IE46359B1 - Improvements in or relating to instruments for measuring the orientation of a borehole - Google Patents

Description

This invention relates to instruments for measuring the direction of a borehole either continuously or at a series of stations along its length.

A spatial survey of the path of a borehole 5 is usually derived from a series of values of an azimuth angle and an inclination angle. Measurements from which values of these two angles can be derived are made at successive stations along the path, the distances betv/een adjacent stations being accurately known.

In a borehole where the earth's magnetic field is unchanged by the presence of the borehole Itself, measurements of the components of the earth's gravitational and magnetic fields in the direction of the case-fixed axes can be used to obtain values for the azimuth angle and the inclination angle, the azimuth angle being measured with respect to an earthfixed magnetic reference, for example magnetic north. However, in situations where the earth's magnetic field is modified by the local conditions in a borehole, for example when the borehole is cased with a steel lining, magnetic measurements can no longer.be used to determine an azimuth angle relative to an earth-fixed reference.

In these circumstances, it is necessary to use a gyroscopic instrument.

It has already been proposed to use a gyroscopic compass in which the spin axis is set up along an earth-fixed reference line at the mouth of the borehole’and, so far as possible, held fixed in inertial space. However, this procedure has many disadvantages, largely due to the necessity of constructing such an instrument to operate within a - 2 46359 narrow bore tube. The size of the gyro rotor, mounted with its axis across the tube, is severely limited and makes satisfactory precession drift rates very difficult to attain in practice since gimbal bearing friction must be very low to compensate for the lack of gyrospin inertia. The usual problems associated with gimbal geometry are also encountered when this type of instrument is used.

According to the invention in one aspect, there is provided an instrument for measuring the direction of a borehole comprising a case having a longitudinal axis coincident, in use, with the axis of the borehole, a single-degree-of-freedom gyro comprising an outer gimbal mounted in the case with its axis coincident with the longitudinal axis thereof, an inner gimbal mounted in the outer gimbal with its axis perpendicular to the outer gimbal axis, a gyro rotor mounted in the inner gimbal, means for sensing angular movement of the inner gimbal relative to the outer gimbal and means for applying a torque to the outer gimbal for rotating it in use about its axis so that the inner gimbal precesses back to its initial position, means for measuring the angle of rotation of the case about its longitudinal axis relative to the outer gimbal and a gravity sensor unit for measuring three components of gravity in three non-coplanar directions.

According to the invention in another aspect, there is provided a method of surveying a borehole comprising moving a survey instrument along the borehole, said survey instrument comprising a case having a longitudinal axis coincident with the axis of the borehole, a single-degree-of-freedom gyro comprising an outer gimbal - 3 _J463S8 . ' mounted in the case with Its axis coincident with the longitudinal axis thereof, an inner gimbal mounted in the outer gimbal with its axis perpendicular to the outer gimbal axis, a gyro rotor mounted in the inner gimbal; continually sensing angular movement of the Inner gimbal relative to the outer gimbal as the instrument moves along the borehole and applying a torque to, the outer gimbal to rotate it about its axis so that the inner gimbal precesses back to its initial position; determining at each of a. series of survey stations spaced along the borehole a set of three components‘ of gravity in three non-coplanar directions relative to the outer gimbal; calculating, from the sensed components of gravity at each of the said survey stations, the inclination of the borehole and the non-rotative high-side angle of the instrument relative to a reference direction which does not rotate about the longitudinal axis of the instrument as it travels along the borehole at each station; and calculating, from said inclination and non-rotative high-side angle, the azimuth of the bore at each station.

The use of a gyro-stabilised single.axis platform has the advantage that, since a torque is , 'applied to the outer gimbal, friction at its bearings is not critical. In the case of the inner gimbal, where hearing friction is critical, angular movement is restricted to small values. This increases the range of techniques which can be used in bearing design. For example, the inner gimbal may be floated within the outer gimbal and ligaments used for power transmission to the driving motor for the rotor.

Ah embodiment of the invention will now be /-/./.-. // - 4 46359 described by way of example, with reference to the accompanying drawings, in which:Figure 1 is a schematic perspective view of an Instrument in accordance with the invention, Figure 2 is a schematic perspective view illustrating a transformation between two sets of reference axes.

Figures 5 to 5 are diagrams illustrating, in two dimensions, the various stages of the transformation shown in Figure 2, Figure 6 is a diagram showing the effect of rotating the instrument shown in Figure 1 about its axis, Figure 7 is a block diagram of the information storage section of the instrument shown in Figure 1, Figure 8 Is a block diagram of the surface information processing equipment for use with the instrument shown in Figures 1 and 7, and Figure 9 is a block diagram of an alternative form of information storage section for a down-hole instrument similar to that shown in Figure 1.

Figure 1 shows an instrument in accordance with the invention having a cylindrical casing 10.

A gyro rotor 12 is mounted in a pair of gimbals 14 and 16, outer gimbal 16 having an axis coincident with that of the housing. The inner gimbal 14 has low friction bearings 18. allowing only a limited amount of angular movement. A position pick-off sensor 20 is arranged to provide an error signal indicating departure of the inner gimbal 14 from orthogonality with the outer gimbal 16.

The error signal from the position pick-off - 5 463 5 9 sensor 20 of the inner gimbal 14 is used to control a torque motor 22 which is coupled to the shaft 24 of the outer gimbal 16 and arranged to apply a torque to rotate the outer gimbal 16 so that the inner gimbal 14 precesses back to orthogonality with the outer gimbal 16.

The outer gimbal shaft 24 also has a resolver 26 mounted, thereon. The resolver 26 has a stator comprising a pair of coils with their axes , orthogonal to one another and a rotor with a corresponding pair of mutually orthogonal coils. The coils of the I rotor are magnetically coupled to those of the stator.

A reference signal is applied to one of the coils of the rotor and the other coil is grounded. Then, if the outputs from the two coils of the stator are a and b respectively, then a/b is equal to the tangent of the angle between the rotor and the stator, i.e. the angle between a reference directional the housing perpendicular to its axis and a corresponding reference direction on the outer gimbal 16.

The instrument also incorporates a gravity sensor unit 28 comprising three gravity sensors mounted on the outer gimbal and arranged to sense components of gravity gxt, g^, and g2, in three orthogonal directions OX8, OY8 and OZ8 as described below, the direction OZ8 being coincident wffii the bore axis. For each station, at which measurements are taken as the instrument is lowered down a borehole, the set (gxs, gy^ gzsf yields sufficient information to allow the set (^, Θ) to be derived whereψis the azimuth angle of the bore hole and 0 is the inclination angle thereof, as will be apparent from the following - 6 46359 description. Alternatively, if the three gravity sensors were mounted on the case 10 instead of on the outer gimbal 16, the resulting set (gx, gy, gz, also yields sufficient information.

Figure 2 shows a borehole 30 schematically and illustrates various reference axes relative to which the orientation of the bore hole 30 may be defined. A set of earth-fixed axes (ON, OE, OV) are illustrated with OV vertically down and ON being a horizontal reference direction. A corresponding case-fixed set of axes (OX, OY, OZ) are illustrated where OZ is the longitudinal axis of the bore hole (and therefore of the instrument) and OX and OY are in a plane perpendicular to the bore hole axis and represented by a chain-dotted line. The earth-fixed set of axes rotate into the instrument-fixed set of axes via the following three clockwise rotations: Rotation about the axis OV through the azimuth angle as shown in Figure 3, Rotation about the axis OE^ through the inclination angle Θ illustrated in Figure 4, and Rotation about the axis OZ through the high-side angle 0 as shown in Figure 5.

The relationship between the high-side angle f6 and the angle measured by the resolver 26 in the instrument is illustrated in Figure 6. 0X‘, OY* and 0Z‘ are the outer-gimbal-fixed axes along which the three components of gravity gx,, gy,, pnd gzj are sensed. is the high-side angle which would be obtained if the instrument was taken to a station without rotation about the case-fixed axis Z.

If the gravity sensors are mounted on the - 7 10 3 5 9 case then the gravity vector g = g, .U + g .U+ g-.U λ λ y . y ζ. z where ϋχ, U^ and U2 are the unit vectors in the casefixed axes directions OX, OY and OZ respectively. If the gravity sensors are imunted on the outer gimbal then the gravity vector g =.δχ»Πχί + gy,Uyt + gzt^z« where υ„», u.. 'x uy5 ^2» are unit vectors in the outer gimbal frame directions OX’, OY’ and OZ' respectively. ' · Thus, εχί = g^cos - gySin ..........(A) gyS = gxsin + gyCos gzs = gz If U.T, U„ and .........(B) ..........(c) are unit vectors in the earthN’ YE----v fixed axes directions ON, OE and OV respectively, then according to the definition of the angles fS, Θ and the vector operation equation Uj^y = the transformation relationship between the sets of unit vectors in the two frames where, cosy -sin^ 0 siny> · cosy- 0 and cos Θ -sin Θ cos $ sin ¢3 0 0 . -sin cos sin Θ 0. cos Θ , The vector operation = ' ^NEV represents the transformation relationship in the opposite direction.

Operating with j vector g .Uv yields on the gravity - 8 4 6 3 5 9 gx = -g.sinO cos/ gy = g.sin© sin/ gz = g.cosO (i) (ii) (iii) Thus, gx, = -g.sinO cos/ cos/^ - g.sin© sin/ sin/^ = -g.sin©.cos(/-/^) and gyt = -g.sinO cos/ sin/^ + g.sinO sin/ cos/^ = g.sin©.sin(/-/^) If the earth-fixed, case-fixed and outergimbal fixed axes coincide at the mouth of the borehole immediately prior to the survey run, then / = /1 + /2 and gx, = -g.sinO.cos/2 gyt = g.sinO.sin/2 g2, = g.cosO (iv) (v) (vi) Thus, if sets of (gx,,gyt,gzt) are recorded at each station then corresponding values of Θ and /2 for each station can be derived from Consider vector V = x.U +y.u + z.U_ at station χ y z (/2>θ, ψ1) rotated through small rotations Δ«Ζ.ϋχ + Δ/S.Uy to yield vector V1 = χ.ϋχ1 + y.Uz1 + z.Uz1 where Πχ·)»^γΐ and Uz1 are unit vectors in the case frame at station (/2 + Δ /2, θ + Δθ, ψ +Δψ). (The use of suffix 2 is permissible here since there is no rotation of the vector about OZ between the two adjacent stations).

Then the components of in the earth-fixed frame can be derived from the operator equation N. '1 E. v. '1 [φ +Δψ j(e+ie j{/2 + Δ /2j - 9 46359 Now, 71 = (χ.Πχ +y.Ux+ζ.Πζ) + ^<Ζ.ϋχχ(χ.ϋχ+γ.ϋγ+ζ.ϋ2) + Ja/5.Uyx(x.Dx+y.iTy+z.Dz) or, if Δ «Λ and Δ/3 are small = (χ+ζ.^Χί) .ϋχ + (y-z. ^-<).Uy + (z + y. Δ·=<—χ. Δβ ).UZ Thus, the components of in the earth fixed frame can also be derived from the operatdr equation ' Ni X + z.E1 y - ζ.Δ tX. ;vi. .z + y.Z) oi - ..(viii) If the operators of (vii) and (viii) are applied to the - 10 46359 selection of the appropriate matrix elements: ^/(sin^g.cose.cosi^ + cos^g.sini/') + ^(005^2.0036.008^sln/g.sinlj/) = - Δ ι/',sin6.sin^+ι3 θ.οοεθ.οοεψ . Aflti-sin^g-sinO) + Δβ (-cos/g.sin6) = -Ζθ.εΐηθ (ix) (x) If the operators of (vii) and (viii) are applied to the vector Γ 1 the following equation can be obtained from a suitable selection of the appropriate matrix elements: -Z/?.cos6 =Δ /g.sin/g.sin6 -Z 6.cos/g.cos6 ........

From equation (χ) Δ& = A^.cos/g + £/-sin/g ........

From equations (x) and (xi) /)^,= 00(-Zyff.sin^g + Δ oi. cos^g) ..........(xiii) sin® .........(xi) .........(xii) and from equations (ix) and (x) Αψ “ 1 (Zl^.sinfig - Zdf.cosjig) (xiv) sine Finally, from results (xiii) and (xiv) (E) Δψ = - 1 . Δ i2 ..........(E) cos Θ Thus, if the set (gx,gv,gz,/x) is known at each station along the path of the borehole, then the corresponding sets of (gY,,g„,,g„») can be derived from equations (A), (B) and (C). Corresponding sets of (6,jig) can then be derived using equations (D) and the increment in azimuth Zty between any two adjacent stations can be derived from the increment 4 jig between those stations by the use of equation (E). Provided that the outer gimbal fixed axes and the earth fixed axes coincide at the mouth of the borehole immediately prior to the survey run, the azimuth at each station along the path measured with respect to the ON direction can be arrived at by continuous Jk summation of the azimuth increments along the path to each station. In practice,however,the necessity to align the spin axis with ON at the mouth of the well 6 3 5 8 ' is obviated provided that the initial angle between OX* and ON is knov/n. The azimuth is then derived by applying the correction θ such that + Σ (4 φ) where the . summation is taken along the path to the station considered In addition to the gyro-stabilised single-axis platform section and the gravity sensor unit as described above, the down-hole instrument contains an information storage section as shown in Figure 7. Since the gravity sensor unit 28 is mounted on the outer gimbal with the sensing axes of the sensors along the OX*, 0Y‘ and 02 directions, the outputs from these· sensors are directly equal to gx,,gyt and ggl respectively. These outputs are applied directly to a recorder 32. This obviates the need to use equations (A), (B), and (C).

The outputs from the resolver 26 are also connected . to the recorder 32 and used to determine the Initial value of the angle between the spin axis of the gyro rotor 12 and the earth-fixed reference direction ON at the start of each run, The recorder 32 also . records the output from a clock 34 to provide a record of the time at which each reading of the outputs from the gravity sensor unit 28 is made.

Figure 8. shows the corresponding surface equipment to the down-hole information storage equipment shown in Figure 7- Hie outputs from a surface clock 3.6 and a wire line gauge 38, which measures the length of the wire line on which the down-hole instrument is suspended, are recorded on a surface recorder 40 during each measuring run. After completion of each run, the . recording made on the down-hole recorder 32 is transferred to a signal processing unit 42 where the recording is replayed simultaneously with the replaying of the - 12 '16 3 5 9 recording made by the surface recorder 40. The recorded output from the down-hole clock 34, together with time and path length outputs from the recorder 40 are applied to a time comparator 44 which provides a station identification signal comprising the path length signal synchronized with the replaying of the recorded values of the output from the gravity sensor unit 28 which is applied to one input of a printer 46. The outputs gx,,gyt,gzr and are applied to a surface computing unit 48 which computes the inclination angle β and the azimuth angle if and applies signals representing these singles to the printer 46 which thus provides a record of the inclination angle Θ and the azimuth angle i^· at each of the stations at which a reading is taken together with information identifying the relevant station.

The gravity sensor unit 28 mounted on the outer gimbal 16 (Figure 1) may be replaced by three gravity sensors mounted on the instrument case 10 with their sensing axis lying along the OX, OY, OZ, directions, so that the sensor outputs are gx, gy and g2> Figure 9 shows a down-hole instrument section for use in the circumstances.

The output from the resolver 26 and the clock 34 are connected to the recorder 32 as before. The g2 output from a gravity sensor unit 50 mounted on the case 10 is also applied directly to the recorder 32 but the outputs g2 and gy from the gravity sensor 50 are applied to respective stator coils of a second resolver 52 which is also mounted between the outer gimbal 16 and the case 10. The outputs from the rotor coils of the resolver 52 comprise the signals g . and g t and these signals are applied to the recorder y x 52. The recorded signals are thus the same as those recorded using the instrument section shown in Figure 7. - 13 46 3 39 As a modification to the arrangement shown in Figure 9» all three outputs g , g , and g from the gravity X y Z sensors 50 may be applied to the recorder 32. In this case, the signals from the resolver 26 are’ used to provide an indication of the angle betvzeen the outer gimbals and the case throughout each measuring run and not merely 'to indicate the initial angle and calculations in accordance with equations A, B, and C are performed on the surface.

If a suitable signal path is available on the 10 one line on which the down-hole instrument is suspended the output from the down-hole instrument can be transmitted directly to the surface and no down-hole time reference is required. The surface equipment shown in Figure 8 is then modified hy omission of the surface clock 36, recorder 40 and time comparator 44, the output of the wire line gauge 38 being connected directly to the printer 46. The signal processing unit 42 is also modified to receive the signals transmitted from the down-hole instrument instead of to replay a recording.

If the instrument is used to record information . during the run prior, to processing at the end of the run, measured parameter storage can be used conveniently in the form of integrated-circuit memory storage packs.

The instrument used in this mode would he battery powered from a battery pack built within the case.

The invention is also applicable to a directional drilling process in which it is required to drill at an increased inclination angle in a known azimuth direction · from a previously drilled shallow near-vertical cased hole. In these circumstances, the near-verticality prohibits the use of a conventional high-side steering tool and the / - 14 4 6 3 3 9 casing prohibits the use of a conventional magnetic steering tool. If a single-axis stabilised platform instrument in accordance with the invention is used to establish the direction of the spin axis with respect to a horizontal reference ON at the mouth of the hole, then this axis will remain substantially referenced with respect to ON as the instrument is lowered through the near-vertical section of the hole. Thus, if the instrument is lowered to locate with a bent-sub/mud-motor arrangement as with the conventional steering tool, then the rotation of the case about the spin axis can be used to establish the direction of the bent-sub/mud-motor With respect to the earth-fixed direction ON.

Claims (9)

1. An instrument for measuring the direction of a borehole comprising a case having a longitudinal axis coincident, in use, with the axis of the borehole, . 5 a single-degree-of-freedom gyro comprising an outer gimbal mounted in the case with its axis coincident with the longitudinal axis thereof, an inner gimbal mounted in the outer gimbal with its axis perpendicular to the outer gimbal axis, a gyro rotor mounted in the Ιθ inner gimbal, means for sensing.angular movement of the inner gimbal relative to the outer gimbal and means for applying a torque to the outer gimbal for rotating It, in use, about its axis so that the inner gimbal precesses back to its initial position, means for 15 measuring,the angle of rotation of the case about its longitudinal axis relative to the outer gimbal and a gravity sensor unit for measuring three components of gravity in three non-coplanar directions.

2. An instrument according to claim 1, in which 20 the gravity sensors are mounted on the outer gimbal.

3. An instrument according to claim 1, in which the gravity sensors are mounted on the case of the instrument.

4. An instrument according to claim 3, including 25 a resolver mounted on the instrument case and having Its rotor connected to the outer gimbal, two of the gravity sensors being arranged to sense components of gravity in directions perpendicular to the longitudinal axis of the instrument and having their outputs 30 connected to the inputs of the resolver. - 16 46359

5. A method of surveying a borehole comprising moving a survey instrument along the borehole, said survey instrument comprising a case having a longitudinal axis coincident with the axis of the 5 borehole, a single-degree-of-freedom gyro comprising an outer gimbal mounted in the case with its axis coincident with the longitudinal axis thereof, an inner gimbal mounted in the outer gimbal with its axis perpendicular to the outer gimbal axis, a gyro 10 rotor mounted in the inner gimbal; continually sensing angular movement of the inner gimbal relative to the outer gimbal as the instrument moves along the borehole and applying a torque to the outer gimbal to rotate it about its axis so that the inner gimbal precesses 15 back to its initial position; determining at each of a series of survey stations spaced along the borehole a set of three components of gravity in three non-coplanar directions relative to the outer gimbal; calculating, from the sensed components of gravity at each of the 20 said survey stations, the inclination of the borehole and the non-rotative high-side angle of the instrument relative to a inference direction which does not rotate about the longitudinal axis of the instrument as it travels along the borehole at each station; and calculating, 25 from said inclination and non-rotative high-side angle, the azimuth of the bore at each station.

6. A method of surveying a borehole according to claim 5, wherein the set of three components of gravity is measured, at each station, by three sensors 30 mounted on the outer gimbal. - 17 463 88

7. A'method of surveying a borehole according to claim 5, wherein a second set of three components of gravity is measured, at each station, by three sensors mounted on the instrument case, the method ' further comprising measuring, at each station, the angle of rotation of the instrument case relative to the outer gimbal and calculating said first mentioned set.of components of gravity from said second set and said angle of rotation.

8. A method of surveying a borehole according to claim-7, wherein one component of each of said setsof components of gravity is parallel to the outer gimbal axis and the other two components of the firsi/mentioned set are calculated by setting the stator and rotor of a resolver; at a relative angle equal to said angle of rotation and applying the other two components of the second set to the input of the resolver.

9. „ An Instrument for measuring the direction of a borehole substantially as hereinbefore described· with reference to the accompanying drawings.