A fractal model for real gas transport in porous shale

A model for real gas flow in shale gas matrices is proposed and consists of two main steps: (a) developing a microscopic (single pore) model for a real gas flow by generalizing our previously reported Extended Navier‐Stokes Equations (ENSE) method and (b) by using fractal theory concepts, up‐scaling the single pore model to the macroscopic scale. A prominent feature of the up‐scaled model is a predictor for the apparent permeability (AP). Both models are successfully validated with experimental data. The impact of the deviation of the gas behavior from ideality (real gas effect) on the gas transport mechanisms is investigated. The effect of the structural parameters (porosity Ф, the maximum pore diameter D_max, and the minimum pore diameter D_min) of the shale matrix on the apparent permeability is studied and a sensitivity analysis is performed to evaluate the significance of the parameters for gas transport. We find that (1) the real gas transport models for a single pore and porous shale matrix are both reliable and reasonable; (2) the real gas effect affects the thermodynamic parameters of the free gas and the adsorption and transport capacity of the adsorbed gas; (3) the real gas effect decreases the effective permeability for convective flow and surface diffusion; i.e., the derivation degree of the effective permeability for bulk diffusion and Knudsen diffusion increases with increasing pressure but presents a bathtub shape when the pore diameter is smaller than 10 nm; and (4) the apparent permeability increases with Ф, D_max, and D_min. It is more sensitive to D_max, followed by the porosity. D_min has a minor impact. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1430–1440, 2017     

页岩岩心活性水浸泡前后渗透率变化规律实验研究

运用非稳态脉冲超低渗透率测量仪对页岩储层渗透率进行测量,得到页岩渗透率随孔压、轴压和围压的变化规律并且确定了页岩的应力敏感性和各向异性,为页岩储层渗流特性研究提供了理论基础。通过页岩在特定的工作液中污染不同的时间前后的渗透率变化,来反映工作液对页岩储层的伤害程度。研究成果可为页岩气井钻完井、增产改造和开采过程中储层保护提供基础参数。     

A mesoscale model for diffusion and permeation of shale gas at geological depth

The demand on energy is rising and shale gas as an important unconventional energy resource has received worldwide attention. It has shown a significant effect on the world's energy structure after the commercial exploitation of shale gas in the United States. Understanding diffusion and permeation of shale gas at geological depths is quite essential, but it cannot be described by traditional Fick or Knudsen diffusion models. In this work, we use dual control volume–grand canonical molecular dynamics method to systematically investigate the permeation process of shale gas in montmorillonite (i.e., a clay mineral of shale) at different geological depths. Results indicate that temperature, pressure, and pore size have an important effect on the permeability, and Knudsen equation cannot describe the permeability of shale gas. Accordingly, on the basis of these simulated data, we propose a new mesoscale model to describe the permeability of shale gas at geological depths. The new mesoscale model shows extensive applicability and can excellently reproduce the extrapolation testing data, and it satisfactorily bridges the gap between Knudsen diffusion and Fick diffusion, which provides important fundamentals for exploitation of shale gas. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1059–1066, 2018     

Displacement of shale gas confined in illite shale by flue gas: A molecular simulation study

The shale gas is an unconventional supplementary energy to traditional fossil energy, and is stored in layered rocks with low permeability and porosity, which leads to the difficulty for exploration of shale gas. Therefore, using CO₂ gas to displace shale gas has become an important topic. In this work, we use molecular simulations to study the displacement of shale gas by flue gas rather than CO₂, in which flue gas is modeled as a binary mixture of CO₂ and N₂ and the shale model is represented by inorganic Illite and organic methylnaphthalene. CH₄ is used as a shale gas model. Compared to the pure CO₂, flue gas is easily available and the cost of displacement by flue gas would become lower. Results indicate that the pore size of shale is an important factor in the process of displacing shale gas and simultaneously sequestrating flue gas, while the flue gas N₂-CO₂ ratio shows a small effect on the process of CH₄ displacement, because the high partial pressure of flue gas is the main driving force for displacement of shale gas. Moreover, the geological condition also has a significant effect on the process of CH₄ displacement by flue gas. Therefore, we suggest that the burial depth of 1 km is suitable operation condition for shale gas displacement. It is expected that this work provides a useful guidance for exploitation of shale gas and sequestration of greenhouse gas.     

页岩气及其吸附与扩散的研究进展

随着当前能源消费的迅速增加,常规天然气资源短缺,很难满足日益增长的能源需求。页岩气作为一种非常规天然气,具有资源潜力大和低碳排放等优点,加之美国和加拿大成功实现商业化开采,因此页岩气资源勘探开发近年来备受世界瞩目。中国作为重要的页岩气开采国家,近几年发展势头良好,部分页岩气田也已经实现商业化生产。由于页岩的致密和低渗特性,导致页岩气的开采难度较大。页岩的孔隙结构和气体吸附扩散研究,对于气藏产能的评估及其高效开采有至关重要的作用。本文介绍了国内外页岩气勘探开发现状以及页岩的孔隙结构,综述了储层中页岩气吸附、扩散与渗流的研究进展,总结了分子模拟方法在页岩气研究中的应用,并对页岩气相关研究的前景进行了展望。     

A semianalytical model for simulating real gas transport in nanopores and complex fractures of shale gas reservoirs

An efficient gridless semianalytical model was developed to simulate real gas transport in shale formation with nanopores and complex fracture geometry. This model incorporates multiple physics such as gas desorption, adsorbed gas porosity, gas slippage and diffusion, residual water saturation, non‐Darcy flow, choke skin, and pressure‐dependent matrix permeability, and fracture conductivity. Additionally, this model is easy to handle complex fracture geometry through dividing fractures into a number of segments and nodes. We verified the model against a numerical model and an analytical model for bi‐wing hydraulic fractures. After validation, the impacts of all these physics on well performance were evaluated in detail through a series of case studies. The simulation results confirm that modeling of gas production from complex fracture geometry as well as modeling important physics in shale gas reservoirs is significant. This study improves our understanding of critical physics affecting gas recovery in shale gas reservoirs. © 2017 American Institute of Chemical Engineers AIChE J, 63: 326–337, 2018     

煤岩气储层滑脱效应实验研究

岩石的孔隙度与岩石的渗透率成正相关。岩石的气透测渗率高于液测渗透率,且具有压力依赖性,该现象被称作滑脱效应或Klinkenberg效应。气测渗透率的压力依赖性是由于在计算渗透率时气体黏度取值不当所致,气体黏度在低压下随压力变化很大,但计算渗透率时却选用了定值。滑脱效应将使气体的黏度无法测量,从而出现测试悖论。气体分子每时每刻都在做不规则的热运动,会不停地与孔隙壁面发生碰撞,致使气体无法出现滑脱。岩石渗透率的气测值高于液测值,是测试介质的分子尺度与孔隙尺度对比的结果。     

页岩压裂裂缝气测导流能力实验研究

页岩遇水容易膨胀以及实际生产时页岩气并不含水,液测导流能力并不能真实反映页岩气在地层裂缝中的渗透能力。压裂用支撑剂多是以钢板夹持支撑剂、蒸馏水为驱替流体完成测试,由于钢板表面平滑、刚性不可嵌入,无法反映页岩储层裂缝壁面不规整、支撑剂在页岩壁面特性,蒸馏水属于液相介质,无法反映天然气气相流动,因此需要改进实验方法。通过对API标准的导流室改进后,采用露头页岩岩心加工成的岩板模拟储层人工裂缝,氮气模拟天然气在页岩岩板夹持下的气相流动,测试其导流能力,优化铺砂浓度。理论分析认为,支撑裂缝只要有一定的宽度,其渗透率仍远高于页岩基质的渗透率。气测裂缝导流能力结果能够真实反映地层情况,为铺砂浓度优选和页岩气藏压裂设计施工等提供可靠依据。     

微纳多孔结构中稀薄气体流动渗透率的解析型预测模型

针对多孔结构内气体表观渗透率受稀薄效应的影响而显著高于其固有渗透率的现象,从孔隙尺度流线的几何拓扑特性出发,提出了利用固有渗透率、孔隙率、弯曲度和收缩-扩张因子来表示多孔结构的有效孔隙尺寸的方法,并将该有效孔隙尺寸与经典的稀薄气体管道流动模型相结合,理论推导出一种新的多孔结构稀薄气体渗透率模型。利用该模型,可以在孔隙几何结构和物性状态已知的条件下对气体的表观渗透率进行预测。随后,通过高精度的直接模拟Monte Carlo方法(DSMC)对提出模型的准确性进行验证。通过对Knudsen数在0.01~10范围、孔隙率在0.17~0.90范围、不同气体工质以及多种有序性孔隙形式下的气体流动过程进行数值模拟表明,所提出的理论模型与模拟数据的平均偏差小于10%。     

微纳多孔结构中稀薄气体流动渗透率的解析型预测模型

针对多孔结构内气体表观渗透率受稀薄效应的影响而显著高于其固有渗透率的现象,从孔隙尺度流线的几何拓扑特性出发,提出了利用固有渗透率、孔隙率、弯曲度和收缩-扩张因子来表示多孔结构的有效孔隙尺寸的方法,并将该有效孔隙尺寸与经典的稀薄气体管道流动模型相结合,理论推导出一种新的多孔结构稀薄气体渗透率模型。利用该模型,可以在孔隙几何结构和物性状态已知的条件下对气体的表观渗透率进行预测。随后,通过高精度的直接模拟Monte Carlo方法(DSMC)对提出模型的准确性进行验证。通过对Knudsen数在0.01~10范围、孔隙率在0.17~0.90范围、不同气体工质以及多种有序性孔隙形式下的气体流动过程进行数值模拟表明,所提出的理论模型与模拟数据的平均偏差小于10%。     

垃圾土降解过程中孔隙气压变化试验研究

采用人工配制的垃圾样进行降解试验,试验过程中监测试样内部孔隙气压、产气速率、产气量的变化。试验结果表明:试样内部的孔隙气压在很短时间内达到最大值,之后气体逸散导致气压快速降低,且距离反应器边缘越近,气压越小。孔隙气压随着埋深的增大而增大,试验中气压传感器测得的孔隙气压最大值为167Pa。垃圾土降解产气速率存在二次峰值现象,第一次峰值在第1d出现,产气速率为299.784 L/d,第二次峰值在第20d出现,产气速率为6.016L/d,试验过程中总产气量为497.372L。     

An analytical model for real gas flow in shale nanopores with non‐circular cross‐section

An analytical model for gas transport in shale media is proposed on the basis of the linear superposition of convective flow and Knudsen diffusion, which is free of tangential momentum accommodation coefficient. The present model takes into the effect of pore shape and real gas, and is successfully validated against experimental data and Lattice–Boltzmann simulation results. Gas flow in noncircular nanopores can be accounted by a dimensionless geometry correction factor. In continuum‐flow regime, pore shape has a relatively minor impact on gas transport capacity; the effect of pore shape on gas transport capacity enhances significantly with increasing rarefaction. Additionally, gas transport capacity is strongly dependent of average pore size and streamline tortuosity. We also show that the present model without using weighted factor can describe the variable contribution of convective flow and Knudsen diffusion to the total flow. As pressure and pore radius decrease, the number of molecule‐wall collisions gradually predominates over the number of intermolecule collisions, and thus Knudsen diffusion contributes more to the total flow. The parameters in the present model can be determined from independent laboratory experiments. We have the confidence that the present model can provide some theoretical support in numerical simulation of shale gas production. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2893–2901, 2016     

湘鄂西地区牛蹄塘组页岩储层特征

运用有机地球化学、X衍射、氩离子抛光-扫描电镜、等温吸附、岩石力学等分析测试手段,系统研究了湘鄂西地区页岩岩石特征、有机地化特征以及页岩气储层特征。研究发现湘鄂西地区牛蹄塘组页岩主要发育原生孔隙、溶蚀孔隙、微裂缝以及有机质孔隙孔隙等4种主要类型,影响页岩含气量的主要因素有矿物组成、有机地球化特征以及页岩储层物性等,储层埋藏深度、温度、压力因素对页岩气保存起到决定性因素。     

川东北陆相气藏试气作业储层保护技术

在油气田的勘探开发过程中,只有减少对油气层的损害,才有可能获取更大的综合经济效益。油气层损害是指在油气井钻井、完井、增产措施施工中,各种工作液在井周附近储层中造成的减少油气层产能的现象。而维持储层产能的重要条件是岩石的渗透性,渗透率越高,流体导流能力越高,储层产能越高。因此,保护油气层的核心问题就是如何保持储层的渗透率。川东北陆相气藏埋藏深,储层类型复杂,泥质含量高,勘探作业施工过程中的入井液体与地层流体不配伍,易引起水敏、盐敏、压敏等现象。文中对试气作业施工可能造成储层损害的因素及采取的相应保护措施进行了详细分析,目的在于提高陆相气藏储层保护水平,确保气藏产能得以彻底解放。     

页岩气藏中孔裂隙特征及其作用

笔者通过总结国内外泥页岩类气藏发现:在主力产层均发育裂缝.由于此类岩层孔隙度渗透率极低,且空隙曲折度高,裂缝的存在不但可以做为储存空间,而且还能改善孔渗特性,同时更能连接孔隙以及层面微裂隙,极大地缩短气体扩散渗流的通道,对气体的产出有重要意义.     

四川盆地志留系页岩CH4和CO2吸附特征

页岩气是一种非常具有开发潜力的非常规天然气能源。选取四川宜宾地区志留系龙马溪组页岩,对总有机碳、黏土矿物含量和镜质体反射率等储层性质进行表征,通过场发射扫描电子显微镜和低温氮气吸附-解吸方法对页岩的孔隙结构进行分析。从孔隙结构表征发现,页岩中有大量的孔隙发育,并且大多数孔隙的尺寸在100 nm以下,微孔对总比表面积的贡献最大,中孔对孔体积做出了较大贡献。对页岩样品分别进行了CH₄和CO₂单组分气体的等温吸附实验,分析了影响页岩吸附气体能力的因素,考察了页岩样品CO₂/CH₄的选择性。结果表明,页岩对CO₂的吸附量要远大于对CH₄的吸附量;有机质含量TOC和孔隙结构对页岩的吸附有很大影响,呈正相关;温度越高,页岩对气体的吸附能力越差;单位压力变化对吸附的影响随着压力的增高而下降,逐渐趋于平缓;在竞争吸附中,页岩对CO₂有更高的选择性。     

辩证看待中国页岩气开发

页岩气是一种清洁、高效的非常规能源,但页岩层较低的渗透率特性,使得对页岩储层的改造成了主要目标,进而开发了水力压裂技术,而水力压裂页岩层是否会严重污染环境引起业界专家学者争议。针对页岩气水力压裂开采工艺,结合现状分析页岩气开采可能造成的水资源的浪费和对环境的污染。文章针对中国页岩气开发初期不完善的相关环境保护制度和尚未完善的技术而言,应重视页岩气开采的环境污染评估,如何高效的开发和对环境的保护,还有中国是否真的适合大力开发页岩气做出分析讨论。     

页岩吸附性能及作用规律

页岩气(主要组分为甲烷)作为一种新兴的非常规天然气,其对于优化能源消费结构、缓解能源对外依存度具有重要意义。相关研究表明,吸附态是页岩气的主要赋存形态,因此明确页岩吸附性能及作用规律是页岩气有效开采的重要前提。为此,本文结合国内外相关研究工作,分析了页岩的吸附特性,归纳了影响页岩吸附能力的因素,指出了页岩及页岩气后续研发方向。分析表明:页岩储层内部页岩气的赋存形态主要包括游离态、溶解态和吸附态,其中吸附态页岩气含量至少占页岩气总含量的40%;页岩气吸附量与页岩储层理化性质、储层温度和压力均有关。虽然国内外已对页岩气开展大量研究工作,但是相比于煤层气等非常规天然气研究仍显不足。为此,关于页岩吸附性能及作用规律需要在以下方面开展研究工作:①进一步探明页岩储层地质特征;②深入明确甲烷和页岩之间的流固作用关系;③利用页岩对甲烷和CO₂吸附性能的差异,推进注入CO₂强化页岩气采收率技术。     

PEMFC带沟槽气体扩散层内传输特性孔隙网络模拟

采用三维孔隙网络模型计算了不同沟槽参数下气体扩散层(GDL)的液态水突破压力、毛细压力分布、气体扩散率和液相相对渗透率随饱和度变化,并从孔隙尺度角度探究了沟槽的作用机制。研究结果表明:沟槽改变了GDL的毛细压力分布,提供了液态水直接传输路径并优化了GDL内氧气和液态水的分布,从而提高了氧气有效扩散率。沟槽位置对氧气传输有明显影响,对液相传输的影响取决于是否形成贯穿GDL的传输路径;沟槽加深,氧气和液态水传输性能增强,沟槽穿透GDL时传输性能达到最佳;沟槽变宽,液相传输性能增强,氧气传输性能在低饱和度范围内先增强后减弱。综合各因素,给出了氧气和液态水传输性能最优时的沟槽参数。     

涪陵气田页岩气井临界携液流量计算新模型

气井井筒积液会引发产量下降,井口压力降低等一系列问题,影响气井的正常生产。目前对于积液预测所使用的临界携液模型主要有:Tunner模型、Coleman模型、李闽模型等,它们之间的差别为携液系数的变化,无本质区别。为了更准确的预测涪陵气田页岩气井的临界携液流量,基于Tunner模型,考虑返排液量和井斜角对临界携液流量的影响,得到了适用于涪陵气田页岩气井的临界携液模型。