Modeling of multiphase flow in low permeability porous media:Effect of wettability and pore structure properties

Multiphase flow in low permeability porous media is involved in numerous energy and environmental applications.However,a complete description of this process is challenging due to the limited modeling scale and the effects of complex pore structures and wettability.To address this issue,based on the digital rock of low permeability sandstone,a direct numerical simulation is performed considering the interphase drag and boundary slip to clarify the microscopic water-oil displacement process.In addition,a dual-porosity pore network model(PNM)is constructed to obtain the water-oil relative permeability of the sample.The displacement efficiency as a recovery process is assessed under different wetting and pore structure properties.Results show that microscopic displacement mechanisms explain the corresponding macroscopic relative permeability.The injected water breaks through the outlet earlier with a large mass flow,while thick oil films exist in rough hydrophobic surfaces and poorly connected pores.The variation of water-oil relative permeability is significant,and residual oil saturation is high in the oil-wet system.The flooding is extensive,and the residual oil is trapped in complex pore networks for hydrophilic pore surfaces;thus,water relative permeability is lower in the water-wet system.While the displacement efficiency is the worst in mixed-wetting systems for poor water connectivity.Microporosity negatively correlates with invading oil volume fraction due to strong capillary resistance,and a large microporosity corresponds to low residual oil saturation.This work provides insights into the water-oil flow from different modeling perspectives and helps to optimize the development plan for enhanced recovery.

Anisotropic dynamic permeability model for porous media

Based on the tortuous capillary network model,the relationship between anisotropic permeability and rock normal strain,namely the anisotropic dynamic permeability model(ADPM),was derived and established.The model was verified using pore-scale flow simulation.The uniaxial strain process was calculated and the main factors affecting permeability changes in different directions in the deformation process were analyzed.In the process of uniaxial strain during the exploitation of layered oil and gas reservoirs,the effect of effective surface porosity on the permeability in all directions is consistent.With the decrease of effective surface porosity,the sensitivity of permeability to strain increases.The sensitivity of the permeability perpendicular to the direction of compression to the strain decreases with the increase of the tortuosity,while the sensitivity of the permeability in the direction of compression to the strain increases with the increase of the tortuosity.For layered reservoirs with the same initial tortuosity in all directions,the tortuosity plays a decisive role in the relative relationship between the variations of permeability in all directions during pressure drop.When the tortuosity is less than 1.6,the decrease rate of horizontal permeability is higher than that of vertical permeability,while the opposite is true when the tortuosity is greater than 1.6.This phenomenon cannot be represented by traditional dynamic permeability model.After the verification by experimental data of pore-scale simulation,the new model has high fitting accuracy and can effectively characterize the effects of deformation in different directions on the permeability in all directions.

“Rigid-elastic chimera” pore skeleton model and overpressure porosity measurement method for shale: A case study of the deep overpressure siliceous shale of Silurian Longmaxi Formation in southern Sichuan Basin, SW China

Based on analysis of pore features and pore skeleton composition of shale,a“rigid elastic chimeric”pore skeleton model of shale gas reservoir was built.Pore deformation mechanisms leading to increase of shale porosity due to the pore skeleton deformation under overpressure were sorted out through analysis of stress on the shale pore and skeleton.After reviewing the difficulties and defects of existent porosity measurement methods,a dynamic deformed porosity measurement method was worked out and used to measure the porosity of overpressure Silurian Longmaxi Formation shale under real formation conditions in southern Sichuan Basin.The results show:(1)The shale reservoir is a mixture of inorganic rock particles and organic matter,which contains inorganic pores supported by rigid skeleton particles and organic pores supported by elastic-plastic particles,and thus has a special“rigid elastic chimeric”pore structure.(2)Under the action of formation overpressure,the inorganic pores have tiny changes that can be assumed that they don’t change in porosity,while the organic pores may have large deformation due to skeleton compression,leading to the increase of radius,connectivity and ultimately porosity of these pores.(3)The“dynamic”deformation porosity measurement method combining high injection pressure helium porosity measurement and kerosene porosity measurement method under ultra-high variable pressure can accurately measure porosity of unconnected micro-pores under normal pressure conditions,and also the porosity increment caused by plastic skeleton compression deformation.(4)The pore deformation mechanism of shale may result in the"abnormal"phenomenon that the shale under formation conditions has higher porosity than that under normal pressure,so the overpressure shale reservoir is not necessarily“ultra-low in porosity”,and can have porosity over 10%.Application of this method in Well L210 in southern Sichuan has confirmed its practicality and reliability.

Porosity and permeability variations of a dam curtain during dissolution

During reservoir operation,the erosion effects of groundwater change the porosity and permeability of the dam curtain,causing changes to the seepage field.To understand where the changes take place and to what degree the porosity and permeability change,a multi-field coupling model was built and solved.The model takes into account seepage,solution concentration,and solid structure.The model was validated using uplift pressure monitoring data.Then,the variations in curtain porosity,seepage flow,and loss quantity of Ca(OH)2 were calculated.The key time nodes were obtained through curve fitting of the variation of seepage flow with the BiDoseResp function.The results showed that the model could reflect the attenuation trend of curtain performance well.The process and position of the erosion were not homogeneous.Although erosion mainly occurred at the top and bottom of the curtain,it was most developed at the top.The erosion effects developed slowly during the early stage,much fast during the middle and late stages,and culminated in complete dissolution.The model results and the daily monitoring data can provide a scientific basis for the safe operation and management of reservoirs.

The coupling of dynamics and permeability in the hydrocarbon accumulation period controls the oil-bearing potential of low permeability reservoirs:a case study of the low permeability turbidite reservoirs in the middle part of the third member of Shahejie

The relationships between permeability and dynamics in hydrocarbon accumulation determine oil- bearing potential （the potential oil charge） of low perme- ability reservoirs. The evolution of porosity and permeability of low permeability turbidite reservoirs of the middle part of the third member of the Shahejie Formation in the Dongying Sag has been investigated by detailed core descriptions, thin section analyses, fluid inclusion analyses, carbon and oxygen isotope analyses, mercury injection, porosity and permeability testing, and basin modeling. The cutoff values for the permeability of the reservoirs in the accumulation period were calculated after detailing the accumulation dynamics and reservoir pore structures, then the distribution pattern of the oil-bearing potential of reservoirs controlled by the matching relationship between dynamics and permeability during the accumulation period were summarized. On the basis of the observed diagenetic features and with regard to the paragenetic sequences, the reservoirs can be subdivided into four types of diagenetic facies. The reservoirs experienced two periods of hydro- carbon accumulation. In the early accumulation period, the reservoirs except for diagenetic facies A had middle to high permeability ranging from 10 × 10-3 gm2 to 4207 × 10-3 lain2. In the later accumulation period, the reservoirs except for diagenetic facies C had low permeability ranging from 0.015 × 10-3 gm2 to 62× 10-3 -3m2. In the early accumulation period, the fluid pressure increased by the hydrocarbon generation was 1.4-11.3 MPa with an average value of 5.1 MPa, and a surplus pressure of 1.8-12.6 MPa with an average value of 6.3 MPa. In the later accumulation period, the fluid pressure increased by the hydrocarbon generation process was 0.7-12.7 MPa with an average value of 5.36 MPa and a surplus pressure of 1.3-16.2 MPa with an average value of 6.5 MPa. Even though different types of reservoirs exist, all can form hydrocarbon accumulations in the early accumulation per- iod. Such types of reservoirs can form hydrocarbon accumulation with high accumulation dynamics; however, reservoirs with diagenetic facies A and diagenetic facies B do not develop accumulation conditions with low accumu- lation dynamics in the late accumulation period for very low permeability. At more than 3000 m burial depth, a larger proportion of turbidite reservoirs are oil charged due to the proximity to the source rock, Also at these depths, lenticular sand bodies can accumulate hydrocarbons. At shallower depths, only the reservoirs with oil-source fault development can accumulate hydrocarbons. For flat surfaces, hydrocarbons have always been accumulated in the reservoirs around the oil-source faults and areas near the center of subsags with high accumulation dynamics.

Prediction of mushy zone permeability of Al-4.5wt%Cu alloy during solidification by phase field model and CFD simulation

Liquid permeability of the mushy zone is important for porosity formation during the solidification process. In order to investigate the permeability of the mushy zone, an integrated model was developed by combining the phase field model and computational fluid dynamics (CFD) model. The three-dimensional multigrain dendrite morphology was obtained by using the phase field model. Subsequently, the computer-aided design (CAD) geometry and mesh were generated based on calculated dendrite morphologies. Finally, the permeability of the dendritic mushy zone was obtained by solving the Navier-Stokes and continuity equations in ANSYS Fluent software. As an example, the dendritic mushy zone permeability of Al-4.5wt%Cu alloy and its relationship with the solid fractions were studied in detail. The predicted permeability data can be input to the solidification model on a greater length scale for macro segregation and porosity simulations.

Porosity Calculation of Tight Sand Gas Reservoirs with GA-CM Hybrid Optimization Log Interpretation Method

Tight sand gas reservoirs are our country’s fairly rich unconventional natural gas resources, and their exploration and development is of prime importance. Sulige Gas Field which located in the northern Ordos Basin is tight sand gas reservoirs. It is typically featured by low porosity and low permeability, and the error of porosity calculation by traditional methods is larger. Multicomponent explanation model is built by analyzing the thin slice data, and the objective function is got according to the concept of optimization log interpretation method. This paper puts the Genetic Algorithm and the Complex Algorithm together to form the GA-CM Hybrid Algorithm for searching the optimal solution of the objective function, getting the porosity of tight sandstone gas reservoirs. The deviation got by this method is lesser compared with the core porosity, with a high reliability.

Review of permeability evolution model for fractured porous media

The ability to capture permeability of fractured porous media plays a significant role in several engineering applications, including reservoir, mining, petroleum and geotechnical engineering. In order to solve fluid flow and coupled flow-deformation problems encountered in these engineering applications,both empirical and theoretical models had been proposed in the past few decades. Some of them are simple but still work in certain circumstances; others are complex but also need some modifications to be applicable. Thus, the understanding of state-of-the-art permeability evolution model would help researchers and engineers solve engineering problems through an appropriate approach. This paper summarizes permeability evolution models proposed by earlier and recent researchers with emphasis on their characteristics and limitations.

光热驱动的膜分离生物甲烷制氢过程建模与仿真分析

光热驱动的甲烷重整制氢技术,是将太阳能转化为燃料的主要技术路径之一。引入聚光集热技术,可实现太阳能的全光谱利用,有效提高太阳能到燃料化学能的转化效率,同时显著降低系统能耗及碳排放。将多孔碳化硅吸热体和La_(1-x)Sr_(x)Co_(1-y)Fe_(y)O_(3-δ)钙钛矿透氧膜结合,提出一种光热驱动的膜反应制氢的设计思路,实现了连续、高效的制氢过程。完成了该反应过程的概念设计,建立考虑辐射-传热-膜分离-反应的数值模型并进行仿真模拟;重点评估了膜表面的温度均匀性,并分析温度对膜内反应及出口产物的影响规律。结果表明,该模型可以有效精准描述概念设计过程。研究成果可为光热驱动的高温膜反应器设计和放大提供理论依据以及初步设计方法。

Dynamic-Change Laws of the Porosity and Permeability of Low-to Medium-Rank Coals under Heating and Pressurization Treatments in the Eastern Junggar Basin,China

Deep coalbed methane exists in high-temperature and high-pressure reservoirs. To elucidate the dynamic-change laws of the deep coal reservoir porosity and permeability characteristics in the process of coalbed methane production, based on three pieces of low- to medium-rank coal samples in the eastern Junggar Basin, Xinjiang, we analyse their mercury-injection pore structures. We measured the porosity and permeability of the coal samples at various temperatures and confining pressures by high-temperature and confining pressure testing. The results show that the porosity of a coal sample decreases exponentially with increasing effective stress. With increasing temperature, the initial porosity increases for two pieces of relatively low-rank coal samples. The increased rate of porosity decreases with increasing confining pressure. With increasing temperature, the initial porosity of a relatively high-rank coal sample decreases, and the rate of change of the porosity become faster. An exponential relationship exists between the porosity and permeability. With increasing coal rank, the initial porosity and permeability decrease. The change rate of the permeability decreases with increasing porosity.

基于CEL法熔滴冲击基板的TPF/FSI数值模拟

航空发动机和燃气轮机的高温工作环境会对其热端金属部件造成不可逆损伤,热障涂层(thermal barrier coatings,TBCs)能够承受高温和高压的侵蚀性环境,从而保证金属部件使用的安全性及可靠性。以空心Y_(2)O_(3)-ZrO_(2)(yttrium-stabilized zirconia,YSZ)粒子在等离子焰流中的2种形态,即全熔熔滴和空心熔滴为研究对象对其冲击铺展过程进行了模拟。基于ABAQUS/EXPLICIT耦合欧拉-拉格朗日(coupled Eulerian-Lagrangian,CEL)有限元方法,首先针对ABAQUS缺少相变模型,导致使用CEL方法计算熔滴铺展形貌失真的问题,给出了适用的动力黏度随温度变化的经验公式。此外,考虑周围空气对熔滴铺展过程的影响,提出了“两相流(two phase flow,TPF)/流-固耦合(fluid-structure-interaction,FSI)”2.5D模型,对熔滴冲击基板凝固成型及空气卷入的过程进行了模拟,并揭示了2种熔滴铺展形貌存在较大差异的机理,对制备隔热性能更优的热障涂层具有指导意义。

多孔介质各向异性动态渗透率模型

基于迂曲毛细管网络模型,推导建立了各向异性渗透率与岩石正应变间的关系式,即各向异性动态渗透率模型(ADPM),采用孔隙尺度流动模拟对模型进行了验证,并对单轴应变过程进行了计算,分析了变形过程中影响不同方向渗透率变化的主要因素。研究表明:层状油气藏开采的单轴应变过程中,有效面孔率对各方向的渗透率影响规律一致,随着有效面孔率的减小,渗透率对应变的敏感程度增大;垂直于压缩方向的渗透率对应变的敏感程度随迂曲度的增加而减小,沿压缩方向的渗透率对应变的敏感程度随迂曲度的增加而增大。初始各方向迂曲度相同的层状油气藏,在压力下降的过程中,迂曲度对各方向渗透率变化幅度的相对关系起决定性作用,迂曲度小于1.6时,岩石水平方向渗透率的降幅要高于垂向渗透率,迂曲度大于1.6时则相反,传统动态渗透率模型无法表征这一现象。经孔隙尺度模拟实验数据的验证,新模型拟合精度高,可以有效表征不同方向变形对各方向渗透率的影响。

基于组织模型的纬编面料透气性数字化预测

为在投入实际生产前正确评估双面纬编面料的透气性能,构建了双面织物组织模型,并进行织物孔隙率的数字化表达。首先,对双面纬编织物结构中包含的成圈、集圈和浮线这3种基本线圈形态进行孔隙率的单元组织模型构建;然后系统归纳出双层结构中9种正面结构单元和21种反面结构单元,并构建双层结构孔隙率矩阵,得到织物理论孔隙率的计算方法;最后,在3款不同组织的面料基础上通过线性拟合得出理论孔隙率与透气率的关系,并选用2款面料验证预测系统的准确性。结果表明:理论计算的透气率为627.0、688.6 mm/s,实际测量的透气率为600.3、676.3 mm/s,其误差率在±6%以内,说明构建的纬编组织模型和透气性预测方法具有合理性和适用性。

基于双孔模型的黏土渗透特性预测

【目的】粗粒土的渗透模型比较成熟,然而黏性土的渗透系数很难精确预测,改进经典KC(KozenyCarman)渗透方程应用于黏性土具有实际意义。【方法】基于文献中不同孔隙比的孔径分布曲线,提出了界定集聚体间孔隙与集聚体内孔隙的方法。在双孔模型框架下,扣除对渗流不起作用的集聚体内部孔隙,计算出有效孔隙比和有效比表面积,最终对经典KC渗透方程进行了修正。【结果】选用文献中3种不同黏性土验证改进方程的准确性,对比试验值、经典KC方程预测值与改进KC方程预测值,改进KC方程预测效果有了很大提升,与试验值比较吻合。【结论】基于双孔模型的改进KC渗透方程在一定范围内可以应用于黏性土渗透特性预测,具有一定的实用性。

Changes in filtration and capacitance properties of highly porous reservoir in underground gas storage:CT-based and geomechanical modeling

The paper presents the results of comprehensive studies of filtration and capacitance properties of highly porous reservoir rocks of the aquifer of an underground gas storage facility.The geomechanical part of the research included studying the dependence of rock permeability on the stress-strain state in the vicinity of the wells,and physical modeling of the implementation of the method of increasing the permeability of the wellbore zone-the method of directional unloading of the reservoir.The digital part of the research included computed tomography(CT)-based computer analysis of the internal structure,pore space characteristics,and filtration properties before and after the tests.According to the results of physical modeling of deformation and filtration processes,it is found that the permeability of rocks before fracture depends on the stress-strain state insignificantly,and this influence is reversible.However,when downhole pressure reaches 7-8 MPa,macrocracks in the rock begin to grow,accompanied by irreversible permeability increase.Porosity,geodesic tortuosity and permeability values were obtained based on digital studies and numerical modeling.A weak degree of transversal anisotropy of the filtration properties of rocks was detected.Based on the analysis of pore size distribution,pressure field and flow velocities,high homogeneity and connectivity of the rock pore space is shown.The absence of pronounced changes in pore space characteristics and pore permeability after non-uniform triaxial loading rocks was shown.On the basis of geometrical analysis of pore space,the reasons for weak permeability anisotropy were identified.The filtration-capacitance properties obtained from the digital analysis showed very good agreement with the results of field and laboratory measurements.The physical modeling has confirmed the efficiency of application of the directional unloading method for the reservoir under study.The necessary parameters of its application were calculated:bottomhole geometry,stage of operation,stresses and pressure drawdown value.

Seepage-Stress-Damage Coupled Model of Coal Under Geo-Stress Influence

In the seepage-stress-damage coupled process,the mechanical properties and seepage characteristics of coal are distinctly different between pre-peak stage and post-peak stage.This difference is mainly caused by damage of coal.Therefore,in the process of seepage and stress analysis of coal under the influence of excavation or mining,we need to consider the weakening of mechanical properties and the development of fractures of damaged coal.Based on this understanding,this paper analyzes the influence of damage on mechanics and seepage behavior of coal.A coupled model is established to analyze the seepage-stress-damage coupled process of coal.This model implemented into COMSOL and MATLAB software to realize the numerical solving.Two examples are adopted to verify the correctness of the model and some useful conclusions are obtained.The numerical model establishes the relationship between microcosmic damage evolution and macroscopical fracture and simulates the whole process of coal from microcosmic damage to macroscopical fracture,and the dynamic simulation of fluid flow in this process.It provides a numerical tool for further research on the seepage-stress-damage analysis.

页岩“刚-弹嵌合”孔隙骨架模型及超压状态孔隙度测量方法——以四川盆地南部志留系龙马溪组深层超压硅质页岩为例

基于页岩孔隙特点、孔隙骨架组成并结合超压状态下孔隙、骨架应力分析,建立页岩储集层“刚-弹嵌合”孔隙骨架模型,解析弹塑性骨架超压变形引起孔隙度增大的孔隙变形机制,提出1种“动态”变形孔隙度测量方法,并对四川盆地南部(简称川南)志留系龙马溪组超压状态真实地层条件孔隙度进行测算。结果表明:(1)页岩储集层是无机岩石颗粒与有机质的混合体,构成了刚性骨架支撑的无机质孔隙和弹塑性骨架支撑的有机质孔隙“刚-弹嵌合”特殊孔隙结构,有机质孔、无机质孔混杂共生。(2)在地层超压作用下,无机质孔隙变化相对很小以至可以假设其孔隙度保持不变,而有机质孔隙由于骨架压缩可发生较大形变,导致孔隙半径增大,连通性增强,孔隙度增加。(3)通过高注入压力氦气法孔隙度测量及超高压煤油法变孔隙压力孔隙度测量相结合,建立的“动态”变形孔隙度测量方法既能测量到常压条件不连通的微孔孔隙度,又能测量到有机质骨架压缩变形导致的孔隙度增量。(4)页岩孔隙变形机制会产生地层条件孔隙度高于常压孔隙度的“反常”现象,超压页岩并非真正的“超低孔”储集层,孔隙度甚至可以大于10%。通过川南地区L210井实例测量,验证了该方法的实用性和可靠性。

温度作用后花岗岩微观孔隙结构和渗透率的研究

为了研究温度作用下花岗岩的孔隙结构和渗流特性,对热处理后(25~600℃)的花岗岩试件进行了扫描电镜测试、高压压汞实验和渗流实验,从扫描电镜的镜下特征、毛管压力曲线形态特征、孔容、孔径分布、孔隙度、渗透率等多方面进行了研究,修正了常规的渗透率预测模型,对模型计算渗透率与实测渗透率进行了对比分析。研究结果表明:花岗岩内部含有初始微裂隙和孔洞,孔洞形状不规则;随着温度升高,花岗岩孔喉不断发育,花岗岩压汞曲线中进汞曲线逐渐变得平滑,退汞率逐渐升高,试件中微孔、过渡孔、中孔、大孔的孔容和总孔容总体逐渐增大,孔径分布范围扩大,连通性增强,孔隙度呈现增大趋势;花岗岩渗透率随温度总体呈指数函数增加,400℃之后渗透率大幅增大;试件中大孔数量增加是花岗岩渗透率大幅提升的主要原因;对高温花岗岩而言,利用修正的Winland模型得到渗透率的预测值和实验测试值较接近,表明孔喉半径特征值、孔隙度共同影响下花岗岩的渗透率模型更为合理。研究结果将为高温岩体地热开发工程的方案设计提供一定的理论基础。

基于动态小波指纹的冰孔隙率测量方法研究

针对飞机结冰情况下的冰孔隙率测量问题,提出并评估了一种基于动态小波指纹技术的超声波孔隙率测量方法。通过理论模型和有限元仿真分析了超声纵波的传播过程,阐述了孔隙大小等因素对孔隙率测量的影响机理。结合20块冰样品的60组超声波测量数据,生成了小波指纹图像,并提取了11维关键特征。基于主成分分析和多项式拟合,所实现的孔隙率测量均方根误差(Root mean square error,RMSE)达到1.144%,说明本文方法比传统的峰值拟合方法更稳定、准确。