US3462957A

US3462957A - Process for storing a gas in a coal mine

Info

Publication number: US3462957A
Application number: US634824A
Authority: US
Inventors: Yvon Henri Arthur Loir
Original assignee: Individual
Current assignee: Individual
Priority date: 1965-07-02
Filing date: 1967-05-01
Publication date: 1969-08-26
Anticipated expiration: 1986-08-26
Also published as: NL6607457A; FR1499998A; GB1145913A; DE1253659B

Description

3,462,957 PROCESS FOR STORING A GAS IN A COAL MINE Yvon Henri Arthur Loir, 97 Route de Mons, Fontaine-LEveque, Belgium No Drawing. Filed May 1, 1967, Ser. No. 634,824 Claims priority, applicatiorli Belgium, May 5, 1966, 27 7 Int. Cl. E21f 1i/16; B65g 5/00 US. Cl. 61-.5 1 Claim ABSTRACT OF THE DISCLOSURE The present invention relates to a process for storage of gas, especially combustible gas, in an underground reservoir or gas holder.

Underground storage of combustible gas has already been effected in certain geological formations.

According to a known process for storing natural gas, the latter is injected into a spent petroleum or natural gas bed or stratum. The storage capacity of the under ground reservoir formed by this bed is, among other factors, a function of the free space remaining in the said bed due to its previous working.

According to another known process for storing natural .gas, the latter is injected into a previously worked, abandoned coal mine filled with ground water. Such a mine forms a great U-shaped underground reservoir with at least two shafts which serve respectively to convey water and gas. In this reservoir, gas is above the adjustable level of water, under a pressure corresponding to the height difference between said level and the surface of the ground. The storage capacity of said reservoir is proportional to said pressure and depends on the gas volume increasing when exhausting water from the mine. In such an underground reservoir which works according to the principle of communicating conduits, the presence of water reduces the possibility of diffusion, fixation, absorption and adsorption of the gas in the rock of the mine, namely through decreasing of the rock permeability. The storage capacity of said reservoir is thus practically limited to the geometrical volume of the mine cavities above the water level in said mine.

According to another known process for storing combustible gas, the latter is introduced into porous sandbearing rocks, which are saturated With water and confined under an impermeable perianticlinal dome. In this case, in order to prevent an inrush of water into the shafts of boreholes, the injection of a Ininirnum permanent volume of gas is indispensable when putting the reservoir into service (a cushion or gas-lock). The storage capacity of the underground reservoir formed by these rocks is then inferior to the actual inherent volume of this reservoir, Since the introduction of the gas into these rocks involves the pressing back of the water in these and since the extraction of this gas out of the said rocks is effected under the counter-pressure of this water, these reservoirs have to have a very large gas-water contact surface and must then be enormous having regard to their effective capacity. In addition, the gas injected is not necessarily in chemical equilibrium with the rock of the reservoir and there is thus a risk of pollution of the waterbearing stratum.

'nited States Paten 0 ice According to other known processes for storing combustible gas, the gas is introduced into natural or artificial underground reservoirs found in suitable impervious geological structures such as salt formations, non-fractured coherent rocks (argillaceous limestones, schists, crystalline rocks, quartzites, limestones) or plastic rocks (clays). In these latter processes, the filling of natural or artificial geometrical cavities of the stratum is effected. The true storage capacity of these reservoirs is directly dependent on the available geometrical volume of the cavities.

The object of the present invention is a new process for storing gas, especially combustible gas, in which the true storage capacity of the underground reservoir is greater than its own inherent volume.

To this end, according to this new process, the gas to be stored is introduced into the gassy coal deposit previously partially worked, degassed and unsaturated with water, at a relatively high pressure so as to increase the absorption of said gas in the rocks of the coal deposit and, in particular, the adsorption of said gas by said rocks.

The fact that the true storage capacity of the coal deposit is appreciably greater than the inherent geometrical volume of this measure is explained by the property which rocks of coal deposit have of being able to absorb a large quantity of gas amongst others under the effect of pressure, and this quantity of gas absorbed may attain, for example, 40 m. N of methane per metric ton for some coals.

Thus, when a gas is introduced into a coal deposit, it not only fills the cavities, fissures and pores of this deposit but it is also absorbed by the rocks constituting the said deposit.

Therefore, according to the process one can introduce into a coal deposit a quantity of gas, especially combustible gas, greater then that which could be introduced into other geological formations by the known processes, that is, for the same inherent volume of all these strata and for the same injection pressure of the gas therein.

The introduction of the gas into the underground reservoir may be made in an unworked coal deposit. However, a coal mine which has previously been partially worked, degassed and unsaturated with water may preferably be used to store this gas. Such a mine has in fact the advantage of having an inherent volume which is larger than that of the not yet worked deposit and which is due especially to the residual volume of the cavities excavited during he mining operations. In addition, such a mine has also an enormous gas-rock contact surface represented by the surface of all these excavated cavities as well as by the multitude of cracks and fissures caused by the movements of the rocks due to these mining operations.

This considerable fracturing of the deposit increases the permeability of the rocks unsaturated with water and thus facilitates the circulation and diffusion of the gas in the said mine or deposit.

. On the other hand, the partial removal of gas or the desorption of thecoal deposit surrounding the part of the inherent volume formed by the mining operations facilitates the absorption of the gas injected into the mine and in particular, the adsorption of this gas by the rocks of the said deposit.

According to the invention, the gas is introduced into the coal deposit which is previously and partially worked and degassed, also unsaturated with water.

Furthermore, the diffusion and absorption of gas in the rocks of a coal deposit result in phenomena which depend on the internal structure characteristic of some rocks of the coal deposit (coals and coal-bearing schists). The specific internal surface of some coals may attain, for example, mP/gram. The gaseous exchanges between the gas outside the rock and that absorbed by the internal structure of the rock depend especially on the nature and the pressure of the external gas in contact with the rock, the magnitude of the contact surface between the external gas and the rock, the nature, internal structure, permeability, magnitude of the internal surface, temperature, moisture and adsorbent power of the rock. In the present specification, the external gaseous phase means the gas found in the part of the inherent volume of the measure which is exterior to the rock and which is formed by the residual volume of all the cavities due, for example, to former mining operations and to the shafts for access to the deposit; the internal gaseous phase means the gas found in the part of the inherent volume of the deposit which is inside the rock and filling the free spaces of its internal structure; and adsorbed phase means the gas found in the internal structure of the rock and covering the internal surface thereof.

The internal phase and the adsorbed phase of the gas which are adsorbed by the internal structure of the rock are normally in equilibrium with the external gaseous phase, this equilibrium depending among other factors on the pressure of this external gaseous phase.

In particular, the result is that the diffusion of the gas into the rocks of the coal deposit and the absorption of this gas by these rocks are facilitated by an increase of pressure in the external gaseous phase.

On that account, according to the invention, the gas to be stored is introduced at a relatively high pressure in order to facilitate the absorption of this gas in the rocks of the coal deposit.

The adaptation of a gassy coal mine, which has been previously and partly worked, degassed and which is unsaturated with water, to an underground reservoir for combustible gas is specially advantageous because it does not necessitate the injection of any gas for washing and above all because it utilises the pit gas or methane remaining up to then absorbed in the coal mine. To this end and according to an important feature of the invention, there is progressively extracted from the gassy coalmine gas from the external phase previously found in the precinct defining the external volume of the rock of the underground reservoir so as to cause it to pass into this external gas from the pit gas up to then absorbed in the rocks of the deposit. In this way, the calorific power of the gas of the external phase is gradually increased and it is regulated with relation to that of the gas to be stored in order to make is approximately equal to the latter in thermal power. Such an extraction of the external gas from the gassy coal-mine is effected before any injection of gas to be stored into the underground reservoir formed by this mine. The preparation of a storage reservoir from a gassy coal mine previously partly worked, degassed and unsaturated with water is thus both simple and particularly economical.

Other details and features of the invention will appear during the description of an example of storage of combustible gas, given without limiting the invention thereto.

The case envisaged concerns the storage of a natural combustible gas having, for example, the following mean composition:

Percent CH 81.3 C H 2.9 C H 0.4 04H) 0.1 C H 1.1 co 0.8 N 14.4

formed on one hand by the shafts, galleries, workings intentionally excavated during the working and on the other hand, by cracks, fissures and cavities produced during and after this working.

In the example selected, it may be estimated that this residual volume of the underground reservoir formed in this coal mine by the mining operation is about 10,000,- 000 m.

This volume external of the rock of the underground reservoir is filled with a gas which is denoted by external gas or external gaseous phase.

Before the preparation for the storage of gas in this reservoir, this external gas is from the air of the mines having, for example, the following average composition:

Percent CO 0.5 CH 2 O 18.5 N 79 On the other hand, this external gas is found at a pressure of the order of 1 atmosphere and has a calorific power of about calories/m. N.

The above-mentioned external volume of the underground reservoir is defined by the rocks of the coal deposit, which include among others coals and schists.

These rocks contain a certain amount of absorbed pit gas or methane. This absorbed pit gas is composed of an internal gas and an adsorbed gas or, in other words, an internal gaseous phase and an adsorbed phase. This absorbed pit gas is at a pressure of about 1 atmosphere in the vicinity of the contact surface between the external gas and the rocks and which increases progressively, for example up to 50 atmospheres, in proportion as it is removed from the degassed and desorbed zone of this contact surface towards the non-desorbed and virgin zone of the coal deposit.

This pit gas absorbed by the rocks of the coal-measures has approximately the following average composition:

Percent H 0.133 CH 97.2 C H 1.96 N +Ar+Kr 0.32 He+Ne 0.0257 CO 0.38 H 8 Traces and an average calorific power of about 9600 calories/ in. N. Before any storage operation in such a previously Worked, gassy and unsaturated with water coal deposit, there exists in the interior of the said deposit an unbalance of the pressures of the external gaseous phase and the internal and adsorbed phases. This pressure gradient causes a migration of the pit gas absorbed in the rocks of the coal deposit toward the external gaseous phase.

The underground storage reservoir is prepared from this worked coal mine by a first simple operation consisting in extracting, after sealing the shafts, the gas of the external phase while maintaining it at low pressure with respect to that of the internal and adsorbed phases.

For example, the external gaseous phase is maintained at a pressure of 1 atmosphere by drawing off while the internal and adsorbed phases of the virgin deposit are at a pressure of about 50 atmospheres.

The maintenance of this unbalance between the pressure of the external gaseous phase and the internal and adsorbed phases of the rocks of the deposit facilitates the migration of the pit gas absorbed in these rocks towards the external gaseous phase.

The coal mine is degassed and desorbed from its constituent pit gas by the extraction in question, while it is enriched in methane, the said external gaseous phase. The calorific power of this external gaseous phase is thus increased and the adjustment of this calorific power can be carried on by the single extraction mentioned above until, for example, the following average composition is attained:

corresponding to a mean calorific power of about 8300 calories/m. N, equal to that of the calorific power of the gas to be stored.

When this adjustment is finished, the gas to be stored is introduced into the underground reservoir-formed by the said coal mine. This introduction of the gas to be stored is made preferably under a relatively high engine pressure but able to be varied, for example, from 1 to 50 atmospheres according to the amount of gas to be stored.

Throughout and after the introduction of the gas to be stored in the underground reservoir formed by the coal mine, the external gaseous phase is also found at this variable pressure of 1 to 50 atmospheres and consequently this external gaseous phase which fills the volume external to the rocks of the deposit, partly diffuses into these degassed, desorbed and unsaturated with water rocks Where it is absorbed. These quantities of gas absorbed by these rocks increase respectively the internal and adsorbed phases, the internal pressure of which increases as a function of the pressure of the external gaseous phase.

Owing to this fact, the storage capacity of the underground reservoir formed by the aforesaid coal mine is by no means limited to that of the inherent volume of the mine and, under a pressure of 10 atmospheres, a storage capacity of, for example, 500,000,000 111. N may be obtained.

Since the phenomena of diffusion and absorption of the gas into the rocks of the coal mine are reversible as a function of the equilibrium factors between the external gaseous phase, the internal gaseous phase and the adsorbed phase, the operations of removal from storage are easily carried out by reduction of the pressure of the external gaseous phase.

What I claim is:

1. Process for storing hydrocarbonaceous gas, comprising withdrawing from a gassy coal mine which has previously been partially worked and which is unsaturated with water, the gas which is external to the rock of the coal deposit of said mine, until the heating value of the gas being withdrawn is at least about equal to that of the gas to be stored, and thereafter introducing said gas to be stored into said coal mine at superatmospheric pressure so as to promote adsorption of said gas to be stored by the partially worked coal formation.

References Cited UNITED STATES PATENTS 2,508,949 5/1950 Howard 61.5 X 2,810,263 10/1957 Raymond 61--.5 2,817,235 12/1957 Hunter et al. 61-.5 X 3,152,640 10/1964 Marx 61.5 X

EARL J. WITMER, Primary Examiner