Neutron logging tool with multiple detectors

A neutron logging tool has multiple detectors spaced about the circumference of the tool. The detectors are shielded from each other such that each detector detects gamma rays from the area of the borehole and formation to which it is closest. The log readings from each detector can be associated with the orientation of that detector. The orientation-specific log readings can then be aggregated to form an azimuthal log which can be used to analyze pre-fractured and/or post-fractured formations.

BACKGROUND

Many wells are fractured with a fracturing fluid to treat a formation and improve oil and gas production. In a standard fracturing operation, fracturing fluid is pumped down a wellbore with high pressure, causing a formation to fracture around a borehole. The fracturing fluid contains proppant (e.g. sand and/or other particles), which remains in the formation fractures and acts to “prop” open the fractures in the formation to increase hydrocarbon flow into the wellbore. Without proppant, the formation fractures may close, reducing the effectiveness of the fracturing procedure. Sometimes, other unwanted effects may occur. This may include proppant flowing back up the wellbore or an uneven distribution of proppant within the fractures in the formation. The resulting hydrocarbon production from the fractured formation may be less than optimal because of these unwanted effects. An example of a reference for hydraulic fracturing and its evaluation is described in the article “Hydraulic fracture evaluation with multiple radioactive tracers,” by Pemper et al., Geophysics, Vol. 53, No. 10 (October 1998), at 1323-1333, which is incorporated herein by reference.

As a result, it would benefit an operator to know the status of the formation after fracturing. If a formation had been minimally fractured, the operator could fracture the formation again. If it could be determined that the formation was evenly fractured, and that much of the proppant was retained in the formation fractures, then the operator could continue with hydrocarbon production.

Logging tools for measuring formation properties before fracturing are known. These tools have been used in the past to log a formation to detect oil and gas formations adjacent to a wellbore. However, there has not been an ability to measure the azimuthal distribution of proppant in formation fractures.

FIG. 1Ashows a deployed exemplary neutron logging system as known in the prior art as a cased hole reservoir evaluation tool. This system is similar to the system disclosed in U.S. Pat. No. 7,999,220, which is incorporated herein by reference in its entirety. Other systems are disclosed in U.S. Pat. Nos. 5,374,823 and 6,376,838, which are also incorporated herein by reference.

For the system ofFIG. 1A, neutron logging tool10is disposed within a borehole33penetrating earth formation40. The borehole33may be cased with casing35, and the casing-borehole annulus may be filled with a grouting material such as cement. Alternatively, the borehole33may be an uncased open hole.

Subsection11houses an array of detector assemblies100as well as a neutron generator102. More specifically, there are four detector assemblies100, each comprising a LaBr3 detector crystal and digital spectrometer for filtering and pulse inspection. These detectors are referred to as the proximal detector104, the near detector106, the far detector110, and the long detector112. The detectors are disposed at increasing longitudinal (or axial or vertical) distances from the neutron generator102. Between the near detector106and far detector110is a fast neutron detector108that measures the fast neutron output flux and pulse shape of the neutron generator102.

Subsection11is connected to instrument subsection24. Instrument subsection24houses control circuits and power circuits to operate and control the elements of subsection11. Additional elements of neutron logging tool10include telemetry subsection26and connector28. Neutron logging tool10is connected by wireline logging cable30to above-surface elements such as draw works34and surface equipment36.

Another multi-detector neutron logging tool10, known in the prior art as a pulsed neutron decay tool, is shown inFIG. 1B. Additional examples of different neutron logging tools10, in addition to the cased reservoir evaluation tool (CRE) inFIG. 1Aand the pulsed neutron decay tool (PND) inFIG. 1B, are the dual neutron tool (MDN), and the compensated neutron tools (CNT-S and CNT-V), all of which are available from Weatherford International Ltd.

The prior art neutron logging tools, such as tool10inFIGS. 1A-1B, are not able to give azimuthal logging information. Rather, the two or more detector assemblies100are spaced apart longitudinally along the body of the neutron logging tool10a short distance from the neutron source102, and the detector assemblies100are vertically in line with each other along a central axis of the tool. Yet, the detector assemblies100make their detections of the adjacent wall of the borehole without particular regard to direction or orientation. Instead, the intention of the multiple detector assemblies100is to provide different formation and statistical sensitivities during logging operations.

In particular, the effect is that the detector assemblies100closest to the neutron generator102typically are more sensitive to the borehole33, and the detector assemblies100further from the neutron generator102typically are more sensitive to the overall formation40. The sigma (Σ) capture cross-section of the borehole33and formation40of the readings may be computed by giving different weights to the near detectors' readings as compared to the far detectors' readings. For example, in a tool with two detectors, 70% weight may be given for the near detector reading and 30% weight for the far detector reading. The neutron logging tool10is usually run decentralized to the wellbore with an offset spring, or decentralizer, (not shown) such that the neutron logging tool10effectively runs along one wall of the wellbore.

SUMMARY

The subject matter of the present disclosure is directed to developing an azimuthal log that may be used before and/or after fracturing a formation. The azimuthal log can characterize the proppant distribution and can be compared to the pre-fracturing formation distribution. This would help an operator make decisions to optimize formation production.

A neutron logging tool has multiple detectors spaced about the circumference of the tool. The detectors are shielded from each other such that each detector detects gamma rays from the area of the borehole and formation to which it is closest. The log readings from each detector can be associated with the orientation of that detector. The orientation-specific log readings can then be aggregated to form an azimuthal log which can be used to analyze pre-fractured and/or post-fractured formations.

DETAILED DESCRIPTION

FIG. 2Ashows a deployed exemplary neutron logging system in accordance with the present disclosure. Neutron logging tool200is disposed within a borehole33penetrating earth formation40. The borehole33may be cased with casing35, and the casing-borehole annulus may be filled with a grouting material such as cement. Alternatively, the borehole33may be an uncased open hole. Neutron logging tool200may be run decentralized or centralized to the borehole33, and in each circumstance, the appropriate environmental corrections would be made. Further, neutron logging tool200may be attached via a mechanical swivel which allows orientation of the tool independent of conveyance. Subsection11of neutron logging tool200houses gamma ray detectors (or sensors)201a-das well as a neutron source202. In this example, the detectors201a-dare disposed at increasing longitudinal distances from the neutron generator202, although other arrangements are possible, as discussed below.

Neutron logging tools200and300have many of the same components as discussed previously, including instrument subsection24, telemetry subsection26, connector28, etc. Therefore, like reference numerals are used for the similar components, and these details are not repeated here.

Turning instead to the tool200,FIGS. 2B-2Cshow the side view and a top-down view of a portion of the exemplary neutron logging tool200with multiple detectors201a-d(i.e.,201a,201b,201c,201d) according to the present disclosure (although only two gamma ray detectors,201aand201b, are shown inFIG. 2B). At the base of the neutron logging tool200is neutron source202. In general, neutron source202, which emits neutrons, may be a pulsed neutron generator or a chemical neutron source, such as an Americium-Beryllium source. While either may be used, pulsed neutron generators are preferred because they have the benefit of being electronically controlled and cycled, and also have more energetic neutrons.

Gamma ray detectors201a-dmay be placed at different longitudinal distances (i.e., da, db, etc.) from neutron source202along the neutron logging tool200, as shown inFIG. 2B. The gamma ray detectors201a-dmay not align vertically with each other, but be dispersed radially around the circumference as shown inFIG. 2C. Moreover, as seen inFIG. 2D, gamma ray detectors201a-d(201dis not shown, as it is behind201b) may also be placed at similar longitudinal distances (i.e., d) from neutron source202. Further details of the possible placement of the detectors201a-dis discussed later.

Although detectors201a-dcan be disposed at similar or different distances from the source202,FIG. 2Cshows a top-down view of the exemplary neutron logging tool200with multiple detectors201a-daccording to the present disclosure. WhileFIG. 2Cshows a neutron logging tool200with a substantially cylindrical cross-section, the neutron logging tool200may have a different cross-sectional shape, such as an ellipse or other shape. However, as seen from this view inFIG. 2C, multiple gamma ray detectors201a-dare spaced about the circumference of the neutron logging tool200. Although four detectors201a-dare shown inFIG. 2C, the number of detectors in the neutron logging tool200may be fewer or greater. Typically, the gamma ray detectors201a-dwill be spaced evenly about the circumference of the neutron logging tool200to image different quadrants or sections of a formation40or a borehole33, but an non-uniform distribution would also perform the same function. A greater number of gamma ray detectors201a-dwould, therefore, give greater detail for an azimuthal log.

In another embodiment, shown inFIG. 2D, gamma ray detectors201, while placed about the circumference of the neutron logging tool200, may all be the same longitudinal distance (d) away from neutron source202. This arrangement may be preferable because the detectors' individual responses can be directly compared with each other, and a correction for different distances does not have to be implemented. While not seen explicitly inFIG. 2D, it will be understood that each of gamma ray detectors201a-dwill be offset from the central axis (not shown) of the neutron logging tool200. Accordingly, in the side view shown inFIG. 2D, gamma ray detector201d(not shown) is obscured by gamma ray detector201b. Although each tool200inFIGS. 2A-2Dhas one group of detectors201a-d, multiple sets of detectors201a-dmay be placed along the length of the tool200in a manner similar to the proximal detector104, the near detector106, the far detector110, and the long detector112of the tool10shown inFIG. 1A.

As noted above, the detectors201a-dcan be arranged in a number of ways on the tool200. If gamma ray detectors201a-dare spaced at different longitudinal distances from the neutron source202, as shown inFIG. 2B, they still may be placed about the circumference of neutron logging tool200. In such a case, the gamma ray detectors201a-dare offset from the central axis230of the neutron logging tool200, although they may still intersect central axis230depending on the size of the detector and the overall diameter of the tool. As an example, in the neutron logging tool200shown inFIG. 2C, gamma ray detector201aat the top of the neutron logging tool200(i.e. at 0 degrees) may be a distance daof 10 centimeters from neutron source202. The subsequent gamma ray detectors201b,201c, and201d, placed at 90, 180, and 270 degrees, may be longitudinally spaced at distances of 20 (db), 30 (do), and 40 (dd) centimeters from neutron source202, respectively. Having gamma ray detectors201a-dat different distances from the neutron source202provides the advantage of allowing for a tool with a smaller diameter. Additionally, as shown with gamma ray detectors201a-binFIG. 2E, the gamma ray detectors may be radially overlapped but longitudinally separated to reduce the diameter of the neutron logging tool200. The disadvantage is that a correction must be made for the various distances of the detectors201a-dfrom the source202, although this correction can be accounted for using techniques known in the art.

As opposed to the prior art that may have multiple detectors arranged vertically in line along the length of a tool, the disclosed tool200with its multiple detectors201a-dspaced around the tool's circumference at either the same or different vertical distances has shielding203bto isolate the various detectors201. For example,FIG. 2Cshows how shielding203bcan fill the core203bof the neutron logging tool200to isolate the detectors201a-dcircumferentially from one another.FIG. 2Bshows the distances daand dbbetween gamma ray detectors201aand201band neutron source202. This spacing allows for shielding203bbetween the gamma ray detectors201a-d, providing vertical isolation in addition to horizontal isolation. As an alternative or in addition, for purposes of optimizing the effectiveness of the azimuthal measurement, localized shielding203aaround the detectors201a-dcan be modified. The shielding203aand/or203beffectively gives each gamma ray detector201a-da sensing direction (sd), as seen inFIG. 2C. The sensing direction sdand respective dotted lines inFIG. 2Cshow the discrete azimuthal directions from which the respective gamma ray detectors201a-ddetect gamma rays. The angle and arc of the azimuthal direction may be varied by varying the shielding around the gamma ray detectors201a-d.

Given that the detectors201a-dcan be disposed at different vertical distances from the source202, the various detectors201a-dmay have different sensitivities. For consistent detection, the differences in detector sensitivities must be resolved between the gamma ray detectors201a-d. To do this, the gamma ray detectors201a-dcan be calibrated to have the same sigma (ρ) capture cross-sections, using techniques known in the art. Other normalization techniques could also be employed.

In some final details of the disclosed tool200and its detectors201a-dcapable of obtaining azimuthal data, it will be appreciated that the multiple gamma ray detectors201a-din the neutron logging tool200preferably detect gamma rays from the closest respective part of the formation. If gamma rays that passed through one side of the neutron logging tool200were detected by a gamma ray detector201a-don another side of the tool200, an accurate azimuthal log would be difficult to generate. As such, it will be appreciated that it is preferred that each gamma ray detector201a-dwithin the neutron logging tool200be shielded from the other detectors201a-d.

As discussed previously and shown in the embodiment inFIG. 2B, the core of the neutron logging tool200is filled (at least partially) with a shielding material203b. This shielding203babsorbs gamma rays that are released from the doped proppant or from the formation. In the neutron logging tool200with multiple gamma ray detectors201a-das shown in the embodiment inFIG. 4A, shielding203that properly houses the detectors201a-dcan prevent gamma rays from approaching a detector201a-dfrom a direction other than from the adjacent borehole wall toward the neutron logging tool's200center.

It will be appreciated that shielding203can alter the response of the detectors201, which can be accounted for in a particular implementation. Shielding203that partially surrounds a gamma ray detector201a-dmay be adjusted to optimize fracture response, optimize porosity and permeability response, and/or reduce some environmental noise-inducing effects. Shielding203may surround a detector both vertically as well as radially (i.e., towards the center of the neutron logging tool200). Acceptable shielding materials may include, but are not restricted to, tungsten and lead.

With each detector201a-dable to read gamma rays primarily from the direction it faces, an orientation-based reading of the formation may be achieved. With a neutron logging tool200with multiple shielded detectors201, each detector201a-dwill primarily detect gamma rays from the direction of the borehole33and formation40to which it is closest. As will be explained in further detail below, gamma rays may also be used to detect the presence of a doped proppant, such as a proppant doped with gadolinium. For example, the post-fracture log from a detector201a-dinFIG. 4Afacing a particular direction may display a high variance from the pre-fracture baseline log for gamma ray counts at gadolinium's characteristic energy, which originates from the gadolinium being activated from the neutron source202. This would indicate the presence of the gadolinium-doped proppant. If the pre-fracture and post-fracture logs did not display a high variance, then it might be determined that the gadolinium-doped proppant was not present. If only one detector201a-dout of multiple detectors201a-ddisplayed a high variance, it might indicate that the doped proppant within a formation fracture was not evenly distributed about the borehole33. Accordingly, an operator analyzing the log data could make decisions, such as deciding whether additional fracturing was necessary.

The top down view of another embodiment of the present disclosure is shown inFIG. 3A. The neutron logging tool300inFIG. 3Amay have only one gamma ray detector301, which is mounted on rotating member320, which can rotate about the vertical central axis330of neutron logging tool300. In other embodiments, rotating member320may rotate about a different positional axis, such that the positional axis may be offset but substantially parallel to the central axis of the neutron logging tool300. In other respects, neutron logging tool300may be similar to the neutron logging tool200shown in other figures. For example, neutron logging tool300has a neutron source302and shielding303, as shown inFIG. 3B. Shielding303may also be annular and located on rotating member320, as shown inFIG. 3A. Rotating member320also supports rotation orientation instrument310. Further, as shown inFIG. 3A, neutron logging tool300also can have an orientation instrument305that is not on rotating member320. In a variation of this embodiment, multiple sets of rotating detectors301may be placed along the length of the tool300in a manner similar to the proximal detector104, the near detector106, the far detector110, and the long detector112of the tool10shown inFIG. 1A. In this manner, neutron logging tool300may have multiple rotating members320, each with a gamma ray detector301, spaced at increasing longitudinal distances from neutron source302. In still another variation, one rotating member320may support multiple gamma ray detectors301at varying longitudinal distances from neutron source302.

FIG. 3Badditionally shows actuator321, which causes the rotation of rotating member320, and a power source322to power the actuator321. Actuator321may be an electric motor, which would rotate rotating member320with a gear assembly. Actuator322may also be another type of motor, such as a hydraulic motor, which would utilize hydraulic pressure to rotate rotating member320. As noted above, neutron logging tool300would have the components such as the instrumentation subsection and telemetry subsection, and further details are not provided here.

During operation of the neutron logging tool300, the rotating member320causes the rotation of gamma ray detector301. Shielding303can also be placed on the rotating member320such that the gamma ray detector301substantially detects gamma rays from the portion of the borehole33and formation40to which it is nearest. Two possible examples of general and/or localized shielding are seen inFIG. 2C, and these may be adapted to the embodiment shown inFIG. 3A. Accordingly, the gamma ray detector301is able to detect gamma rays from different portions of the formation40at different times during the rotation of the rotating member320. For example, inFIG. 3A, the position of the gamma ray detector301allows it to detect gamma rays from a discrete azimuthal portion of the formation in the sensing direction sd, as emphasized inFIG. 3Awith dotted lines. This allows the detector301to obtain an azimuthal reading of the formation40as it rotates with rotating member320.

Having an understanding of the neutron logging tool200and its various exemplary embodiments, discussion now turns to an example method500for obtaining azimuthal logs using the disclosed neutron logging tool200of a formation pre- and post-fracture, as shown inFIG. 5. Azimuthal logging data may be collected both before and after fracturing (steps510,520, and530). The variance between the pre-fracture and post-fracture logs would indicate the presence of a doped proppant, as described below.

In particular, the initial baseline pre-fracture log (step510) may be completed in multiple ways. If the borehole33has already been drilled, the neutron logging tool200may be used to take the baseline log. To capture a log with the neutron logging system, the neutron source202in the neutron logging tool200sends high energy neutrons into the surrounding formation. The neutrons quickly lose energy as the result of scattering, after which they are absorbed by the various atoms within the ambient environment. The scattered and absorbed neutrons emit gamma rays with characteristic energies, as shown inFIG. 4B. These gamma ray emissions can be measured versus characteristic energy and the presence or absence of certain materials can be determined. An example graph showing the characteristic energies of different elements is shown inFIG. 6, where some identifiable energy peaks are labeled.

Because the disclosed tool200has multiple detectors201a-ddisposed around the circumference of the tool200, the detectors201a-dcapture azimuthally directed logs of portions of the borehole33. Thus, the resulting pre-fracture log data obtained would essentially include log data for each detector201, with each detector's log data logging a portion of the formation40(i.e., a quadrant of the formation40if four detectors201a-dare used).

If the borehole33is in the process of being drilled, logging while drilling (LWD) instruments may be used to capture log information for a baseline log. Such a LWD instrument may be a different tool than the disclosed tool200, so that some additional correlation may be needed to match the pre-fracture log obtained with the LWD tool to the post-fracture log obtained with the disclosed tool200(described below in step540). Correlating a pre-fracture log with a post fracture log may be done by finding an orientation reference point by performing a pattern-matching technique between the two logs. In this manner, although the pre-fracture and post-fracture logs would have been obtained by separate instruments, the logs would still be able to be analyzed and compared with respect to each other.

In step520, the borehole33within the formation40would be fractured with a proppant. As known in the art, wells are fractured with a fracturing fluid to treat the formation40and improve oil and gas production. In a standard fracturing operation, fracturing fluid is pumped down the wellbore with high pressure, causing the formation40to fracture around the borehole33.

The next stage of the fracture operation contains proppant (e.g. sand and/or other particles), which remains in the formation fractures and acts to “prop” open the fractures in the formation to increase hydrocarbon flow into the borehole33. The proppant used in the disclosed fracturing process is preferably doped with neutron-absorbing materials, such as gadolinium. Other neutron-absorbing materials may include boron, strontium, barium, gallium, manganese, tantalum, germanium, cadmium, iridium, or combinations thereof. A particular example of a doped proppant and its usage is shown in U.S. Patent Application Publication No. 2011/0177984.

As shown inFIGS. 4A-4B, the gadolinium or other material present in the doped proppant would similarly absorb neutrons that were emitted from the neutron source202within the neutron logging tool200during post-fracture logging. Upon absorbing a neutron from the neutron source202, the gadolinium or other material will become an isotope of the element. In many cases, the isotope will subsequently release gamma rays with the characteristic energies of the isotope, which can be detected and analyzed by the gamma ray detectors201a-dof the disclosed tool200. As mentioned above, the characteristic energies of the gamma ray emissions can be used to identify the presence or absence of these materials.

Returning to the method ofFIG. 5, the pre-fracture log (step510) can be compared with the post-fracture log (step530) to determine the effectiveness of the fracturing operation and other details consistent with the present disclosure. Unfortunately, the neutron logging tool200in wireline operations may rotate while it is lowered into the borehole33during the separate logs. Typically, in a prior art logging tool (i.e.,10inFIGS. 1A-1B) without azimuthal log capabilities, the rotation of the neutron logging tool10would not affect the resultant log. However, the neutron logging tool's200rotation, whether inadvertent or intentional, should be compensated for to produce a more accurate azimuthal log.

Accordingly, orientation of the tool200during the pre-fracture and post-fracture logs needs to be correlated (Step540). To assist in compensating for rotation, the neutron logging tool200may have an orientation instrument205(as shown inFIG. 4A), such as electronic compass, magnetometer, inclinometer, etc., that calculates and stores orientation data. The orientation instrument205may also be a mechanical device, such as a weighting device or magnetic decentralizer that is used to ensure a particular orientation of the gamma ray detectors201a-d. The placement of the instrument205inFIG. 4Ais only meant to be illustrative; the actual placement of the instrument205may be elsewhere in the tool200.

Software navigation packs could additionally calculate the orientation of the neutron logging tool200as it passes downhole. The detector-specific logging data could then be correlated and combined with the orientation data of the neutron logging tool200for a given data reading, as shown in step540inFIG. 5. These detector-specific data sets could then be combined to give azimuthal log information.

For example, the neutron logging tool200shown inFIG. 4Amay have been lowered downhole via wireline with the gamma ray detector201a(at 0 degrees) pointing north. If neutron logging tool200rotated such that gamma ray detector201a(at 0 degrees) pointed east, the resulting data gathered from the gamma ray detector201a-dwould no longer be restricted to a single direction. However, the orientation instrument's205data could be correlated with the gamma ray detector's201a-ddata, allowing for an azimuthal log of borehole33that accounts for changes in the tool's200orientation (i.e., rotation) in the borehole33.

A similar procedure may be used to correlate orientation data for neutron logging tool300ofFIG. 3A. Rotating member320on neutron logging tool300may have a rotation orientation instrument310, which may be used to determine the position of the detector301as it rotates along with rotating member320. Additionally, orientation instrument305may be on the non-rotating portion of the neutron logging tool300. The orientation instrument's305data could be correlated with the rotation orientation instrument's310data, allowing for an azimuthal log of borehole33that accounts for changes in the tool's300orientation (i.e., from the rotation of the tool300) and also accounts for the detector's301orientation (i.e., from the rotation of the detector301within the tool300) in borehole33.

With this data, the azimuthal log readings which incorporate a direction or orientation variable allow an operator to obtain a more accurate understanding of the acquired log data. Azimuthal log data may be obtained even in the case of having a horizontal borehole. Orientation instruments205are available for horizontal borehole logging as well. The data from this instrument205could be similarly combined with the detector log readings from the multiple detectors201a-das described above to create an azimuthal log of the formation.

Although navigation pack tools are also available within logging while drilling (LWD) systems, the pre-fracture log obtained by an LWD tool may not directly compare to the post-fracture log obtained by the neutron logging tool200with multiple detectors201a-d. This is primarily because there are response differences in wireline and LWD instruments, and thus the pre- and post-fracture logs. As a result, logs taken by different instruments cannot necessarily be directly compared without additional calibration or compensation. Thus, any different response characteristics of the LWD tool and neutron logging tool200in the disclosed method ofFIG. 5can be accounted and compensated for in order to compare logs from the different tools.

Continuing with the method inFIG. 5, now that pre-fracture and post-fracture log data have been correlated for orientation, the method can analyze the log data by counting gamma rays with respect to time, energy, total counts, and subsurface depth (or borehole distance, for example, in horizontal boreholes), (step550), combining data from the multiple detectors201a-d(step560) (including orientation data), and generating a comprehensive image (step570). As noted above, when gamma ray detectors are at different longitudinal distances from the neutron source, as shown inFIG. 2B, a correction for the different distances would have to be made when combining data at step560. Time data provides information regarding formation sigma, and consequently proppant distribution. Gamma ray energy data, as well as total gamma ray counts, can also provide information regarding proppant distribution.

By way of a brief example,FIG. 6displays gamma ray counts along an energy spectrum, which could be obtained by one of the detectors201a-dof the disclosed tool200. If a gadolinium-doped proppant is used for formation fracturing, the characteristic energy of gamma rays emitted from gadolinium could be read along the energy spectrum to detect the gadolinium's presence or absence. Gamma ray spikes for the characteristic energy for gadolinium could indicate the presence of a formation fracture with doped proppant. Variances in gamma ray counts between detectors201a-dwould indicate that the proppant was not evenly distributed within a fractured area.

It will be appreciated that total counts of gamma rays may also be measured, without the need to separate the gamma rays along the energy spectrum. For example, if a baseline pre-fracture log has been taken of total gamma ray counts, then any significant variance in a post-fracture log from the pre-fracture log would also indicate the presence of a doped proppant. Total gamma ray counts could be analyzed with respect to time, subsurface depth, and azimuthal orientation, for example. It is understood that the total counts of gamma rays would have to be properly calibrated and/or normalized for the purposes of comparison among the detectors.

As another brief example inFIG. 7, gamma ray counts may be measured as a function of time. A fast neutron pulse sent from the tool's neutron source202would generate resultant gamma rays emissions that would have to be timely detected by a detector201. The slope of the logarithmic gamma ray counts versus time can be used as an indicator to judge the sigma value of the formation40. Additionally, when a doped proppant used in fracturing the formation40as used herein, the sigma of the various detectors' formation40slopes can be measured. This would also allow for the detectors'201a-dgamma ray counts to be used to determine the effectiveness of the fracture of the formation40.

Finally, in another brief example,FIG. 8displays a count of gamma rays versus depth down the borehole33for one detector201. Each detector201a-dwould generate its own set of data similar toFIG. 8, and each data set would represent gamma ray counts versus depth at the particular orientation of the detector201a-din the borehole33. Each detector's data set may be displayed separately. In addition, another data set could display the sum of all or a subset of detectors'201a-ddata sets. When the individual detector data sets are combined with each other and with an orientation calculated from the navigation pack tool, a log containing full azimuthal data can be generated.

The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. It will be appreciated with the benefit of the present disclosure that features described above in accordance with any embodiment or aspect of the disclosed subject matter can be utilized, either alone or in combination, with any other described feature, in any other embodiment or aspect of the disclosed subject matter. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.