1. Natural gas hydrate evaluation and prediction technology
At present, the evaluation and prediction technologies of natural gas hydrate include seismic technology, logging technology, geochemical technology and marker mineral method.
1. seismic technology
Seismic exploration is the most commonly used and important means for gas hydrate exploration at present. The velocity of gas hydrate deposits is relatively high, but the stratum under gas hydrate deposits is generally a hydrocarbon gas (free gas) accumulation area, and the sound velocity is low, so the strong acoustic impedance at the bottom boundary of hydrate will produce strong reflection, showing a unique reflection interface on the seismic reflection profile. In addition, because the boundary of the gas hydrate stability zone is roughly distributed at the same seabed depth, and the reflection at the bottom of the hydrate stability zone is also roughly parallel to the seabed, this technology is named BSR (Figure 10- 10). With the wide application of multi-channel reflection seismic technology and the improvement of seismic data processing technology, the characteristics of BSR in seismic profile, such as high amplitude, negative polarity, parallel to the seabed and intersecting with submarine sedimentary structures, are easy to identify. It has been proved that hydrocarbon gas above BSR exists in the form of solid natural gas hydrate, and hydrocarbon gas below BSR exists in the form of free gas. BSR is the earliest, most reliable and intuitive geophysical sign to confirm the existence of natural gas hydrate. Up to now, most of the confirmed submarine gas hydrates have been discovered by identifying BSR on the reflection seismic profile.
Fig. 10- 10 BSR seismic profile in Black Ridge area.
2. Logging technology
The functions of logging technology mainly include: ① determining the depth distribution of natural gas hydrate and sediments containing natural gas hydrate; ② Estimation of porosity and methane saturation; ③ Using borehole information to correct seismic and other geophysical data. At the same time, logging data is also an effective means to study the sedimentary environment and evolution of the main strata of natural gas hydrate near the well point.
On the conventional logging curve, gas hydrate deposits mainly show the following anomalies (Figure 10- 1 1):① High resistivity; ② The sound wave time difference is small; ③ The amplitude of spontaneous potential is not large; ④ High neutron logging value; ⑤ High gamma value; ⑥ Large aperture; ⑦ There is obvious gas emission during drilling, and the measured gas value is high.
Figure 10- 1 1 logging response characteristics of natural gas hydrate layer
3. Geochemical technology
Geochemical technology is an effective means to identify the occurrence state of submarine gas hydrate. The fluctuation of temperature and pressure can easily decompose natural gas hydrate, so there are often geochemical anomalies of natural gas in shallow seabed sediments. These anomalies can indicate the possible location of natural gas hydrate, and then the source of natural gas can be judged by its hydrocarbon composition ratio (such as C 1/C2) and carbon isotope composition. At the same time, the application of offshore methane field detection technology can delineate the high methane concentration area and determine the prospective distribution of natural gas hydrate.
Under the current technical conditions, the main signs of geochemical exploration of natural gas hydrate are: the decrease of chlorine or salinity in pore water, the redox potential of water, the low sulfate content and the change of oxygen isotope. When analyzing geochemical data, we should treat them differently and comprehensively according to the specific actual situation.
4. The Mark Minerals Act
Typomorphic minerals that can indicate the existence of natural gas hydrate are usually carbonates, sulfates and sulfides with specific composition and morphology. They are a series of typomorphic minerals formed by the interaction of ore-forming fluids with seawater, pore water and sediments in the process of sedimentation, diagenesis and supergene.
When the fluid under the seabed enters the vicinity of the seabed in the form of overflow or seepage, it will produce a series of physical, chemical and biological effects. When the fluid containing saturated gas moves from deep sea to shallow sea bottom, it quickly cools to form natural gas hydrate, accompanied by authigenic carbonate rocks and chemical energy autotrophic biota dependent on this fluid. Because of its low temperature, these fluids are called "cold spring" fluids, which are different from the high-temperature fluids in the deep crust and are one of the most effective marker minerals for finding natural gas hydrates.
Second, natural gas hydrate development technology
Exploitation of natural gas from the formation where natural gas hydrate has been formed is actually a process to meet the decomposition reaction of natural gas hydrate. Reducing formation pressure or increasing temperature can weaken the van der Waals force between methane molecules and water molecules in natural gas hydrate, thus releasing a large amount of methane gas from solid natural gas hydrate. At present, there are three main development technologies of natural gas hydrate: thermal excitation technology, depressurization technology and chemical inhibitor technology.
65438+
Installing pipelines in the stable zone of natural gas hydrate will heat the formation containing natural gas hydrate and raise the local reservoir temperature, which will lead to the decomposition of natural gas hydrate. Steam, hot water, hot brine or other hot fluids are mainly pumped into the hydrate layer from the ground, and fire flooding method or drill string heater used in heavy oil exploitation can also be adopted. Electromagnetic heating method is more effective than the above conventional methods, and has shown its effectiveness in heavy oil development, among which microwave heating method is the most effective method. The main disadvantages of thermal excitation method are large heat loss and low efficiency, and the difficulty is that the generated gas is not easy to collect.
2. Step-down technology
By reducing the pressure of gas hydrate layer, gas hydrate is decomposed. Generally, natural gas "capsule" is formed by the pressure drop in the borehole or the equilibrium pressure of the free gas accumulation layer under the hydrate layer (by thermal excitation method or chemical reagent), and the hydrate in contact with natural gas becomes unstable and decomposes into water and natural gas. Depressurization development is especially suitable for the case that natural gas hydrate is adjacent to conventional natural gas reservoirs, and it is suitable for the development of gas hydrate reservoirs with high permeability and depth greater than 700 m. This technology is characterized by economy, no need for additional equipment and expensive continuous thermal excitation, and high feasibility. The disadvantage is that the effect is slow, and it can not be used for gas hydrate accumulation with the original reservoir temperature close to or lower than 0℃, so as to avoid freezing and blocking the gas reservoir with decomposed water.
3. Chemical inhibitor technology
By injecting chemical inhibitors (such as salt water, methanol, ethanol, ethylene glycol, glycerol, etc.). ), it can change the phase equilibrium conditions of hydrate formation, reduce the stable temperature of hydrate, change the temperature and pressure conditions of natural gas hydrate stability zone, and lead to the decomposition of some natural gas hydrates. This method is very simple and convenient to use, but it is expensive and slow, so it is not suitable for producing marine hydrate under high pressure.
Judging from the application of the above methods, it is unwise to exploit hydrate by only one method. Only by combining the advantages of different methods can the effective exploitation of hydrate be realized. The combination of depressurization method and thermal shock method is the most respected scheme at present. Thermal shock method is used to decompose gas hydrate, and decompression method is used to extract free gas. From the technical point of view, it is feasible to develop natural gas hydrate resources, but no more economical and reasonable exploitation scheme has been found under the current technical conditions, and the development of natural gas hydrate is basically in the discussion stage.
Third, the potential of natural gas hydrate resources
1. Natural gas hydrate in polar tundra
Under appropriate high pressure and low temperature conditions, natural gas and water combine to form a combustible substance like ice. The vast areas of ocean and polar regions meet the conditions for the formation of natural gas hydrate. A large number of field studies show that natural gas hydrate is widely distributed in permafrost and seabed sediments around the continental margin (figure 10- 12). There are a large number of natural gas resources stored in hydrate reservoirs in the world, and the predicted natural gas resources span is also very large, exceeding three orders of magnitude, from 2.8×10/5m3 to 8×10/8m3 (table 10-3). According to the latest estimation results (Jiang Huaiyou et al., 2008), the global natural gas hydrate resources are about (0.1~ 2.1) ×1016m3. Although all kinds of estimates are speculative and uncertain, even according to the most conservative estimates, the exploration potential of natural gas hydrate resources is huge. At present, it is recognized that it is 3000× 10 12m3. It is generally believed that 98% of the world's natural gas hydrate resources are distributed in seabed sediments, and only 2% are distributed in terrestrial permafrost.
Table 10-3 Global Natural Gas Hydrate Resource Assessment
sequential
Note: the unit of natural gas resources is m, and the standard pressure and temperature conditions are 1atm and 20℃.
Figure 10- 12 Actual investigation and predicted location of natural gas hydrate in permafrost and marine sediments around continental margin.
The global polar-permafrost regions (Arctic, Antarctic and Qinghai-Tibet Plateau) have a land area of1.1×107km2, and the natural gas hydrate resources are1.4×10/3m3 to 3.4×/. McIver， 198 1； Trofimuk et al.,1977; McDonald's,1990; Dobrynin et al., 198 1). The permafrost area of the Qinghai-Tibet Plateau is vast, accounting for 6 1% of the total plateau area and 7% of the world permafrost area, reaching 1.588× 106 km2. Both continental basins and marine basins have good conditions for oil and gas hydrate formation, and it is possible to form hydrate reservoirs of a certain scale, including Qiangtang basin, Hoh Xil continental basin area and Hoh Xil basin area. Chen Duofu et al., 2005; Zhu Youhai et al., 2006; Lu Zhenquan et al, 20 10).
2. Natural gas hydrate around continental margin
The periphery of continental margin includes passive continental margin and active continental margin, and the global marine gas hydrate resources range from 0.2×10/5m3 to 7.6×10/8m3 (Meyer,1981; Milkov et al., 2003; Trofimuk et al.,1977; Klauda et al., 2005; Kvenvolden， 1988； McDonald's,1990; Kvenvolden et al.,1988; Dobrynin et al., 198 1), mainly distributed in: ① separated oceans, including active continental margins or passive continental margins; ② In deep-water lakes; (3) The inner region of the ocean plate. Such as Bering Sea, Okhotsk Sea, Thousand Islands Trench Sea, Japan Sea, Shikoku Trough, South China Sea Trough, Okinawa Trough, Southwest Taiwan Province Sea, Dongtai Bay Sea, Dongsha Trough around the South China Sea, Xisha Trough, Nansha Trough and Nansha Sea, Sulawesi Sea, Northwest Australia Sea and North Island Sea of New Zealand; Central American Trough in the East Pacific Ocean, Northern California-Oregon Offshore, Peru Trough; The western Atlantic Ocean, namely the Black Plateau, the Gulf of Mexico, the Caribbean Sea and the offshore continental marginal sea off the east coast of South America; The offshore waters off the west coast of Africa, the Gulf of Oman in the Indian Ocean, the Bay of Bengal, the Barents Sea and Beaufort Sea in the Arctic, the Ross Sea and Weddell Sea in the Antarctic, and the inland Black Sea and Caspian Sea.
3. Natural gas hydrate in China sea area.
China sea area is rich in hydrate resources, and the main conditions for hydrate formation are the South China Sea (the slope area of the South China Sea is greater than 1.20× 1.04 km2) and the East China Sea (the slope area of the East China Sea is about 6× 104km2).
According to the appearance of BSR, the South China Sea is divided into 1 1 hydrate resource prospect area, and the effective distribution area of hydrate in each area is counted. Finally, the effective distribution area of BSR in the whole South China Sea is 125833.2km2, and the thickness of hydrate stability zone is between 47 ~ 47 ~ 389m (Yang Muzhuang et al., 2008). Yao Bochu et al. (2006) and Yang Muzhuang et al. (2008) predicted that the hydrate resources in the South China Sea were 6.435×10/3m3, 6.9305× 1065438 and 7.632× 1065438 respectively.
For the East China Sea, Yang Muzhuang and others calculated that the distribution area of the hydrate stability zone is 5250km2 and the thickness of the stability zone is 50 ~ 49 1.7m according to the parameters of seabed temperature, geothermal gradient, seawater depth and salinity, and finally predicted the hydrate resources in the East China Sea to be about 3.53x10/m3.
Usually, the distribution range of natural gas hydrate, the thickness of hydrate stability zone, the porosity of sedimentary layer, the concentration of hydrate in cracks and the expansion coefficient of methane decomposed by hydrate are considered to estimate the methane resources in submarine natural gas hydrate, and the thickness of hydrate stability zone is of great significance in the evaluation of natural gas hydrate resources (Xu et al., 1999). Natural gas hydrate stability zone refers to the area where natural gas and hydrate can reach phase equilibrium and combine to form natural gas hydrate under specific temperature and pressure conditions. According to the three important parameters of water depth, seabed temperature and geothermal gradient, the thickness of natural gas hydrate stability zone in a specific area can be calculated and determined. On this basis, according to the comprehensive characteristics of natural gas hydrate hydrocarbon gas system, the possible exploration target area which can form high abundance natural gas hydrate accumulation is further determined. The most favorable practical exploration direction is polar sandstone reservoir and marine sandstone reservoir in hydrate stability zone. Of course, it is necessary to analyze the source rock quality of natural gas, the adequacy of natural gas supply, the development of migration channels and other factors, and finally determine the exploration target.
Natural gas hydrate is a kind of clean energy with high energy density, less impurities and large reserves. Exploring and developing natural gas hydrate and increasing natural gas production can gradually change the present situation of China's energy structure and reduce the environmental pollution caused by a large number of coal burning, which has broad exploration prospects.