GAS ACTIVITY ANALYSIS IN THE GROUND

Abstract

A method of introducing a propellant gas into the ground is described, wherein a trace gas that can be analysed above the ground or the earth's surface is added to said propellant gas. According to the invention hydrogen is used as the trace gas, whereby the proportion of the trace gas hydrogen in the propellant gas/trace gas mixture is a maximum of 10% by vol., preferably a maximum of 5% by vol.

Description

BACKGROUND OF THE INVENTION

The invention relates to a method of introducing a propellant gas into the ground, wherein a trace gas that can be analysed above the ground or the earth's surface is added.

The term “ground” hereinafter denotes all types of soil, rock and bedrock, into which gases or gas mixtures are introduced for particular purposes, as explained below.

Gases or gas mixtures—hereinafter referred to as propellant gases, are used in the ground to move substances or liquids. An example of this is the displacement of ground water charged with noxious substances, which is “driven” to a pumping site, where the water is pumped to the surface. Once purification has taken place, the water can once again be returned to the ground. Another example is the driving of crude oil in reservoir rock, so that a greater volume of crude oil stored in the rock can be extracted using EOR (Enhanced Oil Recovery). Nitrogen and carbon dioxide are the most commonly used propellant gases in this case.

In order to determine whether and, if appropriate, where the propellant gas used flows or flows to in the ground, an easily analysable gas—hereinafter referred to as the trace gas—is added to the propellant gas and the concentration of this trace gas on the earth's surface is measured. This enables conclusions to be drawn as to where the propellant gas actually flowed to. Helium has hitherto been commonly used as the trace gas. The helium molecule is small, compared with the molecular size of the gases used as a propellant. This means that the helium molecule is able to diffuse through cracks, gaps and holes in the ground or rock more easily.

Helium can be analysed in the presence of air. Because air is approx. 79% nitrogen, traces of nitrogen, which may possibly emerge from the ground or rock as a propellant gas, cannot be detected in air. Helium, on the other hand, can still be detected in traces in air too. The presence of helium on the surface indicates where the actual propellant gas has flowed to. The detection threshold for helium stands at around 5 mbar*l/s with state-of-the-art helium analysis devices. The helium concentration in the ambient air is approx. 5.24 vppm. A measuring device must therefore be capable of detecting differences in concentration of around a few vppm. This is entirely possible with the measuring technology known today.

However, helium is a very rare gas, which on the one hand is comparatively expensive and, on the other, is often not available in sufficient quantities. In order to conduct investigations or for EOR or the expulsion of water, however, large quantities or trace gas have to be made available over prolonged periods often lasting several months. A typical gas volume for the expulsion of water charged with noxious substances is, for example, 1000 Nm³/hr N₂ with 10% by vol. helium, which is equivalent to a helium volume of 100 Nm³/hr. This sort of helium volume can often not be made available in practice currently, particularly since a multiplicity of helium-consuming rival applications exist in medical technology, in which human lives sometimes depend on the availability of helium. It is therefore also understandable that these applications are given higher priority in the apportionment of helium.

It was further suggested that other reaction-inert gases, particularly other noble gases like argon, krypton or xenon should be considered as trace gases. Argon, which is obtained in large volumes from air decomposers, exists in the ambient air in a proportion of approx. 1%. For this reason, traces of argon that occur in the trace gas application are not detectable. Another aggravating factor with the other noble gases mentioned above is that mass spectrometers have to be used for detection. However, hydrocarbons are also indicated in the region of the spectral lines of these gases, as a result of which the analysis can be distorted. This is an unacceptable disadvantage, particularly when used for EOR.

BRIEF SUMMARY OF THE INVENTION

The problem addressed by the present invention is that of indicating a generic method of introducing a propellant gas into the ground, which avoids the aforementioned disadvantages.

In order to solve this problem, a generic method of introducing a propellant gas into the ground is suggested, which is characterised in that hydrogen is used as the trace gas.

Further advantageous embodiments of the method according to the invention for introducing a propellant gas into the ground, which are objects of the dependent patent claims, are characterised in that

• • at least one additional trace gas is added to the propellant gas in addition to the hydrogen,
  • nitrogen-rich gas mixtures, carbon dioxide, carbon dioxide-rich gas mixtures and/or saturated hydrocarbons, such as methane, are used as the propellant gas,
  • the proportion of the trace gas hydrogen in the propellant gas/trace gas mixture is a maximum of 10% by vol., preferably a maximum of 5% by vol.,
  • helium is used as a further trace gas,
  • the propellant gas/trace gas mixture is blended immediately before it is introduced into the ground and
  • the propellant gas/trace gas mixture is injected into the ground at a depth of at least one metre below the earth's surface.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, hydrogen is now used as the trace gas. While the use of hydrogen as a test gas for identifying leaks and for tightness testing has been known for some time, the use of hydrogen as a trace gas has not been considered hitherto. Due to its explosion risk when mixed with air, hydrogen can only be used as a trace gas in very special cases.

It is advantageously used as a forming gas—this refers to hydrogen/nitrogen gas mixtures—with a hydrogen share of 5% and a nitrogen share of 95%. According to ISO 10156, the lower explosion threshold is around 5.7% by vol. hydrogen in air. However, this explosion risk must be definitively removed, if hydrogen is to be safely used as a trace gas. It has now surprisingly emerged that, provided it is handled correctly, hydrogen can be safely used as a trace gas at even higher contents of up to approx. 10% by vol..

Investigations have shown that hydrogen used as a trace gas surprisingly diffuses significantly quicker in the ground or rock than gases customarily used as propellants. The hydrogen concentrations found are therefore lower than in the propellant gas/trace gas mixture supplied to the ground, even at a small distance from the injection site. A rapid separation of the propellant gas/trace gas mixture evidently occurs, which means that a lower and therefore uncritical hydrogen concentration results. This effect can now be exploited to enable hydrogen to be used as a trace gas in concentrations of over 5%.

If the hydrogen trace gas emerges at the earth's surface, its concentration in the air is several orders of magnitude below the lower explosion threshold, if the injection site is at least approx. 1 metre beneath the earth's surface. This is because even with a diffusion length of one metre, the hydrogen content has actually dropped from several % by vol. to a few ppm. Consequently, an increase in the hydrogen concentration of the propellant gas/trace gas mixture to values off 10% by vol. and more is possible without there being any risk to the individuals taking the measurements and/or the immediate environment.

Hydrogen can be detected in air in concentrations below 1 ppm. Hydrogen therefore meets the necessary prerequisites as a trace gas:

• • Hydrogen is normally only present in the ambient air in low concentrations of under 1 ppm, which means that trace quantities that are detected also originate from the trace gas and undoubtedly not from the environment.
  • Hydrogen detectable in air in the range <1 ppm.
  • Hydrogen is only slightly reactive at ambient temperature, so that there are few if any secondary reactions with earth or rock. Such reactions could distort the measuring result significantly.
  • Hydrogen is produced in large quantities and is usually readily available in an industrial setting.
  • The hydrogen measuring or analytical equipment needed for the practical execution of the method according to the invention for introducing a propellant gas into the ground is adequately known from the state of the art. This equipment does not usually require modification for the intended application.

If the method according to the invention is used for EOR in large oil fields, it may be necessary to mix hydrogen and the propellant gas, preferably nitrogen, on site. In practice, the upper limit for consumption of the propellant gas/trace gas mixture, which is supplied in bottles, is around 50 Nm³/hr. Propellant gas/trace gas mixture volumes that go beyond this are produced by mixing propellant gas and trace gas on site, in accordance with an advantageous embodiment of the method according to the invention.

Claims

1. A method of introducing a propellant gas into the ground, wherein a trace gas that can be analysed above the ground or the earth's surface is added to said propellant gas, characterised in that hydrogen is used as the trace gas.

2. The method according to claim 1, characterised in that at least one additional trace gas is added to the propellant gas in addition to the hydrogen.

3. The method according to claim 1, characterised in that said propellant gas is selected from the group consisting of nitrogen, nitrogen-rich gas mixtures, carbon dioxide, carbon dioxide-rich gas mixtures and saturated hydrocarbons.

4. The method according to claim 3, characterised in that said saturated hydrocarbons are selected from the group consisting of methane.

5. The method according to claim 1, characterised in that the proportion of the trace gas hydrogen in the propellant gas/trace gas mixture is a maximum of 10% by vol.

6. The method according to claim 1, characterised in that the proportion of the trace gas hydrogen in the propellant gas/trace gas mixture is a maximum of 5% by vol.

7. The method according to claim 2, characterised in that helium is used as a further trace gas.

8. The method according to claim 1, characterised in that the propellant gas/trace gas mixture is blended immediately before it is introduced into the ground.

9. The method according to claim 1, characterised in that the propellant gas/trace gas mixture is injected into the ground at a depth of at least one metre below the earth's surface.