DESCRIPTION

The present invention relates to a radical solution for nuclear magnetic resonance magnetometers with dynamic polarization.

More precisely it relates to a new radical solution whose nuclear magnetic resonance (NMR) signal in a weak field can be used over a wide temperature range, is stable at high temperatures and has little response to magnetic field gradients.

It applies in particular to NMR magnetometry with dynamic polarization under extreme temperature and magnetic environment conditions characteristic of drilling wells.

The performance of NMR magnetometers is strongly related to the radical solution which is a component part of a magnetometer, and therefore to the behavior of this solution over the range of temperatures and magnetic field gradients applied to the magnetometer.

The thesis published by R. Besson (Universite Joseph Fourier de Grenoble, 1973, pages 47 to 49 and 65 to 67) mentions several radical solutions which can be used in magnetometry. Since the use of the first radical (Fremy salt) the technique has developed towards other more stable nitroxide radicals of N-O.sup..cndot. type which allow amplification of the NMR signal of the solvent through stronger dynamic polarization. Nitroxide radicals of this type are described in particular in documents FR-A-2 653 121 and EP-A-O 183 825.

The radical solution currently used in magnetometers for drilling wells is the so-called Thetis solution which consists of the nitroxide radical Tanane (2,2,6,6-tetramethylpiperidine-1-oxyl) at a concentration of 10.sup.-3 mol/l in a solvent consisting of a mixture of water and 5% diglyme (dimethyl ether of glycol diethylene).

This radical solution allows the magnetometer to function over a range of temperatures between 10.degree. C. and 125.degree. C. under a very low magnetic field gradient. With this solution the NMR signal deteriorates considerably in the presence of higher gradients and the operating range of the magnetometer is consequently limited to a gradient of a few hundred nT. Also, the acceptable maximum value decreases when temperature rises; this limit value is 700 nT at 40.degree. C. and decreases to 500 and subsequently to 200 nT at the respective temperatures of 70 and 100.degree. C.

The operating range of the magnetometer is therefore not sufficient for its successful use in wells of considerable depth characterized by a surrounding temperature which may reach over 125.degree. C. and by the presence of magnetic silt which generates field gradients which may exceed 1000 nT/m.

Moreover, radical solutions subjected to high temperatures are not stable due to a vanishing process of the radical -NO.sup..cndot. connected with the formation of hydroxylamine through reaction with the protons of the hydrogenated solvent at high temperature, according to the following reaction process:

-NO.sup..cndot. +H.fwdarw.-NOH

which leads to transformation of the nitroxide radical into hydroxylamine.

It would therefore be of interest to have radical solutions available which are suitable for the magnetometer to function at temperatures exceeding 125.degree. C. with field gradients which may exceed 1000 nT/m.

It is known that the performance of a radical solution is related to the dynamic polarizing factor DPF which may be defined by the following equation: ##EQU1## in which: A is the hyperfine interaction constant of the radical

g is the number of energy levels of the radical,

.omega..sub.1 is the Larmor frequency of the proton

f is the efficiency factor of the coupling between the protons of the solvent and the free electron of the radical,

.phi. is a decreasing function of line width RPE in a high field of the solution, .DELTA.H.sub.RPE, and of the number of lines of the spectrum of electronic paramagnetic resonance.

With the Thetis solution (Tanane solution at a concentration of 10.sup.-3 mol/l in the water +5% diglyme mixture) .DELTA.H.sub.RPE is 32 .mu.T at 20.degree. C.

This behavior of the radical solution is also dependent upon:

NMR relaxation time, T.sub.2, and

the stability of the radical solvent couple for applications at high temperatures.

With a relaxation time of .T.sub.2, the shorter this period the more negligible becomes the effect produced by a magnetic field gradient on the solution. Therefore, with the Thetis solution, T.sub.2 is 1.1 s at 20.degree. C. and T.sub.2 increases approximately exponentially with temperature to reach 5.37 s at 120.degree. C.

Therefore, in order to obtain a radical solution with a good NMR signal, first a stable radical-solvent couple must be found which has efficient coupling, that is to say with an efficiency factor (f) nearest to 1, with fine line width RPE and with a relaxation time T.sub.2 of less than 1.1 s at 20.degree. C. so that the radical solution is more resistant to magnetic field gradients than the Thetis solution currently used.

The present invention relates precisely to a radical solution having these properties so that it may be used for nuclear magnetic resonance magnetometry with dynamic polarization over a temperature range of 0.degree. C. to 150.degree. C. with magnetic field gradients which may exceed 1000 nT/m.

According to the invention, the radical solution contains a nitroxide radical and it is characterized in that it consists of a nitroxide radical whose formula is: ##STR2## in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 which may be identical or different, represent an alkyl group or possibly a deuterated alkoxy group, or in which the couples R.sup.1 -R.sup.2 and/or R.sup.3 -R.sup.4 together form a cycloalkyl or a polycycloalkyl group which may be deuterated, and N represents .sup.14 N or .sup.15 N;

dissolved in a solvent consisted of a dimethyl ether with the formula:

CH.sub.3 O(CH.sub.2 CH.sub.2 O)n CH.sub.3 (II)

in which n equals 3 or 4.

In formula I given above for the nitroxide radical, groups R.sup.1, R.sup.2, R.sup.3 and R.sup.4 may be alkyl or alkoxy groups or may together form cycloalkyl groups. The alkyl or alkoxy groups which may be used may be linear or ramified groups generally having 1 to 5 carbon atoms of which all or some of the hydrogen atoms may be replaced by atoms of deuterium D.

When R.sup.1 -R.sup.2 and/or R.sup.3 -R.sup.4 together form a cycloalkyl group, the latter preferably has 4 to 11 carbon atoms. R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are preferably lower alkyl groups of 1 to 4 carbon atoms, in particular methyl groups.

In this nitroxide radical, the nitrogen atom may be .sup.14 N or .sup.15 N. Preferably .sup.15 N is used. Also, alkyl or deuterated alkoxy groups are preferably used for R.sup.1, R.sup.2, R.sup.3 and R.sup.4.

As an example of appropriate nitroxide radical, mention can be made of those which meet the formulae: ##STR3##

According to the invention, the use of a nitroxide radical meeting the formulae (III and IV) given above can achieve improved stability of the solution in comparison with that achieved with the Tanane type radical.

By adding a 6 carbon atom cycle consisted of a benzene cycle to the nitroxide radical of 2,2,5,5,-tetramethyl-pyrrolidine-1-oxyl type, the volume of the molecule can be increased and it can be made rigid to form a flat molecule and to increase its stability. Therefore, in solution in water at a concentration of 10.sup.-3 mol/l, the nitroxide radical with the formula: ##STR4## has a half-life period of 250 hours at 150.degree. C. in water, whereas the nitroxide radical of formula (III) used in the invention shows at this same temperature a half-life period which is substantially longer than 250 hours since the signal loss in the same solvent is only 10% after 300 hours.

It is specified that the half-life period of a nitroxide radical corresponds to the time elapsed between the time when the radical is brought to the required temperature and the time when the RPR signal of this radical has lost 50% of its initial value.

The nitroxide radical meeting the formula (III) given above is described in particular by R. Bolton et al. In J/Chem. Soc. Perkin Trans. 2, 1993, pages 2049-2052.

In the radical solution of the invention, the solvent used, which consists of a dimethyl ether of formula (II) , has a high degree of purity which may be obtained by bi-distillation. It is important to remove certain impurities which may be found in the solvent which might lead to degradation of the radical. In particular, the bi-distillation stage can remove 2,6 di-tert-butyl-4-methylphenol which is used as stabiliser in marketed formula (II) products in order to prevent the formation of peroxides. With the removal of this stabilizer (antioxidant) the degradation of the radical through hydroxylamine formation can be prevented.

For the radical solution of the invention, dimethyl ether of formula (II) is chosen in order to obtain the efficiency factor (f) that is nearest to 1 without however causing too great an increase in line width RPE. Preferably a dimethyl ether of formula (II) is used in which n is 3 or 4, the best results being obtained when n equals 3.

The nitroxide radical concentration of the solution is chosen in such manner as to obtain a good NMR signal while nevertheless obtaining a good dynamic polarizing factor DPF.

Generally, good results are obtained with concentrations in the range of 10.sup.-3 to 10.sup.-2 mol/l.

The nitroxide radicals of formula (I) used in the invention may be prepared using conventional methods. In particular the synthesis method described in Aust. J. Chem. 1983, 36, p. 397-401 may be used.

The invention also relates to a probe for a nuclear magnetic resonance magnetometer, comprising two units each comprising a coil surrounding a container filled with a sample consisting of a solution containing a nitroxide radical, and two resonating cavities to excite the two samples at two separate frequencies, characterized in that each sample is a solution of a nitroxide radical according to the invention.

Other characteristics and advantages of the invention will be better understood on reading the following description, which is evidently given for illustrative purposes and is non-restrictive, with reference to the appended drawings.

FIG. 1 is a diagram representing the progression of efficiency factor (f) in relation to the temperature (in .degree. C.) of a radical solution of the prior art (reference T)

FIG. 2 is a diagram illustrating the progression of line width (in mTesla) in relation to temperature (in .degree. C.) for radical solutions (1) and (2) according to the invention and for solution T according to the prior art.

FIG. 3 is a diagram representing the progression of NMR relaxation time, T.sub.2 (in s), in relation to temperature (in .degree. C.) for radical solutions 1 and 2 of the invention, radical solution T of the prior art and solution C given for comparative purposes.

FIG. 4 is a diagram representing the amplification factor (ratio r: T.sub.2 solution T/T.sub.2 solution 1 or 2) in relation to temperature (in .degree. C.).

FIG. 5 is a diagram representing the NMR signal (in Volts) in relation to intensity (in mA) for solution 1 of the invention at temperatures of 20, 40, 60 and 80.degree. C. and for solution T of the prior art at the same temperatures.

FIG. 6 is a diagram representing the NMR signal (in Volts) exiting the probe in relation to temperature (in .degree. C.) for intensities of 0 mA, 3.5 mA, 7 mA, 10.5 mA and 14 mA for radical solution 1 of the invention and radical solution T of the prior art.

FIG. 7 is a diagram representing the half-life period of radical solution 1 of the invention in relation to temperature (in .degree. C.).

FIGS. 8 to 10 are diagrams representing the nuclear signal (in mV) of nuclear magnetic resonance captors with dynamic polarization using, either solution 1 of the invention, or solution T of the prior art, in relation to the outer magnetic field gradient applied to the axis of the captor (en nT/m), at a temperature of 20.degree. C. (FIG. 8), of 50.degree. C. (FIG. 9) and 100.degree. C. (FIG. 10).

FIGS. 11 to 14 are diagrams representing the progression of the signal to noise ratio (in dB/.sqroot.Hz) exiting the nuclear oscillators using solution 1 of the invention at 40.degree. C. (FIG. 11) and 70.degree. C. (FIG. 12) or solution T at 40.degree. C. (FIG. 13) and 70.degree. C. (FIG. 14) in relation to the outer magnetic field gradient applied (en nT/m).

FIG. 15 is a diagram representing the variations in the NMR nuclear signal (en V) exiting the probe in relation to temperature (in .degree. C.) for solution 1 of the invention and for solution C given for comparison.

FIG. 16 is a drawing of a nuclear magnetic resonance probe of the invention .

In the following description two radical solutions according to the invention are used comprising the nitroxide radical (Tmio 15D) of formula (I) in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are the deuterated methyl group and N represents .sup.15 N in triglyme, that is to say dimethyl ether of formula (II) with n equalling 3. The triglyme used is the marketed product which we submit to bi-distillation to remove impurities, in particular the stabilizer that is present in the marketed product.

Radical solution 1 comprises 2.10.sup.-3 mol/l of Tmio 15D and solution 2 comprises 10.sup.-3 mol/l of Tmio 15D.

For comparative purposes, solution T of the prior art is also used, consisting of the nitroxide radical Tanane at a concentration of 10.sup.-3 mol/l in the water +5% diglyme mixture (Thetis solution) and solution C consisting of Tmio 15D at a concentration of 10.sup.-3 mol/l in the water/5% diglyme mixture.

A further solution 3 according to the invention comprises Tmio 15D at a concentration of 10.sup.-3 mol/l in tetraglyme, that is to say dimethyl ether of formula (II) where n is 4.

Table I below groups together the properties at 20.degree. C. and 120.degree. C. of solutions 1, 2 and 3 of the invention and of the solution of the prior art (Thetis solution).

                  TABLE I
    ______________________________________
                                        T.sub.2
                         .DELTA.H.sub.RPE
                                   F    (s)
    ______________________________________
    Solution 1
    2.10.sup.-3 M
               20.degree. C.
                         34 .mu.T  0.77 0.48
               120.degree. C.
                         69 .mu.T  0.7  2.5
    Solution 2
    10.sup.-3 M
               20.degree. C.
                         27 .mu.T  0.45 0.34
               120.degree. C.
                         47.9 .mu.T
                                   0.53 3.27
    Solution 3
    10.sup.-3 M
               20.degree. C.
                         30 .mu.T  0.40 0.39
               120.degree. C.
                         40 .mu.T  0.57 2.71
    Solution T
    10.sup.-3 M
               20.degree. C.
                         32 .mu.T  0.66 1.1
               120.degree. C.
                         71.9 .mu.T
                                   0.54 5.37
    ______________________________________