基于竞争吸附的页岩气藏提高采收率机理

为研究注CO₂提高页岩气藏采收率的机理以及确定合理的注入参数,以页岩吸附解吸实验为基础,对比了页岩自然降压开采和在不同注入压力、注入速率下注CO₂开采页岩气藏的实验结果,研究了降压开采和注CO₂开采2种方式下CH₄的采气速率和采收率的变化。结果表明,在相同实验条件下,CO₂吸附能力最强,CH₄次之,N₂最弱,N₂解吸能力最强,CH₄次之,CO₂最弱;3种气体的等温解吸曲线相对于等温吸附曲线均存在一定的滞后,CO₂的滞后现象最为明显;降压开采的采收率较低,注CO₂可以有效提高CH₄的采气速率,延长稳产时间,使累计产气量快速增长;在一定注入压力和注入速率范围内,CH₄采收率随着CO₂注入压力和注入速率的增加而增加,但其增加幅度因岩心孔隙度和渗透率的不同而有所不同。     

陆相页岩气的储集空间特征及赋存过程——以鄂尔多斯盆地陕北斜坡构造带延长探区延长组长7段为例

页岩的储集空间不但影响页岩气的储量,还影响页岩气井的产能。研究页岩气的赋存空间及赋存过程有助于确定勘探靶位。综合利用野外露头、岩心观察、薄片分析及扫描电镜等多种手段,研究了鄂尔多斯盆地陕北斜坡构造带延长探区延长组长7段页岩的储集空间类型及其特征。在此基础上,通过解吸模拟实验进行页岩气气体特征分析,最终重建页岩气的赋存过程。结果表明,延长探区长7段页岩发育原生粒间孔、次生溶蚀孔、有机质生烃孔、构造张裂缝及层间页理缝等多种孔、缝类型。在解吸过程中,分子直径较小的甲烷气最容易解吸,含¹³C的甲烷分子则相对解吸困难。在生气初期,长7段页岩生成的少量重烃气主要吸附于有机质表面及微孔中;在生气期,页岩优先吸附重烃气和具¹³C的甲烷气;当满足页岩的吸附和溶解等残留需要后,气体以游离态赋存。     

新型促页岩气解吸附体系研究

页岩气的赋存方式以吸附态为主,若提高页岩气井的产量,则须使吸附态的页岩气从页岩内表面解吸附变为游离态。升高温度和注入ＣＯ２ 均可提高页岩气的解吸率,为尽可能提高页岩层内吸附气的采收率,首次将２ 种促解吸方法结合在一起,开发出自生热自生ＣＯ２ 体系,并对该体系进行了反应物加量优化、腐蚀性和促解吸效果的评价。结果表明,该体系能够产生大量的热和ＣＯ２ 气体,二者对于提高页岩气解吸率都有良好的效果。     

页岩储层微观孔隙、流体测定方法及吸附—解吸实验 ——以鄂尔多斯榆林地区盒8段储层为例

微观孔隙结构是影响页岩储层吸附和解吸能力的重要因素。基于核磁共振测试原理,采用离心实验与热处理相结合方法,测定了页岩微观孔隙结构特征,定量识别了多种类型孔隙流体,并从微观孔隙尺度研究了吸附解吸过程中多态甲烷赋存量与压力的关系。结果表明：页岩中不可采出流体、可动流体的T₂截止值分别为0.18～0.32 ms、0.67～1.07 ms,不可采出流体和可动流体的饱和度分别为7.39%～15.61%、36.25%～56.86%;吸附态甲烷赋存于驰豫时间小于3.42 ms的孔隙,游离态甲烷赋存于驰豫时间为3.42～121.44 ms的孔隙;吸附过程中甲烷首先赋存于基质和小孔,其次逐步进入大、中孔隙,而解吸过程中则是先由裂缝、大孔隙到中、小孔隙,再到微孔隙;吸附态甲烷赋存量与压力呈非线性关系,当压力小于7.25 MPa时出现解吸滞后现象,且滞后程度随压力的降低而增大,游离态甲烷临界解吸滞后压力为1.17 MPa。研究成果可为鄂尔多斯盆地页岩气的高效开发提供技术支持。     

页岩气解吸规律新认识

自主研发的无压阻微压损型全自动含气量测试仪,不仅可以连续测试页岩气解吸气量,还具备检测甲烷质量分数的功能。依托于该仪器,对渝东南地区页岩气井进行现场含气量测试,首次实现对页岩气解吸中气体组分的连续检测,发现页岩气解吸具有独特规律。页岩气解吸遵循"三段式"变化模式,解吸初期(前3 h),甲烷质量分数快速增加,解吸中期(3～10 h)甲烷质量分数基本稳定不变,解吸后期(10 h至解吸终止)甲烷质量分数跳跃式下降至解吸结束。页岩气解吸是从外到内,从裂缝、大孔到微裂缝、微孔,最后到页岩基质块体,层层递进、逐级深入的过程。解吸初期,页岩释放的气体多来自裂缝或连通较好的大孔,游离气占据主要部分;解吸中期,微裂缝或大孔中呈吸附状态的气体及具一定连通性的中孔和部分微孔中气体开始释放,以游离气和吸附气为主;解吸后期,连通性较差及微孔中呈吸附或束缚状态的气体开始释放,吸附气占据主要部分。     

中国石化无锡石油地质研究所实验地质技术之页岩高压等温吸附技术

正页岩气主要以吸附态和游离态2种赋存形式存在,吸附气的定量评价对页岩气藏的地质储量评估有较大的影响。甲烷等温吸附实验是表征页岩吸附气赋存能力的主要方法。目前国内页岩气地层埋深普遍较大(2 000~4 500 m),所处地层温度和压力高,甲烷已处于高     

页岩吸附与解吸气量实验研究

页岩气主要以游离气和吸附气形式存在于富含有机质页岩中,含气量的大小受页岩储集层压力、温度等多种因素影响。页岩吸附气量是评价页岩气资源量的关键性参数,也是评价页岩气是否具有开采价值的一项重要标准。在分析页岩气吸附与解吸机理的基础上,开展F页岩气田龙马溪组-五峰组岩心等温吸附、解吸实验研究,实验结果表明:利用等温吸附线法是获得朗氏体积(VL)和朗氏压力(pL)的有效途径;钻井现场页岩快速解吸获得的总含气量主要为吸附气量,游离气量占比较小;页岩朗氏体积(VL)与样品的有机碳含量(Cto)成正相关,达到饱和吸附后温度升高,吸附能力明显下降。     

渗吸效应对页岩气赋存状态的影响规律

目前对于渗吸效应改变页岩气赋存状态的定量化认识尚未形成,对页岩储层中压裂液大量滞留所引起的气水动态置换规律也不明确。为此,开展了气水置换实验以模拟水力压裂后近井区域页岩含水状态的变化情况,借助于含氢流体低场核磁共振谱分析技术(1H-NMR)动态监测页岩储层中甲烷的赋存状态,并计算不同赋存状态下的甲烷气量,进而研究了渗吸效应对页岩气赋存状态的影响规律。研究结果表明:①页岩饱和甲烷的过程分为吸附主导阶段和孔隙填充阶段,吸附作用和压力梯度作用在页岩饱和甲烷的过程中同时发挥作用;②页岩饱和甲烷过程前期阶段优先饱和吸附气,游离态甲烷作为外部甲烷转换为吸附态甲烷的中间状态在页岩孔隙中赋存,吸附气达到饱和状态后,甲烷在压力梯度作用下填充页岩孔隙直至孔隙内外压力平衡;③渗吸效应使页岩发生气水置换作用,吸附态甲烷部分解吸为游离态甲烷,吸附气占比降低,渗吸时间达到80 h时吸附气占比由63.58%降低至45.87%,而游离气量增加使页岩孔隙压力升高,同时水分占据部分孔隙体积,压缩游离气赋存空间,部分游离气被排出页岩孔隙,储层含气性降低,页岩样品含气量由渗吸开始前的7.91 mL/g下降至7.34 mL/g;④水力压裂过程中,压裂液的大量滞留使页岩储层近井区域处于富含水状态,渗吸效应引发的气水置换作用和排替作用使得页岩孔隙及井筒等外部空间中甲烷游离气量升高,孔隙压力升高使得地层压力上升,在一定程度上有利于页岩气的开采。     

应用核磁共振在线检测技术研究不同赋存状态下的页岩气动用规律

要合理制订页岩气井生产制度,进而优化页岩气田开发技术政策,弄清不同赋存状态下的页岩气产出规律是前提和基础。为了避免间接测试的不确定性,研发了适用于高温高压条件下的核磁共振在线检测系统,选取四川盆地长宁地区N203井下志留统龙马溪组龙一11层页岩岩心,以甲烷气体为实验流体,测量在页岩气开采过程中游离态和吸附态甲烷产出量的变化情况,并且结合页岩气衰竭式开发物理模拟实验,对不同赋存状态下的页岩气动用特征和产出变化规律进行了研究。研究结果表明:(1)饱和甲烷气体的页岩核磁共振T2谱图具有明显的双峰特征,吸附态甲烷主要赋存于页岩纳米孔隙表面,弛豫时间较短(0.1～1.0 ms),而游离态甲烷则赋存于较大的页岩孔隙中,弛豫时间较长(1～100 ms);(2)采用核磁共振测量的甲烷总含气量及吸附态/游离态甲烷气量与采用间接方法计算的结果较为接近;(3)基于页岩气开发物理模拟实验,在开发初期,产出气以游离态甲烷为主,吸附态甲烷的阶段贡献率低于5%,随着生产的持续,吸附态甲烷的阶段贡献率逐渐增加,尤其是当压力低于15 MPa以后,吸附态甲烷的阶段贡献率迅速上升,至生产后期,吸附态甲烷的阶段贡献率超过50%,累计贡献率达到30%;(4)把实验结果换算为矿场条件下气井的生产动态数据,生产前5年,吸附气对气井累计产气量的贡献率不足5%,至生产末期,吸附气对累计产气量的贡献率可以达到25%。结论认为,核磁共振在线监测技术可以定量表征不同赋存状态下甲烷气体的动用规律,为后续开展页岩气气水两相渗吸、CO_2/CH_4吸附置换等研究提供了新的方法。     

四川盆地焦石坝地区页岩吸附特征室内实验

页岩含气量对页岩气田储量计算至关重要,直接关系到页岩气的产量、递减规律等。页岩气以游离气和吸附气形式赋存,有重量法等温吸附、容量法等温吸附、现场含气量测试等不同方法测试吸附气、游离气。通过测试页岩气降压解吸过程中气体体积变化来测量吸附气量,首次建立了同时测试页岩吸附气、游离气的方法。该方法采用柱状页岩岩心,模拟不同温度、压力、含水、真实孔隙结构等条件,消除了等温吸附曲线的负吸附异常;分析了有机质含量、压力、温度、含水、气体组成等因素对页岩吸附量的影响。实验结果表明,页岩的甲烷吸附量随压力增加而增加,压力大于12MPa后达到吸附/解吸动态平衡,吸附量不再增加;有机质含量TOC增加时吸附量增加;温度增加时吸附量降低;页岩含水后吸附量降低;甲烷吸附量高于氮气吸附量;焦石坝龙马溪组主力层页岩在温度20℃、压力30MPa下页岩气吸附气量介于1.8~3.1m~3/t之间,总含气量介于5.1~6.5m~3/t之间,吸附气占总气量40%左右。     

页岩气储层蠕变特性及其对页岩气开发的影响

页岩气储层渗透率极低,必须经过压裂改造才能形成有效产能,大量狭小自支撑裂缝在天然气解吸及流动中具有重要作用。页岩气储层生产周期长,人工裂缝导流能力对裂缝变形极其敏感。研究结果表明,页岩气储层具有一定的蠕变特性,并随着粘土含量的增加而增强。压裂改造后,储层会产生大量裂缝,裂缝闭合蠕变是蠕变形变的主要形式。裂缝的存在会使储层蠕变速率大幅度提高,并对裂缝导流能力产生不可忽视的影响。裂缝闭合蠕变速率与裂缝界面之间、裂缝界面与支撑剂间的相互作用有关,并与基质蠕变速率成正比。压裂改造形成的裂缝网络越发育、单裂缝宽度越小,蠕变对裂缝导流能力的影响越大。在页岩气数值模拟中,考虑裂缝闭合蠕变的累积产量与未考虑裂缝闭合蠕变的累积产量相差较大。同时,在地应力计算、储层可改造性、支撑剂和施工参数优选以及生产方式的选择等方面也应考虑蠕变的影响,保持流体压力有利于减小蠕变形变,维持裂缝导流能力,提高气井产能和采收率。     

Optimization of shale gas reservoir evaluation and assessment of shale gas resources in the Oriente Basin in Ecuador

The petroleum geological features of hydrocarbon source rocks in the Oriente Basin in Ecuador are studied in detail to determine the potential of shale gas resources in the basin. The favorable shale gas layer in the vertical direction is optimized by combining logging identification and comprehensive geological analysis. The thickness in this layer is obtained by logging interpretation in the basin. The favorable shale gas accumulation area is selected by referring to thickness and depth data. Furthermore, the shale gas resource amount of the layer in the favorable area is calculated using the analogy method. Results show that among the five potential hydrocarbon source rocks, the lower Napo Formation is the most likely shale gas layer. The west and northwest zones, which are in the deep-sea slope and shelf sedimentary environments, respectively, are the favorable areas for shale gas accumulation. The favorable sedimentary environment formed thick black shale that is rich in organic matter. The black shale generated hydrocarbon, which migrated laterally to the eastern shallow water shelf to form numerous oil fields. The result of the shale gas resource in the two favorable areas,as calculated by the analogy method, is 55,500×10~8 m~3. This finding shows the high exploration and development potential of shale gas in the basin.     

页岩气储层综合表征技术研究进展——以涪陵页岩气田为例

页岩气储层综合表征技术作为页岩气勘探开发的核心技术之一,既是勘探开发评价的基础,又是工程工艺方案制定的重要参考依据。但我国页岩气勘探开发起步较晚,大规模商业开发直至2012年底焦页1井试气获工业气流后方才正式启动,加之与北美页岩气成熟开发区相比,地质地表条件复杂,因而页岩气储层综合表征技术的研究无成熟可借鉴的经验。为此,以页岩气与常规天然气储层的差异化分析为基础,重点依托涪陵页岩气田四年多来的开发实践,从沉积环境、生烃潜力、储集性、可压性及含气性等多个方面系统建立了页岩气储层的精细描述参数体系,明确了页岩气储层综合表征方法和技术手段;并进一步阐述了涪陵页岩气田五峰组-龙马溪组下部优质页岩气层段典型地质特征,可为类似页岩气田的勘探开发提供技术指导和示范借鉴。      

Economic appraisal of shale gas resources, an example from the Horn River shale gas play, Canada

Development of unconventional shale gas resources involves intensive capital investment accompanying large commercial production uncertainties. Economic appraisal, bringing together multidisciplinary project data and information and providing likely economic outcomes for various development scenarios, forms the core of business decision-making. This paper uses a discounted cash flow (DCF) model to evaluate the economic outcome of shale gas development in the Horn River Basin, northeastern British Columbia, Canada. Through numerical examples, this study demonstrates that the use of a single average decline curve for the whole shale gas play is the equivalent of the results from a random drilling process. Business decision based on a DCF model using a single decline curve could be vulnerable to drastic changes of shale gas productivity across the play region. A random drilling model takes those drastic changes in well estimated ultimate recovery (EUR) and decline rates into account in the economic appraisal, providing more information useful for business decisions. Assuming a natural gas well-head price of $4/MCF and using a 10 % discount rate, the results from this study suggest that a random drilling strategy (e.g., one that does not regard well EURs), could lead to a negative net present value (NPV); whereas a drilling sequence that gives priority to developing those wells with larger EURs earlier in the drilling history could result in a positive NPV with various payback time and internal rate of return (IRR). Under a random drilling assumption, the breakeven price is $4.2/MCF with more than 10 years of payout time. In contrast, if the drilling order is strictly proportional to well EURs, the result is a much better economic outcome with a breakeven price below the assumed well-head price accompanied by a higher IRR.     

Predicting the total porosity of shale gas reservoirs

The favorable zone with organic-rich shale reserve is the “sweet” in the exploration of shale gas, while the logging evaluation of reservoirs, to which the accurate assessment of total porosity is instrumental, is critical to seek this “sweet.” Taking the reservoirs in the Wufeng-Longmaxi Formation in Jiaoshiba region of Fuling, China as an example, the applicability of Herron Model for total porosity was first analyzed based on the logging data of Well #S1 (including element capture spectroscopy (ECS) and conventional logging) as well as the core analysis data (including total porosity and total organic carbon). Results show that the total porosity of target horizons was not accurately calculated with the Herron Model for the following two reasons: (a) in the calculation of apparent density porosity and apparent neutron porosity, rocks are taken as a simple combination of matrix and pore, neglecting the organic matters in reservoirs; (b) the weight coefficients of apparent density porosity and apparent neutron porosity are constant, ignoring their variation in various geological structures. Based on this analysis, an improved method of evaluating the total porosity of shale gas reservoirs was put forward and used to calculate the total porosity of Well #S2 in Wufeng-Longmaxi Formation in Jiaoshiba. Compared with original Herron Model, the new method offers simpler operation and considerably better accuracy, which make it suitable for wide range of applications.     

Chemical analyses of shale gas and conventional natural gas

Abstract

Shale gas is essentially non-traditional natural gas (NG). Shale gas can be considered an unusual alternative energy source. Shale gas production is a method of obtaining the NG trapped between deep underground rocks. Shale gas production is not economical, except for horizontal drilling and hydraulic fracturing methods. Advanced analysis of shale gaseous samples can be done using gas chromatography, mass spectrometry and other modern testing methods. The Orsat apparatus includes three absorption pipettes containing chemical solutions that absorb gases. Absorbents are a 33% by weight aqueous solution of potassium hydroxide (KOH) for carbon dioxide (CO₂), alkali pyrogallol for oxygen (O₂) and ammoniacal cuprous for carbon monoxide (CO) measurement. Oxygen is absorbed in alkaline pyrogallol or in a chromous solution. Shale gas can be analyzed best gas chromatographically. The capillary column can be separated from all the hydrocarbons and their isomers by alumina, which is used as a stationary phase in the gas chromatographic column, because alumina is highly selective for hydrocarbons. Silica is a specific adsorbent that exhibits greater applicability for hydrocarbons. The chemical contents of shale gas are similar to those of the conventional NG. The processing, transfer and storage and distribution of shale gas are assumed to be similar to the conventional NG.     

页岩气产量递减规律研究

页岩气藏的渗透率极低,储层流体流动性较差,气井产能较低,其产量递减规律有别于常规气藏。研究页岩气产量递减规律,对预测气田未来产量变化与最终开发指标及指导开发措施调整等均有重要意义。以美国德克萨斯州 Haynesville 与 Barnett 页岩气田单井生产动态为例,研究了层状硅质-钙质页岩与层状硅质页岩两大类页岩气田的产量递减规律,分析了复合递减模型以及广义双曲型曲线用于页岩气研究时参数发生的变化及其影响因素。通过 Arps 广义双曲递减法与复合递减模型法应用的对比表明,层状硅质-钙质页岩与层状硅质页岩两大类页岩气田产量变化规律有较大差异。     

页岩气保存机制探讨

常规天然气与页岩气在储集空间上的本质差别就是孔径或孔隙结构的差异。根据四川盆地涪陵焦石坝地区页岩埋藏史、含气量、生产数据及解吸气同位素分馏这4方面资料,指出页岩气是以单个或多个有机质碎片所形成的连通孔隙网络为单元被保存在页岩中,毛管压力与静水压力是页岩气孔隙压力的保存机制,因此页岩的富气程度不受构造形态或圈闭的控制。构造活动对页岩气保存条件的破坏主要决定于构造升降对静水压力破坏及地质应力对页岩页理面的破坏程度,水平挤压条件下层理面张开所引起的毛管封闭能力下降,导致页岩气散失。而断层往往是应力释放区域,使页理面不会受到进一步的破坏,相反可能对下盘起到保护作用,成为页岩气的有利勘探目标。根据毛管力原理可进一步估算页岩气的可采收率,从而为勘探开发提供指导。      

页岩含气量评价方法

作为页岩气资源勘探评价的核心基础,含气量的评价一直作为关键研究内容而受到高度关注。页岩气的成藏和富集是一个动态地质过程,游离和吸附状态天然气的同时存在及比例变化,导致了页岩中天然气赋存状态的复杂性。页岩含气机理与煤层气差异较大,直接和间接成因的页岩气类型各具不同的页岩油气形成条件和含气特点。垂向上的页岩含气相关指征曲线变化特点,可提供更多的沉积、含气及保存等信息。页岩含气量的获得方法可划分为6种基本类型,归属于3个可信度梯度级别,其中的现场解析法是含气量获取方法中的重要方法。在现场解析的升温过程中,只有当岩心在加热至地层温度前见解吸气者,通过线性或多项式逆向回归法计算出来的损失气量才具有明确物理意义。页岩的含气量受页岩的生气能力和强度控制,损失气、解吸气及残余气分别与吸附气和游离气存在内在联系。页岩吸附含气量和总含气量是页岩含气量地质评价中的重要参数,页岩气中游离气的占比不仅能反映页岩中天然气的赋存状态,而且更指示了页岩气的可采性。同时满足总含气量和游吸比双高目标的评价对象,是页岩气的有利目标。含气量、游吸比及可采系数等含气结构参数的同时使用,有助于更准确地进行页岩气评价。机器学习和大数据分析等提高了数据处理工作效率,智能评价是页岩含气量评价研究未来发展的重要方向。