CA2063218C - Stand-off compensated formation measurements apparatus and method - Google Patents

Abstract

Apparatus and method for measuring density, porosity and other formation characteristics while drilling is dis-closed. The apparatus, preferably housed in a drill collar and placed within a drill string, includes a source of neu-trons and a source of gamma rays placed within a tubular body which is adapted to provide for the flow of drilling through it. Two sets of stabilizer blades are provided. One set, associated with the neutron source, includes secondary radi-ation detectors that are placed radially beyond the nominal outer radius of the body. Formation porosity measurement accuracy is substantially enhanced since the standoff of the detectors from the formation is substantially decreased.
Another set, associated with the gamma ray source, includes one or more gamma ray detection assemblies in a single blade.
Each of the gamma ray detector assemblies is also placed radially beyond the nominal outer radius of the tubular wall.
Formation density or absorption coefficient accuracy is sub-stantially enhanced since the standoff between the detection assembly and the borehole wall is decreased. In a particular-ly preferred embodiment, ultrasonic sensors incorporated either or both sets of blades provide a measurement of the borehole diameter and/or standoff of the detectors from the borehole. The bulk density measurement and neutron porosity measurement data can be corrected with information derived from the standoff or borehole diameter. All such information is preferably transmitted to surface instrumentation where the corrections are performed, or performed downhole and the corrected data transmitted to the surface.

Description

-STAND-OFF COMPENSATED FORMATION

MEASUREMENTS APPARATUS AND METHOD

Thls lnventlon relates generally to devlces and methods for measurlng earth formatlon propertles such as poroslty, density and photoelectrlc absorptlon coefflclent.
The inventlon ls preferably embodled ln a drllllng collar such that these measurements may be made whlle drllllng. Stlll more partlcularly, the lnventlon relates to a method and apparatus by whlch neutron poroslty and gamma-gamma denslty measurement data can be collected wlth lncreased accuracy by decreaslng detector-borehole standoff, and through measurement and determlnatlon of detector standoff, the compensatlon of such measurement data as a functlon of such standoff.

BACKGROUND OF THE INVENTION
A measuring whlle drllling apparatus for maklng poroslty, denslty and other formatlon characterlstlc measure-ments ls descrlbed ln U.S. patent 4,879,463 lssued November 7, 1989 and asslgned to the asslgnee of the present lnventlon.
The patent descrlbes a drllllng collar ln whlch two radloac-tlve sources are provlded. A neutron source ls posltloned near the cyllndrlcal axls of the tool whlle a gamma ray source ls eccentered agalnst an lnterlor slde of the collar's cylln-drlcal body. Both sources can be axlally lnserted lnto and removed from the body's interlor vla one end of the body.
Secondary radlatlon detectors for the ~r 206~218 porosity measurement are provided within an interior cylindrical body secured within the collar cylindrical body. Similarly, gamma radiation detectors are arranged within the interior body.
Stabilizer blades provided about the outer radius of the cylindrical tool aid in the drilling process. Opening~ in a blade angularly aligned with the gamma radiation detectors are aligned with openings in the steel cylindrical body which are also aligned with the detectors. Radiation transparent materials are provided in the openings of the steel body and the ad~acent stabilizer blade.
The apparatus described above represent~ significant advances in the field of performing porosity and neutron measurements of surrounding formations while drilling a borehole. Fir~t, the nuclear ~ources are placed within the body of the collar on a ~retrievable carrier which i~ loaded into the collar from its end.
When inserted, the gamma ray source automatically is properly placed in an eccentered position in the collar; the neutron source - is placed on the center-line of the collar. Advantageously, if the drill collar were to become stuck in the hole, a fishing head placed at the top of the carrier may be latched by means of fishing equipment such a~ a wireline-conveyed over~hot through the center or mud flow path of the drill string. The carrier with both nuclear sources may then be brought to the ~urface.
Performing porosity measurements and density measurements while drilling results in certain advantages over conventional wireline porosity and density measurements. Longer sample periods due to the slower nature of the drilling process reduce the statistical variations and uncertainty in measuring while drilling porosity and density measurements. Many of the borehole effects that perturb wireline measurements of porosity or density are reduced because the drill collar substantially fills the borehole while drilling. Also, formation effects, lithology and salinity changes under drilling conditions are comparable to or less than those for an open hole wireline measurement which may occur hours or even days after the borehole iB drilled. However, the washing action of drilling fluid while drilling can produce variations in borehole size. Increased variations in borehole diameter are called washouts. Separation, or "standoff", of the tool from the borehole wall causes measured data perturbations. The occurrence of washouts exacerbates the standoff effect.
The apparatus of U.S. patent 4,879,463 described above performs well under ordinary drilling conditions. For example, where an eight and one half inch (8 1/2n) drill bit is used, a six and one half inch (6 1/2n) drill collar is used above it. With the detectors within the coll~r cylindrical body, approximately a one inch standoff exists between the tool and the borehole wall.
However, where larger size holes are drilled, for example with a twelve and one-quarter inch (12 1/4") drill bit, an eight inch (8") drill collar is typically used above it. The combination of such a 12 1/4" bit and an 8" collar results in a nominal two inch standoff between the tool and the borehole wall. Such large ~tandoffs are disadvantageous as explained above.

2~63218 .
One -measuring while drilling assembly is schematically illustrated in an advertisement brochure of Gearhart Geodata Services. As best can be understood from the schematic illustrations, a radiation source and near and far detectors are placed in one of four stabilizer fins of a cylindrical body of a MWD porosity tool. Except for the diameter of the device, its physical characteristics appear to be e~sentially the same as a conventional wireline compensated neutron poro~ity tool in that a conventional compensated neutron porosity tool is designed to be run eccentered in the borehole. In other words, a single source, near detector, and far detector eccentered alignment appears to be contemplated in the p~G~O-^~ Gearhart Geodata Services device with a stabilizer blade serving as the mechanism for providing eccentering of the source and detectors.
A similar measurement while drilling neutron porosity tool i~
described in a brochure of TELECO OI~FIELD SERVICES, INC. bearing a copyright notice of 1990 with a further notation of 5/90. The brochure describes a similar drill collar with a source and a single pair of near and far detectors aligned with the source. The tool includes a 6 3/4~ diameter mandrel with a 7 1/2" upset. Three fluted chAnn~ls located in the upset allow for return mud circulation. The source and detector~ are aligned with one of the three resulting "vanes" but apparently are not placed radially beyond the nominal 6 3/4 n diameter.

20~3218 OBJECTS OF THE INVENTION
A general ob~ect of the present invention is to extend radially the position of radioactivity sensors in a logging while drilling tool where the nominal standoff of the cylindrical body of the tool and thQ borehole wall is relatively large.
Another ob~ect of this invention is to place both ultrasonic sensors and radiation detectors radially beyond a nominal cylindrical body radius of a logging while drilling tool in order to enhance the measurement of tool standoff and formation characteristic~ by reducing the distancQ between sensors and detectors and the formation wall.
It is an ob~ect of this invention to provid~ a neutron porosity while drilling tool with a plurality of symmetrically azimuthally placed secondary radiation detectors exten~ng beyond ~the nominal outer radius of the tool and partially into stabilizer blades to enhance porosity data acquisition accuracy.
It i8 another ob~ect of thi~ invention to provide a neutron porosity while logging tool with ultrasonic sensors for mea~uring the diameter of the borehole and thereby providing a correction to the neutron porosity measurement.
Another ob;ect of the invention is to provide a gamma-gamma density while drilling tool with near and far gamma ray detectors exten~ng beyond the nominal outer radius of the tool and partially into a ~tabilizer blade azimuthally aligned with an eccentered gamma ray source to enhance density data acquisition accuracy.
Anothe~ ob~ect of the invention i~ to provide a gamma-gamma density while drilling tool with ultrasonic sensors ~or the measurement of standoff between the tool and the borehole wall.
Another ob~ect of the invention is to provide a measuring while drilling tool with neutron porosity measuring apparatus, gamma-gamma density measuring apparatus, and ultrasonic caliper/standoff measuring apparatus.
Another ob;ect of the invention is to provide a measuring while drilling tool with detachable stabilizer blade covers which facilitate placement in and removal of radiation detectors and also changing the size of the stabllizers.

SUMMARY OF THE I~v~~ ON
The ob~ects identified above along with other advantages and features of the invention are achieved with a logging-while-~drilling (LWD) apparatu~ such a~ a drill collar equipped withporosity, density or photoelectric absorption coefficient, and standoff measurement sensor~. The porosity radioactivity detectors are placed at least partially outwardly of the tubular body of the collar within stabilizer blades. Such placement results in the detectors being relatively closer to the borehole wall and decreases the measurement error of collected data caused by borehole fluid and formation cuttings which exist in the standoff space between blade and borehole wall.
The density or absorption coefficient gamma ray sensors are placed outwardly of the tubular body of the collar within one blade of an additional set of stabilizer blades. Such placement 2063~ls decreases the measurement error of collected data caused by gamma rays travelling through the standoff between the tool and the formation.
In order to further increase the ultimate accuracy of the determination of formation porosity and formation density or photoelectric absorption coefficient, ultrasonic sensors are placed on stabilizer blades to collect acoustic data useful in determining tool standoff and borehole diameter. Such acoustic data and radioactivity data as a function of borehole depth are transmitted to surface instrumentation for correction of porosity and/or density or absorption coefficient data as a function of tool standoff. Alternatively, the borehole diameter and tool standoff corrections to the porosity and density data, respectively, are performed downhole and the corrected data transmitted to the surface in real time, or stored in downhole memory for later retrieval and playback.
In accordance with the present invention, there is provided a logging-while-drilling tool for measuring characteristics of earth formations surrounding a borehole including a tubular body having upper and lower ends adapted for coupling to a drill string and providing a fluid flow path between its upper and lower ends, said tubular body having a tubular outer radius and a source of neutrons disposed therein, and a plurality of longitudinally spaced radiation detectors carried by said tool and including means responsive to said longitudinally spaced radiation detectors to generate first radiation signals useful for determining a first characteristic of said earth formations, wherein said tool is characterized by a plurality of stabilizer blades secured symmetrically about the outer periphery of said tubular body, a plurality of detector cavities in registration with each of said stabilizer blades, each of said cavities formed by an outwardly facing slot of said body which faces an inwardly facing slot of a stabilizer blade, each of said cavities having an outer radial extent substantially greater than said nominal tubular outer radius, and each of said radiation detectors being placed within one of said cavities.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects, advantages and features of the invention will become more apparent by reference to the drawings which are appended hereto and wherein like numerals indicate like parts and wherein an illustrative embodiment of the invention is shown, of which:
Figure 1 schematically illustrates a measuring while drilling tool placed in a drill string, where the tool measures neutron porosity and formation density or photoelectric absorption 7a -~. ,, ~ ~

coefficient, and preferably simultaneously measures borehole diameter or tool standoff for correcting the measured neutron porosity and/or density or photoelectric absorption coefficient;
Figure 2 i8 a longitudinal cross-section through the measuring while drilling tool illustrating an upper neutron source and detectors and lower gamma ray source and deteetion assemblies and ultrasonie standoff measurement sensors:
Figure 3 is a cross-seetion of the tool of Figure 2 looking downwardly along lines 3-3;
Figure 4 is a eross-seetion of the tool of Figure 2 looking downwardly along lines 4-4; and Figures 5A, 5B, 6A and 6B illustrate with more detail the arrangement of sourees, deteetors, stabilizer fins ete., of the measuring while drilling tool of the invention.

DESCRIPTION OF THE lNv~ lON
Figure 1 illustrates a typieal rotary drilling rig system 5 having apparatus for measurement while drilling of formation porosity, formation bulk den~ity, formation photoelectric absorption coefficient, and borehole diameter associated therewith.
Downhole measurements are conducted by instruments placed in drill collar 20. Sueh measurement~ may be stored in memory apparatus of the downhole in~truments, or may be telemetered to the surface via eonventional measuring-while-drilling telemetering apparatus and methods. For that purpose, an MWD tool sub, ~chematieally illustrated as data signaling module 23, reeeives signals from instruments of collar 20, and telemeters them via the mud path of drill string 6 and ultimately to ~urface instrumentation 7 via a pressure sen~or 21 in ~tand pipe 15.
Drilling rig 5 include~ a motor 2 which turns a kelly 3 by means of a rotary table 4. A drill string 6 includes sections of drill pipe connected end-to-end to the kelly and turned thereby.
A drill collar 20 of this invention, as well as other conventional collars and other MWD tools, are attached to the drilling string 6.
Such collars and tools form a bottom hole drilling assembly between the drill string 6 and the drilling bit 30.
As the drill string 6 and the bottom hole assembly turn, the drill bit 30 bores ths borehole 9 through earth formations 32. An annulus 10 i8 defined as the portion of the borehole 9 between the outside of the drill string 6 including the bottom hole assembly and the earth formation~ 32.
Drilling fluid or ~mud" is forced by pump 11 from mud pit 13 via stand pipe 15 and revolving in~ector head 17 through the hollow center of kelly 3 and drill string 6 to the bit 30. The mud acts to lubricate drill bit 30 and to carry borehole cuttings upwardly to the surface via annulus 10. The mud is delivered to mud pit 13 where it is separated from borehole cuttings and the like, degassed, and returned for application again to the drill string.
The preferred embodiment of the invention is incorporated in a drill collar 20 which is an important component of an improved MWD nuclear logging system from that disclosed in U.S. patent 4,879,463 described above, which is hereby incorporated herein by 20632~8 reference. As shown in more detail in Figure 2 through Figure 6B, the drill collar 20 includes poro~ity measurement section 500 at the upper end of the collar and a gamma-gamma density section 600 at the lower end.
Referring now to Figure 2, the collar or "tool" 20 ("tool" and "collar" are used interchangeably herein) includes a generally tubular outer body 14 preferably including upper thread~ 25 and lower threads 26 by which it is connected to other collars, subs, drill pipe, etc., in drill string 6. Tubular body 14 has a nominal outer radius (or diameter) which is generally a constant dimension as a function of its longitudinal or axial length, except at portions of the neutron porosity section 500 and gamma density section 600 where stabilizer blades are provided.
An inner body 28 is ~ecured and fluidly sealed within outer body 14. O-ring seals 27 illustrated at Figure 5A seal the inner body 28 with the outer body 14. Other seal~ (not shown) at the bottom of the tool provide additional sealing. A mud flow path 29 i~ provided within inner body 28 that i~ sized for carrying drilling fluid from the drill string 6 to bit 30. A~ illustrated best in Figure 2, mud path 29 enters the top of the tool 20 coaxially within inner body 28, extends around fishing neck 54, continues about coaxially po~itioned data signalling cartridge 23 and electronic cartridge 45, and exits coaxially via the bottom end of the tool.
The preferred structure and design for placing a neutron radiation sgurce 57 (~ee Figure 5B) in neutron porosity ~ection 500 and a gamma radiation source in gamma-gamma density section 600 (see Figure 6A) i~ substantially the same as that described in the above mentioned U.S. patent 4,879,463. As best illustrated in Figures 5B and 6A, the neutron source 57 is mounted at the upper end of an elongated flexible carrier 53. The gamma ray source 55 is secured to the lower end of carrier 53. Retrievable carrier 53 is preferably a relatively stiff, yet flexible, solid rod which can be inserted through passage 29 at the top of tool 20, or withdrawn.
However, other types of source location and placement designs may also be used in practicing the present invention including those tools where the source~ are placed in pockets or chambers accessible from the exterior of the tool. Retrievable carrier 53 is described in more detail in commonly-assigned U.S. Patent Application Serial No. 670,850 filed conc~rently herewith and hereby incorporated herein by reference.
To correctly po~ition the radiation ~ources 55 and 57 in the longitudinal bore 29 in the preferred embodiment of the present invention, a centralizing member 64 i di~o~od in the inner body 28. Member 64 include~ a central passage 66 with an upper axially-aligned portion cooperatively arranged for centering the upper end portion of the retrievable carrier 53 in the tubular outer body 14.
An extension 65 of the central passage 66 is further arranged with a lower downwardly-inclined portion 67 that diverts the intermediate portion of the retrievable carrier 53 to the side of the inner body 28' so that the lower portion of the retrievable carrier 53 can be loosely retained with a laterally-offset 2~3218 longitudinal passage 68 that extends along one ~ide of the inner body 28'.
Passage 68 is aligned with the source chamber 47 in the inner body 28'. Accordingly, it will be seen from Figures 5B and 6A that the two interconnected passages 66 and 68 cooperate to correctly position the sources 55 and 57 in the body 14, by virtue of the flexibility of the carrier 53 and the curvatures of the transitional portions of the two passages. The retrievable carrier 53, including neutron source 57 and gamma ray source 55, can be readily inserted into and removed from the tool body. Should tool 20 of the present invention become stuck in the borehole, the removal of the radiation sources 55 and 57 can be ~ccomplished by lowering a suitable wireline-conveyed or tubing-conveyed overshot (not shown) through ths drill string 6 and into the upper end of ~the body 14 until the overshot is ~ecurely coupled to the upstanding fishing neck S4. Removal of th- lower and upper sources SS and 58 may be carried out without disconnecting any electrical - connections. Advantageously, even though the removal of the sources 55 and 57 will render the radioactivity measurement capability of the tool 20 thereafter inoperative, the tool 20 will still be functional ~o that it can continue to provide the other downhole measurements that are independent of either of the radiation sources 55 and 58. So long as the radiation sources 55 and 57 are po~itioned within the tool 20 of the invention, they will be operative to provide radiation to produce successive data signals representative of the formation density and porosity of the earth formations 32 that have been penetrated by the drill bit 30.
Detection o~ ~uch ~ignals is described below with an explanation of the placement of detectors to generate such signals and the correction of such signals with measurement of borehole diameter and tool standoff.
The preferred coaxial placement Or neutron source 57 is advantageous over prior eccentering of such source, a~ is typical in wireline logging tool~. As explained in U.S. patent 4,879,463, coaxial placement of neutron source 57 allows a larger sized source to be used with corre~ponding greater radiation output strength.
An americium-beryllium "chemical" neutron source is preferred, but alternatively, an electronic neutron generator o~ a type well known in the wireline logging industry, if properly ruggedized, would offer advantage~ of electronic shut-off Or neutron radiation.
The gamma-ray radiation source 55 secured to the bottom end of retrievable carrier 53 is preferably an encapsulated chemical source such as a quantity of cobalt or cesium or other suitable radioactive substance that produce~ gamma rays in its decay.
Radiation chamber 47, in which source 55 is placed when carrier 53 is fully inserted within tool 20, is angularly arranged within inner body member 28' 80 as to be angularly and longitudinally aligned with op~n~ng 49 in body 14. As will be explained more fully below, "gamma radiation" stabilizer blade 208 i~ angularly aligned with opening 49.
The opening 49 i~ fluidly sealed by a radiation-transparent member 50. A plug or window 51 in stabilizer blade 208 is placed ` 20~3218 in a hole in blade 208 which i8 aligned with opening 49 of body 14.
Such plug is fabricated with a radiation transparent material such as beryllium, nitrile rubber, or titanium for excluding mudcake or other borehole materials.
The measuring-while-drilling tool or collar 20 exhibits many features not found in the tool of U.S. patent 4,879,463 and other prior radiation type measuring-while-drilling tools. A first feature relates to the placement of secondary radiation detectors 59 and 60 radially beyond the nominal body radiu9 RB of body wall 14. Near radiation detector 59 and far radiation detector 60 may comprise parallel connected helium 3 detector~, which are directly respon~ive to neutron~, or parallel connected Geiger-Mueller tubes, which are 6ensitive to gamma ray~ produced by the interaction between neutrons and the formation nuclei. In order to reduce the effect of borehole materials, (such as drilling fluid, cuttings, etc.) on the radiation transmitted through and returning from the surrounding formation to the detector, the near and far detectors 59, 60 are placed in cavities 58 which are radially provided at least partially in the associated ~tabilization blades 56.
Figures 5A, SB and 3 illustrate the orientation of cavities 58 in which near detectors 59 and far detectors 60 are placed. Figure 3, a cros~-section along lines 3-3 of Figure 2, shows that stabilizer blade- s6 are pre~erably constructed by providing three symmetrically increased diameter or radiu~ sections of body 14.
Such section of increased radius of body 14 i~ illustrated with a radius arrow RIl (representing radiu~ of first increased 20G321~

dimension). Each increased radius section is defined by an angular width, a~ ~een in Figure 3, and a longitudinal or axial length as illustrated in Figure~ 5A and 5B. Over a substantial portion of each increased radius portion, a reduced radius outwardly facing slot is formed of minimum radius RDl (for first decreased radius).
An external cover 155, which is secured to increased diameter bases by a plurality of upper and lower threaded bolts 62, is wider in angular extent and longer in longitudinal length, as illustrated in Figures 2, 3, 5A and 58, than is each increa5ed diameter base. An inwardly facing slot within each cover 155 is substantially aligned with a corresponding slot in the increased radius section to create detector spaces or cavities 58. Radiation near-detectors 59 and radiation far-detector~ 60 are secured within cavities 58 in the positions illu~trated by suitable securing means. Although it is preferred to create the stabilizer blade~ 56 a9 illustrated in a "sandwiched" for~, such blades 56 may be integral with body 14, or covers 155 may be constructed to slide on increased radius sections of body 14.
Longit~ n~l 810t 61 extend~ downwardly from each cavity 58 to lateral slot 63. A channel 165 i~ provided at lateral slot 63 downwardly between inner body 28 and outer body 14. The channel 165, lateral slot 63, and longit~ Al slot 61 provide a cable path for electrical lead~ connected to detectors 59 and 60. Such cable path leads downwardly to electronic cartridge 45 via other passages (not illustrated).
The blade~ 56 may be changed at a well site by providing covers 155 of di~erent radial sizes. For example, stabilizer blades 56 be made "full gauge" for straight drilling or be made "under gauge" for deviated drilling by providing "full gauge"
covers 155 or "under gauge" cover~ 155.
The measuring-while-drilling tool or collar 20 further exhibits features not found in tool of U.S. patent 4,879,463 in that near and far gamma ray detectors are placed at least partially radially beyond the nominal outer radius RB f body wall 14. In a similar construction as described for the stabilizer blades, cavities, etc for the neutron detectors 59 and 60, an increased radius RI2 of an angular width and longit~ nAI length (as shown in Figures 4 and 6A) is provided in the lower part of body wall 14.
An outwardly facing slot of reduced radiu~ RD2 is formed in the increased radiu~ section. A cover 208 is sandwiched over the increased radius wall section and extends beyond the longitudinal ends of the increased radius section and extend~ beyond the angular width of the increased radius section. Such cover 208 is preferably ~ecured to the lower increased radius sections by a plurality of upper and lower threaded bolt~ 63. An inwardly facing slot in cover 208 cooperates with the outwardly facing slot of reduced radius RD2 of wall body 14 to create cavity 90 in stabilizer blade 210. A corresponding blade 210' is provided of similar con~truction a~ shown at Figure~ 2, 4, 6A and 6B but does not include a cavity for placement of detectors. The stabilizer blades 210, 210' and 210~ may, like blades 56 described above, be changed at the well site by providing covers of different radial sizes.

Near gamma ray detector 300 and far gamma ray detector 310 are secured within cavity 90 by conventional securing means. Near and far gamma ray detectors each preferably comprise an inorganic scintillator coupled to a photomultiplier. A suitable scintillation detector may be provided of sodium iodide. Plugs 43 and 44, which are preferably made of material substantially transparent to gamma radiation, fill holes in cover 208 adjacent the scintillators of detectors 300, 310. Beryllium, nitrile rubber, or titanium are the preferred materials for such plugs.
Longit~ n~l slot 212 open~ into radial slot 214 which in turn leads to a space 216. Space 216 communicates with a cable path leading to electronic cartridge 45. A cable including electric leads (not shown) runs from electronic cartridge 45 to detectors 300 and 310.
The measuring-while-drilling apparatus of a particularly preferred embodiment of this invention includes additional sensors to those described in U.S. patent 4,879,463. Ultra~onic sensors 400 (see Figures 1 and 4) are preferably placed in collar 20 in the lower part of body at the same general level as the gamma-gamma density section 600 of the tool. The construction and placement in opposing blades 210' is generally described in commonly-assigned U.S. Patent Application Serial No. 07/525,268, filed on May 16, 1990, which is incorporated herein by reference as if its specification were written here.
Briefly, the ultrasonic sensors 400 of tool 20 are preferably transceivers which emit high frequency acoustic or "sonic" pulses 20~3218 and receive echoes from the borehole wall. TransCeivers 400 provide a tool standoff measurement to determine the hole diameter when the tool is rotating (which is the normal case during drilling), or when the tool is stationary. When the tool is rotating, the transceiver sends the sonic pulse through the mud-filled gap or annulus between the tool and borehole wall. The gap typically varies with the rotation angle. The measured standoffs are accumulated for statistical processing, and the average hole diameter is calculated after several turns. Several standoff measurements are preferably evaluated each second. Because the typical drill string rotation speed is between about 50 to 200 RPM, an average accumulation time from about 10 to about 60 seconds creates enough data for accurate averaging.
Providing a second transceiver diametrically o~ood from the fir~t improves the diameter measurement when the tool axi~ moves from side to side in the well-bore during drilling. One transceiver measures the standoff on its side. Then immediately thereafter or simultaneously, the other tran~ceiver measures the standoff on the other side of the tool. Simultaneous firing of diametrically o~yo~cd transceivers is of course possible but not preferred in that more complicated and duplicative firing electronics i~ required. An instantaneous firing of both transceivers i~ not required as long as tool movement in the time between the two transceiver measurements is relatively ~mall.
The hole diameter is determined by adding the tool diameter to the standoffs as measured by the two opposed transceivers. A

206321~

number of borehole diameter determinations are accumulated and averaged to produce a borehole measurement Additional signal processing rejects false echoes caused by, for example, large cuttings in the drilling fluid by identifying formation echoes which occur after echoes from drilling cuttings in the drilling fluid. The signal processing also distinguishes formation echoes from its multiple arrivals, and from sensor noise.
An important aspect of the placement of acoustic transceivers 400 on the stabilizer blades 210' of the collar 20 is that such placement improves the accuracy of the tool standoff and borehole diameter measurements. The improvement in accuracy results from reducing the gap between the outer surface of the transceivers 400 and the borehole wall.
Knowledge of gamma ray attenuation caused by the drilling fluid exi~ting between the gamma ray detectors and the formation wall iB desirablQ for generating an improved formation gamma density determination which is compensated for standoff or cave effects. In making a drilling fluid attenuation correction to gamma ray detection data, the difference between formation density calculated from the far detector 310 and that calculated from the near detector 300 iB generated. Thi~ difference i8 functionally related to an increment, which should be added to the density determined from the far detector. Such increment i~ a function of the standoff of the tool 20 from the borehole wall and to the gamma ray absorption property of tho drilling fluid density being used and its photoelectric adsorption coefficient. Thus, knowledge of 20632~8 the denslty and the photoelectric adsorption coefficient of the drilling fluid in addition to the tool standoff i8 desirable to make an appropriate correction to the determined formation density.
In a similar manner, near and far detector data from radiation sensors 59, 60 of the neutron porosity section are affected by the amount of drilling fluid existing between such detectors and the formation wall. Measurement of borehole diameter by means of sensors 400 as described above provides the essential data in the determination of such volume of drilling fluid between the formation wall and the detectors.
Accordingly, near and far neutron porosity data from detectors 59, 60 and borehole diameter data from ultrasonic sensors 400 are collected in electronic cartridge 45 as a function of borehole depth or position. Such data may be stored and later retrieved when tool 20 is returned from the borehole to the well surface.
Preferably, however, such data iB transmitted to the surface via data signaling module 23 in the form of acoustic or pressure pulses via the drilling fluid within drill string 6. Such pulses are sensed by sensor 21 in standpipe 15 and the data is collected in surface instrumentation unit 7 of Figure 1. To practice such data communication via drilling fluid, the data signalling cartridge illustrated in Figures 1 and 2 is preferably arranged similarly with the arrangement disclosed in u.S. patent 4,479,564 which is incorporated herein by reference.
Correction of data from near and far detector~ 59 and 60 of the neutron porosity section 500 and of the near and far detectors 20~3218 300, 310 of the gamma-gamma denaity section 600 by the standoff measurement derived from ultrasonic sensors 400 is either carried out after transmission of such data to surface instrumentation 7, or done downhole with suitable downhole instrumentation with the corrected data transmitted uphole or stored downhole.
Various modifications and alterations in the described methods and apparatus will be apparent to those skilled in the art of the foregoing description which does not depart from the spirit and scope of the invention. For this reason, these changes are desired to be included in the appended claims. The descriptive manner which ia employed for setting forth the embodiments should be interpreted as illustrative but not limitative.

Claims (10)

1. A logging-while-drilling tool for measuring characteristics of earth formations surrounding a borehole including a tubular body having upper and lower ends adapted for coupling to a drill string and providing a fluid flow path between its upper and lower ends, said tubular body having a tubular outer radius and a source of neutrons disposed therein, and a plurality of longitudinally spaced radiation detectors carried by said tool and including means responsive to said longitudinally spaced radiation detectors to generate first radiation signals useful for determining a first characteristic of said earth formations, wherein said tool is characterized by a plurality of stabilizer blades secured symmetrically about the outer periphery of said tubular body, a plurality of detector cavities in registration with each of said stabilizer blades, each of said cavities formed by an outwardly facing slot of said body which faces an inwardly facing slot of a stabilizer blade, each of said cavities having an outer radial extent substantially greater than said nominal tubular outer radius, and each of said radiation detectors being placed within one of said cavities.

2. The tool of claim 1 having a gamma ray source positioned within said body and at least one gamma ray detection assembly carried by said body and including means responsive to said at least one gamma ray detection assembly to generate second radiation signals useful for determining a second characteristic of said earth formations, wherein said tool is further characterized by a second plurality of stabilizer blades disposed about the outer periphery of said tubular body, each of said second plurality of stabilizer blades extending radially outwardly from said tubular body at a second maximum radial distance, and said at least one gamma ray detection assembly is associated with at least one of said second plurality of stabilizer blades, each gamma ray detection assembly placed radially at least partially beyond said nominal outer radius of said tubular wall of said body in an associated gamma ray detection cavity, said gamma ray detection cavity extending radially into an associated blade.

3. The tool of claim 1 or claim 2 further characterized by means including ultrasonic sensor means carried by said body for producing standoff signals representative of standoff of said body from said borehole while drilling said borehole, wherein signals used to measure characteristics of earth formations may be corrected as a function of measured standoff of said body.

4. The tool of claim 1 or 2 wherein said first characteristic of earth formations is porosity.

5. The tool of claim 2 or 4 wherein said second characteristic of earth formationsis density or photoelectric absorption coefficient.

6. The tool of claim 1 further characterized by each of said blades having a longitudinal length and an angular width dimension such that the angular width of each blade is substantially less than the angular distance between each blade.

23a

7. The tool of claim 3 further characterized by said ultrasonic means including at least one ultrasonic sensor disposed in at least one of said second plurality of stabilizer blades said ultrasonic sensor facing radially outwardly from said tubular body and means for combining radiation signals with said standoff signals to correct a measurement of a characteristic of said earth formations for the effect of standoff of said tool from said borehole.

8. The tool of claim 7 further characterized by said one blade of said second plurality of stabilizer blades is a blade other than a blade in which a gamma ray detection assembly is disposed.

9. The tool of claim 7 further characterized by means for generating borehole diameter signals from said ultrasonic sensor indicative of the diameter of said borehole opposite said tool, means for combining said standoff signals with said first signals to correct said first formation characteristic, and means for combining said borehole diameter signals with said second radiation signals to correct said second characteristic of said earth formations.

10. The tool of claim 2 further characterized by said stabilizer blade in which said at least one gamma ray detection assembly is associated includes a plug of substantially gamma ray transparent material adjacent said gamma ray detection assembly, and said tubular body includes a path of substantially gamma ray transparent material through its wall adjacent said gamma ray detection assembly.