Crossed-coil nuclear magnetism well logging apparatus and method utilizing a polarizing field and an additional polarizing field to shift the direction of polarization

Claim 2

In a cross coiled nuclear magnetism logging sonde for detecting in an earth formation penetrated by a well bore the presence of formation fluids through generation and detection of nuclear magnetic signals from protons of said formation fluid, by alignment of said protons into a first direction by means of a rather strong polarizing field HP whereby said protons acquire a nuclear polarization M, interrupting a said polarizing field HP and, as said protons and said acquired polarization M precesses about the earth''s magnetic field, detecting said protons as relaxation into alignment with said earth field occurs, the improvement for providing more effective nuclear magnetism logging results in earth formations in magnetically equatorial regions of the world or in slanted boreholes in which the earth''s magnetic field is substantially perpendicular to the borehole axis, comprising: a. separate crossed coil polarizing and receiving coil means housed within said sonde having substantially perpendicular magnetic axis with respect to each other, b. means for cyclically connecting current source means and proton precession detection means relative to said polarizing and receiving coils so as to generate and detect nuclear magnetism signals of said formation protons, c. said cyclically connecting means including interrupt and current control means connected to said coil means for generating, in addition to said strong polarizing field HP, a separate polarizing field Hl overlapping in time and space with respect to said strong polarizing field HP but being of substantially different resultant direction therefrom for angularly and adiabatically shifting with respect to the earth''s field said polarization M to more effective directions prior to their precession about the earth''s field.

Claim 3

The improvement of claim 1 in which said interrupt and current control means includes switch means connecting said current source means to said polarizing coil as well as selectively connecting said current source means relative to said receiving coil to generate said polarizing field Hl of a rather weak magnitude and thereafter, for selectively disconnecting said current source means from said polarizing coil means after said weak polarizing field Hl is produced thereby terminating the generation of said strong HP polarizing field in the presence of said weak polarizing field Hl.

Claim 4

The improvement of claim 2 in which said interrupt and current control means includes damping resistor means selectively connected between said current source means and said polarizing coil means to control the rate of collapse of said polarizing field HP in the presence of said weak polarizing field Hl whereby a resultant magnetic field Hr formed therefrom is caused to rotate at a rate which is small compared to the Larmor frequency of protons in said Hr field.

Claim 5

Apparatus for conducting cross-coil nuclear magnetism logging in an earth formation in which the earth''s magnetic field is substantially perpendicular to the axis of a borehole penetrating said formation wherein nuclear magnetism polarization M acquired by polarized protons of fluids in said earth formation is manipulated prior to precession about the earth''s magnetic field to more effectively indicate the presence of said precessing protons, comprising: a. a polarizing coil having a first magnetic axis, in selective and cyclic electrical communication for a first time period with a source means of electrical current to generate a polarizing magnetic field HP whereby protons in said formation adjacent to said polarizing coil are polarized and acquire said polarization M, b. a receiving coil adjacent to said polarizing coil having a second magnetic axis substantially perpendicular to said first magnetic axis in selective electric communication for a second time period with said source means of electrical current to generate a rather weak magnetic field Hl, said second time period commencing before and terminating after termination of said first time period producing an overlapping time increment of relatively short time duration, said receiving coil also being in selective contact with a proton precessing detection means during a third time period to detect precession of said protons about the earth''s field, c. means for controllably connecting said receiving and polarizing coils relative to said current source means and said proton precession detection means during said first, second and third time periods, d. said last-mentioned means including interrupt and current control means for controllably connecting said current source means relative to said receiving and polarizing coils during said overlapping time increment whereby said polarization M acquired by said formation protons is angularly and adiabatically shifted with respect to the earth''s field to more effective directions prior to precession about the earth''s field.

Claim 6

Apparatus for conducting cross-coil nuclear magnetism logging in an earth formation in which the earth''s magnetic field is substantially perpendicular to the axis of a borehole penetrating said formation wherein nuclear magnetism polarization M acquired by polarized protons of fluids in said earth formation is manipulated prior to precession about the earth''s magnetic field to more effectively indicate the presence of said precessing protons, comprising: a. a polarizing coil having a first magnetic axis, in selective and cyclic electrical communication for a first time period with a source means of electrical current to generate a polarizing magnetic field HP whereby protons in said formation adjacent to said polarizing coil are polarized and acquire said polarization M, b. a receiving coil adjacent to said polarizing coil having a second magnetic axis substantially perpendicular to said first magnetic axis in selective electric communication for a second time period with said source means of electrical current to generate a rather weak magnetic field Hl, said second time period commencing before and terminating after termination of said first time period producing an overlapping time increment of relatively short time duration, said receiving coil also being in selective contact with a proton precessing detection means during a third time period to detect precession of said protons about the earth''s field, c. means for controllably connecting said receiving and polarizing coils relative to said current source means and said proton precession detection means during said first, second and third time periods, d. said last-mentioned means including interrupt and current control means for controllably connecting said current source means relative to said receiving and polarizing coils during said overlapping time increment whereby said polarization M acquired by said formation protons is angularly and adiabatically shifted with respect to the earth''s field to more effective directions prior to precession about the earth''s field.

Claim 7

The improvement of claim 3 in which said dampening resistor means and said polarizing coil form a parallel R-L circuit with respect to ground whereby polarizing current through said polarizing coil decays exponentially with time.

Claim 8

ApParatus of claim 4 in which said interrupt and current control means includes dampening resistor means selectively connected between said current source means and said polarizing coil to control the rate of collapse of said polarizing field HP in the presence of said weak polarizing field Hl whereby a resultant magnetic field Hr formed therefrom is caused to rotate at a rate which is small compared to the Larmor frequency of protons in said Hr field.

Claim 9

Apparatus of claim 6 in which said dampening resistor means and said polarizing coil form a parallel R-L circuit with respect to ground whereby polarizing current through said polarizing coil decays exponentially with time.

Claim 10

Apparatus for conducting crossed-coil nuclear magnetism logging of claim 4 in which said polarizing and receiving coils each have a substantially rectangular shape defining a longitudinal axis substantially parallel to said borehole axis but each of said first and second magnetic axes being substantially perpendicular to said borehole axis and to each other.

Claim 11

Apparatus of claim 8 wherein said substantially rectangular polarizing coil has a greater effective length within said sonde than said substantially rectangular receiving coil, the differential length of said polarizing coil extending in the direction in which the sonde is logged to permit the protons along successive sections of said formation to be effectively polarized before arrival of said receiving coil.

Claim 12

In a method for detecting in an earth formation penetrated by a well bore the presence of formation fluids through generation and detection of nuclear magnetic signals from protons of said formation fluid, by alignment of said protons into a first direction by means of a rather strong polarizing field HP whereby said protons acquire a nuclear polarization M, interrupting said polarizing field HP and, as said protons and said acquired polarization M precesses about the earth''s magnetic field, detecting said protons as relaxation into alignment with said earth field occurs, the improvement for providing more effective nuclear magnetism logging results in earth formations in magnetically-equatorial regions of the world or in slanted boreholes in which the earth''s magnetic field is substantially perpendicular to the borehole axis which comprises: a. positioning separate cross-oriented polarizing and receiving coils in said borehole adjacent to a longitudinal region of said earth formation so that respective magnetic axes of said coils are substantially perpendicular to each other, b. applying polarizing current to said polarizing coil for a first time period to generate said magnetic field HP so that protons within said region of formation are polarized and acquire said polarization M, c. applying a weak current to said receiving coil to produce a weak magnetic field, Hl, for a second time period, d. controlling said first and second time periods so that said second period commences before and terminates after said first time period to define an overlapping time increment of relatively short time duration, e. controlling during said overlapping time increment cut-off of said polarizing current and hence collapse of said polarizing field HP in the presence of said weak magnetic field Hr whereby said polarization M acquired by said formation protons is angularly and adiabatically shifted with respect to the earth''s field to more effective directions prior to precession about the earth''s field, and f. after said second time period has terminated, connecting said receiving coil to a proton precessing detection means for a third time period to detect the precessing protons.

Claim 13

In a method for detecting in an earth formation penetrated by a well bore the presence of formation fluids through generation and detection of nuclear magnetic signals from protons of said formation fluid, by alignment of said protons into a first direction by means of a rather strong polarizing field HP whereby said protons acquire a nuclear polarization M, interrupting said polarizing field HP and, as said protons and said acquired polarization M precesses about the earth''s magnetic field, detecting said protons as relaxation into alignment with said earth field occurs, the improvement for providing more effective nuclear magnetism logging results in earth formations in magnetically-equatorial regions of the world or in slanted boreholes in which the earth''s magnetic field is substantially perpendicular to the borehole axis which comprises: a. positioning separate cross-oriented polarizing and receiving coils in said borehole adjacent to a longitudinal region of said earth formation so that respective magnetic axes of said coils are substantially perpendicular to each other, b. applying polarizing current to said polarizing coil for a first time period to generate said magnetic field HP so that protons within said region of formation are polarized and acquire said polarization M, c. applying a weak current to said receiving coil to produce a weak magnetic field, Hl, for a second time period, d. controlling said first and second time periods so that said second period commences before and terminates after said first time period to define an overlapping time increment of relatively short time duration, e. controlling during said overlapping time increment cut-off of said polarizing current and hence collapse of said polarizing field HP in the presence of said weak magnetic field Hr whereby said polarization M acquired by said formation protons is angularly and adiabatically shifted with respect to the earth''s field to more effective directions prior to precession about the earth''s field, and f. after said second time period has terminated, connecting said receiving coil to a proton precessing detection means for a third time period to detect the precessing protons.

Claim 14

The method of claim 10 in which the cut-off of said polarizing current in step (e) and hence the rate of collapse of said polarizing field HP is such that a resultant magnetic field Hr formed from said polariZing fields HP and Hl is angularly shifted at a rate less than the Larmor frequency of protons in the Hr field.

Claim 15

The method of claim 11 in which the cut-off is exponential with time.