The present invention relates to determination of the gas composition in soils or surface rocks. The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.

It is important to know which gases are present in a soil, or which gases are emitted or trapped in surface rocks, in terms of organic gases (volatile hydrocarbons) or inorganic gases (dihydrogen H ₂ , carbon dioxide C0 ₂ , hydrogen sulfide H ₂ S,...). For example, knowing the existence of H ₂ S in soil can prevent accidents in the framework of hydrocarbon exploration. On the contrary, the presence of a large amount of volatile hydrocarbons or of H ₂ may encourage starting prospections. Knowing the gas composition of a soil also makes it possible to identify and characterize polluted areas. In the solutions of the prior art, to perform a gas analysis in a surface rock or a hard soil, it is first necessary to perforate the rock or the soil, then introduce a rigid tube into the hole in order to recover the gases. The composition of the recovered gases may then be obtained thanks to a gas analyzer. Flowever, a portion of the gases escapes as soon as the hole is formed, so that the gas composition may have changed at the time of introduction of the rigid tube. This is particularly true for volatile gases such as H ₂ .

There is thus a need for a system for accurately determine the gas composition of a soil or of a surface rock.

SUMMARY OF THE INVENTION

The invention relates to a collecting device for collecting gas, said collecting device being adapted to be connected to a drilling equipment. The collecting device may comprise:

- a skirt adapted to define an internal volume of the skirt;

- a connection element adapted to connect the drilling equipment to the skirt, in such a way that the drilling equipment partially extends in the internal volume of the skirt; and - a gas collecting system adapted to collect gas contained in the internal volume of the skirt.

During drilling, the gas initially contained in the soil is therefore trapped into the internal volume of the skirt. This gas may be collected and analyzed to determine is composition.

Advantageously, the connection element may be adapted to connect the drilling equipment to the skirt in a sealed manner. This prevents the gas from escaping at the connection element.

In one embodiment, the gas collecting system may be located on the connection element. Alternatively, the gas collecting system may be located on the skirt.

The collecting device may further comprise a first sealing element between the skirt and the connection element. When the skirt and the connection element are two distinct elements, this first sealing element prevents a gas leak at the junction between the skirt and the connection element.

In one or several embodiments, the skirt may be at least partially flexible. For example, the skirt may comprise a bellows.

The partial flexibility of the skirt allows the skirt to be deformed during drilling, in particular as the drill bit sinks into the soil.

In one or several embodiments, the skirt may substantially extend along a longitudinal axis corresponding to a drilling direction.

Furthermore, the gas collecting system may comprise a gas analyzer adapted to analyze a composition of the collected gas.

In one or several embodiments, the collecting device may further comprise, at an end of the skirt opposite to the connection element, a second sealing element. This second sealing element prevents a gas leak at the junction between the skirt and the surface to be drilled.

In addition, the collecting device may further comprise a guidance system adapted to guide a drill bit of the drilling equipment in a drilling direction. Such guidance system increases the stability of the drilling system (composed from the collecting device and the drilling equipment) during drilling. In one example, the skirt may substantially extend along a longitudinal axis, said longitudinal axis corresponding to the drilling direction, and the guidance system may comprise a first plate, a second plate and at least one shaft, wherein the first plate may be connected to the connection element, wherein the second plate may be connected to an end of the skirt opposite to the connection element, wherein each shaft among the at least one shaft may be connected to the second plate, wherein the first plate may comprise at least one hole, each hole among the at least one hole being able to receive a respective shaft among the at least one shaft, wherein the first plate may be able to slide along the drilling direction and to be guided by the at least one shaft.

Another aspect of the invention relates to a drilling system comprising:

- a drilling equipment; and

- a collecting device adapted for collecting gas, said collecting device being connected to the drilling equipment, the collecting device comprising: a skirt adapted to define an internal volume of the skirt; a connection element adapted to connect the drilling equipment to the skirt, in such a way that the drilling equipment partially extends in the internal volume of the skirt; and a gas collecting system adapted to collect gas contained in the internal volume of the skirt.

Yet another aspect of the invention relates to a method for collecting gas escaping from a drilled surface. The method may comprise:

- connecting a drilling equipment to a first end of a skirt, in such a way that a drill bit of the drilling equipment partially extends in an internal volume of the skirt;

- placing a second end of the skirt, opposite to the first end, against a surface to be drilled;

- drilling the surface with the drilling equipment; and

- collecting gas contained in the internal volume of the skirt.

In one embodiment, the surface may be drilled in a drilling direction that is substantially perpendicular to a geological fault. Indeed, this direction generally corresponds to a maximum amount of gas trapped in the rock. Alternatively the surface may be drilled in a drilling direction that is substantially perpendicular to said surface. By“substantially perpendicular” it is meant that the angle between the drill bit and the surface 6 may not be exactly equal to 90°, there may be a deviation of a few degrees from the perpendicular axis to the surface 6. For instance, this deviation may be less than or equal to 5°, for instance 1 °, 2°, 3°, 4°, or 5°. This deviation may be higher, for instance between 5° and 10°.

Other features and advantages of the method and apparatus disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which like reference numerals refer to similar elements and in which:

- Figure 1 represents the gas collecting device according to a possible embodiment;

- Figure 2 represents the gas collecting device according to another possible embodiment; - Figures 3a and 3b represent two possible configurations of the gas collecting device according to an embodiment of the invention;

- Figure 4 represents the gas collecting device according to another possible embodiment; - Figures 5a and 5b represent two possible configurations of the gas collecting device according to another embodiment of the invention;

- Figure 6 is a flow chart describing a method for collecting gas according to an embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 represents the gas collecting device according to a possible embodiment.

As represented in Figure 1 , the gas collecting device 1 comprises a connection element 2, a skirt 3 and a gas collecting system 4. The skirt 3 may be adapted to define an internal volume 3a of the skirt 3. The connection element 2 is adapted to connect a drilling equipment to the skirt 3, in such a way that the drilling equipment may partially extend in the internal volume 3a of the skirt 3. The gas collecting system 4 may be adapted to collect gas contained in the internal volume 3a of the skirt 3. The collected gas may be for instance a gas initially contained in a soil or a surface rock 6, and released in the internal volume 3a of the skirt 3 during the drilling of the soil or of the surface rock 6.

In one or several embodiments, the connection element 2 may be a rigid element, composed for example of metal. Alternatively, the connection element 2 may be composed of a material other than metal, for instance a flexible material, such as flexible polyvinyl chloride (PVC), plastic, leather, rubber, etc.

In order to connect the drilling equipment to the skirt 3, the connection element 2 may comprise, at a top end 2a of the connection element 2 opposite to the surface 6 to be drilled, an orifice adapted to fix a piece of the drilling equipment. Advantageously, the connection element 2 is configured for connecting in a sealed manner the drilling equipment to the skirt 3.

In one embodiment, the connection element 2 and the skirt 3 belong to a single piece. Alternatively, the connection element 2 and the skirt 3 belong to two distinct pieces. In that case, the gas collecting device 1 may further comprise an optional sealing element 5 between the connection element 2 and the skirt 3. The sealing element 5 may be a seal, an air chamber, or any element capable of preventing gas leak at the junction between the skirt 3 and the connection element 2.

In one or several embodiments, the gas collecting system 4 may be located on the connection element 2. Alternatively, the gas collecting system 4 may be located on the skirt 3.

The gas collecting system 4 may be any known system of the prior art configured for collecting gas. For example, the gas collecting system 4 may comprise a gas sampling pump, eventually connected to a gas collecting (or “sampling”) tube. Alternatively, the gas collecting system 4 may comprise a vacuum tube that can be opened to collect the gas accumulated in the internal volume of the skirt 3. According to another example, the gas collecting system 4 may comprise a gas sampling tube connected to a first valve (inlet valve) located between the connection element 2 and the gas sampling tube, and also connected to a second valve (outlet valve) located at the end opposite to the end connected to the first valve.

When the gas collecting system 4 comprises a gas sampling tube, the collected gas may be sent in laboratory for delayed chemical and isotopic analysis for example.

Alternatively or in addition, the gas collecting system 4 may comprise a gas analyzer, configured for analyzing in real time the composition of the collected gas. This embodiment is represented in Figure 2. Advantageously, the gas analyzer is portable.

Of course, the gas collecting system 4 may comprise a gas sampling tube and a gas analyzer. Thereby, a part of the collected gas may be analyzed in real time by the gas analyzer, and another part of the collected gas may be analyzed in laboratory later. This allows for both accurate analyzes (by the laboratory analysis, offline) and quick analyzes (by the gas analyzer, in“real time”, but probably less accurate than in laboratory).

Figures 3a and 3b represent two possible configurations of the gas collecting device according to an embodiment of the invention.

Figures 3a and 3b represent two configurations of the gas collecting device when it is connected to a drilling equipment 7. The drilling equipment 7 comprises a drill bit 8. The drilling equipment 7 may be any tool equipped with a drill bit 8 and adapted for making holes in a soil or a rock, as an electric drill (for instance, a pistol-grip drill, a hammer drill, a rotary hammer, etc.), a magnetic drilling machine, a drilling rig, etc.

When the system (composed by the gas collecting device 1 and the drilling equipment 7) is installed for drilling, the drill bit 8 is oriented in the drilling direction 9. In one or several embodiments, the skirt 3 may substantially extend along a longitudinal axis corresponding to the drilling direction 9.

Advantageously, the drilling direction 9 may be substantially perpendicular to a geological fault. Indeed, this direction generally corresponds to a maximum amount of gas trapped in the rock. By “substantially perpendicular” it is meant that the angle between the drill bit and the surface 6 may not be exactly equal to 90°, there may be a deviation of a few degrees from the perpendicular axis to the surface 6. For instance, this deviation may be less than or equal to 5°, for instance 1 °, 2°, 3°, 4°, or 5°. This deviation may be higher, for instance between 5° and 10°. Alternatively, the drilling direction may be substantially perpendicular to the surface 6 to be drilled.

As represented in Figures 3a and 3b, a piece of the drilling equipment 7 may be connected to the connection element 2. As already mentioned, the connection element 2 may comprise to that purpose an orifice adapted to fix a piece of the drilling equipment 7. The piece of the drilling equipment 7 may be for instance the body of the drilling equipment, or advantageously any piece that does not rotate during drilling. Fixing a piece of the drilling equipment 7 that does not rotate during drilling ensures stability of the whole system (composed by the gas collecting device 1 and the drilling equipment 7).

Advantageously, the skirt 3 may be at least partially flexible. For instance, the skirt may be composed of a flexible material, such as flexible polyvinyl chloride (PVC), plastic, leather, rubber, etc. In one embodiment, the skirt 3 may comprise a bellows, as represented in Figures 3a and 3b. Of course, the skirt may have any other form, as represented for instance in Figure 4.

The partial flexibility of the skirt 3 allows the skirt 3 to move between two positions:

- a“rest” position, represented in Figure 3a and corresponding to the position of the skirt 3 before drilling, i.e. when no pressure is applied on the skirt 3 in the drilling direction 9; and

- a “compressed” position represented in Figure 3b and corresponding to a maximum position of drilling, when a pressure is applied on the skirt 3 via the connection element 2 and when the skirt 3 cannot deform anymore in the drilling direction.

Advantageously, in the rest position, the drill bit 8 is totally included in the internal volume of the skirt 3. This prevents the gas to escape outside the skirt when drilling begins. During drilling, a pressure is applied on the skirt 3 in the drilling direction 9 via the connection element 2. When the drill bit 8 begins to drill the surface 6, some of the gas stuck in the soil or in the surface rock escapes and ends up in the skirt. This gas may then be collected in the gas collector 4.

Figure 4 represents another embodiment of the gas collecting device. When the drill bit 8 drills the surface 6, a part of gas contained in the surface 6 may follow a movement mostly vertical in an opposite direction with respect to gravity, i.e. from the surface to the sky (represented by dashed arrows). This is particularly true for volatile gas, such H ₂ . However, some of gas particles may also have a movement mostly horizontal along the surface 6 (represented by dotted arrows). To avoid loss of these particles, which would distort the analysis of the composition of the gas contained in the soil or the surface rock, the skirt 3 may comprise an additional sealing element 10 at the end opposite to the connection element, i.e. at the end which is placed on the surface 6 to be drilled. The skirt 3 is then configured to define a sealed volume when it is placed on the surface 6. Therefore, the composition of the gas collected in the gas collector 4 corresponds more accurately to the composition of the gas escaped from the soil or the surface rock during drilling. The additional sealing element 10 may be a seal, an air chamber, or any element capable of preventing gas leak at the junction between the skirt 3 and the surface 6 to be drilled. The additional sealing element 10 is optional. As mentioned above, it is likely that a large part of gas escapes in a vertical direction and remains trapped inside the skirt, so that the composition of the gas collected by the collecting device 1 without the additional sealing element 10 is probably quite close to the composition of the gas escaped from the ground surface 6. Furthermore, during drilling, a pressure is applied on the skirt 3 in the drilling direction, so that the bottom of the skirt 3 (i.e. the end of the skirt 3 that has to be placed on the surface 6) is pressed against the ground surface 6, thus preventing (more or less effectively, depending e.g. on the nature of the ground surface 6 or the flexibility of the skirt 3) gas escaping through the bottom of the skirt 3.

The additional sealing element 10 may also be adapted at the bottom of the skirt 3 in the embodiments described in Figure 1 , 2, 3a and 3b if the sealing capacity of the skirt 3 is insufficient.

Figures 5a and 5b represent two possible configurations of the gas collecting device according to another embodiment of the invention.

In one or several embodiments, the gas collecting device may further comprise a guidance system adapted to guide the drill bit of the drilling equipment in the drilling direction. An example of the guidance system is represented in Figures 5a and 5b, in the rest position and in the compressed position, respectively.

As represented in Figures 5a and 5b, the guidance system may comprise a first plate 1 1 , a second plate 12 and at least one shaft. In the specific example of Figures 5a and 5b, the guidance system comprises four shafts 13a, 13b, 13c, 13d. In other embodiments, the guidance system may comprise a number of shafts other than four, for instance one, two, three, or more than four.

The first plate may be connected to the connection equipment. To that end, the first plate may comprise an aperture 14 for inserting a part of the drilling equipment. When the drilling equipment is connected to the gas collecting system, the aperture 14 may be aligned to an orifice of the connection element.

The second plate 12 may be connected to the end of the skirt opposite to the connection element, i.e. the end of the skirt 3 that has to be placed on the ground surface. For instance, the second plate may comprise an aperture (not represented in Figures 5a and 5b) for receiving the end of the skirt 3.

Each shaft 13a, 13b, 13c, 13d may be connected to the second plate 12. For connecting the at least one shaft 13a, 13b, 13c, 13d to the second plate 12, the second plate 12 may comprise at least one hole, each hole being able to receive a respective shaft 13a, 13b, 13c, 13d. When the guidance system is positioned for drilling, the at least one shaft 13a, 13b, 13c, 13d are therefore in abutment against the surface to be drilled. Alternatively, the second plate 12 may comprise at least one fixing element (for instance, a notch or a nut), each fixing element being able to fix a respective shaft 13a, 13b, 13c, 13d to the second plate 12.

The first plate 1 1 may comprise at least one hole, each hole being able to receive a respective shaft 13a, 13b, 13c, 13d. Accordingly, the first plate 1 1 may be able to slide along the drilling direction and to be guided by the at least one shaft 13a, 13b, 13c, 13d.

Figure 6 is a flow chart describing a method for collecting gas according to an embodiment of the present invention.

As recited above, the drilling equipment may be connected 601 to a first end of the skirt. For instance, the body drilling equipment may be connected to the connection element of the gas collecting system. This connection may be performed such that the drill bit of the drilling equipment partially extends in the internal volume of the skirt. For instance, in the rest position, the drill bit may be entirely contained in the internal volume of the skirt.

Then, the second end of the skirt, opposite to the first end, may be placed 602 against the surface to be drilled.

The surface is then drilled 603 with the drilling equipment. At this stage, the gas originally trapped into the soil escapes and ends up inside the internal volume of the skirt. The gas contained in the internal volume of the skirt may then be collected 604 as described above, via the gas collecting system.

According to one embodiment, the surface is drilled over a length between 0,5 meter (m) and 2 m, and preferentially between 0,5 m and 1 m.

Such method makes it possible to collect gas contained in the soil during drilling, instead of collecting it after the soil has been perforated, as in the methods of the prior art. This prevents some of the gas from escaping directly into the air and from being permanently lost. Comparative measurements carried out with the methods of the prior art on the one hand, and with the method of the present invention on the other hand, have shown a ratio between some quantities (e.g., a concentration of a given gas) up to 10. It has to be noted that gases can be created at the time of drilling, because of the friction of the drill bit on the rock for example. The composition and quantity of these gases depends on a set of parameters, such as the type of rock, the humidity of the air, whether the rock is wet or not, etc. It is possible to construct predictive models to determine the type of gas produced and their quantity, according to this set of parameters. Such determination may be used for“correcting” the analysis of the collected gas.

Expressions such as "comprise", "include", "incorporate", "contain", "is" and "have" are to be construed in a non-exclusive manner when interpreting the description and its associated claims, namely construed to allow for other items or components which are not explicitly defined also to be present. Reference to the singular is also to be construed in be a reference to the plural and vice versa.

A person skilled in the art will readily appreciate that various parameters disclosed in the description may be modified and that various embodiments disclosed may be combined without departing from the scope of the invention.