Abstract:
An apparatus and method for a wellsite γ-ray analysis of core samples is implemented. A wheeled carriage supporting a γ-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 γ 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 γ 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 γ emission of the core may be observed by traversing the apparatus along the core without the radionuclide source.

Description:
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 (γ) rays from an artificial source can be used to determine the density of a core sample taken from a stratigraphic exploration well. A γ-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 γ 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 γ 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 γ-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 γ-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 γ-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 γ-ray analysis apparatus stably straddling the core and detecting natural γ activity from the core. There is also provided a method of core analysis including the steps of providing a γ-ray analysis apparatus stably straddling the core, and detecting γ-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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1A illustrates an apparatus for determining core quality and recovery in cores within a core barrel liner; 
     FIG. 1B illustrates a portion of an embodiment of the apparatus of FIG. 1A; 
     FIG. 1C illustrates a portion of another embodiment of the apparatus of FIG. 1A; 
     FIG. 2 illustrates a calibration structure used with the apparatus of FIG. 1; and 
     FIG. 3 illustrates an exemplary calibration curve obtained with the calibration structure of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     A portable γ-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. Aγ-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 γ-ray source, such that the γ-source is disposed opposite the γ-ray detector and with the core barrel, and the core within, disposed therebetween. γ-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 γ-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 γ-ray apparatus  100  in accordance with the present invention. Carriage  101  supports γ-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 γ-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.  1 B. 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, ρ. In an exemplary embodiment wherein the contour is a circular arc, the arc may have a radius, ρ 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.  1 C. 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, φ, substantially determined by 1−h/a=sinφ. 
     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 γ-source  116  and γ-ray scintillator  104 . γ-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 γ-ray apparatus  100 , FIG.  1 . In this way, γ-source  116  is disposed on an opposite side of core barrel  106  from γ-ray scintillator  104 . Gamma rays emitted by γ-source  116  in the direction of γ-ray scintillator  104 , first pass through core barrel  106  and core sample  108  contained within. The flux of such γ-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 s   137 . However, other γ-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 γ-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 γ-ray apparatus of the present invention may be used without γ-source  116  to measure the natural γ 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 γ-source  116 , or naturally emitted γ radiation from core sample  108  passing to γ-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™ gamma system manufactured by Oxford Instruments, Inc. Gamma-ray scintillator  104  emits lights in response to the γ 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™ software of Oxford Instruments, Inc., and included in the NanoSpec-2CS system, which software is compatible with the Windows 95™ 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 γ-ray spectrum providing a γ-ray intensity as a function of the energy of the γ-rays, which as noted hereinabove may be the γ-rays emitted from a γ-source  116 , or alternatively, naturally occurring γ radiation from core sample  108 . 
     In order to reduce γ-ray backgrounds reaching scintillator  104 , γ-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™ 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 γ-ray signal within a defined energy window, after calibration of the γ-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 γ-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 γ spectrum. Calibration of the MCA is performed using techniques that are known in the γ-ray detection art. In an embodiment of the present invention using a commercial detector assembly, such as the NanoSpec-2CS™ system of Oxford Instruments, Inc., calibration of the MCA may be performed in accordance with procedures provided with the ASSAYER™ software instruction manual. 
     After calibration of the MCA, the γ-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 γ-source  116  to correct for the presence of the background γ 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 γ 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 γ 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, γ 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 γ 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): 
     
       
         Density=0.0003×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 γ 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):                  Porosity                   (   %   )       =           Grain                 density     -     Bulk                 density           Grain                 density     -     Fluid                 density         ·   100       ,           (   2   )                                
     where the bulk density is the density value obtained from the γ 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 γ 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 γ 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 γ bulk density determination, may be useful in order that remedial measures may be expeditiously taken, thereby saving both time and expense. 
     A portable γ-ray apparatus, which may be used for well-site measurements of core density, and the natural γ spectra of a core sample has been provided. The γ 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 γ apparatus of the present invention may be suited to the γ 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.