FR3028554A1 - METHOD AND SYSTEM FOR TREATING AND SEPARATING NON-CONVENTIONAL GAS - Google Patents

Abstract

The system comprises: - an initial hydraulic stimulation unit (3), with a unit (4) for injecting liquid CO2 under pressure into a well (11) for operating an atypical deposit, - a unit (10) for recovering at low temperature after a stimulation operation, a mixture of natural gas and CO2, this recovery unit (10) comprising in particular an evaluation unit (42), in the mixture of natural gas and recovered CO2. , the amount of CO2 and a unit (42) for comparing this amount of CO2 with respect to a threshold (S), - a unit for separating at least a portion of the CO2 in the liquid, semi-liquid state or solid from the mixture of natural gas and recovered CO2, with at least different separation units (51, 52, 53) depending on whether the amount of CO2 is above or below the threshold (S), - at least one unit (81) Temporary storage of separated CO2 in liquid, semi-liquid or solid state, e at least one land vehicle (501, 502, 503, 504) for moving a temporary storage unit (81) to another site to reuse the separated CO2 and stored in the liquid, semi-liquid or solid state, so supplying a unit (4) for injecting liquid CO2 into a new stimulation unit (3) on a new neighboring well (11).

Description

FIELD OF THE INVENTION The present invention relates to the technical fields of unconventional gas treatment and separation and more particularly relates to a method and an initial hydraulic stimulation system, using carbon dioxide injection ( CO2), of any rock of low porosity, for the production of natural gas trapped in this rock. PRIOR ART Unconventional gases, which are trapped in atypical deposits, include gas trapped in a rock, with in particular shale gas (known as "shale gas"), coal gas (referred to as "coal"). bed methane ") and the compact tank gas (referred to as" tight gas "). The extraction of these unconventional gases requires the use of specific techniques to release them. In particular, hydraulic rock stimulation techniques are used to increase its permeability.

Hydraulic stimulation is often performed by a technique using the injection of water and a proppant. Water, a proppant and chemical additives are introduced under high pressure into the wellbore to effect stimulation. The proppant, which may be sand or an artificial material, serves to keep open the microcracks created by the hydraulic stimulation. Hydraulic stimulation using water has drawbacks in a context where one wishes to preserve the environment, partly because there are desert regions rich in shale gas where this water is scarce, whereas it is necessary to have about 10,000 m3 per well, and secondly 3028554 2 because the use of chemical additives, such as gelling agents, disinfectants, gel breakers, friction reducers, acids, corrosion inhibitors or decalcifiers have potential reactions with rock that remain unknown. On the other hand, the treatment of the water after the stimulation operation causes stresses and when the chemicals are injected into the well when it is closed after production, the seismic stability of the production zone is not affected. not assured. This is why alternative hydraulic stimulation techniques without water have been proposed. For example, it has already been proposed to perform stimulation with fluoro-propane. However, fluoro-propane is expensive and it is necessary to separate the fluoro-propane gas produced, for reasons of safety. For example, in France, it is required that the total quantity of fluorinated compounds introduced when biomethane is injected into a natural gas network should be less than 10 mg / m3 (n), and draft standards even fix this. amount to 3.5 mg / m3 (n). Another alternative technique is to inject under high pressure liquid CO2 instead of water (see in particular documents US 20090260828, CA 1134258, CA 2255413, US 4627495 and US 5883053), which allows in particular to find synergies with industries producing CO2 that can then be recycled. These industries may include gas production plants associated with acidic natural gas in conventional fields, but also synthesis gas production plants associated with the production of ammonia, fertilizers and hydrogen or plants. treatment plants associated with biogas or synthetic natural gas. CO2 sources can also be found in energy production plants, in cement plants, in the iron and steel industry, or in facilities for the capture and geological sequestration of carbon dioxide. Hydraulic stimulation with liquid CO2 has many advantages over hydraulic stimulation using water.

3028554 3 100% liquid CO2 is injected and there is no chemical additive. The volumes of liquid CO2 injected are lower than the volumes of water required for water stimulation.

The cracks are wider and there are no water condensates in the well, which improves the productivity of the well. Unlike water stimulation, you do not need to use a valuable natural resource. Unlike the case of stimulation with LPG (liquefied petroleum gas), stimulation with liquid CO2 does not impose additional constraints with respect to flammability or explosion risks. In the case of liquid CO2 stimulation, the well can be cleaned soon after the stimulation, which facilitates the operation of the well. The CO2 is returned in gaseous form for a few days, until the return flow includes only natural gas and the usual proportion of CO2 present in the natural gas, which facilitates the recovery and the treatment of the return flow. . The supply of CO2 is however sometimes difficult.

Also, it has already been proposed to recover the CO2 that has been used for a first stimulation, or at least a large part of it, to store it in a liquid state in a tank placed on a truck and to reuse it on another site requiring a stimulation operation, on a new well next to the same shale gas deposit or in a nearby shale gas field. It should be noted that in this case, in the post-stimulation period, the CO2, then the shale gas, circulate at a relatively low pressure, which is much lower than a range of 2 to 10 bar. During the few days following the stimulation, the composition of the recovered stream varies. In the first period, only CO2 gas is released. Then, slowly with some delay due to the inertia of the desorption, the natural gas that was absorbed in the shale is released and flows into the fractures and into the well. The recovered stream is then a gaseous mixture of CO2 and mainly methane, the percentage of CO2 decreasing with time until only natural gas remains in production. It should be noted that the natural gas produced is generally recovered and stored only after the end of the post-stimulation phase, when there is practically no CO2 return, while during the post-stimulation phase, stimulation the natural gas produced is generally not upgraded and feeds a torch or is simply released into the atmosphere, which in any case contributes to increasing the greenhouse effect and is not conducive to the preservation of the 'environment.

In order to remedy this disadvantage, it is appropriate, in the days immediately following the stimulation: a) to recover pure CO2, in liquid form, to allow storage and recycling; and b) recovering a stream of natural gas that can be directly transported for commercialization or for use in the field. However, it is necessary to implement a whole process of treatment to separate CO2 from natural gas and then to liquefy it. Since this treatment is only necessary for the period of a few days corresponding to post-stimulation, it is preferable to place the treatment plant on a trailer, in order to allow it to be moved to another site. where should be a stimulation. These existing solutions have disadvantages.

Thus, in the first phase of the post-stimulation period, the return flux very rich in CO2 can be liquefied, but the CO2 level varies regularly decreasing from 100% to 5% and the CO2 liquefaction systems do not are not suitable for treating varying percentages of CO2 in a range as large as 5 to 100%. Furthermore, in the second phase of post-stimulation, CO2 should be separated from natural gas before liquefaction, but conventional separation techniques are not suitable for this.

With chemical absorption using solvents such as amines, CO2 is recovered in the vapor phase at 40 ° C and under a pressure of 1 to 2 bar. Such low pressure makes the reliquefaction step more expensive because lower temperatures and higher volume containers should be used. In addition, the size of such an installation does not make it suitable for placement on the trailer of a semitrailer. With membranes, again, the CO2 is recovered in the vapor phase at 40 ° C and under a pressure of 1 to 2 bar, which makes the reliquefaction more expensive. Although more suitable for placement on a trailer, a membrane installation requires a flexible and expensive compression step, as long as the inlet pressure has to be adapted to correspond to a variable percentage of CO2. Economically speaking, with conventional acid gas treatment techniques, there is a tendency to release CO2 and natural gas into the atmosphere in the second phase of the post-stimulation period, which induces an increase in greenhouse effect, higher cost of CO2 supply and losses of natural gas.

DEFINITION AND OBJECT OF THE INVENTION The present invention aims at overcoming the above-mentioned drawbacks and in particular making it possible to perform hydraulic stimulation with CO2 which is more environmentally friendly and more efficient and which, moreover advantageously: - to limit the quantity of CO2 necessary for the stimulation 30 while making it easier to recycle for another stimulation operation, and to limit the loss of natural gas and the impact on the greenhouse effect of the flux recovered after stimulation. These objects are achieved according to the invention by a method of treating and separating an unconventional gas comprising a) an initial hydraulic stimulation step, using pressure injection of carbon dioxide. liquid in an operating well of an atypical deposit in which said unconventional gas is trapped in a rock, characterized in that it further comprises the following steps: b) recovering at low temperature after the stimulation operation , a mixture of natural gas and CO2, this low temperature recovery step comprising in particular a step b1) of evaluation, in the mixture of natural gas and recovered CO2, of the amount of CO2 and a step b2) of comparison of this amount of CO2 with respect to a predetermined threshold (S), c) separating at least a portion of the CO2 in the liquid, semi-liquid or solid state from said mixture of natural gas and CO2 recovered, with a different technique depending on whether the amount of CO2 is higher or lower than said predetermined threshold (S), d) temporarily store in at least one portable tank on a land vehicle the separated CO2 in the liquid state, semi-liquid or e) moving the land vehicle with said at least one tank to another site to reuse the separated CO2 and stored in a liquid, semi-liquid or solid state in order to perform a new stimulation operation on a new one. neighboring well, in the same unconventional gas field or in a nearby unconventional gas field. According to one aspect of the process according to the invention, when the amount of CO2 recovered is between 100% and said predetermined threshold (S) of the mixture of natural gas and recovered CO2, step c) of separating the CO2 from the liquid, semi-liquid or solid state from said mixture comprises a step of liquefying CO2 using a cryogenic distillation system.

According to another aspect of the process according to the invention, when the amount of CO2 recovered is between said predetermined threshold (S) and 20% of the mixture of natural gas and recovered CO2, step c) of separation of at least a portion of the liquid, semi-liquid or solid CO 2 from said mixture comprises a rapid cooling CO 2 condensation step passing CO 2 from a gaseous phase to a solid phase.

The evaluation step, in the mixture of natural gas and recovered CO2, of the amount of CO2 can be carried out using gas analysis by micro-chromatography. The predetermined threshold is advantageously equal to a value of between 55 and 65% and preferably equal to 60%. After step c) of separating at least a portion of the CO 2 at low temperature after the stimulating operation, the natural gas is recovered in gaseous form for later use. The mixture of natural gas and CO2 recovered at low temperature after the stimulation operation may further comprise an additional component such as nitrogen N2 and, in step c) separation of at least a portion At low temperature after the stimulation operation, the natural gas is recovered with the at least one additional component in gaseous form for later use. The invention also relates to a system for treating and separating an unconventional gas comprising an initial hydraulic stimulation unit, with a unit for injecting liquid carbon dioxide under pressure into a well of exploitation of a deposit. atypical in which said unconventional gas is trapped in a rock, characterized in that it further comprises: a low temperature recovery unit after a stimulation operation, a mixture of natural gas and CO2, this unit of low temperature recovery comprising in particular an evaluation unit, in the mixture of natural gas and recovered CO2, the amount of CO2 and a unit for comparing this amount of CO2 with respect to a predetermined threshold (S), a unit for separating at least part of the CO2 in the liquid, semi-liquid or solid state from said mixture of natural gas and CO2 recovered by said unit of recovery, with at least first and second different separation units depending on whether the amount of CO2 is greater or less than said predetermined threshold (S), at least one temporary storage unit, separated CO2 in the liquid state, semi-liquid or solid, and at least one land vehicle for moving said at least one temporary storage unit to another site to reuse the separated CO2 and stored in the liquid, semi-liquid or solid state, to feed a unit for injecting liquid carbon dioxide under pressure into a new stimulation unit on a new neighboring well, in the same unconventional gas field or in a nearby unconventional gas field. Advantageously, the first separation unit comprises a CO2 liquefaction unit using a cryogenic distillation system to ensure the separation of the CO2 in the liquid, semi-liquid or solid state from said mixture when the amount of CO2 recovered. is between 100% and said predetermined threshold (S) of the mixture of natural gas and recovered CO2. The second separation unit comprises a fast-cooling CO2 condensing unit for passing CO2 from a gas phase to a solid or semi-liquid phase when the amount of CO2 recovered is between said predetermined threshold (S ) and 20% of the mixture of natural gas and CO2 recovered. The unit of evaluation, in the recovered mixture of natural gas and CO2, of the amount of CO2, may comprise a gas analyzer by micro-chromatography.

According to a particular characteristic, the first and second different separation units are connected to said recovery unit by a system of valves controlled from the output information of said comparison unit. According to a possible embodiment, at least one of the first and second different separation units comprises a self-cooling unit in which the mixture of natural gas and recovered CO2 is subjected to a heat exchange with a previously separated natural gas stream before reuse. According to another possible embodiment, at least one of the first and second different separation units comprises a pre-cooling unit in which the mixture of natural gas and recovered CO2 is subjected to a heat exchange with the heat. latent a flow of CO2 previously separated in the second condensation separation unit and passed from the solid state to the liquid state before reuse.

According to another particular embodiment, at least one of the first and second different separation units comprises a complementary cooling unit comprising a Stirling cycle type refrigeration system.

According to another particular embodiment, at least one of the first and second different separation units comprises a distillation column with liquid, semi-liquid or solid CO2 recovery means at the bottom of the column at the bottom of the column. column and natural gas recovery means at the head of the column.

According to another particular embodiment, at least one of the first and second separation units comprises an expansion unit in a Joule-Thomson valve. According to yet another particular embodiment, at least one of the first and second separation units comprises a two-phase expansion turbine followed by a centrifugal device for separating the solid CO2. According to yet another possible embodiment, at least one of the first and second separation units comprises a convergent-divergent nozzle provided in its diverging portion with a liquid or solid CO2 capture slit. The system may comprise different storage units associated respectively with the first and second separation units. According to one particular characteristic of the invention, the system may furthermore comprise a unit for separating CO2 in the gas phase without a liquefaction device to ensure the separation of the CO2 in the gaseous state and the recovery of the natural gas from the mixture. of natural gas and recovered CO2, when the amount of CO2 is less than 20% in the mixture of natural gas and recovered CO2. Other features and advantages of the invention will become apparent from the following description of the particular embodiments given by way of example with reference to the accompanying drawings, in which: schematic of a hydraulic stimulation system according to a particular embodiment of the invention; FIG. 2 is a diagram showing the evolution of the concentration of the stimulation agent at the wellhead as a function of time from the end of the stimulation; Figure 3 is a block diagram showing a first example of a low temperature CO 2 separation technique using a bi-phasic turbo expander; FIGS. 4 and 5 are block diagrams showing second and third examples of low temperature CO 2 separation techniques each using a converging / diverging nozzle; and FIG. 6 is a block diagram showing a fourth example of a low temperature CO2 separation technique using a distillation column. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 symbolically shows an exemplary hydraulic stimulation system according to a particular embodiment of the invention. Module 3, which is shown as being mounted on a movable platform 5, such as a trailer or a truck, includes a liquid CO2 tank 8 and symbolizes hydraulic stimulation equipment with the use of liquid CO2 which, at through an injection device comprising at least one valve 4 and a pipe 6 makes it possible to inject liquid CO2 into a main pipe of a well 11 making it possible to carry out an initial hydraulic stimulation step, with the aid of injection under pressure of liquid carbon dioxide into the well 11 of exploitation of an atypical deposit 7 in which an unconventional gas is trapped in a rock, buried in relation to the ground level 9.

The reservoir 8 of liquid CO2 may itself contain liquid CO2 which has been recovered in another operating well of the same atypical deposit or of another atypical deposit nearby, according to the process according to the invention, which will be described below. Reference is first made to FIG. 2, which shows the evolution of the concentration of a carbon dioxide fracturing agent at the head of the well 11, when after the first stimulation step one recover a mixture of gas and fracturing agent. Initially, there is a proportion of fracking agent that is close to 100% and remains important in the first few moments (of the order of a few hours) of the well 11 operating process.

After a variable time t1, of the order of a few hours or a day, the concentration of fracturing agent drops to a predefined threshold S, which is advantageously defined between 55 and 65%, preferably about 60%. The CO2 concentration then continues to decrease over time to become after a few days equal to the usual concentration of CO2 in the natural gas extracted from the deposit to be exploited, that is to say a concentration in the order of 2 or a few percent. Reference 1 designates that part of the curve of FIG. 2 corresponding to a CO2 concentration above the threshold S and 2 denotes the portion of the curve of FIG. 2 corresponding to a CO 2 concentration below the threshold S. The The process according to the invention takes into account this decreasing evolution of the concentration of fracturing agent in order to apply different techniques for recovering CO2 at low temperature, depending on whether the concentration of CO2 is greater (curve 1) or lower (curve 2) than In the following description, the case will essentially be described in which two different techniques for recovering CO2 at low temperature are used, depending on whether the CO2 concentration is greater (curve 1) or lower (curve 2) than threshold S, but it is also possible to provide several different thresholds which would be staggered on the curve of FIG. 2 and to use a technique of ifferent CO2 recovery at low temperature at each crossing threshold.

According to the invention, after the stimulation operation, a mixture of natural gas and CO2 is recovered at the head of well 11, at low temperature. This low-temperature recovery step includes, in particular, a step of evaluating, in the mixture of natural gas and of recovered CO2, the amount of CO2 and a step of comparing this quantity of CO2 with respect to at least one predetermined threshold. S. The CO2 in the liquid, semi-liquid or solid state is then separated from the recovered natural gas and CO2 mixture with a different technique depending on whether the amount of CO2 is above or below the predetermined threshold S. For example, if a threshold S of 60% is predicted, when the CO2 concentration is between 100% and 60%, a CO2 liquefaction technique can be used, for example by adapting a carbon dioxide system. cryogenic distillation of a carbon purification unit used in an oxy-combustion process, whereas, when the CO 2 concentration is between 60% and 20%, it is possible to use a CO2 condensation technique with in fast cold (refrigeration), by direct passage from a gaseous phase to a solid phase. Then temporarily stored in at least one transportable tank 81 on a land vehicle 504, such as a trailer or a truck, CO2 separated in the liquid state, semi-liquid or solid. The land vehicle 504 is then moved with the tank 81 to another site to reuse the separated CO2 and stored in a liquid, semi-liquid or solid state in order to perform a new stimulation operation on a new neighboring well, in the same unconventional gas deposit or in a nearby unconventional gas field. The tank 81 will then play the same role as the tank 8 having been used in the stimulation module 3 used for the well 11. This is done for a succession of stimulation operations in a succession of operating wells with a stock There is a limited amount of CO2 that is recycled without the need for an expensive liquefaction operation, since the CO2 separated from the product gas is simply subjected to low temperature recovery operations that are relatively inexpensive and easy to implement. It is especially avoided to waste and liberate in nature large amounts of frac- tion agent as in conventional methods. Naturally, when the amount of CO2 recovered is less than about 20% in the recovered natural gas and CO2 mixture, while being greater than the residual rate of about 2% of CO2 present in the natural gas, it is possible to proceed with using conventional techniques for the separation of CO2 in the gas phase without a liquefaction device to ensure the separation of CO2 in the gaseous state and the recovery of natural gas from the mixture of natural gas and recovered CO2. The installation shown diagrammatically in FIG. 1 makes it possible to implement the method according to the invention. The system for treating and separating an unconventional gas according to the invention comprises a hydraulic stimulation unit 3, already mentioned above, mounted on a land vehicle 5 and equipped with a unit 4 for injection under pressure. liquid carbon dioxide in a well 11 for operating an atypical deposit 7, in which the unconventional gas is trapped in a rock, buried in relation to the level of the ground 9. The injection unit 4 is connected at the well via a pipe 6 comprises a system of valves allowing connections and disconnections between the hydraulic stimulation unit 3 and the well 11. According to an important characteristic, the installation according to the invention comprises a low temperature recovery unit 10 25 after a stimulation operation, a mixture of natural gas and CO2. This low-temperature recovery unit 10 comprises in particular an analysis module 42 comprising an evaluation unit, in the mixture of natural gas and of CO2 recovered at the head of the well 11, the amount of CO2 and a unit of analysis. comparing this amount of CO2 with respect to at least a predetermined threshold S. The evaluation unit, in the recovered mixture of natural gas and CO2, of the amount of CO2 may comprise a gas analyzer by micro-chromatography .

The low temperature recovery unit 10 comprises a liquid, semi-liquid or solid CO2 separation unit from the mixture of natural gas and CO2 recovered via line 13 from the well 11. , with at least first and second separation units 51, 52 different depending on whether the amount of CO2 is greater or less than the predetermined threshold S. The number of these 5 separation units 51, 52 is not limited to two and, FIG. 1 is an optional dotted line showing a third separation unit 53 for treating the CO 2 in a concentration lower than another threshold which is itself below the threshold S. FIG. 1 still shows at least one unit 81 for temporary storage of separated CO2 in the liquid, semi-liquid or solid state in the separation units 51, 52, 53. The temporary storage unit 81, but also the separation units 51, 52, 53 his Advantageously disposed on land vehicles 501 to 504, such as trailers or trucks, 15 to allow in particular to move a temporary storage unit 81 to another site to reuse the separated CO2 and stored in the liquid state, semi- liquid or solid, in order to supply a unit for injecting liquid carbon dioxide under pressure into a stimulation unit on a new well near well 11, in the same unconventional gas deposit or in a non-conventional gas deposit. conventional neighbor. Naturally vehicles 501 to 504 can be grouped into one or two vehicles for example. The analysis module 42 may itself be mounted on one of the vehicles 501 to 504 provided for the other modular elements of the system.

If we still consider Figure 1, we see that a pipe 13 connected to the wellhead 11 is connected by a valve 21 to a pre-cooling unit 41. This unit 41 may constitute a self-cooling. In this case, the mixture of natural gas and CO2 recovered through line 13 is heat-exchanged with a previously separated stream of natural gas flowing in lines 91, 92, 93 prior to local reuse or in a Natural gas network 94. A secondary pipe 12 is derived from the main pipe 13 to provide the analysis module 42 with a gas sample 35 for determining the CO2 concentration. Similarly, a secondary pipe 14 may be derived from the main pipe 15 receiving the mixture of gas and CO2 at the outlet of the pre-cooling unit 41 to provide the analysis module 42. a gas sample to determine the CO2 concentration. A three-way valve 22 makes it possible to connect the pipe 15 5, either via a pipe 31, to a first separation unit 51, or, via a pipe 17, to a second three-way valve 23. A secondary pipe 16 is derived the line 17 to provide the analysis module 42 with a gas sample for determining the concentration of CO2.

A three-way valve 23 is used to connect the pipe 17, either via a pipe 32, to a second separation unit 52 or, via a pipe 19, to a third three-way valve 24. A secondary pipe 18 is derived. of line 19 to provide the analysis module 42 with a gas sample to determine the CO2 concentration. The valve 24 allows a connection, via a pipe 33, with a third separation unit 53, which is optional, and optionally a connection to yet another valve associated, if necessary, with yet another separation unit, not shown in FIG. FIG. 1, for example a unit for separating CO2 in the gas phase without a liquefaction device for separating the CO2 in the gaseous state and recovering the natural gas from the mixture of natural gas and recovered CO2, when the amount of CO2 is less than 20% in the mixture of natural gas and recovered CO2, while remaining greater than the residual amount of CO2 in the natural gas, which is of the order of 2% or a few percent . Naturally, if there are for example only the two separation units 51 and 52, the valve 24 does not exist and the valve 23 may be a two-way valve.

Pipes 34, 35, 36 can connect the separating units 51 to 53 to each other and to a vent 37. At the outlet of each of the separation units 51, 52, 53, a pipe 91, 92, 93 can be used to recover the natural gas to send it to a storage area or a natural gas network 94, 35 which may be a transmission network or a distribution network.

The liquid, solid or semi-liquid CO2 is recovered at the outlet of each of the separation units 51, 52, 53 via the lines 61, 62, 63. The lines 62 and 63 are connected by a three-way valve. to a pipe 64 which is itself connected by a three-way valve 71 to the outlet pipe 61 of the separation unit 51 and to a pipe 65 connecting a tank 81 for storing liquid, solid or semi-solid CO2 liquid. Of course, again, if the separation unit 53 does not exist, the valve 72 may be a simple two-way valve.

Note that only the line 12 or 14 connecting with the analysis module 42 could be used, the lines 16 and 18 being optional. Moreover, FIG. 1 does not show the control links of the different valves from the information supplied by the analysis module 42 which is in fact a complete control and command system. During the entire recovery process of the mixture of natural gas and CO2 extracted from the well 11 through the pipe 13, the valve 4 associated with the module 3 remains closed, while the valve 21 is open.

At the beginning of the process of recovering the mixture of natural gas and CO2 extracted from the well 11 by the pipe 13, the analysis carried out within the analysis module 42 from the mixture transmitted via the pipe 12 or 13 normally indicates that the concentration of CO2 is greater than the threshold S of for example 60%. As a result, the valve 22 connects the conduit 15 to the inlet conduit 31 to the first separation unit 51, while the subsequent valves 23, 24 are closed. The analysis module 42 can be installed for example on any of the land vehicles 501, 502, 503. The evaluation unit 30 of the analysis module 42 can determine the concentration of CO2 in the return flow of the well 11, either continuously or intermittently, but with a good frequency, for example every thirty minutes. At the beginning of the recovery process of the mixture of natural gas and CO2 extracted from the well 11 via the pipe 13, or more generally, whenever the CO2 concentration is greater than the threshold S of for example 60%, is the separation unit 51 which is active and the valves 23 and 24 remain closed. On the other hand, when the CO2 concentration falls below the threshold S by, for example, 60%, while remaining greater than 5, ie at a minimum threshold, for example of the order of 20%, or at a second intermediate threshold, for example, close to 40%. %, the separation unit 51 is deactivated by the switching of the valve 22 in a position that closes the access to the pipe 31 and puts the pipe 15 in communication with the pipe 17. The valve 23 10 is then placed manually or automatically in a position that connects the pipe 17 to the inlet pipe 32 of the second separation unit 52, while the next valve 24 remains closed. Similarly, if there is an additional separation unit 53 with which a valve 24 is associated, and which is provided to ensure a separation of the CO2 when the CO2 concentration is between the intermediate threshold and the minimum threshold, when the analysis module 42 has defined that the CO2 concentration is in this range, the separation unit 51 is deactivated by the switching of the valve 22 in a position which closes the access to the pipe 31 and communicates the line 15 with the pipe 17, the separation unit 52 is also deactivated by the switching of the valve 23 in a position that closes the access to the pipe 32 and communicates the pipe 17 with the pipe 19. The valve 24 is then manually or automatically placed in a position which connects the pipe 19 to the inlet pipe 33 of the third separation unit 53, while any valve s following remain closed. The control of such subsequent valves associated with any additional separation units to be implemented in other ranges of CO2 concentration value is similar. The control of the various valves 22, 23, 24 is preferably reversible, that is to say that in the case where the CO2 concentration would rise to go from a value below the threshold S to a value located at above the threshold S, the separation unit 52 would be deactivated and the separation unit 51 would be reactivated.

As indicated above, at the outlet of the separation units 51, 52, 53, the natural gas is discharged via pipelines 91, 92, 93 to a network 94 of natural gas or to a storage zone, whereas the CO2 liquid, solid or semi-liquid is transferred through lines 61 to 65 and valves 71, 72 to the tank 81 mounted on a land vehicle 504. The pre-cooling unit 41 may be associated with one or the other of the separation units 51 to 53. More particularly, in the pre-cooling unit 41, the mixture from the well 11 or tubing 13 associated with the well 11 is subjected to a heat exchange cooling with: - either the cold natural gas stream separated in the separation units 51 to 53, which can then be recycled to one or more of these separation units 51 to 53 to undergo further purification, before being sent to a res water distribution, a storage area or be used on site; or the flow of liquid, solid or semi-liquid CO2 before it is introduced into the tank 81. In the case where a second unit of separation 52 is used, a condensing unit of CO 2 rapid cooling to pass the CO2 from a gaseous phase to a solid or semi-liquid phase when the amount of CO2 recovered is between the predetermined threshold S and about 20% of the mixture of natural gas and CO2 recovered, it is possible to recover the latent heat of the solid or semi-liquid CO2 before the CO2 is transferred in liquid form into the tank 81. The pre-cooling unit 41 may comprise a complementary cooling unit comprising a refrigeration system which may be a system optimized refrigeration cycle 30 or a Stirling cycle system (which is suitable for medium-sized mobile applications for example in submarines). The first separation unit 51 can comprise a CO2 liquefaction unit using a cryogenic distillation system, such as for example those used in carbon purification units, to ensure the separation of the CO 2 in the liquid state, semi-liquid or solid from the mixture from the pre-cooling unit 41 and allows to separate the largest part of the CO2 present in the mixture, when the proportion of CO2 in the mixture is between 100% and the predetermined threshold S included for example between 55% and 65%.

It is also possible to use a cryogenic distillation system with a rapid cooling CO 2 condensing unit to pass CO2 from a gas phase to a solid or semi-liquid phase in the separation unit 52. when the amount of CO2 recovered is between the predetermined threshold S and a lower threshold of, for example, between 20% and 40% of the mixture of natural gas and recovered CO2. FIG. 6 shows an example of a cryogenic distillation system 400 that can be implemented in the separation unit 52 and that can use a CFZ type of technique (for the English expression "Controlled Freeze Zone" designating a zone controlled freezing). In this case, a distillation column 400 comprises a lower stage 401, which is a conventional lower distillation stage, an intermediate stage 402, which constitutes the controlled freezing zone 20 and an upper stage 403, which is a higher distillation stage. classic. A mixture containing CO2 is introduced via a line 404 into the intermediate stage 402. The liquid or semi-liquid CO2 is collected at the base of the lower stage 401 and is heated in a heater 405. The shale gas is removed. at the top of the upper stage 403, is partially condensed in a condenser 406 and is then separated in the separator 407 into a gaseous phase which is evacuated and into a liquid phase which is recycled to the upper stage 403. The liquid produced in the upper stage 403 is collected and stored in a tank 408, from which it can be pumped to be sprayed into the intermediate stage 402. Installations such as those of FIG. 6 are described for example in the documents US 4533372. , US 8312738 and US 2009/0266107. As can be seen in FIG. 1 with the connecting pipe 37, it is possible to implement successively the separation units 51 and 52 which then operate in cascade, a first CO 2 separation being carried out within the separation unit 51 with a first separation process and the residual mixture with a reduced proportion of CO2 being introduced into the separation unit 52 with a second separation process more suitable for a mixture containing a lower CO2 concentration.

Similarly, with the connecting line 35 between the separation unit 52 and the separation unit 53, it can be seen that it is possible to continue the gas purification process within one or more additional units. , such as unit 53, which provide cooling and liquefaction of CO 2 or rapid cooling to pass CO 2 from a gas phase to a solid or semi-liquid phase. In this case, it is possible to use, for example, an expansion unit in a Joule-Thomson valve, as described for example in documents US Pat. No. 7,325,415, WO 2010/141996, WO 2011/026170 and WO 2010/141995. It is also possible to use a two-phase expansion turbine, as described, for example, in documents US 2011/226010, WO 2010/105820 and WO 2012/068588, which supplies energy for example for re-compressing natural gas or to provide additional refrigeration. A non-stick coating prevents CO2 from settling on the blades of the turbine and the solid CO2 can be recovered in a centrifugal device after the turbine. Figure 3 shows an example of a bi-phasic expansion turbine system 100. The gaseous mixture to be treated is introduced via an inlet 111 into a heat exchanger 101 providing condensation. The flow at the outlet of the heat exchanger-condenser 101 is applied to a compressor 102 whose output stream is applied to a heat exchanger 103. The output of the heat exchanger 103 is applied to a small refrigeration loop 104. An additional feedstock 30 itself feeds an exchanger-freezer 105 which produces on line 114 a stream of solid CO2 and also feeds an expansion stage 106, followed by a solid-gas separation unit 107, with an exit line 113. providing a solid CO2 stream and an exit line 112 providing a depleted CO2 stream which is reapplied to the exchanger 103. The lines 113, 114 in which a solid CO2 stream flows are applied to a compressor 108 of solids. At the outlet of exchanger 103, a line 116 feeds a liquid pump 109 to provide a pressurized liquid CO2 stream, a line 117 provides a depleted CO2 stream, and a line 118 provides SOx. and other freezer components.

At least one of the first and second separation units 51, 52, 53 may comprise a convergent-divergent nozzle provided in its diverging portion with a liquid or solid CO2 capture slit. FIG. 4 shows an example of a "twister", that is to say, swirling, convergent-divergent nozzle separation system 200, which is described, for example, in US 8475572 B2. A saturated gas flow 211 is introduced into a body 201 defining a chamber 202 generating a vortex and equipped with static guide vanes 203. The chamber 202 is followed by a Laval nozzle 204 with a converging tapered internal body 205 followed by a neck and a diverging portion. The supersonic velocities reached at the level of the neck correspond to a transformation of the energy in speed and provoke relaxation and cooling. By the cyclone effect of the vortex flow, a separation of CO2 droplets or flakes 206, which are projected outwardly of the nozzle and can be captured by a side slot 209, while a central diverging diffuser 207 provides at exit 208 a dry gas. The embodiment of FIG. 5 shows a separation system 300 similar to a convergent-divergent nozzle, which is described in particular in document US Pat. No. 7985278. The inlet 301 of the convergent portion 302 of the nozzle receives from a compressor a gaseous flow under subsonic regime. At the neck 303 of the nozzle equipped with valves 304 creating a swirling effect, a transient regime is obtained creating shock waves with a Mach number equal to 1. In the diverging portion 305, supersonic speeds with Mach number typically between 3 and 5 and there is a rapid isentropic relaxation. The CO2 freezing commences and solid particles 306 created in the outer zone 35 can be evacuated and captured by a lateral opening 307.

The technologies described with reference to Figures 3 to 6 are only examples and can be adapted for example to be as compact as possible. For example, the distillation column of FIG. 6 may be equipped with gaskets or the various heat exchangers may comprise different types of fins. Each separation unit 51, 52, 53 can itself be divided into several units operating in parallel and can be installed on different land vehicles.

Each separating unit 51, 52, 53 may itself also be divided into several units operating in series, each carrying out part of the process and being able to be installed on different land vehicles. As mentioned above, CO2 recovered in liquid form or semi-liquid slurry is stored in one or more tanks 81 placed on one or more trailers or the like for reuse. Each reservoir 81 may be associated with a separation unit 51, 52, 53, but a reservoir 81 may also be used with several or all of the separation units 51 to 53, particularly as shown in the example of FIG. The natural gas recovered during the separation phase with CO2 immediately following the CO2 stimulation stage, that is to say before the normal production of well 11, can be evacuated by a pipeline. to a storage or consumption area or can be consumed on the spot, for example to supply motors, such as motors of electrical energy production installations used for example for driving CO2 injection pumps, or for the operation of low temperature separation systems. 30

Claims (21)

CLAIMS1.A method of treatment and separation of an unconventional gas comprising a) an initial hydraulic stimulation step, using injection pressure of liquid carbon dioxide in a well (11) operating a atypical deposit in which said unconventional gas is trapped in a rock, characterized in that it further comprises the following steps: b) recovering at a low temperature after the stimulation operation, a mixture of natural gas and CO2, this low-temperature recovery step comprising in particular a step b1) of evaluating, in the mixture of natural gas and recovered CO2, the amount of CO2 and a step b2) of comparing this quantity of CO2 with a threshold predetermined (S), c) separating at least a portion of the CO2 in the liquid, semi-liquid or solid state from said mixture of recovered natural gas and CO2, with a different separation technique depending on whether the amount of CO2 is above or below said predetermined threshold (S), d) temporarily storing in at least one transportable tank (81) on a land vehicle (504) the separated CO2 in the liquid, semi-liquid state. liquid or solid, and e) moving the land vehicle (504) with said at least one tank (81) to another site to reuse the separated CO2 and stored in a liquid, semi-liquid or solid state to realize a new stimulation operation on a new neighboring well, in the same unconventional gas field or in a nearby unconventional gas field. 2. Method according to claim 1, characterized in that when the amount of CO2 recovered is between 100% and said predetermined threshold (S) of the mixture of natural gas and recovered CO2, step c) of separation of the CO2 in the liquid state, serai-liquid or solid from said mixture comprises a step of liquefying CO2 using a cryogenic distillation system. 35 3028554 24 3. Method according to claim 1, characterized in that when the amount of CO2 recovered is between said predetermined threshold (S) and 20% of the mixture of natural gas and recovered CO2, step c) of separation of at least a portion of the liquid, semi-liquid or solid CO2 from said mixture comprises a rapid cooling CO 2 condensation step passing CO 2 from a gaseous phase to a solid phase. 4. Method according to claim 1, characterized in that the step 10 evaluation, in the mixture of natural gas and recovered CO2, the amount of CO2, is carried out using gas analysis by micro-chromatography. 5. Method according to any one of claims 1 to 4, characterized in that said predetermined threshold is equal to a value between 55 and 65% and preferably equal to 60%. 6. Method according to any one of claims 1 to 5, characterized in that, after step c) of separation of at least a portion of the CO2 at low temperature after the stimulation operation, the natural gas is recovered in gaseous form for later use. 7. Process according to any one of claims 1 to 6, characterized in that the mixture of natural gas and CO2 recovered at low temperature after the stimulation operation further comprises at least one additional component such as nitrogen N2 and in that during step c) separation of at least a portion of the CO 2 at low temperature after the stimulation operation, the natural gas is recovered with said at least one additional component in gaseous form for further use. 8. An unconventional gas treatment and separation system comprising an initial hydraulic stimulation unit (3), with a unit (4) for injecting liquid carbon dioxide under pressure into a well (11). operating an atypical deposit in which said unconventional gas is trapped in a rock, characterized in that it further comprises: a unit (10) for recovery at low temperature after a stimulation operation, a mixture of natural gas and CO2, this low temperature recovery unit (10) comprising in particular an evaluation unit (42), in the mixture of natural gas and recovered CO2, the amount of CO2 and a unit (42). ) for comparing this quantity of CO2 with respect to a predetermined threshold (S), a unit for separating at least a portion of the CO2 in the liquid, semi-liquid or solid state from said mixture of natural gas and of CO2 recovered by said one recovery unit, with at least different first and second separation units (51, 52, 53) depending on whether the amount of CO2 is higher or lower than said predetermined threshold (S), at least one temporary storage unit (81) Separated CO2 in the liquid, semi-liquid or solid state, and at least one land vehicle (501, 502, 503, 504) for moving said at least one temporary storage unit (81) to another site for re-use. CO2 separated and stored in a liquid, semi-liquid or solid state, in order to supply a unit (4) for injecting liquid carbon dioxide under pressure into a new stimulation unit (3) on a new neighboring well (11), in the same unconventional gas field or in a nearby unconventional gas field. 9. System according to claim 8, characterized in that the first separation unit (51) comprises a CO2 liquefaction unit using a cryogenic distillation system to ensure the separation of the CO2 in the liquid state, liquid or solid from said mixture when the amount of CO2 recovered is between 100% and said predetermined threshold (S) of the mixture of natural gas and CO2 recovered. 10. System according to claim 8, characterized in that the second separation unit (52) comprises a fast-cooling CO2 condensing unit for passing CO2 from a gaseous phase to a solid or semi-solid phase. liquid when the amount of CO2 recovered is between said predetermined threshold (S) and 20% of the mixture of natural gas and recovered CO 2. 11. System according to any one of claims 8 to 10, characterized in that the evaluation unit (42), in the mixture of natural gas and of recovered CO2, of the amount of CO2, comprises an analyzer of gas by micro-chromatography. 12. System according to any one of claims 9 to 11, characterized in that the first and second different separation units (51, 52, 53) are connected to said recovery unit (81) by a system of valves ( 22, 23, 24, 71, 72) controlled from the output information of said comparison unit (42). 13. System according to any one of claims 9 to 12, characterized in that at least one of the first and second different separation units (51, 52, 53) comprises a unit (41) of self pre- cooling in which the mixture of recovered natural gas and CO2 is subjected to heat exchange with a separate natural gas stream before reuse. 14. System according to any one of claims 9 to 13, characterized in that at least one of the first and second different separation units (51, 52, 53) comprises a pre-cooling unit in which the mixture of recovered natural gas and CO2 is subjected to heat exchange with the latent heat of a CO2 stream previously separated in the second condensing separation unit (52) and subjected to a passage from the solid state to the liquid state before reuse. 15. System according to any one of claims 9 to 14, characterized in that at least one of the first and second different separation units (51, 52, 53) comprises a complementary cooling unit comprising a refrigeration system. Stirling cycle type. 3028554 27 16. System according to any one of claims 9 to 15, characterized in that at least one of the first and second different separation units (51, 52, 53) comprises a distillation column (400) with means recovering liquid, semi-liquid or solid CO2 at the bottom of the column and means for recovering the natural gas at the top of the column. 17. System according to any one of claims 9 to 16, characterized in that at least one of the first and second separation units (51, 52, 53) comprises an expansion unit in a Joule-Thomson valve. . 18. System according to any one of claims 9 to 16, characterized in that at least one of the first and second separation units (51, 52, 53) comprises a two-phase expansion turbine followed by a device centrifuge to separate solid CO2. 19. System according to any one of claims 9 to 16, characterized in that at least one of the first and second separation units (51, 52, 53) comprises a convergent-divergent nozzle (200; 300). provided in its diverging portion with a slot (209; 307) for capturing liquid or solid CO2. 20. System according to any one of claims 9 to 19, characterized in that it comprises different storage units associated respectively with the first and second separation units (51, 52, 53). 21. System according to any one of claims 9 to 20, characterized in that it further comprises a gas phase CO2 separation unit without liquefaction device for separating the CO2 in the gaseous state and the recovering natural gas from said recovered natural gas and CO2 mixture, when the amount of CO2 is less than 20% in the mixture of natural gas and recovered CO2.