Fracture characterization and permeability prediction by pore scale variables extracted from X-ray CT images of porous geomaterials

Pore scale variables(e.g.,porosity,grain size)are important indexes to predict the hydraulic properties of porous geomaterials.X-ray images from ten types of intact sandstones and another type of sandstone samples subjected to triaxial compression are used to investigate the permeability and fracture characteristics.A novel double threshold segmentation algorithm is proposed to segment cracks,pores and grains,and pore scale variables are defined and extracted from these X-ray CT images to study the geometric characteristics of microstructures of porous geomaterials.Moreover,novel relations among these pore scale variables for permeability prediction are established,and the evolution process of cracks is investigated.The results indicate that the porescale permeability is prominently improved by cracks.In addition,excellent agreements are found between the measured and the estimated pore scale variables and permeability.The established correlations can be employed to effectively identify the hydraulic properties of porous geomaterials.

3D pore-scale modeling of nanofluids-enhanced oil recovery

The numerical modeling of oil displacement by nanofluid based on three-dimensional micromodel of cores with different permeability was carried out by the volume of fluid(VOF)method with experimentally measured values of interfacial tension,contact angle and viscosity.Water-based suspensions of SiO_(2) nanoparticles with a concentration of 0–1%and different particle sizes were considered to study the effect of concentration and size of nanoparticles,displacement fluid flow rate,oil viscosity and core permeability on the efficiency of oil displacement by nanofluid.The oil recovery factor(ORF)increases with the increase of mass fraction of nanoparticles.An increase in nanoparticles’concentration to 0.5% allows an increase in ORF by about 19% compared to water flooding.The ORF increases with the decrease of nanoparticle size,and declines with the increase of displacing rate.It has been shown that the use of nanosuspensions for enhanced oil recovery is most effective for low-permeable reservoirs with highly viscous oil in injection modes with capillary number close to the immobilization threshold,and the magnitude of oil recovery enhancement decreases with the increase of displacement speed.The higher the oil viscosity,the lower the reservoir rock permeability,the higher the ORF improved by nanofluids will be.

Pore scale simulation of liquid and gas two-phase flow based on digital core technology

Two-phase flow in two digital cores is simulated by the color-gradient lattice Boltzmann method.This model can be applied totwo-phase flow with high-density ratio(on order of 1000).The first digital core is an artificial sandstone core,and itsthree-dimensional gray model is obtained by Micro-CT scanning.The gray scale images are segmented into discrete phases(solid particles and pore space) by the Otsu algorithm.The second one is a digital core of shale,which is reconstructed usingMarkov Chain Monte Carlo method with segmented SEM scanning image as input.The wettability of solid wall and relativepermeability of a cylindrical tube are simulated to verify the model.In the simulations of liquid and gas two phase flow in digital cores,density ratios of 100,200,500 and 1000 between liquid and gas are chosen.Based on the gas distribution in the digital core at different times,it is found that the fingering phenomenon is more salient at high density ratio.With the density ratioincreasing,the displacement efficiency decreases.Besides,due to numerous small pores in the shale,the displacement efficiency is over 20% less than that in the artificial sandstone and the difference is even about 30% when density ratio is greaterthan 500.As the density ratio increases,the gas saturation decreases in big pores,and even reaches zero in some small pores orbig pores with small throats.Residual liquid mainly distributes in the small pores and the edge of big pores due to the wettability of liquid.Liquid recovery can be enhanced effectively by decreasing its viscosity.

Numerical simulation of displacement characteristics of CO_2 injected in pore-scale porous media

Pore structure of porous media, including pore size and topology, is rather complex. In immiscible twophase displacement process, the capillary force affected by pore size dominates the two-phase flow in the porous media, affecting displacement results. Direct observation of the flow patterns in the porous media is difficult, and therefore knowledge about the two-phase displacement flow is insufficient. In this paper, a two-dimensional(2D) pore structure was extracted from a sandstone sample, and the flow process that CO_2 displaces resident brine in the extracted pore structure was simulated using the Navier eStokes equation combined with the conservative level set method. The simulation results reveal that the pore throat is a crucial factor for determining CO_2 displacement process in the porous media. The two-phase meniscuses in each pore throat were in a self-adjusting process. In the displacement process,CO_2 preferentially broke through the maximum pore throat. Before breaking through the maximum pore throat, the pressure of CO_2 continually increased, and the curvature and position of two-phase interfaces in the other pore throats adjusted accordingly. Once the maximum pore throat was broken through by the CO_2, the capillary force in the other pore throats released accordingly; subsequently, the interfaces withdrew under the effect of capillary fore, preparing for breaking through the next pore throat.Therefore, the two-phase displacement in CO_2 injection is accompanied by the breaking through and adjusting of the two-phase interfaces.

Pore-scale simulation of gas-water flow in low permeability gas reservoirs

A novel method was developed to establish a realistic three dimensional(3D) network model representing pore space in low permeability sandstone.Digital core of rock sample was established by the combination of micro-CT scanning and image processing,then 3D pore-throat network model was extracted from the digital core through analyzing pore space topology,calculating pore-throat parameters and simplifying the shapes of pores and throats.The good agreements between predicted and measured porosity and absolute permeability verified the validity of this new network model.Gas-water flow mechanism was studied by using pore-scale simulations,and the influence of pore structure parameters,including coordination number,aspect ratio and shape factor,on gas-water flow,was investigated.The present simulation results show that with the increment of coordination number,gas flow ability in network improves and the effect of invading water on blocking gas flow weakens.The smaller the aspect ratio is,the stronger the anisotropy of the network is,resulting in the increase of seepage resistance.It is found that the shape factor mainly affects the end points in relative permeability curves,and for a highly irregular pore or throat with a small shape factor,the irreducible water saturation(Swi) and residual gas saturation(Sgr) are relatively high.

Numerical simulation of pore-scale flow in chemical flooding process

Chemical flooding is one of the effective technologies to increase oil recovery of petroleum reservoirs after water flooding.Above the scale of representative elementary volume(REV), phenomenological modeling and numerical simulations of chemical flooding have been reported in literatures,but the studies alike are rarely conducted at the pore-scale,at which the effects of physicochemical hydrodynamics are hardly resolved either by experimental observations or by traditional continuum-based simulations.In this paper,dissipative particle dynamics(DPD),one of mesoscopic fluid particle methods,is introduced to simulate the pore-scale flow in chemical flooding processes.The theoretical background,mathematical formulation and numerical approach of DPD are presented.The plane Poiseuille flow is used to illustrate the accuracy of the DPD simulation,and then the processes of polymer flooding through an oil-wet throat and a water-wet throat are studies, respectively.The selected parameters of those simulations are given in details.These preliminary results show the potential of this novel method for modeling the physicochemical hydrodynamics at the pore scale in the area of chemical enhanced oil recovery.

Three-Dimensional Imaging of Pore-Scale Water Flooding Phenomena in Water-Wet and Oil-Wet Porous Media

The penetration of water during water flooding has been observed over many years using several methods. A microfocused X-ray computed tomography scanner can be used to directly observe 3D water flooding in a nondestructive manner. To eliminate the possibility of false images being produced because of X-ray broadening effects, we developed a visualization method by arranging the brightness distribution of all phases involved. Water flooding experiments were conducted using oil-wet and water-wet porous media. The water phase was injected upward into packed glass beads containing an oil phase, and the process was scanned every minute until steady state was reached. Using this scheme, real-time, the water invasion pattern and oil trapping process in clusters of pores and individual pores can be observed clearly. By eliminating false images, the boundary of each phase could be identified with high precision, even in a single pore. Porelevel phenomena, including snap off (which has never before been captured in a real 3D porous medium), piston-like displacement, and the curvature of the interface, were also observed. Direct measurement of the pore throat radius and the contact angle between the wetting and nonwetting phases gave an approximation of the capillary pressure during the piston-like displacement and snap-off processes.

Influence mechanism of pore-scale anisotropy and pore distribution heterogeneity on permeability of porous media

Based on micro-CT scanning experiments, three-dimensional digital cores of tight sandstones were established to quantitatively evaluate pore-scale anisotropy and pore-distribution heterogeneity. The quartet structure generation set method was used to generate three-dimensional anisotropic, heterogeneous porous media models. A multi-relaxation-time lattice Boltzmann model was applied to analyze relationships of permeability with pore-scale anisotropy and pore distribution heterogeneity, and the microscopic influence mechanism was also investigated. The tight sandstones are of complex pore morphology, strong anisotropy and pore distribution heterogeneity, while anisotropy factor has obvious directivity. The obvious anisotropy influences the orientation of long axis of pores and fluid flow path, making tortuosity smaller and flowing energy loss less in the direction with the greater anisotropy factor. The strong correlation of tortuosity and anisotropy is the inherent reason of anisotropy acting on permeability. The influence of pore distribution heterogeneity on permeability is the combined effects of specific surface area and tortuosity, while the product of specific surface area and tortuosity shows significantly negative correlation with heterogeneity. The stronger the pore distribution heterogeneity, the smaller the product and the greater the permeability. In addition, the permeability and tortuosity of complex porous media satisfy a power relation with a high fitting precision, which can be applied for approximate estimation of core permeability.

页岩储层应力敏感性的时间效应

有效应力及其作用时间是控制岩石孔隙和裂缝变形的重要外部因素。页岩储层的多尺度孔隙结构特征及孔缝配置关系使得其应力敏感时间效应具有特殊性,然而,以往研究尚未考虑有效应力加载及卸载时间对页岩应力敏感行为的影响。以川南威荣气田龙马溪组页岩气藏为研究对象,钻取不同孔缝发育程度的页岩样品,设计并测试了变有效应力状态下变加载及卸载持续稳定时间对岩样渗透率的影响实验。结果表明:1)有效应力从5 MPa升至35 MPa时,微裂缝岩样和基块岩样渗透率分别降低了66.82%和27.19%,卸载过程中渗透率恢复率为74.73%和79.82%。页岩渗透率越低,有效应力滞后效应越显著;2)恒定有效应力加载持续稳定时间从2.5 h延长至10.0 h,应力敏感系数增加3.76%∼80.17%,且当有效应力较小时的时间效应更显著。研究认为,页岩组分及多尺度孔缝结构是导致其孔缝变形行为差异性的主因。基于实验研究结果,推荐页岩样品老化处理为6.0 h,提出了优化页岩储层敏感性评价实验方案和优化气井工作制度的建议。

致密砂岩气藏注CO_(2)提高天然气采收率微观机理

中国致密砂岩气藏资源储量丰富,复杂的气水渗流关系和气水同产特征制约了单井产能的发挥和天然气采收率提高,注CO_(2)是提高气藏采收率(EGR)和实现碳埋存的双赢途径。为明确致密砂岩气藏CO_(2)驱替微观渗流和提高天然气采收率机理,指导致密砂岩气藏CO_(2)-EGR方案设计,基于格子玻尔兹曼方法(LBM)建立了孔隙尺度多相多组分流动模型,揭示了致密砂岩气藏储层微观气水分布特征和CO_(2)-EGR的微观渗流机理,并明确了CO_(2)-EGR的主控因素。研究结果表明:①驱动压差显著影响了致密砂岩气藏的气水微观分布和水锁程度,使得气水流动能力和气水相对渗透率特征不同;②CO_(2)-EGR微观渗流过程包括气水两相的非混相驱替和CO_(2)-CH_(4)的混相驱替,对应EGR机理为分别受生产压差和地层压力控制的黏性驱替和混相扩散;③注入的CO_(2)可有效缓解水锁现象和贾敏效应,与CH_(4)良好的混相能力能促进沟通分散气泡,微观驱气效率可达42%~94%;④含水饱和度、孔隙结构和驱动压差显著影响微观驱气的作用机制和驱气效率的改善幅度。结论认为,在进行致密砂岩气CO_(2)-EGR的方案设计时,可优先考虑中—低含水饱和度的区块作为试验靶区,并根据靶区储层孔隙结构特征,优化不同注气阶段的注采参数,可充分发挥CO_(2)对CH_(4)的黏性驱替和混相扩散作用。

致密砂岩孔隙尺度下气-水界面动态演化可视化实验研究

水相圈闭是非常规天然气开发面临的主要储层损害问题之一,采用岩心驱替实验无法直观揭示孔隙内部气水两相分布和流动演化过程,因此,对水相圈闭损害微观作用机理的认识有待提高.根据致密砂岩铸体薄片,设计出一维通道和二维孔隙网络两种玻璃芯片模型,可视化研究了孔隙尺度下渗吸和返排过程中气-水界面演化和两相流动行为;结合岩心驱替实验,探讨了孔隙尺度气-水界面演化与致密砂岩宏观气体流动的关联机制.结果表明:(1)孔隙尺度下气-水界面随含水饱和度的增加由水膜水气-水界面向毛管水气-水界面演化,并主要通过卡断和绕流两种形式破坏气体流动连续性;(2)优势通道既是孔隙尺度下水相返排的快速走廊,也是气泡运移和贾敏效应发生的主要通道,是造成储层产水但不产气的一个重要原因;(3)孔隙尺度下气-水界面演化引起的水封气现象是水相圈闭损害的具体微观作用形式,宏观表现为岩心尺度上水相难以彻底返排和气体渗透能力不能完全恢复.实验结果可为储层岩石孔隙尺度下水相圈闭作用机理和气水可动性分析提供理论依据.

致密油藏混合润湿对孔隙尺度渗吸的影响

以毛细管压力为基础的渗吸作用可有效动用致密储层中的原油并补充能量。为了研究混合润湿程度与渗吸采出程度之间的关系,以及典型混合润湿孔喉通道渗吸规律,基于相场理论和Navier-Stokes方程,建立静态孔隙尺度渗吸数值模型,对孔隙尺度渗吸过程进行数值模拟,分析不同混合润湿程度对渗吸采油效果的影响。结果表明:混合润湿占比为10%的混合润湿岩石孔隙渗吸采出程度比纯水湿的采出程度高。当混合润湿占比低时,混合润湿通道为渗吸的主力通道,渗吸效果与纯水湿的类似;当混合润湿占比超过40%时,渗吸采出程度低于10%,渗吸效果差;当混合润湿占比高时,连通性好且成片存在的水湿孔喉成为混合润湿体系渗吸排驱采油的主要优势通道,连续的油湿孔喉抑制水相流动,导致渗吸作用停止。该结果为阐明混合润湿程度和混合润湿分布的渗吸规律和流动潜力、提高致密油藏的采收率提供指导。

济阳页岩油渗流机理及开发规律研究

济阳页岩油资源潜力巨大,但受复杂地质条件影响,对压裂后多尺度空间渗流机理及开发规律缺乏系统认识,规模效益开发难度大。为厘清济阳页岩油开发规律的内在机制,针对济阳页岩油多尺度裂缝发育特征,综合物理实验与数值模拟等多种手段,研究压裂后复合缝网的形成机制及组合模式,开展多尺度裂缝应力敏感、多级孔缝内流体非线性渗流、孔缝系统渗吸置换等多重介质空间渗流机理研究。依据压裂改造后多尺度缝网的分布及其渗流规律的差异性,建立了页岩油“易流-缓流-滞流”三区渗流模式,明确了济阳页岩油不同流动阶段的渗流特征与开发规律。研究结果表明:层理缝发育可改善储层物性条件,提高渗吸置换效率,降低启动压力梯度;弹性开发阶段随地层压力下降,渗流通道减小,缝网导流能力降低,具有明显的应力敏感性。济阳页岩油开发过程中,开发初期流态以易流区供液的裂缝线性流和裂缝椭圆流为主;开发中期缓流区向易流区供能,流态转变为地层椭圆流;开发后期滞流区内的页岩油逐渐动用,流态转变为边界控制流。受地质条件和生产制度影响,不同水平井能量变化、含水率下降和产量变化规律具有较大差异。合理优化闷井时间和生产制度,能够保证渗吸置换效果,高效利用地层能量,提高页岩油产能。

不同胶结度的多孔介质中溶质横向弥散的孔隙尺度模拟研究

深入理解、精确刻画溶质孔隙尺度横向弥散行为是揭示溶质运移现象内在机制、提高溶质运移模拟精度的关键。基于孔隙尺度模拟方法研究了不同胶结度的多孔介质中溶质横向弥散特征。结果表明:当胶结度增大时,溶质横向弥散程度有所增强,但与纵向弥散相比增强幅度较小,且在模拟的胶结度范围内横向弥散未展现显著的尺度效应。研究还发现多孔介质中横向弥散系数D_(T)与纵向弥散系数D_(L)间的差异程度不是固定不变的,而是随着溶质运移距离逐渐增大最终达到稳态的过程,并且D_(L)/D_(T)的渐进值及达到渐进值所需时间均受胶结度的影响。因此,将D_(T)视为比D_(L)小一个数量级的值与溶质实际弥散行为存在较大偏差。同时,通过对比不同介质中流体流速横向、纵向分量概率密度及溶质对流速率概率密度的差异,揭示了以上溶质横向弥散特征变化及其与纵向弥散特征变化不同的内在机制。

不同胶结度的饱和土壤孔隙尺度结构对地下水渗流特性的影响

饱和土壤中地下水渗流过程的研究在诸多领域具有重要意义,土壤孔隙尺度结构差异对地下水渗流特性有显著影响。土壤颗粒胶结度是土壤的基本属性之一,但是目前针对不同胶结度的土壤孔隙尺度结构差异对地下水渗流影响的研究尚少。采用整体重排算法构建不同胶结度的土壤介质孔隙尺度结构,基于此运用有限元软件模拟介质中地下水渗流过程开展相关研究。结果表明:土壤颗粒胶结度对饱和土壤中地下水渗流具有显著影响。随着胶结土壤颗粒百分数Pc从0增至60.20%,流场的变异函数增大了70.15%(从1.233到2.098),即随着胶结度增大,流速空间非均质性显著增强。此外,沿地下水主要流动方向和垂直于主要流动方向上的流速概率密度分布函数均越来越发散。流速接近平均流速的区域减少,不流动区、优势流区同时显著增大。当Pc从0增至60.20%时,地下水中不流动区比例增大了23倍(从2.06%到48.31%),而优势流区比例增大了将近9倍(从0.27%到2.41%)。当平均流速不同时,渗流特征的以上变化趋势保持不变。此外,本研究发现不同胶结度介质间的孔隙尺度结构差异正是引起地下水渗流特性发生上述变化的内在原因。

粉煤灰基陶瓷膜支撑体的制备与尺寸放大试验

选取固体废物粉煤灰作为支撑体骨料,TiO_(2)为烧结助剂,羧甲基纤维素(CMC)为黏结剂,木炭粉为造孔剂,采用固态粒子烧结法制得初始陶瓷膜支撑体样品,并对其进行等比尺寸(直径)放大制备。探究支撑体尺寸放大制备中烧结温度、造孔剂添加量、尺寸(直径)放大倍数等因素对支撑体性能的影响,对物质组成、微观形貌、抗折强度、纯水通量、酸碱腐蚀率及孔径分布等性能进行表征。结果表明:最高烧结温度为1 050℃,造孔剂木炭粉添加量为15%(质量分数),放大到原尺寸(直径)的2倍时,所制得支撑体性能最佳,其内部孔隙均匀,颈型结构明显,纯水通量为4 728.26 L/(m^(2)·h·MPa),抗折强度为25.15 MPa,中值孔径为3.06μm,孔隙率为38.56%,酸、碱质量损失率分别为0.33%、0.25%。

整合压汞、N_(2)和CO_(2)吸附的中−高阶煤多重分形特征

煤储层孔隙结构多重分形特征控制着煤层气的运移和可持续产出,直接决定了煤层气的开采效率,对煤层气开采具有重要意义。为了研究中−高阶煤孔隙结构多重分形特征及其在煤化作用过程中的演化趋势,针对取自沁水煤田生产矿井的中−高阶煤样,整合高压压汞、低温氮气吸附实验和二氧化碳吸附实验,结合多重分形理论,表征并探究了中−高阶煤储层宏孔(>50 nm)、介孔(2~50 nm)和微孔(<2 nm)的多重分形特征在煤化作用过程中的演化趋势及其影响因素。结果表明,中−高阶煤宏孔、介孔和微孔的广义维数谱(D_(q)−q)和多重分形奇异谱(f(α)−α)均满足多重分形特征,这意味着中−高阶煤宏孔、介孔和微孔均表现出多重分形行为。相对于宏孔和介孔,微孔表现出更大的奇异性指数α_(0)和谱宽(ΔD)与较小的赫斯特指数(Hurst,H),即微孔具有更强的非均质性和更差的孔隙连通性。煤化作用促进了煤中大分子的聚合,使煤储层由宏孔优势型和宏孔−微孔并存型储层转变为更为致密的微孔优势型储层,煤中不同尺度孔隙孔径分布趋于均质化,导致中−高阶煤储层孔隙结构均质性的增强和孔隙连通性的改善。宏孔和微孔体积分数对其相应孔径范围内孔隙结构非均质性分别存在积极和消极的影响,而介孔体积分数并不是介孔孔隙孔径分布非均质性的有效约束。镜质组和惰质组对孔隙孔径分布非均质性表现出相反的影响,镜惰比(V/I)与H之间存在正相关性,与α_(0)之间呈负相关性,富镜质组煤发育更多的微孔从而表现出更强的孔隙结构均质性和较好的孔隙连通性。

特低渗油藏CO_(2)/水交替微观驱油机理研究

苏北盆地断块特低渗透油藏具有“小、碎、深、薄、低”的典型特征,在经过短期弹性开发后如何优选合理高效的提高油藏采收率方法成为该类油藏面临的瓶颈问题。水气交替注入(Water Alternating Gas,WAG)方法能有效抑制水窜和气窜,兼具水驱和CO_(2)驱的优点,在特低渗透油藏取得了成功应用,但其微观驱油机理及实际效果仍需进一步研究和评价。文中以苏北ZD油田为研究对象,基于特低渗透天然岩心的三维数字模型,应用孔隙尺度微观数值模拟方法,研究了CO_(2)/水交替驱油过程中气水相界面的动态演化过程,并深入阐明了特低渗油藏WAG提高采收率的微观机理,结果表明:水气交替驱具有更大的波及面积和更少的剩余油体积,CO_(2)驱过程具有更好的流动性和克服黏滞力的能力,同时水驱过程中的渗吸作用和封堵作用能有效提高驱油效率。因此,CO_(2)/水交替驱可以有效启动特低渗储层,起到调剖调驱协同增益的作用,研究成果对于指导特低渗透油藏提高采收率技术的优选和实施具有重要的理论意义和矿场应用价值。

考虑级配影响的透水性基床填料细观空隙特征研究

大空隙级配碎石填料因其较好的渗透性能广泛应用于透(排)水性基床,然而级配对细观三维空隙特征的影响尚不明确。针对此不足,对3种代表性级配的透水性基床填料试样开展室内新型平板振动压实试验,并对压实后的试样进行高精度工业XCT扫描,重构其三维模型并获取其内部空隙,计算面空隙率、等效体积和三维形态等空隙结构特征指标,分析不同级配(颗粒堆积结构)对其三维形态及分布规律的影响。研究结果表明,不同级配的试样其内部面空隙率沿高度方向总体呈对称分布,即两端大中部小,骨架密实型颗粒堆积结构试样其面空隙率处于最低水平且沿高度方向的差异性最小,骨架空隙型颗粒堆积结构试样其面空隙率最大且不均匀性也最大;随着颗粒堆积结构从悬浮密实型过渡至骨架密实型再至骨架空隙型,试样内部空隙总数量大幅减小,孤立微空隙和小空隙数量减小,连通性较好的中空隙与大空隙增多;空隙形态逐渐演化为不规则型和长条型,并从较为圆润丰满型过渡至边缘残缺型,具有更好的连通性,可提供水气运移的通道。优化颗粒堆积结构有利于形成连通空隙并提供渗流通道,进而提升基床填料的水−力特性。研究结果可为透水性级配碎石基床填料的优化设计提供理论依据和技术参考。