Gamma logging tool assembly

Disclosed embodiments include a gamma logging detector assembly that includes a detector support structure comprising one or more high density alloy materials and including a first cylindrical drill collar segment and a second cylindrical drill collar segment each having a radius of at least R1. A third cylindrical drill collar segment is disposed axially between the first and second cylindrical drill collar segments to form an annular channel over the third cylindrical drill collar segment and between the first and second cylindrical drill collar segments. The third cylindrical drill collar segment includes an inwardly defined open cavity and a radius, R2, that is less than R1. An annular pressure sleeve comprising one or more low density alloy materials is disposed within the annular channel.

BACKGROUND

Production and injection wells are frequently formed by drilling boreholes that traverse subterranean formations. Borehole measurements such as nuclear magnetic resonance (NMR) and gamma radiation measurements may be performed using a wireline configuration in which drilling equipment is withdrawn from the borehole and the measurement instruments inserted by wireline. Another technique for deploying measurement instrumentation at various positions along a borehole is known as logging while drilling (LWD) also referred to as measurement while drilling (MWD). LWD is a measurement technique that deploys logging tools, such as gamma ray measurement tools, within a borehole during a drilling operation as part of the lowermost bottom hole assembly (BHA) portion of the drill string.

DETAILED DESCRIPTION OF EMBODIMENTS

The description that follows includes example systems, methods, techniques, and program flows that embody aspects of the disclosure. However, it is understood that this disclosure may be practiced without one or more of these specific details. In some instances, well-known instruction instances, protocols, structures and techniques have not been shown in detail in order not to obfuscate the description.

Overview

LWD may be used to implement natural gamma ray logging in which naturally occurring, or non-induced, gamma radiation is measured by scintillation detectors deployed in a BHA. Measurement of naturally occurring gamma radiation may be used for borehole imaging, and other purposes, during or after drilling. Borehole imaging may be utilized as an input for directional drilling to optimize the positioning of wellbores within target hydrocarbon formations. The encoded imaging information is utilized to guide the directional drilling of boreholes such as for determining the direction of drilling during LWD operations.

Naturally occurring gamma radiation is typically characterized by low count intensity. Natural gamma radiation logging, including spectral gamma radiation logging, is particularly sensitive to material composition and configuration of BHA components, which may attenuate incoming gamma radiation. Additionally, mechanical disturbances, including shock and vibration during LWD operation, may distort the results of spectral gamma measurements. Disclosed embodiments include a detector assembly apparatus and drilling system configured to increase measured gamma ray counts during LWD logging in which a natural gamma ray logging tool is deployed within a drill string.

The detector assembly may include a detector support structure comprising first and second cylindrical segments having a first radius, and a third cylindrical segment disposed axially between the first and second cylindrical segments. The third cylindrical segment has second radius that is less than the first radius and also includes one or more inwardly defined open cavities. The first, second, and third cylindrical segments form a detector support structure for a logging tool. The logging tool may comprise a gamma radiation sensor and an electronics package disposed within a respective one or more of the open cavities. An annular pressure sleeve is disposed within an open channel formed over the third cylindrical segment and between the first and second cylindrical segments.

Forming part of the drill collar, the first, second, and third cylindrical segments are fabricated using one or more high density alloy materials. The pressure sleeve is fabricated using an alloy having a relatively low density, low atomic number, and relatively high metallurgic strength. For example, the pressure sleeve may comprise a titanium alloy having low gamma scattering and high metallurgic toughness characteristics such as a Ti-6AL-2SN-4ZR-6MO alloy. The pressure sleeve material is substantially less dense and has a lower effective atomic number than the materials of which the first, second, and third cylindrical segments are fabricated. The low density, low effective atomic number material presents a reduced gamma attenuation barrier that also provides sufficient metallurgic toughness to withstand substantial downhole fluid pressure. The circumferentially recessed third cylindrical segment in combination with the pressure sleeve material further permits flexure of the drill string proximate to the detector assembly that relieves bending stresses otherwise incurred by logging tool components during drilling.

Example Illustrations

FIG. 1is a block diagram depicting a drilling system100that is configured to collect and utilize spectral gamma radiation information during logging while drilling (LWD) operations in accordance with some embodiments. Drilling system100includes a drilling rig102comprising various mechanical and electronic systems, subsystems, devices, and components configured to lower, rotate, and otherwise operate a drill string. The drill string includes, among other components, a section of drill piping104coupled at one end to a top drive (not depicted) within drilling rig102. Drill piping104coupled at the other end to a bottom hole assembly (BHA)115that includes a drill bit110on its lower end. BHA115further includes a steering actuator116configured either as a rotary steering system or motor driven device to determine the drilling direction by adjusting the direction in which drill bit110is pointed.

Drill bit110may be actuated by rotation imparted to the drill string by the top drive within drilling rig102. A borehole106having a cylindrically contoured borehole wall108is formed as drill bit110is rotated within a formation140. As drill bit110rotates, a pump (not depicted) within drilling rig102pumps drilling fluid, sometimes referred to as “drilling mud,” downward through a drilling fluid conduit114that is formed within the various sections of the drill string. The drilling fluid cools and lubricates drill bit110as it exits drill bit110.

BHA115further includes a drill collar112that provides downward weight force on drill bit110for drilling. Drill collar112comprises one or more thick-walled cylinders machined from various relatively high density metals or metallic alloys such as carbon steel or a nickel alloy. While not expressly depicted inFIG. 1, drill collar112may comprise multiple distinct cylindrical members that are interconnected using releasable connections such as threaded connectors integral to the individual drill collar members. In some instances, multiple interconnected cylinders may be collectively referred to as a drill collar and in other instances each cylinder may be referred to individually as a drill collar. In addition to providing the downward force on drill bit110, drill collar112includes structures for protectively deploying downhole measurement logging tools used to detect geological formation information as well as measuring various environmental drilling parameters.

A measurement tool113is deployed within a section of drill collar112and is configured, using various sensor and support electronics components, to measure and record drilling parameters such as the position and orientation of BHA115. Measurement tool113may be further configured, using various sensor and support electronics components, to measure and record downhole environment conditions such as downhole pressure and temperature proximate BHA115. The drilling parameters and downhole environment information collected by measurement tool113is transmitted to a surface processing system, such as a data processing system130via a telemetry link125. Telemetry link125includes transmission media and endpoint interface components configured to employ a variety of communication modes. The communication modes may comprise different signal and modulation types carried using one or more different transmission media such as acoustic, electromagnetic, and optical fiber media.

Drill collar112is further configured to support a gamma logging detector assembly117that includes components for measuring and recording total and/or spectral natural gamma radiation emitted from radioelement concentrations within formation140. Detector assembly117further includes information processing and communication components for transmitting the collected total and/or spectral gamma radiation information via telemetry link125to data processing system130. Detector assembly117includes, in part, a detector support structure comprising all or portions of one or more interconnected cylindrical drill collar members that make up the overall drill collar112. The drill collar member(s) and/or portions thereof forming the detector support structure may be referred to as drill collar segments and comprise relatively dense metallic members, such as steel and/or nickel alloys having a density of at least 7 grams per cubic centimeter.

In contrast to typical drill collar construction, the drill collar segments forming the detector support structure within detector assembly117have axially varying circumferential contouring. As shown, the dense metallic material portions of the detector support structure include cylindrical drill collar segments123and127forming the axial ends of detector assembly117. Drill collar segments123and127of detector assembly117are structurally and materially consistent, each comprising a cylindrical volume of one or more metals and/or alloys displaced only by a corresponding section of the relatively narrow drilling fluid conduit114.

Detector assembly117further includes a mandrel section131bounded by the opposing inner surfaces of drill collar segments123and127. Mandrel section131comprises a recessed cylindrical mandrel segment129axially disposed between drill collar segments123and127. The position and outer contour of mandrel segment129with respect to drill collar segments123and127forms an annular channel over mandrel segment129and between drill collar segments123and127in which an annular pressure sleeve122is disposed. In some embodiments, annular pressure sleeve122is a structurally and compositionally distinct component that is not materially integral with the drill collar metals/alloys of which cylindrical segments123and127and mandrel segment129are fabricated. As a discrete component, pressure sleeve122has an inner cylindrical surface that contacts an outer cylindrical surface of mandrel segment129. The recessed mandrel segment129may be compositionally consistent with and structurally integral with one or both of the dense metallic end segments123and127of detector assembly117.

Within mandrel section131, mandrel segment129includes an inwardly defined open cavity118within which a gamma radiation sensor137, such as a scintillation-type gamma sensor, is disposed. Mandrel segment129further includes an additional inwardly defined open cavity120within which an electronics package139for the sensor is disposed. Unlike the annular channel within which pressure sleeve122is disposed, open cavities118and120result in a circumferentially uneven contour of the section of dense drill collar material over the respective axial extents of the cavities.

As depicted, pressure sleeve122is disposed directly over the recessed mandrel member129, providing a fluid and fluid pressure seal for gamma sensor137and electronics package139disposed within open cavities118and120, respectively. The thickness of pressure sleeve122is determined based, at least in part, on the surface areas of cavities118and120in combination with the material composition of pressure sleeve122. The material composition of pressure sleeve122is selected to minimize gamma ray attenuation while maintaining a pressure barrier within borehole106that is sufficient to protect components carried in the pressure sleeve122, and to carry out drilling operations. In some embodiments, pressure sleeve122is uniformly fabricated of a low density alloy such as a titanium alloy having adequate metallurgic toughness to withstand high pressure and other forces at high temperatures. Such forces may include column fluid pressure within borehole106and flexural stresses imparted by operation of the drill string. In some embodiments, pressure sleeve122comprises an alpha-beta titanium alloy that maintains high strength to at least a temperature of 450° C. For example, pressure sleeve122may comprise a Ti-6AL-2SN-4ZR-6MO alloy comprising 82% titanium, 6% aluminum, 2% tin, 4% zirconium, and 6% molybdenum. The titanium alloy used to fabricate pressure sleeve122is substantially lower in bulk density and effective atomic number than the metals/alloys used to fabricate the denser cylindrical drill collar segments123and127and mandrel segment129.

In combination, gamma sensor137and electronics package139form a gamma logging tool configured to detect, records, and transmit naturally occurring gamma radiation emitted from formation materials, particularly radioelements that are present at various amounts and concentrations. Such radioelements typically includes various isotopes of the thorium and uranium decay chains, and potassium. The low gamma scattering material properties of pressure sleeve122, such as when comprising Ti-6AL-25N-4ZR-6MO alloy, enable increased gamma ray counts to penetrate and be detected by gamma sensor137than would otherwise occur using a denser pressure sleeve material.

During drilling operations, information from the logging tool that includes gamma sensor137and from measurement tool113are processed by a surface data processing system130to determine and modify the drilling direction of drill bit110within formation140. For instance, data processing system130may comprise processing components configured to derive formation material properties from raw and/or pre-processed natural gamma radiation measurements collected by the logging tool components including gamma sensor137and electronics package139. The logging tool may be configured as a spectral gamma logging tool with gamma sensor137configured as a spectral gamma sensor that collects spectral gamma radiation information that may be transmitted to data processing system130in real-time or subsequent to drilling operation.

The radiation and/or spectral information that is collected by components within detector assembly117may include counts and detected energy levels that may be transmitted as information via telemetry link125to data processing system130. In some embodiments, data processing system130comprises a spectral processing components135configured to generate spectral component data from the detected gamma radiation. For embodiments in which the spectral gamma information is transmitted in real-time, the formation material properties information generated by data processing system130from the spectral information may be used to generate drill bit direction instructions. The direction instructions may be transmitted from data processing system to steering actuator116via telemetry link125.

Data processing system130includes a processor132, a display device136, and a memory device134into which spectral processing components135are loaded. Spectral processing components135comprise program instructions configured to determine formation properties such as material composition based, at least in part, on the measurement information from detector assembly117. In some embodiments, spectral processing components135may further include an imaging component comprising program instructions configured to generate borehole azimuthal image information. The borehole azimuthal image information may describe, such as by color-coding or otherwise, the material composition of formation materials proximate to BHA115based on the determined formation properties. The azimuthal image information may be displayed as an optical image on display device136. Data processing system130may further include a user input device138that may be used to generate and send directional drilling instructions based, at least in part, on the formation properties. The directional drilling instructions may be transmitted by data processing system130to steering actuator116via telemetry link125.

FIG. 2Aillustrates a side cross-section view of a portion of a drill collar such as drill collar112comprising a detector assembly202in accordance with some embodiments. The depicted drill collar portion is positioned within a cylindrical borehole206defined by a borehole wall204and containing formation and drilling fluids that exert substantial pressure on the outer components of the drill collar. The depicted drill collar portion includes a first drill collar member208and a second drill collar member210. Drill collar members208and210each comprise a discrete thick-wall tubular manufactured from a solid bar of high density alloy material such as alloy steel or other relatively dense metallic material such as a nickel alloy. Drill collar members208and210may be fabricated of a same alloy or a different alloy. The outer contours of drill collar members208and210are substantially cylindrical although the respective surfaces may be finished as slick or spiraled depending on the desired mechanical and fluid dynamic properties. For example, the outer surfaces of drill collar members208and/or210may be finished to have spiral grooves to promote even flow of drill fluid around the collar diameter within borehole206.

The mutually opposing ends of drill collar members208and210are releasably connected by rotational engagement of a threaded end of232of drill collar member208with a threaded end234of drill collar member210. Similarly, the overall drill collar that includes members208and210may further include a third drill collar member236connected by threaded engagement at the other end of drill collar member210. Each of drill collar members208,210, and236include respective portions of a drilling fluid conduit211, which in the depicted embodiment is disposed at the center of the overall apparatus.

Detector assembly202includes drill collar member210as well as at least a portion of drill collar member208. The drill collar member portions of detector assembly202are contoured to form a detector support structure229comprising portions of the steel or nickel alloy of which drill collar members208and/or210are fabricated. Detector support structure229includes a first end comprising a first cylindrical segment231formed by the depicted lower portion of drill collar member208. Detector support structure229includes a second end axially opposing the first end and comprising a second cylindrical segment233formed by the lower portion of drill collar member210. In the depicted embodiment, the first and second cylindrical segments231and233forming the ends of detector support structure229have a radius R1and terminate at the depicted bounds of a depicted mandrel section212of detector assembly202. In some embodiments, drill collar members208and210and/or or segments thereof such as cylindrical segments231and233may have different radii. For example, the radius of cylindrical segment231may be greater than or less than the radius of cylindrical segment233.

Mandrel section212includes a recessed portion of dense drill collar material forming the intermediary portion of detector support structure229in the form of a recessed cylindrical mandrel235that is materially integral to drill collar member210. As shown, the recessed mandrel235forming the depicted upper portion of drill collar member210has a radius of R2that is substantially less than R1to form an annular channel237in which an annular pressure sleeve225is disposed. Annular channel237is formed over the surface area of the recessed cylindrical drill collar segment comprising mandrel235and between the first and second cylindrical segments231and233that bound mandrel section212. In some embodiments in which the outer radius, R1, of the drill collar members is between 8.89 cm (3.5 inches) and 11.43 cm (4.5 inches), the difference between R1and R2is at least 0.572 cm (0.225 inches).

Detector assembly202further includes a cavity214inwardly defined at a first axial position within mandrel235. A gamma radiation detector216is disposed within cavity214. Gamma radiation detector216may comprise a scintillometer device configured to measure the number and energy intensity of naturally occurring gamma rays emitted by formation material. Gamma radiation detector216is configure in to detect incoming natural gamma radiation through annular pressure sleeve225. During a measurement cycle, individual gamma rays of the overall gamma radiation pass through pressure sleeve225and strike a measurement medium within detector216such as a scintillation crystal and may or may not be absorbed. In response to absorption of a gamma ray, the crystal generates a light energy flash. The light energy flash may be detected by a photomultiplier within detector216that converts the light flash into an electrical pulse that is counted by detector216. Detector216may be secured within cavity214using mechanical and/or adhesive fasteners.

Detector assembly202includes at least one other inwardly defined cavity218within mandrel235. Cavity218is defined at a second position that is both radially and axially offset from the first position of cavity214along the length of mandrel235. An electronics package220is disposed within cavity218and includes components for providing electrical power and control for radiation detector216. Electronics package220may further include data processing circuitry configured to process measurement information generated by radiation detector216. For example, electronics package220may include programmed circuitry configured to determine intensity in terms of count rate and energy levels of the detected naturally occurring gamma radiation.

Forming part of detector assembly202, pressure sleeve225is disposed within an annular channel237formed by the difference in radius between the recessed cylindrical segment forming mandrel235and cylindrical segments231and233. As shown, pressure sleeve225is disposed along the recessed segment such that the inner cylindrical surface of pressure sleeve225non-bindingly contacts the outer cylindrical surface of mandrel235. In this manner, pressure sleeve225may be slidably inserted and removed from the annular channel when drill collar member208is rotationally disengaged from drill collar member210to install or otherwise provide access to detector216and electronics package220within exposed open cavities214and218. In the depicted embodiment, the thickness of pressure sleeve225is selected to match the difference between radius, R1, and the radius, R2, of mandrel235.

For nuclear radiation logging systems, measurement precision is determined in significant part by maximizing count rate. To optimize natural gamma detection, pressure sleeve225is comprised of a low density metallic alloy that provides fluid pressure shielding for radiation detector216and electronics package220while minimizing attenuation of incoming gamma radiation. The amount of gamma radiation attenuation within pressure sleeve225is an exponential function of the sleeve thickness structural feature of pressure sleeve225. Gamma radiation attenuation is also a function of the material density and gamma scattering cross-section compositional features of pressure sleeve225. To reduce gamma attenuation while maintaining sufficient mechanical strength and fatigue resistance, pressure sleeve225comprises a low-density, low effective atomic number (low-Zell), and high-strength alloy material.

In some embodiments, pressure sleeve225is an integral annular member having a thickness between 0.572 cm (0.225 inches) and 0.699 cm (0.275 inches) and comprising an alloy having a bulk density equal to or less than approximately 5 grams per cubic centimeter, and an effective atomic number equal to or less than approximately 30. To this end, and in some embodiments, pressure sleeve225may be an integral annular component uniformly comprising a Ti-6AL-2SN-4ZR-6MO alloy. During drilling, bending moments may be imparted to various portions of a drill string such as the depicted drill string portion depicted inFIG. 2A. The dense alloy cross-section is reduced along the recessed segment formed by mandrel235and cavities214and218formed within mandrel235. The reduced cross-section results in a potentially increased bending moment over the span of detector assembly202. In addition to imparting relatively low gamma attenuation, a Ti-6AL-2SN-4ZR-6MO alloy formed into an annular sleeve having a thickness between 0.572 and 0.699 cm (0.225 and 0.275 inches) provides flexure proximate to the detector assembly202. Having a thickness of between 0.572 and 0.699 cm (0.225 and 0.275 inches) and fabricated of a Ti-6AL-2SN-4ZR-6MO alloy, pressure sleeve225allows mechanical flexure of mandrel235proximate to radiation detector216and electronic package220thereby relieving mechanical bending stresses the logging tool components would otherwise incur during drilling.

FIG. 2Billustrates the transverse A-A′ cross-section view of mandrel section212of the gamma detector assembly illustrated inFIG. 2A. Included in the cross-section view are cavities214and218, and drilling fluid conduit211all diametrically aligned within the mandrel235portion of drill collar member210. Cavity214is semi-cylindrically contoured to securely and efficiently encase cylindrically contoured gamma radiation detector216. Cavity214may vary in contour as well as dimensions depending on the dimensions and contouring of radiation detector216.

Cavity218is rectangularly contoured to securely encase rectangularly contoured electronics package220. As is the case for cavity214, cavity218may vary in contour and dimension depending on the dimensions and contour of electronics package220. Cavity218and electronics package220are depicted as shadow components using dashed lines to indicate the axial offset between cavity214and cavity218. Consistent withFIG. 2A, the cross-section view ofFIG. 2Bdepicts the relative disposition of pressure sleeve225as disposed on the outer surface of the recessed mandrel235portion drill collar member210to provide fluid and pressure shielding over the otherwise open cavities214and218.

FIG. 3illustrates a transverse cross-section view of a portion of a drill collar on which a gamma detector assembly is deployed in accordance with some embodiments. In combination with the dimensions and material composition features of the pressure sleeve, the sizing and relative disposition of the gamma detector may affect gamma count rates. As shown inFIG. 3, the detector assembly components include a recessed mandrel segment of a drill collar member302equivalent to mandrel235depicted inFIGS. 2A and 2B. The embodiment further includes a pressure sleeve308that provides fluid and pressure shield from column fluid pressure within a borehole306. Pressure sleeve308may be contoured and composed of a titanium alloy, similar to or identical to that depicted and described with reference to pressure sleeve225inFIG. 2A.

In contrast to the embodiment shown inFIGS. 2A and 2B, the structural components within the recessed mandrel segment of a drill collar member302are not diametrically aligned. Instead, an electronics package cavity312is disposed approximately at a circumferentially 90° offset position with respect to a gamma detector cavity310. Furthermore, a drilling fluid conduit304may be positioned in a non-centered position and offset in a direction away from the position of gamma detector cavity310in order to provide sufficient space to increase the detector/cavity size. The depicted offset positioning of conduit304and electronics package cavity312provides a larger cross-section space within which a larger gamma detector may be disposed in the larger gamma detector cavity310. The configuration depicted inFIG. 3may increase gamma count rates for natural gamma logging due to the provision of additional gamma absorption volume for the relatively high energies of characteristic potassium, thorium, and uranium gamma radiation.

FIG. 4illustrates a transverse cross-section view of a mandrel section400of a drill collar on which a gamma detector assembly is configured to include gamma windows in accordance with some embodiments. As shown inFIG. 4, the detector assembly components include a recessed cylindrical mandrel402of a drill collar member equivalent to the recessed mandrel235depicted inFIGS. 2A and 2B. In the depicted embodiment, four gamma detector cavities including detector cavity408are distributed evenly around the circumference of mandrel402that includes a drilling fluid conduit410at its center. The depicted detector assembly further includes a pressure sleeve that is not uniformly comprised of a single low-density, low-Zeff alloy. Instead, the depicted pressure sleeve includes gamma measurement windows in the form of four azimuthal sections414a-414dcomprised of a low density, low-Zeff alloy.

The depicted pressure sleeve further includes four azimuthal sections416a-416dcomprising a higher density alloy disposed in an interleaved manner with respect to azimuthal sections414a-414d. As shown, the pressure sleeve is azimuthally aligned with respect to cylindrical mandrel402such that each of sections414a-414dis disposed over an opening of a respective one of the gamma detector cavities and each of sections416a-416dis disposed between a respect pair of detector cavities. In this configuration, the alloy of which azimuthal sections414a-414dare fabricated may comprise Ti-6AL-2SN-4ZR-6MO to provide a low-attenuation gamma window into the detector cavities. In some embodiments, azimuthal sections414a-414dmay be fabricated using a beryllium alloy. Sections416a-416dmay be fabricated using an alloy having a higher density such as the same or substantially similar alloys used to fabricate cylindrical mandrel402.

Variations

While the aspects of the disclosure are described with reference to various implementations and exploitations, it will be understood that these aspects are illustrative and that the scope of the claims is not limited to them. Many variations, modifications, additions, and improvements are possible. Plural instances may be provided for structures, components, or operations described herein as a single instance. In general, structures and functionality presented as separate components in the example configurations may be implemented as a combined structure or component. Similarly, structures and components presented as a single component may be implemented as separate structures and components.

Computer program code for carrying out operations for aspects of the disclosure may be written in any combination of one or more programming languages, including an object oriented programming; a dynamic programming language; a scripting language; and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on a stand-alone machine, may execute in a distributed manner across multiple machines, and may execute on one machine while providing results and or accepting input on another machine.

Use of the phrase “at least one of” preceding a list with the conjunction “and” should not be treated as an exclusive list and should not be construed as a list of categories with one item from each category, unless specifically stated otherwise.

EXAMPLE EMBODIMENTS

Example embodiments include the following:

Embodiment 1: A gamma logging detector assembly comprising: a detector support structure comprising one or more high density alloy materials and including a first cylindrical drill collar segment and a second cylindrical drill collar segment each having a radius of at least R1, and a third cylindrical drill collar segment having a radius, R2, that is less than R1and disposed axially between the first and second cylindrical drill collar segments to form an annular channel over the third cylindrical drill collar segment and between the first and second cylindrical drill collar segments, the third cylindrical drill collar segment including an inwardly defined open cavity, and an annular pressure sleeve comprising one or more low density alloy materials disposed within the annular channel.

Embodiment 2: The gamma logging detector assembly of Embodiment 1, wherein the second and third cylindrical drill collar segments are included in an integral drill collar member, said gamma logging detector assembly further including a connector that releasably connects the first cylindrical drill collar segment to the third cylindrical drill collar segment.

Embodiment 3: The gamma logging detector assembly of Embodiments 1-2, wherein the connector comprises a first threaded component at a first end of the second cylindrical drill collar segment and a second threaded component at an end of the first cylindrical drill collar segment that rotationally engages the first threaded component.

Embodiment 4: The gamma logging detector assembly of Embodiments 1-3, wherein the annular pressure sleeve has a density of less than approximately 5 grams per cubic centimeter and an effective atomic number of less than approximately 30.

Embodiment 5: The gamma logging detector assembly of Embodiments 1-4, further comprising a gamma radiation sensor disposed within the open cavity and configured to measure spectral gamma radiation through the annular pressure sleeve.

Embodiment 6: The gamma logging detector assembly of Embodiments 1-5, wherein the annular pressure sleeve has a thickness of at least 0.56 cm (0.22 inches).

Embodiment 7: The gamma logging detector assembly of Embodiments 1-6, wherein the annular pressure sleeve has a thickness approximately equal to the difference between R1and R2.

Embodiment 8: The gamma logging detector assembly of Embodiments 1-7, wherein third cylindrical drill collar segment includes N inwardly defined open cavities, and wherein the annular pressure sleeve comprises an annular body having N azimuthal sections comprising low density alloy material interleaved with N azimuthal sections comprising a high density alloy material, where N is an integer greater than 1.

Embodiment 9: The gamma logging detector assembly of Embodiments 1-8, wherein the third cylindrical drill collar segment includes one or more inwardly defined open cavities at a first position and one or more inwardly defined open cavities at a second position axially offset from the first position.

Embodiment 10: The gamma logging detector assembly of Embodiments 1-9, wherein the annular pressure sleeve comprises an alpha-beta titanium alloy that maintains structural integrity to at least a temperature of 400° C.

Embodiment 11: The gamma logging detector assembly of Embodiments 1-10, wherein the annular pressure sleeve comprises a Ti-6AL-2SN-4ZR-6MO alloy.

Embodiment 12: A drill string comprising: a first end configured to connect to a drive component; a drill bit disposed at a second end of the drill string; and a drill collar disposed between the drill bit and the first end and that comprises one or more interconnected cylindrical members, wherein a detector assembly portion of the drill collar comprises, first and second cylindrical segments each having a radius of at least R1; a cylindrical mandrel having one or more inwardly defined open cavities and a radius, R2, that is less than R1, and is axially disposed between the first and second cylindrical segments to form an annular channel over the cylindrical mandrel and between the first and second cylindrical segments, and wherein the cylindrical mandrel and the first and second cylindrical segments comprise one or more first alloy materials each having a density of at least D1; and an annular pressure sleeve disposed within the annular channel, wherein the annular pressure sleeve comprises a second alloy material having a density, D2, that is less than D1.

Embodiment 13: The drill string of Embodiment 12, wherein the cylindrical mandrel and first and second cylindrical segments comprise a first alloy having a density of at least 7 grams per cubic centimeter, and wherein the annular pressure sleeve comprises a second alloy having a density less than or equal to about 5 grams per cubic centimeter and an effective atomic number less than or equal to 30.

Embodiment 14: The drill string of Embodiments 12-13, wherein the second alloy material comprises a Ti-6AL-2SN-4ZR-6MO alloy.

Embodiment 15: The drill string of Embodiments 12-14, wherein the second cylindrical segment and the cylindrical mandrel are included in an integral cylindrical member, said drill string further including a releasable connector that connects the first cylindrical segment to the cylindrical mandrel.

Embodiment 16: The drill string of Embodiments 12-15, wherein the cylindrical mandrel includes one or more inwardly defined open cavities at a first axial position and one or more inwardly defined open cavities at a second axial position offset from the first axial position, and wherein the second alloy material comprises a Ti-6AL-2SN-4ZR-6MO alloy.

Embodiment 17: A drilling system comprising: a bottom hole assembly including a detector assembly that comprises one or more drill collar members forming a detector support structure that includes; first and second cylindrical segments each having a radius of at least R1; and a cylindrical mandrel disposed axially between the first and second cylindrical segments and having a radius, R2, that is less than R1to form an annular channel over the cylindrical mandrel and between the first and second cylindrical segments, wherein the cylindrical mandrel includes an inwardly defined open cavity; a logging tool including a gamma radiation sensor disposed within the open cavity; and an annular pressure sleeve disposed within the annular channel and comprising an alloy having a density equal to or less than 5 grams per cubic centimeter and an effective atomic number equal to or less than 30; and a data processing system communicatively coupled with the logging tool and configured to determine a drill bit direction modification based, at least in part, on measured gamma radiation data generated by the logging tool.

Embodiment 18: The drilling system of Embodiment 17, wherein the annular pressure sleeve has a thickness approximately equal to the difference between R1and R2.

Embodiment 19: The drilling system of Embodiments 17-18, wherein the annular pressure sleeve comprises a Ti-6AL-2SN-4ZR-6MO alloy.

Embodiment 20: The drilling system of Embodiments 17-19, wherein the cylindrical mandrel includes one or more inwardly defined open cavities at a first axial position and one or more inwardly defined open cavities at a second axial position.