Nuclear magnetic resonance borehole logging apparatus and method for ascertaining a volume of hydrocarbons independent of a diffusion coefficient
AssigneeSchlumberger Technology Corporation
InventorsRobert L. Kleinberg, Lawrence L. Latour, Abdurrahman Sezginer
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Abstract
A nuclear magnetic resonance (NMR) logging technique reveals the volume of hydrocarbons in an earth formation. The differences in viscosity among gas, oil and water are used to distinguish among these three fluids. During NMR logging, a parameter of a sequence of magnetic field pulses is altered, causing echo signal strengths to vary due to the diffusion of the fluids in the formation. The volume of hydrocarbons in the formation is determined from detected spin echo strengths. The invention applies to wireline NMR logging and NMR logging-while-drilling techniques.
Claims34
A method for determining the volume of hydrocarbons in a region of investigation of earth formations surrounding a borehole, comprising the steps of:
- a) providing a logging device that is moveable through the borehole;
- b) generating, from said logging device, a static magnetic field in a volume of the formation directed to one side of the logging device;
- c) generating, from said logging device, a sequence of magnetic field pulses in the region of investigation of the formations, and detecting the nuclear magnetic resonance spin echo strength from the formations;
- d) varying a parameter of said sequence of magnetic field pulses that causes said echo strength to vary due to diffusion of fluid species in the formations, and repeating step (c) a plurality of times to detect the echo strength of further nuclear magnetic spin echoes from the formations;
- and
- e) determining the volume of hydrocarbons, independent of a diffusion coefficient, in the region of investigation of the earth formations from the detected spin echo strengths.
A nuclear magnetic resonance method for determining the volume of hydrocarbons in a region of investigation in formations surrounding a borehole, comprising the steps of:
- a) providing a logging device that is moveable through the borehole;
- b) generating, from said logging device, a static magnetic field in said region of investigation of the formations;
- c) generating, from said logging device, a sequence of RF magnetic field pulses in said region of investigation of the formations, and detecting nuclear magnetic resonance spin echoes from said region of investigation of the formations;
- the sequence of pulses and echoes being:
- 90-t.sub.1 -90-t.sub.s -90-t.sub.1 -echo.sub.1 -t.sub.i -180-t.sub.i -echo.sub.i+1 !.sub.i
- where t.sub.1 is the time between the initial 90 degree pulse and the second 90 degree pulse, t.sub.s is the time between the second and third 90 degree pulses, and t.sub.i are the Carr-Purcell spacings (times) for the subsequent 180 degree reversing pulses and echoes (i=1,2 . . . );
- d) repeating step (c) a plurality of times with different times t.sub.s ;
- and
- e) computing the volume of oil in said region of investigation of the formations from the spin echoes detected in steps (c) and (d).
Nuclear magnetic logging apparatus for determining a characteristic of formations surrounding a borehole, comprising:
- a logging device that is moveable through the borehole;
- means in said logging device for generating a static magnetic field in a volume of the formation directed to one side of the logging device;
- means in said logging device for generating a sequence of RF magnetic field pulses and magnetic field gradient pulses in the formations, and for detecting nuclear magnetic resonance spin echoes from the formations, comprising:
- an RF antenna comprising a trough-shaped conductor, disposed in said logging device with the open end of said trough facing the borehole wall, and wherein a gradient field antenna comprises a split body of the trough-shaped conductor shorted at one end, means for applying a sequence of RF pulses to said RF antenna, and a detector circuit coupled with said RF antenna;
- and
- means for determining said characteristic of the formations from the detected spin echoes.
A method for characterizing a parameter of a region of investigation of an earth formation surrounding a borehole, comprising the steps of:
- a) providing a logging device that is moveable through the borehole;
- b) generating, from said logging device, a static magnetic field in a volume of the formation directed to one side of the logging device;
- c) generating, from said logging device, a sequence of magnetic field pulses in the region of investigation of the formation, and detecting the nuclear magnetic resonance spin echo strength from the formation;
- d) varying a parameter of said sequence of magnetic field pulses that causes said echo strength to vary due to diffusion of fluid species in the formation, and repeating step (c) a plurality of times to detect the echo strength of further nuclear magnetic spin echoes from the formation;
- e) producing, with said logging device, a signal indicating a volume of hydrocarbons, independent of a diffusion coefficient, in the region of investigation of the earth formation from the detected spin echo strengths;
- and
- f) characterizing the earth formation parameter using the signal indicating the volume of hydrocarbons.
A method for characterizing a parameter of a region of investigation of an earth formation surrounding a borehole, comprising the steps of:
- a) providing a logging device that is moveable through the borehole;
- b) generating, from said logging device, a static magnetic field in a volume of the formation directed to one side of the logging device;
- c) generating, from said logging device, a sequence of magnetic field pulses in the region of investigation of the formation, and detecting the nuclear magnetic resonance spin echo strength from the formation;
- d) varying a parameter of said sequence of magnetic field pulses that causes said echo strength to vary with the diffusion times of fluid species in the formation, and repeating step (c) a plurality of times to detect the echo strength of further nuclear magnetic spin echoes from the formation;
- e) producing, with said logging device, a signal indicating a volume of hydrocarbons in the region of investigation of the earth formation from the detected spin echo strengths;
- f) characterizing the earth formation parameter using the signal indicating the volume of hydrocarbons;
- and
- g) characterizing the parameter while drilling the borehole into the formation.
Dependencies are inferred from the claim text.