0 引言
天然气水合物是由水分子笼和客体分子结合构成的类冰状晶体化合物,在低温、高压以及孔隙水充足的环境下形成,一般赋存于水深超过300 m的海域和距地表130 m以下的陆域冻土区中。鉴于其高能量、高密度、清洁无污染和分布广等特点,天然气水合物在全球能源利用和环境保护中发挥着重要作用,是未来人类开发和利用的主要新型能源[1-2]。针对不同含水合物储层介质的特性差异,研究其所含天然气水合物的赋存状态对水合物的勘探具有深远意义。现有的探测手段主要为地球物理探测和地球化学探测。近年来,超声波探测技术、时域反射(TDR)技术和阻抗勘测在实验领域不断被深入研究,同时X-CT射线扫描、激光拉曼光谱法、核磁共振等探测技术也在更新,但仍需细化和改进。
电阻率法探测技术作为储层测井响应方法之一,不仅可以确定水合物赋存位置,还能够探测水合物的成核、生成以及饱和度,在天然气水合物勘探和开采方面具有不可替代的作用[3]。阿尔奇(Archie)公式作为评价水合物饱和度的重要方式,其岩性附加导电系数(a)、岩性比例系数(b)、孔隙度指数(m)和饱和度指数(n)的选取,对Archie公式能否准确计算饱和度具有决定性作用,且m、n的影响程度远大于a和b[4]。目前,Archie公式在计算天然气水合物饱和度时更多地依赖于固定的m、n值,很少采用双对数坐标下的非线性计算;另外,在不同地区天然气水合物饱和度的研究中,对不同形成条件下m、n的影响因素分析较少。考虑到m、n的物理意义以及它们的取值对水合物饱和度计算精度的影响,Archie公式在天然气水合物饱和度评价中的应用范围还有待进一步扩大。
1 含水合物沉积物的电阻率演化规律
1.1 电阻率法探测的基本原理
1.2 影响因素
1.2.1 气源与沉积物性质
气源与含水合物沉积物的数量和种类同样影响水合物的电学性质。研究人员利用四氢呋喃水合物作为水合物的勘测模拟物进行室内模拟实验,结果表明:随着温度和盐度的递增,电阻率逐渐降低,相比之下,声速的变化并不显著[8]。不同粒径的多孔介质中(砂、黏土、泥砂),不同浓度四氢呋喃体系下的含水饱和度和电性参数是不同的,电阻率会随着压力增加而略有上升,其孔隙流体的导电性与流体体积分数密切相关[9]。此外,采用电阻率分析二氧化碳水合物的生成过程,不仅能够对气体在溶液中的溶解度做计算,还可以通过具有类似晶体结构或者物理性质的水合物相变化图分析甲烷水合物的性质[10]。由于四氢呋喃和二氧化碳不能代表所有的水合物结构,且不同影响因素下导致的电学性质变化不同,考虑到甲烷水合物最接近天然气水合物,因此后期更多以甲烷为水合物的主要气体来源。
测井数据显示,近年来在南海水合物钻探区,以水合物富集为主的混合层中粉砂岩的比例逐渐增加[11-12]。Chen和Kim等[13-14]的研究成果表明,多数情况下水合物分布于细粒、粗砂或粉砂的沉积物混合层中。水合物在不同形成阶段具有不同的孔隙充填模式,主要分为接触、悬浮和胶结模式,且随水合物饱和度的增加,电阻率的上升趋势变得更陡[15]。考虑沉积物粒径效应对水合物储层环境中孔径分布的影响,Dufrane等[16]以砂子或玻璃珠模拟多孔介质,研究甲烷水合物的动力学过程和稳定性,发现玻璃珠对甲烷水合物电学性能的影响小于砂,且随着砂浓度的增加,活化能降低,电导性增加,符合阿伦尼乌斯(Arrhenius)表达式。但在实际储层中,杂质的种类以及不同的沉积物含量对电阻率的影响可能不同[17],从而导致水合物分布与电阻率的测试结果有所差异。
1.2.2 温度、压力
一定的气体含量、适宜的温度和压力是形成水合物的主要条件。依据水合物在生成和分解过程中的温压曲线和电导性分布实验结果,认为电阻率能够作为水合物定性分析的一项指标[18]。实验证明,粒径为1~2 mm的粗砂中水合物的饱和度在10 ℃温度梯度下率先达到最大值,且更高的温度梯度增加了电阻率的上升时间[19]。而在分散型沉积物孔隙中,水和冰均能生成水合物,随着温度周期次数的增加电阻率仍有递增趋势[20-21]。在甲烷、水溶液和多孔介质形成的多成分混合系统中,离子浓度和电阻率随时间和温度的变化而变化,且温度与电导率在形成和解离时均呈指数关系,适用于Arrhenius表达式:σ(Τ)=σ0
1.3 演化规律
这些基本因素和规律可为复杂储层中水合物非均质分布和运移的分析提供理论依据和定量信息,而对于整体的天然气水合物或沉积物中电阻率的探究,还需更多的理论和实践分析。
2 电阻率法探测技术在天然气水合物勘探中的应用进展
图1
图2
2.1 含天然气水合物沉积物的电学特性
Nobes等[31]采用海洋水合物区域内粉土和黏土为多孔介质建立了热导率和电阻率二个关系式,指出电阻率对水合物的存在反应比沉积物性质的变化更灵敏,因此认为电阻率法在评估水合物的分布上有一定的潜在价值。到目前为止,几乎所有已知的勘探或开采区域都采用过电阻率法探测技术,这代表了电阻率法在识别含天然气水合物储层及估算水合物饱和度中具有不可替代的作用。
Waite和Yousif等[32-33]利用Berea固结岩心进行了天然气水合物合成分解模拟实验研究,该装置不仅能测量系统压力的变化,还可以通过四电极系统来测量体系电阻率参数,并通过电阻率的变化来识别水合物合成分解的速率,从而确定天然气水合物合成的数量及生成方式。Zatsepina、Yama、Buffet等[34⇓-36]通过测量水合物合成分解过程中的电阻率变化,分析了在没有游离气体的多孔介质中水合物的动力学和热力学机理,并建立了溶解气体生成水合物模型,发现对电阻率变化速度规律的研究可量化水合物的成核规模。陈强等[37]考虑孔隙流体的影响,以二氧化碳、甲烷水合物作对比,发现在垂向上甲烷水合物会受到排盐效应的影响,导致离子分布、盐水浓度和孔隙中水合物的分布模式发生区域性变化,电阻率总体会呈现出某一处的降低随后逐渐升高。为增加孔隙流体的多种可能性,Lech等[38]分别测量黏性和非黏性土壤在不同盐浓度溶液中的电阻率变化,结合孔隙度与弯曲度的函数关系[39],指出依据孔隙水电阻率,可以分析孔隙度、含水饱和度和相关背景下的水合物饱和度。金学彬等[40]证实在电容和电阻均存在阻抗系统中,阻抗在高频率下呈现单调增高趋势,低频率下相对现象不明显。对于相角—频率的规律分析,曲线的整体趋势相同,与水合物饱和度关联性较小。由于复电阻率在不同频率下的频散特性不同,利用Archie公式进行天然气水合物饱和度的计算后,发现水合物饱和度在不同频率下的变化不同,原因是电性数据提供不够广泛,导致水合物在高饱和度范围内的分析并不充分。该方法仍有待进一步研究,目前不具有普适性[41]。牛佳乐等[42]结合对称电极和多频率点(5~400 Hz)发现:阻抗幅值、流体和孔隙壁界面的极化反应均与频率呈反比,且在一定范围内,水合物饱和度指数与频率为线性关系。
Chen等[43]利用岩心数据分析孔隙流体对电阻率反演数据的影响,发现当水合物饱和度较低时(≤15%),孔隙水矿化度增大,电阻率逐渐减小。当饱和度在50%左右时,沉积物电阻率和孔隙度指数m均增加了2倍,此时矿化度不再是主导因素。Pandey等[44]结合地震反射成像和纵波速度方程就第二次印度远征勘探的富砂层进行深度分析,对比了井下水合物的电阻率测井曲线与压力岩心的饱和度数据的误差,而后以孔隙填充型和裂缝型二种填充方式对水合物—电阻率关系进行孔隙度的细化分析,认为孔隙度密度误差通过影响Archie公式的岩性参数间接影响水合物饱和度的评价。但该结论存在一定缺陷,首先设定了储层性质为各向同性,导致成像中的水合物几乎垂直填充高电阻率裂缝;其次,方程中取黏土和砂的体积为具体比例,从而限制了地层条件。为进一步研究含水合物沉积物中孔隙度对岩电参数及电阻率检测的影响,Feng等[45]将Archie公式与核磁共振T2时间分布相结合提出了一种修改模型,预测低渗透砂岩中含水饱和度和电阻率指数的变化,将实际测井获得的岩心数据与修改后的Archie公式计算结果对比,发现其符合度高且相关性较强,而岩性比例系数b和饱和度指数n与实验测量相比,相对误差在5%以内,表明模型具有较高的可靠性。
2.2 电阻率法测量实验装置的发展
“电极法”经常被用来测量室内砂土的电阻率,其测量电极和供电电极一般布置在一个水平面上。其中,四电极法具有可操作性强、便捷等优点[46]。赵仕俊等[47]利用早期的一维装置(图3a),得到了水合物区域内的电阻率演化规律,验证了电阻率法在一维电极装置下探测水合物动态变化的可行性。杜燕等[48]围绕电容法对二维模拟装置进行优化(图3b),发现水合物饱和度增加后,电容量会逐渐减小。而在电极材料的选择上,除恒压恒流的环、点和片状铜电极外,Birkedal等[49]利用银滤纸材料结合核磁共振,观察水合物生成中三维空间和微观上的电阻率特征,发现电阻率随频率值的增加而减小,在水合物饱和度接近20%时,水合物的形成对沉积物内部的电阻率变化影响不大。由于水合物钻探装置的室内研究对反应釜的要求仅在一维、二维会有所局限,李小森等[50]引进了一个三维空间反应釜系统(图3c),是天然气水合物电阻率反演的一大进步。三维反应釜实验揭示了水合物的形成在空间上分布不均匀,在传热效应的影响下,水合物主要沿壁分布,并由于孔隙堵塞、连通性不好等原因,电阻率和水合物饱和度的线性关系有待探究,并指出孔隙流体的流动对于水合物的开采有重要影响。
图3
考虑到电极与砂子之间接触电阻和接触面积的影响,为了减少误差,逐渐引申至小范围内测试水合物的电阻率[51],尽量采用多接触点、短距离的电极探针和环状电极探测水合物的电阻率,这样不仅能够提高精准度,还可以准确测量水合物的生成特性和盐分变化[52]。Wu等[53]在阐述环电极和点电极二种体系对不同粒径砂砾间接触电阻的影响时,指出环形电极可以监测气体运移的能力和类型,点状电极对于测量气体饱和度更适用,另外,还可通过提高压力密度来保证样品与电极之间的良好接触。陈玉凤等[54]通过数字交流电桥组成的电阻系统,以降温速率为变化点,指出孔隙水的电阻率基本不变,水合物饱和度在不同含量界限区间内呈非均匀性,且与饱和度指数n有关。
2.3 数值模拟
图4
对于松散的含水合物沉积物模型,考虑到毛细管力、粒度、湿润性和孔隙度的影响,认为水合物饱和度指数和模型的尺寸呈正相关,与水合物饱和度具有强依赖性[56]。为研究Archie经验公式计算模型对电阻率计算的影响, Dong等[57]利用CT分别扫描黏附型、胶结式和分散型这3种不同的含水合物沉积物,将得到的水合物饱和度与校准岩电参数后的 Archie 公式计算结果进行对比,结果表明,修正后的Archie公式比传统Archie模型在水合物饱和度计算精度上提高了20%。若使用纳米聚焦X—CT在微观层面探讨3种填充模式下的不同边界水合物薄膜变化,不仅可以观察到不同水合物饱和度下孔喉的迁移变化,还得到了砂砾—水合物—水之间的有效孔隙度、孔数以及最大孔径等参数随水合物饱和度变化对应的动态图像,这加速了水合物饱和度分析的研究进展[58]。用分形维数分析各部分电导性的细节变化时,孔隙水电导率和孔隙率的变化趋势相同,而沉积物骨架的电导率在水合物饱和度Sh≤80%时为平稳状态,在Sh>80%时呈指数突增[59]。结合电极探针,发现不同多孔介质背景下水合物的生成模式、饱和度存在一个与孔隙相关的临界水合物饱和度值,在孔隙堵塞之前电阻率会随水合物饱和度的升高而持续增长[60]。
基于上述讨论,电阻率不仅受沉积物本身、外在因素(温压条件)、水合物的赋存状态以及孔隙流体的影响,还与外界的传热效应和储层性质各向分布计算等因素有关。目前,电性数据提供不够广泛,基于水合物高饱和度范围内的分析不够充分,应根据电极装置、电极体系以及电性参数进一步优化,从而更好地反演地层水合物的确切状态及储量。对于复杂的砂岩储层,细化含水合物沉积物中天然气水合物的分子运移路径模型,可使得饱和度的定量评价更加明确。微观动力学和可视成像分析应更多结合饱和度评价公式(如Archie公式),对多元化储层提供可变的计算拓展式。
3 Archie公式的应用进展
3.1 Archie公式研究背景
电阻率测井作为一种探测碳氢化合物的重要手段,早期广泛应用于油气储层中液体和化合物的性质识别,并在储层响应特征的定性评价中占有一席之地。直至Archie公式出现,人们逐渐将电阻率法测井技术作为水合物储层探测手段,同时结合饱和度计算模型对天然气水合物进行定量分析。利用Archie公式[64]计算水合物饱和度由地层因子和电阻率增大指数二部分组成,即
据此可得出电阻率与含油气饱和度之间的关联式:
式中:F为地层因子;R0为饱含水时的地层电阻率,Ω·m;Rt为地层电阻率,Ω·m;Rw为孔隙水电阻率,Ω·m;Φ为储层岩石孔隙度,%;Sw为含水饱和度,%;a、b、m和n均为Archie经验参数。
3.2 岩电参数m、n的研究现状
近年来,人们常以堆放的纯净散沙孔隙度指数(m=1.3)作为水合物饱和度计算值[67]。饱和度指数(n)也早在很久之前就被认为与含水量具有一定关联,在过去几十年中,大部分以n=1.938 6计算砂岩储层中的水合物饱和度[68]。然而,多年冻土区和海域中的含水合物沉积物内部结构随地质孔隙空间移动路径的多变而呈现出复杂化,外界附加导电因素、有效孔径分布、渗透率和粒度等的影响也导致水合物的分布表现出随机性、不均匀性[69⇓-71],同时,受温度、盐度变化的影响,沉积物孔隙内部流体流动路径发生变化也将引起含水合物沉积物电阻的变化[8,72];从m、n本身的物理意义来看,这些变量因素都会间接影响m、n的取值。但目前,大多数实验仍采用固定值计算天然气水合物的饱和度,只有少量通过测井或实验模拟数据拟合得出胶结指数和水合物饱和度指数。
针对日益复杂的含水合物沉积物储层,目前正在不断探索适用于泥质砂岩、低渗透砂岩以及复杂孔隙储层等不同地质环境的m、n值。Shankar等[73]认为在沉积物以黏土为主时,m的取值在0.8~1之间。此外,颗粒圆润度的降低、孔隙结构连续分形的不断减小等也会使岩层渗透率、电导率等发生变化,间接影响m的取值。Jackson等[74]拟合了8种球度为0.8的天然海沙曲线,发现m在1.39~1.58之间,约为1.49,而人工样品砂砾的m在1.2~1.9,且球度为0.78时约为1.5,因此指出孔隙度指数受砂粒形状的影响大于砂粒本身的大小和分布。可见降低球形度是增加m的一种方式。Anderson等[75]通过研究发现,当含水饱和度(Sw)在30%左右变化时,受孔隙连通性或粘度改变的影响,n值的取值范围将增大到2~3.5之间。Spangenberg等[56]通过观察水合物在储层内的形成方式,验证了n值可在0.5~4之间变化。然而,当水合物饱和度(Sh)较高时(Sh>70%),水合物作为沉积物基质起到承载作用,此时n值最高可达6[76]。Bahuguna等[77]认为同一介质中不同层段内的m、n值也不同,上部为1.89和1.46,中部为1.88和1.4,计算精度提高了近10%。Li等[78]进一步对孔隙水溶液的影响做了补充,利用5组不同初始盐水饱和度(12%、20%、30%、40%和50%)形成的甲烷水合物样品进行水合物饱和度—电阻率曲线的岩电参数拟合,选取水合物饱和度膨胀系数(Kh)为1.25,确定了m=0.167 7、n=1.601 9,其中水合物膨胀系数随不同储层环境和变量条件而变化,间接影响m、n的拟合取值。Moahamad等[79]在通过传统的Archie公式、三维标准化回归方程和CAPE三种方式对砂岩和碳酸盐进行岩电参数的拟合时,得出传统方法计算中n值在0.84~3.4之间变化且幅度较大,CPAE技术由于样本分布的各向异性,碳酸盐的m值在1.49~2.19范围内不稳定变化。
综合以上结论,3D空间技术最适合实际参考使用,能够在三维空间上对非均质分布的储层进行水合物饱和度准确计算。对于Archie公式的模型架构,除了孔隙填充、周围胶合和接触胶合模式,Cook等[80]采取承载架构结合纵波探讨了适用于粗砂储层的n值,指出n=2.5±0.5在粗颗粒储层中是可行的,此时参考数据m=1.7±0.1。依据天然气水合物自身对电阻率的影响,Jin等[81]对比原始多孔介质模型(OPM)和演化多孔介质理论模型(EPM),非线性回归了地层电阻率、孔隙水电阻率和含水饱和度,得出OPM模型m=1.5、n=2.6,EPM模型m=2.0、n=0.9,证实了天然气水合物会在实验中部分堵塞通道且增加孔隙弯曲度的观点。
3.3 岩电参数m、n的影响因素
除有效孔径外,Rosales[84]提出了一个残余孔隙度(φr)概念:当孔隙度趋于极限但不等于零时,取极值为残余孔隙度,并把孔隙分成有效区域和停滞区域。当残余孔隙度应用到分散或接触的水合物填充模式时,取φr=0.02、M=1.64(M为几何参数,其值随着颗粒球度变小而变大)、m=1.3(为固定参数值),总体变化趋势为0.02<φr<0.1时M=1.85。在后期,人们更多地研究了岩石骨架对孔隙度的影响。解茜草等[85]通过变换三维空间上岩石的排列和数量,发现孔隙的复杂程度和弯曲度越大则电阻率越高;考虑到弯曲度,对X-CT扫描图像进行微观分形切割,指出孔隙结构的分形不断减小时,弯曲度增加幅度不等,导致水合物饱和度指数(n)在不同程度上产生差异[86]。目前,在研究孔隙影响方面,关注岩石结构本身变化的模拟较多,对于含天然气水合物沉积物中随水合物生成孔隙结构与m、n之间变化的模拟较少,且一般在实验模拟时均采用定性水膨胀系数计算水合物饱和度。但是,m、n的物理意义分别与孔隙弯曲度和流通性有关;而随着天然气水合物在孔隙中无规则形成,孔隙中的水不同程度膨胀成水合物,多孔介质有效孔隙度将被不同程度地减小、分形,这些因素间接影响了m、n值的适用性。
实验数据证实,与常温、常压相比,温度、压力升高后,岩石的孔隙度减小,通道曲折度变大,孔隙中的流体因传热温差会重新分布,m、b值增加,a、n值减小,但变化幅度不等[87]。熊学川等[88]探究了围压对岩电参数的影响,得出a=0.961 5、m=1.832、b≈1.16、n≈1.156,并发现随着围压的增加,a、b基本不变,m增加幅度明显,n则缓慢下降。另外,矿化度也是重要影响因素之一,若矿化度增加,导电离子会增多,电阻率逐渐减小,此时水合物饱和度逐渐减小,孔隙流体连通性增强,n值逐渐增加,而m值对于矿化度的改变会呈现出不规律变化。以频率衡量岩电参数时,同一砂岩颗粒在不同频率变化下,m、n的变化程度不同,且频散现象在低饱和度时更明显[89-90]。
综上所述,不同方法和影响因素下的岩电参数m、n的误差程度存在差异,这不仅涉及到孔隙结构、水合物的胶结方式和岩石中含水程度等因素,还与温度、压力及矿化度有关。目前,针对天然气水合物不同形成条件设置的变量较少,对天然气水合物领域中m、n值的拟合和影响因素研究范围不够。
4 结语
目前,电阻率测井法已经大量应用于天然气水合物领域,Archie公式的应用范围也变得更加广泛,且围绕水合物饱和度的计算正在向成熟阶段过渡。建议下一步的研究重点在以下几个方面:
1)电阻率随含天然气水合物饱和度的增加而单调递增,这不仅与沉积物组成和性质有关,还会受到体系温压、孔隙中流体导电性以及测量频率、储层各向电性分布等因素的影响。针对高水合物饱和度沉积物中电阻率测量数据的短缺和误差,未来需提供更多高水合物饱和度范围内的电性数据,从而为分析不同形成条件下含水合物沉积物的电阻率特性提供支撑。
2)在现有的电阻率钻探装置和工艺上,进一步优化电学装置、数值模拟软件、可视化装置等测量方式,以便更好地反演地层水合物的确切状态及储量。
3)应对复杂储层的岩性多样化,需要更多的测井数据、实际案例以及水合物变化分析的模拟软件,为含天然气水合物储层的动态响应分析提供理论依据。从而实现天然气水合物与地层参数的实时关联,进一步提高天然气水合物饱和度的计算精度。
4)岩电参数需进行细化和完善,特别是m、n值的选取。应采用不同水合物形成条件下非线性参数的优化值代替定值,通过与实际耗气量计算和直接估算法的天然气水合物饱和度进行对比,从而验证参数的可靠性。这将更准确地反映天然气水合物相应的电学特性,为Archie方程在天然气水合物饱和度评估中提供理论数据。
5)岩电参数m、n影响因素的优化。由于孔隙度、温度、压力以及多孔介质等不同因素引起的水合物饱和度的变化都间接影响m、n的取值。因此,建议在非线性计算时选取计算水膨胀系数不要单一,应尽量模拟不同水合物形成条件下多孔介质中水合物的微观填充形态和实时孔隙度变化;另外,应考虑更多天然气水合形成过程中的变量条件,如不同降温过程、砂土配比含量等条件下m、n值的拟合和影响。
参考文献
中国陆域冻土层可燃冰勘探开发现状与前景
[J].
Current situation and prospect of exploration and development of combustible ice in continental frozen soil in China
[J].
Distribution and characteristics of natural gas hydrates in the Shenhu sea area,South China Sea
[J].DOI:10.1016/j.marpetgeo.2018.07.028 URL [本文引用: 1]
The status of natural gas hydrate research in China: A review
[J].
岩电参数对储层饱和度计算精度的影响分析
[J].
Analyzing the effect of rock electrical parameters on the calculation of the reservoir saturation
[J].
天然气水合物储层测井响应与评价方法综述
[J].
Review on logging responses and evaluation methods of natural gas hydrated reservoir
[J].
含水合物沉积物电阻率演化规律实验研究进展
[J].
Progress in experimental research on resistivity evolution of hydrate bearing sediments
[J].
中国天然气水合物赋存特征
[J].
Reservoir characteristics of natural gas hydrates in China
[J].
Acoustic and resistivity measurements on rock samples containing tetrahydrofuran hydrates: Laboratory analogues to natural gas hydrate deposits
[J].
Parametric study of the physical properties of hydrate-bearing sand,silt,and clay sediments: Electromagnetic properties
[J].
Experimental study of the stability of CO2-hydrate in a porous medium
[J].DOI:10.1016/S0378-3812(01)00636-7 URL [本文引用: 1]
The promoting effect of natural sand on methane hydrate formation: Grain sizes and mineral composition
[J].DOI:10.1016/j.fuel.2016.04.017 URL [本文引用: 1]
南海神狐海域天然气水合物饱和度的数值模拟分析
[J].
Numerical modeling of gas hydrate saturation for the Shenhu area,South China Sea
[J].
Hydrate formation in sediments from free gas using a one-dimensional visual simulator
[J].DOI:10.1016/j.fuel.2017.02.034 URL [本文引用: 1]
Geotechnical and geophysical properties of deep marine fine-grained sediments recovered during the second Ulleung Basin Gas Hydrate expedition,East Sea,Korea
[J].DOI:10.1016/j.marpetgeo.2013.05.009 URL [本文引用: 1]
南海神狐海域水合物对岩心电阻率的影响
[J].
Effect of hydrate on core resistivity in Shenhu area,South China Sea
[J].
Electrical properties of methane hydrate+sediment mixtures
[J].DOI:10.1002/jgrb.v120.7 URL [本文引用: 1]
Physical properties of hydrate-bearing sediments
[J].
用电导性监测天然气水合物的形成和分解
[J].
Detecting formation and decomposition of NGH by measuring conductivity
[J].
温度梯度对粗砂中甲烷水合物形成和分解过程的影响及电阻率响应
[J].
Effect of temperature gradient on methane hydrate formation and decomposition process and resistivity response in coarse sand
[J].
温度周期变化下分散型天然气水合物的沉积物电阻率测量
[J].
Electricity resistivity measurement of disseminated gas hydrates bearing under cyclic temperature
[J].
Physical properties of gas hydrates: A review
[J].
The effect of brine on the electrical properties of methane hydrate
[J].
DOI:10.1029/2019JB018364
[本文引用: 1]
Gas hydrates possess lower electrical conductivity (inverse of resistivity) than either seawater or ice, but higher than clastic silts and sands, such that electromagnetic methods can be employed to help identify their natural formation in marine and permafrost environments. Controlled laboratory studies offer a means to isolate and quantify the effects of changing individual components within gas-hydrate-bearing systems, in turn yielding insight into the behavior of natural systems. Here we investigate the electrical properties of polycrystalline methane hydrate with >= 25% gas-filled porosity and in mixture with brine. Initially, pure methane hydrate was synthesized from H2O ice and CH4 gas while undergoing electrical impedance measurement, then partially dissociated to assess the effects of pure pore water accumulation on electrical conductivity. Methane hydrate + brine mixtures were then formed by either adding NaCl (0.25-2.5 wt %) to high-purity ice or by using frozen seawater as a reactant. Conductivity was obtained from impedance measurements made in situ throughout synthesis while temperature cycled between +15 degrees C and -25 degrees C. Several possible conduction mechanisms were subsequently determined using equivalent circuit modeling. Samples with low NaCl concentration show a doping/impurity effect and a log linear conductivity response as a function of temperature. For higher salt content samples, conductivity increases exponentially with temperature and the log linear relationship no longer holds; instead, we observe phase changes within the samples that follow NaCl-H2O-CH4 phase equilibrium predictions. Final samples were quenched in liquid nitrogen and imaged by cryogenic scanning electron microscopy (cryo-SEM) to assess grain-scale characteristics.
电阻率测试技术在沉积物—盐水—甲烷水合物体系中的应用
[J].
Application of resistivity measurement in the system of sediment-brine-methane hydrate
[J].DOI:10.1007/BF02329132 URL [本文引用: 1]
多孔介质中甲烷水合物形成与分解实验研究
[J].
DOI:10.7623/syxb200904019
[本文引用: 1]
基于实际海洋水合物资源的赋存状态及温度、压力条件,在人工多孔介质中物理模拟海底水合物稳定带的水合物藏进行了水合物的形成与分解的实验研究。分析甲烷水合物在多孔介质中的形成与降压开采过程,揭示了其温度、压力和产气速率的变化规律。采用逐步降压的方法测定了多孔介质中水合物在特定温度下最小分解推动力,比较了不同降压模式下的累计产气量。结果表明,水合物形成过程中通过不断注水保持系统压力,甲烷可完全生成水合物,最终水合物藏中仅有水和水合物两相;实验条件下水合物的分解主要受压差影响,压差越大,分解速率越大,累计产气量越高;在一定温度下水合物的分解需有一个最小推动力。比较不同降压模式发现,累计产气量只与压差有关,而与降压模式无关。
Experimental study on formation and decomposition of methane hydrate in porous media
[J].
Experimental investigation of production behaviors of methane hydrate saturated in porous rock
[J].DOI:10.1080/00908310390207873 URL [本文引用: 1]
Experimental investigation into formation/dissociation characteristics of methane hydrate in consolidated sediments with resistance measurement
[J].DOI:10.1016/j.fuel.2018.07.101 URL [本文引用: 1]
松散沉积物中天然气水合物生成、分解过程与声学特性的实验研究
[J].
Experimental study on the formation,decomposition and acoustic characteristics of natural gas hydrate in loose sediments
[J].
Acoustic velocity and electrical resistance of hydrate bearing sediments
[J].
世界天然气水合物钻探历程与试采进展
[J].
World progress of drilling and production test of natural gas hydrate
[J].
自然界天然气水合物勘探开发概述
[J].
An overview on gas hydrate exploration and exploitation in natural fields
[J].
Estimation of marine sediment bulk physical properties at depth from seafloor geophysical measurements
[J].DOI:10.1029/JB091iB14p14033 URL [本文引用: 1]
Methane hydrate formation in partially water-saturated Ottawa sand
[J].DOI:10.2138/am-2004-8-906 URL [本文引用: 1]
Depressurization of natural gas hydrate in Berea sandstone cores
[J].
Nucleation of CO2-hydrate in a porous medium
[J].DOI:10.1016/S0378-3812(02)00032-8 URL [本文引用: 1]
Paleostresses inferred from mesoscale faults in the Late Pliocene Hitoyoshi Formation;implications for the formation of vein-type ore deposits in southern Kyushu
[C]//
Formation of gas hydrate from dissolved gas in natural porous media
[J].DOI:10.1016/S0025-3227(99)00127-9 URL [本文引用: 1]
孔隙水垂向不均匀分布体系中水合物生成过程的电阻率变化
[J].
DOI:10.7623/syxb201602008
[本文引用: 1]
天然气水合物具有极强的储气能力,被认为是一种潜力巨大的新型能源矿产。电阻率测井技术在海洋天然气水合物勘探工作中发挥着重要作用,然而涉及水合物电阻率数据的处理与Archie公式应用等方面的工作仍需进一步完善。利用自主研发的水合物电学特性模拟实验装置开展了孔隙水垂向不均匀分布体系中甲烷水合物的合成实验,获得了不同饱和度下含水合物沉积物电阻率数据,并在此基础上试算了Archie公式的经验参数。结果表明:电阻率变化能够指示水合物反应过程,其变化特征是反应不同阶段排盐效应、孔隙含水量和孔隙填充方式等多种影响因素共同作用的结果;含水合物的天然海砂电阻率参数存在非Archie现象,地层因子、电阻率增大指数等Archie公式参数受水合物饱和度变化的影响。
Resistivity variation during hydrate formation inhomegeneous distribution system of pore water
[J].
Applications of electrical resistivity surveys in solving selected geotechnical and environmental problems
[J].
DOI:10.3390/app10072263
URL
[本文引用: 1]
Standard test methods may not be suitable or sufficient for determining the geotechnical conditions of a structure’s subsoil and the effects of the designed structures on the environment. Geophysical test methods, validated with other methods, may prove useful. In recent years they have found many new applications in engineering practice, both geotechnical and environmental. The advantages of geophysical methods include the non-destructive and non-invasive nature of the tests, their low costs and quick results, as well as compatibility with different materials, including soils, solid rocks, wastes and anthropogenic formations. The paper presents the analysis of laboratory and field investigations including research in a modified oedometer, resistivity chamber, electrical resistivity tomography (ERT) and resistivity cone penetration test (RCPT). Laboratory tests allowed for the assessment of the degree of saturation and porosity of sandy and clayey soils. The tests were carried out on saturated and unsaturated soil samples and allowed for the determination of some relationships between electrical conductivity and porosity. The proposed equations were used to assess parameters in in situ studies using RCPT tests and showed good agreement with reference values based on undisturbed soil samples. ERT tests confirmed the usefulness of electrical measurements in the quality assurance of subsoil and hydrotechnical structures. The tests showed weakening zones in the levee body, discontinuity of the vertical sealing system on the modernized section of the embankment, and location of the top of clay deposits.
On the relationship between formation resistivity factor and porosity
[J].
DOI:10.2118/10546-PA
URL
[本文引用: 1]
A theory on the relationship between formation resistivity factor and porosity is presented. This theory considers that, from the standpoint of the flow of electric current within a porous medium saturated with a conducting fluid, the pore space can be divided into flowing and stagnant regions. This assumption leads to a general expression, and formulas currently used in practice are special cases of this expression. The validity of the new expression is established by the use of data corresponding to sandstones and packings and suspensions of particles. For the case of natural rocks, the theory confirms Archie's equation and gives an interpretation of the physical significance of the so-called cementation exponent.
多孔介质中甲烷水合物聚散过程的交流阻抗谱响应特征
[J].
Response characteristics of AC impedance spectrum during the formation and dissociation of methane hydrate in porous media
[J].
含甲烷水合物多孔介质的复电阻率频散特性与模型
[J].
Frequency dispersion characteristics and modeling of complex resistivity of porous medium containing methane hydrate
[J].
基于宽频电阻抗特性与阿尔奇公式的含水合物饱和度计算模型
[J].
Calculation model of hydrate saturation based on broadband electrical impedance characteristics and Archie formula
[J].
In situ measurement of electrical resistivity of ocean sediments containing gas hydrates
[J].
Estimate of gas hydrate saturations in the Krishna-Godavari basin,eastern continental margin of India,results of expedition NGHP-02
[J].DOI:10.1016/j.marpetgeo.2018.12.009 URL [本文引用: 1]
A novel method to estimate resistivity index of tight sandstone reservoirs using nuclear magnetic resonance logs
[J].DOI:10.1016/j.jngse.2020.103358 URL [本文引用: 1]
四极法电阻率测量在石油管道防腐蚀中的应用
[J].
Application of the four-electrode resistivity method to anti-corrosion design of oil pipelines
[J].
基于电阻率测量的一维天然气水合物模拟实验装置
[J].
One-dimensional gas hydrate simulation experiment device based on resistivity measurement
[J].
水合物合成电容特性与降压开采二维实验研究
[J].
Two-dimensional experimental study on capacitance characteristics of hydrate synthesis and depressurization mining
[J].
Electrical resistivity measurements of CH4 hydrate-bearing sandstone during formation
[C]//
电阻率在天然气水合物三维生成及开采过程中的变化特性模拟实验
[J].
An experimental study of resistivity variation in the 3D simulation of methane hydrate generation and production
[J].
饱和砂土不同孔隙率的电阻率特性研究
[J].
Electrical resistivity characteristics of saturated sand with varied porosities
[J].
Resistivity in formation and decomposition of natural gas hydrate in porous medium
[J].DOI:10.1016/S1004-9541(08)60320-1 URL [本文引用: 1]
Flume experiment evaluation of resistivity probes as a new tool for monitoring gas migration in multilayered sediments
[J].DOI:10.1016/j.apor.2020.102415 URL [本文引用: 1]
南海沉积物天然气水合物饱和度与电阻率的关系
[J].
DOI:10.7623/syxb201303012
[本文引用: 1]
在天然气水合物勘探中,阿尔奇公式是由电阻率测井数据估算沉积层含水合物饱和度的基本公式,是对含油(气)岩心进行实验总结出的规律。但是对于水合物填充于多孔介质孔隙的沉积物,其电阻率与沉积物的物性以及水合物在孔隙的微观分布状态有关,可能存在一定的非阿尔奇现象,因此采用电阻率估算饱和度需要进行一定的校正。采用交流电桥法测量了3.5 % 盐水饱和的南海沉积物以及水合物在水饱和的沉积物中形成过程中的电阻率数据。水合物形成过程中其电阻率随着含水合物饱和度的增大而增大,尤其在低水合物饱和度(S<sub>h</sub><22 % ),其电阻率随着水合物的生成异常增大,含水合物沉积物的电阻率由水饱和的1.667Ω·m增大到含水合物饱和度为45 % 的2.661Ω·m。对于含水合物的沉积物,其双对数坐标系的电阻率增大指数和含水饱和度并不是阿尔奇公式所描述的直线关系,其饱和度指数n不是定值1.938 6,而随水饱和度S<sub>w</sub>的增加而增加。当54.8 % <S<sub>w</sub><78.6 % 时,n小于1.938 6;当S<sub>w</sub>>78.6 % 时,n大于1.938 6。
Relationship between gas hydrate saturation and resistivity in sediments of the South China Sea
[J].
DOI:10.7623/syxb201303012
[本文引用: 1]
<p>The Archie’s formula is a fundamental equation to calculate gas hydrate saturation of sediments with downhole electric resistivity log data from gas hydrate wells. This formula is based on measurements of a large number of oil/gas-bearing cores. However, the electric resistivity of porous hydrate-bearing sediments depends on physical properties of sediments and the microscopic distribution of gas hydrate in pores, thus a certain non-Archie’s behavior in hydrate-bearing sediments may exist in these porous sediments. Therefore, an experimental calibration for the hydrate saturation calculated by electric resistivity in hydrate-bearing sediments should be conducted. So laboratory studies on properties of hydrate-bearing sediments as a function of methane saturation are very important. Herein, we conducted a series of laboratory experiments to measure electric properties of South China Sea sediments initially saturated with 3.5 % salinity water and variations in electric resistivity for these sediments as gas hydrate was formed. The results show that the sediment resistivity increases with the increase of hydrate saturation during the formation of gas hydrate, especially at a low gas hydrate saturation (<em>S</em><sub>h</sub><22 % ), where the electric resistivity of sediments increases dramatically with the formation of gas hydrate, rising from 1.667Ω·m to 2.661Ω·m as the gas hydrate saturation increases from 0 % to 45 %. Besides, the dependence of the resistivity index versus hydrate saturation for gas hydrate-bearing sediments is inconsistent with the Archie’s law, therefore, the saturation exponent (n) is not a constant but a function of hydrate saturation, i.e. it increases with the increase of water saturation (<em>S</em><sub>w</sub>), thus, it is lower than 1.938 6 when water saturation ranges between 54.8 % and 78.6 % but higher than 1.938 6 when water saturation exceeds 78.6 %. The results are of important implication for quantitative laboratory and field calibration of geophysical measurements within gas hydrate-bearing intervals.</p>
Cementation failure behavior of consolidated gas hydrate-bearing sand
[J].
Modeling of the influence of gas hydrate content on the electrical properties of porous sediments
[J].DOI:10.1029/2000JB900434 URL [本文引用: 2]
Developing a new hydrate saturation calculation model for hydrate-bearing sediments
[J].DOI:10.1016/j.fuel.2019.03.038 URL [本文引用: 1]
Investigation on the multiparameter of hydrate-bearing sands using nano-focus X-ray computed tomography
[J].DOI:10.1029/2018JB015849 URL [本文引用: 1]
基于分形孔隙模型的含天然气水合物沉积物电阻率数值模拟
[J].
Numerical simulation of resistivity of gas hydrate bearing sediments based on fractal pore model
[J].
多孔介质中甲烷水合物的微观分布对电阻率的影响
[J].
Effect of microscopic distribution of methane hydrate on resistivity in porous media
[J].
Hydrate formation in sediments from free gas using a one-dimensional visual simulator
[J].DOI:10.1016/j.fuel.2017.02.034 URL [本文引用: 1]
电阻率层析成像技术在岩芯尺度水合物可视化探测中的应用
[J].
Application of resistivity tomography in visualization detection of hydrate at core scale
[J].
沉积物中天然气水合物生成过程的二维电阻层析成像观测
[J].
2-D electrical resistivity tomography assessment of hydrate formation in sandy sediments
[J].
The electrical resistivity log as an aid in determining some reservoir characteristics
[J].
DOI:10.2118/942054-G
URL
[本文引用: 1]
The usefulness of the electrical resistivity log in determining reservoircharacteristics is governed largely by:the accuracy with which the trueresistivity of the formation can be determined;the scope of detailed dataconcerning the relation of resistivity measurements to formationcharacteristics;the available information concerning the conductivity ofconnate or formation waters;the extent of geologic knowledge regardingprobable changes in facies within given horizons, both vertically andlaterally, particularly in relation to the resultant effect on the electricalproperties of the reservoir.
变形后阿尔奇公式在特低渗储层解释中的应用
[J].
Application of Archie formula in the interpretation of extra low permeability reservoir after deformation
[J].
阿尔奇公式a,m值对饱和度计算结果的影响
[J].
Effect of different values of a and m in Archie formula on water saturation
[J].
电阻率测井在确定某些储层特性中的作用
[J].
The electrical resistivity log as an aid in determining some reservoir characteristic
[J].
Natural gas hydrate deposits: A review of in situ properties
[J].
基于主控因素识别低阻油层的评价方法
[J].
Evaluation method for identifying low resistance reservoir based on main control factors
[J].
Pore-Scale investigation of the electrical property and saturation exponent of Archie’s law in hydrate-bearing sediments
[J].
DOI:10.3390/jmse10010111
URL
[本文引用: 1]
Characterizing the electrical property of hydrate-bearing sediments is essential for hydrate reservoir identification and saturation evaluation. As the major contributor to electrical conductivity, pore water is a key factor in characterizing the electrical properties of hydrate-bearing sediments. The objective of this study is to clarify the effect of hydrates on pore water and the relationship between pore water characteristics and the saturation exponent of Archie’s law in hydrate-bearing sediments. A combination of X-ray computed tomography and resistivity measurement technology is used to derive the three-dimensional spatial structure and resistivity of hydrate-bearing sediments simultaneously, which is helpful to characterize pore water and investigate the saturation exponent of Archie’s law at the micro-scale. The results show that the resistivity of hydrate-bearing sediments is controlled by changes in pore water distribution and connectivity caused by hydrate formation. With the increase of hydrate saturation, pore water connectivity decreases, but the average coordination number and tortuosity increase due to much smaller and more tortuous throats of pore water divided by hydrate particles. It is also found that the saturation exponent of Archie’s law is controlled by the distribution and connectivity of pore water. As the parameters of connected pore water (e.g., porosity, water saturation) decrease, the saturation exponent decreases. At a low hydrate-saturation stage, the saturation exponent of Archie’s law changes obviously due to the complicated pore structure of hydrate-bearing sediments. A new logarithmic relationship between the saturation exponent of Archie’s law and the tortuosity of pore water is proposed which helps to calculate field hydrate saturation using resistivity logging data.
Effect of pore size distribution on dissociation temperature depression and phase boundary shift of gas hydrate in various fine-grained sediments
[J].DOI:10.1021/acs.energyfuels.8b00074 URL [本文引用: 1]
The effect of brine on the electrical properties of methane hydrate
[J].
DOI:10.1029/2019JB018364
[本文引用: 1]
Gas hydrates possess lower electrical conductivity (inverse of resistivity) than either seawater or ice, but higher than clastic silts and sands, such that electromagnetic methods can be employed to help identify their natural formation in marine and permafrost environments. Controlled laboratory studies offer a means to isolate and quantify the effects of changing individual components within gas-hydrate-bearing systems, in turn yielding insight into the behavior of natural systems. Here we investigate the electrical properties of polycrystalline methane hydrate with >= 25% gas-filled porosity and in mixture with brine. Initially, pure methane hydrate was synthesized from H2O ice and CH4 gas while undergoing electrical impedance measurement, then partially dissociated to assess the effects of pure pore water accumulation on electrical conductivity. Methane hydrate + brine mixtures were then formed by either adding NaCl (0.25-2.5 wt %) to high-purity ice or by using frozen seawater as a reactant. Conductivity was obtained from impedance measurements made in situ throughout synthesis while temperature cycled between +15 degrees C and -25 degrees C. Several possible conduction mechanisms were subsequently determined using equivalent circuit modeling. Samples with low NaCl concentration show a doping/impurity effect and a log linear conductivity response as a function of temperature. For higher salt content samples, conductivity increases exponentially with temperature and the log linear relationship no longer holds; instead, we observe phase changes within the samples that follow NaCl-H2O-CH4 phase equilibrium predictions. Final samples were quenched in liquid nitrogen and imaged by cryogenic scanning electron microscopy (cryo-SEM) to assess grain-scale characteristics.
Assessment of gas hydrate saturation in marine sediments from resistivity and compressional-wave velocity log measurements in the Mahanadi Basin,India
[J].DOI:10.1016/j.marpetgeo.2013.10.007 URL [本文引用: 1]
Resistivity-porosity-particle shape relationships for marine sands
[J].
DOI:10.1190/1.1440891
URL
[本文引用: 1]
A laboratory investigation has been made of formation factor‐porosity relationships (formation factor being the ratio of the resistivity of a porous medium to the resistivity of the pore‐fluid), using natural and artificial sand samples whose grains varied widely in both size and shape. All samples obeyed Archie’s law, [Formula: see text] (where FF is the formation factor and n is the porosity) including mixtures of two differently shaped particle types. The exponent m was dependent on the shape of the particles, increasing as they became less spherical, while variations in size and spread of sizes appeared to have little effect. The results have been combined to produce an FF/n relationship, with an error “envelope”, which may be applicable to marine sediments in general, being in agreement with published data for marine clays. It is also suggested that the exponent m may be a better measure of the “tortuosity” of porous media than the formulas quoted in the literature.
Wettability literature survey-Part 3:The effects of wettability on the electrical properties of porous media
[J].
DOI:10.2118/13934-PA
URL
[本文引用: 1]
This paper examines the effects of wettability on the Archie saturation exponent and the formation factor, which are determined experimentally in cores. These parameters are important in the investigation of the hydrocarbon saturation of a formation by use of resistivity data obtained from well logging. The Archie saturation exponent, n, typically has a value of about in water-wet formations and cleaned cores, while in native-state, non-water-wet cores and formations it is generally larger than 2. In uniformly oil-wet cores with low brine saturations, n can reach values of 10 or more. The exponent is higher in oil-wet cores at low saturations because a portion of the brine is trapped or isolated in dendritic fingers where it is unable to contribute to electrical conductivity. If a cleaned water-wet core is used to measure n and the reservoir is actually oil-wet, interstitial water will be underestimated during logging. No definite conclusions can be drawn about the effects of wettability on the formation factor. However, the wettability of clays in a core is likely to affect this parameter.
Pore space hydrate formation in a glass bead sample from methane dissolved in water
[J].
Impact on estimation of water saturation values using laboratory determined a,m & n parameters: A case study
[C]//
Experimental studies on the evolvement of electrical resistivity during methane hydrate formation in sediments
[J].DOI:10.1021/ef301257z URL [本文引用: 1]
Determination techniques of Archie’s parameters: a,m and n in heterogeneous reservoirs
[J].DOI:10.1088/1742-2140/aa805c URL [本文引用: 1]
Archie’s saturation exponent for natural gas hydrate in coarse-grained reservoirs
[J].DOI:10.1002/jgrb.v123.3 URL [本文引用: 1]
Experimental and theoretical quantification of the relationship between electrical resistivity and hydrate saturation in porous media
[J].DOI:10.1016/j.fuel.2020.117378 URL [本文引用: 1]
Resistivity of brine-saturated sands in relation to pore geometry
[J].
Formation factors of unconsolidated porous media: Influence of particle shape and effect of cementation
[J].
DOI:10.2118/223-G
URL
[本文引用: 1]
The literature reveals that scant attention has been paid to the systematic experimental determination of the formation factors of unconsolidated porous media. No experiments appear to have been made on the effect of increasing cementation on the formation factor of an initially unconsolidated porous medium.
Generalization of the maxwell equation for formation resistivity factors
[J].
DOI:10.2118/5502-PA
URL
[本文引用: 1]
A formula relating porosity and formation resistivity factor is presented.This equation is applicable not only to consolidated and unconsolidatedmaterials, but also to dispersive systems. A comparison of calculatedvalues with experimental data shows the equation yields satisfactory results.
砾岩地层电阻率与孔隙度和孔隙形状关系数值模拟
[J].
Numerical simulation of relationship between resistivity and porosity and pore shape in conglomerate formation
[J].
Fractal analyses on saturation exponent in Archie’s law for electrical properties of hydrate-bearing porous media
[J].DOI:10.1016/j.petrol.2020.107642 URL [本文引用: 1]
不同温压条件下砂岩m、a、n、b值的变化特征
[J].
Characteristics of m,a,n and b values of sandstone under different temperature and pressure conditions
[J].
围压对页岩岩电参数的影响研究
[C]//
The experimental study of confining pressure effect on shale rock electrical parameters
[C]//
阿尔奇参数实验影响因素分析
[J].
Analysis of influencing factors of Archie parameter experiment
[J].
