多孔介质中不同饱和度天然气水合物的分解特性研究

开采天然气水合物对缓解世界能源危机、解决中国天然气资源短缺问题具有重要现实意义。降压法是开采天然气水合物一种较为简单、安全、有效的方法,但是目前对水合物饱和度较高条件下的降压分解特性缺乏深入认识。在多孔介质（石英砂）体系开展了天然气水合物的生成与分解实验,研究了3种不同水合物饱和度（38.1%、42.1%、46.4%）条件下的水合物降压分解特性,分解压力分别为2 MPa和3 MPa。实验结果表明：2 MPa对应的产气速率较高,但是模拟储层温度会降至冰点以下,在实际水合物开采中会引起冰堵,阻碍水合物进一步分解。当分解压力为3 MPa时,在0～2 h内,随着水合物饱和度增加,产气速率减小;从2 h到水合物分解结束,水合物饱和度越高,产气速率越高;在降压阶段,不同饱和度水合物的储层温度（T1、T2、T3）快速下降且最低温度相似;在恒压阶段,不同饱和度水合物的储层温度开始回升,水合物饱和度越高,储层温度波动越大,并且恢复至实验设定温度需要的时间越长。研究结果将为水合物试采过程防止冰堵现象发生、提高气体回收率提供重要的理论参考。     

热水驱替开采Ⅱ类水合物藏规律研究

天然气水合物广泛分布于陆地冻土带和深海地层,资源潜力巨大,其中Ⅱ类水合物藏占有重要地位。为加强对Ⅱ类水合物藏开采规律的认识,结合实际水合物藏参数,使用数值模拟方法研究了热水驱替开采Ⅱ类水合物藏的动态规律,并与降压法的开采效果进行了对比分析。结果表明:①热水驱替开采Ⅱ类水合物藏时,产气速率和分解气速率首先快速上升,然后以较快速度下降至趋于相对稳定;累产气和累分解气上升较快;气体采出程度和水合物分解程度均处于较高水平(>60%)。②热水驱替对Ⅱ类水合藏的开采具有一定的适应性,与降压法开采相比,热水驱替方式下储层水合物的分解更彻底,气体采出程度、水合物分解程度也更优,但具有较低的累积气水比,产水量较大。     

Investigation of the hydrate reservoir production under different depressurization modes

Abstract

Large reserves of natural gas hydrates exist, and the depressurization method has the greatest potential for gas hydrate reservoir recovery. Currently, the most commonly adopted depressurization simulation method is a constant bottom-hole pressure production scheme. This study proposes a new depressurization mode with decreasing bottom-hole pressure. The production characteristic was numerically investigated using this method. The results show the following: (1) As the depressurization exponent (n) decreases, the development effect improves, and production indexes including cumulative gas production/dissociation and gas-water ratio increase. However, the reservoir energy consumption is higher and the hydrate reformation is more severe. (2) Compared to the proposed depressurization mode, the hydrate production index of the constant bottom-hole pressure production (n?=?0) is better. However, the hydrate reservoir energy consumption is higher and the hydrate reformation is more severe using constant bottom-hole pressure production. (3) To achieve a balance between production and reservoir energy consumption during depressurization production, the bottom-hole pressure should be controlled by selecting a suitable depressurization exponent between n_min and n_max, which can be determined through numerical simulations.     

极地天然气水合物资源利用前景

极地天然气水合物资源潜力巨大,储层类型主要为富砂沉积物储层,最可能实现远景勘探和商业利用,是一种重要的战略能源。综述了天然气水合物在极地的分布情况、极地主要地区天然气水合物矿藏资源量以及水合物开发利用的资源金字塔。结合美国、加拿大和俄罗斯在北极冻土区天然气水合物勘探开发的案例进行了4种开采技术优缺点对比,并对极地区和非极地区5次天然气水合物试采效能进行对比评价。根据目前国际油气价格、能源结构、各国水合物研究目标和我国天然气水合物勘查开发现状等,提出了我国参与极地天然气水合物研究和开发的思路。     

海洋天然气水合物开采方法及产量分析

海洋天然气水合物的巨大储量刺激了世界各国能源部门努力研究如何从天然气水合物储层生产天然气。根据水合物形成的条件,只有当水合物处在其相平衡条件以外,水合物才能分解。因此,水合物的开采方法只能为热熔法、抑制剂刺激法、减压法和地面分解法。为了对天然气水合物储层中气体的生产有个定量的评估,本文以水合物开采井为例,运用数学方法推导了水合物井中气体的产生量。结果表明,在天然气水合物储层中,天然气释放量是井内水合物分解温度、压力及水合物层气体渗透性的敏感函数。该函数可以用于天然气水合物井气体开采量的计算及对水合物储层可开采性评价。     

天然气水合物开采井孔出砂问题研究

天然气水合物是一种重要的非常规能源,但水合物开采出砂问题成为制约其走向商业化开发的关键因素。南海北部陆坡是我国天然气水合物的重要赋存区,其水合物主要以分散状弱胶结方式赋存,研究其出砂问题显得极为迫切。研究表明储层出砂的根本原因是储层介质强度降低,而水合物储层介质强度受到水合物胶结模式、水合物饱和度、孔隙流体压力、流速及生产压差等影响。本文在对比分析天然气水合物储层出砂与常规油气藏出砂特征的基础上,总结了弱胶结疏松砂岩油藏出砂研究方法,为天然气水合物储层开采井孔出砂的研究提供思路和参考。     

非成岩水合物储层降压开采过程中出砂和沉降实验研究

天然气水合物多赋存在非成岩地层中,在开采过程中易出现出砂和沉降情况,制约了天然气水合物的安全高效长期开采。为研究水合物开采过程中的温压、产气、产水、出砂和沉降情况,在自主研发水合物出砂及防砂模拟装置上进行了不同条件下的开采模拟实验。研究表明,在前两个生产阶段,产水含砂率和出砂粒径随着水合物开采而逐渐增大;水合物细砂储层产气速率增加会增大携液能力,导致携砂能力增强而增大出砂风险,同时高产气速率促进井筒温度降低导致冰相生成,存在冰堵的风险;开采过程中的储层沉降与储层水合物含量相关性较大,而产气速率和降压速率对储层沉降的影响与产气模式有关。水合物开采中后期进行增产作业会增加储层出砂风险和沉降速率,进一步探讨了该实验对日本2013年第一次海域水合物试采出砂情况的推测,提出水合物开采分阶段分级防砂的概念。     

天然气水合物降压开采过程中储层应力规律分析

为了研究天然气水合物降压开采过程的储层应力及其稳定性,运用线性多孔弹性力学和岩石力学知识,考虑水合物储层原始应力、孔隙压力、渗流附加应力及降压开采水合物过程中水合物饱和度的变化,建立了降压开采天然气水合物储层的力学模型,结合墨西哥湾某处水合物藏的基本参数,对降压开采水合物储层应力变化和开采过程的储层稳定性进行研究。结果表明:井底压力是影响水合物储层应力变化的关键因素之一;渗流附加应力在一定程度上减小了储层的应力;水合物分解储层应力发生变化,储层应力在井壁处的波动最大,井壁处是整个储层所受轴向偏应力最大的位置,因此井壁处是优先发生剪切破坏的位置;为了储层的稳定性,降压开采水合物生产压差应小于2.19 MPa。     

Formation history and physical properties of sediments from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

The synthesis of available geological information and surface temperature evolution in the Alaska North Slope region suggests that: biogenic and deeper thermogenic gases migrated through fault networks and preferentially invaded coarse-grained layers that have relatively high hydraulic conductivity and low gas entry pressures; hydrate started forming before the beginning of the permafrost; eventually, the permafrost deepened and any remaining free water froze so that ice and hydrate may coexist at some elevations. The single tested specimen (depth 620.47-620.62 m) from the D unit consists of uncemented quartzitic fine sand with a high fraction of fines (56% by mass finer than sieve #200). The as-received specimen shows no evidence of gas present. The surface texture of sediment grains is compatible with a fluvial-deltaic sedimentation environment and shows no signs of glacial entrainment. Tests conducted on sediments with and without THF hydrates show that effective stress, porosity, and hydrate saturation are the major controls on the mechanical and geophysical properties. Previously derived relationships between these variables and mechanical/geophysical parameters properly fit the measurements gathered with Mount Elbert specimens at different hydrate saturations and effective stress levels. We show that these measurements can be combined with index properties and empirical geomechanical relationships to estimate engineering design parameters. Volumetric strains measured during hydrate dissociation vanish at 2-4 MPa; therefore, minimal volumetric strains are anticipated during gas production at the Mount Elbert well. However, volume changes could increase if extensive grain crushing takes place during depressurization-driven production strategies, if the sediment has unexpectedly high in situ porosity associated to the formation history, or if fines migration and clogging cause a situation of sustained sand production.     

天然气水合物分解诱发海底滑坡影响因素分析及致灾风险评价

天然气水合物主要赋存于低温高压下的海底沉积物层中,当周围环境的温度或者压力发生变化时,其稳定性会受到破坏,诱发坍塌、海底滑坡等地质灾害,对钻井平台、海底电缆等造成巨大破坏。结合南海北部海底陆坡的实际地震资料,首先获得了符合实际情况的储层属性参数,然后基于改进的地质力学模型,获得了相应的力学模型参数,利用孔隙压力平衡方程计算得到地层的有效应力,对天然气水合物分解诱因的海底滑坡的数值模拟,基于强度折减法讨论分析了初始水合物分解量、水合物分解总量等因素引起的水合物储层变化的力学响应,获得了对应的安全系数,实现了水合物分解对海底边坡稳定性影响的分析,为今后水合物开采过程中可能诱发边坡失稳的程度及失稳位置分布预测提供了指导和帮助。     

Analysis of the Black Sea sediments by evaluating DSDP Leg 42B drilling data for gas hydrate potential

Many gas seepages, temperature, pressure, salinity, anoxic environment and high source gas potential of the Black Sea indicates that the Black Sea might have huge potentials for biogenic and thermogenic gas hydrates. However, the last important parameter to consider gas hydrate as an energy source is the type of sediments. Coarse marine sands are considered as good hydrate reservoirs because of high porosity and high permeability. Only very limited data is available related to the types of lithology of the Black Sea sediments. Hence, in this study, the literature data (especially the drilling and coring data of DSDP Leg 42B program) about gas seepages, temperature gradient, pressure gradient, salinity, anoxic environment and high source gas potential, and the types of the sediments in the Black Sea were investigated and analyzed. Although gas seepages, temperature gradient, pressure gradient, salinity, anoxic environment and high source gas potential of the Black Sea are appropriate for producible gas hydrate reservoirs, the sediments of the Black Sea appear to be generally fine grained with high clay content. Sandy-silt and silty sand layers in turbidites of the Black Sea might be potential producible hydrate reservoirs but these sediments are fine. As well as turbidites, separate thin sand layers might be potential gas hydrate reservoirs as an energy source in the Black Sea.     

世界热点地区高饱和度天然气水合物有利沉积储集条件

天然气水合物是全球未来能源的接替资源,高饱和度（Sh＞50%）水合物储层是未来面向工业化开采的首要选择。截止到目前,高饱和度天然气水合物有利沉积相带与储层条件之间的关系仍缺乏系统研究。根据公开发表的文献资料,系统总结了墨西哥湾、日本南海海槽、韩国郁陵盆地、印度Krishna-Godavari盆地以及南海神狐海域等全球5个天然气水合物热点钻探区64口井取芯及井-震联合资料,对含水合物储层岩性、沉积环境、水合物饱和度等参数进行的详细总结分析表明：在必要的温压环境和气源条件下,深海平原区块体搬运沉积和浊流等高沉积速率的深水砂质沉积物赋存孔隙型水合物,水合物可分布在砂岩、极细砂岩、粉砂岩、粉砂质黏土和泥等粒级沉积物中,但高饱和度水合物主要赋存于粉砂-细砂岩中,储层孔隙度与饱和度具有一定的正相关性。中国南海神狐海域发现含有孔虫黏土质粉砂或粉砂质黏土这种特殊的细粒沉积物,其水合物饱和度可达到中高水平（20%～76 %）。上述研究成果及认识奠定了下一步寻找优质天然气水合物储层的地质基础,也可为高饱和度水合物商业化勘探开发提供理论依据。     

Numerical studies of methane production from Class 1 gas hydrate accumulations enhanced with carbon dioxide injection

Class 1 gas hydrate accumulations are characterized by a permeable hydrate-bearing interval overlying a permeable interval with mobile gas, sandwiched between two impermeable intervals. Depressurization-induced dissociation is currently the favored technology for producing gas from Class 1 gas hydrate accumulations. The depressurization production technology requires heat transfer from the surrounding environment to sustain dissociation as the temperature drops toward the hydrate equilibrium point and leaves the reservoir void of gas hydrate. Production of gas hydrate accumulations by exchanging carbon dioxide with methane in the clathrate structure has been demonstrated in laboratory experiments and proposed as a field-scale technology. The carbon dioxide exchange technology has the potential for yielding higher production rates and mechanically stabilizing the reservoir by maintaining hydrate saturations. We used numerical simulation to investigate the advantages and disadvantages of using carbon dioxide injection to enhance the production of methane from Class 1 gas hydrate accumulations. Numerical simulations in this study were primarily concerned with the mechanisms and approaches of carbon dioxide injection to investigate whether methane production could be enhanced through this approach. To avoid excessive simulation execution times, a five-spot well pattern with a 500-m well spacing was approximated using a two-dimensional domain having well boundaries on the vertical sides and impermeable boundaries on the horizontal sides. Impermeable over- and under burden were included to account for heat transfer into the production interval. Simulation results indicate that low injection pressures can be used to reduce secondary hydrate formation and that direct contact of injected carbon dioxide with the methane hydrate present in the formation is limited due to bypass through the higher permeability gas zone.     

储层和开采参数对天然气水合物开采产能的影响分析

开展储层参数和开采参数对天然气水合物开采产能影响的研究有助于其实际开采场址和开采方法的选择。以中国南海神狐海域SH7站位的地质参数为背景,采用TOUGH+HYDRATE软件系统地分析了储层压力、温度、孔隙度、水合物饱和度、渗透率、上覆层和下伏层渗透率等储层参数,以及降压幅度、降压井长度和出砂堵塞（通过改变井周网格渗透率反映出砂堵塞）等开采参数对天然气水合物降压开采产能的影响。数值模拟结果表明：①随着储层渗透率的增大,产气量有明显的增加;随着储层压力、孔隙度的增大以及上覆层和下伏层渗透率的减小,产气量有较大的增加;随着储层温度的增大,产气量有一定的增加;产气量随饱和度的增大先增大后减小。因此,实际开采时可优先选择渗透率大、上覆层和下伏层渗透率小、孔隙度大、温度较高、水合物饱和度适中的储层。②随着降压幅度的增大以及降压井长度增大,产气量有明显的增加;而随着出砂堵塞程度的加剧,产气量有非常明显的减少。因此,实际开采时可以通过增大降压幅度和降压井长度以及采取减轻出砂堵塞的措施来提高产气量。研究结果可以为我国将来天然气水合物开采区域及开采方式的选择和确定提供参考。     

天然气水合物的重要特征与评价及其在中国海域的勘探前景

从勘探技术和资源评价的角度综述了甲烷水合物生成和聚集的重要特征, 如地震反射剖面、测井曲线资料、地球化学特点等以及对未知区的地质勘探和选区评价 .甲烷水合物在地震剖面上主要表现为BSR(似海底反射)、振幅变形(空白反射)、速度倒置、速度-振幅结构(VAMPS)等,大规模的甲烷水合物聚集可以通过高电阻率(＞100欧姆.米)声波速度、低体积密度等号数进行直接判读.此项研究实例表明,沉积物中典型甲烷水合物具有低渗透性和高毛细管孔隙压力特点,地层孔隙水矿化度也呈异常值,并具有各自独特的地质特征.现场计算巨型甲烷水合物储层中甲烷资源量的方法可分为:测井资料计算法公式为:SW＝(abRw/φm.Rt)1/n;地震资料计算法公式为:ρp＝(1-φ)ρm+(1-s)φρw+sφρh、VH＝λ.φ.S.对全球甲烷水合物总资源量预测的统计达20×1015m3以上.甲烷水合物形成需满足高压、低温条件,要求海水深度＞300 m.因此,甲烷水合物的分布严格地局限于两极地区和陆坡以下的深水地区,并具有3种聚集类型:1.永久性冻土带;2.浅水环境;3.深水环境.深海钻探计划(DSDP)和大洋钻探计划(ODP)已在下述10个地区发现大规模的甲烷水合物聚集,他们是:秘鲁、哥斯达黎加、危地马拉、墨西哥、美国东南大西洋海域、美国西部太平洋海域、日本海域的两个地区、阿拉斯加和墨西哥湾地区.在较浅水沉积物岩心样中发现甲烷水合物的地区,包括黑海、里海、加拿大北部、美国加里福尼亚岸外、墨西哥湾北部、鄂霍茨克海的两个地区.在垂向上,甲烷水合物主要分布于海底以下2 000 m以浅的沉积层中.最新统计表明又主要分布于二个深度区间:200～450 m和700～920 m,前者是由ODP995～997站位发现的;后者在加拿大麦肯齐河三角洲马立克2L-38号井中897～922 m处发现.中国海域已发现多处甲烷水合物可能赋存地区,包括东沙群岛南部、西沙海槽北部、西沙群岛南部以及东海海域地区.姚伯初报道了南海地区9处地震剖面速度异常值的发现,海水深度为420～3 920 m,海洋地质研究所则在东海海域解释了典型BSR反射的剖面,具有速度异常、弱振幅、空白反射、与下伏反射波组具不整合接触关系(VAMPS)等,大致圈定了它们的分布范围,表明在中国海域寻找甲烷水合物具有光明的前景.     

The thermal structure of the Dvurechenskii mud volcano and its implications for gas hydrate stability and eruption dynamics

The sediment temperature distribution at mud volcanoes provides insights into their activity and into the occurrence of gas hydrates. If ambient pressure and temperature conditions are close to the limits of the gas hydrate stability field, the sediment temperature distribution not only limits the occurrence of gas hydrates, but is itself influenced by heat production and consumption related to the formation and dissociation of gas hydrates. Located in the Sorokin Trough in the northern Black Sea, the Dvurechenskii mud volcano (DMV) was in the focus of detailed investigations during the M72/2 and M73/3a cruises of the German R/V Meteor and the ROV Quest 4000 m in February and March 2007. A large number of in-situ sediment temperature measurements were conducted from the ROV and with a sensor-equipped gravity corer. Gas hydrates were sampled in pressurized cores using a dynamic autoclave piston corer (DAPC). The thermal structure of the DMV suggests a regime of fluid flow at rates decreasing from the summit towards the edges of the mud volcano, accompanied by intermittent mud expulsion at the summit. Modeled gas hydrate dissociation temperatures reveal that the gas hydrates at the DMV are very close to the stability limits. Changes in heat flow due to variable seepage rates probably do not result in changes in sediment temperature but are compensated by gas hydrate dissociation and formation.     

南海东沙海域天然气水合物与地质构造的关系

构造作用和构造过程是控制天然气水合物发育和赋存的重要地质因素之一。陆坡区复杂的构造运动能够形成良好的气体运移通道以及欠压实、高孔隙的水合物储集空间。东沙群岛邻近海域具有水深变化大、沉积厚度大、沉积速率高和有机质丰富等天然气水合物有利赋存条件,最新的研究已经在该海域发现了天然气水合物赋存的地球物理证据BSR,针对东沙群岛海域广泛发育的断裂、底辟、海底滑坡等构造,开展了其与天然气水合物成藏的关系研究,可以进一步深入了解天然气水合物在东沙群岛不同地质构造中的分布特征与演化,为该区天然气远景评估提供参考。     

Occurrence and exploration of gas hydrate in the marginal seas and continental margin of the Asia and Oceania region

Supplies of conventional natural gas and oil are declining fast worldwide, and therefore new, unconventional forms of energy resources are needed to meet the ever-increasing demand. Amongst the many different unconventional natural resources are gas hydrates, a solid, ice-like crystalline compound of methane and water formed under specific low temperature and high pressure conditions. Gas hydrates are believed to exist in large quantities worldwide in oceanic regions of continental margins, as well as associated with permafrost regions in the Arctic. Some studies to estimate the global abundance of gas hydrate suggest that the total volume of natural gas locked up in form of gas hydrates may exceed all known conventional natural gas reserves, although large uncertainties exist in these assessments. Gas hydrates have been intensively studied in the last two decades also due to connections between climate forcing (natural and/or anthropogenic) and the potential large volumes of methane trapped in gas hydrate accumulations. The presence of gas hydrate within unconsolidated sediments of the upper few hundred meters below seafloor may also pose a geo-hazard to conventional oil and gas production. Additionally, climate variability and associated changes in pressure-temperature regimes and thus shifts in the gas hydrate stability zone may cause the occurrence of submarine slope failures.Several large-scale national gas hydrate programs exist especially in countries such as Japan, Korea, Taiwan, China, India, and New Zealand, where large demands of energy cannot be met by domestic supplies from natural resources. The past five years have seen several dedicated deep drilling expeditions and other scientific studies conducted throughout Asia and Oceania to understand gas hydrates off India, China, and Korea. This thematic set of publications is dedicated to summarize the most recent findings and results of geo-scientific studies of gas hydrates in the marginal seas and continental margin of the Asia, and Oceania region.     

Occurrence and exploration of gas hydrate in the marginal seas and continental margin of the Asia and Oceania region

Supplies of conventional natural gas and oil are declining fast worldwide, and therefore new, unconventional forms of energy resources are needed to meet the ever-increasing demand. Amongst the many different unconventional natural resources are gas hydrates, a solid, ice-like crystalline compound of methane and water formed under specific low temperature and high pressure conditions. Gas hydrates are believed to exist in large quantities worldwide in oceanic regions of continental margins, as well as associated with permafrost regions in the Arctic. Some studies to estimate the global abundance of gas hydrate suggest that the total volume of natural gas locked up in form of gas hydrates may exceed all known conventional natural gas reserves, although large uncertainties exist in these assessments. Gas hydrates have been intensively studied in the last two decades also due to connections between climate forcing (natural and/or anthropogenic) and the potential large volumes of methane trapped in gas hydrate accumulations. The presence of gas hydrate within unconsolidated sediments of the upper few hundred meters below seafloor may also pose a geo-hazard to conventional oil and gas production. Additionally, climate variability and associated changes in pressure-temperature regimes and thus shifts in the gas hydrate stability zone may cause the occurrence of submarine slope failures.Several large-scale national gas hydrate programs exist especially in countries such as Japan, Korea, Taiwan, China, India, and New Zealand, where large demands of energy cannot be met by domestic supplies from natural resources. The past five years have seen several dedicated deep drilling expeditions and other scientific studies conducted throughout Asia and Oceania to understand gas hydrates off India, China, and Korea. This thematic set of publications is dedicated to summarize the most recent findings and results of geo-scientific studies of gas hydrates in the marginal seas and continental margin of the Asia, and Oceania region.