CN106854984A - A kind of enhancing methane exploitation of combination injection hot sea water and the gas hydrates method of replacing of carbon dioxide sequestration - Google Patents

Abstract

The invention belongs to the field of natural gas hydrate exploitation, and relates to a natural gas hydrate replacement method combining injection of hot seawater to enhance methane exploitation and carbon dioxide sequestration. First, after the drilling is completed, the pressure of the hydrate reservoir decreases, and the gas hydrate around the wellbore is in a metastable state; then, the wellbore is used to inject carbon dioxide into the reservoir, and the replacement reaction occurs in the shallow layer of the hydrate reservoir near the wellbore; and then Use the wellbore to simultaneously inject carbon dioxide hot seawater into the reservoir, decompose the dense carbon dioxide and methane mixed hydrate formed by the replacement reaction, and develop the methane in the small pores of the hydrate that cannot be replaced; finally stop injecting hot seawater , to continuously inject carbon dioxide into the reservoir, so that carbon dioxide migrates to the deep layer of hydrate reservoir mining, and promotes more hydrate replacement reactions. The invention simultaneously realizes the purpose of safe and efficient mining of hydrate and sequestration of carbon dioxide to reduce the greenhouse effect.

Description

A natural gas combined with hot seawater injection for enhanced methane recovery and carbon dioxide storage Hydrate replacement method

technical field

The invention belongs to the field of natural gas hydrate exploitation, and relates to a natural gas hydrate replacement method combining injection of hot seawater to enhance methane exploitation and carbon dioxide sequestration.

Background technique

Natural gas hydrate widely exists in continental permafrost and deep seabed, and has the characteristics of wide distribution, shallow burial, large scale, and high energy density. It is regarded as a potential unconventional energy source, and its proven reserves are equivalent to the global Twice the total amount of proven fossil fuels (coal, oil, natural gas). The traditional natural gas hydrate production methods mainly include depressurization method, thermal shock method and inhibitor injection method, but all of them have defects: low production efficiency of depressurization method, large energy dissipation of heat shock method, and damage to the environment by injection of inhibitor method. Carbon dioxide replacement is a new and environmentally friendly natural gas hydrate mining method, which can simultaneously realize the purpose of methane development as an energy supply and carbon dioxide sequestration to alleviate the greenhouse effect, and the formation of carbon dioxide hydrate during the mining process can ensure the reservoir stability and avoid marine geological disasters.

As the most promising natural gas hydrate extraction method, carbon dioxide replacement still has many problems in practical engineering: First, methane hydrate consists of 6 large cavities and 2 small cavities. The volume of carbon dioxide molecules is larger than that of methane, and its size is between methane Between the large and small cavities of the hydrate, under ideal conditions, the maximum recovery rate of methane can only reach 75%. Secondly, in the early stage of mining, carbon dioxide is rapidly replaced on the surface of methane hydrate, and gradually forms on the surface of hydrate as the replacement reaction progresses. The dense mixed hydrate layer of methane and carbon dioxide leads to a decrease in permeability, which is not conducive to the diffusion of carbon dioxide into the hydrate, and the replacement rate decreases rapidly. In severe cases, the replacement reaction basically stops. Therefore, in view of the problems of low methane recovery rate, carbon dioxide storage rate and low replacement rate, a new natural gas hydrate replacement method is urgently needed.

Contents of the invention

In order to overcome the problems existing in the prior art, the present invention provides a novel natural gas hydrate replacement method, which combines the injection of hot seawater in the middle of the replacement to decompose the dense carbon dioxide and methane mixed hydrate layer, thereby increasing the methane recovery rate and carbon dioxide Storage rate, while promoting the diffusion of carbon dioxide and increasing the replacement rate.

In order to achieve the above functions, the technical solution provided by the present invention is a natural gas hydrate replacement method that combines injection of hot seawater to enhance methane recovery and carbon dioxide sequestration. The specific steps are as follows:

1) Select the target area for hydrate reservoir exploitation, build an offshore injection platform and a gas production platform, and drill carbon dioxide and hot seawater injection wells and methane gas production wells respectively; in a metastable state.

2) Inject carbon dioxide into the hydrate reservoir through carbon dioxide and hot seawater injection wells, and control the reservoir pressure above the equilibrium pressure of the carbon dioxide hydrate phase corresponding to the reservoir temperature, so as to promote the exploitation of shallow gas hydrate reservoirs closer to the wellbore A displacement reaction occurs, and the displaced methane gas and residual carbon dioxide gas are collected through the methane production well. According to the phase equilibrium conditions and gas separation conditions of carbon dioxide, carbon dioxide hydrate and methane hydrate, the optimal injection conditions of carbon dioxide can be selected.

3) Simultaneously inject hot seawater and carbon dioxide into hydrate reservoirs through carbon dioxide and hot seawater injection wells, and use hot seawater to partially decompose dense carbon dioxide and methane mixed hydrates formed by replacement reactions in the shallow layers of natural gas hydrate reservoirs, Open the channel for the flow and migration of carbon dioxide to deep hydrate reservoirs, increase the replacement rate of carbon dioxide, and use hot seawater to release methane in the small pores of natural gas hydrate that cannot be directly replaced by carbon dioxide, increasing the methane recovery rate and carbon dioxide sequestration rate; The released methane gas and carbon dioxide are collected by methane production wells.

The above-mentioned simultaneous injection of hot seawater and carbon dioxide, on the one hand, partially decomposes the mixed hydrate, avoids large-scale decomposition and collapse of hydrate reservoirs, and avoids marine geological disasters to ensure reservoir stability; on the other hand, compared with conventional In the heat shock method, the injection process of hot seawater combined with carbon dioxide does not need to heat the entire hydrate reservoir, so it has low energy dissipation. According to different reservoir conditions such as temperature, pressure, permeability and hydrate saturation, the optimal injection conditions can be selected.

4) Stop injecting hot seawater, use carbon dioxide and hot seawater injection wells to inject carbon dioxide into the hydrate reservoir, and control the reservoir pressure above the carbon dioxide hydrate phase equilibrium pressure corresponding to the reservoir temperature, so as to promote the hydration of natural gas far away from the wellbore. The replacement reaction occurs in the deep layer of the reservoir mining, and the displaced methane gas and residual carbon dioxide gas are collected through the methane gas production well; at the same time, the residual hot seawater and the injected carbon dioxide generate carbon dioxide hydrate, which improves the stability of the reservoir and enhances the storage capacity of carbon dioxide .

5) For the products collected from the methane gas production wells of the offshore gas production platform in steps 2), 3) and 4), the gas and water are separated first, and then the gas is separated to obtain pure methane gas, and finally the storage of methane gas is completed with transport.

Further, after the methane purification described in the above step 5), the residual carbon dioxide gas is reinjected into the hydrate reservoir for storage to increase the storage capacity of carbon dioxide.

Compared with prior art, the beneficial effect of the present invention is:

The invention simultaneously realizes the purpose of safe and efficient mining of hydrate and sequestration of carbon dioxide to reduce the greenhouse effect. Compared with the traditional displacement mining, the combination of hot water injection can effectively increase the displacement rate, methane recovery rate and carbon dioxide sequestration rate; compared with the traditional thermal shock method, the simultaneous injection of hot seawater and carbon dioxide can partially decompose the mixed hydrate without Large-scale decomposition and collapse of hydrate reservoirs will cause marine geological disasters. On the other hand, the injection process does not need to heat the entire hydrate reservoir, so it has low energy dissipation.

(1) The present invention adopts the combination of carbon dioxide replacement and hot seawater injection, so that small cavities that cannot be replaced by methane hydrate can be developed, and the recovery rate of methane can be improved. The free water generated by the decomposition of small cavities combines with carbon dioxide to form carbon dioxide hydration substances to increase the carbon dioxide sequestration rate;

(2) The present invention combines the injection of hot seawater in the middle stage of replacement, which can effectively destroy the dense methane and carbon dioxide mixed hydrate layer, improve the permeability of the reservoir, promote the diffusion of carbon dioxide, and increase the replacement rate.

(3) Combined with the injection of hot seawater and carbon dioxide, the mixed hydrate is partially decomposed, and the replacement rate is increased without large-scale decomposition and collapse of the hydrate reservoir, causing marine geological disasters, and ensuring the stability of the reservoir.

(4) Compared with the traditional heat shock method, the injection process of hot seawater combined with carbon dioxide does not need to heat the entire hydrate reservoir, so it has lower energy dissipation.

(5) The residual hot seawater will form carbon dioxide hydrate with the injected carbon dioxide, which will enhance the storage capacity of carbon dioxide while further improving the stability of the reservoir.

Description of drawings

Figure 1 is a schematic diagram of a natural gas hydrate replacement method that combines injection of hot seawater to enhance methane recovery and carbon dioxide sequestration.

Accompanying drawing 2 is the schematic diagram of mining shallow layer displacement.

Accompanying drawing 3 is the schematic diagram of hot seawater combined with carbon dioxide injection stage.

Accompanying drawing 4 is the schematic diagram of mining deep layer replacement.

detailed description

The technical solutions of the present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

Example 1

As shown in Figure 1, natural gas hydrate pools often exist in marine sediments 1200-1500 meters below sea level, and the temperature and pressure conditions are 275-285K, 3.2-11MPa. The hydrate reservoir at 1,200 meters below sea level, with a reservoir temperature of 275K and a reservoir pressure of 3.2MPa is now selected as the target area to describe the standardized mining process and describe the optimal conditions.

(1) Build an offshore injection platform and a gas production platform, as shown in Figure 1, to drill carbon dioxide and hot seawater injection wells and methane gas production wells. After the drilling is completed, the pressure of the hydrate reservoir decreases, and the gas hydrate around the wellbore is in a metastable state.

(2) As shown in Figure 2, inject carbon dioxide into the hydrate reservoir from the carbon dioxide and hot seawater injection well, and control the reservoir pressure to be above the equilibrium pressure of the carbon dioxide hydrate phase corresponding to the reservoir temperature. Here, the pressure is controlled above 1.5MPa (the phase equilibrium pressure of carbon dioxide hydrate at 275K), so that the replacement reaction occurs in the shallow layer of the hydrate reservoir near the wellbore, and the replaced methane gas and residual carbon dioxide gas pass through the methane Gas production well collection.

According to the phase equilibrium conditions of carbon dioxide, carbon dioxide hydrate and methane hydrate, the temperature and pressure conditions for carbon dioxide to replace natural gas hydrate can be divided into three replacement regions: A region (above the phase equilibrium curve of carbon dioxide, carbon dioxide hydrate and methane hydrate ), B region (below the carbon dioxide phase equilibrium curve, above the carbon dioxide hydrate and methane hydrate phase equilibrium curve) and C region (below the carbon dioxide and methane hydrate phase equilibrium curve, above the carbon dioxide hydrate phase equilibrium curve), Among them, the replacement rate and replacement rate of region A (liquid carbon dioxide has a strong diffusion ability) and region C (methane hydrate is in an unstable region) are better than those of region B.

Based on the above description, when the reservoir temperature is 275K, it is better to control the reservoir pressure at 1.5-3.2MPa and above 3.7MPa; considering the problems of gas separation and methane collection, the optimal condition is to control the reservoir pressure at 1.5-3.2MPa Excellent mining conditions.

(3) Simultaneously inject hot seawater and carbon dioxide into hydrate reservoirs from carbon dioxide and hot seawater injection wells, and collect products through methane gas production wells.

As shown in Figure 3, the purpose of injecting hot seawater is to partially decompose the dense carbon dioxide and methane mixed hydrate formed due to the replacement reaction in the shallow layer of hydrate reservoir mining: on the one hand, open the flow of carbon dioxide to the deep layer of hydrate reservoir mining On the other hand, the methane in the small hydrate pores that cannot be replaced can be developed, and the methane recovery rate and carbon dioxide sequestration rate can be improved. The injection temperature and flow rate of hot seawater are particularly critical, and better injection conditions can ensure reservoir stability and lower energy dissipation: on the one hand, mixed hydrates are partially decomposed due to better injection conditions, and hydrate accumulation does not occur. Large-scale decomposition and collapse of large-scale hydrates can cause marine geological disasters, which is also the purpose of injecting carbon dioxide at the same time; energy dissipation.

Based on the above description, there are different optimal injection conditions according to different conditions such as reservoir temperature, pressure, permeability and hydrate saturation. Here, under the condition that the reservoir temperature is 275K, the pressure is 1.5-3.2MPa, and the hydrate saturation is 40-60%, the injection temperature of hot seawater is selected as 40℃, and the injection time is 20min. The injection ratio is 1:20.

(4) Stop injecting hot seawater, use carbon dioxide and hot seawater injection wells to inject carbon dioxide into the hydrate reservoir, and control the reservoir pressure above the equilibrium pressure of the carbon dioxide hydrate phase corresponding to the reservoir temperature. Here, the pressure is controlled at 1.5- 3.2MPa, as shown in Figure 4, causes a displacement reaction to occur in the deep layer of the hydrate reservoir far from the wellbore, and the displaced methane gas and residual carbon dioxide gas are collected through the methane gas production well. It is worth noting that the residual hot seawater will form carbon dioxide hydrate with the injected carbon dioxide, which will enhance the storage capacity of carbon dioxide while further improving the stability of the reservoir.

(5) For the products collected from the methane gas production wells of the offshore gas production platform in steps (2), (3) and (4), the gas and water are separated first, and then the pure methane gas is obtained by the cryogenic separation process, and finally completed Storage and transportation of methane gas. The carbon dioxide-containing gas remaining in the methane purification step can be reinjected into the hydrate reservoir for storage.

The above embodiment is one of the specific implementation methods of the present invention, and the usual changes and substitutions made by those skilled in the art within the scope of the technical solution shall be included in the present invention.

Claims (2)

1. a kind of combination injection hot sea water enhancing methane exploits the gas hydrates method of replacing with carbon dioxide sequestration, and it is special Levy and be, comprise the following steps：

1) choose hydrate and hide exploitation target area, build marine injection platform and aerogenesis platform, carry out respectively carbon dioxide and Hot sea water injection well, methane gas-producing well drilling well；After the completion of drilling well, the reduction of hydrate reservoir pressure, the gas water around pit shaft Compound is in metastable state state；

2) by carbon dioxide and hot sea water injection well to hydrate reservoir injecting carbon dioxide, and reservoir pressure is controlled in correspondence More than the carbon dioxide hydrate vapor pressure of reservoir temperature, promote shallow in the gas hydrates exploitation close to pit shaft There is displacement reaction in layer, the methane gas being displaced is collected with the carbon dioxide of remnants by methane gas-producing well；

3) hot sea water and carbon dioxide are injected to hydrate reservoir by carbon dioxide and hot sea water injection well simultaneously, using hot sea Water makes gas hydrates exploit fine and close carbon dioxide, the generation of methane blended hydrate that shallow-layer is formed by displacement reaction Decomposed, opens the passage that carbon dioxide hides flowing migration to deep layer hydrate, improves the replacement rate of carbon dioxide, while Methane release in the gas hydrates foveola by carbon dioxide direct replacement is will be unable to using hot sea water out, improve first Alkane coefficient of mining and carbon dioxide sequestration rate；Methane gas and carbon dioxide is released to be collected by methane gas-producing well；

4) stop injection hot sea water, using carbon dioxide and hot sea water injection well to hydrate reservoir injecting carbon dioxide, and control Reservoir pressure processed promotes from pit shaft farther out natural more than the carbon dioxide hydrate vapor pressure of correspondence reservoir temperature Gas hydrate hides exploitation deep layer and occurs to replace reaction, and the methane gas and the carbon dioxide of remnants being displaced pass through methane Gas-producing well is collected；Remaining hot sea water and the carbon dioxide of injection generation carbon dioxide hydrate, improve reservoir stability simultaneously And strengthen the amount of sealing up for safekeeping of carbon dioxide；

5) for step 2), 3) and 4) in the product collected from the methane gas-producing well of marine aerogenesis platform, gas and water point is carried out first From, then isolated pure methane gas is carried out to gas, it is finally completed the storage to methane gas and transport.

2. a kind of combination injection hot sea water enhancing methane according to claim 1 exploits the natural gas with carbon dioxide sequestration Hydrate method of replacing, it is characterised in that step 5) described in methane purification after, the carbon dioxide of residual is re-injected Sealed up for safekeeping in hydrate reservoir.