FR2496263A1 - Sampling tube for gas present in gas mixt. of liq. - using polymer tube with selective permeability for gases, esp. to detect helium in drilled holes when prospecting for uranium ores - Google Patents

Abstract

A tube closed at both ends is filled with a reception fluid (f) for the gas to be determined. The tube is selectively permeable w.r.t. gases, and is immersed in a gas mist. or liq. for a specific time, so the gas to be determined diffuses through the tube into fluid (f). The tube is then removed and its contents are analysed. The results obtd. are used to calculate the quality of a gas in the gas mixt. or liq. in which the tube was immersed. Fluid (f) is pref. a gas when a gas mixt. is to be analysed, but is pref. a liq. when a liq. is to be analysed. The tube is pref. made of a polymer, esp. a polydiene, polyalkene, polymethacrylate, polyamide, polyester, or polysiloxane. Used esp. to analyse gases present in a drilling hole, e.g. to determine He arising from the radioactive decay of uranium and used to detect the presence of uranium. The sampler has a very low cost.

Description

Method and device for sampling gases
Present in a mass of fluid
The present invention provides for the sampling of gases present in a mass of fluid, situated at a distance from an installation of analysis. This gas may be mixed with a carrier gas phase or in solution in a liquid phase.

 The invention finds a particularly important, but not exclusive, application consisting of the measurement of gas in the atmosphere of unsaturated soils in water or gas in solution in open water and in groundwater.

 It is known that this assay is particularly used in the prospection of uranium ore, the presence of offspring gases such as helium and radon constituting an important index.

 Remote gas sampling devices are already known for use in prospecting surveys. These devices comprise a sealed-wall sampling chamber which is brought to the location to be explored in order to collect in situ a given quantity of the fluid carrying the gases to be analyzed.

The enclosure has an opening with a remotely controlled shutter. The sequence of use is as follows: the sample chamber, initially empty, is placed in the medium where the fluid containing the gases to be analyzed is located. A suction device is remotely operated to fill the sample chamber with fluid that penetrates through the opening. The shutter is operated remotely to seal the sample chamber that is recovered and the gases present in the fluid contained in the chamber are dosed in a conventional manner.

The sealed closure of the chamber after sampling avoids uncontrollable outgassing under the effect of pressure and / or temperature variations that may occur between the sampling site and the place of recovery and analysis. In the absence of degassing, the measurement of the gases in the sample taken is directly representative of their initial contents in situ.

 In return for this quality, the known devices have serious defects.

 - Their operation is unreliable when the fluid to be sampled is mixed with a solid phase (plant fibers, dust, sand, gravel, etc.) or with fluids having a high viscosity (sludge, clay, etc.) capable of hindering the proper operation of the remote control, filling and closing devices. This situation often occurs in natural environments and especially in polls.

 - They require individual active control for each speaker used, which tends to limit the amount of recovery recoverable with the same equipment during a sampling sequence (set up, filling, recovery). For example, it is difficult to place several enclosures that must be independently controlled in staggered positions along a borehole or a vertical, or a horizontal in a body of water, to determine the gas contents at the same time and deduce a distribution profile.

 - They risk polluting the environment to be studied since the same equipment must be set up successively in various environments (because of its high cost that requires reuse) unless we take precautions that constitute a constraint.

 The aim of the invention is to provide a method and a sampling device which better than those previously known meet the requirements of the practice, in particular from the point of view of cost, the possibilities of use and the results obtained.

 For this purpose, the invention proposes in particular a method for sampling at least one gas present in a mass of fluid to be analyzed, characterized in that said fluid is exposed to a closed chamber containing a receiving fluid, delimited by an envelope a non-porous material having gas-selective permeance, for a given period of time, sufficient for the concentration of the gas to be metered within the shell to reach an appreciable fraction of the outside content, in that the chamber is withdrawn for the purpose of dosing the gas present in the receiving fluid in a time which is a small fraction of the previous time, and in that the content of the gas in the fluid to be analyzed is deducted. dosing of the gases present in the receiving fluid.

 The term "non-porous material having selective permeance" means a material having no appreciable open porosity, which the gases can pass through molecular diffusion, which excludes in particular crystallized materials and those consisting of agglomerated fibers, it is that is, having a structure similar to that of the usual filters. The material must also have homogeneity on a global scale. It will generally be a plastic material, the term plastic to be interpreted in a broad sense and designating high molecular weight materials, carbon or silicone, fluorinated or not, even original.

natural, although they are usually of synthetic origin.

 It can be seen that the process according to the invention is carried out with purely passive, inexpensive and therefore possibly disposable elements after a single use, especially when it is desired to avoid any risk of pollution. The simplicity of implementation in situ is reflected. by saving time and saving. The absence of any opening or closing command makes it possible to have a large number of speakers in a string, for example along a sounding, so as to perform a simultaneous sampling. The method further allows for gas sampling in environments where the operation of previously known devices is difficult or even impossible.

 It is desirable to use a gas as the receiving fluid when the mass of fluid to be analyzed is a gas. Correlatively, it is desirable to use a liquid having the same general properties as the mass of fluid to be analyzed, for sampling in the liquid phase. It is thus easier to determine, from the composition of the receiving fluid, when the chamber is recovered, the gas content to be assayed in the mass.

 The invention also proposes a device for implementing the above method, comprising an enclosure whose envelope is at least partly constituted by the material having a selective permeance vis-à-vis the gas. The thickness of the material of the envelope will be chosen according to the nature of this material and the working conditions, in particular the time required to recover the enclosure. For the analyzed sample to be representative, it is indeed necessary that the exchanges through the envelope during this recovery time are negligible. The choice of the material and the thickness of the envelope also has an influence on the minimum duration during which it will be necessary to expose the envelope to the mass of fluid to be analyzed so that the gas exchanges by diffusion through the envelope are translated. by a significant change in the gas concentration in the receiving fluid.

The invention will be better understood on reading the following description of particular embodiments, given by way of non-limiting examples. The description refers to the accompanying drawings in which
FIG. 1 is a block diagram showing a chamber that can be used in the implementation of the invention, placed in a mass of the fluid to be analyzed
FIG. 2 schematically shows the variation of the concentration of gas to be analyzed as a function of the radial distance, in an enclosure of the type shown in FIG. 1, at different times of the exposure time;
- Figures 3 and 4 are charts giving the
variation over time of the gas concentration,
respectively in linear scale and in logarithmic scale
FIG. 5 is a block diagram showing
the implementation of the invention in a borehole
FIG. 6 is a representative curve of the
concentration variation as a function of time.

 Before describing a complete device for implementing the invention, the physical mechanisms it implements will be explained for the sake of clarity.

According to the invention, a closed chamber 10 (FIG. 1) of volume v, containing initially a receiving fluid f, is placed in a volume V of fluid F, containing the gas or gases to be assayed, the initial content of which unknown to be determined is Ko. Part of the envelope limiting the enclosure 10 is constituted by a non-porous material having a selective permeance with respect to the gases. In the embodiment illustrated in FIG. 1, this envelope comprises a cylindrical wall 12 closed by two bottoms 14. The cylindrical wall 12 is made of a material which selectively passes the gases and has a constant thickness. The funds are on the contrary impermeable. One of them is equipped with a manual control valve 15 for connecting the chamber to a sampling or analysis installation. The chamber 10 is closed after having been filled with the fluid f (which will be assumed monophasic for simplicity) whose content
initial C0 in gas is known. Throughout the sampling period t, the chamber remains closed but a gas exchange occurs by diffusion through the wall 12, between the fluids F and f. This exchange tends to establish a balance of the concentrations C and K of each of the gases present initially, respectively in the receiving fluid f and in the fluid to be sampled F.

Inside the sample chamber, the concentration C of the ca z varies progressively and tends asymptotically towards a limit value of equilibrium Coe
After the exposure time t, the chamber 10 is recovered and the gases are metered into the fluid f.

 During the exposure time t and as long as the equilibrium is not established, the gaseous concentrations in volumes v and V, as well as in the thickness of the wall 12, have transient variations in space .

FIG. 2 shows the variation of the concentration C in the radial direction, at two successive instants t1 (solid line curve) and t2 (dashed curve). Initial and equilibrium concentrations are denoted by the indices 0 and oe. r, R and e respectively denote the free diffusion distance, in the sampling chamber (radius of the chamber 10 if it has an axis of revolution) and in the mass of fluid to be sampled, and the thickness of the 12.The duration of exposure that will be required to approach the equilibrium is often too long to be acceptable, and therefore a duration of exposure which is only a fraction of the previous one must be allowed and will correspond to example to the curve at time t2. It is necessary, from the concentration obtained in the chamber at this instant t2, to determine the concentration Ko
An assay performed after any exposure time t gives a mean value Ct in the sample chamber; therefore the amount of gas
Q introduced into the sampling chamber 10 or extracted from it since the beginning of the exposure is Q = v. (Ct - C0).

 The laws of gaseous diffusion theoretically make it possible to directly calculate the gaseous concentrations by the resolution of differential equations.

But this method of direct inversion requires the introduction into the calculations of multiple subsidiary data whose determination is often imprecise.

 A much simpler solution is to use reference charts, established experimentally for each type of enclosure that will be used.

These charts can take into account experimentally the influence of all the variable parameters. These charts are particularly convenient to use by normalizing them with respect to the initial and final values of the function C (t), which leads to a representation (Ct - C0) / C. In particular, it is possible to draw a network of standardized abacuses making it possible to easily calculate the equilibrium concentration C from a measurement C t at any time t:
for the same gas at different temperatures in the same receiving fluid,
for different gases at the same temperature in the same fluid,
for different fluids, at a given temperature.

 By way of example, FIG. 3 shows the shape of the curves corresponding to helium 4 with hydrogen sulphide and with carbon dioxide in the same fluid and at a given temperature.

 Even charts of the same kind can be plotted on a logarithmic scale, which leads to substantially rectilinear curves. By way of example, FIG. 4 shows the variation curves of the function log / (x - xt) / (x - xot7 x 100 as a function of time, for several materials.

 When C is available in the fluid f, it is possible to deduce the corresponding equilibrium content K in the fluid to be sampled.

Indeed, the classical laws of mixtures in gas phase and in solution then apply directly and the calculation shows that
C xa = = K x
In the formula above, a is equal to 1 if the carrier fluids are gases. a is equal to the solubility coefficient solubility of B unsen if the carrier fluids are liquids the indices f and F respectively correspond to the receiving fluid f and the fluid F to be sampled.

 As indicated above, it will always be desirable to use a gas as a receiving fluid for sampling in a mass of gaseous fluid and a receiving fluid of the same nature as the mass of fluid to be analyzed for liquid phase sampling. : then we have α f = α F and K = C #, expression independent of the temperature.

 It remains to deduce the initial concentration Kg of the equilibrium concentration K #.

 In fact, one will almost always be in one or the other of two simple cases.

 In the case of sampling in wells, rivers, lakes, the sea, marshy soils, etc., the volume v will be negligible in relation to the volume V and in this case we will have Ko K ,,, C, .

 In other cases, on the other hand, the volume V is contained inside a sealed wall and it is equal to n.v.

The calculation then shows that we have
K0 = C # (1 + 1 / n) - C0 / n (1)
This case is also frequent: it is the one encountered during sampling in environments limited externally by an impermeable wall, for example a metal pipe or a drilling in line with an impermeable formation.

 By way of example, reference will be made below to the conditions of application of the process to particular cases.

 FIG. 5 shows the implementation of the method in a menaob borehole 16 in impervious grounds and where it is desired to determine the concentration profile. If the drilling has been performed with sludge, it will be advantageous to use as the receiving fluid a slurry of the same composition. If the hole is flooded, the receiving fluid may be water. It will be the same when the survey must be carried out in a body of water.

 The enclosure 10, constituted by a section of plastic tube 12 provided with impervious bottoms 14, is placed in the borehole by conventional means of descent. Several identical enclosures may be distributed at regular intervals along the hole to allow the determination of a concentration profile at the same time. The nature and the thickness of the tube will be chosen according to the nature of the gases to be detected and also according to the depth.

If indeed the hole is shallow, the time required to recover the speakers will be low and we can admit a relatively fast diffusion. If, on the contrary, the hole is deep and makes a duration of the order of the hour necessary to raise the enclosures, the nature and the thickness of the wall will be chosen so that the diffusion is slow and the variations of concentration encountered. during the ascent do not result in an appreciable error. We will then be led to accept lengths of stay in place of several hours, or even more than ten hours.

The materials used to form the wall are of a very diverse nature. If we limit ourselves to polymers, mention may be made especially of poly (dienes), poly (alkenes), poly (methacrylates) and various poly (amides), polyesters and polysiloxanes. The properties of many bodies belonging to these categories are given in the literature. By way of example, the table below gives the permeation coefficient for various gases, through usable compounds. The permeation coefficient P is given in cm3. cm. cm-2,
-l.

sec. cm Hg
The value given must be multiplied by 7.5 x 10-4 if the pressures are given in Pascals.

<Tb>

<SEP> Polymer <SEP> Gas <SEP> P <SEP> x <SEP> 101 <SEP>
<tb> Polyethylene <SEP> He <SEP> 4.9
<tb> (density <SEP> 0.914) <SEP> C02 <SEP><SEP> 12.6
<tb><SEP> CH4 <SEP> 2.88
<tb> polymethacrylate <SEP> He <SEP> 6.82 <SEP>
<tb> ethyl <SEP> CO2 <SEP> 5.00 <SEP>
<tb><SEP> H2O <SEP> 3200
<tb> chloride <SEP> of <SEP> He <SEP> 2.05
<tb> polyvinyl <SEP> H2 <SEP> 1.70
<tb><SEP> CO2 <SEP> 0.157 <SEP>
<tb><SEP> CH4 <SEP> 0.0286
<tb><SEP> H2O <SEP> 275 <SEP>
<tb> chloride <SEP> of <SEP> He <SEP> 0.31
<tb> polyvinyl- <SEP> CO2 <SEP> 0.03 <SEP>
<tb> indene <SEP>("SARAN")<SEP>
<tb><SEP> H20 <SEP> 0.5
<Tb>
In practice, it will often be sufficient to use commercially available sections of polyvinyl chloride (PVC) pipe. Depending on their density, these products have various uses.

The lower the density, the higher the permeability, while the molecular weight of the polymer generally has little effect on permeability. The above table, corresponding to a temperature of 250 ° C., shows a selectivity of the variable usable materials according to these.

To show the influence of the ratio between the volume of the chamber and that occupied by the mass of fluid to be sampled, it will be assumed that the tubular enclosure 12 has a diameter of 20 mm and is placed in a hole of 100 mm. diameter. In that case,
V / v is in the order of 24, dot l / n - 4%. We see that it suffices to increase C by 4t to obtain Kg (according to formula (1)).

 More generally, whenever v is not negligible in relation to V, the formula (1) must be applied to determine Kg from CO and C.

The curve of variation of Ct-Co / Ko as a function of the duration of exposure has the appearance shown in full lines in FIG. 6. It can be seen that it tends asymptotically towards 1 - 1 / n after having followed up to instant at which the diffusion front in the mass of fluid to be sampled reaches the sealed wall of the hole 16) a common portion with the curve corresponding to a negligible value of v in front of V.

 it will therefore be desirable, when possible, to choose a duration of exposure t less than tk, which will lead to an exact determination of Ko by use of abacuses corresponds to an infinite extension of the mass of fluid to be sampled.

If this condition is not respected, the simple recourse to the abacuses leads to an error, all the more weak as t * is greater, that is to say that the diffusion rate is low in the mass of fluid to be sampled and that the free diffusion distance between the wall of the sampling chamber and that of the hole is high
One can obviously still be in the intermediate situation of a sample fluid mass contained within a permeable wall and having a diffusivity different from that of the mass of fluid to be sampled. In this case, the representative curve of the concentration variation as a function of time will be placed between the solid lines and dashed curves of Figure 6. This is the.

a case that is frequently encountered in boreholes.

To determine the concentration Kg, it will generally be advantageous to apply the determination mode valid for an infinite medium. Figure 6 shows that we will then know the maximum possible error, which corresponds to a sealed wall.

 It can be seen that the method according to the invention makes it possible to determine the concentration of gas at different locations along a borehole, which represents a particularly valuable advantage when this concentration is likely to vary in time or when one seeks to determine a renewal time for example. But the invention is not limited to this particular case and it finds applications in all areas of practice, including nuclear safety.

 The speaker can have very different shapes.

For example, it will often be possible to use, instead of a tube section (which however has the advantage of the indeformability and the mechanical strength), a simple flexible plastic bag, the cost of which is so low that it can be thrown away after a single job.

Claims (8)

claims  1. A method for sampling at least one gas present in a mass of fluid to be analyzed, characterized in that said fluid is exposed to a closed chamber (10) containing a receiving fluid (f) delimited by a casing. a non-porous material having gas-selective permeance, for a specified time, sufficient for the concentration of the gas to be metered within the shell to reach an appreciable fraction of the outside content, removing the enclosure (10) for metering the gas present in the receiving fluid in a time that is a small fraction of the previous time, and in that the gas content in the analyze the measurement of its concentration in the receiving fluid.  2. Process according to Claim 1, characterized in that a gas is used as the receiving fluid when the mass of fluid to be analyzed is a gas, and a liquid having the same general properties as the mass of fluid to be analyzed for liquid phase sampling.  3. Method according to claim 1 or 2, characterized in that the exposure time of the chamber to the fluid mass is generally several hours.  4. A method according to any one of the preceding claims for sampling at least one gas in a borehole or in a vertical or horizontal in a body of water, charac terized in that s sunultanornent a rosary of speakers distributed along the hole or the vertical or the horizontal.  5. Device for implementing the method according to any one of the preceding claims, characterized in that it comprises an enclosure, the casing is at least partially constituted by the material having selective permeance vis-à-vis gases, and and means for setting up said chamber in a mass of fluid to be analyzed and recovering it for analysis of the contents of the chamber in a small delay with respect to the duration of exposure.  6. Apparatus according to claim 5, characterized in that the enclosure or each enclosure is constituted by a pipe section of said material, closed at the ends and provided with a manual opening and closing system.  7. Device according to claim 5, characterized in that the enclosure is constituted by a flexible plastic bag.  8. nispositif according to claim 5, 6 or 7, characterized in that the material having a selective permeance is constituted by a polymer, preferably selected from poly (dienes), poly (alkenes), poly (methacrylates) and various poly (amides), polyesters and polysiloxanes.