WO2019134220A1 - 一种天然气水合物开采采气方法及系统 - Google Patents

一种天然气水合物开采采气方法及系统 Download PDF

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WO2019134220A1
WO2019134220A1 PCT/CN2018/076449 CN2018076449W WO2019134220A1 WO 2019134220 A1 WO2019134220 A1 WO 2019134220A1 CN 2018076449 W CN2018076449 W CN 2018076449W WO 2019134220 A1 WO2019134220 A1 WO 2019134220A1
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gas
liquid
hydrate
storage tank
valve
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PCT/CN2018/076449
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English (en)
French (fr)
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陈朝阳
李小森
张晋铭
张郁
李刚
吕秋楠
王屹
颜克凤
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中国科学院广州能源研究所
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Priority to US15/765,652 priority Critical patent/US10683736B2/en
Publication of WO2019134220A1 publication Critical patent/WO2019134220A1/zh

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    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/12—Methods or apparatus for controlling the flow of the obtained fluid to or in wells
    • E21B43/121—Lifting well fluids
    • E21B43/122—Gas lift
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B41/00—Equipment or details not covered by groups E21B15/00 - E21B40/00
    • E21B41/0099—Equipment or details not covered by groups E21B15/00 - E21B40/00 specially adapted for drilling for or production of natural hydrate or clathrate gas reservoirs; Drilling through or monitoring of formations containing gas hydrates or clathrates
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/01—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells specially adapted for obtaining from underwater installations
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/16—Enhanced recovery methods for obtaining hydrocarbons
    • E21B43/166—Injecting a gaseous medium; Injecting a gaseous medium and a liquid medium
    • E21B43/168—Injecting a gaseous medium
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/34—Arrangements for separating materials produced by the well
    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B43/00—Methods or apparatus for obtaining oil, gas, water, soluble or meltable materials or a slurry of minerals from wells
    • E21B43/34—Arrangements for separating materials produced by the well
    • E21B43/36—Underwater separating arrangements
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P90/00—Enabling technologies with a potential contribution to greenhouse gas [GHG] emissions mitigation
    • Y02P90/70—Combining sequestration of CO2 and exploitation of hydrocarbons by injecting CO2 or carbonated water in oil wells

Definitions

  • the invention belongs to the field of energy technology, and relates to a marine gas hydrate mining gas extraction method, in particular to a marine gas hydrate mining gas transmission system and a control method.
  • Natural Gas Hydrate is a non-stoichiometric, ice-like, cage-like crystalline compound formed from low molecular weight hydrocarbons in water and natural gas under low temperature and high pressure conditions.
  • the natural gas hydrates in nature are mainly methane hydrates, most of which are found on the seabed. They have the advantages of large reserves, wide distribution, shallow burial, high energy density, no pollution and residual after combustion.
  • the decomposition of methane hydrate per unit volume produces 150-180 standard volumes of methane gas. It is estimated that the organic carbon stored in the form of natural gas hydrates on the earth accounts for 53% of the total organic carbon in the world, and is twice the total carbon of fossil fuels of coal, oil and natural gas. Therefore, natural gas hydrates are considered to be the ideal clean alternative energy source for the 21st century.
  • Natural gas hydrates are deposited in solid form on the loose sediments of the muddy seabed, and phase transformation occurs during the mining process. Compared with the exploitation of oil and natural gas, it has great mining difficulty. According to the location of natural gas hydrate decomposition during the mining process, the exploitation of natural gas hydrate can be divided into two categories: underground decomposition and above-ground decomposition.
  • Chinese patent CN1294648A proposes to use high-pressure airflow to impact natural gas hydrate reservoirs and transport solid natural gas hydrates to the sea surface through airflow entrainment.
  • Chinese patent CN1587642A refers to the mining and sorting mode of land mines, and proposes to use underwater automatic excavation machinery to extract solid natural gas hydrate, and then use the process of sediment separation and natural gas hydrate decomposition to extract seabed natural gas hydrate.
  • Chinese patent CN105587303A discloses a green mining method and a mining device for shallow seabed diagenetic natural gas hydrates
  • CN105064959A discloses a green mining method for submarine non-diagenetic natural gas hydrates, which mainly uses seabed mining to extract natural gas.
  • the mixture of natural gas hydrate solid particles and seawater is used in a confined space, and the natural gas hydrate is decomposed into natural gas and water by the thermal energy of the sea surface hot sea water, and the gas lifting effect is increased to The sea.
  • These above-ground decomposing methods have a small range of mining adaptation, high technical requirements for underwater automatic mining equipment, difficult technical implementation, serious damage to the submarine geological structure, and easily cause problems such as bottom collapse and landslide.
  • the underground decomposing and mining research reports the most, mainly referring to the oil and natural gas mining process.
  • the wellbore is constructed in the submarine stratum, and measures are taken to destroy the thermodynamic conditions such as temperature and pressure in which the natural gas hydrate is stable, and promote the natural gas hydrate in the original place.
  • the bit is decomposed into water and natural gas, and then natural gas and water are collected.
  • the gas and water are separated and transported to the surface through a marine riser.
  • Natural gas hydrate underground decomposition and mining includes thermal excitation, depressurization and chemical methods.
  • most underground decomposition research focuses on how to economically, safely and efficiently decompose natural gas hydrates in situ in the formation.
  • Japan used the electric submersible pump to pump the bottom gas and water mixture through the production well to the seabed gas-liquid separator in the first trial mining of marine gas hydrate in the South China Sea trough.
  • the separated gas phase and liquid phase were respectively passed through two
  • the root marine riser is transported to the surface of the sea.
  • 2017, the China Geological Survey conducted natural gas hydrate trial mining in the Shenhu area of the South China Sea. It also used a high-power electric submersible pump to transport the gas-water mixture geological fluid well production well and marine riser of the hydrate layer to the sea surface to separate methane. Gas and water.
  • the object of the present invention is to overcome the above-mentioned shortcomings of the prior art and to provide an economical and efficient natural gas hydrate mining method and system.
  • a natural gas hydrate mining gas extraction method which utilizes the gas lift effect of methane gas released by natural gas hydrate decomposition, and conveys the gas-water mixture of the bottom of the production well through the marine riser to the sea platform to realize the controllability of the marine natural gas hydrate Self-spray mining, the specific steps are as follows:
  • Step 1 Start-up phase of mining: Firstly, a certain amount of nitrogen or methane gas is injected into the submarine gas storage tank through the compressor to make the pressure higher than the static pressure of the seabed; the wellhead device and the marine riser are opened, and the submarine gas storage tank and the ocean
  • the deep sea automatic control gate valve between the bottom of the pipe injects gas into the marine riser through the submarine gas storage tank, and uses gas stripping to lift the bottom hole liquid to the sea platform to reduce the pressure in the seabed hydrate layer to the hydrate phase equilibrium. Below the pressure, the hydrate in the seabed hydrate layer is decomposed into methane gas and water, and the gas-water mixture seeps into the production well under the pressure of the hydrate reservoir;
  • Step 2 The liquid-gas ratio of the gas-liquid mixed fluid produced from the hydrate reservoir is measured online by a sensor. For example, if the ratio of liquid to gas is greater than the ratio of self-injection to gas of the gas-liquid mixed fluid, the submarine gas storage tank is passed. Replenishing the gas to the marine riser; if the ratio of liquid to gas is less than the ratio of self-injection to gas of the gas-liquid mixed fluid, the valve between the subsea gas storage tank and the marine riser is closed, the gas supply is stopped, and the seafloor gas-liquid cyclone separation is started.
  • the valve to the marine riser divides part of the gas-liquid mixed fluid to the submarine gas-liquid cyclone separator, and the separated gas is pressurized by the booster pump to supplement the gas consumption in the seabed gas storage tank, and the remaining gas-liquid mixture Returning to the bottom of the marine riser, it is lifted to the sea platform by its own pneumatic force. After the gas-liquid mixed fluid is separated by the gas-liquid separator on the sea platform, the produced water can be directly discharged, and the produced methane gas is stored in the gas storage tank. delivery.
  • the self-ejection liquid-gas ratio of the gas-liquid mixed fluid increases as the production pressure of the bottom hole increases; at the same bottom hole production pressure, the self-jet liquid-gas ratio of the gas-liquid mixed fluid follows the water depth Decrease and increase.
  • the gas recovery method can be applied to the following natural gas hydrate production methods, including: ocean natural gas hydrate depressurization method, heat shock method, chemical injection method, and CO 2 displacement mining method.
  • Natural gas hydrate mining gas production system including sea platform, gas-liquid separator, gas storage tank, compressor, submarine gas storage tank, booster pump, submarine gas-liquid cyclone separator, gas buffer tank, marine riser a wellhead device, a production well;
  • the offshore platform is disposed at a sea level, the gas-liquid separator, a gas storage tank, and a compressor are disposed on an offshore platform;
  • the production well is vertically disposed above the seabed rock layer Passing through the seabed sediment layer and the gas hydrate layer, the top of the production well is connected to the wellhead device;
  • the bottom of the marine riser is connected to the wellhead device through the first valve, and the top of the marine riser passes through the pipeline and the sea surface
  • the gas-liquid separator, the gas storage tank and the compressor on the platform are connected in sequence;
  • the submarine gas storage tank, the booster pump, the submarine gas-liquid cyclone separator and the gas buffer tank are arranged beside the wellhead device, the sea bottom
  • a ball valve is disposed between the seabed gas storage tank and the compressor.
  • the production well is provided with a sand control device.
  • the first valve, the second valve, the third valve, the fourth valve, and the fifth valve are deep sea automatic control gate valves.
  • the present invention utilizes the gas lift effect of natural gas hydrate to raise the gas-liquid mixture decomposition gas gas mixture from the bottom of the well to the sea platform, which can reduce the significant gas production energy consumption; compared with the use of the downhole electric submersible pump lift, The seabed is resistant to high pressure moving equipment, and the process and equipment are greatly simplified;
  • the invention adopts the gas stripping action of the decomposition gas to transport the gas-liquid mixed fluid, and the pressure of the gas-liquid mixed fluid in the marine riser is significantly reduced compared with the use of the underground electric submersible pump, which can effectively avoid the gas and liquid in the marine riser.
  • the mixed fluid re-forms the hydrate, which leads to the risk of blockage of the marine riser.
  • the pressure in the marine riser can easily exceed the phase equilibrium pressure formed by the hydrate due to the pressurization of the electric submersible pump. Lead to the formation of secondary hydrates in the marine riser, posing a risk of clogging;
  • a natural gas hydrate gas production method the gas production process and equipment are simple, the operation is convenient, the energy consumption is low, the cost is low, the submarine moving equipment is not needed, the industrialization and the automatic production are easy to be realized, and the application range is wide. It can effectively avoid the risk of clogging caused by the formation of secondary hydrate in marine riser. It can be applied to various natural gas hydrate mining methods such as ocean natural gas hydrate depressurization method, heat shock method, chemical injection agent and CO2 displacement mining. Market prospects.
  • FIG. 1 is a schematic view showing the principle of a marine gas hydrate mining gas production system according to the present invention.
  • Figure 2 is a graph showing the relationship between the pressure at the bottom of the well and the ratio of the self-spraying liquid to gas.
  • the offshore platform 9 is erected by the prior art, and a vertical production well 13 is drilled above the seabed rock layer, and the production well 13 penetrates the seabed sediment layer and the natural gas hydrate layer.
  • a sand control device 14 is arranged in the production well, and the top of the production well is connected to the wellhead device 12; at the same time, the marine riser 10 is installed, and the bottom of the marine riser 10 is connected to the wellhead device 12 through the deep sea automatic control gate valve 205, and the top of the marine riser 10 passes through the pipeline and the sea platform 9
  • the gas-liquid separator 8, the gas storage tank 7, and the compressor 6 are connected in sequence; a subsea gas storage tank, a subsea gas-liquid cyclone separator 11, a gas buffer tank 4, and a booster pump 3 are installed beside the subsea production well.
  • the gas-liquid mixture inlet of the subsea gas-liquid cyclone separator 11 is connected to the wellhead device 12 through a pipeline and a deep sea automatic control gate valve 204, and the liquid outlet of the subsea gas-liquid cyclone separator 11 passes through the pipeline and the deep sea automatic control gate valve 203 and the ocean
  • the bottom of the tube 10 is connected, and the gas outlet at the top of the subsea gas-liquid cyclone separator 11 is sequentially connected to the gas buffer tank 4, the booster pump 3 and the deep sea automatic control gate valve 202 through a pipeline, and is connected It is connected to the top of the subsea gas storage tank 1; in addition, the subsea gas storage tank 1 is connected to the compressor 6 on the sea platform 9 through a pipeline and a ball valve 5, and is also connected to the bottom of the marine riser 10 through a pipeline and a deep sea automatic control gate valve 201.
  • a certain amount of nitrogen or methane gas is first injected into the seabed gas storage tank 1 through the compressor 6, so that the pressure is greater than the seabed static pressure; the deep sea automatic control gate valves 205 and 201 are opened, and the pressure is passed.
  • the gas is injected from the submarine gas storage tank 1 to the bottom of the marine riser 10, and the gas enters the marine riser 10 and rises by its own buoyancy, and the gas bottoming is used to lift the bottom hole liquid of the production well 13 to the sea platform, thereby reducing the bottom of the well.
  • the gas-liquid mixed fluid produced from the hydrate reservoir can be sprayed along the marine riser 10 to the sea platform under the lifting action of the methane gas contained therein. 9.
  • the deep sea automatic control gate valve 201 of the subsea gas storage tank 1 to the marine riser 10 is closed, the gas injection is stopped, and the hydrate mining enters the self-jet mining stage.
  • the liquid-gas ratio of the gas-liquid mixed fluid produced from the hydrate reservoir is measured online by a sensor, and if the liquid-gas ratio is greater than the gas-liquid mixed fluid self-spraying liquid-gas ratio, the deep sea automatic control gate valve 201 is opened.
  • the gas is supplied to the bottom of the marine riser 10 through the subsea gas storage tank 1 so that the liquid-to-gas ratio of the mixed fluid in the marine riser 10 is equal to or smaller than the ratio of the mixed fluid to the liquid-to-liquid ratio; if the ratio of liquid to gas is smaller than that of the liquid-liquid mixed fluid In the gas ratio, the deep sea automatic control gate valve 201 is closed, the gas supply is stopped, and the deep sea automatic control gate valves 202, 203, 204 are opened, and part of the gas-liquid mixed fluid is branched to the submarine gas-liquid cyclone separator 11, and the separated gas passes through the gas. After the buffer tank 4 and the booster pump 3 are pressurized, the gas consumption in the seabed gas storage tank 1 is supplemented, and the remaining gas-liquid mixture is returned to the bottom of the marine riser 10, and is lifted to the sea platform by its own pneumatic force.
  • the produced water can be directly discharged, and the produced methane gas is stored in the gas storage tank 7 and then transported to the outside.
  • the inner diameter of the marine riser is 200 mm
  • the gas-liquid mixed fluid production rate is 37.5 kg/s, such as a well.
  • the bottom pressure-reducing pressure is 8.0 MPa
  • the production fluid self-spraying liquid-to-gas ratio is 13.5 kg H 2 O/m 3 CH 4
  • the liquid-gas ratio of the mixed fluid that controls the bottom of the marine riser 10 is less than 13.5 kg H 2 O/ m 3 CH 4
  • the self-spray production can be carried out normally; if the pressure at the bottom of the well is 5.5 MPa, the ratio of the production fluid to the spray liquid is 9 kg H 2 O/m 3 CH 4 , and the control enters the bottom of the marine riser 10
  • the liquid-gas ratio of the mixed fluid is less than 9 kg H 2 O/m 3 CH 4 , and the self-spray production can be carried out normally.

Abstract

一种天然气水合物开采采气方法和采气系统。该采气方法利用天然气水合物分解释放的甲烷气的气举作用，将开采井井底的气水混合物通过海洋立管输送至海面平台，实现海洋天然气水合物的可控自喷开采；该采气系统包括具有气液分离器(8)、储气罐(7)、压缩机(6)的海面平台(9)，垂直设置海底岩石层上的开采井(13)，海洋立管(10)，海底气液旋流分离器(11)等装置。该采气方法及设备简单，操作方便，能耗低，无需海底动设备，易于实现工业化和自动化生产，可有效避免海洋立管中二次水合物形成导致的堵塞风险，应用范围广。

Description

一种天然气水合物开采采气方法及系统 技术领域
本发明属于能源技术领域，涉及一种海洋天然气水合物开采采气方法，尤其是一种海洋天然气水合物开采采气输气系统与控制方法。
背景技术
天然气水合物(Natural Gas Hydrate，简称Gas Hydrate)是在低温、高压条件下水和天然气中低分子量的烃类化合物形成的一种非化学计量型、类冰状、笼型结晶化合物。自然界存在的天然气水合物以甲烷水合物为主，其中绝大部分赋存于海底，具有储量大、分布广、埋藏浅、能量密度高、燃烧后无污染和残留等优点。单位体积的甲烷水合物分解可产生150-180标准体积的甲烷气体。据估计，地球上以天然气水合物形式储藏的有机碳占全球总有机碳的53％，是煤、石油、天然气三种化石燃料总碳量的2倍。因此，天然气水合物被认为是21世纪的理想清洁替代能源。
天然气水合物以固体形式赋存于泥质海底的松散沉积层中，在开采过程中发生相转化，与石油、天然气的开采相比，具有很大的开采难度。根据开采过程中天然气水合物分解的地点不同，天然气水合物的开采可分为地下分解开采和地上分解开采两大类。
地上分解开采主要针对浅埋藏、非成岩水合物藏的开采。中国专利CN1294648A提出采用高压气流冲击天然气水合物储层，并通过气流夹带输送固体天然气水合物至海面。中国专利CN1587642A参照陆地矿山采矿分选模式，提出采用水下自动挖掘机械开采固体天然气水合物、然后采用泥沙分离，天然气水合物分解等工艺开采海底天然气水合物。中国专利CN105587303A公开了一种海底浅层非成岩天然气水合物的绿色开采方法及开采装置，CN105064959A公开了一种海底非成岩天然气水合物的绿色开采方法，其主要是采用海底采矿的方式，采掘天然气水合物沉积物固体，并进行二次粉碎后，将天然气水合物固体颗粒与海水的混合物在密闭空间内，利用海面热海水的热能将天然气水合物分解为天然气和水，利用气力提升效应提升至海面。这些地上分解开采方法均存在开采适应 范围小，水下自动开采设备技术要求高，实施技术难度大，对海底地质构造破坏严重，容易引起井底塌陷、滑坡等问题。
地下分解开采研究报道最多，主要是参考石油、天然气的开采工艺，首先在海底地层中构筑井筒，采取措施破坏天然气水合物稳定存在的温度、压力等热力学条件，促进天然气水合物在赋存地原位分解为水和天然气，然后收集天然气和水，气水分离后通过海洋立管输送至海面。天然气水合物地下分解开采包括热激发法、降压法和化学法等，目前，大多数地下分解开采研究主要专注于如何采取经济、安全、高效的方法将天然气水合物在地层中原位分解，而对于分解后的气、水、砂混合物如何从井底输送至海面平台研究较少。2013年，日本在南海海槽开展的海洋天然气水合物首次试采中，主要采用电潜泵将井底气水混合物经开采井抽至海底气液分离器，分离后的气相和液相分别经两根海洋立管输送至海面。2017年中国地质调查局在南海神狐海域开展的天然气水合物试开采，也是采用大功率电潜泵将水合物层的气水混合物地质流体井开采井和海洋立管输送至海面，分离出甲烷气和水。这些采用海底电潜泵采气输气的方法，电潜泵能量消耗大、运行寿命短，开采成本高。因此，需要研究开发经济、高效的天然气水合物开采采气输气方法与技术，应用于海洋天然气水合物资源开发利用。
发明内容
本发明的目的是克服上述现有技术的缺点，提供一种经济、高效的天然气水合物开采采气方法及系统。
本发明是通过以下技术方案来实现的：
一种天然气水合物开采采气方法，利用天然气水合物分解释放的甲烷气的气举作用，将开采井井底的气水混合物通过海洋立管输送至海面平台，实现海洋天然气水合物的可控自喷开采，具体步骤如下：
步骤1.开采启动阶段：首先通过压缩机向海底气体储罐中注入一定量的氮气或甲烷气，使其压力大于海底静压；开启井口装置与海洋立管，以及海底气体储罐和海洋立管底部之间的深海自动控制闸阀，通过海底气体储罐向海洋立管注入气体，利用气体的气提作用将井底液体提升至海面平台，降低海底水合物层中的压力至水合物相平衡压力以下，使海底水合物层中的水合物分解为甲烷气和水， 在水合物藏压力的推动下，气水混合物渗流至开采井内；
步骤2.自喷开采阶段：通过传感器在线测定从水合物藏产出的气液混合流体的液气比，如液气比大于气液混合流体的自喷液气比，则通过海底气体储罐向海洋立管补充气体；如液气比小于气液混合流体的自喷液气比，则关闭海底气体储罐和海洋立管之间的阀门，停止供气，同时开启海底气液旋流分离器至海洋立管的阀门，将部分气液混合流体分流至海底气液旋流分离器，分离出的气体经增压泵增压后，补充海底气体储罐中气体的消耗，其余气液混合物返回海洋立管底部，通过自身的气力提升至海面平台，气液混合流体在海面平台经气液分离器分离后，产出的水可直接排放，产出甲烷气采用储气罐储存后，对外输送。
作为上述方案的改进，所述气液混合流体的自喷液气比随井底生产压力的升高而增大；在同等井底生产压力下，气液混合流体的自喷液气比随水深减小而增大。
作为上述方案的改进，所述的采气方法可应用于以下天然气水合物开采方法中，包括：海洋天然气水合物降压法、热激法、注化学剂法、CO 2置换开采法。
一种天然气水合物开采采气系统，包括海面平台、气液分离器、储气罐、压缩机、海底气体储罐、增压泵、海底气液旋流分离器、气体缓冲罐、海洋立管、井口装置、开采井；所述的海上平台设置在海平面上，所述的气液分离器、储气罐、压缩机设置在海上平台上；所述的开采井垂直设置海底岩石层之上，其贯穿海底沉积物层和天然气水合物层，开采井顶部与所述的井口装置连接；所述的海洋立管的底部通过第一阀门与井口装置连接，海洋立管的顶部通过管线与海面平台上的气液分离器、储气罐、压缩机依次连接；所述的海底气体储罐、增压泵、海底气液旋流分离器、气体缓冲罐设置在井口装置旁，所述的海底气液旋流分离器的气液混合物进口通过管线和第二阀门与井口装置连接，海底气液旋流分离器的液体出口通过管线和第三阀门与海洋立管底部连接，海底气液旋流分离器的气体出口通过管线依次与气体缓冲罐、增压泵、第四阀门连接后接入海底气体储罐；所述的海底气体储罐通过管线与压缩机连接，海底气体储罐通过管线和第五阀门与海洋立管的底部连接。
作为上述方案的改进，所述的所述的海底气体储罐与压缩机之间设有球阀。
作为上述方案的改进，所述的开采井中设有防砂装置。
作为上述方案的改进，所述的第一阀门、第二阀门、第三阀门、第四阀门、第五阀门为深海自动控制闸阀。
本发明具有以下有益效果：
(1)本发明利用天然气水合物的气举作用，将天然气水合物分解气液混合物从井底提升至海面平台，可降低显著采气能源消耗；与采用井下电潜泵举升相比，省略了海底耐高压动设备，工艺及设备大大简化；
(2)本发明所述产出流体液气比低于气液混合流体的自喷液气比时，通过海底气液分离器及海底储气罐收集和储存气体；在产出流体液气比高于自喷液气比时，通过海底储气罐向海洋立管供给气体，使之达到自喷开采要求。这样既有利于控制高气液比是流体的自喷速度，又可保证产出流体气液比低时的自喷开采需求，不仅节约了气举能耗，而且改善了自喷采气的操作稳定性；
(3)本发明采用分解气的气提作用输送气液混合流体，与采用井下电潜泵输送相比，海洋立管中气液混合流体的压力显著降低，可有效避免海洋立管中气液混合流体重新形成水合物，导致海洋立管堵塞的风险；而采用井下电潜泵输送，由于电潜泵的增压作用，极易使海洋立管中的压力超过水合物形成的相平衡压力，导致海洋立管中二次水合物生成，产生堵塞风险；
(4)本发明所述一种天然气水合物开采采气方法，采气工艺及设备简单，操作方便，能耗低，成本低，无需海底动设备，易于实现工业化和自动化生产，应用范围广，可有效避免海洋立管中二次水合物形成导致的堵塞风险，可应用于海洋天然气水合物降压法、热激法、注化学剂、CO2置换开采等多种天然气水合物开采方法中，未来市场前景广阔。
附图说明
图1为本发明的一种海洋天然气水合物开采采气系统的原理示意图。
图2为井底降压开采压力与自喷液气比关系图。
具体实施方式
实施例1
如图1所示，在海洋天然气水合物藏所在处，采用现有技术架设海面平台9， 在海底岩石层之上钻垂直开采井13，开采井13贯穿海底沉积物层和天然气水合物层，开采井中设防砂装置14，开采井顶部连接井口装置12；同时安装海洋立管10，海洋立管10底部通过深海自动控制闸阀205连接至井口装置12，海洋立管10顶部通过管线与海面平台9上的气液分离器8、储气罐7，以及压缩机6依次相连；在海底生产井旁安装海底气体储罐1、海底气液旋流分离器11、气体缓冲罐4和增压泵3，海底气液旋流分离器11的气液混合物进口通过管线和深海自动控制闸阀204与井口装置12相连，海底气液旋流分离器11的液体出口通过管线和深海自动控制闸阀203与海洋立管10底部相连，海底气液旋流分离器11顶部的气体出口通过管线依次与气体缓冲罐4、增压泵3和深海自动控制闸阀202相连，并连接至海底气体储罐1顶部；另外，海底气体储罐1还通过管线和球阀5与海面平台9上的压缩机6相连，还通过管线和深海自动控制闸阀201与海洋立管10底部相连。
在进行降压法水合物开采时，首先通过压缩机6向海底气体储罐1中注入一定量的氮气或甲烷气，使其压力大于海底静压；开启深海自动控制闸阀205和201，通过压力从海底气体储罐1向海洋立管10底部注入气体，气体进入海洋立管10后通过自身浮力上升，利用气体的气提作用将开采井13的井底液体提升至海面平台，从而降低井底及海底水合物层中的压力至水合物相平衡压力以下，使海底水合物层中的水合物分解，释放出甲烷气和水，在水合物藏压力梯度的推动下，渗流至开采井13的井底。
当海底水合物层产出的水和甲烷气达到一定值后，从水合物藏产出的气液混合流体就可以在所含甲烷气的提升作用下，沿海洋立管10自喷至海面平台9，此时关闭海底气体储罐1至海洋立管10的深海自动控制闸阀201，停止注气，水合物开采进入自喷开采阶段。
在自喷开采过程中，通过传感器在线测定从水合物藏产出的气液混合流体的液气比，如液气比大于气液混合流体自喷液气比，则开启深海自动控制闸阀201，通过海底气体储罐1向海洋立管10底部补充气体，使海洋立管10中的混合流体液气比等于或小于混合流体自喷液气比；如液气比小于气液混合流体自喷液气比，则关闭深海自动控制闸阀201，停止供气，同时开启深海自动控制闸阀202、203、204，将部分气液混合流体分流至海底气液旋流分离器11，分离出的气体经气体 缓冲罐4、增压泵3增压后，补充海底气体储罐1中气体的消耗，其余气液混合物返回海洋立管10底部，通过自身的气力提升至海面平台。
经自喷开采输送至海面平台的气液混合流体经气液分离器8分离后，产出的水可直接排放，产出甲烷气采用储气罐7储存后，对外输送。
如图2所示，对于水深2000米(海洋立管与生产井长度之和)的天然气水合物藏，海洋立管内径为200毫米，气液混合流体开采速率为37.5kg/s时，如井底降压开采压力采用8.0MPa，则生产流体自喷液气比为13.5kg H 2O/m 3CH 4，控制进入海洋立管10底部的混合流体的液气比小于13.5kg H 2O/m 3CH 4，则自喷生产可以正常进行；如井底降压开采压力采用6.0MPa，则生产流体自喷液气比为9kg H 2O/m 3CH 4，控制进入海洋立管10底部的混合流体的液气比小于9kg H 2O/m 3CH 4，则自喷生产可以正常进行。
上列详细说明是针对本发明可行实施例的具体说明，该实施例并非用以限制本发明的专利范围，凡未脱离本发明所为的等效实施或变更，均应包含于本案的专利范围中。

Claims (7)

  1. 一种天然气水合物开采采气方法，其特征在于，利用天然气水合物分解释放的甲烷气的气举作用，将开采井井底的气水混合物通过海洋立管输送至海面平台，实现海洋天然气水合物的可控自喷开采，具体步骤如下：
    步骤1.开采启动阶段：首先通过压缩机向海底气体储罐中注入一定量的氮气或甲烷气，使其压力大于海底静压；开启井口装置与海洋立管之间的深海自动控制闸阀，以及海底气体储罐和海洋立管底部之间的深海自动控制闸阀，通过海底气体储罐向海洋立管注入气体，利用气体的气提作用将井底液体提升至海面平台，降低海底水合物层中的压力至水合物相平衡压力以下，使海底水合物层中的水合物分解为甲烷气和水，在水合物藏内压力的推动下，气水混合物渗流至开采井内；
    步骤2.自喷开采阶段：通过传感器在线测定从水合物藏产出的气液混合流体的液气比，如液气比大于气液混合流体的自喷液气比，则通过海底气体储罐向海洋立管补充气体；如液气比小于气液混合流体的自喷液气比，则关闭海底气体储罐和海洋立管之间的阀门，停止供气，同时开启海底气液旋流分离器至海洋立管的阀门，将部分气液混合流体分流至海底气液旋流分离器，分离出的气体经增压泵增压后，补充海底气体储罐中气体的消耗，其余气液混合物返回海洋立管底部，通过自身的气力提升至海面平台，气液混合流体在海面平台经气液分离器分离后，产出的水可直接排放，产出甲烷气采用储气罐储存后，对外输送。
  2. 根据权利要求1所述的一种天然气水合物开采采气方法，其特征在于，所述气液混合流体的自喷液气比随井底生产压力的升高而增大；在同等井底生产压力下，气液混合流体的自喷液气比随水深减小而增大。
  3. 根据权利要求1所述的一种天然气水合物开采采气方法，其特征在于，所述的采气方法可应用于以下天然气水合物开采方法中，包括：海洋天然气水合物降压法、热激法、注化学剂法、CO 2置换开采法。
  4. 一种天然气水合物开采采气系统，其特征在于，包括海面平台、气液分离器、储气罐、压缩机、海底气体储罐、增压泵、海底气液旋流分离器、气体缓冲罐、海洋立管、井口装置、开采井；所述的海上平台设置在海平面上，所述的 气液分离器、储气罐、压缩机设置在海上平台上；所述的开采井垂直设置海底岩石层之上，其贯穿海底沉积物层和天然气水合物层，开采井顶部与所述的井口装置连接；所述的海洋立管的底部通过第一阀门与井口装置连接，海洋立管的顶部通过管线与海面平台上的气液分离器、储气罐、压缩机依次连接；所述的海底气体储罐、增压泵、海底气液旋流分离器、气体缓冲罐设置在井口装置旁，所述的海底气液旋流分离器的气液混合物进口通过管线和第二阀门与井口装置连接，海底气液旋流分离器的液体出口通过管线和第三阀门与海洋立管底部连接，海底气液旋流分离器的气体出口通过管线依次与气体缓冲罐、增压泵、第四阀门连接后接入海底气体储罐；所述的海底气体储罐通过管线与压缩机连接，海底气体储罐通过管线和第五阀门与海洋立管的底部连接。
  5. 根据权利要求4所述的一种天然气水合物开采采气系统，其特征在于，所述的所述的海底气体储罐与压缩机之间设有球阀。
  6. 根据权利要求4所述的一种天然气水合物开采采气系统，其特征在于，所述的开采井中设有防砂装置。
  7. 根据权利要求4所述的一种天然气水合物开采采气系统，其特征在于，所述的第一阀门、第二阀门、第三阀门、第四阀门、第五阀门为深海自动控制闸阀。
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