 1, 2 or 3 comprising a first coil for inducing a magnetic field at the lower end of a drill string, and means for detecting the strength of the induced field.

6. Drilling apparatus as claimed in claim 4 or 5 wherein the coil is elliptical and the axis of coil is offset from the axis of the lower end of the drill string.

7. Drilling apparatus as claimed in claims 4, 5 or 6 comprising a second coil remote from the first coil for determining the induced field strength.

8. Drilling apparatus as claimed in any one of claims 4 to 7 wherein the field strength detecting means comprises two coils electrically connected to each other and positioned on different sides of the axis of the first coil.

9. Drilling apparatus as claimed in any one of claims 4 to 8 wherein the field strength detecting means comprises four coils arranged substantially equidistant about the axis of the first coil, each opposite pair of coils being