FR3104739A1 - Method for evaluating the uranium content by gamma spectrometry in a wellbore and associated device - Google Patents

Abstract

Method for evaluating the uranium content by gamma spectrometry in a wellbore and associated device The invention relates to a method for evaluating the uranium content of at least one region of interest (10) of a sub -sol (11) crossed by a wellbore (12) by gamma spectrometry. The method comprises the following steps: a) acquiring a first and a second energy spectra of calibration gamma radiation in a first and a second calibration regions (18, 20), b) obtaining respectively at least a first and a second radioactive imbalance uranium, c) calculate a first and a second ratio between the area of a first energy band and the area of a second energy band of the first and second calibration energy spectra, d) calculate a first and a second coefficient from the radioactive imbalances and ratios, e) acquire an energy spectrum associated with the region of interest (10), f) calculate the uranium content in the region of interest (10) using in particular the first and second coefficients. Figure for abstract: Figure 1

Description

Method for evaluating the uranium content by gamma spectrometry in a wellbore and associated device

The present invention relates to a method for evaluating the uranium content of at least one region of interest of a subsoil traversed by a wellbore by gamma spectrometry, the subsoil comprising at least a first region of calibration and a second calibration region traversed by the wellbore, the first calibration region exhibiting a first radioactive imbalance, the second calibration region exhibiting a second radioactive imbalance distinct from the first radioactive imbalance.

Uranium occurs naturally in the form of three isotopes: ²³⁸ U, ²³⁵ U and ²³⁴ U, the latter coming from the decay chain of ²³⁸ U. ²³⁸ U is very widely majority and represents more than 99.3% by mass fraction of total uranium.

²³⁸ U and ²³⁵ U successively disintegrate into different chemical elements called daughter elements until the resulting chemical element is stable. Each disintegration is most often accompanied by the emission of high energy photons also called X or gamma radiation whose energy spectrum is typically between a few tens of keV and more than 2000 keV. The unit of measurement of radiation is expressed in counts per unit of time, for example in counts per second.

In mining exploration, it is known to use the gamma radiation emitted by geological formations to characterize the latter and more particularly the uranium content of underground geological formations. This type of measurement is carried out in particular in boreholes. These measurements typically make it possible to characterize the uranium potential of a region. When a deposit is discovered, gamma radiation measurements in drilling are also used to estimate the uranium reserves of the deposit. The accuracy of these measurements and the processing that is done to obtain the associated uranium contents are therefore critical.

Conventionally, in boreholes, the total gamma radiation is measured over the entire energy spectrum, in the form of a total count, all along the borehole by moving a scintillator detector. The scintillator detector comprises, for example, a crystal of sodium iodide (NaI). This method works relatively well when there is a balance between the different chains of uranium parentage. In this case, the total gamma radiation is roughly proportional to the uranium content of the formation. A calibration coefficient, specific to the gamma radiation detector, relates the total radiation measured and the uranium content of the formation. The calibration coefficient is obtained by simulation or experimentally from standard samples.

Nevertheless, in the case of an imbalance in the uranium filiation chains, this method leads to underestimating or overestimating the uranium content in the geological formations. Such an imbalance is generally observed for low-grade uranium deposits subject to differential leaching phenomena (e.g. uranium deposit of the "roll fronts" type) which constitute a significant part of the uranium deposits exploited in the world.

The object of the invention is to overcome these drawbacks, by proposing a method for evaluating the uranium content of at least one region of interest of a subsoil traversed by a wellbore which is reliable and which allows to obtain accurate uranium content values, even in the event of an imbalance between the different uranium parentage chains.

To this end, the invention relates to a method of the aforementioned type, comprising the following steps:

a) acquiring at least a first and a second calibration gamma radiation energy spectra respectively in the first and second calibration regions, each of the calibration energy spectra comprising at least:

- a first energy band relating to a uranium contribution, the first band comprising at least one energy line at 92keV of ²³⁴ Th and an energy line of the fluorescence X-radiation of uranium at 98keV,

- a second energy band relating to a radon contribution, the second band comprising at least one energy line at 609keV of ²¹⁴ Bi, and

- an energy band centered on an energy line at 1001keV of ^234m Pa,

b) respectively obtaining at least the first radioactive uranium imbalance associated with the first calibration energy spectrum and the second radioactive uranium imbalance associated with the second calibration energy spectrum,

c) calculating at least a first ratio and a second ratio between the area of the first energy band and the area of the second energy band respectively of the first calibration energy spectrum and of the second calibration energy spectrum,

d) calculating a first coefficient and a second coefficient from the at least first and second radioactive imbalances of uranium obtained and the at least first and second ratios calculated,

e) acquiring at least one energy spectrum of gamma radiation associated with the region of interest, the energy spectrum comprising at least the first energy band and the second energy band,

f) calculate the uranium content in the region of interest using the area of the first energy band, the area of the second energy band and the net area of the 609keV energy line of ²¹⁴ Bi of the energy spectrum acquired in the region of interest, and the first and second calculated coefficients.

Thus, the method according to the invention makes it possible to reliably evaluate the uranium content of the formations of the subsoil from a calibration carried out in situ and taking into account the possible presence of a radioactive imbalance.

According to particular embodiments, the method according to the invention comprises one or more of the following characteristics, taken in isolation or in all technically possible combinations:

- the first energy band is substantially between 87keV and 110keV and the second energy band is substantially between 560keV and 660keV;

- the first radioactive imbalance of uranium and/or the second radioactive imbalance of uranium are calculated using respectively the net area of the energy line at 1001 keV and the net area of the energy line at 609keV of each of the first and second calibration energy spectra;

- the method comprises a preliminary step of identifying the first calibration region and/or the second calibration region by measuring the total gamma radiation count or by acquiring a gamma radiation spectrum and calculating a ratio between the area of the first energy band and the area of the second energy band along the wellbore;

- the first coefficient and the second coefficient are calculated by linear regression of the at least first and second ratios calculated as a function of the at least first and second radioactive imbalances obtained;

- the first coefficient and the second coefficient are calculated using the equation:

being the area of the first energy band of the energy spectrum of the calibration region (18, 20, 22) considered, being the area of the second energy band of the energy spectrum of the calibration region considered, being the uranium radioactive imbalance of the calibration region of index k, α being the first coefficient, β being the second coefficient;

- the wellbore passes through at least a third calibration region, the third calibration region having a third imbalance different from the first and second imbalances, the method comprising the acquisition of at least a third calibration gamma radiation spectrum associated with the third calibration region, obtaining a third radioactive imbalance, calculating a third ratio, the first and second coefficients being calculated further using the third radioactive imbalance and the third ratio;

- the second calibration region is located between the first calibration region and the third calibration region along the wellbore.

The invention also relates to a device for evaluating the uranium content of a region of interest of a subsoil crossed by a wellbore by gamma spectrometry, the subsoil comprising at least a first calibration region and a second calibration region traversed by the wellbore, the first calibration region having a first imbalance, the second calibration region having a second imbalance different from the first imbalance, the device comprising:

- a spectrometric probe comprising a scintillator detector,

- an acquisition module connected to the spectrometric probe, the acquisition module being configured to acquire at least a first energy spectrum of calibration gamma radiation associated with the first calibration region, a second energy spectrum of associated calibration gamma radiation to the second calibration region and a gamma radiation energy spectrum associated with the region of interest, each of the calibration energy spectra comprising at least:

- a first energy band relating to a uranium contribution, the first band comprising at least one energy line at 92keV of ²³⁴ Th and an energy line of the fluorescence X-radiation of uranium at 98keV,

- a second energy band relating to a radon contribution, the second band comprising at least one energy line at 609keV of ²¹⁴ Bi, and

- an energy band centered on an energy line at 1001keV,

- a module for obtaining the at least first and second radioactive imbalances of uranium associated with the first and second calibration energy spectra,

- a first calculation module for calculating at least a first ratio and a second ratio between the area of the first energy band and the area of the second energy band respectively of the first calibration energy spectrum and of the second spectrum calibration energy,

- a second calculation module for calculating a first coefficient and a second coefficient from at least the first and second radioactive imbalances of uranium obtained by the obtaining module and the first and second ratios calculated by the first calculation module,

- a third module for calculating the uranium content in the region of interest using the area of the first energy band, the area of the second energy band and the net area of the energy line of the spectrum acquired in the region of interest by the acquisition module, and the first and second coefficients calculated by the second calculation module.

According to a particular embodiment, the module for obtaining comprises a calculation sub-module for calculating a radioactive imbalance of uranium by using the net area of the energy line at 1001 keV and the net area of the energy line at 609keV of the energy spectrum acquired by the acquisition module.

The invention will be better understood on reading the following description, given solely by way of example, and made with reference to the drawings, including:

- Figure 1 is a schematic representation of a wellbore and a device according to the invention,

- FIG. 2 is an example of a gamma radiation spectrum acquired by the acquisition module of the device of FIG. 1, and

- Figure 3 is a schematic representation of a method according to the invention.

In the rest of the description, the terms “mass content”, “mass concentration” and “content” are considered to be synonyms. Similarly, the terms 'line', 'energy ray', 'spike' or 'energy peak' are also considered synonymous.

A device 100 for evaluating the uranium content of at least one region of interest 10 of a subsoil 11 traversed by a wellbore 12 is represented schematically in FIG. 1.

For simplicity, the wellbore 12 is shown in Figure 1 vertical, that is to say with a dip equal to 90 °. Alternatively, the wellbore 12 is not vertical and has any dip and azimuth.

The wellbore 12 is carried out through a plurality of geological formations 14. For example, the wellbore 12 is carried out for exploration purposes to search for a possible deposit of uranium 16. Alternatively, the well of drilling 10 is carried out for the purpose of developing an identified deposit. The information collected in borehole 12 is then used to estimate the uranium reserves of deposit 16.

Referring to Figure 1, the wellbore 12 notably passes through at least a first region 18, called the first calibration region, and a second region 20, called the second calibration region.

The wellbore 12 also passes through at least one reference region, preferably located above the first calibration region 18.

The reference region is an unmineralized region. By "non-mineralized region" is meant a region in which the uranium content is low, below a reference threshold.

The reference threshold is for example between 10ppm and 50ppm.

The first calibration region 18 has a first uranium content. The second calibration region 20 has a second uranium content.

In the same way, the first calibration region 18 presents a first radioactive imbalance of uranium D ₁ and the second calibration region 20 presents a second radioactive imbalance of uranium D ₂ .

Preferably, the first calibration region 18 and the second calibration region 20 are mineralized regions. By “mineralized region” is meant a region in which the uranium content is greater than the reference threshold, preferably greater than a multiple of the reference threshold.

The multiple is for example equal to 2, 3, 4 or 5.

In the remainder of the description, it is understood that the contents of the various calibration regions 18, 20 and/or region of interest 10 correspond to average uranium contents for said regions considered. In the same way, the radioactive imbalances of uranium correspond to average imbalances for the said regions considered.

The device 100 comprises a spectrometric probe 102 arranged to be inserted into the wellbore 12 and a surface installation 104 allowing the spectrometric probe 102 to be moved along the wellbore 12 up and down according to the direction of the well. drilling. For example, the surface installation 104 comprises a winch 106 and a device 108 adapted to know the position of the spectrometric probe inside the wellbore 12, for example a code wheel. The spectrometric probe 102 is connected by a cable 110 to the surface installation 104. The cable 110 allows both the displacement of the spectrometric probe 102 inside the wellbore 12, the electrical supply of the spectrometric probe 102 and the transfer of the measurements made by the spectrometric probe 102.

Spectrometric probe 102 includes a scintillator detector. The scintillator detector comprises for example a lanthanum bromide (LaBr ₃ ) crystal or a sodium iodide (NaI) crystal.

The device 100 comprises a computer 112 for evaluating the uranium content of the region of interest 10 of the wellbore 12, a display unit 114 connected to the computer 112 to display the results provided by the computer and an interface man-machine 116 to control the device 100.

The computer 112 includes for example a database 118.

The database 118 is intended for example to record the results provided by the computer 112.

The computer comprises a processor 120 and a memory 122 receiving software modules. The processor 120 is capable of executing the software modules received in the memory 122 to calculate the uranium content of the region of interest 10.

The memory 122 includes an acquisition module 124 of at least a first radiation energy spectrum. More particularly, by using the surface installation 104, the acquisition module 124 makes it possible to acquire an energy spectrum of radiation in the first calibration region 18, in the second calibration region 20 and in a region of interest 10 from the borehole 12.

The spectrometric probe 102 is connected to the acquisition module 124 by the cable 110.

FIG. 2 presents an example of an energy spectrum 200 acquired by the acquisition module 124.

The energy spectrum 200 represents the gamma count as a function of the energy of the radiation. Gamma counting is usually expressed in counts per second. Radiation energy is usually expressed in kilo-electron-volts (keV).

Each 200 energy spectrum includes at least:

- a first energy band C _U relating to a uranium contribution, the first band comprising at least one energy line at 92keV of ²³⁴ Th and an energy line of the fluorescence X-radiation of uranium at 98keV,

- a second energy band C _Rn relating to a radon contribution, the second band comprising at least one energy line at 609keV of ²¹⁴ Bi, and

- an energy band centered on an energy line at 1001keV (not shown in figure 2).

The first energy band corresponds more particularly to the contribution of radionuclides from the ²³⁵ U isotope decay chain, assumed to be in secular equilibrium, and radionuclides from the top of the ²³⁸ U isotope decay chain which are in secular balance. This group of radionuclides includes the elements ^234m Pa, ²¹¹ Pb, ²²³ Ra, ²¹⁹ Rn, ²²⁷ Th, ²³⁰ Th, ²³¹ Th, ²³⁴ Th, ²³⁵ U, ²³⁸ U.

The 92keV energy line is dominated mainly by emissions of two radionuclides:

- the emission of an X-ray at 93.35keV (which contributes to the signal at 92keV due to the resolution of the scintillator) linked to ²³⁵ U. This emission comes from the reorganization of the electronic procession of the thorium atom following the α decay of ²³⁵ U to ²³¹ Th, and

- the emission of a ²³⁴ Th gamma doublet at 92.38keV and 92.80keV (located at the top of the ²³⁸ U chain).

The 98keV energy line is dominated by four emissions:

- the XK _α1 fluorescence of uranium at 98.439keV,

- a spontaneous X emission at 98.439keV linked to ^234m Pa (at the top of the ²³⁸ U chain) which follows the internal conversion of its ²³⁴ U child nucleus,

- the spontaneous emission of two X-rays linked to ²²³ Ra at 97.90keV ( ²³⁵ U chain) resulting from the reorganization of the electronic procession of the radon atom following the α disintegration of ²²³ Ra towards ²¹⁹ Rn, And

- the spontaneous emission of two X-rays linked to ²²⁶ Ra at 97.90keV ( ²³⁸ U chain) resulting from the reorganization of the electronic procession of the radon atom following the α decay of ²²⁶ Ra towards ²²² Rn .

The second energy band corresponds to the contribution of the end of the chain of uranium 238.

The first energy band and second energy band are chosen by those skilled in the art so as to contain at least the emissions mentioned above. In particular, the intervals of the first and second energy bands are chosen according to the resolution of the scintillator which influences the width at mid-height of the peaks.

For example, with a sodium iodide (NaI) crystal scintillator, the first energy band is for example between 87keV and 110keV. The second energy band is for example between 560 keV and 660 keV. Alternatively, the second energy band is much wider, for example from 250 keV to 950 keV, to increase the counting statistics without however including the 185 keV and 1001 keV contributions from uranium.

The memory 122 further comprises a module 126 for obtaining at least one radioactive imbalance . More particularly, the obtaining module 126 makes it possible to obtain a first radioactive imbalance uranium and a second radioactive imbalance uranium associated with the first and second calibration energy spectra.

It is recalled that secular equilibrium is a situation where the activity of a radioisotope remains constant because its rate of production (due to the decay of a parent isotope) is equal to its rate of decay. Otherwise, it is called a radioactive imbalance. In the present application, the radioactive imbalance of uranium refers to the average imbalance observed on the one hand between radon ²²² Rn and uranium ²³⁸ U due to the volatility of radon, and on the other hand between radium ²²⁶ Ra (father of radon ²²² Rn) and uranium ²³⁸ U due to differential leaching between uranium and radium.

For example, the module for obtaining 126 includes a sub-module 128 for calculating a radioactive imbalance of uranium using the net area of the energy line at 1001 keV and the net area of the energy line at 609 keV of the energy spectrum acquired by the acquisition module 124. More particularly, the calculation sub-module 128 allows the calculation of the first radioactive imbalance uranium and/or the second radioactive imbalance uranium using the net area of the 1001 keV energy line and the net area of the 609 keV energy line of each of the first calibration energy spectrum and the second calibration energy spectrum.

The net area at 609keV comes only from the gamma emission of ²¹⁴ Bi. Bismuth-214 is located downstream of radium-226 and radon-222 in the uranium-238 decay chain.

The net area at 1001keV comes solely from the emission of ^234m Pa. Protactinium 234 is located upstream of radium 226 and radon 222 in the decay chain of uranium 238.

The net areas are obtained after subtraction of the continuous Compton background, for example carried out using software for processing energy spectra.

The memory 122 further comprises a first calculation module 130 of at least one ratio between the area of the first energy band and the area of the second energy band of the energy spectrum acquired by the acquisition module 124.

More particularly, the first calculation module 130 allows at least the calculation of a first ratio and a second ratio between the area of the first energy band , and the area of the second energy band , respectively of the first calibration energy spectrum and of the second calibration energy spectrum.

The areas of the first energy band and the second energy band correspond to the raw areas without correction of the continuous Compton background.

The memory 122 further comprises a second calculation module 132 for calculating a first coefficient and a second coefficient from the at least first and second radioactive imbalances uranium obtained by the obtaining module 126.

Finally, memory 122 includes a third calculation module 134 to calculate the uranium content in the region of interest 10 of the wellbore 12 using the area of the first energy band , the area of the second energy band and the net area of the 609keV energy line of ²¹⁴ Bi of the energy spectrum acquired in the region of interest 10 of the wellbore 12, and the first and second coefficients calculated by the second calculation module.

The modules 124, 126, 130, 132, 134 are programmed to implement the method according to the invention, described below.

FIG. 3 presents the steps of a method 300 for evaluating the uranium content of at least one region of interest 10 of a subsoil 11 by a wellbore 12 according to the invention.

The method 300 firstly comprises an acquisition step 310 using the installation 104 and the acquisition module 124 of at least a first and a second energy spectra of calibration gamma radiation respectively in the first and second calibration regions 18, 20 of the borehole 10.

Each of the first and second energy spectra of calibration gamma radiation comprises at least the first energy band, the second energy band, and an energy band centered on an energy line at 1001 keV, as represented for example in FIG. .

The first and second energy bands are chosen as explained above.

Preferably, the acquisition time of the first and second energy spectra is between a few tens of minutes and several hours, for example, depending on the uranium content and the time required to detect the line at 1001 keV.

Advantageously, the first and second calibration regions 18, 20 are identified during a preliminary step of identifying the first and second calibration regions 18, 20.

For example, method 300 includes a preliminary step of identifying by measuring the total count of gamma radiation along the wellbore. To do this, a radiometric probe measuring by total counting of gamma radiation is moved into the borehole 12.

More specifically, this preliminary step includes the identification of a non-mineralized reference region with a uranium content below the reference threshold. The reference region has a reference gamma count.

The first calibration region 18 and the second calibration region 20 are then identified as regions having a gamma count rate significantly higher than the reference gamma count, that is to say higher than a multiple of the reference gamma count.

The first calibration region 18 and the second calibration region 20 are preferably chosen so that they have a different ratio between the area of the first energy band and the area of the second energy band. .

The method 300 then comprises a step 320 of respectively obtaining at least a first radioactive imbalance uranium from the first calibration region 18 and a second radioactive imbalance uranium from the second calibration region 20.

The step of obtaining 320 comprises for example a step of calculating 325 each of the radioactive imbalances uranium using respectively the net area of the 1001 keV energy line and the net area of the 609 keV energy line of each of the first and second calibration energy spectra.

Specifically, the imbalance in each of the calibration regions 18, 20 is obtained by equation (1):

With the radioactive imbalance of uranium, the net area of the energy line at 1001keV, the net area of the energy line at 609 keV, and a calibration coefficient.

The calibration coefficient is obtained by numerical simulation or experimentally using standard blocks or reference wells.

The first imbalance and the second imbalance are for example obtained by calculation from respectively the net areas at 609 keV and 1001 keV of the first calibration energy spectrum acquired in the first calibration region 18 and of the second calibration energy spectrum acquired in the second calibration region 20 .

The method 300 then comprises the calculation 330 of at least a first ratio and a second ratio between the area the first energy band , and the area of the second energy band , respectively of the first calibration energy spectrum and of the second calibration energy spectrum.

Ratios And are related to radioactive imbalances uranium by equation (2):

With the radioactive imbalance of uranium, And respectively the areas of the first energy band and of the second energy band.

Thus, the method 300 comprises from the areas And of each of the first and second energy spectra and of the radioactive imbalances, the calculation 340 of the coefficients And .

In the example, this leads to solving a system of two equations with two unknowns ( And ).

The method 300 then comprises the acquisition 350 of at least one gamma radiation energy spectrum of the region of interest 10. The region of interest 10 corresponds to a region of the wellbore 12 in which the uranium content is evaluated. .

Preferably, the method 300 includes acquiring a plurality of gamma radiation energy spectra along the wellbore 12. For example, the method includes acquiring an energy spectrum between a plurality of regions of interest 10 spaced by a predetermined pitch. The predetermined pitch is for example between 50cm and 2m.

The method 300 then comprises a step 360 of calculating the uranium content in the region(s) of interest 10 using the area of the first energy band the area of the second energy band and the net area of the 609keV energy line of ²¹⁴ Bi of each energy spectrum acquired in the region(s) of interest 10 and the first and second coefficients , calculated above.

More specifically, the area of the first energy band and the second energy band of the spectrum acquired in the region of interest 10 make it possible to calculate the radioactive imbalance D _i of the uranium in the region of interest 10, using equation (2).

The uranium content in region of interest 10 is then calculated using equation (3):

Or is a calibration coefficient obtained by numerical simulation or experimentally, is the uranium radioactive imbalance of the region of interest and is the “radon” activity of region of interest 10.

The “radon” activity of region of interest 10 is obtained using equation (4):

Or is the net area of the 609keV line of the energy spectrum in the region of interest 10,

is the efficiency of the scintillator detector at 609keV,

is the emission intensity of the at 609keV, i.e. the number of photons emitted by decay,

is the counting time (in seconds) corrected for the dead time.

The efficiency of the scintillator detector at 609 keV is for example calculated by numerical simulation.

According to a particular embodiment, the method 300 comprises the acquisition of a third energy spectrum of gamma radiation in a third calibration region 22 of the borehole 12.

For example, the third calibration region 22 is a mineralized region distinct from the first calibration region 18 and from the second calibration region 20 .

The third calibration region 22 has a third imbalance , preferably distinct from the first imbalance and the second imbalance .

The third calibration region 22 is preferably selected similarly to the first calibration region 18 and to the second calibration region 20 .

For example, the first calibration region 18 is located above the second calibration region 20 along the direction of the wellbore 12. The third calibration region 22 is located below the second calibration region 20 .

The method 300 then comprises the acquisition of a third energy spectrum of gamma radiation in the third region 22 of calibration and the obtaining of a third imbalance radioactive uranium for the third region 22 of calibration.

More particularly, similarly to what is done for the first calibration region 18, the method comprises a sub-step of calculating a third radioactive imbalance from the sharp areas at 609 keV and 1001 keV of the third calibration energy spectrum acquired in the third calibration region 22 .

The method then also comprises the calculation of at least a third ratio between the area of the first energy band and the area of the second energy band of the third calibration energy spectrum.

From radioactive imbalances of uranium , , and areas And of each of the first, second and third energy spectra, the coefficients And are calculated.

Preferably, the first coefficient and the second coefficient are calculated by linear regression of the at least first, second and third ratios calculated and the at least first, second and third radioactive imbalances obtained.

As a further variant, the method comprises the acquisition of calibration energy spectra in any number of calibration regions.

In addition or as a variant, in each of the calibration regions 18, 20, 22, a plurality of calibration energy spectra are acquired and steps 310, 320, possibly 325 and 330 are performed on each of said calibration spectra.

This makes it possible to evaluate an uncertainty associated with the uranium content(s) in the region(s) of interest obtained by the process by plotting a calibration curve representing the multiple pairs (D _i ; C _Ui /C _Rni ) from from which it is possible to estimate the uncertainty on the coefficients And linear regression.

As a further variant, the step of obtaining the radioactive imbalance of at least one of the calibration regions 18, 20, 22 comprises a sub-step of assigning a value to said imbalance.

Indeed, in this embodiment, at least one reference region 18, 20, 22 is selected on the basis of a priori information coming for example from measurements carried out on core samples taken from the wellbore 10.

Preferably, at least one calibration region 18, 20, 22 is a region with a very high imbalance. The step of obtaining the radioactive imbalance of said region then comprises assigning a zero value to said radioactive imbalance.

Claims (10)

Method for evaluating the uranium content of at least one region of interest (10) of a subsoil (11) traversed by a wellbore (12) by gamma spectrometry, the subsoil comprising at least a first calibration region (18) and a second calibration region (20) traversed by the wellbore (12), the first calibration region (18) having a first radioactive imbalance (D ₁ ), the second calibration region (20) exhibiting a second radioactive imbalance (D ₂ ) distinct from the first radioactive imbalance (D ₁ ), the method comprising the following steps:
a) acquiring (310) at least a first and a second calibration gamma radiation energy spectra respectively in the first and second calibration regions (18, 20), each of the calibration energy spectra comprising at least:
- a first energy band relating to a uranium contribution, the first band comprising at least one energy line at 92keV of ²³⁴ Th and an energy line of the fluorescence X-radiation of uranium at 98keV,
- a second energy band relating to a radon contribution, the second band comprising at least one energy line at 609keV of ²¹⁴ Bi, and
- an energy band centered on an energy line at 1001keV of ^234m Pa,
b) obtaining (320) respectively at least the first radioactive uranium imbalance (D ₁ ) associated with the first calibration energy spectrum and the second radioactive uranium imbalance (D ₂ ) associated with the second calibration energy spectrum,
c) calculating (330) at least a first ratio (C _U1 /C _Rn1 ) and a second ratio (C _U2 /C _Rn2 ) between the area of the first energy band (C _U1 , C _U2 ) and the second energy band area (C _Rn1 , C _Rn2 ) respectively of the first calibration energy spectrum and of the second calibration energy spectrum,
d) calculating (340) a first coefficient (α) and a second coefficient (β) from the at least first and second radioactive imbalances (D ₁ , D ₂ ) of the uranium obtained and from the at least first and second ratios calculated (C _U1 /C _Rn1 , C _U2 /C _Rn2 ),
e) acquiring (350) at least one energy spectrum of gamma radiation associated with the region of interest (10), the energy spectrum comprising at least the first energy band and the second energy band,
f) calculating (360) the uranium content in the region of interest (10) using the area of the first energy band (C _Ui ), the area of the second energy band (C _Rni ) and the net area of the 609keV energy line of ²¹⁴ Bi of the energy spectrum acquired in the region of interest (10), and the first and second coefficients (α, β) calculated. A method according to claim 1, wherein the first energy band is substantially between 87keV and 110keV and the second energy band is substantially between 560keV and 660keV. Method according to Claim 1, in which the first radioactive imbalance (D ₁ ) of uranium and/or the second radioactive imbalance (D ₂ ) of uranium are calculated using respectively the net area of the energy line at 1001 keV and the net area of the energy line at 609keV of each of the first and second calibration energy spectra. Method according to any one of Claims 1 to 3, comprising a preliminary step of identifying the first calibration region (18) and/or the second calibration region (20) by measuring the total count of gamma radiation or by acquiring a gamma radiation spectrum and calculating a ratio between the area of the first energy band and the area of the second energy band along the wellbore (12). Method according to any one of Claims 1 to 4, in which the first coefficient (α) and the second coefficient (β) are calculated by linear regression of the at least first and second calculated ratios (C _U1 /C _Rn1 , C _U2 / C _Rn2 ) as a function of the at least first and second radioactive imbalances (D ₁ , D ₂ ) obtained. Method according to claim 5, in which the first coefficient (α) and the second coefficient (β) are calculated using the equation (C _Uk /C _Rnk ) = α × D _k + β, C _Uk being the area of the first energy band of the energy spectrum of the calibration region (18, 20, 22) considered, C _Rnk being the area of the second energy band of the energy spectrum of the calibration region (18, 20, 22 ) considered, D _k being the radioactive imbalance (D ₁ , D ₂ , D ₃ ) of the uranium of the calibration region (18, 20, 22) of index k. A method according to any of claims 1 to 6, wherein the wellbore (12) passes through at least a third calibration region (22), the third calibration region (22) having a third imbalance different from the first and second imbalances, the method comprising acquiring at least a third calibration gamma radiation spectrum associated with the third calibration region (22), obtaining a third radioactive imbalance (D ₃ ), calculating a third ratio (C _U3 /C _Rn3 ), the first and second coefficients (α, β) being calculated by further using the third radioactive imbalance (D ₃ ) and the third ratio (C _U3 /C _Rn3 ). A method according to claim 7, wherein the second calibration region (20) is located between the first calibration region (18) and the third calibration region (22) along the wellbore (12). Device (100) for evaluating the uranium content of a region of interest (10) of a subsoil (11) traversed by a borehole (12) by gamma spectrometry, the subsoil comprising at at least a first calibration region (18) and a second calibration region (20) traversed by the wellbore (12), the first calibration region (18) having a first imbalance (D ₁ ), the second calibration region (20) having a second imbalance (D ₂ ) different from the first imbalance (D ₁ ), the device (100) comprising:
- a spectrometric probe (102) comprising a scintillator detector,
- an acquisition module (124) connected to the spectrometric probe (102), the
(124) being configured to acquire at least a first calibration gamma radiation energy spectrum associated with the first calibration region (18), a second calibration gamma radiation energy spectrum associated with the second calibration region (20 ) and a gamma radiation energy spectrum associated with the region of interest (10), each of the calibration energy spectra comprising at least:
- a first energy band relating to a uranium contribution, the first band comprising at least one energy line at 92 keV of ²³⁴ Th and an energy line of the fluorescence X-radiation of uranium at 98 keV,
- a second energy band relating to a radon contribution, the second band comprising at least one energy line at 609 keV of ²¹⁴ Bi, and
- an energy band centered on an energy line at 1001 keV,
- a module for obtaining (126) for obtaining the at least first and second radioactive imbalances (D1, D2) of the uranium associated with the first and second calibration energy spectra,
- a first calculation module (130) for calculating at least a first ratio (C _U1 /C _Rn1 ) and a second ratio (C _U2 /C _Rn2 ) between the area of the first energy band (C _U1 , C _U2 ) and the area of the second energy band (C _Rn1 , C _Rn2 ) respectively of the first calibration energy spectrum and of the second calibration energy spectrum,
- a second calculation module (132) for calculating a first coefficient (α) and a second coefficient (β) from the at least first and second radioactive imbalances (D ₁ , D ₂ ) of the uranium obtained by the module d obtaining (126) and first and second ratios (C _U1 /C _Rn1 , C _U2 /C _Rn2 ) calculated by the first calculation module (130),
- a third calculation module (134) of the uranium content in the region of interest (10) using the area of the first energy band, the area of the second energy band and the area net of the energy line of the spectrum acquired in the region of interest (10) by the acquisition module (124), and the first and second coefficients (α, β) calculated by the second calculation module (132). Apparatus (100) according to claim 9, wherein the obtaining module (126) comprises a calculation sub-module (128) for calculating a radioactive imbalance of uranium (D ₁ , D ₂ ) using the area net of the energy line at 1001 keV and the net area of the energy line at 609 keV of the energy spectrum acquired by the acquisition module (124).