Summary of Numerical Models for Predicting Productivity of Shale Gas Horizontal Wells

Shale gas production has gradually achieved high and stable output, which makes it possible to make up for the shortage of oil and gas energy as an alternative energy source. Shale reservoir is compact, with well-developed nano-pore, and has the characteristics of adsorption and desorption, diffusion and slippage. At the same time, there are a large number of natural cracks, bedding and foliation. Hydraulic fractures expand irregularly after volume fracturing in horizontal wells. The whole system has multi-field coupling and cross-scale flow effects. Productivity prediction of shale gas is difficult and uncertain, which restricts the efficient development and evaluation of shale reservoirs. In this paper, the development status of productivity numerical models for shale gas horizontal wells is reviewed in consideration of the multi-scale transport characteristics of shale gas. These models include dual media capacity models, multiple media capacity models, and complex seam productivity models. It is considered that the dual medium and multi-media productivity models weaken the large permeable flow area and channel provided by the complex seam network system after shale reservoir lamination, and cannot comprehensively characterize the full-scale coupled transport characteristics of shale gas. The numerical model for productivity prediction of shale gas horizontal wells based on complex fracture network provides a multi-scale flow embedded fracture network system, which solves the problem of systematic flow without losing the ability to accurately characterize each scale flow. It is necessary to obtain the complex fracture network morphological characterization which conforms to reservoir geological characteristics, rock mechanical behavior and fluid-solid coupling mechanism. Fracture network characterization is the key to the productivity prediction of shale gas horizontal wells.     

离散裂隙网络对页岩气井产能影响的数值模拟

页岩作为典型的非常规储层,基质孔隙小,渗透率极低,水平井多级水力压裂为其商业开发的主要手段。准确模拟页岩气产能,应同时考虑水力裂隙和天然裂隙的渗流。基于离散裂隙模型和等效连续模型建立页岩气渗流数学模型,利用有限元分析方法进行数值求解,研究不同走向裂隙组对页岩气井产能的影响。研究认为,页岩基质为气体的生产提供了主要气源,天然裂隙作为渗流的主要通道,将气体输送到水力裂缝,进而到达井筒。模拟结果表征,离散裂隙的渗流特征对于页岩气井的产能有重要影响。根据页岩储层的天然裂隙走向,可以优化相应的水平井方位。对于二维离散裂隙网络模型,水平井沿着2个裂隙组夹角的平分线更有利于生产。      

An analytical model for multiple fractured shale gas wells considering fracture networks and dynamic gas properties

Multi-stage fractured horizontal wells play an important role in developing shale gas reservoirs by significantly improving productivity. By considering fracture networks, gas desorption, stress-sensitive fracture permeability, and pressure-dependent gas PVT properties, an analytical model is developed for shale gas wells. Fracture networks are handled based on transient linear flow, gas desorption is handled by defining a new total compressibility, stress-dependent hydraulic fracture permeability is handled by variable substitution, and pseudo-pressure and pseudo-time are used to handle pressure-dependent PVT properties. After obtaining the solution of the linearized model, a material balance method and successive substitution iteration procedure are proposed to convert the pseudo-time into real time and calculate the production contribution from gas desorption. The results show that induced fractures also have a great impact on the production of the well. Production contribution from free gas and adsorbed gas could be quantified using the proposed material balance principle and iterative method. The rank of parameters that influence the ultimate recovery is the following: half-length of hydraulic fracture, induced fracture length/hydraulic fracture spacing, hydraulic fracture spacing, conductivity of induced fractures, conductivity of hydraulic fracture, and induced fracture spacing.     

页岩水平井多段分簇压裂起裂压力数值模拟

页岩储层孔隙度和渗透率极低,天然裂缝和水平层理发育,常规压裂增产措施无法满足页岩气的开发要求,水平井多段分簇压裂是页岩气开发的关键技术之一,该技术能够大幅度提升压裂改造的体积、产气量和最终采收率。为确定页岩储层水平井多段分簇射孔压裂的起裂点和起裂压力,采用有限元方法建立了水平井套管完井（考虑水泥环和套管的存在）多段分簇射孔的全三维起裂模型。数值模型的起裂压力与室内试验结果吻合较好,证明了数值模型的准确性和可靠性。利用数值模型研究了页岩水平井多段分簇射孔压裂的起裂点和起裂压力的影响因素,研究发现：射孔孔眼附近无天然裂缝或水平层理影响,起裂点发生在射孔簇孔眼的根部;射孔簇间距越小,中间射孔簇的干扰越大,可能造成中间的射孔簇无法起裂;射孔密度和孔眼长度增大,起裂压力降低;天然裂缝的存在,在某些情况能够降低起裂压力且改变起裂位置,主要与天然裂缝的分布方位及水平主应力差有关;水平层理可能会降低起裂压力,但与垂向主应力与水平最小主应力的差值有关。获得的起裂压力变化规律,可作为进一步研究水平井多段分簇射孔条件下的裂缝扩展规律的基础,可以为压裂设计和施工的射孔参数确定及优化给出具体建议。     

页岩水平井体积压裂设计的一种新方法

页岩水平井常采用体积压裂技术获得产能,压裂形成的缝网体积、渗透率是影响压裂效果的关键因素。目前页岩体积压裂设计借用产能预测模型优化缝网参数,此模型较复杂,不便于现场应用。根据等效渗流原理,将页岩储层压裂后形成的缝网系统等效为一个高渗透带,建立了体积压裂缝网参数与施工规模关系模型,提出了体积压裂设计的3个步骤:体积压裂可行性研究、数值模拟优化缝网参数和施工参数优化。根据QY2页岩油水平井特征,进行了体积压裂设计和现场实施。结果表明:压裂形成的高渗透带对产能的贡献最大;高渗透带数量、体积和渗透率增加,压裂后的累积产量和采出程度逐渐增加,存在最优的高渗透带参数。现场应用表明这种设计方法方便实用,可以推广。     

A shear‐dilation‐based model for evaluation of hydraulically stimulated naturally fractured reservoirs

The role of shear dilation as a mechanism of enhancing fluid flow permeability in naturally fractured reservoirs was mainly recognized in the context of hot dry rock (HDR) geothermal reservoir stimulation. Simplified models based on shear slippage only were developed and their applications to evaluate HDR geothermal reservoir stimulation were reported. Research attention is recently focused to adjust this stimulation mechanism for naturally fractured oil and gas reservoirs which reserve vast resources worldwide. This paper develops the overall framework and basic formulations of this stimulation model for oil and gas reservoirs. Major computational modules include: natural fracture simulation, response analysis of stimulated fractures, average permeability estimation for the stimulated reservoir and prediction of an average flow direction. Natural fractures are simulated stochastically by implementing ‘fractal dimension’ concept. Natural fracture propagation and shear displacements are formulated by following computationally efficient approximate approaches interrelating in situ stresses, natural fracture parameters and stimulation pressure developed by fluid injection inside fractures. The average permeability of the stimulated reservoir is formulated as a function of discretized gridblock permeabilities by applying cubic law of fluid flow. The average reservoir elongation, or the flow direction, is expressed as a function of reservoir aspect ratio induced by directional permeability contributions. The natural fracture simulation module is verified by comparing its results with observed microseismic clouds in actual naturally fractured reservoirs. Permeability enhancement and reservoir growth are characterized with respect to stimulation pressure, in situ stresses and natural fracture density applying the model to two example reservoirs. Copyright © 2002 John Wiley & Sons, Ltd.     

页岩气开采的相关实验、模型和环境效应

页岩气是一种重要的非常规天然气资源,正在改变世界能源、经济和政治格局。渗透率是评价页岩气藏商业开采可行性的重要参数之一,由于页岩的致密性,页岩气的流动机理不同于常规气藏,因此,页岩储层渗透率测试和页岩气流动模型已成为当前国际研究的热点课题之一。在对页岩气开采技术简单介绍的基础上,综述了页岩储层渗透率测试的试验和理论研究现状,分析了气体吸附对页岩渗透率的影响。阐述了页岩气流动模型的最新进展,分析了双重孔隙模型描述气体迁移的准确性,提出了描述均匀储层中页岩气解吸-扩散-渗流多级运移模型。评述了页岩气开采的温室效应和对地下水的影响,并简单介绍了适合页岩气开采的新技术即无水压裂开采技术,即采用CO2对页岩气藏分段压裂,同时将CO2埋存于废弃井中。最后,对页岩储层渗透率测试和页岩气流动模型研究的新发展以及无水压裂技术进行了展望。     

裂缝参数对考虑滑脱效应页岩气水平井产能的影响

低渗透页岩气藏中,气体渗流时会受滑脱效应的影响。建立了考虑滑脱效应的气、水两相页岩气藏渗流数学模型,并建立了理想地质模型,采用数值模拟方法,研究了水力压裂的不同裂缝参数对水平井产能的影响。模拟结果表明:裂缝条数、长度和间距是影响页岩气井产能的重要参数,而裂缝宽度和渗透率对产能的影响相对较弱;页岩气井的产能随着裂缝条数和裂缝长度的增加而增大;水平井的水平段长度及裂缝条数一定时,可通过增大裂缝间距来减少裂缝间的相互干扰。      

页岩气藏体积压裂有效改造体积计算方法

页岩气藏矿场压裂实践表明,储层有效改造体积（effective stimulated reservoir volume,简称ESRV）是影响页岩气藏体积压裂水平井生产效果的关键因素,ESRV的准确计算对页岩气藏压裂方案评价与体积压裂水平井产量预测具有重要作用.基于页岩储层改造体积（stimulated reservoir volume,简称SRV）多尺度介质气体运移机制,建立了SRV区域正交离散裂缝耦合双重介质基质团块来表征单元体渗流模型（representation elementary volume,简称REV）,并结合北美页岩储层实例研究了次生裂缝间距、宽度等缝网参数对页岩气藏气体运移规律的影响.在此基础上根据SRV区域次生裂缝分布特征,采用分形质量维数定量表征裂缝间距分布规律,结合页岩气藏次生裂缝间距对基质团块内流体动用程度的影响规律,得到了页岩气藏体积压裂ESRV计算方法.结果表明SRV区域次生裂缝间距对基质团块内吸附及自由气影响较大,次生裂缝间距小于0.20 m时可以实现SRV区域基质团块内流体向各方向裂缝的"最短距离"渗流.选取北美典型页岩储层生产井体积压裂数据进行ESRV计算,页岩气藏目标井ESRV占体积压裂SRV的37.78%.因此ESRV受改造区域次裂缝分布规律及SRV有效裂缝间距界限的影响,是储层固有性质及人工压裂因素综合作用的结果.      

微支撑剂对页岩油气的增产机理及选配原则

水力压裂是页岩油气增产的主要手段,其核心是向储层连续泵入压裂液迫使储层产生水力裂缝/缝网形成油气运移高效通道,从而达到油气增产的目的。相较人工主缝,微裂缝具有数量大、缝宽小、走向复杂等特点,常规支撑剂无法对其形成有效支撑,如何提高微裂缝的导流能力是进一步提高页岩油气产能的关键。微缝导流能力实验表明:微支撑剂嵌入页岩裂缝表面导致一定程度的渗透率损害,但支撑微裂缝的渗透率仍比页岩基质渗透率高出2～3个数量级,仍可为页岩油气运移提供有效渗流通道。本文系统分析了微支撑剂对微裂缝的支撑特性、微支撑剂的长距离输运性能及辅助降滤等特征,提出了微支撑剂对致密油气增产的内在机制。结合我国致密油气储层基质孔隙尺度,形成了考虑微裂缝开度及基质孔隙尺寸的微支撑剂选配原则。     

Beyond dual-porosity modeling for the simulation of complex flow mechanisms in shale reservoirs

The state of the art of modeling fluid flow in shale reservoirs is dominated by dual-porosity models which divide the reservoirs into matrix blocks that significantly contribute to fluid storage and fracture networks which principally control flow capacity. However, recent extensive microscopic studies reveal that there exist massive micro- and nano-pore systems in shale matrices. Because of this, the actual flow mechanisms in shale reservoirs are considerably more complex than can be simulated by the conventional dual-porosity models and Darcy’s law. Therefore, a model capturing multiple pore scales and flow can provide a better understanding of the complex flow mechanisms occurring in these reservoirs. This paper presents a micro-scale multiple-porosity model for fluid flow in shale reservoirs by capturing the dynamics occurring in three porosity systems: inorganic matter, organic matter (mainly kerogen), and natural fractures. Inorganic and organic portions of shale matrix are treated as sub-blocks with different attributes, such as wettability and pore structures. In kerogen, gas desorption and diffusion are the dominant physics. Since the flow regimes are sensitive to pore size, the effects of nano-pores and micro-pores in kerogen are incorporated into the simulator. The multiple-porosity model is built upon a unique tool for simulating general multiple-porosity systems in which several porosity systems may be tied to each other through arbitrary connectivities. This new model allows us to better understand complex flow mechanisms and eventually is extended into the reservoir scale through upscaling techniques. Sensitivity studies on the contributions of the different flow mechanisms and kerogen properties give some insight as to their importance. Results also include a comparison of the conventional dual-porosity treatment and show that significant differences in fluid distributions and dynamics are obtained with the improved multiple-porosity simulation.     

新场气田须二气藏天然裂缝有效性定量表征方法及应用

天然裂缝是地层中广泛分布的一种地质构造现象,当其在油气开发过程中保持一定有效性时具有重要作用,其有效程度高低是裂缝性油气藏高产富集的关键.本次研究以川西新场气田须二气藏裂缝特征及成因认识为基础,利用气藏各类动静态资料对裂缝张开度、裂缝渗透率、裂缝孔隙度等参数进行解释和评价,明确了不同资料计算获取裂缝参数的物理含义及相互之间的关系,为裂缝有效性评价奠定了基础.文中以井筒附近、地质模型网格单元体内裂缝网络系统作为裂缝有效性定量表征对象,通过裂缝网络系统裂缝参数的分布特征,选取并组合了参数分布的特征变量从而建立了裂缝有效性定量表征指标;基于裂缝有效性定量表征方法和建立的定量表征指标对新场气田须二气藏单井产层段裂缝的有效性及气藏裂缝有效性的纵横变化规律进行了研究和评价,其评价结果与区域构造、应力场分布、井下监测、生产动态具有很好的一致性.本文对油气藏中天然裂缝有效性的认识和定量表征方法为裂缝性油气藏地质建模中裂缝有效参数场的建立和数值模拟工作奠定了基础,为裂缝性油气藏的描述和生产动态研究提供了方向和思路.     

川南地区龙马溪组页岩储层裂缝特征

我国页岩气资源储量巨大,四川盆地更是我国主要的页岩气产区。在页岩气勘探开发中一个重要指标便是裂缝,它承载着气体储存和运输通道两大功能。因此了解储层裂缝的微观性质对于判断储层含气量以及开采远景具有重要意义。本文以页岩气主要产区——川南地区页岩气气田泸201井的岩芯为主要研究对象,以手持显微镜、体式显微镜为主要观察工具,对岩芯进行了细观尺度下的观察,借助于图像软件对岩芯表面进行了图像重构,对储层裂缝的类别及特征、发育特征及矿物特征等进行了统计分析,并成功构建出研究区页岩岩芯二维裂缝模型。通过宏观到微观角度观察研究发现:(1)龙马溪组龙一1亚段1、2小层与五峰组页岩储层裂缝多为裂缝长度/岩芯周长小于25%的短裂缝,平均占比79.8%。(2)龙一1亚段1小层岩芯裂缝发育,其中以平行于层理的裂缝为主,龙一1亚段2小层天然微裂缝数量略低于1小层,而五峰组岩芯仅发现极少量微观可见的天然微裂缝。(3)岩芯表面观测到的主要矿物为黄铁矿、方解石。本文的主要创新点在于通过直观的方法对川南龙马溪组储层页岩的裂缝特征进行了统计分析,对于后续页岩气开采以及储层气体流动分析具有十分重要的意义。     

高含硫气藏非稳态水平井产量预测模型研究

元素硫从含硫天然气中析出沉降会影响流体流动,常规的水平井产量预测模型无法用于高含硫气藏水平井非稳态产量预测。本文基于非稳态椭圆流理论,耦合基质内的椭圆流动和裂缝内的非达西流动,考虑元素硫沉积及缝间干扰效应的影响,建立了适用于酸化水平井与酸压水平井的非稳态产量预测模型。分析了硫沉积对渗透率、孔隙度的影响,计算了不同裂缝参数对产能的影响,该模型适用于变井底流压生产条件。计算结果表明：科学准确的元素硫溶解度模型能较好地预测高含硫气藏水平井产量。酸压改造后缝间干扰现象会降低气井产能,且随着时间逐渐增强。硫沉积会降低储层渗透率和孔隙度,使得产量下降。增大裂缝导流能力,增加裂缝半长或者裂缝数量都能有效提高水平井产量,与裂缝导流能力相比,裂缝半长和裂缝数量对产量的影响更大。     

Coupled Numerical Evaluations of the Geomechanical Interactions Between a Hydraulic Fracture Stimulation and a Natural Fracture System in Shale Formations

Due to the low permeability of many shale reservoirs, multi-stage hydraulic fracturing in horizontal wells is used to increase the productive, stimulated reservoir volume. However, each created hydraulic fracture alters the stress field around it, and subsequent fractures are affected by the stress field from previous fractures. The results of a numerical evaluation of the effect of stress field changes (stress shadowing), as a function of natural fracture and geomechanical properties, are presented, including a detailed evaluation of natural fracture shear failure (and, by analogy, the generated microseismicity) due to a created hydraulic fracture. The numerical simulations were performed using continuum and discrete element modeling approaches in both mechanical-only and fully coupled, hydro-mechanical modes. The results show the critical impacts that the stress field changes from a created hydraulic fracture have on the shear of the natural fracture system, which in-turn, significantly affects the success of the hydraulic fracture stimulation. Furthermore, the results provide important insight into: the role of completion design (stage spacing) and operational parameters (rate, viscosity, etc.) on the possibility of enhancing the stimulation of the natural fracture network (‘complexity’); the mechanisms that generate the microseismicity that occurs during a hydraulic fracture stimulation; and the interpretation of the generated microseismicity in relation to the volume of stimulated reservoir formation.     

新场气田分段压裂水平井裂缝布局优化

新场气田属于大型多层致密砂岩异常高压气藏,储层物性差,非均质性强,气井动用储量低。目前,为大幅度提高气井产能,提高储量动用长度,该气藏多采用多段压裂水平井开发。因此,迫切需要论证裂缝参数及其组合对多段压裂水平井开发效果的影响,为气藏下步科学高效开发和持续高产稳产提供理论基础。以川西新场气田为研究对象,采用数值模拟法深入研究了压裂水平井裂缝几何布局对气井产能的影响,包括非均匀裂缝长度、非均匀裂缝间距、压裂规模与裂缝数量、裂缝长度与间距的匹配、裂缝夹角与间距的匹配。结果表明:对于多段压裂水平井,U型模式的裂缝长度布局最优;均匀裂缝间距开发效果优于非均匀裂缝间距;水力压裂时,少段数长缝能取得更佳的开发效果;0. 67~1倍缝长的裂缝间距布局、垂直于井筒的正交裂缝布局有利于改善压裂水平井开发效果,裂缝间距的增大能有效降低非正交裂缝低夹角对产量的影响。     

Numerical Modeling of Natural Fracture Distributions in Shale

The production efficiency of shale gas is affected by the interaction between hydraulic and natural fractures. This study presents a simulation of natural fractures in shale reservoirs, based on a discrete fracture network (DFN) method for hydraulic fracturing engineering. Fracture properties of the model are calculated from core fracture data, according to statistical mathematical analysis. The calculation results make full use of the quantitative information of core fracture orientation, density, opening and length, which constitute the direct and extensive data of mining engineering. The reliability and applicability of the model are analyzed with regard to model size and density, a calculation method for dominant size and density being proposed. Then, finite element analysis is applied to a hydraulic fracturing numerical simulation of a shale fractured reservoir in southeastern Chongqing. The hydraulic pressure distribution, fracture propagation, acoustic emission information and in situ stress changes during fracturing are analyzed. The results show the application of fracture statistics in fracture modeling and the influence of fracture distribution on hydraulic fracturing engineering. The present analysis may provide a reference for shale gas exploitation.     

低渗透油藏水平井注CO2井网优化与影响因素

针对低渗透油藏直井井网CO₂驱油的不足,以大庆某低渗区块为例,建立反七点水平井井网CO₂驱油地质模型。在物性参数相态拟合的基础上,对反七点水平井和五点直井两种不同井网模式的开发效果进行了对比,对反七点水平井井网参数进行了优化,同时对储层非均质(渗透率、储层非均质性、裂缝的发育情况等)进行了影响因素分析。研究表明：利用水平井注CO₂可以适当地增大井距、排距,减少井的数量,降低钻井成本;在生产制度相同的情况下水平井井网的开发效果要比直井井网的开发效果好;在水平井井网中,采出程度随着油井到水平井的垂直距离的比值变化而变化,当比值为1.4时采出程度最大;水平井注CO₂更适合低渗透正韵律储层;在储层裂缝发育的情况下,裂缝条数越多,CO₂在储层中的不均匀推进现象越明显,最终采出程度越低。     

基于嵌入离散裂缝的页岩气藏视渗透率模型

页岩储层具有不同类型的储集空间,但综合考虑不同储集空间,对页岩储层渗透率进行评价的模型未见报道.基于嵌入离散裂缝模型,建立的页岩气藏视渗透率模型包括4个步骤:（1）构建天然裂缝、有机质和无机质的空间分布模型;（2）筛选不同类型储集空间的渗透率计算方法;（3）基于嵌入离散裂缝模型,结合空间分布模型和渗透率计算方法,建立数值模拟模型;（4）在模型的入口和出口端施加压差,求得一定压差下通过该岩心的气体流量,采用达西定律得到该页岩气藏的视渗透率.其计算结果与文献报道的渗透率实验值吻合较好.通过对不同因素的探讨,结果表明,天然裂缝对页岩气藏视渗透率的贡献大于无机质和有机质孔隙.因此,计算页岩视渗透率时有必要对天然裂缝、有机质和无机质孔隙进行综合考虑.      

页岩储层体积压裂裂缝扩展机制研究

页岩储层天然裂缝、水平层理发育,水力压裂过程中可能形成复杂的体积裂缝。针对页岩储层体积裂缝扩展问题,基于流-固耦合基本方程和损伤力学原理,建立了页岩储层水力压裂体积裂缝扩展的三维有限元模型。将数值模型的模拟结果与页岩储层裂缝扩展室内试验结果进行对比,二者吻合较好,从而证明了数值模型的可靠性。通过一系列数值模拟发现：（1）水力压裂过程中水平层理可能张开,形成水平缝,水平与垂直缝相互交错,形成复杂的体积裂缝网络;（2）水平主应力差增大,体积裂缝的分布长度（水平最大主应力方向压裂裂缝的展布距离）增加、分布宽度（水平最小主应力方向压裂裂缝的展布距离）减小,体积裂缝的长宽比增加;（3）压裂施工排量增大,体积裂缝的分布长度减小、宽度增加,压裂裂缝的长宽比降低;（4）天然裂缝的残余抗张强度增大,体积裂缝分布宽度减小、分布长度增加,体积裂缝的长宽比增加。研究成果可以为国内的页岩气的压裂设计和施工提供一定的参考和借鉴。