CN-117248870-B - Method for improving recovery ratio of natural gas hydrate in frozen soil area

CN117248870BCN 117248870 BCN117248870 BCN 117248870BCN-117248870-B

Abstract

The invention provides a method for improving recovery ratio of natural gas hydrate in a frozen soil area. The method comprises the steps of discharging free gas from a natural gas hydrate reservoir in a target frozen soil area in a depressurization mode, enabling the temperature of the natural gas hydrate reservoir in the target frozen soil area to be reduced to be less than or equal to 0 ℃ and more than or equal to minus 6 ℃ and enabling the pressure of the reservoir to reach 0.8-1.2 times of the balance pressure of the natural gas hydrate, then injecting mixed gas of CO 2 and H 2 into the natural gas hydrate reservoir in the target frozen soil area for carrying out multi-round throughput purging, wherein a braising well is carried out between each round of throughput purging, the temperature of the natural gas hydrate reservoir in the target frozen soil area is kept to be less than or equal to minus 6 ℃, and after the throughput purging is finished, injecting CO 2 into the natural gas hydrate reservoir in the target frozen soil area for purging to recover free H 2 and carrying out geological sealing of CO 2 . The method can obviously improve the recovery ratio of the natural gas in the frozen soil area, effectively ensure the stability of the reservoir and realize the geological sequestration of the carbon dioxide.

Inventors

SUN CHANGYU
ZHU YIJIAN
HUANG XING
LI HAO
CHEN GUANGJIN

Assignees

中国石油大学(北京)

Dates

Publication Date: 20260512
Application Date: 20231009

Claims (9)

1. A method for enhancing recovery of natural gas hydrate from a frozen earth region, wherein the method comprises: The method comprises the step of depressurization exploitation, namely discharging free gas from a natural gas hydrate reservoir in a target frozen soil area in a depressurization mode, so that the reservoir pressure of the natural gas hydrate reservoir in the target frozen soil area is reduced to a first rated pressure and the temperature of the reservoir reaches a first rated temperature, wherein the first rated pressure is 0.8-1.2 times of the balance pressure of the natural gas hydrate, and the first rated temperature is less than 0 ℃ and is more than or equal to minus 6 ℃; The method comprises the steps of throughput purging, namely, after the reservoir pressure of a natural gas hydrate reservoir in a target frozen soil area is reduced to a first rated pressure and the reservoir temperature reaches the first rated temperature, injecting mixed gas of CO 2 and H 2 into the natural gas hydrate reservoir in the target frozen soil area for multi-pass throughput purging, wherein a well is closed in each pass of throughput purging, the temperature of the natural gas hydrate reservoir in the target frozen soil area is maintained to be less than or equal to 0 ℃ and more than or equal to minus 6 ℃ in the multi-pass throughput purging process, and the mole percentage of CO 2 and H 2 in the mixed gas of CO 2 and H 2 is 80:20-20:80; After the huff and puff purging step is finished, injecting CO 2 into the natural gas hydrate reservoir of the target frozen soil area to purge and recover free H 2 and performing geological sequestration of CO 2 ; In the process of discharging free gas from the natural gas hydrate reservoir in the target frozen soil area in a depressurization mode, if the reservoir temperature of the natural gas hydrate reservoir in the target frozen soil area is higher than a first rated temperature, the reservoir temperature of the natural gas hydrate reservoir in the target frozen soil area is reduced to the first rated temperature by controlling the depressurization rate and the depressurization degree in the process of discharging the free gas in the depressurization mode; if the reservoir temperature of the natural gas hydrate reservoir in the target frozen soil area is the first rated temperature, maintaining the reservoir temperature of the natural gas hydrate reservoir in the target frozen soil area at the first rated temperature by controlling the depressurization rate and the depressurization degree in the process of discharging free gas in a depressurization manner; Injecting CO 2 into the natural gas hydrate reservoir of the target frozen soil area for purging and recovering free H 2 and for geological sequestration of CO 2 comprises the following steps: 1) Injecting CO 2 into the natural gas hydrate reservoir of the target frozen soil area until the reservoir pressure of the natural gas hydrate reservoir of the target frozen soil area reaches a second rated pressure, stopping injection and conducting well stewing until the reservoir pressure of the natural gas hydrate reservoir of the target frozen soil area is no longer reduced, wherein the second rated pressure is higher than the natural gas hydrate formation pressure at the natural gas hydrate reservoir temperature of the target frozen soil area and is lower than the liquefaction pressure of CO 2 at the natural gas hydrate reservoir temperature of the target frozen soil area at the moment; 2) And repeating the step 1) until the geological sequestration step of CO 2 without generating carbon dioxide hydrate after CO 2 is injected is finished, namely injecting CO 2 into the natural gas hydrate reservoir in the target frozen soil area until the reservoir pressure of the natural gas hydrate reservoir in the target frozen soil area reaches the second rated pressure, and stopping injection for well-flushing, wherein the reservoir pressure does not drop.
2. The method of claim 1, wherein during the multi-pass throughput purging of the mixed gas of CO 2 and H 2 injected into the natural gas hydrate reservoir of the target frozen soil zone, each pass of throughput purging is completed after the mole percentage of CH 4 in the purged gas is reduced to the rated content, wherein the rated content is 10% -20%.
3. The method of claim 1, wherein the next round of throughput purging is performed after the well is braised between rounds of throughput purging until the rate of increase of the CH 4 mole percent in the gas in the natural gas hydrate reservoir in the target frozen soil zone is no more than 5%/h.
4. The method of claim 1, wherein the mixed gas of CO 2 and H 2 is injected into the natural gas hydrate reservoir in the target frozen soil zone for a plurality of rounds of throughput purging until the molar content of CH 4 in the reservoir is no longer increased after the mixed gas of CO 2 and H 2 is injected, and the throughput purging step is ended.
5. The method of claim 1, wherein, The initial hydrate saturation of the natural gas hydrate reservoir layer in the target frozen soil area is 40-60 percent, and/or The initial water saturation of the natural gas hydrate reservoir in the target frozen soil area is 40% -70%.
6. The method of claim 1, wherein the temperature of the mixture of CO 2 and H 2 injected into the target frozen earth zone natural gas hydrate reservoir is 15-30 ℃.
7. The method of claim 1, wherein the injection pressure of the mixture of CO 2 and H 2 is greater than 25% -95% of the production pressure during multiple passes of the throughput purge of the mixture of CO 2 and H 2 into the target frozen earth zone natural gas hydrate reservoir.
8. The method of claim 1, wherein during the injecting of CO 2 into the target frozen natural gas hydrate reservoir for purging to recover free H 2 and for geological sequestration of CO 2 , the CO 2 injection pressure is lower than the liquefaction pressure of CO 2 at the target frozen natural gas hydrate reservoir temperature at that time and higher than the phase equilibrium pressure of CO 2 hydrate at the target frozen natural gas hydrate reservoir temperature at that time.
9. The method of claim 1, wherein the second rated pressure is greater than 95% of the liquefaction pressure of CO 2 at the target frozen earth zone natural gas hydrate reservoir temperature.

Description

Method for improving recovery ratio of natural gas hydrate in frozen soil area Technical Field The invention belongs to the technical field of exploitation of natural gas hydrate in a frozen soil area, and particularly relates to a method for improving recovery ratio of natural gas hydrate in the frozen soil area. Background Natural gas hydrates are mainly composed of water and hydrocarbon gases such as methane, which form ice-like solids at high pressure and low temperature, and are also known as "combustible ice" because of their ability to burn. The large reserves of natural gas hydrates, high energy density and no pollution by combustion are recognized as an alternative clean energy source in the 21 st century. In recent years, natural gas hydrates with huge reserves are successively found in submarine sediments and frozen earth zones in countries around the world. Although the natural gas hydrate reserves in the frozen soil area are huge, the natural gas hydrate has a strong self-protection effect below the freezing point, so that the gas production efficiency and the gas production amount are greatly reduced, and the process of commercial exploitation of the natural gas hydrate in the frozen soil area is greatly restricted. In order to solve the problem of the strong self-protection effect of the natural gas hydrate in the frozen soil zone, the most effective solution is to mine the natural gas hydrate in the frozen soil zone by combining depressurization with various heat injection modes. The ice shell on the surface of the natural gas hydrate is melted by injecting a large amount of heat, so that the natural gas hydrate can be continuously decomposed to produce gas, but most of the injected heat is used for heating a natural gas hydrate reservoir, so that the heat for decomposing the natural gas hydrate is really utilized to be very little, and the energy efficiency of a heat injection mode is very low. Meanwhile, the cementing effect of natural gas hydrate or ice and reservoir sediments is destroyed by injecting heat, so that the mechanical strength of the reservoir is greatly reduced, thereby causing geological disasters such as reservoir sedimentation, stratum collapse and the like in the natural gas hydrate exploitation process, and seriously threatening the underground facilities and gas production safety. While conventional carbon dioxide displacement recovery is considered the safest recovery method above and below freezing, further development of the method is limited by the dramatic decrease in post-gas production efficiency caused by carbon dioxide hydrate coating on natural gas hydrate. Therefore, on the premise of ensuring the stability of the reservoir in the frozen soil area, the improvement of the recovery ratio of the natural gas hydrate in the frozen soil area becomes a problem to be solved urgently in the exploitation of the natural gas hydrate in the frozen soil area. Disclosure of Invention The invention aims to provide a method capable of improving the recovery ratio of natural gas hydrate in a frozen soil area and simultaneously ensuring the stability of a reservoir. In order to achieve the above object, the present invention provides a method for improving recovery efficiency of natural gas hydrate in a frozen soil region, wherein the method comprises: The method comprises the step of depressurization exploitation, namely discharging free gas from a natural gas hydrate reservoir in a target frozen soil area in a depressurization mode, so that the reservoir pressure of the natural gas hydrate reservoir in the target frozen soil area is reduced to a first rated pressure and the temperature of the reservoir reaches a first rated temperature, wherein the first rated pressure is 0.8-1.2 times of the balance pressure of the natural gas hydrate, and the first rated temperature is less than 0 ℃ and is more than or equal to minus 6 ℃; The method comprises the steps of throughput purging, namely, after the reservoir pressure of a natural gas hydrate reservoir in a target frozen soil area is reduced to a first rated pressure and the reservoir temperature reaches the first rated temperature, injecting mixed gas of CO 2 and H 2 into the natural gas hydrate reservoir in the target frozen soil area for multi-pass throughput purging, wherein a well is stewed between each pass of throughput purging, and the temperature of the natural gas hydrate reservoir in the target frozen soil area is maintained to be less than 0 ℃ and more than or equal to minus 6 ℃ in the multi-pass throughput purging process; And (3) after the geological storage step of CO 2 is finished, injecting CO 2 into the natural gas hydrate reservoir of the target frozen soil area for purging and recovering free H 2 and carrying out geological storage of CO 2. The method for improving the recovery ratio of the natural gas hydrate in the frozen soil region can break the self-protection effect of the hydrate below the freezing poin