Patent GB-1463738-A (GB)
Title: Well surfeying apparatus
IPC: E21B47/022
Filed: 1974-12-10
Abstract: 1463738 Well surveying apparatus SCIENTIFIC DRILLING CONTROLS 10 Dec 1974 53265/74 Heading G1N [Also in Division H3] Well surveying apparatus comprises a probe 10, gravity actuated sensors 38, 39, 40 for sensing the orientation of the probe relative to the vertical, further sensors 41, 42, 43 for sensing the orientation of the probe relative to a further direction such as that of the earths magnetic field, and a computor responsive to signals from the sensors to calculate well characteristics. The probe comprises an inner housing 30 mounted within outer housing 28 by a shock absorbing structure 32 received in a T-slot 34 of housing 30. The probe is lowered into the well by signal carrying cable 12 which passes over a sheave 16 driving a pulse producing odometer 17 connected to computor 19. The gravity responsive sensors 38, 39, 40 sense the component of the earths gravity acting along respective axes OY, OX, OZ, Fig. 6 of the probe. Each sensor is constituted by an accelerometer, Fig. 3, having a weight 44 integral with a disc 46 having a thin hinge portion 48 which permits the weight to move resiliently along the respective axis, the weight and the disc being of quartz. The position of the weight is sensed by a capacitative pick-up 51 and a servo amplifier 52 supplies current to a coil 53 to maintain the weight at its zero position by interaction with magnets 49, 50. An output voltage is produced at 55 and applied to a signal multi-plexer 57. The further sensors 41, 42, 43 sense the component of the earths magnetic field along respective probe axes OY, OX, OZ, Fig. 6. Each sensor is constituted by a flux gate magnetometer, Fig. 4, having a pair of rods 66, 67 of magnetic material aligned with the respective axis. The coils 69, 70 are energized by A.C. source 72 such that the rods 66, 67 are saturated twice during each source cycle and any second harmonic signal appearing on line 73 is amplified at 173, and fed to a phase sensitive demodulator 74 the output of which is integrated at 276. The integrator produces a D.C. current which is fed back to coils 69, 70 to create a field equal and opposite to the component of the earths field acting along the rods, the system acting to reduce the output of demodulator 74 virtually to zero. An output signal is taken at 63 to the multiplexer 57. The signals a-f applied to multi-plexer 57 are selected in turn by means of counter 77 and switch device 62 to each determine the time interval between two adjacent pulses in a series of pulses fed to wire 12. When a particular switch 62 is closed, a capacitor 79 is momentarily connected to a-6 volt source by switch 80, and is then charged by a constant current source 78'. When the capacitor potential is - 4 volts, comparator 81  is actuated and furns on a bi-stable 82. When the capacitor potential equals the signal voltage comparator 83 is actuated and turns the bi-stable off. When thus turned off, the bi-stable causes pulse driver 84' to supply a pulse to line 12, switch 80 to connect capacitor 79 to the - 6 volt source, and counter 77 to advance one step whereby a different signal is selected by switch device 62. A sync. signal is provided by means of a long interval between two successive pulses at the end of a series of pulses corresponding to the signals a-f. The signals in wire 12 are decoded at the earths surface by circuit 20. A counter 89 is advanced through successive counts by the pulses, and the length of time it remains in each count condition is measured by a counter 92 connected to a clock oscillator 93 the output of the counter being applied to computer 19. Each time that the length of wire 12 in the well equals a multiple of 100', the computor prints out the drift angle azimuth angle, true depth, N-S and E-W shifts from the surface opening, and curvature of the well, Formulae for deriving these values from the signals a-f being given. In addition the computor may actuate a recorder 24 to plot one of horizontal shift versus true depth and N-S shift against E-W shift data for the other being stored so that it may be plotted on command. The flux gate magnetometers may be replaced by gyroscopic devices, and the probe may be incorporated in a drilling string.