Sources of dissolved oxygen in monitoring and pumping wells

Groundwater monitoring and pumping wells set in anoxic aquifers require attention to keep the groundwater free of dissolved oxygen (DO). In properly constructed monitoring or pumping wells, two processes can however still introduce oxygen to anoxic groundwater: (1) permeation of oxygen through polymer materials such as silicone, PVC, HDPE or Teflon, and (2) thermally driven convection, which can occur in all types of piezometers or wells, regardless of construction material, when the water table or pressure head is close (<10 m) to the land surface. Here, field measurements (temperature and DO well loggings) from a monitoring well in Bilthoven, the Netherlands, are combined with analytical and numerical modelling to investigate the role of both processes on oxygenation of anoxic groundwater in wells. The results of numerical and analytical modeling show that both permeation and convection can introduce oxygen into anoxic wells to near saturation concentrations. In the field data gathered, convection is primarily responsible for oxygen intrusion up to a depth of around 12 m. Oxygen intrusion through convection and permeation in monitoring and pumping wells may influence groundwater sampling and analyses, and may contribute to well clogging, depending on site conditions. The combination of field and modelling provides new insights into these processes, which can be used for both groundwater sampling and pumping well design.     

基于微地震向量扫描的煤层气井天然裂缝监测

天然裂缝发育程度是影响煤层气产能的主要因素,为了准确获取沁水盆地南部A煤层气田井组的天然裂缝发育程度和分布位置,采用地面微地震向量扫描技术对区域内7口二次压裂井进行天然裂缝发育情况监测。在压裂井周围部署一定量的三分量检波器,采集压裂过程中周边储层的微地震事件,进行Semblance叠加后得到监测区内不同时刻的破裂能量切片,解释出监测区内天然裂缝发育情况。对比井组单井产能,与监测到的天然裂缝表现出良好的相关性,揭示了天然裂缝是影响煤层气单井产能的主控因素,同时表明煤层气储层非均质性强,天然裂缝呈现局部发育特征,且比较分散、面积小,常规的三维地震预测方法难以有效的识别。应用该技术能够准确地识别出煤层气储层的天然裂缝发育情况,为调整井位的部署及优选层位提供可靠的指导。     

南海北部陆坡天然气水合物区海水甲烷浓度分布特征及其影响因素探讨

2011—2015年对东沙和神狐水合物钻探区进行了连续5年7个航次的海上调查工作,获取了大量研究区海水水文、水化学及溶解甲烷含量数据,为天然气水合物勘查与试采环境评价提供了良好的基础数据及采前环境基线。调查期间,研究区海水甲烷浓度范围为0~31.4nmol·L~(-1),平均浓度为6.7nmol·L~(-1),高于全球平均海水甲烷浓度,表明南海海水甲烷浓度本底值高于全球平均水平;研究区海水溶解甲烷浓度及其分布特征不受区域海水水文特征、海水化学特征及季节等因素影响,且表层海水-大气甲烷交换并非单一的汇或者源的关系,而是根据时间的不同,海水-大气甲烷交换存在汇源转换;综合调查结果表明,研究区甲烷渗漏对海水、大气甲烷含量没有明显影响,且水合物钻探对区域环境没有明显的影响。     

深海甲烷电化学原位长期监测技术及其在海洋环境调查和天然气水合物勘探中的意义

深海溶解甲烷浓度数据连续获取的方法技术,对于海洋环境和天然气水合物开发过程中甲烷扩散作用及通量的动态监测,具有重要的科学意义和实际应用价值。本文较详细地介绍了依据"海水脱气、气体样品定量输入、电化学高精度检测"技术思路,采用"增压排液整机系统控制的海水循环、减压稳流、气液分离、烃类组分高精度检测技术改进"方法,研发"深海甲烷电化学原位长期监测技术"的关键环节和技术方法。结合原位传感器在胶州湾港口为期94天底水长期监测实验获取的数据成果,对原位传感器的技术性能、数据质量、地质效果进行了研究评价。结果表明:(1)原位传感器量程甲烷指标达到0.01～10 000 nmol/L,灵敏度达到0.01 nmol/L,对烃类组分检测具有较好的稳定性和选择性;(2)监测水域溶解甲烷数值范围19.01～106.87 nmol/L,正常甲烷背景32.41 nmol/L,局部异常甲烷背景80.60 nmol/L,资料显示异常与污水排放过程对海水环境污染有关;(3)实测甲烷数据成果地球化学特征与胶州湾海域海水环境以往调查研究成果符合,证明了实测数据的客观性和科学性;(4)海试监测试验成果证明,原位传感器测试性能可靠、结构设计合理、设计思路科学,基本具备了海洋科学调查中对海水甲烷浓度数据获取的能力,在未来海洋天然气水合物开发过程中对甲烷扩散作用的动态监测及深海甲烷浓度通量的长期监测中,具有实际应用价值和科学意义。     

华北地区煤层气井压裂裂缝监测及其扩展规律

介绍了井温测试法、放射性同位素法、大地电位测试法和微地震测试法监测煤层水力压裂裂缝的基本原理。使用上述4种方法对华北地区施工的煤层气井压裂裂缝进行测量,得到了大量压裂井的裂缝方位和高度的监测数据。通过统计分析发现:压裂后的煤层裂缝一般都穿越其上下隔层,最大裂缝高度是压裂层厚度的6倍,裂缝长度大部分为50~90 m,裂缝形状基本以垂直裂缝为主,裂缝方向存在着随机性,扩展方向受地应力、局部地层构造和煤层割理共同作用。     

煤层气直井产气曲线特征及其与储层条件匹配性

煤层气产气曲线类型与地质条件的匹配与否直接影响产气效果。以沁水盆地柿庄南区块排采4 a以上的直井为研究对象,在产气曲线类型划分基础上,分析不同产气曲线特征,进一步分析产气曲线与储层参数的匹配性。结果表明：研究区产气曲线可划分为单峰快速上升、单峰稳定上升、双峰后低和双峰后高4种类型。产气曲线所表现出的特征受控于储层原始渗透率,储层动力及压裂效果。单峰快速上升型适用于含气量大于12 m³/d、临储比大于0.4和渗透率大于0.1×10^-3 μm²的储层,该种曲线容易造成产气量的骤降;单峰稳定上升型适用的储层条件广泛,与储层参数匹配性较高;双峰后低型产气效果整体不佳,与储层参数的匹配性差;双峰后高型适用于压裂效果较好的井、对储层原始参数要求较低,其后峰产气量的增加速率影响整体的排采效果。基于上述分析,将储层划分为七种类型,对研究区及其相邻区块实施\     

沁水盆地南部煤层气直井合层排采产气效果数值模拟

随着煤层气勘探开发的深入,多煤层合层排采受到广泛关注。合层排采管控工艺是确保煤层气合采井高产稳产的关键,而多煤层组合条件下复杂的地质条件增加了合层排采管控的难度。数值模拟技术是研究煤层气井合层排采管控工艺的有效手段,科学、可靠的模拟结果可为合采井排采管控提供依据。考虑温度效应、煤基质收缩效应、有效应力作用对煤层流体运移规律以及渗透率等煤层物性参数的影响,建立煤层气直井合层排采生产动态过程多物理场耦合数学模型,并进行有限元法的多物理场耦合求解。通过对沁水盆地南部郑庄区块煤层气合采井组的模拟,探讨不同排采速率下煤层气直井合层排采产气效果及渗透率等煤层物性参数动态演化特征,提出煤层气直井合层排采工程建议。模拟结果显示,郑庄区块3号、15号煤层整体含气量较高,煤层气合采井组具有较大增产潜力,提高排采速率对提高煤层气采收率的效果不显著;排采过程中,煤基质收缩效应对渗透率的影响强于有效应力作用,是提高煤层气井排采速率的保障,在确保排采速率不超过煤层渗流能力上限的基础上,适当提高排采速率可实现煤层气井增产。基于模拟结果,建议排采速率的调整以控制动液面或液柱压力为主;以3号、15号煤层气合采井增产为目标,产水阶段和憋压阶段,郑庄区块煤层气直井合层排采速率以液柱压力降幅0.12~0.20 MPa/d或动液面降幅12~20 m/d为宜,既可实现煤层气增产,又可避免储层伤害。      

含沙量对过饱和总溶解气体释放过程影响分析

高坝泄水常为高含沙水流,对含沙水体过饱和总溶解气体(TDG)释放规律的研究具有重要的理论和现实意义。为探讨泥沙含量以及紊动强度等特性对过饱和TDG释放过程的影响,设计了静置水柱、搅拌诱发紊动以及明渠水流3种实验条件,通过对不同含沙量工况下过饱和TDG释放过程的观测研究,分别计算各工况下的释放系数。结果表明,泥沙含量和紊动强度均对过饱和TDG的释放起着促进作用,而且随着含沙量和紊动强度的增大,释放系数逐渐增大。本研究对于过饱和TDG数值模拟研究中释放系数的取值具有指导意义,同时也可为过饱和TDG影响的减缓措施研究提供基础数据和参考依据。     

煤层气直井开发井网适应性优选

利用直井井网开发煤层气,优选合理的井网对于煤层气藏的高效开发关系重大,因此,有必要针对不同储层条件的煤层气藏研究优化井网类型。通过数值模拟方法,针对不同渗透率、不同各向异性的煤层气藏,进行了正方形、矩形和菱形3种不同井网的数值模拟,得出适合不同井网开发的储层渗透率和各向异性的范围。模拟研究表明:正方形井网不适合煤层气的开发;矩形井网适合于中渗、低渗至特低渗煤储层;菱形井网适合于中渗至高渗煤储层。     

北京西山煤炭采空区地面塌陷危险性分析

北京西山煤炭资源主要分布在百花山、庙安岭—髫髻山、九龙山—香峪大梁、北岭等向斜区，范围达1000km^2以上。含煤层位属石炭—二叠纪和侏罗纪，在已开采的煤炭产量中，有76％出自侏罗纪煤系，24％出自石炭—二叠纪煤系。在800余年开采历史中，已采出煤炭数亿吨。地表及地下浅部煤炭已被采完，在地下深部也形成了大面积多层次采空区。20世纪80年代以来，与采煤有关的地面塌陷灾害频繁凸现。本文作者通过对不同历史阶段采煤方法的差异、多层叠置的采空区结构特点等的分析，提出进入中—晚期开采时期的煤田，塌陷坑、地裂缝、山体滑塌、地面不均匀沉降、破坏性矿震等已构成灾害链，各类煤炭采空区进一步塌陷的危险性依然存在。在北京西山，不论是进一步的煤炭开采，还是地表居民的生产生活，以及国家未来的经济建设，都不应忽视煤炭采空区地面塌陷的危险性。     

Experiments of methane gas solubility in formation water under high temperature and high pressure and their geological significance

The solubility of methane in formation water and water content in the coexisting gas phase were measured under the conditions of high temperature and high pressure, using an ultra-high-pressure fluid PVT system, where the experimental temperature reached up to 453 K and pressure reached up to 130 MPa. Experimental results show the following (1) The two phases of gas and liquid still exhibit an obvious interphase interface even under high temperatures and pressures. (2) When temperatures exceed 353 K, the solubility of methane in formation water increases as the temperature and pressure rise. The growth rate of solubility is faster under a relatively low temperature and pressure, and slower at a relatively high temperature and pressure, but the solubility will not increase without limit. In this experiment, the solubility of methane in formation water reached its peak when the temperature was at 453 K and the pressure at 130 MPa. (3) Water content in the coexisting gas phase increases as temperature rises, with a smaller increase at relatively low temperatures and a much greater increase at relatively high temperatures but decreases with the increasing pressure, more rapidly under low pressure and more slowly under high pressure. The solubility of methane in formation water and the water content in the coexisting gas phase are controlled by both temperature and pressure, but using classic calculation models, these two parameters under high temperatures and pressures are inconsistent with our experimental data. Therefore, the study is significant and highlights other possible effects on solubility and condensate water content. Additionally, an example from the Yinggehai Basin in the South China Sea, where the temperature and the pressure are high, demonstrates the influence of solubility and phase behaviour on natural gas migration, its formation and the distribution of gas reservoirs.     

A review of the geochemistry of methane in natural gas hydrate

The largest accumulations on Earth of natural gas are in the form of gas hydrate, found mainly offshore in outer continental margin sediment and, to a lesser extent, in polar regions commonly associated with permafrost. Measurements of hydrocarbon gas compositions and of carbon-isotopic compositions of methane from natural gas hydrate samples, collected in subaquatic settings from around the world, suggest that methane guest molecules in the water clathrate structures are mainly derived by the microbial reduction of CO₂ from sedimentary organic matter. Typically, these hydrocarbon gases are composed of > 99% methane, with carbon-isotopic compositions (δ¹³C_PDB) ranging from − 57 to − 73‰. In only two regions, the Gulf of Mexico and the Caspian Sea, has mainly thermogenic methane been found in gas hydrate. There, hydrocarbon gases have methane contents ranging from 21 to 97%, with δ¹³C values ranging from − 29 to − 57‰. At a few locations, where the gas hydrate contains a mixture of microbial and thermal methane, microbial methane is always dominant. Continental gas hydrate, identified in Alaska and Russia, also has hydrocarbon gases composed of > 99% methane, with carbon-isotopic compositions ranging from − 41 to − 49‰. These gas hydrate deposits also contain a mixture of microbial and thermal methane, with thermal methane likely to be dominant. Published by Elsevier Science Ltd     

Effects of clay minerals and biosurfactants on isotopic and molecular characteristics of methane encaged in pressure vessel gas hydrates

The stable isotope values of carbon (δ¹³C_methane) and hydrogen (δ²H_methane) from methane molecules trapped in gas hydrates are useful for differentiation of methane from microbial and thermal origins, providing valuable information during hydrocarbon exploration. Recent studies have reported catalysis of methane hydrates when smectite clays and biosurfactants are present in hydrate-hosting sediments, but catalytic influences on the values of δ¹³C_methane and δ²H_methane are not well documented. In this study, pressure vessel methane hydrates were formed from solutions in contact with smectite clays (montmorillonite and nontronite) and biosurfactants (rhamnolipids and surfactin). Experiments show less than 1‰ differences in values of δ¹³C_methane between free and encaged molecules and up to 10‰ variations in values of δ²H_methane between free and encaged molecules. Notably, methane consumption increased in methane hydrates formed from solutions containing biosurfactants and biosurfactant–smectite mixtures. Results presented here indicate that a hydrate formed in the presence of smectite clays and biosurfactants are characterized by small shifts in free and encaged values of δ¹³C_methane and δ²H_methane and do not complicate interpretation of gas origin. In contrast, methane consumption in hydrates formed under the catalytic effect of smectite clays and biosurfactants modifies gas wetness, obscures gas origin and complicates interpretation of thermal maturity.     

浙西北地区临安区块页岩气勘探前景评价

对浙西北地区临安区块及周缘早寒武世荷塘组页岩进行了露头及野外路线地质调查、样品测试与邻井资料分析认为:临安区块荷塘组黑色页岩厚度大(150 ~ 300 m),有机碳含量高(0.22% ~ 13.56%),干酪根类型好(以Ⅰ型为主,含少量Ⅱ1),热演化适中,脆性矿物含量高(大于50%),页岩含气量高,具有形成页岩气藏的潜力.综合考虑目的层位、地表出露地层及产状、构造形态、断裂发育情况、页岩有效厚度及埋藏深度、地形地貌等因素,优选出临安区块3个勘查有利区,并预测了下一步勘探方向.     

水体溶解态黑碳的分子标志物苯多羧酸的气相色谱-质谱分析

苯多羧酸分子标志物法是定量水体中溶解态黑碳的常用方法,然而由于部分苯多羧酸,尤其是硝基苯多羧酸商业化标准品的缺失,采用该方法定量溶解态黑碳仍需进一步完善和优化。本文通过已有的苯多羧酸商业化标准品,建立了气相色谱-质谱联用仪(GC-MS)分离和定量 9 种缺少商业化标准品苯多羧酸的分析方法。并首次将菲-D10 作为溶解态黑碳的替代物,指示苯多羧酸法定量溶解态黑碳过程中的损失率。结果表明,该方法具有良好的重现性(RSD=10.34%),方法检出限为 0.67~5.38 ng/L。本文采用该方法分析了大亚湾表层水体溶解态黑碳的分布特征,其浓度范围为 30.38~46.19 μg/L,平均值为(38.93±6.17)μg/L,空间分布表现为从湾内到外海递减的趋势。