天然气水合物开发的水平井控水控砂完井研究进展

2020年第二次南海水合物试采证明水平井是实现产业化的重要途径,计划在2030年南海天然气水合物商业开发中补齐粤港澳大湾区天然气供给的短板。但我国海洋水合物甜点多赋存在高含水、边底水丰富的非成岩泥质粉砂储层,水平井开发过程中储层水（排液）易携泥砂（出砂）脊进突入井筒导致产量降低,水平井控水控砂完井是产业化的瓶颈问题之一。针对第二次水平井开发水合物出现的新问题,分析了海洋天然气水合物储层开发过程中的水平井非均衡排液出砂情况,总结了国内外水平井控水控砂实验、模拟和现场的进展,提出了水平井开采水合物控水控砂的难点及我国面临的挑战。分析结果表明,天然气水合物储层开发的水平井控水控砂与常规油气开发存在共性问题,也有其自身分解特点及其赋存的非成岩储层有关的特性问题。针对我国海域天然气水合物储层间各向异性明显、潜在的“四气合采”和“碳封存”,对水合物水平井控水控砂抽取和注入提出了具体的研究思路及建议,以期推动海洋天然气水合物产业化开发进程。     

海洋天然气水合物开发产能及出砂研究

海洋天然气水合物多分布在深水非成岩储层,其开采过程中易出现泥砂运移（出砂）的情况且难以避免,然而出砂条件下的水合物产能估算偏差较大。本文通过室内海洋水合物降压开采出砂实验数据和海洋水合物试采公开资料,首次推导了出砂条件下现场尺度海洋水合物储层产能情况：在相当于1 200 m水深覆压、30.5 m厚的水合物细砂储层和7寸垂直井不防砂的情况下,得到最大产气速率4.63 ~ 14.1 m³/s,折合40.01×10⁴ ~ 121.84×10⁴ m³/d,综合出砂率0.16% ~ 10.74%;泥质储层在不防砂垂直井和水平井单个半径12 mm射孔下,其最大产气速率达到79.95×10⁴ m³/d和170 m³/d,但其综合出砂率是灾难性的。由于时空限制,产气速率、综合产能和出砂率还有很大提升空间,在平衡综合出砂率（控砂精度）和产气效率（产能）的情况下,有望达到产业化规模。本研究为合理估算出砂条件下的海洋天然气水合物产能提供支撑。     

自然界天然气水合物勘探开发概述

我国南海北部神狐区域海洋天然气水合物首次试采成功表明这种新型能源的巨大资源潜力,天然气水合物也越来越受到全社会关注,国家已将水合物开发列入中长期发展规划之中。本文首先介绍了当前自然界中水合物在全球的分布范围及资源量评估情况,接着分析了全球水合物勘探开发进展状态,然后从地震波反演、钻探取芯/测井以及室内实验室测试三个方面概述了野外水合物储层的勘探手段和赋存产状情况,最后根据不同水合物藏分类,阐述了相应的开采评估和技术方案,以及仍然面临的工艺难点和不确定性。借鉴水合物试验开采的经验,提出了我国水合物海陆并举、加快节奏的建议,可为水合物相关人员提供一定的参考。     

南海北部陆坡天然气水合物研究进展

综合调查已揭示了一系列显示南海北部陆坡存在天然气水合物的地质、地球物理和地球化学指标。结果表明两类天然气水合物系统即低通量扩散型水合物和高通量渗漏型水合物共存于南海北部陆坡,特别是东沙海域和神狐海域。天然气水合物钻探航次获得的资料为进一步理解神狐海域和东沙海域的天然气水合物系统提供了基础。钻探结果证明强似海底反射往往与低饱和度的含天然气水合物沉积物薄层联系在一起;高饱和度的天然气水合物一般不需要与地震剖面上识别的似海底反射对应,而与气体渗漏和断裂构造等特征相关。地球化学资料显示神狐海域和东沙海域的天然气水合物气源主要为微生物成因气。神狐海域钻探证实的天然气水合物分布区具有160亿立方米的甲烷地质储量,在目前天然气水合物开采技术和工艺条件下,开采具有较大困难,有待于开发更为先进的技术。随钻测井资料显示东沙海域浅部存在中等-高饱和度水合物,而深部水合物稳定带底界上方存在低饱和度天然气水合物。南海北部陆坡天然气水合物有待于深入探究其赋存状态、饱和度、储层特性和资源前景。     

南海可燃冰研究列入973计划

重大科技专项——“南海天然气水合物富集规律与开采基础研究”项目,通过国家重大基础研究发展计划（973计划）组织的审查,于2009年1月正式启动。广州海洋地质调查局通过连续9a的调查研究已发现了天然气水合物赋存的地质、地球物理、地球化学和生物证据,并于2007年5-6月经钻探在南海北部神狐海域获取了天然气水合物的岩芯样品。     

Micro-CT造影剂在多孔介质内水合物-水两相分辨中的应用评价

天然气水合物资源开采过程中涉及多孔介质内固(水合物)-液(水)-气(天然气)复杂相态转变及多相流体流动与热质传递,因此实现多相流体的相分辨是开展研究的关键问题之一。利用Micro-CT技术,以四氢呋喃水合物(THF)作为研究对象,开展了固(水合物)-液(水)两相分辨的造影剂效果评价研究。发现溶液内添加造影剂KI的分辨效果要远好于NaCl,KI的分辨效果随着质量分数的增加先增加后减小,最后处于较低值维持不变,KI质量分数为6%时分辨效果最好,可以实现多孔介质内小密度差的水与水合物相分辨及水合物分解过程相界面变化的动态可视化检测。基于此,利用阈值法可以进一步实现分解过程中水合物饱和度的定量分析,获取多相流动过程的各相流体原位和度变化数据。     

CO2乳液置换开采天然气水合物研究进展

CO₂置换开采及封存的碳汇是降低我国南海水合物产业化开采成本的重要路径,不仅能保障粤港澳大湾区经济建设所需的能源供给,而且能降低大湾区过量的碳排放,与气态、液态CO₂相比,注入CO₂乳液置换效率更高。综述CO₂乳液在置换开采甲烷水合物中的应用,以及乳液的稳定性问题,讨论CO₂乳液开采南海水合物的优势及难点,并对下一步CO₂乳液法置换开采南海水合物进行分析,为未来天然气水合物开采及同步碳封存提供参考和建议。     

天然气水合物储层水力压裂研究进展

天然气水合物是一种广泛分布于海底地层中重要的未来战略能源,但在开采过程中,由于水合物储层介质颗粒粒径较小,孔隙多被固态水合物占据,储层渗透率低,制约着天然气水合物开采的产业化进程。当今水力压裂技术已广泛应用于低渗透油气藏的增产作业中,本文总结了近年来国内外对天然气水合物储层应用水力压裂技术的研究现状,从压裂实验、数值模拟和压裂液等方面进行了讨论。结果表明,水力压裂可以创造人工裂缝,扩大水合物解离面积,提高储层渗透率和天然气产量,有利于商业开发。储层的脆性响应问题、开发新型压裂液以及压裂对水合物储层地质安全的影响,都是水合物储层水力压裂研究亟待解决的问题。     

天然气水合物气藏开发技术探讨

天然气水合物是本世纪最具开发前景的替代能源，开发天然气水合物资源，对我国宏观能源战略决策和可持续发展具有重大的现实意义。文章概述了天然气水合物气藏的特点和国外水合物气藏勘探项目的进展．对热激发技术、降压技术和化学试剂技术等3种技术进行了对比分析，并着重研究了微波加热技术和CO2置换技术。微波加热技术的优点是作用速度快、设备简单、灵活性高、不会对储层造成任何污染。CO2置换技术的优点是利用天然气水合物生成和分解的机理，不仅考虑了经济地开采资源，并且还提出了在开采后消除对海底环境产生有害影响的对策。     

南海可燃冰研究列入973计划

重大科技专项“南海天然气水合物富集规律与开采基础研究”项目,日前通过国家重大基础研究发展计划（973计划）组织的审查,将于2009年1月正式启动。广州海洋地质调查局通过连续9年的调查研究已发现了天然气水合物赋存的地质、地球物理、地球化学和生物证据,     

Experimental study of sand control in a natural gas hydrate reservoir in the South China sea

Gas hydrate is considered to be one of the most promising energy sources of the 21st century, however, with the deepening of research on hydrate resources and the acceleration of trial exploitation processes, it has become apparent that sand production is one of the key factors restricting hydrate exploitation. Here we focus on the key issues of sand production in natural gas hydrate reservoirs in the Liwan Sea area in the northern part of the South China Sea. Innovative systematic studies of the mode of hydrate sand production using a multi-channel hydration acoustic wave monitoring system were conducted. The results show that a hydrate formation composed of very fine silt is prone to excessive sand production requirement. When the production pressure difference is only 1 MPa, the sand output already accounts for 19% of total liquid production, making sand production an important issue under these conditions. Finally, using filter screens with different pore sizes, hydrate reservoir sand control simulations were carried out. The results were analysed to determine properties such as: sand yield, permeability, sand content, and productivity. The design criterion for the filter screen of the fine sand particles in hydrate formations was finally obtained as D₅₀ = 11d₅₀ (where D₅₀ is median grain size of the gravel, and d₅₀ is the median grain size of the formation).     

Analysis of physical properties of gas hydrate-bearing unconsolidated sediment samples from the ultra-deepwater area in the South China Sea

Marine natural gas hydrate has recently attracted global attention as a potential new clean energy source. Laboratory measurements of various physical properties of gas hydrate-bearing marine sediments can provide valuable information for developing efficient and safe extraction technology of natural gas hydrates. This study presents comprehensive measurement results and analysis of drilled hydrate-bearing sediments samples recovered from Qiongdongnan Basin in the South China Sea. The results show that the gas hydrate in the core samples is mainly methane hydrate with a methane content of approximately 95%, and the other components are ethane and carbon dioxide. The saturation of the samples fluctuates from 2%–60%, the porosity is approximately 38%–43%, and the water content is approximately 30%–50%, which indicate that high water saturation means that timely drainage should be paid attention to during hydrate extraction. In addition, the median diameter of the sediment samples is mainly distributed in the range of 15 to 34 μm, and attention should be paid to the prevention and control of sand production in the mining process. Moreover, the thermal conductivity is distributed in the range of 0.75 to 0.96 W/(m∙K) as measured by the flat plate heat source method. The relatively low thermal conductivity of hydrates at this study site indicates that a combined approach is encouraged for natural gas production technologies. It is also found that clay flakes and fine particles are attached to the surface of large particles in large numbers. Such characteristics will lead to insufficient permeability during the production process.     

天然气水合物开采技术研究进展

天然气水合物以其储量大、能量密度大、分布广的特点被认为是一种非常具有潜力的替代能源。勘测数据表明迄今已至少在全球116个地区发现了天然气水合物。天然气水合物广泛存在于冻土区和海底沉积物中。但目前实现实际试开采的区域仅有四处,分别位于：美国的阿拉斯加北坡地区、俄罗斯的西伯利亚玛索亚哈气田、加拿大西北部的麦肯齐三角洲及日本的南海海槽。目前主要的开采技术研究主要集中于实验室模拟阶段。美国、德国、日本、中国分别建立了自己的水合物模拟开采实验装置并且进行了相关研究。提出了不同的水合物开采方案,并且对水合物开采过程中的关键技术问题进行了研究。     

海洋天然气水合物地层钻井安全问题研究进展

由于天然气水合物的特性及海底天然气水合物地层地质环境的复杂性,在海洋天然气水合物地层进行钻井作业时会面临诸多的安全问题。目前科研工作者主要通过实地钻井项目、实验研究及数值模拟的方式对海洋天然气水合物地层钻井安全问题展开研究。本文基于对国内外海洋天然气水合物钻井安全问题研究的广泛调研,针对海洋天然气水合物地层钻井面临的主要安全问题及其最新的研究进展进行了讨论和总结。同时,分析了海洋天然气水合物地层钻井安全问题研究目前存在的问题以及未来研究的重点方向。     

Mechanical experiments and constitutive model of natural gas hydrate reservoirs

Natural gas hydrate is a new type of green energy resources and has great development prospects, and it has attracted worldwide attentions. The exploitation of natural gas hydrate may result in a series of geological disasters. Therefore, the constitutive model of natural gas hydrate bearing sediments needs to be established to reveal deformation laws of the reservoir sediments and accurately evaluate mechanical properties of hydrate reservoirs. This is the basic guarantees for the effective exploitation of natural gas hydrate resources. The triaxial compressive tests were conducted on samples of natural gas hydrate sediment. Furthermore, the Duncan-Chang hyperbolic model was modified by considering the influences of hydrate saturation based on the test results to obtain the constitutive model according with the deformation characteristics of natural gas hydrate reservoirs. The results show that the stress-strain curves of natural gas hydrate reservoirs show unobvious compaction stage and peak strength, short elastic stage, long yield stage, and significant strain hardening characteristics. After applying loads on natural gas hydrate bearing sediments, the internal solid particles were dislocated and slid. When the loads were small, the sediments demonstrated elastic deformation. With the increase of loads, plastic flows appeared in the interior, and the hydrate crystals were re-orientated, thus the sediments showing plastic deformation. Initial tangent elastic modulus increased with the effective confining pressures, which had little correlations with hydrate saturation. Furthermore, the damage ratio increases with the increase of effective confining pressures, while slightly decreases with the increase of natural gas hydrate saturation. The predicted results of stress-strain curves of sediments with different hydrate saturations well coincide with the results of triaxial compressive tests, indicting the feasibility and rationality of this model.     

Latest progress in numerical simulations on multiphase flow and thermodynamics in production of natural gas from gas hydrate reservoir

Natural gas hydrates are promising potential alternative energy resources. Some studies on the multiphase flow and thermodynamics have been conducted to investigate the feasibility of gas production from hydrate dissociation. The methods for natural gas production are analyzed and several models describing the dissociation process are listed and compared. Two prevailing models, one for depressurization and the other for thermal stimulation, are discussed in detail. A comprehensive numerical method considering the multiphase flow and thermodynamics of gas production from various hydratebearing reservoirs is required to better understand the dissociation process of natural gas hydrate, which would be of great benefit to its future exploration and exploitation.