山西沁水盆地中-南部煤储层渗透率物理模拟与数值模拟

通过对山西沁水盆地中南部上主煤层宏观裂隙观测,力学参数测量和应力渗透率实验,分别建立了裂隙面密度、裂隙产状、裂隙宽度与煤储层渗透率之间的预测数学模型;利用FLAC—3D软件,模拟了该区上主煤层内现代地应力状态,结合煤层气试井渗透率资料,构建了应力与渗透率之间关系预测的数学模型,并对该区上主煤层渗透率进行了全面预测。通过吸附膨胀实验,揭示了各煤类煤基质的收缩特征,构建了有效应力、煤基质收缩与渗透率之间的耦合数学模型,并对煤层气开发过程中渗透率动态变化进行了数值模拟。     

高煤级煤基质力学效应与煤储层渗透率耦合关系分析

基于吸附膨胀物理模拟实验，揭示了瘦煤至无烟煤阶段煤基质的自调节特征，分别探讨了煤层气排采过程煤基质收缩和有效应力变化对煤储层渗透率的影响，构建了有效应力、煤基质收缩与煤储层渗透率之间耦合的数学模型，揭示出高煤级煤储层渗透率在煤层气排采过程中的递减变化规律，并讨论了这一发现的地质意义。     

煤样渗透率有效应力与基质收缩双重耦合效应及其与煤阶关系

为研究煤层气在排采过程中不同煤阶煤储层渗透率动态变化规律,利用煤岩三轴应力应变（基质收缩膨胀）测试系统,对褐煤、气煤和无烟煤样开展了有效应力与基质收缩双重效应物理模拟实验。固定轴压和围压不变,改变气体平衡压力,模拟开发过程中储层压力变化特征,测试其动态渗透率。利用实验结果,分析了不同煤阶煤岩在排采过程中动态渗透率反弹特征,并对比分析煤岩动态渗透率改善效果的差异性。研究表明：气体平衡压力从5 MPa降至1 MPa过程中,在有效应力和基质收缩双重效应作用下,褐煤样的归一化渗透率依次为1.00、0.60、0.57、0.57、0.52,气煤样依次为1.00、0. 64、0.50、0.54和0.55,无烟煤样依次为1.00、0.74、0.58、0.50和0.56。随气体平衡压力下降,中阶及高阶煤样动态渗透率先下降后上升,整体呈不对称“V”型变化规律,但拐点略有不同;低阶煤样动态渗透率呈先下降后基本稳定的趋势,整体呈斜“L”型变化规律。在有效应力和基质收缩双重效应影响下,中阶及高阶煤样动态渗透率改善效果优于低阶煤样。     

高煤级煤储层煤层气产能“瓶颈”问题研究

基于山西沁水盆地高煤级煤储层宏观裂隙、显微裂隙的连续观测,孔隙的系统测量,结合应力渗透率、气-水相对渗透率、吸附膨胀等实验成果,分析了高煤级煤储层三级渗流特征,探讨了有效应力和煤基质收缩对高煤级煤储层渗透率的耦合作用,系统揭示了在地面排水降压开发煤层气的过程中,高煤级煤储层初期产气量高,数月后急剧衰减之“瓶颈”现象,找出了造成高煤级煤储层产气缺陷的根本原因。鉴于高煤级煤储层物性的特殊性,指出了高煤级煤储层煤层气开发的技术和措施。     

气体运移导致煤体结构变形演化特征研究——以注入氦气为例

气体运移引起煤体变形是研究煤层气抽采、预防瓦斯突出与温室气体的地质封存的核心问题。一般认为,有效应力变化是控制岩土类材料骨架变形的关键因素。但大量测试结果表明,煤的渗透率与有效应力(或者孔隙压力)表现出非线性关系。为此,应实时观测在静孔隙压力与三轴应力状态下氦气流动导致原煤变形演化全过程。在静孔隙压力状态下煤体积经历从收缩到回弹过程。注气压力越大,煤的收缩与回弹量越大,且收缩量总是大于回弹量。在三轴应力状态下注气初期煤样迅速膨胀。随着注气达到平衡状态,煤变形过程与约束条件表现出紧密相关性,即在应力约束下煤的膨胀率相比注气初期明显减缓;在位移约束下煤由膨胀转向收缩。上述试验结果表明,仅有孔隙压力作用下,煤基质与裂隙之间孔隙压力差可以压缩煤体,随着气体扩散的进行,可恢复煤的部分压缩变形量。在三轴应力状态下,煤的总体变形是裂隙与基质两者变形共同作用的结果。在应力约束下煤基质与裂隙可以自由膨胀。而煤体在位移约束下,因气体扩散导致煤基质膨胀只能挤压裂隙。根据上述实测结果探讨注气导致煤骨架变形演化机制,为深入理解煤裂隙与基质相互作用对煤渗透率演化提供试验依据。     

煤样渗透率有效应力与基质收缩双重耦合效应及其与煤阶关系

为研究煤层气在排采过程中不同煤阶煤储层渗透率动态变化规律,利用煤岩三轴应力应变（基质收缩膨胀）测试系统,对褐煤、气煤和无烟煤样开展了有效应力与基质收缩双重效应物理模拟实验。固定轴压和围压不变,改变气体平衡压力,模拟开发过程中储层压力变化特征,测试其动态渗透率。利用实验结果,分析了不同煤阶煤岩在排采过程中动态渗透率反弹特征,并对比分析煤岩动态渗透率改善效果的差异性。研究表明：气体平衡压力从5 MPa降至1 MPa过程中,在有效应力和基质收缩双重效应作用下,褐煤样的归一化渗透率依次为1. 00、0. 60、0. 57、0. 57、0. 52,气煤样依次为1.00、0. 64、0. 50、0. 54和0. 55,无烟煤样依次为1.00、0. 74、0. 58、0. 50和0. 56。随气体平衡压力下降,中阶及高阶煤样动态渗透率先下降后上升,整体呈不对称“V”型变化规律,但拐点略有不同;低阶煤样动态渗透率呈先下降后基本稳定的趋势,整体呈斜“L”型变化规律。在有效应力和基质收缩双重效应影响下,中阶及高阶煤样动态渗透率改善效果优于低阶煤样。     

CO2地质埋藏深度对高阶煤孔隙结构的影响

高阶煤中的CO₂地质埋藏具有存储CO₂和提高煤层气采收率的双重意义。通过压汞测试和低温液氮吸附实验对经过CO₂地质埋藏模拟实验处理前后的煤样品进行分析测试,探讨了不同埋藏深度下煤中孔隙演化的特征与机理。研究表明:煤的真密度、视密度、孔隙体积、煤基质体积变化、有机质膨胀与收缩等参数均表现出不同的演化特征;埋藏过程中温度压力的增大对H₂O–CO₂–煤的地球化学反应效应的影响并非线性,而是存在一个对孔隙特别是微孔孔容和比表面积改造最大的深度范围,该深度将使得高阶煤孔隙结构得到最佳的改造效果,从而进一步更有利CO₂的地质埋藏和提高煤层气的采收率。      

基于COMET3软件的煤储层数值模拟方法

煤储层数值模拟技术是进行产能预测、地面开发前景评价和生产工艺优选等的重要手段。基于煤储层数值模拟软件的发展历史,阐述了运用第三代专用软件（COMET3）进行煤储层数值模拟的主要步骤,通过实例展示了煤储层排采历史拟合和煤层气井产能预测的效果。结果显示,COMET3软件考虑了三重孔隙结构、双扩散特性、煤基质收缩膨胀效应等煤储层特点,可在较大程度上反演和修正煤储层测试数据,有利于提高煤储层特性分析和煤层气井产能预测的客观性。     

变渗透率条件下煤层气井的产能预测

煤层气储集特征、渗流机理均不同于常规砂岩气藏,导致产能评价难度较大。引入煤层气多组分的基质收缩效应,建立了近井地带紊流效应影响下的二项式产能模型,并进行了实际单井的应用及分析。结果表明:煤储层渗透率受到应力形变和基质收缩效应的双重影响,呈现先下降后上升的特征,且杂质气体含量越高,煤层渗透率恢复程度越弱。实际应用证明采用变渗透率的产能模型可对常规方法无法解释的煤层气井测试数据进行解释,解释结果能用于准确评价煤层气井的产能。      

欠饱和煤储层渗透率动态变化模型及实例分析

开发过程中因受应力压实效应和基质解吸收缩效应的共同影响,导致煤储层渗透率发生复杂的变化。目前,已有诸多学者建立一系列的煤储层渗透率动态模型。然而,对欠饱和煤层气藏开发过程中的不同生产阶段,何种效应对煤储层渗透率起主导作用仍未达成共识。本研究在总结已有的渗透率变化模型的基础上,分析欠饱和煤层气藏开发过程中的降压解吸特征、有效应力效应、基质收缩效应和克林肯伯格效应,并对现有的渗透率模型进行改进与对比分析,以达到定量分析欠饱和煤层气藏储层渗透率变化规律的目的,最后通过鄂尔多斯东南缘韩城煤层气田实例分析煤储层渗透率动态变化特征及其主控因素。结果表明欠饱和煤层气藏开发过程中渗透率动态变化特征可以临界解吸压力划分为两个阶段,前一阶段仅为有效应力效应作用阶段,后一阶段则受有效应力效应、基质收缩效应和克林肯伯格效应影响,且后两种效应随着储层压力的降低而进一步显现。对比分析显示SD改进模型在描述欠饱和煤层气藏渗透率动态变化上优于PM改进模型。因此,借助SD改进模型对韩城煤层气井进行实例计算,分析结果显示煤储层渗透率改善效果依次为3#>11#>5#,区内煤储层渗透率改善效果取决于含气饱和度,而渗透率应力伤害受控于地解压差。     

考虑滑脱效应的低阶煤动态渗透率预测新模型

为准确预测低阶煤动态渗透率变化规律,在煤岩立方体模型基础上,考虑基质孔隙和滑脱效应对渗透率的影响,建立低阶煤动态渗透率预测新模型,并对影响绝对渗透率和滑脱系数的因素进行敏感性分析,讨论了甲烷和氮气对基质收缩与滑脱效应的影响。研究表明：基于“火柴棍”假设建立的模型是新模型不考虑基质孔隙时的一个特例,P-M、S-D模型与新模型相比基质收缩作用更加明显,考虑基质收缩与滑脱效应的新模型更具实用性。气体郎格缪尔应变是影响基质收缩的关键,煤岩绝对渗透率能否反弹是割理压缩、基质孔隙膨胀、基质弹性形变和基质收缩4个因素共同作用的结果。相同条件下,甲烷的基质收缩强于氮气,氮气的滑脱效应强于甲烷,影响滑脱系数的因素包括内因和外因,滑脱系数与割理宽度随孔隙压力变化时呈现相反规律。滑脱效应和基质收缩效应共同提升气测渗透率,煤岩孔隙压力越低,二者对渗透率的提升作用越明显。     

Gas sorption and transport in coals: A poroelastic medium approach

In this paper, single-component gas sorption and transient diffusion processes are described within coal matrix exhibiting bimodal pore structure. The coal matrix is treated as a poroelastic medium manifesting swelling and shrinkage effects due to the sorption of gas under effective overburden stress. Gas transport is considered Fickian with molecular (bulk) and surface diffusion processes simultaneously taking place in the macro- and micropores of coal, respectively. The numerical formulation is intended to be explicit in nature to investigate the influences of sorption phenomena on the macropore volumes and on the overall gas transport for the cases of gas uptake by and release from coal.Results of the study show the presence of hysteresis during a sorption–desorption cycle of the gas. It is also found that the overall gas transport takes place at a rate significantly less than that in the macropores only. Thus the existence of a retardation effect in the overall gas transport is concluded. This retardation effect is primarily due to the micropore resistances, in particular gas adsorption, and is independent of the changes in the macropore volumes. It is shown that macroporosity of the coal matrix may change during gas transport due to combined effects of pressure and sorption-induced swelling or shrinkage of the coal. It is estimated that the macroporosity variation is non-uniform in space and time, as it is expected in reality, and typically taking values less than ± 10 percent of the initial porosity.     

Determination of the Effective Stress Law for Deformation in Coalbed Methane Reservoirs

Effective stress laws and their application are not new, but are often overlooked or applied inappropriately. The complexity of using a proper effective stress law increases when analyzing stress variation in coal as a result of gas production or mining. In this paper, an effective stress law is derived analytically for coalbed methane reservoirs, combining the concepts of matrix shrinkage/swelling and external stress by including the effect of sorbing gas pressure on the elastic response of the reservoir. The proposed law reduces to that of Terzaghi when the compressibility of bulk material is sufficiently greater than the compressibility of the solid grain, and without the strain associated with matrix shrinkage/swelling effect. Moreover, it is shown that the Biot coefficient (α) can have a value larger than unity for self-swelling/dilation materials, such as coal. The proposed stress–strain relationship was validated using experimental results. Overall, the effective stress law for deformation was extended for sorptive materials, providing a new and unique technique to analyze the elastic behavior of coal by reducing three variables, namely, external stress, pore pressure and matrix shrinkage/swelling along with the associated stress, down to one variable, “effective stress”.     

考虑基质收缩效应的煤层气应力场-渗流场耦合作用分析

在煤层气的初级生产过程中,为了获取较高的生产率,需要降低储层压力,储层压力下降对于煤层气的渗透率具有两个相反的效应：（1）储层压力下降,有效应力增加,煤层裂隙压缩闭合,渗透率降低;（2）煤层气解吸,煤基质收缩,煤层气流动路径张开,渗透率升高。Shi和Durucan、Palmer-Mansoori以及Gray等都建立了包含了基质收缩效应以及有效应力的影响的渗透率模型,其模型都基于以下两个关键假设：煤岩体处于单轴应变状态以及竖向应力恒定。为了检验上述两个假设的合理性,建立了一个考虑基质收缩效应以及渗流场-应力场耦合作用下的煤层气流动模型,对煤层气初级生产过程中渗透率的变化进行了耦合分析。分析结果表明：单轴应变的假设具有合理性,而竖向应力是随指向生产井的应变梯度的变化而变化的,其对于渗透率的变化具有重要影响,因此,竖向应力恒定的假设可能导致渗透率预测出现误差;上述渗透率模型都可能低估煤层气初级生产过程中渗透率的变化。     

Shrinkage and swelling of coal induced by desorption and sorption of fluids: Theoretical model and interpretation of a field project

Geologic sequestration in deep unmineable coal seams and enhanced coalbed methane production is a promising choice, economically and environmentally, to reduce anthropogenic gases such as carbon dioxide in the atmosphere. Unmineable coal seams are typically known to adsorb large amounts of carbon dioxide in comparison to the sizeable amounts of sorbed methane, which raises the potential for large scale sequestration projects. During the process of sequestration, carbon dioxide is injected into the coalbed and desorbed methane is produced. The coal matrix is believed to shrink when a gas is desorbed and swell when a gas is sorbed, sometimes causing profound changes in the cleat porosity and permeability of the coal seam. These changes may have significant impact on the reservoir performance. Therefore, it is necessary to understand the combined influence of swelling and shrinkage, and geomechanical properties including elastic modulus, cleat porosity, and permeability of the reservoir.The present paper deals with the influence of swelling and shrinkage on the reservoir performance, and the geomechanical response of the reservoir system during the process of geologic sequestration of carbon dioxide and enhanced coalbed methane production in an actual field project located in northern New Mexico. A three-dimensional swelling and shrinkage model was developed and implemented into an existing reservoir model to understand the influence of geomechanical parameters, as well as swelling and shrinkage properties, on the reservoir performance. Numerical results obtained from the modified simulator were compared to available measured values from that site and previous studies. Results show that swelling and shrinkage, and the combination of geomechanical and operational parameters, have a significant influence on the performance of the reservoir system.     

考虑应力敏感性的煤层气井排采特征

与裂缝性砂岩气藏相比,煤层气藏是一种具有阶段性应力敏感特征的特殊裂缝性气藏。在煤层气排采初期, 有效水平应力起主导作用, 随着有效应力的增大,渗透率逐渐减小;当割理内部流体压力降低到解吸压力之后, 由于基质收缩,渗透率可得到一定程度恢复。所以,提高煤层气排采效果的重要举措,是尽可能提高煤层气压降-解吸的面积。在煤层气开采初期,不合理的高排采速率将引起近井地带渗透率降低,影响压降漏斗的传播,造成增排不增产的后果。通过岩心应力敏感实验,得到了岩心渗透率随有效应力的变化规律。以煤层气开采井为例,利用ECLIPSE E300三维双重孔隙介质多组份模拟器,证明了初期排采量并非越大越好,而是存在一个合理值。该结论可用于指导煤层气井的开采。      

Modelling of anisotropic coal swelling and its impact on permeability behaviour for primary and enhanced coalbed methane recovery

Coal swelling/shrinkage during gas adsorption/desorption is a well-known phenomenon. For some coals the swelling/shrinkage shows strong anisotropy, with more swelling in the direction perpendicular to the bedding than that parallel to the bedding. Experimental measurements performed in this work on an Australian coal found strong anisotropic swelling behaviour in gases including nitrogen, methane and carbon dioxide, with swelling in the direction perpendicular to the bedding almost double that parallel to the bedding. It is proposed here that this anisotropy is caused by anisotropy in the coal's mechanical properties and matrix structure. The Pan and Connell coal swelling model, which applies an energy balance approach where the surface energy change caused by adsorption is equal to the elastic energy change of the coal solid, is further developed to describe the anisotropic swelling behaviour incorporating coal property and structure anisotropy. The developed anisotropic swelling model is able to accurately describe the experimental data mentioned above, with one set of parameters to describe the coal's properties and matrix structure and three gas adsorption isotherms. This developed model is also applied to describe anisotropic swelling measurements from the literature where the model was found to provide excellent agreement with the measurement. The anisotropic coal swelling model is also applied to an anisotropic permeability model to describe permeability behaviour for primary and enhanced coalbed methane recovery. It was found that the permeability calculation applying anisotropic coal swelling differs significantly to the permeability calculated using isotropic volumetric coal swelling strain. This demonstrates that for coals with strong anisotropic swelling, anisotropic swelling and permeability models should be applied to more accurately describe coal permeability behaviour for both primary and enhanced coalbed methane recovery processes.     

Influence of gas production induced volumetric strain on permeability of coal

Summary The gas permeability of a coalbed, unlike that of conventional gas reservoirs, is influenced during gas production not only by the simultaneous changes in effective stress and gas slippage, but also by the volumetric strain of the coal matrix that is associated with gas desorption. A technique for conducting laboratory experiments to separate these effects and estimate their individual contribution is presented in this paper. The results show that for a pressure decrease from 6.2 to 0.7 MPa, the total permeability of the coal sample increased by more than 17 times. A factor of 12 is due to the volumetric strain effect, and a factor of 5 due to the gas slippage effect. Changes in permeability and porosity with gas depletion were also estimated using the measured volumetric strain and the matchstick reservoir model geometry for flow of gas in coalbeds. The resulting variations were compared with results obtained experimentally. Furthermore, the results show that when gas pressure is above 1.7 MPa, the effect of volumetric strain due to matrix shrinkage dominates. As gas pressure falls below 1.7 MPa, both the gas slippage and matrix shrinkage effects play important roles in influencing the permeability. Finally, the change in permeability associated with matrix shrinkage was found to be linearly proportional to the volumetric strain. Since volumetric strain is linearly proportional to the amount of gas desorbed, the change in permeability is a linear function of the amount of desorbing gas.