Because natural gas hydrates are mainly distributed in permafrost and deep-water areas, the difficult geographical environment and complex geological conditions cast a difficult shadow on the development of this resource. The exploration and development technology of natural gas hydrate is a comprehensive natural gas hydrate industry, which integrates natural gas geology, engineering geology in permafrost regions, deep-sea geology and deep-sea drilling technology, and it is also one of the frontier topics in today's geological circles.
Since the Soviet Union discovered Mesoyaha gas field in 196s, the development idea of natural gas hydrate is basically to consider how to decompose the natural gas hydrate contained in sediments first, and then to extract natural gas to the surface. Generally speaking, artificially breaking the stable temperature and pressure conditions of natural gas hydrate and causing its decomposition is the main method to develop methane resources in natural gas hydrate at present. The methods proposed at this stage can be classified into the following categories: heating method, depressurization method, adding chemical agent method (Figure 8.17), gas lifting method, CO2 displacement mining method, etc.
fig. 8.17 schematic diagrams of three natural gas hydrate exploitation methods
(1) heating method
Steam, hot water, hot brine or other hot fluids are pumped into hydrate formation from the ground, and the fire flooding method used in heavy oil exploitation can also be adopted. In short, any method that can promote the temperature rise to hydrate decomposition can be called thermal excitation method. The main disadvantage of thermal mining technology is that it will cause a lot of heat loss and its efficiency is very low. Especially in permafrost regions, even if insulated pipes are used, the permafrost layer will reduce the effective heat transferred to the reservoir. In the thermal stimulation model, there are two kinds of heat conduction control technologies for hydrate production: ① Hot water or steam is injected into the preheating well circularly. The numerical simulation experiment shows that the hydrate reservoir should have a minimum porosity of 15% and a thickness of 7.5cm. If the temperature of the injection is between 34 and 395 K, it can meet the needs of its economic feasibility. ② Direct heating by electromagnetic or microwave. In order to make more effective use of heat energy, a heating device can be installed underground, with complicated equipment, or microwave heating can be used, and microwaves can be introduced into the bottom of the well through a waveguide to directly heat hydrate or water.
in recent years, in order to improve the heating efficiency, the heating technology of underground device is adopted when heating heavy oil, and the underground electromagnetic heating method is one of them. Practice has proved that the electromagnetic heating method is a more effective method than the conventional mining technology. This method is to put different electrodes in the upper and lower layers (or gas hydrate layers) adjacent to the gas hydrate zone along the extension direction of the vertical (or horizontal) well, and then directly heat the reservoir with alternating current. Electromagnetic heat also reduces the viscosity of fluid and promotes the flow of gas. The simulation results show that it is feasible to decompose hydrate by this method.
(2) depressurization method
The stable phase equilibrium curve of natural gas hydrate is shifted by reducing the pressure, so as to promote the decomposition of hydrate. Generally, it is to "reduce" the pressure of natural gas or form a natural gas cavity (which can be artificially formed by thermal excitation or chemical reagent) in the free gas accumulation layer below the hydrate layer, so that the hydrate in contact with natural gas becomes unstable and decomposes into natural gas and water. Exploitation of free gas under hydrate layer is an effective method to reduce reservoir pressure. In addition, by adjusting the extraction speed of natural gas, the reservoir pressure can be controlled, and then the hydrate decomposition can be controlled. The biggest feature of decompression method is that it does not need expensive continuous excitation, so it may become one of the effective methods for large-scale exploitation of natural gas hydrate in the future. However, it is very slow to exploit natural gas only by decompression method, which is a kind of weakened passive exploitation.
(3) method of adding chemicals
Some chemicals, such as brine, methanol, ethanol, ethylene glycol, glycerol, etc., can change the phase equilibrium conditions of hydrate formation and lower the stable temperature of hydrate. When the above chemicals are pumped into the borehole, it will cause the decomposition of natural gas hydrate. Adding chemicals is slower than heating, but it does have the advantage of reducing the initial energy input, and its biggest disadvantage is that the cost is too high. Table 8.6 comments on various natural gas hydrate production methods.
Table 8.6 Review of natural gas hydrate exploitation methods Table
(4) Gas lifting method
Figure 8.18 Schematic diagram of gas lifting system for seabed hydrate exploitation
The principle is that a pipe is inserted into the seabed hydrate-bearing layer, and gas is blown in from the center of the pipe. The gas lifting causes an updraft in the pipe, and the solid hydrate rises with the gas flow. When it approaches the sea surface, the hydrate in the pipe is decomposed due to the increase of temperature and the decrease of pressure (p) Through the experiment and simulation of the relationship between two-phase fluid, the decomposition rate of hydrate and the inlet parameters of the riser, it is considered that it is economical and feasible to use gas lifting method to exploit submarine hydrate. The mathematical analysis of gas-liquid-solid three-phase fluid is carried out, and the results are consistent with the experimental results, which shows that it is feasible to predict the relationship of three-phase fluid in the actual system. In the experiment, the decomposition rate of HCFC141 hydrate was obtained, and the relationship between Reynolds number and Nusselt number, which determined the decomposition rate of hydrate in fluid, was obtained. The mathematical simulation analysis of hydrate decomposition in the lifting pipeline shows that it is an economical mining method to exploit hydrate by using the lifting effect of gas itself in the gas lifting system. The mathematical simulation results of fluid movement at the inlet of pipeline are in good agreement with the experimental results. Discrete Element Method (DEM) is used to simulate the movement of hydrate block, and an ideal entrance shape-umbrella shape is obtained.
(5)CO2 replacement mining method
The method of injecting CO2 into gas hydrate zone and replacing CH4 in hydrate with CO2 (Figure 8.19). It has some remarkable characteristics: ①CO2 replacing CH4 in hydrate is thermodynamically beneficial; ② The heat of forming CO2 hydrate is 2% greater than that of decomposing methane hydrate, so the formation of CO2 hydrate counteracts the cooling caused by the decomposition of CH4 hydrate; ③ The pore space refilled with ③CO2 hydrate is expected to maintain the mechanical stability of gas products, thus ensuring the safety of gas exploitation; (4) This process is beneficial to the climate, because CO2 leaves the atmosphere by sinking and produces clean burning natural gas.
Early experimental studies have discussed the exploitation of hydrate by CO2 displacement. These studies emphasize that thermodynamic forces promote the displacement reaction, but they still have some limitations. Early experiments mostly placed methane hydrate in liquid or gaseous CO2 environment, which limited the effective contact area of displacement. Some experiments show that when the temperature and pressure conditions are close to the hydrate equilibrium conditions or the CO2 content reaches saturation, the methane production rate in the sediment will slow down.
fig. 8.19 schematic diagram of CO2 displacement mining method
nuclear magnetic resonance imaging is an effective method to study the formation and decomposition of hydrate in porous media, because it can detect hydrogen in free water and methane gas, but not solid hydrogen, and the weakening and strengthening of signal intensity can well reflect the formation and decomposition process of hydrate. The experimental results of Stevens＆Howard and Huseba show that when CH4 in the hydrate in the core is released, the accumulation of CH4 in the crevice can enhance the image signal of nuclear magnetic resonance. When the displacement reaction reaches equilibrium, the continuous increase of CO2 will promote the hydrate to release CH4 gas again. The hydrate structure can be maintained in the process of CO2 replacing CH4, which shows that almost no liquid water is discharged during hydrate exploitation, that is, the hydrate-bearing sediments can remain intact during exploitation, so the CO2 replacement exploitation method is feasible. Using nuclear magnetic resonance imaging technology to monitor the reaction process of CO2 displacement mining method, it is concluded that the structure of hydrate can be maintained during the mining process, and almost no liquid water is discharged, so that the sedimentary layer containing hydrate can be kept intact, so it is a potential method. The early experimental research is encouraging, but further work is still needed to confirm this process, especially the scale of reserves, in order to evaluate the overall economic potential. The northern slope of Alaska is an ideal area for this experiment because of the discovery of gas hydrate deposits, potential CO2 gas sources nearby and the infrastructure that can bring gas to the market.
(6) fluorine gas+microwave mining technology
fluorine gas+microwave mining technology is a new hydrate mining method, which uses a microwave antenna, which is placed in the wellbore and connected with wires, and can emit microwaves with a frequency as high as 245MHz. At such high energy, it can melt the hydrate and turn it into water and methane-based substances (which are similar to ice), thus breaking the thermodynamic equilibrium state of hydrate. The methane-based substance reacts with the injected fluorine gas (halogenation reaction), which is a strong exothermic reaction. The released heat further promotes the halogenation reaction. The solubility of methyl fluoride produced by the halogenation reaction in water is very high, reaching 166cm3/1mL of water, forming a concentrated solution with high methyl fluoride content. The concentrated solution is pumped to the ground through a production well, and then methane gas is obtained through a series of steps such as Vilt reaction, electrolysis and cracking. The main advantage of using this technology is that the microwave action is selective, which is stronger for some materials and weaker for others. The absorption of energy mainly depends on microwave frequency, sample composition and temperature. Moreover, fluorine gas is rich in nature (.54), and methyl fluoride is more environmentally friendly. In this technology, the pressure of fluid and hydrate is reduced, and the hydrate is below the phase equilibrium point, which can achieve the purpose of decomposition.
From the use of methods, it is uneconomical to exploit natural gas hydrate by only one method. Only by combining the advantages of different methods can we achieve effective exploitation of gas hydrate. For example, if the depressurization method is combined with thermal recovery technology, that is, the natural gas hydrate is decomposed by thermal excitation method first, and then the free gas is extracted by depressurization method, the effect may be better. Recently, decompression method, thermal stimulation method and their combined schemes have been used as possible methods to produce natural gas in hydrate. Decompression method and thermal stimulation method have been used to produce a small amount of gas in Mallik SL-38 research well in Mackenzie Delta in northwest Canada. However, the economic value of these methods for commercial oil and gas exploitation is still uncertain. Combined mining with various principles and methods is the development trend in the future, and it will certainly show attractive prospects.