低渗透突出煤的瓦斯渗流规律研究

 为了解低渗透突出煤体的瓦斯渗流规律,利用自行研制的煤岩体三轴渗透仪,在不同轴压和围压条件下,对以南桐矿区矿井低渗透突出煤层的原煤而制备的试样采用稳态渗流法进行瓦斯渗流试验;比较传统的渗透率计算方法与考虑瓦斯渗流的Klinkenberg效应的渗透率拟合方法在低渗透煤体渗流试验数据处理中的差异。研究结果表明：(1) 低渗透煤体中的瓦斯渗流具有显著的Klinkenberg效应;(2) 对于低渗透煤体,Klinkenberg系数b值与煤体的绝对渗透率呈显著的幂函数关系,而煤体的绝对渗透率与体积应力呈显著的二次多项式函数关系;(3) Klinkenberg系数b值随着煤体绝对渗透率的降低而逐渐增大,煤体的绝对渗透率随着煤体体积应力的增大而逐渐降低;(4) 采用考虑瓦斯渗流的Klinkenberg效应的渗透率拟合方法处理试验数据所得到的结果更为合理;(5) 试验得到的煤体渗透率表达式反映了瓦斯压力和应力对瓦斯渗流的共同作用,能很好地模拟低渗透煤层的瓦斯渗流。     

不同含水饱和度低渗透岩石气体滑脱效应研究

低渗透岩石气体滑脱效应的研究是油气开采与存储领域十分重要的内容,但目前关于低渗透岩石气体滑脱效应的研究大多是在气体单相流下进行的,对于气–液两相流时,液体对气体滑脱效应的影响,所做的研究不足。因此,利用研发的低渗透岩石惰性气体渗透性测试系统,对含水饱和度为0~70%的低渗透砂岩,进行了不同含水饱和度的低渗透岩石气体滑脱效应及有效渗透率变化规律的研究,试验结果表明:(1)二次公式k_g=k_∞(1+b/q-a/p~2)可以较为准确的解释低渗透岩石的气体滑脱效应,准确性明显高于Klinkenberg公式。(2)含水饱和度对低渗透岩石的气体滑脱效应有明显影响,气体滑脱效应随着含水饱和度增大而减少,在含水饱和度超过50%时,气体滑脱效应几乎完全被限制。(3)由于水的作用,含水的低渗透岩石随着围压增大,气体滑脱效应减少,这与克氏理论的结论相反。(4)含水饱和度对低渗透岩石的有效渗透率影响显著,随含水饱和度的增大有效渗透率减少,且围压越大,低渗透岩石的有效渗透率对含水饱和度变化越敏感。(5)低渗透岩石的有效渗透率与含水饱和度符合幂函数关系,即k_∞=k_0(1-S_w)~c。     

考虑Klinkenberg效应的压实膨润土渗气特性研究

利用非饱和三轴渗气仪研究了压实高庙子膨润土的渗气特性。发现当渗气系数小于10~(-14) m~2时,压实膨润土中的Klinkenberg效应较为显著,渗气系数会随气压力的增大而降低;气体滑脱因子随Klinkenberg渗气系数与体积含气率之比的变化满足幂函数关系,试验得到的二者之间关系与干密度、含水率无关,并与Klinkenberg理论模型相近;Klinkenberg渗气系数与体积含气率在双对数坐标中呈线性关系,其斜率与含水率基本无关。基于试验规律,建立了考虑Klinkenberg效应的高庙子膨润土非饱和渗气系数的数学模型。     

In situ rock salt permeability measurement for long term safety assessment of storage

A field test was carried out in the Amelie Mine owned by Mines de Potasse d'Alsace (MDPA, France) for the purpose of measuring rock salt permeability away from underground facilities. The selected salt bed is about 1 m thick and located 16 m away from the gallery floor. The permeability was measured by means of nitrogen and saturated brine injection. Theoretical studies indicated that in order to control the `disturbing effects' (thermal effects, chemical reaction between halite and brine), the work should be done at constant temperature and pressure. The field experiment results confirm that rock salt is permeable to gas and brine, even relatively far from underground openings.The measurements of the tests with brine were interpreted satisfactorily by a model based on Darcy's law, with an intrinsic permeability value of 2×10⁻²¹ m² and an initial pore pressure value of 1 MPa. Interpretation of the measured gas flow rate shows that: (a) after brine percolation, the capillary pressure effect is significant and (b) gas migration in salt is not controlled by Darcy's law; the Knudsen effect and partial saturation may play an important role.     

Research on anisotropic permeability and porosity of columnar jointed rock masses during cyclic loading and unloading based on physical model experiments

In this paper, the permeability and porosity of self-prepared artificial columnar jointed rock masses (CJRM) with different columnar dip angles under different pore pressures during cyclic loading-unloading of confining pressure were measured. The experimental results indicate that the gas permeability of the artificial CJRM gradually decreases with the rise of pore pressure due to the existence of Klinkenberg effect, and Klinkenberg effect gradually decreases with the rise of columnar dip angle. The existence of it improves significantly the intrinsic permeability. Besides, the intrinsic permeability and porosity of artificial CJRM are more sensitive to the first loading than that of the intact cement mortar specimen. The permanent deformation of pores in the specimens mainly appears in the first cycle, and the subsequent cycles have smaller effects on the intrinsic permeability and porosity. The permanent deformation results in the intrinsic permeability, and porosity in the loading process is always higher than those in the unloading process. The intrinsic permeability gradually increases with the growth of columnar dip angle, exhibiting obvious permeability anisotropy. Under high confining pressure, the permeability anisotropy of artificial CJRM is less than that under low confining pressure. Meanwhile, under the same confining pressure, during the repeated loading-unloading cycles, the permeability anisotropy decreases gradually. The existence of a representative element volume (REV) for CJRM is verified; thus, the equivalent continuum media model can be utilized to analyse the seepage characteristics of CJRM.

     

多场耦合作用下页岩渗透特性实验研究

页岩气开采过程,其储层页岩渗透率受多重因素的影响。采用自主研发的多场耦合作用下不同相态CO₂致裂驱替CH₄实验装置,实验研究了有效应力、孔隙压力、温度以及吸附膨胀效应等因素对裂隙页岩体与型岩渗透特性的影响。结果表明:(1)页岩渗透率随有效应力增加呈负指数关系减小,且型岩对于有效应力变化具有更强的敏感性;(2)在相同应力状态下,由于吸附引起的差异性膨胀效应会降低页岩渗透率,不同气体作用下测得的页岩渗透率表现为He＞N₂＞CO₂。(3)两种页岩中渗透率随孔隙压力变化规律具有差异性。SC-CO₂致裂页岩在低压条件下Klingkenberg效应不明显,渗透率随平均孔隙压力的增大而增大,型岩在低压阶段由于Klingkenberg效应对渗透率变化起主导作用,随着孔隙压力增加,其渗透率降低,在达到极小值后,随着孔隙压力的增加,Klingkenberg效应减弱,有效应力起主导作用,渗透率随孔隙压力增加而增加;(4)温度作用通过改变页岩的孔隙结构、力学性质等控制页岩渗透率的变化,随着温度增加,页岩渗透率降低。因此,在页岩气开采过程以及CO₂强化页岩气开采过程需要考虑多因素耦合作用对页岩渗透率的影响。     

Multi-scale modelling of gas flow in nanoscale pore space with fractures

In this work,a multi-scale pore network with fractures is developed against experimental data in a wide range of degrees of water saturation.The pore network is constructed based on the measured microstructure information at several length scales.The gas transport is predicted by different gas transport equations(e.g.Javadpour,dusty gas model(DGM),Civan and Klinkenberg),which can consider the fundamental physics mechanisms in tight porous media,such as Knudsen diffusion and viscous flow.Then,the model is applied to simulating the gas permeability of the Callovo-Oxfordian(COx) claystone.The predicted gas permeability is basically in good agreement with the experimental data under different degrees of water saturation.Then the effects of micro-fissures are studied.The results suggest that this model can predict the gas flow in other tight porous media as well and can be applied to other fields such as carbon capture and storage.     

Analysis of coupled gas flow and deformation process with desorption and Klinkenberg effects in coal seams

Coupled gas flow and solid deformation in porous media has received considerable attention because of its importance in pneumatic test analysis, contaminant transport, and gas outbursts during coal mining. Gas flow in porous media is quite different from liquid flow due to the large gas compressibility and pressure-dependent effective permeability. The dependence of gas pressure and gas desorption on gas permeability has a significant effect on gas flow, but has been ignored in most previous studies. Moreover, solid deformation has a direct impact on the porosity, which also leads to desorption or sorption of methane in the coal seam. In this study, a coupled mathematical model for solid deformation and gas flow is proposed and is implemented using a finite element method. The numerical code is used to solve the gas flow equation with Klinkenberg effect, and is validated by comparison with available analytical solutions. Then, it is used to simulate the coupled process during gas migration in a deformable coal seam. The numerical results indicate that the desorption and Klinkenberg effects and mechanical process effect make a significant contribution to gas flow in the coal seam. Without considering the desorption and Klinkenberg effects and the coupling action of mechanical process, the gas pressure in the coal seam would be underestimated.     

Effective stress law for the permeability of a limestone

The effective stress law for the permeability of a limestone is studied experimentally by performing constant-head permeability tests in a triaxial cell with different conditions of confining pressure σ and pore pressure p_f. Test results show that a pore pressure increase and a confining pressure decrease both result in an increase of the permeability, and that the effect of the pore pressure change on the variation of the permeability is more important than the effect of a change of the confining pressure. A power law is proposed for the variation of the permeability with the effective stress (σ′=σ–n_kp_f). The permeability-effective stress coefficient n_k increases linearly with the differential pressure and is greater than unity as soon as the differential pressure exceeds few bars. The test results are well reproduced using the proposed permeability-effective stress law. A conceptual pore-shell model based on a detailed observation of the microstructure of the studied limestone is proposed. This model is able to explain the experimental observations on the effect of the total stress and of the pore pressure on the permeability of the limestone. Effective stress coefficients for the stress-dependent permeability which are greater than one are obtained. It is shown that the controlling factor is the ratio of the different bulk moduli of the various constituents of the rock. This ratio is studied experimentally by performing microhardness tests.     

一种现场测定地基固结系数与渗透系数的方法

地基固结系数及渗透系数是地基计算及设计中的关键参数。本文利用IFCOBAT系统测试地基原位水平固结系数和渗透系数,采用考虑塑性区内外孔压系数不同的圆柱形孔洞扩张理论和一维径向固结方程模拟滤头周围的初始超静孔隙水压力分布及其消散过程,在渗透系数分析中,同时考虑了针头对流经水的阻力和滤头及容器内水柱的影响。现场测试结果与薄壁土样的室内试验结果的比较分析表明,现场测定的固结系数和渗透系数与室内试验结果接近。另外,还讨论了两种测试结果之间差别的原因及其合理性。     

Determination of relevant parameters influencing gas permeability of mortars

This paper is devoted to the study of the influence of the microstructure of cementitious materials on their gas permeability. For that purpose, cement pastes with four water to cement ratios W/C and mortar specimens with two aggregate volume contents have been cast. On the one hand gas permeability tests with a low pressure device have been carried out on desaturated materials and on the other hand porosity features have been determined by mercury porosimetry measurements. Gas permeability depends upon the mixture parameters (W/C, aggregate volume content, aggregate size) and the moisture content. Correlation between gas permeability and pore size distribution has been discussed from Katz–Thompson equation too. Our results showed that permeability predicted from the threshold radius of the capillary porosity for cement pastes and mortars whatever the aggregate volume content corresponds to permeability measured after drying at relative humidity, RH equal to 90%. For preconditioning with lower RH, the measured permeability is influenced by microcracking generated by drying shrinkage.     

Measuring low permeabilities of gas-sands and shales using a pressure transmission technique

Liquid and gas permeability measurements for tight gas-sand and shales were done using a pressure transmission technique in specially designed apparatus in which confining pressure, pore pressure, and temperature are independently controlled. Downstream pressure changes were measured after increasing and maintaining upstream pressure constant. The initial pressure difference changes only after the pressure pulse propagates across the sample. For low permeability samples, the downstream pressure increase is delayed but the measurement senses a greater sample volume. On the other hand, conventional pulse decay techniques provide a more rapid response but are sensitive to local sample permeability heterogeneity. Permeability measured for the rocks studied varies from 1.18×10⁻¹⁵ to 3.95×10⁻²¹ m². The measured permeability anisotropy ratio in gas shale varies from 20% to 31%. The magnitudes of permeability anisotropy remain almost constant, but the absolute permeability values decrease by a factor of 10 with a 29.79 MPa effective pressure. All samples showed a nonlinear reduction in permeability with increasing effective pressure. The rate of reduction is markedly different from sample to sample and with flow direction. This reduction can be described by a cubic k–σ law and explained by preferential flow through microcracks.     

致密岩石气体渗流滑脱效应试验研究

由于致密结构和低的渗透率,气体在孔隙喉道小的致密岩石中流动会受到滑脱效应的影响。以湖南某试验场地致密砂岩为研究对象,对岩样进行了微观结构的SEM分析,通过一系列围压和孔隙压力作用下的砂岩气体流量和渗透率测试,研究气体在致密岩石中渗流特征,证明了致密砂岩气体流动存在滑脱效应现象,其渗流不符合Darcy定律。分析了孔压对滑脱效应的影响、滑脱效应对气测渗透率的影响以及滑脱因子与绝对渗透率的函数关系。研究结果表明,滑脱效应对气测渗透率的影响随着围压和气体孔隙压力的变化有所不同。同等围压下,孔隙压力越小,滑脱效应越明显,导致气测渗透率大于砂岩绝对渗透率。同等孔压下,当围压达到某一值后,其对滑脱效应的影响有限,同时也说明围压对岩石的压密是有限的。砂岩的平均气体孔隙压力与气测渗透率关系更加符合二次项曲线方程。计算获得的克努森数Kn说明了在相对高的围压和低的孔隙压力条件下,气体渗流过程位于滑脱流和过渡流之间,传统的N-S方程可能不再适用,应用Knudsen扩散方程更加合理,特别是当克努森数Kn比较高时。     

Experimental research on water retention and gas permeability of compacted bentonite/sand mixtures

Highly compacted bentonite-based materials are often considered as buffer or sealing materials for deep high-level radioactive waste repositories. In situ, the initial state of bentonite-based materials is only partially saturated, which has a very high suction that will promote water absorption from the host rock. In addition, a gradient of water saturation will be formed between the external part and the central part of the compacted bentonite blocks. In this paper, water retention tests, under both constant-volume and free-swelling conditions, were performed to investigate the suction behavior of a compacted bentonite/sand mixture. In order to investigate the sealing ability of the partially saturated bentonite/sand mixture, gas permeability tests were also carried out under the in situ confining stress. It was found that the confining conditions have a limited effect on the water retention capacity of the compacted bentonite/sand mixture at lower levels of relative humidity (RH), while this influence is significant at higher RH levels. The results of gas permeability tests show that gas permeability is very sensitive to the water content and the confining pressure. When the sample (stable at RH=98%) was subjected to a in situ confining pressure (7–8 MPa), the gas permeability was very low (1.83×10^–14 m/s) which indicates that gas tightness can be obtained even though the sample is not fully saturated.     

Effects of dry-wet cycles on three-dimensional pore structure and permeability characteristics of granite residual soil using X-ray micro computed tomography

Due to seasonal climate alterations, the microstructure and permeability of granite residual soil are easily affected by multiple dry-wet cycles. The X-ray micro computed tomography (micro-CT) acted as a non-destructive tool for characterizing the microstructure of soil samples exposed to a range of damage levels induced by dry-wet cycles. Subsequently, the variations of pore distribution and permeability due to dry-wet cycling effects were revealed based on three-dimensional (3D) pore distribution analysis and seepage simulations. According to the results, granite residual soils could be separated into four different components, namely, pores, clay, quartz, and hematite, from micro-CT images. The reconstructed 3D pore models dynamically demonstrated the expanding and connecting patterns of pore structures during dry-wet cycles. The values of porosity and connectivity are positively correlated with the number of dry-wet cycles, which were expressed by exponential and linear functions, respectively. The pore volume probability distribution curves of granite residual soil coincide with the χ² distribution curve, which verifies the effectiveness of the assumption of χ² distribution probability. The pore volume distribution curves suggest that the pores in soils were divided into four types based on their volumes, i.e. micropores, mesopores, macropores, and cracks. From a quantitative and visual perspective, considerable small pores are gradually transformed into cracks with a large volume and a high connectivity. Under the action of dry-wet cycles, the number of seepage flow streamlines which contribute to water permeation in seepage simulation increases distinctly, as well as the permeability and hydraulic conductivity. The calculated hydraulic conductivity is comparable with measured ones with an acceptable error margin in general, verifying the accuracy of seepage simulations based on micro-CT results.     

基于位错理论的饱和土渗流特性CPTU评价研究

孔压静力触探（CPTU）测试数据通常用于土层划分和岩土工程设计参数评价。国际上CPTU测试技术的最新进展之一在于采用测得的锥尖阻力、侧壁摩阻力和孔隙水压力快速、连续地获得饱和土的渗透系数,以避免孔压消散试验费时的缺点。首先简要回顾了基于CPTU测试资料确定饱和土渗透系数的的研究成果,并进行了分析与改进,提出了基于位错理论和圆柱面径向流模型的水平向渗透系数预测新方法。选取典型场地进行了CPTU测试,并在黏性土场地采用薄壁取土器获得无扰动的原状试样,进行室内水平向渗透试验,在无黏性土场地进行现场钻孔抽水试验。以室内渗透试验和现场抽水试验的结果为参考,对3种基于CPTU连续评价饱和土渗透系数的方法进行了对比分析。研究表明,位错理论适用于评价饱和土的原位渗透系数,然而准确性与所采用的渗流模型有关。对国际标准CPTU探头,孔压过滤环位于锥肩（u₂）位置时,采用圆柱面的径向流模型能够获得最为准确的水平向渗透系数预测结果,采用球面流或半球面流模型时趋于低估土层的渗透性。     

Study on the permeability evolution law and the micro-mechanism of CCL in a landfill final cover under the dry-wet cycle

This paper aims to investigate the failure mechanism of compacted clay liner (CCL) impervious structures caused by the dry-wet cycles in a landfill final cover. Experimental research is performed on the evolution of permeability characteristics and microstructure of CCL with different initial compactness under repeated dry-wet cycles. The research results show that the effect of dry-wet cycle varies significantly on clays with different compactness. The permeability of the low-compacted clay (90 %) gradually decreases as the times of dry-wet cycles increases, while those of the high-compacted clay (98 %) gradually increases. A non-linear variation relationship between the permeability and dry-wet cycle times with compactness of CCL has been established. The tests and the prediction outcomes of the models both show that, after several dry-wet cycles, the permeability of all compacted clays are greater than 1 × 10^?9 m/s, which can not meet the anti-seepage requirement of the landfill cover systems. The change in pore connectivity caused by the damage of soil structure under the wet-dry cycles is the main factor for the change of the permeability characteristics of CCL. We’ve determined the quantitative indexes (critical pore size) that can present the connectivity characteristic of the pore structure of clay, for which the evolution law under the dry-wet cycles is the same as that of the permeability of the CCL.     

不同渗透压力下盐岩的渗透率测试研究

地下盐腔被广泛应用于核废料地下处置、石油天然气地下储存和二氧化碳封存,由于盐岩的低渗透性,目前很难通过试验获得其渗透率。通过稳态法对3种不同成分盐岩进行渗透特性试验研究,获得盐岩的渗透参数,并对拟压力法和考虑克氏效应的2种渗透率计算方法的结果进行对比研究。结果表明：(1) 盐岩的渗透率与孔隙度极低,孔隙度为0.3%～3.0%,纯盐岩渗透为10－20 m2左右,而含杂质盐岩渗透率更低,为10－20～10－21 m2;(2) 对盐岩的气测渗透率进行测试,进气压力为1～5 MPa时,Klinkenberg 效应影响明显,当渗透力超过5 MPa后,岩体在渗透力作用下会产生损伤,渗透率升高;(3) 通过对比,考虑Klinkenberg效应的方法较拟压力法在盐岩渗透率计算中更为理想。     

Gas relative permeability and pore structure of sandstones

Accurate quantification of gas relative permeability is important in several engineering problems including gas storage and production, drying and wetting processes, oil production etc. In this study, experimental curves of gas relative-permeability for three different kinds of sandstone were defined using a modified version of the pulse-decay method. In order to understand the role played by the porous structure in defining the nonwetting phase permeability, the porous networks of the rocks tested were identified by two complementary methods: mercury porosimetry and sorption techniques. The results obtained show two distinct types of behaviour which are closely dependent on the pore structure. The pores located near the peak of the mercury intrusion curve seem to control the variation in gas relative-permeability. In order to clarify the influence of wetting fluid viscosity on the nonwetting phase relative permeability, each sample was subjected to two series of tests using two different saturating fluids. Our results show that, for a viscosity ratio μ_nw/μ_w1, the gas relative-permeability remains unchanged even if the wetting fluid viscosity is twenty times higher than that of water. A simple empirical relation (the Brooks and Corey equation) was used to correlate the gas relative-permeability with the degree of saturation. The comparison shows that this correlation accurately predicts the nonwetting-phase relative permeability.     

In situ gas permeability measurements to delineate damage in rock salt

In situ gas permeability measurements can be used to distinguish regions within rock salt formations which have experienced disturbance or damage. High-resolution hydrologic testing reveals rock salt well removed from an excavation (i.e. undisturbed) is a low permeability, low porosity, porous medium with a significant pore (brine) pressure. In this region, rock salt is effectively impermeable to gas due to threshold capillary pressures which preclude gas from displacing the resident brine. In a region confined to a depth of less than about one effective radius of an excavation, rock salt is typically permeable to gas. The measured gas permeabilities are consistent with a partially saturated, dilated zone developing adjacent to the excavation. The extent and nature of the damaged region interpreted from gas permeability measurements is consistent with other measurements and analyses. Thus, gas permeability measurements serve as effective means for detecting and delineating the damaged region.