Logging while drilling method and apparatus for measuring formation characteristics as a function of angular position within a borehole

A method for determining density of an earth formation surrounding an inclined borehole in which a generally cylindrical logging while drilling tool is received, including the steps of,

• determining a bottom contact point of said tool which contacts said inclined borehole while said tool is rotating in said borehole
• defining a bottom angular distance of said tool which includes said bottom contact point,
• applying gamma rays into the borehole from a radiation source,
• recording, as a function of angular distance of said tool with respect to the borehole for a predetermined time period, a count rate of gamma rays which return to the tool which result from interaction with said formation, and
• determining density of the formation from the count rate of certain gamma rays which occur solely within said bottom angular distance of said tool.

In a logging while drilling tool and system, having a source of gamma ray radiation and long and short spaced gamma ray detectors, a method for assessing density of an earth formation surrounding an enlarged borehole, including the steps of

• dividing a cross section of said borehole into plural borehole angular distance segments,
• detecting signals representing hard window count rates of gamma rays from said formation during successive time increments from said long spaced gamma ray detector and from said short spaced gamma ray detector while said tool is rotating and associating each of said count rate signals with one of said plural borehole angular distance segments,
• for at least one borehole angular segment, determining a signal proportional to the difference in the variance of all said count rates from one of said gamma ray detectors for such at least one borehole angular segment and an expected variance of such count rates for a circular borehole for such at least one borehole segment,
• for said at least one borehole angular segment, determining a density rotation correction factor, called .DELTA..rho..sub.ROT, representative of a density measurement correction needed to correct a density measurement of said borehole angular segment for borehole irregularity along said angular segment,
• determining from said long and short spaced gamma ray detectors hard window count rates and associated with said one of said plural borehole angular distance segments, signals representative of density, called .rho..sub.segment,L, .rho..sub.segment,S, respectively, and
• determining a signal proportional to density of said angular distance segment, called .rho..sub.b ROT segment,as a function of said .rho..sub.segment,L, .rho..sub.segment,S and .DELTA..rho..sub.ROT signals.

A method for determining photoelectric effect, called PEF, of earth formations surrounding a borehole in which a logging while drilling tool is received, said tool including a source of radiation, a short spaced gamma ray detector and a long spaced gamma ray detector, the method including the steps of

• identifying particular angular segments of said borehole through which said short spaced detector and said long spaced detector pass while said tool is rotating in said borehole,
• recording for a predetermined time period a count rate of gamma rays in said short spaced detector and in said long spaced detector as a function of said particular angular segments, where said gamma rays result from interaction of gamma rays from said source with said formations, and where said count rate of gamma rays of said short spaced detector and of said long spaced detector are recorded as to their respective energy levels called windows, thereby producing a spectrum of count rates with certain higher energy level windows being designated as hard windows and with certain lower energy level windows being designated as soft windows,
• determining average density, called .rho..sub.AVG, of the entire formation, and
• determining a macroscopic cross section, called U.sub.AVG, of the entire formation as a function of total soft window count rate of one of said detectors and total hard window count rate of said one of said detectors, and
• determining an average PEF of said formation as a ratio of said macroscopic cross section to said average density, that is, ##EQU10##

A method for determining a rotational value of photoelectric effect, called PEF.sub.ROT, of earth formations surrounding a borehole in which a logging while drilling tool is received, said tool including a source of radiation, a short spaced gamma ray detector, and a long spaced gamma ray detector, the method including the steps of

• identifying particular angular segments of said borehole through which said short spaced detector and said long spaced detector pass while said tool is rotating in said borehole,
• recording for a predetermined time period a count rate of gamma rays in said short spaced detector and in said long spaced detector as a function of said particular angular segments, where said gamma rays result from interaction of gamma rays from said source with said formations, and where said count rate of gamma rays of said short spaced detector and of said long spaced detector are recorded as to their respective energy levels called windows, thereby producing a spectrum of count rates with certain higher energy level windows being designated as hard windows, such count rates of hard windows from one of said detectors called HARD COUNT RATE, and with certain lower energy level windows being designated as soft windows, such count rates of soft windows from said one of said detectors called SOFT COUNT RATE,
• determining rotational correction factors .DELTA.CR.sub.SOFT and .DELTA.CR.sub.HARD respectively from statistical distributions of HARD COUNT RATE and SOFT COUNT RATE,
• determining a signal representative of macroscopic rotational cross section factor U.sub.ROT as a function of said HARD COUNT RATE, SOFT COUNT RATE, and said rotational correction factors .DELTA.CR.sub.SOFT and .DELTA.CR.sub.HARD,
• determining a density correction signal called .DELTA..rho..sub.ROT representative of a density correction factor to correct a density measurement of said borehole for borehole irregularity as a function of said HARD COUNT RATE and said rotational correction factor .DELTA.CR.sub.HARD,
• determining from said long and short spaced detectors hard window count rates, signals representative of density called .rho..sub.L, .rho..sub.S, respectively,
• determining a signal proportional to density corrected for borehole irregularity, called .rho..sub.b ROT, as a function of .rho..sub.L, .rho..sub.S and .DELTA..rho..sub.ROT, and
• determining a signal representative of said rotational value of photoelectric effect as the ratio of said U.sub.ROT and .rho..sub.b ROT signals, that is, ##EQU12##

A method of determining neutron porosity of an earth formation surrounding an inclined borehole in which a logging while drilling tool is received, said tool including a source of neutrons, near spaced and far spaced detectors of neutrons which result from interaction of neutrons from said source of neutrons with said formation, and an ultrasonic sensor, the method including the steps of;

• determining a bottom contact point of said tool which contacts said inclined borehole while said tool is rotating in said borehole;
• defining a bottom angular distance segment, called SEGMENT.sub.BOTTOM of said borehole which includes said bottom contact point;
• for a predetermined length of time, recording a far neutron count rate of a said far spaced detector of neutrons and a near neutron count rate of said near spaced detector of neutrons for said bottom angular distance segment;
• with said ultrasonic sensor, determining a measured average BOTTOM STANDOFF from ultrasonic measurements in said bottom angular distance segment;
• and
• determining average neutron porosity as a function of said near neutron count rate and said far neutron count rate measured in said SEGMENT.sub.BOTTOM corrected by said BOTTOM STANDOFF.

A method for determining formation heterogeneity surrounding a borehole in which a logging while drilling tool is received including the steps of

• defining a cross-section of said tool which is orthogonal to a longitudinal axis of said tool;
• separating said cross-section into a plurality of angular distance segments;
• applying energy into said formation surrounding said borehole from an energy source disposed in said tool as said tool turns in said borehole during drilling;
• recording measurement signals received at a sensor disposed in said tool where said signals are in response to returning energy which results from the interaction of applied energy with said formation;
• associating said measurement signals with energy returning from said formation while said sensor is in each particular one of said plurality of angular distance segments;
• deriving at least one signal characteristic of said formation surrounding said borehole as a function of said measurement signals for each of said angular distance segments, and
• identifying formation heterogeneity as a function of said angular distance segments by comparing said at least one signal characteristic of said formation from one segment to another.

In a logging while drilling tool system, having a source of gamma radiation and long and short spaced gamma ray detectors and an ultrasonic transducer, a method for determining whether apparent mud density in a borehole is greater than or less than the apparent density of an earth formation surrounding said borehole comprising the steps of

• dividing a cross-section of said borehole into plural borehole angular distance segments,
• with said ultrasonic transducer, determining the STANDOFF of all such angular distance segments, identifying that angular distance segment characterized by the greatest STANDOFF of all such angular distance segments as SEGMENT.sub.STANDOFF, and identifying that angular distance segment characterized by the least STANDOFF of all such angular distance segments as SEGMENT.sub.NO STANDOFF ;
• detecting signals representing hard window count rates of gamma rays from said formation during successive time increments from said long spaced gamma ray detector and from said short spaced gamma ray detector while said tool is rotating and associating each of said count rate signals with one of said borehole angular distance segments,
• determining the count rates called CR.sub.STANDOFF, and CR.sub.NO STANDOFF respectively of said SEGMENT.sub.STANDOFF and said SEGMENT.sub.NO STANDOFF, and
• determining that apparent formation density is greater than apparent mud density if CR.sub.STANDOFF, is greater than CR.sub.NO STANDOFF, and vice versa.

Apparatus for determining a characteristic of an earth formation surrounding an inclined borehole comprising:

• a generally cylindrical logging while drilling tool having a radial cross-section which is orthogonal to its longitudinal axis,
• means for determining a bottom contact point of said cross-section of said tool which contacts said inclined borehole as said tool rotates in said borehole,
• computer program means for separating said cross-section into at least two segments, where one of said segments is called a bottom segment of said borehole which includes said bottom contact point of said cross-section of said tool with said inclined borehole,
• energy source means for applying energy into and circumferentially around said borehole from an energy source disposed in said tool, as said tool is turning inside said borehole,
• sensor means disposed in said tool for producing signals in response to energy stimuli,
• means for recording measurement signals received at said sensor means, from circumferentially spaced locations around said borehole, where said measurement signals am in response to returning energy which results from the interaction of the applied energy with said formation,
• computer program means for associating said measurement signals with a particular segment of said borehole during the time such signals are produced in response to energy returning from said formation as said tool is turning in said borehole, and
• computer program means for deriving an indication of a characteristic of said formation as a function of said measurement signals associated with said bottom segment of said borehole.

Apparatus for determining density of an earth formation surrounding an inclined borehole comprising

• a generally cylindrical logging while drilling tool,
• means for determining a bottom contact point of said tool which contacts said inclined borehole while said tool is rotating in said borehole;
• computer program means for defining a bottom angular distance of said tool which includes said bottom contact point and for defining other distinct angular distances of said tool about said borehole,
• a radiation source for applying gamma rays into the borehole,
• means for recording, as a function of angular distance of said tool with respect to the borehole for a predetermined time period, a count rate of gamma rays which return to the tool which result from interaction with said formation, and
• means for determining the density of the formation for each such angular distance from the count rate of gamma rays which occurs solely within each defined angular distance about said borehole.

In a logging while drilling tool and system, having a source of gamma ray radiation and long and short spaced gamma ray detectors, apparatus for assessing density of an earth formation surrounding an enlarged borehole comprising

• means for dividing a cross section of said borehole into plural borehole angular distance segments,
• means for detecting signals representing hard window count rates of gamma rays from said formation during successive time increments from said long spaced gamma ray detector and from said short spaced gamma ray detector while said tool is rotating and associating each of said count rate signals with one of said plural borehole angular distance segments,
• means for determining, for at least one borehole angular segment, a signal proportional to the difference in the variance of all said count rates from at least one of said gamma ray detectors for such at least one borehole angular segment and an expected variance of such count rates for a circular borehole for such at least one borehole angular segment,
• means for determining, for said at least one borehole angular segment, a density rotation correction factor, called .DELTA..rho..sub.ROT, representative of a density measurement correction needed to correct a density measurement of said at least one borehole angular segment for borehole irregularity along said at least one angular segment,
• means for determining from said long and short spaced gamma ray detectors hard window count rates and associated with said one of said plural borehole angular distance segments, signals representative of density, called .rho..sub.segment,L, .rho..sub.segment,S, respectively, and
• means for determining a signal proportional to density of said angular distance segment, called .rho..sub.b ROT segment, as a function of said .rho..sub.segment,L, .rho..sub.segment,S and .DELTA..rho..sub.ROT signals.

Apparatus for determining photoelectric effect, called PEF, of earth formations surrounding a borehole comprising,

• a logging while drilling tool including a source of radiation, a short spaced gamma ray detector and a long spaced gamma ray detector,
• means for identifying particular angular segments of said borehole through which said short spaced detector and said long spaced detector pass while said tool is rotating in said borehole,
• means for recording for a predetermined time period a count rate of gamma rays in said short spaced detector and in said long spaced detector as a function of said particular angular segments, where said gamma rays result from interaction of gamma rays from said source with said formations, and where said count rate of gamma rays of said short spaced detector and of said long spaced detector are recorded as to their respective energy levels called windows, thereby producing a spectrum of count rates with certain higher energy level windows being designated as hard windows and with certain lower energy level windows being designated as soft windows,
• computer program means for determining average density, called .rho..sub.AVG, of the entire formation, and
• computer program means for determining a macroscopic cross section, called U.sub.AVG, of the entire formation as a function of total soft window count rate of one of said detectors and total hard window count rate of said one of said detectors, and
• computer program means for determining an average PEF of said formation as a ratio of said macroscopic cross section to said average density, that is, ##EQU15##

Apparatus for determining a rotational value of photoelectric effect, called PEF.sub.ROT, of earth formations surrounding a borehole comprising,

• a logging while drilling tool including a source of radiation, a short spaced gamma ray detector, and a long spaced gamma ray detector,
• means for identifying particular angular segments of said borehole through which said short spaced detector and said long spaced detector pass while said tool is rotating in said borehole,
• means for recording for a predetermined time period a count rate of gamma rays in said short spaced detector and in said long spaced detector as a function of said particular angular segments, where said gamma rays result from interaction of gamma rays from said source with said formations, and where said count rate of gamma rays of said short spaced detector and of said long spaced detector are recorded as to their respective energy levels called windows, thereby producing a spectrum of count rates with certain higher energy level windows being designated as hard windows, such count rates of hard windows from one of said detectors called HARD COUNT RATE, and with certain lower energy level windows being designated as soft windows, such count rates of soft windows from said one of said detectors called SOFT COUNT RATE,
• computer program means for determining rotational correction factors .DELTA.CR.sub.SOFT and .DELTA.CR.sub.HARD respectively from statistical distributions of HARD COUNT RATE and SOFT COUNT RATE,
• computer program means for determining a signal representative of macroscopic rotational cross section factor U.sub.ROT as a function of said HARD COUNT RATE, SOFT COUNT RATE, and said rotational correction factors .DELTA.CR.sub.SOFT and .DELTA.CR.sub.HARD,
• computer program means for determining a density correction signal called .DELTA..rho..sub.ROT representative of a density correction factor to correct a density measurement of said borehole for borehole irregularity as a function of said HARD COUNT RATE and said rotational correction factor .DELTA.CR.sub.HARD,
• computer program means for determining, from said long and short spaced detectors hard window count rates, signals representative of density called .rho..sub.L and .rho..sub.S, respectively,
• computer program means for determining a signal proportional to density corrected for borehole irregularity, called .rho..sub.b ROT, as a function of .rho..sub.L, .rho..sub.S and .DELTA..rho..sub.ROT, and
• computer program means for determining a signal representative of said rotational value of photoelectric effect as the ratio of said U.sub.ROT and .rho..sub.b ROT signals, that is,
• PEF.sub.ROT =U.sub.ROT /.rho..sub.b ROT.

Apparatus for determining neutron porosity of an earth formation surrounding an inclined borehole comprising

• a logging while drilling tool including a source of neutrons, near spaced and far spaced detectors of neutrons which result from interaction of neutrons from said source of neutrons with said formation, and an ultrasonic sensor;
• means for determining a bottom contact point of said tool which contacts said inclined borehole while said tool is rotating in said borehole;
• computer program means for defining a bottom angular distance segment, called SEGMENT.sub.BOTTOM of said borehole which includes said bottom contact point;
• means for recording, for a predetermined length of time, a far neutron count rate from said far spaced detector of neutrons and a near neutron count rate from said near spaced detector of neutrons for said bottom angular distance segment;
• means for determining a measured average BOTTOM STANDOFF from ultrasonic sensor measurements in said bottom angular distance segment;
• and
• computer program means for determining average neutron porosity as a function of near neutron count rate and far neutron count rate measured in said SEGMENT.sub.BOTTOM corrected by said BOTTOM STANDOFF.

Apparatus for determining formation heterogeneity surrounding a borehole comprising

• a logging while drilling tool having a cross-section which is orthogonal to a longitudinal axis of said tool, said tool including an energy source and a sensor,for generating signals from energy stimuli returning to said tool from said formation surrounding said borehole;
• means for separating said cross-section into a plurality of angular distance segments;
• means for applying energy into said formation surrounding said borehole from said energy source disposed in said tool as said tool turns in said borehole during drilling;
• means for recording measurement signals received at said sensor disposed in said tool where said signals are in response to returning energy which results from the interaction of applied energy with said formation;
• means for associating said measurement signals with energy returning from said formation while said sensor is in each particular one of said plurality of angular distance segments;
• computer program means for deriving at least one signal characteristic of said formation surrounding said borehole as a function of said measurement signals for each of said angular distance segments, and
• computer program means for identifying formation heterogeneity as a function of said angular distance segments by comparing said at least one signal characteristic of said formation from one segment to another.

Apparatus for determining whether apparent mud density in a borehole is greater than or less than the apparent density of an earth formation surrounding said borehole comprising

• a logging while drilling tool system including a source of gamma radiation, long and short spaced gamma ray detectors and an ultrasonic transducer;
• means for dividing a cross-section of said borehole into plural borehole angular distance segments,
• means for determining a representation of STANDOFF from measurements of said ultrasonic transducer of all such angular distance segments, identifying that angular distance segment characterized by the greatest STANDOFF of all such angular distance segments as SEGMENT.sub.STANDOFF, and identifying that angular distance segment characterized by the least STANDOFF of all such angular distance segments as SEGMENT.sub.NO STANDOFF ;
• means for producing count rate signals representing hard window count rates of gamma rays from said formation during successive time increments from said long spaced gamma ray detector and from said short spaced gamma ray detector while said tool is rotating and associating each of said count rate signals with one of said borehole angular distance segments,
• means for determining the count rates called CR.sub.STANDOFF, and CR.sub.NO STANDOFF respectively of said SEGMENT.sub.STANDOFF and said SEGMENT.sub.NO STANDOFF, and
• means for determining that apparent formation density is greater than apparent mud density if CR.sub.STANDOFF is greater than CR.sub.NO STANDOFF, and vice versa.