Portable gamma apparatus for core analysis and method therefor

An apparatus and method for a wellsite .gamma.-ray analysis of core samples is implemented. A wheeled carriage supporting a .gamma.-ray detector stably straddles a core sample, which may be encased in a core barrel. A bracket attached to the carriage may hold a radionuclide source of .gamma. radiation positioned distally from the detector, thereby forming a space therebetween for passage of the core. The density of the core may be determined by counting the .gamma. flux attenuated by the core; by traversing the carriage along a length of the core, the density may be determined as a function of position, and disrupted core or partial recovery detected thereby. Additionally, the natural .gamma. emission of the core may be observed by traversing the apparatus along the core without the radionuclide source.

TECHNICAL FIELD
 The present invention relates in general to the determination of porosity
 in petroleum well cores, and in particular, to the determination of core
 density using gamma-ray densitometry.
 BACKGROUND INFORMATION
 The attenuation of gamma (.gamma.) rays from an artificial source can be
 used to determine the density of a core sample taken from a stratigraphic
 exploration well. A .gamma.-ray analysis of the core at the wellsite is
 used in order to make the preliminary selection of portions of the core
 which are to be further analyzed in a laboratory. Furthermore, preliminary
 analysis of the core at the drilling site may be useful in guiding the
 drilling of additional core samples. A system for the wellsite analysis of
 core samples, for natural .gamma. activity, has been described in U.S.
 Pat. No. 4,854,163 to Mount, et al. Mount, et al. is directed to an
 analysis of the natural .gamma. activity of the core sample, which is
 useful for correlating positions along the core sample with locations
 within the borehole.
 Modern coring technology uses coring techniques in wells which are lined
 with an inner barrel. These yield core samples which are clad by the
 barrel. Thus, visual observation of the core quality and of recovery is
 precluded because the barrel is opaque. (Recovery, the length of core
 obtained, may be less than the length attempted.) Typically, it is
 impractical to remove the core from the pipe at the wellsite for such
 observations. Moreover, barrel-clad core samples are used to obtain core
 samples in unconsolidated strata. In such strata, the core sample, if not
 confined by the barrel, would disintegrate into an unstratified mix of the
 constituent core material, rendering the analysis of the core useless.
 Such strata are commonly encountered at off-shore wellsites. Therefore, a
 wellsite analysis system similar to that taught in Mount, et al. may not
 be usable for the well-site analysis of barrel-liner-clad cores. Thus, in
 order to access the core within the pipe for analysis, the core sample
 must be frozen, typically using cryogenic means. The frozen core may then
 be sectioned for analysis. However, these methods are usually impractical
 at the wellsite.
 Consequently, barrel-clad cores must be transported off site for analysis,
 which is costly in both time and expense. Or, wellsite analysis relies on
 simple handheld Geiger counters to preliminarily analyze the cores, a
 process which is prone to inaccuracy because the Geiger counters are not
 shielded from background radiation and counts are not energy analyzed
 Therefore, there is a need in the art for a portable apparatus, and method
 of using the same, for performing a .gamma.-ray analysis of barrel
 liner-clad cores.
 SUMMARY OF THE INVENTION
 The aforementioned needs are addressed by the present invention.
 Accordingly there is provided, in a first form, an apparatus for core
 analysis having a carriage operable for supporting a .gamma.-ray detector,
 wherein the carriage is operable for straddling the core and stably
 traversing a length thereof. The apparatus also includes a support
 attached to the carriage operable for mounting a .gamma.-ray source
 distally of the detector and forming a space therebetween operable for
 passage of the core.
 Additionally, there is provided, in a second form, a method of core
 analysis including the steps of providing a .gamma.-ray analysis apparatus
 stably straddling the core and detecting natural .gamma. activity from the
 core. There is also provided a method of core analysis including the steps
 of providing a .gamma.-ray analysis apparatus stably straddling the core,
 and detecting .gamma.-rays emitted from a radionuclide source and passing
 from the source through the core.
 The foregoing has outlined rather broadly the features and technical
 advantages of the present invention in order that the detailed description
 of the invention that follows may be better understood. Additional
 features and advantages of the invention will be described hereinafter
 which form the subject of the claims of the invention.

DETAILED DESCRIPTION
 A portable .gamma.-ray spectrometer apparatus which may be used for
 determining the core density and porosity of a core sample within a core
 barrel liner is provided. A.gamma.-ray detector is mounted on a carriage,
 or skate, assembly. The carriage assembly engages a portion of the core
 barrel, and is traversable along the core barrel on rollers which run on
 an outer surface of the core barrel. A bracket assembly attached to the
 skate supports an artificial .gamma.-ray source, such that the
 .gamma.-source is disposed opposite the .gamma.-ray detector and with the
 core barrel, and the core within, disposed therebetween. .gamma.-rays from
 the artificial source pass through the core barrel and core sample and
 then into the detector.
 In the following description, numerous specific details are set forth, such
 as specific .gamma.-source radioisotopes, to provide a thorough
 understanding of the present invention. However, it will be obvious to
 those skilled in the art that the present invention may be practiced
 without such specific details.
 Refer now to the drawings wherein depicted elements are not necessarily
 shown to scale and wherein like or similar elements are designated by the
 same reference numeral through the several views. FIG. 1 illustrates, in
 front elevation section and right-side elevation section, a portable
 .gamma.-ray apparatus 100 in accordance with the present invention.
 Carriage 101 supports .gamma.-ray scintillator 104 in proximity to core
 barrel 106 containing core sample 108. Carriage 101 includes a detector
 support 102 and a base 103.
 Contact rollers 110 are rotatably attached to base 103 of carriage 101.
 Contact rollers 110 contact an upper portion of an outer surface 112 of
 core barrel 106, as illustrated in the side elevation in FIG. 1, showing
 the .gamma.-ray apparatus of the present invention in a right-side
 elevational section. Contact rollers 110 have a surface 114 having a
 contour adapted for contacting surface 112 of core barrel 106 in a
 substantially tangential fashion.
 A portion 140 of apparatus 100 illustrating surfaces 114 and 112 in further
 detail is shown in FIG. 1B. Surface 114 of each contact roller 110
 contacts surface 112 of core barrel 106, thereby supporting apparatus 100
 and rollers 110 allow it to traverse the length of the core barrel. In an
 embodiment of the present invention, the contour of surface 114 may be a
 substantially circular arc, having a radius, .rho.. In an exemplary
 embodiment wherein the contour is a circular arc, the arc may have a
 radius, .rho. of approximately 0.2 inches, in association with a roller
 having a width, W, of approximately 0.5 inches and a diameter, D, of
 approximately 1.125 inches. However, it would be understood by an artisan
 of ordinary skill that alternative embodiments having circular arc
 contours of other radii would be within the spirit and scope of the
 present invention.
 In an alternative embodiment, the contour may be a bevel. Such an
 embodiment is illustrated in FIG. 1C. Core barrel 106 may be substantially
 a circular cylinder having a predetermined outer radius a, wherein contact
 roller 110 has a line of contact with surface 112 substantially parallel
 to a generatrix of core barrel 106 and located a predetermined distance,
 h, below a top of core barrel 106. The beveled portion of surface 114 may
 have an angle, .phi., substantially determined by 1-h/a=sin.phi..
 Each contact roller 110 is attached to base 103 of carriage 101 using a
 shoulder bolt 142 and a retaining nut 144. Roller 110 is separated from
 base 103 by a pair of washers 146 and a thrust bearing 148 (shown exploded
 in FIG. 1B for clarity). In this way, carriage 101 may be stably supported
 by core barrel 106, mitigating transverse slipping of apparatus 100, and
 may also be translated along the length of core barrel 106.
 Additionally, the support of carriage 101 by core barrel 106 maintains a
 substantially uniform spacing between .gamma.-source 116 and .gamma.-ray
 scintillator 104. .gamma.-source 116 is supported by an "L-shaped" bracket
 having lower portion 118, and upper portion 119 that is attached to
 carriage 101. Lower portion 118 and upper portion 119 of the L-shaped
 bracket are more clearly illustrated in the right-side elevational section
 of .gamma.-ray apparatus 100, FIG. 1. In this way, .gamma.-source 116 is
 disposed on an opposite side of core barrel 106 from .gamma.-ray
 scintillator 104. Gamma rays emitted by .gamma.-source 116 in the
 direction of .gamma.-ray scintillator 104, first pass through core barrel
 106 and core sample 108 contained within. The flux of such .gamma.-rays is
 thereby attenuated by the material of core barrel 106 and core sample 108.
 By measuring the attenuation for known samples of material within a given
 core barrel 106, calibration curve can be obtained whereby the density of
 core sample 108 may be inferred. This will be discussed further in
 conjunction with FIGS. 2 and 3 below. Gamma source 116 may be
 C.sub.s.sup.137. However, other .gamma.-emitting radioisotopes are known
 in the art, and it would be understood by a practitioner of ordinary skill
 that such other radioisotopes may be used with the present invention. By
 maintaining a substantially uniform distance between .gamma.-source 116
 and scintillator 104, an improved measurement of the properties of core
 sample 108 in a well-site environment may be obtained with the present
 invention.
 Additionally, the .gamma.-ray apparatus of the present invention may be
 used without .gamma.-source 116 to measure the natural .gamma. activity of
 core sample 108. As previously described, such measurements, for example,
 be used in correlating positions along core sample 108 with locations
 within the borehole.
 Gamma rays, either from a .gamma.-source 116, or naturally emitted .gamma.
 radiation from core sample 108 passing to .gamma.-ray scintillator 104,
 which may be a commercially available NaI scintillator. In an embodiment
 of the present invention, scintillator 104 may be included in a commercial
 detector assembly, such as a NanoSpec-2CS.TM. gamma system manufactured by
 Oxford Instruments, Inc. Gamma-ray scintillator 104 emits lights in
 response to the .gamma. radiation, and such light is detected by
 photomultiplier assembly 120. Photomultiplier assembly 120 generates a
 signal in response to the emitted scintillation light impinging thereon,
 which signal is encoded in a serial data format and input to a serial
 input of computer 122 via cable 124. Photomultiplier signals are
 interpreted by multichannel analyzer (MCA) software running on computer
 122 which may be commercially available MCA software, such as the
 ASSAYER.TM. software of Oxford Instruments, Inc., and included in the
 NanoSpec-2CS system, which software is compatible with the Windows 95.TM.
 operating system. Computer 122 may be a commercial "laptop" computer as
 are well known in the data processing art, running an operating system
 compatible with the analyzer software. Computer 122 may typically include
 program storage media and circuitry for storing information well-known in
 the data processing art, such as disk storage devices including a hard
 disk and a floppy disk drive.
 Photomultiplier assembly 120 may be included in a commercial detector
 assembly such as the NanoSpec-2CS detector manufactured by Oxford
 Instruments, Inc. Computer 122, under the control of the multichannel
 analyzer software, outputs a .gamma.-ray spectrum providing a .gamma.-ray
 intensity as a function of the energy of the .gamma.-rays, which as noted
 hereinabove may be the .gamma.-rays emitted from a .gamma.-source 116, or
 alternatively, naturally occurring .gamma. radiation from core sample 108.
 In order to reduce .gamma.-ray backgrounds reaching scintillator 104,
 .gamma.-ray scintillator 104 is surrounded by lead shield 126.
 Additionally, aluminum housing 128 surrounds lead shield 126, thereby
 protecting the soft lead shield, and isolating it from the environment.
 Electrical power is supplied to photomultiplier assembly 120 by battery
 pack 129, shown in the right-side elevation in FIG. 1. Battery pack 129 is
 held in place by battery bracket 130 and electrical power provided to
 photomultiplier assembly 120 by power cable 131. It would be understood by
 an artisan of ordinary skill that battery pack 129 includes a number of
 battery cells of a size and type sufficient to power photomultiplier
 assembly 120 in accordance with a design thereof. For example, in an
 embodiment of the present invention using the NanoSpec-2CS.TM. gamma
 system manufactured by Oxford Instruments, Inc., battery pack 129 includes
 four size "D" cells serially connected, each of which provides a voltage
 of approximately 1.5 volts.
 The density of core sample 108 may be determined by measuring the
 .gamma.-ray signal within a defined energy window, after calibration of
 the .gamma.-ray apparatus of the present invention, using materials of
 known density. The materials are formed into a geometry which simulates a
 core measurement geometry. Such materials may be referred to as
 "calibration billets." An arrangement of a core barrel and calibration
 billets which may be suitable for calibrating the .gamma.-ray apparatus of
 the present invention is illustrated in FIG. 2. At position 1 along core
 barrel 106, the core barrel 106 is empty, forming a void space 202, such
 as may be found in a core sample 108 having a missing core interval. At
 position 2 along core barrel 106, a polycarbonate billet 204 is located,
 and at position 3 along core barrel 106 an aluminum billet 206 is located.
 Source 116 is first used to calibrate the MCA. Calibration of the MCA
 associates one or more energy channels with a known .gamma. spectrum.
 Calibration of the MCA is performed using techniques that are known in the
 .gamma.-ray detection art. In an embodiment of the present invention using
 a commercial detector assembly, such as the NanoSpec-2CS.TM. system of
 Oxford Instruments, Inc., calibration of the MCA may be performed in
 accordance with procedures provided with the ASSAYER.TM. software
 instruction manual.
 After calibration of the MCA, the .gamma.-ray source is removed from
 bracket 118, and a measurement of background radiation is made. The
 background counts may then be subtracted from the counts made in the
 presence of .gamma.-source 116 to correct for the presence of the
 background .gamma. radiation. Source 116 is then replaced, and a number of
 counts is made for a fixed, pre-selected, time interval. An exemplary
 count result from such a measurement made with .gamma. apparatus 100 in
 accordance with the principles of the present invention is illustrated in
 the graph shown in FIG. 3.
 In FIG. 3, plots of density and porosity versus the number of .gamma.
 counts is illustrated. The number of counts corresponding to the void
 spaces indicated by the point labeled "void space," and is plotted having
 an ordinate corresponding to a density of zero grams per cubic centimeter
 (g/cc).
 Returning to FIG. 2, .gamma. apparatus 100 is positioned over polycarbonate
 billet 204, at position 2. The measured number of counts is then plotted
 on the abscissa, in FIG. 3, at a density value on the ordinate
 corresponding to the density of polycarbonate, approximately 1.2 g/cc.
 Apparatus 100 is then positioned over aluminum billet 206 and a .gamma.
 count over the pre-selected interval of time is made. The number of counts
 is then plotted on the abscissa in FIG. 3 at the known density of
 aluminum, approximately 2.7 g/cc. This point is labeled "Al" in FIG. 3. A
 linear regression of the billet density in counts yields the straight line
 labeled "Density" in FIG. 3, from which the density of an unknown core
 sample 108 may be inferred. Such a plot may be referred to as a
 "calibration transform." Alternatively, the density may be computed by
 using the equation for the straight line density curve in FIG. 3, Equation
 (1):
EQU Density=0.0003.times.counts+3.94189 (1)
 In the exemplary calibration transform of FIG. 3, core barrel 106 was made
 of aluminum. It would be understood by an artisan of ordinary skill that
 for other core barrels, a different calibration transform would be
 obtained and, consequently, a regression equation different from Equation
 (1) would result. By taking a .gamma. count of an unknown core sample 108
 in a core barrel 106 of the same composition used in the calibration, for
 the pre-selected interval of time, the density of the core sample may be
 inferred by locating the number of counts on the density curve of FIG. 3,
 and reading the value of the density at that point on the density scale,
 which is the "left-hand" scale in FIG. 3 or, alternatively, using a
 calibration transform equation similar to Equation (1). Gamma measurement
 data may be stored on a computer readable storage medium, for example, a
 hard disk or a floppy disk, for subsequent analysis.
 From the measurement of the density of the core sample 108, a porosity may
 be inferred. The porosity of core sample 108 may be related to the density
 using Equation (2):
 ##EQU1##
 where the bulk density is the density value obtained from the .gamma. count
 measurement on core 108, and the calibration transform corresponding to
 barrel 106. The grain density is the density of the mineral composition
 from which the core sample is formed, and is initially assumed based on
 the lithology of the formation from which the sample is taken. The fluid
 density is the density of any fluids which are trapped in the formation
 from which the core is taken, and may include a mixture of fluid types,
 wherein the fluid density is a weighted average calculated from the
 density of each fluid in the mixture and the fractional amount of the
 fluid in the mixture. With a value of 2.65 g/cc for the grain density,
 typical of Gulf Coast and Mid-Continent, or similar, sands, and a value of
 1.0 g/cc for the fluid density (i.e. water), and using the density
 transform in FIG. 3 to determine a density in terms of an observed .gamma.
 count, the exemplary porosity curve, labeled "Porosity," illustrated in
 FIG. 3, is obtained. This curve may be used to determine the porosity of
 core sample 108, in FIG. 1, from the measured .gamma. count. It would be
 understood by an artisan of ordinary skill, that other porosity curves
 would be obtained for other, predetermined grain and fluid densities.
 Density and porosity information may be stored on a computer readable
 storage medium, such as a hard disk or floppy disk.
 It is seen from the curves in FIG. 3, as well as Equations (1) and (2),
 that as the sample porosity increases, the density correspondingly
 decreases. Thus, an anomalously high porosity, for example greater than
 sixty percent (60%), the density is correspondingly anomalously low, less
 than 1.66 g/cc, for the exemplary curves in FIG. 3. Such a density would
 indicate a partial recovery or a disrupted or disturbed interval wherein
 the core sample may be fragmented. If the density approaches 1.00 g/cc,
 then an inference of missing core would be made. Such information obtained
 at the wellsite, predicated on a reliable .gamma. bulk density
 determination, may be useful in order that remedial measures may be
 expeditiously taken, thereby saving both time and expense.
 A portable .gamma.-ray apparatus, which may be used for well-site
 measurements of core density, and the natural .gamma. spectra of a core
 sample has been provided. The .gamma. apparatus of the present invention
 is suitable for well-site measurements on core samples enclosed in a core
 barrel, without the need for removing the core sample from the barrel. In
 this way, the .gamma. apparatus of the present invention may be suited to
 the .gamma. analysis of unconsolidated core samples.
 Although the present invention and its advantages have been described in
 detail, it should be understood that various changes, substitutions and
 alterations can be made herein without departing from the spirit and scope
 of the invention as defined by the appended claims.