WO2011001792A1

WO2011001792A1 - Method and equipment for dissociation of methane hydrate

Info

Publication number: WO2011001792A1
Application number: PCT/JP2010/059597
Authority: WO
Inventors: 秀紀皆川; 英夫成田
Original assignee: 独立行政法人産業技術総合研究所
Priority date: 2009-07-02
Filing date: 2010-06-07
Publication date: 2011-01-06
Also published as: JP5067814B2; JP2011012451A

Abstract

In order to collect methane from methane hydrate (MH) present in a methane hydrate bed with low electric-power consumption safely and efficiently, provided is a method which comprises: reducing the ambient pressure of a methane hydrate sediment bed to a prescribed level; injecting an electrolyte solution into the resulting methane hydrate sediment bed; passing an electric current between multiple electrodes inserted into the methane hydrate sediment bed to raise the temperature of the methane hydrate sediment and thus dissociate the methane hydrate into methane and water; and continuing the passing of an electric current therebetween, while preferentially discharging the methane thus separated and thereby keeping the rate of penetration of water into the methane hydrate sediment bed.

Description

Methane hydrate decomposition method and apparatus

The present invention relates to a method for separating and producing methane gas from methane hydrate deposits and an apparatus therefor.

Methane hydrate (hereinafter referred to as MH) in the waters near Japan has a three-dimensional network structure of water molecules under low temperature and high pressure conditions, and methane molecules enter the internal gaps and become ice-like crystals. Therefore, the amount of resources equivalent to about 100 years of Japan's annual natural gas consumption has been estimated, and development as domestic energy in the future is expected.
The methods for recovering methane gas from MH reservoirs currently under investigation include thermal stimulation, decompression, and inhibitor injection, etc., all of which separate and decompose MH into gas and water in situ. Is a way to produce.
Unlike oil and natural gas, which are conventional energy resources, MH is solid and has no fluidity. Therefore, MH is treated as a substance that hinders the flow of gas and water, and the permeability value indicating the ease of fluid flow in the reservoir in the presence of MH is an important factor in evaluating gas productivity. It becomes.

As shown in Patent Document 1 and Patent Document 2, the hot water injection method, which is one of the thermal stimulation methods, is a method of promoting the decomposition of MH by injecting hot water to raise the temperature of the reservoir. Therefore, high gas productivity is expected compared to other recovery methods.
However, the growth and regeneration of MH due to the methane gas and water generated by the temperature drop and decomposition of the hot water inside the sedimentary layer are promoted as a result of MH flowing into the undecomposed and low temperature downstream area, and as a result There is a concern that the permeability will be significantly reduced and it will become impossible to continue the hot water injection.

In addition, the depressurization method in which the pressure of the MH layer is reduced and maintained in the decomposition region causes ice or MH to be regenerated due to the hydrate decomposition and the endothermic reaction that accompanies the rapid depressurization process, and the permeability is significantly reduced. .

On the other hand, as a thermal attack method (heating method) different from the thermal stimulation method, Patent Document 3 proposes conducting electricity between two wells and electrically heating the reservoir. As a result of experiments by a person, even in an experiment simulating 5 cm between wells, current heating at AC 200 V is not performed unless salt water with a seawater concentration is passed through the MH core deposit in advance as an electrolyte solution before the current is passed. Little was done.

There are two possible causes for this.
First of all, immediately after the methane hydrate embryo state, residual water and gas and methane hydrate are present in the pores of the sediment, but the electrical resistance is low because the amount of water required for energization is very small. Large energization is impossible.
Next, when the methane hydrate deposit is saturated with deionized / distilled water, the pores are filled with deionized / distilled water, etc., but the electrical resistance of deionized / distilled water is very high (1 .67X10 ⁵ ohms m) for the energization current at the applied voltage AC200V becomes less than 50 .mu.A, it is conceivable that electrical heating is not performed effectively.
Certainly, if a high voltage of 1000 V or higher is applied, there is a possibility that electric heating will be performed, but at such a high voltage, there is a possibility that a discharge will occur between the electrodes that have entered the well, such as an explosion. Risk increases.

JP 2009-30378 A JP 2005-213824 A U.S. Pat. No. 3,916,993

The object of the present invention is to enable effective MH heating even at low voltage and low power, thereby enabling more efficient MH decomposition over a long period of time compared to the conventional MH decomposition method described above. And providing a safe MH decomposition method and apparatus therefor.

The inventors of the present invention reduced the MH deposit layer to the decomposition region by the reduced pressure method, and then maintained the electrolyte solution and moisture separated from the MH between the electrodes for current heating, and the flow of fluid in the MH deposit. By increasing the permeability value representing the ease, it is possible to promote the temperature rise and decomposition of the MH deposit over a long period of time, even with a low voltage, and to produce highly efficient methane gas from the MH deposit. Based on the knowledge that it is possible, the following methane hydrate decomposition method was developed.

A step of reducing the pressure around the methane hydrate deposit layer to a predetermined pressure, a step of injecting an electrolyte solution into the methane hydrate deposit layer, and energization between a plurality of electrodes inserted in the methane hydrate deposit layer Preferentially treating the methane decomposed in the step of performing the treatment, raising the temperature of the methane hydrate deposit by the energization treatment, decomposing the methane hydrate into methane and moisture, and decomposing the methane hydrate. The methane hydrate comprising the step of continuing the energization treatment while maintaining the permeability value of the water generated in the step of discharging and decomposing the electrolyte solution and methane hydrate in the layer of the methane hydrate deposit Disassembly method.

Furthermore, in order to realize this methane hydrate decomposition method automatically and efficiently, the following methane hydrate decomposition equipment was developed.
That is, via a plurality of wells reaching the methane hydrate deposit layer, a pressure reducing device for reducing the pressure around the methane hydrate deposit layer to a predetermined pressure, and via the plurality of wells, the methane hydrate An electrolyte solution injection device for injecting an electrolyte solution into the deposit layer, and a methane discharge passage that communicates with the plurality of wells and discharges decomposed methane from the upper or middle height of the methane hydrate deposit layer. And a plurality of well electrodes inserted into the methane hydrate deposit layer through each of the plurality of wells, and a predetermined voltage and a predetermined current between the plurality of well electrodes. An apparatus for decomposing methane hydrate comprising a power supply device to be applied, and a control device for controlling power application by the pump, the electrolyte solution injection device, a discharge valve, and the power supply device

In the above methane hydrate decomposition apparatus, a plurality of electrodes are arranged at one well electrode and the apex of an equilateral triangle centering on this, so that the above methane hydrate decomposition can be performed with lower power. The predetermined voltage is applied alternately between the well electrode located at the center and each well electrode arranged at the apex of the equilateral triangle.

According to the methane hydrate decomposition method of the present invention, the MH deposit layer is depressurized to a predetermined pressure, and a low voltage is applied between a plurality of electrodes inserted in the MH deposit layer in the presence of an electrolyte solution. The initial energization heating is smoothly started to increase the temperature of the MH deposit, and MH is decomposed into methane and moisture, and the separated methane is discharged preferentially from the MH deposit layer. The electrolyte solution and the water generated in the step of decomposing MH can be maintained, and an efficient energization process can be continued for a long time while maintaining the permeability value.

According to the methane hydrate decomposition apparatus of the present invention, the above methane hydrate decomposition method can be carried out efficiently.

It is the figure which showed the whole structure of the methane hydrate decomposition | disassembly apparatus by this invention. It is the figure which showed the work process of the well until it performs the gas production from the methane hydrate by this invention. It is the figure which showed the whole structure of the experimental apparatus for verifying the decomposition | disassembly performance of the methane hydrate by this invention. It is the figure which contrasted the decomposition equilibrium temperature pressure phase diagram (a) of methane hydrate, and the decomposition equilibrium temperature pressure phase diagram (b) of xenon hydrate. The core temperature (a), the core electrical resistivity (b), and the amount of released gas (c) for each hydrate saturation rate when AC energization (300 mA) was performed under the decomposition equilibrium pressure condition of xenon hydrate used in the experiment. ) Is a diagram showing experimental results of time variation (horizontal axis elapsed time) of discharged water (d). It is the figure which showed the whole structure of another Example by this invention. It is the schematic at the time of applying another Example to a horizontal well.

Example 1
FIG. 1 shows the overall configuration of a methane hydrate collection plant, and FIG. 2 shows steps (a) to (g) from the construction of such a plant to gas production.
In the process of FIG. 2 (a), first, a plurality of wells 1 reaching the MH deposit are excavated, and the well openings are sealed by the wellhead device 2, thereby preventing the inflow of seawater. An underwater pipe 3 extends to the sea at the top of the anti-outlet device 2, and a pipe introduction port 4 and a methane gas recovery valve 5 are installed at the tip.

In the process of Fig. 2 (b), the wall of the excavation hole is cemented with a casing 6 so that the entire wall is electrically insulated, and the formation water from the sedimentary layer is transferred to the well at the excavation stage. The well is filled with water up to a height corresponding to the formation pressure (initial head pressure: about 9.0 MPa).

2 In the steps shown in FIGS. 2 (c) and 2 (d), holes and cracks are formed in the wall and part of the deposit by perforation 7, and then water in the well is pumped up by an anti-bottom pump 8 as a decompression device. The pressure is reduced by lowering the pressure (water head pressure during decompression: about 4.0 MPa).
By this decompression operation, MH in the deposited layer is partly decomposed and gas may be released through 7 parts of perforation, but the core temperature decrease due to the endothermic effect is caused by the energization heating in the step (g) described later. Compensated and the degradation rate can be maintained.

In the process of Fig. 2 (e), seawater, which is an electrolyte solution, is injected into the well by a water pump 9 as an electrolyte solution injection device, and the seawater is injected into the sedimentary layer by filling the seawater to the initial head pressure. After that, the water pump 9 is disconnected.

Next, in the step of FIG. 2 (f), the sea bottom in the well is pumped up again by the anti-bottom pump 8 and the head pressure is reduced (head pressure during decompression: about 4.0 MPa). 8 is extracted from the well and the well electrode 10 is inserted so as to correspond to 7 parts of perforation.

The power from the power supply device 11 is supplied to the well electrode 10 inserted into each of the plurality of wells 1 in the process of FIG. 2 (g) via the pipe introduction port 4, and decomposition and collection of MH are started. To do.

For example, if the diameter of the well electrode 10 is 5.04 centimeters, the length is about 1.0 m, and the distance between the two electrodes is 3 m, when 50 Hz AC 10 A to 40 A (100 V to 500 V) is energized The internal temperature rises and hydrate decomposition proceeds around the well electrode 10.
At that time, the endothermic effect accompanying the progress of decomposition is compensated by energizing the well electrode 10.

As MH is decomposed, MH is separated into methane gas and moisture, and methane gas having a small specific gravity rises in the well 1. Therefore, methane gas can be collected by opening the methane gas recovery valve 5 provided in the methane discharge passage at the top of the subsea pipe 3 in accordance with the start of MH decomposition.
Thus, by collecting methane gas preferentially from the upper end of the well 1, moisture generated by the decomposition of MH fills the pores in the sedimentary layer, and the electric resistance around the well 1 is reduced and penetrated. The rate can be maintained or increased. Furthermore, if a gas phase remains in the MH layer from the beginning of energization, this can also be eliminated to increase the penetration rate.

In this embodiment, the methane gas is collected from the upper end of the well 1 through the methane gas recovery valve 5, but a methane extraction pipe may be separately provided in the middle portion of the MH deposit layer. In the MH deposit layer, the permeability should be maintained to such an extent that the electrolyte solution and the separated water can sufficiently flow.

As described above, in the present invention, after passing the electrolyte solution, the pressure is reduced to the MH equilibrium decomposition pressure and energized to raise the temperature of the deposited layer, decompose MH, and then infiltrate the deposit. Prevents the rate from decreasing and even increases the penetration rate.
That is, in the present invention, the water generated by decomposing MH by energization stays below and generates a new energized region, thereby increasing the energized region of the MH deposited layer, that is, increasing the energized / heated region. The decomposition of MH can be promoted, and the methane gas production amount accompanying MH decomposition can be increased.
The present invention is also applicable to a horizontal well in which MH deposits are energized and heated in the horizontal direction at a predetermined depth.

Note that the methane gas recovery valve 5, the anti-bottom pump 8, the power supply device 11 and the like are used to detect pressure and temperature sensors installed at appropriate locations in the well 1 as well as the permeability of the electrolyte solution and separated water. While monitoring the detection value of the solution electrical resistance sensor, the operator may adjust the value to an optimum value by using an adjuster provided on the control panel. Based on these detection values, a microcomputer (not shown) or the like may be used. It may be controlled sequentially by the control device.

By the way, in order to simulate methane gas production from an MH deposit layer in a submarine underground having a depth of 1000 m or more, which is a very general MH deposit layer, the present inventors have developed a xenon gas hydrate (hereinafter referred to as “xenon gas hydrate”) as shown in FIG. And XH) and an experimental apparatus using an isostatic core holder 12 was used.
Referring to FIG. 4 (a) showing the MH decomposition equilibrium temperature and pressure phase diagram (a), in the case of MH deposits, the reduced pressure from 9 MPa to 4 MPa is the equilibrium temperature from 12.1 ° C. to 4. This corresponds to a temperature change up to 2 ° C.
On the other hand, referring to FIG. 4 (b) showing the decomposition equilibrium temperature pressure phase diagram of XH, in the case of XH deposits, the pressure change from 0.9 MPa to 0.4 MPa is from the equilibrium temperature of 17.7 ° C. This corresponds to a temperature change up to 9.7 ° C.
Therefore, by using the XH deposit, it is possible to investigate the effect of the same degree of equilibrium temperature change with a pressure change of about 1/10 compared with the MH deposit.

In this experimental apparatus, an XH deposit is produced in the hydrostatic core holder 12, the core permeability is measured by passing water through the core, and the core pressure is reduced by controlling the back pressure control valve 13. To do. When producing the XH deposit, first, a deposit obtained by shaping Toyoura standard sand with a controlled moisture content into a diameter of 5.08 cmφ × 12 cm in length is placed in the rubber sleeve 14 inside the core holder 12. Using the restraint pressure control device 15, the restraint pressure is monitored by the restraint pressure pressure gauge 18 through the rubber sleeve 14 and the end caps 16 and 17, and a restraint pressure of 5 MPa is applied to the deposit 19. Xenon gas is flowed from the xenon gas cylinder 20 to the deposit 18 through the gas flow rate controller 21, the back pressure control valve 13 is controlled, the core internal pressure is monitored by the core pressure gauge 22, and the pressure is increased to 0.9 MPa. Thereafter, the output of the core thermocouple 23 is monitored, the core temperature is cooled to 1 ° C. by the jacket-type core temperature controller 24, xenon gas hydrate is allowed to develop in the pores of the deposit, and the xenon gas hydrate deposit Get 19. Thereafter, the core temperature is raised to 9 ° C. by the jacket type core temperature controller 24.

While maintaining a temperature of 9 ° C. and a pressure of 0.9 MPa, distilled water was injected from the end cap 16 at one end of the core located at the lower part of the core by the liquid injection device 25 in the core, and the core inlet pressure and the outlet pressure were adjusted by the differential pressure gauge 26. The pressure difference is measured and the permeability Kw is measured. Next, an electrolyte solution simulating seawater with NaCl 3.5% is injected by the core liquid injection device 25 instead of distilled water, and the permeability Ke is measured. Thereafter, the pressure is reduced to 0.4 MPa, and the temperature / pressure equilibrium state in the xenon gas hydrate formation decomposition is maintained.
By supplying 50 Hz AC 100 mA to 500 mA (AC voltage 20 V to 600 V) with the AC power supply device 27 using the end caps 16 and 17 at both ends of the core as electrodes, the internal temperature of the core increases by 1 ° C. to 40 ° C. The decomposition of the rate proceeds smoothly, and the xenon gas is partially discharged from the piping together with the decomposed water through the back pressure control valve 13 on the side opposite to the solution press-fitting side of the core holder 12. The discharged gas and water are measured for discharge by a gas integrated flow meter 28 and a water integrated recorder 29, respectively. The remaining moisture is retained inside and penetrates into the MH decomposition region, so that the electric resistance gradually decreases and the permeability increases gradually.

When XH deposits having different hydrate saturation rates are subjected to constant current (AC 300 mA) by alternating current (50 Hz) under the decomposition equilibrium temperature and pressure conditions, the input power and the core electrical resistance increase as the hydrate saturation rate increases.
However, when energization is performed as described above in a state where the electrolyte solution has permeated and the hydrate is decomposed as the core temperature rises, the input power / core electric resistance decreases with time and gradually approaches a constant value. This is because the water produced by the decomposition of the hydrate stays in the core and increases the current-carrying area. That is, it turns out that the water produced | generated by decomposition | disassembly of the hydrate by electricity supply has increased electricity supply efficiency and heating efficiency.

FIG. 5 shows the core temperature (a), the core electrical resistivity (b), and the amount of released gas (c) with respect to each hydrate saturation rate when AC energization (300 mA) is performed under the XH decomposition equilibrium pressure condition. The experiment result of the time change (horizontal axis elapsed time) of discharged water (d) is shown.
In particular, when the XH saturation rate is 10% or more, the core temperature increases, the amount of gas released increases, the core electrical resistance decreases, and the input electrical resistance decreases as XH decomposes. From this, it can be confirmed that the water generated along with the decomposition of XH stays in the core and increases the energization region.

Regarding the production behavior of gas and water released during hydrate decomposition, the effect of gravity was evaluated by changing the core outlet in the upper, lower, and horizontal directions.
That is, when the discharge port is at the bottom, the residual water inside the core is discharged first, so that the electrical resistance inside the core increases, the heating efficiency decreases, and the gas production also decreases.
However, when the discharge port is provided at the top or in the horizontal position as in the experimental example, no significant increase in electrical resistance and no change in the gas / water production ratio are observed.
When the discharge port is upper and horizontal, it means that the water generated by the hydrate decomposition stays in the lower part, reducing the electrical resistance of the deposit and increasing the energization efficiency and heating efficiency. This experimental result agrees with the energization experimental result of the deposits having different hydrate saturation rates.

Based on the above experimental results, for example, current heating of an actual hydrate well by two electrodes with a diameter of 5.08 cm (2 inches), a length of 1.0 m (about 3.0 feet), and a distance of 3 m. Assuming the case of performing the above, it is possible to sufficiently reduce the electric power necessary to maintain the decomposition of MH by energization of AC25A (300V).

(Example 2)
In energization heating between two wells, the length of the energization / heating area is limited by the length between the wells and the well electrode, and the range is limited by the distance between wells × the length of the well electrode. Equivalent to. It may also be assumed that a wide range of MH deposition layers need to be effectively energized and heated with low power.
Therefore, in Example 2, as shown in FIG. 6, the energization region of the electrode is limited to 1 m (about 3 feet) in length, and an equilateral triangle having one side of 10.0 m centered on one well electrode 30. After the three well electrodes 31a to 31c are arranged at the apex, the center well electrode 28 similar to that of the first embodiment and the three well electrodes 31a to 31c arranged at the apexes of the equilateral triangle are arranged. If the heating is performed between the

well electrodes

31a, 31b, and 31c after each of them is energized and heated by the power supply device 32, a diameter of 8.67m centered on the well electrode 7 is obtained. It is possible to decompose the MH deposition layer uniformly in the depth direction.
The steps shown in FIGS. 2A to 2G are the same in this embodiment.
In this embodiment, the current heating between the well electrode 30 and each of the well electrodes 31a to 31c is about AC25A (800V), and the current heating between the

well electrodes

31a, 31b, 31c. With regard to MH, it is possible to maintain the decomposition of MH at about AC 15 to 25 A (700 to 1000 V).

Furthermore, if these wells are connected to each other at a plurality of locations in the vertical direction by horizontal wells as shown in FIG. 7, and the current is heated at each horizontal well position, the thickness of the MH layer to be mined Even if the MH layer is distributed over a wide range in the layer direction, it is possible to sequentially promote the decomposition of the hydrate.
In this case as well, as in the first embodiment, the operator may adjust the value to an optimum value using an adjuster provided on the control panel, or based on these detected values, a control device such as a microcomputer may be used. It may be controlled sequentially.

As described above, according to the present invention, it is possible to extract methane from MH safely, efficiently and with low power, and have a resource amount corresponding to about 100 years of natural gas consumption. Can contribute to promoting the use of MH.

DESCRIPTION OF SYMBOLS 1 Well 2 Wellhead device 3 Underwater pipe 4 Piping inlet 5 Methane gas recovery valve 6 Casing 7 Perforation 8 Bottom pump 9 Water pump 10 Well electrode 11 Power supply device 12 Hydrostatic core holder 13 Back pressure control valve 14 Rubber sleeve 15 Restraint pressure control device 16 End cap (lower part)
17 End cap (top)
18 Constrained Pressure Pressure Gauge 19 Deposit 20 Xenon Gas Cylinder 21 Gas Flow Controller 22 Core Pressure Gauge 23 Core Thermocouple 24 Jacket Type Core Temperature Controller 25 In-core Liquid Press-In Device 26 Differential Pressure Gauge 27 AC Power Supply Device 28 Gas Integrated Flow Meter 29

Water Accumulation Recorder

30, 31a-31c Well Electrode 32 Power Supply for Well Heating

Claims

Reducing the pressure around the methane hydrate sediment layer to a predetermined pressure;
Injecting an electrolyte solution into the methane hydrate deposit layer;
Conducting a current treatment between a plurality of electrodes inserted into the methane hydrate deposit layer;
Increasing the temperature of the methane hydrate deposit by the energization treatment, and decomposing the methane hydrate into methane and moisture;
The methane decomposed in the step of decomposing the methane hydrate is preferentially discharged, and the permeability value of the water generated in the step of decomposing the electrolyte solution and methane hydrate in the layer of the methane hydrate deposit is maintained. A method for decomposing methane hydrate comprising the step of continuing the energization process above.
A pressure reducing device for reducing the pressure around the methane hydrate sediment layer to a predetermined pressure through a plurality of wells reaching the methane hydrate sediment layer;
An electrolyte solution injection device for injecting an electrolyte solution into the methane hydrate deposit layer through the plurality of wells;
A methane gas recovery valve provided in a methane discharge passage for discharging decomposed methane from the upper or middle height of the methane hydrate sediment layer, and communicating with the plurality of wells;
A plurality of well electrodes inserted into the methane hydrate deposit layer through each of the plurality of wells;
A power supply device that applies power of a predetermined voltage and a predetermined current between the plurality of well electrodes;
An apparatus for decomposing methane hydrate comprising the pump, the electrolyte solution injection device, a methane gas recovery valve, and a control device for controlling power application by the power supply device.
The plurality of electrodes are composed of one well electrode and a well electrode arranged at each vertex of the equilateral triangle centered on the well electrode, and the well electrode located at the center and the vertex of the equilateral triangle The apparatus for decomposing methane hydrate according to claim 2, wherein the predetermined voltage is alternately applied to each well electrode arranged.