Well-logging apparatus including azimuthally spaced radiation detectors

A well-logging device may include a housing to be positioned within a larger borehole of a subterranean formation and thereby define a stand-off distance with respect to the larger borehole. The well-logging device may also include at least one radiation source carried by the housing to direct radiation into the subterranean formation, and radiation detectors carried by the housing in azimuthally spaced relation to detect radiation from the subterranean formation. The well-logging device may further include a controller to cooperate with the radiation detectors to determine at least one property of the subterranean formation corrected for the stand-off distance.

BACKGROUND

To determine a porosity of a subterranean formation, it may be desirable to make several simultaneous measurements. One tool for measuring porosity is based on neutron transport through the subterranean formation. The neutron flux attenuated with distance from the source may depend strongly on the hydrogen content of the subterranean formation. For a neutron source, radioisotopic sources or accelerator based sources are used in existing tools.

If pore spaces are filled by liquid, the higher porosity corresponds to a higher hydrogen index. The detected neutron counts are generally lower in this case. A properly calibrated tool may increase the accuracy of the porosity measurement in liquid-filled formations if the matrix composition is known. However, the measurement may be affected by various environmental conditions.

On the other hand, the same measurement may be less accurate for gas-filled subterranean formations when the hydrogen content in the pore spaces is lower due to the relatively low density of the gas. A density measurement may address this ambiguity. For the same porosity of the subterranean formation, the gas-filled and liquid-filled matrix have different densities.

One environmental condition that may affect the porosity measurement is tool position or stand-off in the borehole. The stand-off and the borehole fluid (liquid or gas) may impact the count rate in radiation detectors.

SUMMARY

A well-logging apparatus may include a housing to be positioned within a larger borehole of a subterranean formation and thereby define a stand-off distance with respect to the larger borehole. The well-logging apparatus may also include a radiation source carried by the housing to direct radiation into the subterranean formation, and radiation detectors carried by the housing in azimuthally spaced relation to detect radiation from the subterranean formation. A controller may cooperate with the plurality of radiation detectors to determine at least one property of the subterranean formation corrected for the stand-off distance based upon the detected radiation from the radiation detectors.

A method aspect is directed to a method of determining a stand-off distance defined by a housing of a well-logging apparatus to be positioned within a larger borehole of a subterranean formation. The stand-off distance is defined with respect to the larger borehole. The method may include directing radiation from a radiation source carried by the housing into the subterranean formation, and detecting radiation from the subterranean formation using radiation detectors carried by the housing in azimuthally spaced relation. The method may also include using a controller to cooperate with the plurality of radiation detectors and determine at least one property of the subterranean formation corrected for the stand-off distance based upon the detected radiation from the radiation detectors.

DETAILED DESCRIPTION

Referring initially toFIGS. 1 and 2, a well-logging apparatus10includes a housing11to be positioned within a larger borehole12of a subterranean formation13and thereby define a stand-off distance d with respect to the larger borehole. The housing11illustratively has a rounded shape, but may be another shape. The housing11may be coupled to a tether16to position the housing in the borehole12. For example, the tether16may be in the form of a wireline, coiled tubing, or a slickline. Of course, the tether16may be another type of tether that may use other techniques for conveying the housing11within the borehole12.

A radiation source14is carried by the housing11. The radiation source14may be a neutron generator (accelerator based, pulsed), for example, or may be radioisotopic radiation source, such as, for example,241AmBe or252Cf. Of course, the radiation source14may be another type of radiation source. The radiation source14directs radiation into the subterranean formation13.

Radiation detectors15a-15fare carried by the housing11in azimuthally spaced relation to detect radiation from the subterranean formation13. More particularly, the radiation detectors15a-15fare equally azimuthally spaced from one another. Of course, in some embodiments, the radiation detectors15a-15fmay not be equally azimuthally spaced. The radiation detectors15a-15fmay be carried by the housing11to cover 360-degrees. The radiation detectors15a-15fgenerate a count rate of detected neutrons, for example, in the presence of an active neutron source14. Moreover, while six radiation detectors15a-15fare illustrated, any number of radiation detectors may be carried by the housing11.

The radiation detectors15a-15fmay be neutron detectors, for example, when the radiation source14is a neutron generator. In some embodiments, the radiation detectors15a-15fmay be epithermal, thermal, or high energy (>100 keV) neutron detectors.

The well-logging tool10may advantageously provide increased compensation for stand-off. Porosity, for example, neutron porosity, generally depends on the count rates of each radiation detector. If the count rate is sensitive to the stand-off, the asymmetry of the count rates may provide a measure of stand-off that can be used for correction. If, however, the count rate is not sensitive to the stand-off, for example, when the borehole fluid hydrogen index is low, (e.g. the borehole fluid is methane gas) then the asymmetry is also less sensitive to the stand-off and a small or no correction factor may be applied to the radiation detector reading, for example.

Referring additionally to the graph30inFIG. 3, the dependence of porosity on the detector count rates is illustrated. A Monte Carlo simulation was performed for a water-filled borehole. The three curves31a-31ccorrespond to different stand-offs of the radiation detector. In particular, curve31acorresponds to no stand-off, while curve31bcorresponds to 0.4 inches of stand-off. Curve31ccorresponds to 0.85 inches of stand-off.

Referring additionally to the graph40inFIG. 4, the same dependences are illustrated when the borehole fluid is high density methane, for example. For the modeling it was assumed that methane pressure was 5000 psi, density 0.18 g/cm3, and the hydrogen index 0.5. The data show that for relatively low hydrogen index borehole fluid, stand-off has a reduced affect on the radiation detector reading as compared to a water-filled borehole. Curve41acorresponds to no stand-off, while curve41bcorresponds to 0.4 inches of stand-off. Curve41ccorresponds to 0.85 inches of stand-off.

A controller20cooperates with the radiation detectors15a-15fto determine at least one property of the subterranean formation corrected for the stand-off distance based upon the detected radiation from the radiation detectors. More particularly, the controller20determines a correction factor that may be used to correct a porosity or the stand-off distance from the respective count rates. The controller20may also determine property of the subterranean formation corrected for the stand-off distance based upon an asymmetry of the detected radiation from the radiation detectors15a-15f, as will be explained in further detail below.

The controller20may include a processor21and a memory22coupled thereto. Of course, the controller20may include more than one processor. The controller20may be remote from the borehole12, carried within the borehole, or positioned outside the borehole above the subterranean formation13, for example. Of course, the controller20may be positioned elsewhere or in more than one location so that its functionality is shared.

To correct for the stand-off, the controller20uses azimuthal information from the radiation detectors15a-15f. For ease in explanation, reference is made to two radiation detectors15b,15e(FIG. 2). In general, if the housing11of the well-logging apparatus10touches the wall of the borehole12, there is little or no stand-off for the radiation detector15eadjacent or close to the subterranean formation13. However, the radially opposite radiation detector15bhas a maximum stand-off for the given borehole size.

If the hydrogen index of the borehole12is larger than the hydrogen index of the subterranean formation13, the radiation detector15bwith stand-off collects fewer counts than the radiation detector15ewith little or no stand-off. As mentioned above, the radiation detector15bwith the increased stand-off sees the borehole12together with the subterranean formation13. The effective hydrogen index is higher in this case than the hydrogen index of the subterranean formation13. The count rate of the radiation detector15ewith little or no stand-off is more than the count rate of the radiation detector15bwith the increased stand-off. Therefore, the asymmetry of the count rates of the radiation detectors15a-15fcan be used by the controller20as a variable that measures the stand-off.

Referring to the equation:

Det(i) is the count rate of the ithdetector, and n is the total number of detectors, which in the example described herein in six. The variable A(i) depends also on the borehole hydrogen index. If the housing11of the well-logging tool10, for example, is centralized in the borehole12, the count rates of the radiation detectors15a-15fare the same, and the asymmetry=1 (assuming that the subterranean formation surrounding the borehole is uniform). If the hydrogen index of the borehole fluid is relatively small, then count rates do not depend strongly on the housing11position, and the asymmetry may be close to one.

Referring now additionally to the graph50inFIG. 5, the distribution of A versus porosity for a water-filled borehole illustrated. Curve51acorresponds to no stand-off, while curve51bcorresponds to 0.4 inches of stand-off. Curve51bcorresponds to 0.85 inches of stand-off.

The asymmetry for radiation detectors with a maximum stand-off is less than one. Without stand-off the asymmetry is more than one. Likewise, asymmetry is relatively close to one for radiation detectors with a medium stand-off.

For a methane-filled borehole, the radiation detector count rate may not depend strongly on the stand-off, and the asymmetry may be close to one. Asymmetry represents the combined effects of borehole fluid and stand-off on each radiation detector count rate. By the controller20using the azimuthal information from the radiation detectors15a-15f, the asymmetry in this particular case, the counts of each radiation detector may be corrected to derive a stand-off independent porosity.

Referring to the graphs60and61inFIGS. 6aand6b, respectively, the dependence of porosity on the combination of count rates and asymmetry for radiation detectors with different stand-offs for water-filled and methane-filled boreholes is illustrated. Using the asymmetry A for the count rate of each radiation detector, the porosity can be determined as a function of the corrected detector count rate and independent of the stand-off. The same combination corrects the stand-off for both borehole fluids. The curve63in the graph60ofFIG. 6aillustrates the porosity dependence on the average counts of radiation detectors having no stand-off, 0.4 inches of stand-off, 0.85 inches of stand-off and the average stand-off. In addition to the individual radiation detectors, the curve62in the graph61ofFIG. 6billustrates the porosity dependence on the average counts of radiation detectors having no stand-off, 0.4 inches of stand-off, and 0.85 inches of stand-off, corrected with its corresponding asymmetry factor, which is close to one in this case. The average count rate and the individual radiation detector count rates after correction may agree.

In the examples illustrated inFIGS. 6aand6b, the relationship between porosity and the uncorrected count rates of the different detectors (countuncorr) becomes unique if each radiation detector count rate is corrected by the formula given below to determine the corrected count rate countuncorr:
log(countcorr)=log(countuncorr)−α·A
where is a factor determined from experiments or modeling. In the case of the example given inFIGS. 6aand6b, =0.833. The value may vary depending on such parameters as tool diameter, detector geometry and spacing, and borehole diameter for example. The functional form of the relationship between the asymmetry and the stand-off correction may be different and more complex for other tool geometries.

Referring now toFIG. 7, in another embodiment of the well-logging tool10′, the azimuthally radiation detectors15′ are at a first axial spacing from the radiation source14′. An additional set of radiation detectors17′ is carried by the housing11′ in azimuthally spaced relation and at a second axial spacing from the radiation source14′ to detect additional radiation from the subterranean formation13′. The controller20′ determines the stand-off distance d also based upon the additional detected radiation from the additional radiation detectors17′. Each of the additional radiation detectors17′ may be a scintillation detector, for example. The additional radiation detectors17′ may be gamma ray detectors and/or neutron detectors, or a combination thereof. In some embodiments, each of the additional radiation detectors17′ may be another type of detector. Moreover only one additional radiation detectors may be included. It should be noted that the additional radiation detectors17′ are azimuthally spaced similarly to radiation detectors15′ as illustrated above inFIG. 2, for example, however, the additional radiation detectors may not be azimuthally spaced and may be different in number from the radiation detectors15′.

A method aspect is directed to determining a stand-off distance defined by a housing11of the well-logging apparatus10, for example, as described above. The method includes directing radiation from a radiation source14carried by the housing11into the subterranean formation13. The method also includes detecting radiation from the subterranean formation13using radiation detectors15carried by the housing11in azimuthally spaced relation. The method further includes using a controller20to determine the stand-off distance based upon the detected radiation from the radiation detectors15.

The radiation detectors15generate a count rate, and the controller20is used to determine the stand-off distance from the count rates. The controller20is also used to determine the stand-off distance based upon an asymmetry of the detected radiation from the radiation detectors15.

In another method embodiment, the radiation detectors15are at a first axial spacing from the radiation source14. The method includes detecting radiation from the subterranean formation13using an additional set of radiation detectors17, for example, as described above, carried by the housing11in azimuthally spaced relation and at a second axial spacing from the radiation source14. The controller20is used to determine the stand-off distance also based upon the additional detected radiation from the additional radiation detectors17.