Cement evaluation with X-ray tomography

A tool can include an X-ray tomography device to evaluate cement in a downhole environment. The X-ray tomography device includes an X-ray beam source configured to transmit an X-ray beam at a first predetermined angle. The beam angle may be set by a capillary device coupled to the X-ray beam source. An energy dispersive, multi-pixel photon detector is configured to count detected backscatter photons received at a second predetermined angle and determine an energy spectrum for the detected photons. A density map of the cement may be generated in response to the number of detected photons. Additional apparatus, systems, and methods are disclosed.

BACKGROUND

Natural resources such as gas, oil, and water residing in a geological formation may be recovered by drilling a wellbore into the formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe (e.g., casing) is run into the wellbore in order to provide structural support for the wellbore sides. The casing may be metal (e.g., steel).

Primary cementing may be performed whereby a cement slurry is injected into the annulus between the casing and the geological formation. The cement is permitted to set into a hard mass (i.e., a sheath) to thereby support the string of pipe within the wellbore and seal the annulus.

To determine the condition of the hardened cement, an evaluation log was introduced in the 1960s, using sonic attenuation measurements. Through years of evolution, acoustic solutions now include sonic attenuation measurements with borehole compensation and ultrasonic impedance imaging with azimuthal sensitivity. These evaluation technologies are being challenged to meet new environmental conditions (such as wellbores located in deep water) and new regulatory requirements. The acoustic impedance contrasts between heavy drilling mud and light cement, for example, may be close to or beyond the measurement sensitivity limitations of existing sensors, while heavy casings present an additional technical barrier for traditional acoustic solutions.

DETAILED DESCRIPTION

Some of the challenges noted above, as well as others, can be addressed by implementing the apparatus, systems, and methods described herein. In many embodiments, an X-ray backscattering process may be used for cement evaluation using an X-ray source and a photon detector. The resulting spectrometric information may be used to investigate possible gaps, bubbles, or other features behind casing material.

FIG. 1is a cross-sectional diagram of a cased hole completion geometry, according to various embodiments of the invention. The cased-hole completion geometry includes a borehole101formed in a geological formation104. The borehole101is lined with the casing102that is often a metal (e.g., steel). Cement103is injected into the borehole101such that, after it reaches the bottom of the borehole, it returns upward in the annulus region between the casing102and the formation104. Thus, the cement stabilizes the casing102within the borehole101.

Due to possible imperfections introduced into the cement103during construction and/or subsequent wear damage caused by use of the borehole, it would be desirable to perform non-destructive testing of the cement. Using an x-ray tomography device in the borehole, possible air gaps, bubbles, and/or other imperfections in the cement may be discovered. Various embodiments of the invention may be used to detect the present of these imperfections.

FIG. 2is a block diagram showing an x-ray tomography device within a cased hole, according to various embodiments of the invention. The device210uses an X-ray backscattering process to penetrate the casing material, with reflections back to an X-ray detector, to generate a density map of the casing and the cement in order to investigate defects behind the casing. The device210may be located in a wireline tool housing to be used during a wireline logging operation (seeFIG. 7) or a drill string tool housing to be used during a logging while drilling (LWD) operation (seeFIG. 8).

The X-ray tomography device210includes an x-ray source200(e.g., X-ray tube) for generating an X-ray beam (i.e., a photon beam) through the casing102and into the cement103. The X-ray source200may be a relatively high flux X-ray source (e.g., >109photon count per second) using a relatively high tube voltage (e.g., 300 keV). The X-ray source200may operate in continuous or pulsed modes. Heat generated by source operation may be dissipated through cooling fluid (e.g., air, water, oil).

The X-ray beam becomes broader along its path towards the casing. In order to keep the beam width relatively small (as compared to the raw beam emanating from the X-ray source) and concentrated to one location in the cement103, a capillary device201may be placed in front of the X-ray source200. The capillary device201confines the coverage width of the X-ray beam without reducing the intensity of the beam. The capillary device201is also configured to direct the X-ray beam from the X-ray source at an angle α (as measured from vertical), such that the beam enters the casing at the same angle. The X-ray beam then enters the casing102, the gap material220(if any) and the cement103.

The photons in the beam react with the cement103, which scatters the photons back through the gap material220(if any) and the casing102. In order to detect only those photons exiting the cement/gap/casing at a desired angle β, a slant-hole collimator205is located in front of a detector204. Thus only those photons traveling in a direction that is parallel to the collimator205may enter the collimator205and reach the detector204.

The detector204is a multi-pixel detector configured to perform energy-dispersive photon counting. Each pixel is shown having a diameter D. The multi-pixel capability of the detector204records the energy spectrum at different depths in the wellbore with a single measurement. The photon count capability records the photon count arriving at each pixel and the energy-dispersive capability sorts the count into different energy bins and generates a spectrum. Various detector embodiments are shown inFIG. 5and discussed subsequently.

A radiation shield203is located between the X-ray source200and the detector204. The shield203blocks photons from traveling directly from the X-ray source200to the detector204without passing through the cement103. The radiation shield203may be any photon blocking material (e.g., tungsten, lead) appropriate for blocking photons. For example, a photon blocking metal may be used as the radiation shield203.

The photons entering the cement130are reflected back from certain points230in the cement103. Although there are many, only one of these points is illustrated to provide clarity in the illustration. As the X-ray tomography tool rotates in the azimuthal direction in the wellbore, this point230moves around the cement encircling the wellbore so that the entire diameter of the cement is investigated as the tool moves through the wellbore.

Also noted inFIG. 2are representations of the photon energies and counts prior to entering the casing102as well as the photon energies and counts after reacting with the cement103and gap material220(if present) and exiting the casing102. For example, E0represents the energy of the photons prior to entering the casing102and N0represents the count of photons. Similarly, E represents the energy of the photons exiting the casing102and N3represents the count of photons.

Representations of the mass attenuation coefficients of each of the material(s) comprising casing102, cement103, and gap material220(if any), respectively, are illustrated inFIG. 2for subsequent discussion. For example, μs represents the attenuation of the casing, μgrepresents the attenuation of the gap (if present), and μcrepresents the attenuation of the cement.

Representations of the densities of each of the material(s) comprising casing102, cement103, respectively, are illustrated inFIG. 2for subsequent discussion. For example, ρxrepresents the density of the casing, ρgrepresents the density of the gap (if present), and ρcrepresents the density of the cement.

The intensity of an incident X-ray beam is attenuated by casing, gap materials (if any), and cement through elastic and inelastic interaction. The inelastic interaction includes photoelectric absorption, Compton scattering and pair production. In the expected range of X-ray energy (less than 300 keV), photoelectric absorption and Compton scattering are the relevant inelastic interactions. The total mass attenuation coefficient is determined by:
μ=μelastic+μphotoelectric+μcompton
where μelasticis the mass attenuation coefficient for elastic scattering, μphotoelectricis the mass attenuation coefficient for photoelectric absorption, and μcomptonis the mass attenuation coefficient for Compton scattering.

The total intensity attenuation is given by:
N=N0e−μρl
where ρ is the density of material and l is the distance of photon travelling inside the particular material (e.g., steel, air, water, cement).

When there are various materials along the path, the total intensity attenuation is written as:
N=N0e−Σiμiρili
where i is the count for each material.

For the medium-high energy X-ray photons, the Compton scattering process dominates. The incident photons interact with the electrons inside cement (or gap materials) and get scattered away from its original direction. The scattered photon energy can be described by the Compton equation:

E=E01+E0m0⁢c2⁢(1-cos⁢⁢θ)
where E0is the incident photon energy, m0is the rest mass of electrons, c is the speed of light, and θ is the scattering angle.

After the interaction, the photons have a probability of being scattered in any direction. The angular distribution of the scattering probability is described by the Klein-Nishima formula:

d⁢⁢σd⁢⁢Ω=r02⁡(E2E02)⁢(EE0+E0E-sin⁡(θ)2)
where r0is the classical electron radius, E0is the incident photon energy, E is the energy of scattered photons, θ is the scattering angle, and Ω is the solid angle.

With reference toFIG. 2, the percentage of photons scattered at the angle toward a single detector pixel is given by:

where Ω is the solid angle of the detector204as seen from the scattering point230, D is the diameter of the collimator205, ρ is the density of the scattering media, α is the incident beam angle, and β is the collimator angle. The total photon count detected at each pixel is the number of photons scattered toward the detector direction after intensity attenuation in the forward-passing and back-scattering path. These angles are shown inFIG. 2.

FIG. 3is a plot showing the modeling result of total detected photon counts without a gap between the casing and the cement, according to various embodiments of the invention. The plot includes the total photon count, in counts per second (cps) along the y-axis versus a detector pixel distance (pixel location) from a reference point 0 (e.g., top of detector), in centimeters (cm) along the x-axis. This plot shows that there is no increase in the reflected photon count as a result of a change in material density due to a gap being present. This plot may be contrasted to the plot ofFIG. 4showing photon counts with a gap.

FIG. 4is a plot showing the modeling result of total detected photon counts with a gap between the casing and the cement, according to various embodiments of the invention. The plot includes the total photon count, in counts per second (cps) along the y-axis versus a detector pixel distance (pixel location) from a reference point 0 (e.g., top of detector), in centimeters (cm) along the x-axis. In the illustrated embodiment, the gap is assumed to be filled with water (1 g/cm3). This plot shows an increase of the photon count between 1 and 2 cm from the reference at the location of the gap. Thus, the gap causes the detector pixel total photon count to respond (e.g., increase). Detecting such an increase in the total photon count may indicate a void in the cement or a gap between the cement and the casing. As the X-ray tomography tool rotates and moves within the borehole, a density map may be constructed using this information.

FIG. 5is a diagram showing classifications of X-ray detectors, according to various embodiments of the invention. In general, there are three types of X-ray detector500: imaging501, counting502, and imaging with counting ability503.

X-ray imaging detectors501collect a total charge produced by the incident X-ray photons over a period of time. X-ray imaging detectors501may generate a two-dimensional image of the object but may not detect the photon energy information. Examples of X-ray imaging detectors501include flat panel imaging detectors511used in medical and non-destructive testing industries.

X-ray counting detectors502generate a count of the number of detected incident photons according to photon energies. This information may be used to generate an energy spectrum of the detected X-ray photons. The X-ray counting detector502may provide spectrum information for the detected photons but may not have a sensitivity to the detected location on the detector. Examples of X-ray counting detectors502include single pixel detectors512, such as a scintillator, coupled to a photomultiplier tube522and a semiconductor photo diode523.

X-ray imaging and counting detectors503combine the advantages of the above two types of detectors501,502. The imaging and counting detectors503are often configured as counting multi-pixel detectors513to produce images of the detected photons with position information for each detected photon. In this type of detector, each pixel can count every incident photon.

Due to complexity of fabrication, some sub-types of counting multi-pixel detectors513may only operate to count photons above a particular energy threshold. These types of detectors are considered non-energy-dispersive detectors (i.e., cannot differentiate the energy of detected photons). Counting multi-pixel detectors513that can sort photons into different energy categories and generate a full spectrum are considered energy-dispersive detectors (i.e., can differentiate the energy of detected photons, thus resulting in a received spectrum of photon energies).

In some embodiments, the detector204(seeFIG. 2) may comprise an energy-dispersive photon counting multi-pixel detector. There are a plurality of types of such a detector.

One detector of this type couples a scintillator (continuous or pixelated) with a multi-anode photomultiplier tube (MAPMT)533. The MAPMT is position-sensitive and records the photon counts and energy information through a multiple-channel (e.g., 4 channels, 8 channels) multi-channel analyzer (MCA).

Another detector of this type couples a scintillator (continuous or pixelated) with a counting imager534(e.g., complementary metal oxide semiconductor (CMOS) imager, charged coupled device (CCD)). Such a detector534may be non-energy dispersive543or energy dispersive544.

Yet another detector of this type may be constructed by bump bonding a pixelated semiconductor chip to a solid state counting application specific integrated circuit (ASIC) readout535. The semiconductor chip may comprise Cd(Zn)Te or Hgl with pixelated Ohmic or Schottky contacts. The ASIC readout is designed to provide arrays of pixels with multiple energy categories per channel and may be fabricated with CMOS technology.

A detector535of this type may be non-energy dispersive555or energy dispersive556. The energy-dispersive multi-pixel photon counting detector556may achieve high image contrast through photon counting and materials information may be extracted through the spectrum information.

FIG. 6is a flowchart of a method for performing X-ray inspection of the cement in a downhole environment, according to various embodiments of the invention. In block601, a tool comprising the X-ray tomography device is lowered into the wellbore to be inspected. The tool may be part of a wireline tool or a drill string tool, as described subsequently.

In block603, an X-ray beam is transmitted into the wellbore in the direction of the casing and cement. This transmission may be performed while the tool is rotating in the azimuthal direction so that a three dimensional mapping of the cement density at that elevation in the wellbore may be performed.

In block605, a photon detector (e.g., an energy dispersive multi-pixel area detector) is used with a collimator to detect backscatter photons from one or more reflection points in the cement. As discussed previously, only those photons that are reflected back at the angle determined by the collimator are detected, thus providing the detector with an image and photon count of a particular reflection point in the cement.

The system measurement sensitivity can be changed by adjusting X-ray source power, X-ray source capillary angle and detector collimator angle. These changes may be accomplished to provide measurements through substantially the entire thickness of the cement.

In block607, the backscatter photons received from the reflection point in the cement, at the angle of the collimator, are counted. The quantity of photons provides an indication of density, as seen previously with respect toFIGS. 3 and 4. For example, if the photon count increases at a particular point, this may be due to a void in the cement. The energy dispersive multi-pixel photon detector may also determine a pixel location on the detector for the detected photons to generate a density map of the cement and measure a three-dimensional energy spectrum for the backscattered photons. This information may be used to determine elemental composition in the wellbore.

For example, abnormalities inside the cement may be determined and/or quantified based on comparing photon count-rate differences between measurements for each pixel at different depths. In some embodiments, the abnormalities may be determined and/or quantified by comparing count-rate differences between measurements for each pixel at different depths and predicted measurements for those respective depths. Still further embodiments may be realized.

For example,FIG. 7is a diagram showing a wireline system764, according to various embodiments of the invention andFIG. 8is a diagram showing a drilling system864, according to various embodiments of the invention. The systems764,864may thus comprise portions of a wireline logging tool body720as part of a wireline logging operation or of a downhole tool824as part of a drilling operation. The wireline logging tool body720and/or downhole tool824may include the X-ray tomography device210as described previously.

Turning now toFIG. 7, a drilling platform786equipped with a derrick788that supports a hoist790can be seen. Drilling oil and gas wells is commonly carried out using a string of drill pipes connected together so as to form a drillstring that is lowered through a rotary table710into a wellbore or borehole712. Here it is assumed that the drillstring has been temporarily removed from the borehole712to allow a wireline logging tool body720, such as a probe or sonde with the X-ray tomography device210, to be lowered by wireline or logging cable774(e.g., slickline cable) into the borehole712. Typically, the wireline logging tool body720is lowered to the bottom of the region of interest and subsequently pulled upward at a substantially constant speed.

During the upward trip, at a series of depths various instruments may be used to perform X-ray measurements on the casing and cement lining the borehole712. The wireline data may be communicated to a surface logging facility792for processing, analysis, and/or storage. The logging facility792may be provided with electronic equipment for various types of signal processing. Similar formation evaluation data may be gathered and analyzed during drilling operations (e.g., during LWD/MWD operations, and by extension, sampling while drilling).

In some embodiments, the wireline logging tool body720is suspended in the wellbore by a wireline cable774that connects the tool to a surface control unit (e.g., comprising a workstation754). The tool may be deployed in the borehole712on coiled tubing, jointed drill pipe, hard wired drill pipe, or any other suitable deployment technique.

Referring now toFIG. 8, it can be seen how a system864may also form a portion of a drilling rig802located at the surface804of a well806. The drilling rig802may provide support for a drillstring808. The drillstring808may operate to penetrate the rotary table710and bushing898for drilling the borehole712through the subsurface formations714. The drillstring808may include a drill pipe818and a bottom hole assembly820(e.g., drill string), perhaps located at the lower portion of the drill pipe818.

The bottom hole assembly820may include drill collars822, a down hole tool824including the X-ray tomography device210, and a drill bit826. The drill bit826may operate to create the borehole712by penetrating the surface804and the subsurface formations714. The downhole tool824may comprise any of a number of different types of tools besides the X-ray tomography device210including MWD tools, LWD tools, and others.

During drilling operations, the drillstring808(perhaps including the drill pipe818and the bottom hole assembly820) may be rotated by the rotary table710. Although not shown, in addition to, or alternatively, the bottom hole assembly820may also be rotated by a motor (e.g., a mud motor) that is located down hole. The drill collars822may be used to add weight to the drill bit826. The drill collars822may also operate to stiffen the bottom hole assembly820, allowing the bottom hole assembly820to transfer the added weight to the drill bit826, and in turn, to assist the drill bit826in penetrating the surface804and subsurface formations714.

During drilling operations, a mud pump832may pump drilling fluid (sometimes known by those of ordinary skill in the art as “drilling mud”) from a mud pit834through a hose836into the drill pipe818and down to the drill bit826. The drilling fluid can flow out from the drill bit826and be returned to the surface804through an annular area840between the drill pipe818and the sides of the borehole712. The drilling fluid may then be returned to the mud pit834, where such fluid is filtered. In some embodiments, the drilling fluid can be used to cool the drill bit826, as well as to provide lubrication for the drill bit826during drilling operations. Additionally, the drilling fluid may be used to remove subsurface formation cuttings created by operating the drill bit826.

The workstation754and the controller796may include modules comprising hardware circuitry, a processor, and/or memory circuits that may store software program modules and objects, and/or firmware, and combinations thereof. The workstation754and controller796may be configured to create a density and energy spectrum map of the borehole cement, according to the methods described previously.

Thus, in various embodiments, components of a system operable to conduct X-ray tomography measurements and analyze the measurements, as described herein or in a similar manner, can be realized in combinations of hardware and/or processor executed software. These implementations can include a machine-readable storage device having machine-executable instructions, such as a computer-readable storage device having computer-executable instructions. Further, a computer-readable storage device may be a physical device that stores data represented by a physical structure within the device. Such a physical device is a non-transitory device. Examples of machine-readable storage devices can include, but are not limited to, read only memory (ROM), random access memory (RAM), a magnetic disk storage device, an optical storage device, a flash memory, and other electronic, magnetic, and/or optical memory devices.

FIG. 9is a block diagram of an example system900operable to implement the activities of multiple methods, according to various embodiments of the invention. The system900may include a tool housing906having an X-ray tomography device210such as that illustrated inFIG. 2. The system900may be configured to operate in accordance with the teachings herein to perform X-ray tomography measurements and to determine the quality of cement between a casing and a formation.

The system900may include a controller920, a memory930, an electronic apparatus940, and a communications unit935. The memory930can be structured to include a database. The controller920, the memory930, and the communications unit935can be arranged to operate as a processing unit to control operation of the X-ray tomography device210. A processing unit925, structured to conduct such evaluation using X-ray measurement, can be implemented as a single unit or distributed among the components of the system900including electronic apparatus940. The electronic apparatus940can provide other circuitry for operation of the system900. The controller920and the memory930can operate in concert to control activation of the X-ray source(s)200(e.g. as shown inFIG. 2) of X-ray tomography device210to generate X-ray flux. The controller920and the memory930can also operate together to control selection of the detector(s)915in the tool906and to manage processing schemes. The controller920, the memory930, and other components of the system900can be configured, for example, to operate similar to or identical to the components discussed herein or similar to or identical to any of methods discussed herein.

The communications unit935can include downhole communications for appropriately located sensors in a wellbore. Such downhole communications can include a telemetry system. The communications unit935may use combinations of wired communication technologies and wireless technologies at frequencies that do not interfere with on-going measurements.

The system900can also include a bus937, where the bus937provides electrical conductivity among the components of the system900. The bus937can include an address bus, a data bus, and a control bus, each independently configured or in an integrated format. The bus937can be realized using a number of different communication mediums that allows for the distribution of components of the system900. The bus937can include a network. Use of the bus937can be regulated by the controller920.

In various embodiments, the peripheral devices950can include additional storage memory and other control devices that may operate in conjunction with the controller920and the memory930. In an embodiment, the controller920can be realized as a processor or a group of processors that may operate independently depending on an assigned function. The controller may be configured to generate an energy spectrum for the detected photons based on a number of detected photons received at the second predetermined angle. The controller may also be configured to controller configured to generate a density map of cement in response to a photon energy and a detected photon count.

The system900can include display unit(s)960as a distributed component on the surface of a wellbore, which can be used with instructions stored in the memory930to implement a user interface to monitor the operation of the tool906or components distributed within the system900. The user interface may be used to input parameter values for thresholds such that the system900can operate autonomously substantially without user intervention in a variety of applications. The user interface can also provide for manual override and change of control of the system900to a user. Such a user interface can be operated in conjunction with the communications unit935and the bus937. Many embodiments may thus be realized. A few examples of such embodiments will now be described.

Example 1 is an X-ray tomography device comprising: an X-ray beam source configured to transmit an X-ray beam at a first predetermined angle in a downhole environment; a single pixel or multi-pixel photon detector configured to count detected photons received at a second predetermined angle; and a controller coupled to the X-ray beam source and the detector and configured to generate an energy spectrum for the detected photons based on a number of detected photons received at the second predetermined angle.

In Example 2, the subject matter of Example 1 can include a radiation shield located between the X-ray beam source and the detector.

In Example 3, the subject matter of Examples 1-2 can include a radiation shield that comprises a photon block metal.

In Example 4, the subject matter of Examples 1-3 can include a detector that is an energy dispersive, multi-pixel photon detector.

In Example 5, the subject matter of Examples 1-4 can include a detector collimator coupled to the detector and configured such that only photons received at the second predetermined angle are detected by the detector.

In Example 6, the subject matter of Examples 1-5 can include an X-ray beam source that comprises an X-ray tube.

In Example 7, the subject matter of Examples 1-6 can include a capillary device coupled to the X-ray source, the capillary device configured to transmit the X-ray beam at the first predetermined angle, with a smaller beam width than the X-ray beam from the X-ray source.

In Example 8, the subject matter of Examples 1-7 can include a detector configured to record photon counts arriving at each pixel and sort each count into different energy bins to generate the energy spectrum associated with the detected photons.

In Example 9, the subject matter of Examples 1-8 can include a detector comprising a semiconductor and counting application specific integrated circuit detector.

In Example 10, the subject matter of Examples 1-9 can include a detector comprising a scintillator and counting complementary metal oxide semiconductor imaging detector.

In Example 11, the subject matter of Examples 1-10 can include a detector comprising a continuous or pixilated detector.

Example 12 is a method for performing X-ray inspection of cement, the method comprising: transmitting an X-ray beam at a first predetermined angle; detecting backscatter photons, received at a second predetermined angle, from the cement using an energy dispersive, multi-pixel photon detector; and counting a number of photons detected at the second predetermined angle to generate a density map of the cement.

In Example 13, the subject matter of Example 12 can include use of a detector that comprises a plurality of pixels, the method further comprising determining and/or quantifying abnormalities inside the cement based on: comparing photon count-rate differences between measurements for each pixel at different depths or comparing count-rate differences between a measurement for each pixel at different depths and a predicted measurement for the respective pixel at each respective depth.

In Example 14, the subject matter of Examples 12-13 can include determining a location of a pixel on the detector at which a photon is received to determine a reflective point in the cement from which the photon was reflected.

In Example 15, the subject matter of Examples 12-14 can include the detector generating an energy spectrum for detected photons.

In Example 16, the subject matter of Examples 12-15 can include transmitting the X-ray beam in a downhole environment from a wireline tool or a drill string tool.

In Example 17, the subject matter of Examples 12-16 can include causing the wireline tool or the drill string tool to rotate such that the X-ray beam is rotated within the cement along an azimuthal angle.

Example 18 is a system comprising: a downhole tool including an X-ray tomography device, the device comprising an X-ray beam source configured to transmit an X-ray beam at a first predetermined angle; an energy dispersive, multi-pixel photon detector configured to count detected photons received at a second predetermined angle; and a controller coupled to the X-ray beam source and the detector and configured to generate an energy spectrum for the detected photons received at the second predetermined angle.

In Example 19, the subject matter of Example 18 can include a downhole tool that is a wireline tool.

In Example 20, the subject matter of Examples 18-19 can include a downhole tool that is a drill string tool.

In Example 21, the subject matter of Examples 18-20 can include an X-ray tomography device that comprises a radiation shield located between the X-ray beam source and the detector; a capillary device coupled to the X-ray beam source, the capillary device configured to transmit the X-ray beam at the first predetermined angle and reduce the X-ray beam width to a width that is less than the X-ray beam from the X-ray beam source; and a detector collimator coupled to the detector, the detector collimator configured to receive only backscatter photons received at the second predetermined angle.

In Example 22, the subject matter of Examples 18-21 can include a controller configured to generate an energy spectrum of received photons and a density map of cement in response to determining a photon energy and a detected photon count, respectively.