Geochemical Characteristics and Development Significances of Constant and Trace Elements from Coalbed Methane Co-Produced Water:A Case Study of the Shizhuangnan Block,the Southern Qinshui Basin

As an unconventional natural gas resource,coalbed methane(CBM)development releases a large amount of CBM wells co-produced water.Geochemical characteristics of the co-produced water provide an essential foundation for the production dynamics of CBM reservoirs if the impacts of fracturing fluids and other aquifers can be ignored.In the Shizhuangnan Block of the southern Qinshui Basin,constant and trace elements in CBM co-produced water from the wellheads were collected and determined,which is applied to assess water source,fracturing fluid effect,and CBM production.Based on principle component analysis and hierarchical clustering analysis,the water samples are divided into four categories.It suggests that different characteristics affected by water-rock interaction,reservoir environment,aquifer recharge,and hydraulic fracturing result in the various ratios of Na^(+)/Cl^(-),alkalinity(HCO_(3)^(-)+CO_(3)^(2-))/Cl^(-)and other specific rules.Moreover,Cl^(-)is selected as a dividing line for complete fracturing fluid flow back,associated with organic-bound chlorine complexes in the original coal seam water.Compared to constant elements,there is a significant correlation between Li and Sr concentrations and CBM productivity,so templates regarding trace elements can be used to distinguish various sources of the co-produced water.

Extreme massive hydraulic fracturing in deep coalbed methane horizontal wells:A case study of the Linxing Block,eastern Ordos Basin,NW China

Deep coal seams show low permeability,low elastic modulus,high Poisson’s ratio,strong plasticity,high fracture initiation pressure,difficulty in fracture extension,and difficulty in proppants addition.We proposed the concept of large-scale stimulation by fracture network,balanced propagation and effective support of fracture network in fracturing design and developed the extreme massive hydraulic fracturing technique for deep coalbed methane(CBM)horizontal wells.This technique involves massive injection with high pumping rate+high-intensity proppant injection+perforation with equal apertures and limited flow+temporary plugging and diverting fractures+slick water with integrated variable viscosity+graded proppants with multiple sizes.The technique was applied in the pioneering test of a multi-stage fracturing horizontal well in deep CBM of Linxing Block,eastern margin of the Ordos Basin.The injection flow rate is 18 m^(3)/min,proppant intensity is 2.1 m^(3)/m,and fracturing fluid intensity is 16.5 m^(3)/m.After fracturing,a complex fracture network was formed,with an average fracture length of 205 m.The stimulated reservoir volume was 1987×10^(4)m^(3),and the peak gas production rate reached 6.0×10^(4)m^(3)/d,which achieved efficient development of deep CBM.

Characteristics of dissolved inorganic carbon in produced water from coalbed methane wells and its geological significance

Based on long-term dynamic tracing of dissolved inorganic carbon(DIC)and stable carbon isotope(δ13CDIC)in produced water from 20 coalbed methane(CBM)wells in western Guizhou,the spatial-temporal dynamic variations ofδ13CDIC of the GP well group produced in multi-layer commingled manner were analyzed,and the relationship between the value ofδ13CDIC and CBM productivity was examined.The produced water samples of typical wells in the GP well group were amplified and sequenced using 16S rDNA,and a geological response model ofδ13CDIC in produced water from CBM wells with multi-coal seams was put forward.The research shows that:δ13CDIC in produced water from medium-rank coal seams commonly show positive anomalies,the produced water contains more than 15 species of methanogens,and Methanobacterium is the dominant genus.The dominant methanogens sequence numbers in the produced water are positively correlated withδ13CDIC,and the positive anomaly of v is caused by reduction of methanogens,and especially hydrogenotrophic methanogens.Vertical segmentation of sedimentary facies and lithology in stratum with multi-coal seams will result in permeability and water cut segmentation,which will lead to the segmentation ofδ13CDIC and archaea community in produced water,so in the strata with better permeability and high water cut,theδ13CDIC of the produced water is abnormally enriched,and the dominant archaea is mainly Methanobacterium.In the strata with weak permeability and low water cut,theδ13CDIC of the produced water is small,and the microbial action is weak.The shallow layer close to the coal seam outcrop is likely to be affected by meteoric precipitation,so theδ13CDIC of the produced water is smaller.The geological response model ofδ13CDIC in produced water from multi-coal seams CBM wells in the medium-rank coal reveals the geological mechanism and microbial action mechanism of theδ13CDIC difference in the produced water from the multi-coal seams CBM wells.It also provides effective geochemical evidence for the superimposed fluid system controlled by sedimentary facies,and can also be used for the contribution analysis of the produced gas and water by the multi-layer CBM wells.

A method for identifying coalbed methane co-production interference based on production characteristic curves: A case study of the Zhijin block, western Guizhou, China

Efficient detection of coalbed methane(CBM) co-production interference is the key to timely adjusting the development plan and improving the co-production efficiency. Based on production data of six typical CBM co-production wells in the Zhijin block of western Guizhou Province, China, the production characteristic curves, including production indication curve, curve of daily water production per unit drawdown of producing fluid level with time, and curve of water production per unit differential pressure with time have been analyzed to explore the response characteristics of co-production interference on the production characteristic curves. Based on the unit water inflow data of pumping test in coal measures, the critical value of in-situ water production of the CBM wells is 2 m^(3)/(d·m). The form and the slope of the initial linear section of the production indication curves have clear responses to the interference, which can be used to discriminate internal water source from external water source based on the critical slope value of 200 m^(3)/MPa in the initial linear section of the production indication curve. The time variation curves of water production per unit differential pressure can be divided into two morphological types: up-concave curve and down-concave curve. The former is represented by producing internal water with average daily gas production greater than 800 m^(3)/d, and the latter produces external water with average daily gas production smaller than 400 m^(3)/d. The method and critical indexes for recognition of CBM co-production interference based on the production characteristic curve are constructed. A template for discriminating interference of CBM co-production was constructed combined with the gas production efficiency analysis, which can provide reference for optimizing co-production engineering design and exploring economic and efficient co-production mode.

Microbial Geochemical Characteristics of the Coalbed Methane in the Shizhuangnan Block of Qinshui Basin, North China and their Geological Implications

Methanogens and sulfate reducing bacteria were detected by the 16SrRNA sequencing of coalbed methane(CBM)co-produced water in the south of the Qinshui Basin,which is indicative of the presence of secondary biological gas in the south of this basin,in contradiction to the previous understanding of thermogenic gas.This work systematically collected water samples from the CBM wells in the Shizhuangnan Block and analyzed the microbial geochemical characteristics from the aspects of water ions,hydrogen and oxygen isotopes,dissolved inorganic carbon and microbial diversity.It is shown that the Shizhuangnan Block has a nearly SN-trending monoclinic structure,and the elevation of coal seam decreases gradually from the east to west.Because of the water blocking effect of Sitou fault in the west,the precipitation flowed from the east to west,and gradually transited to stagnant flow area.The concentration variation of some ions such as Na^+,K^+,Ca2^+,Mg2^+,Cl^-,HCO3^-and total dissolved solids(TDS)suggest the variation of redox condition in the coal reservoir water.The 16SrDNA sequencing analysis of the collected water samples detected the presence of methanogens and sulfate reduction bacteria.The presence of methane production zone and sulfate methane transition zone(SMTZ)was identified.The effect of methanogens in the methane production zone leads to an increase in the methane concentration,resulting in a high gas content in the study area.In the SMTZ,most methane is consumed by anaerobic oxidation due to high sulfate concentrations.

Geochemical Characters of Water Coproduced with Coalbed Gas and Shallow Groundwater in Liulin Coalfield of China

To reveal the geochemical characters of water coproduced with coalbed gas and shallow groundwater,water samples were collected from 12 wells of coalbed methane and 7 wells of shallow groundwater.The pH,CODMn,fCO2,total dissolved solids （TDS）,total hardness,and concentrations of metasilicic acid,sodium and kalium,calcium ion,magnesium ion,ammonium iron,bicarbonate ion,carbonate,chloride,sulfate ion,nitrate ion,fluoride,lithium,zinc,nickel,manganese,iron,boron,barium,etc.of the samples were measured.Research results showed the following：（1） Concentrations of TDS,chloride,fluoride,sodium and kalium,ammonium,iron,and barium in the water coproduced with coalbed gas exceeded the national standards of China; however,physical,chemical,and biological properties of shallow groundwater could meet the national standard.（2） The water produced from coalbed contained mainly Na-Cl·HCO3,with average TDS of 4588.5 ppm,whereas shallow groundwater contained a mixture of chemicals including Na.Mg.Ca-HCO3·SO4 and Na.Mg-HCO3·SO4,with average TDS of 663.8 ppm.（3） In general,it was observed that bicarbonate and sodium accumulated in a reducing environment and deeper system,while depletion of hydrogen ions and dissolution of sulfate,calcium,and magnesium occurred in a redox environment and shallow system.（4） Sodium and kalium,ammonium,chloride,and bicarbonate ions were the main ions found in the study area.

深部煤层气水赋存机制、环境及动态演化

准确认识深部条件下气体和水分的赋存状态、相对含量及分布特征,对煤层气高效勘探开发具有重要指导意义。基于理论模型、分子模拟和气水演变分析,明确了煤层中气、水的赋存状态,揭示了气、水动态运聚界限和动态演化过程。考虑煤-水界面作用、水的可动性及赋存状态,煤中水可分为可动水(重力水和毛细水)、束缚水(吸附水、沸石水、结晶水和层间水)、结构水,其中毛细水、重力水和吸附水由孔隙主导,沸石水、结构水、结晶水和层间水由矿物主导。分子模拟结果显示,水分子在0.7 nm孔隙中可以饱和充填,吸附和解吸路径一致,在更大孔隙中出现弱吸附层和自由态。水分子吸附过程表现为单分子含氧基团吸附、单层强吸附、多层弱吸附、水团簇形成和充填孔隙等阶段。甲烷分子在1.5 nm孔隙可存在3层稳定充填吸附,在较大孔隙中(>1.5 nm)即以单层吸附和游离态共存,游离态在介孔及更大孔隙中普遍存在。结合上述吸附-游离气存在界限,改进了游离气和吸附气理论计算公式,为含气量计算提供新思路。深部热成因煤层气是煤大规模生排烃之后的残余气,在排烃过程中发生气驱水和水分蒸发扩散,残余水分为束缚水和结构水,后期无法改变。假定静水压力20 MPa,在0、5、10、15和20 MPa储层压力下,外来水分可入侵最大孔径为7、9、13、27 nm和不侵入。受差异保存条件控制,煤成气除了形成超压和欠压等差异含气系统外,还可能在煤系形成多类型含气模式。上述研究明确了煤层气、水微观赋存机制及形成演化模式,对深部煤层气富集特征及高效开发设计具有指导意义。

水分对CH_(4)和CO_(2)在煤中竞争吸附特性影响研究

为研究水分对CH_(4)和CO_(2)及混合气体在煤中竞争吸附的影响,通过自主研发的含瓦斯煤多元气体竞争吸附试验系统进行不同含水率煤对CH_(4)和CO_(2)单组分吸附研究,试验结果表明:水分并没有改变CH_(4)和CO_(2)等温吸附曲线的基本规律,在相同条件下,水分也未影响煤对单组分气体吸附能力的排序,始终是煤对CO_(2)的吸附能力大于对CH_(4)的吸附能力。相同条件下,水分对CH_(4)的抑制率大于CO_(2)吸附量抑制率,说明水分对煤吸附弱吸附性气体的抑制程度更大。在进行水对混合气体的竞争吸附影响研究试验,发现:当注入煤体的气体比例保持不变时,在同一吸附平衡压力下,混合气体的吸附总量随着水分含量的升高而降低,并且吸附平衡后的CH_(4)吸附量和CO_(2)吸附量都随着煤体中水分含量的增大而减小;而当含水率不变时,同一注气比例下的气体吸附平衡压力越大,吸附平衡后的CH_(4)吸附量和CO_(2)吸附量越大。游离相中的CO_(2)的体积分数始终低于气源,而CH_(4)体积分数始终高于气源。不同条件下CO_(2)/CH_(4)选择系数的数值均大于1,范围处在4.8~5.4,煤对CO_(2)的吸附亲和能力大于CH_(4)。相同组分混合气体吸附条件下,CO_(2)/CH_(4)选择系数均随着试验煤样含水率变大而降低。该研究进一步完善影响煤对气体吸附的因素的理论分析,以及为工程实践为井下注气促抽瓦斯技术提供理论依据。

五里堠井田煤层气井单相水流阶段合理降幅的确定

为了确定煤层气井单相水流阶段的合理降幅,最大化地保证煤层气井的水压传播距离,以五里堠井田煤层气井为研究对象,对研究区地下水势进行计算,拟合得出地下水势与平均日产水量的关系;以研究区地下水势平均值为界限,将研究区地下水势分为高地下水势和低地下水势,分析了高地下水势区、低地下水势区煤层气井产水特征差异。基于地下水势、地层供液指数、单相水流阶段压裂液量返排率的关系,建立了单相水流阶段合理降幅数学模型,研究区排采数据验证了模型的准确性。在此基础上,根据研究区地下水势差异性,预测了相应的单相水流阶段合理降幅范围。研究结果表明:研究区地下水势与平均日产水量为正相关关系,不同水势区煤层气井产水特征差异明显,其中,高地下水势区煤层气井排采初期日产水量增长迅速,排采中后期日产水量衰减明显;低地下水势区煤层气井排采初期日产水量增长较慢,排采中后期日产水量无明显变化。单相水流阶段,随着地下水势的增加,地层供液指数、压裂液返排率也随之增大;不同地下水势区域,单相流阶段合理降幅范围不同。研究区东北部及中部,单相水流阶段合理降幅范围为0.018~0.024 MPa/d;研究区西北部、南部,单相水流阶段合理降幅范围为0.007~0.017 MPa/d。该研究成果为未投产区及相似地质条件下煤层气井单相水流阶段排采制度的制定提供了借鉴与指导。

深煤层气井低伤害完井液分析

深煤层气井的开采通常是采用水敏损害的完井液,但这类完井液存在着严重的伤害问题,对煤层气井的开采效率有着不利影响。在煤层气开采过程中,需要不断向井底注入液体,在注入过程中不可避免地会对煤层造成一定的伤害。从深煤层气井开采过程中出现的问题出发,对煤层气井中常见的完井液进行了分析和讨论,并在此基础上提出了相应的低伤害完井液方案。分析结果表明:低伤害完井液能有效降低深井煤层气开采过程中出现的问题。

深部煤层气水平井大规模极限体积压裂技术--以鄂尔多斯盆地东缘临兴区块为例

针对深煤层渗透性差、弹性模量较低、泊松比较高、塑性强、破裂压力高、裂缝扩展难度大、加砂困难等特点,提出缝网改造规模化、缝网扩展均衡化、缝网支撑有效化的压裂设计理念,形成以高排量大规模注入、高强度加砂、等孔径限流射孔、暂堵转向、一体化可变黏滑溜水、多粒径组合支撑剂为核心的深煤层水平井大规模极限体积压裂技术。将该技术应用于鄂尔多斯盆地东缘临兴区块首口深煤层多级压裂水平井先导性试验,施工排量18 m^(3)/min、加砂强度2.1 m^(3)/m、用液强度16.5 m^(3)/m,压裂后形成了复杂裂缝网络,平均缝长为205 m,储层改造体积为1987×10^(4)m^(3),最高产气量达6×10^(4)m^(3)/d,实现了深部煤层气的高效开发。

二连盆地富气凹陷低阶煤煤层气成因及成藏机制

内蒙古低阶煤煤层气资源丰富,煤层气成因与成藏机制研究对低阶煤煤层气资源选区评价至关重要。以二连盆地重点富气凹陷低阶煤煤层气为研究目标,利用煤层气组分、碳/氢同位素、煤层水水质、氢/氧同位素及放射性同位素^(3)H和^(14)C测试等多种实验手段,分析煤层气、水地球化学特征,揭示低阶煤煤层气成因来源及成藏机制。结果表明,二连盆地煤层气组分以甲烷为主,均为干气,其中甲烷体积分数随埋深增加而增大,CO_(2)体积分数随埋深增加呈先增加后降低趋势,在300~500 m范围出现高值区。甲烷碳、氢同位素普遍偏轻,δ^(13)C(CH_(4))分布在-70.3‰~-48.0‰,δD(CH_(4))分布在-285.5‰~-189.0‰,δ^(13)C(CO_(2))在-37.6‰~1.94‰变化。煤层水化学类型主要为HCO_(3)-Na型和Cl·HCO_(3)-Na型,现今煤层水体环境较为稳定,水动力较弱,煤层水表观年龄在1020~47490 a,主要来源于第四纪大气降水,没有或较少有现今地表水补给。二连盆地煤层气主要为原生生物成因气,混有少量早期热成因气,随着埋深加大,地层环境和产甲烷古菌类型发生变化,生物甲烷生成途径发生转变。其中吉尔嘎朗图凹陷早期以乙酸发酵产气为主,晚期转变为CO_(2)还原产气为主,并混有少量低熟热成因气;巴彦花和霍林河凹陷微生物产气途径均以乙酸发酵为主,其中霍林河凹陷还混有少量甲基发酵型生物气。研究区具有适合生物气生成的低地温、低矿化度和低热演化程度的“三低”煤层条件,其中,吉尔嘎朗图凹陷属于地堑式浅部厚煤层生物气成藏模式,巴彦花和霍林河凹陷属于半地堑式中深部承压区水力封堵生物气成藏模式。寻找适合生物成因气形成和富集的有利目标区,应是二连盆地煤层气未来勘探开发的重点方向,也是二连盆地低阶煤煤层气增储上产的现实保障。

煤层气井产出水重复利用配制压裂液指标

煤层气生产过程中会产生大量的产出水,为了解决产出水处理过程难、处理费贵等问题,以沁水盆地安泽区块为例,开展了煤层气井活性水压裂后产出水离子矿化度连续性监测、产出水稳定性评价及产出水配制活性水压裂液配伍性实验评价。实验结果表明,压裂液进入煤层很快与煤层水混合,当压裂后快速返排,72 h大部分压裂液返排出煤层;煤层产出水最少静置48 h后,水质变清澈稳定,产出水固相含量小于50 mg/L,达到压裂重复再利用固相含量标准;煤层产出水配制压裂液与地层水混合无沉淀及絮凝,配伍性好,对储层伤害小。利用煤层气井产出水配制压裂液是可行的,设计了现场静置处理产出水的积液池,可使产出水充分循环利用,节约水处理成本,实现降本增效。

基于气−水两相流的注热CO_(2)增产CH_(4)数值模拟研究

注CO_(2)增产煤层气技术(CO_(2)−ECBM)可以降低温室气体排放,具有清洁能源生产和环境保护的功能。为研究气−水两相流条件下,在含水煤层中注CO_(2)增产CH_(4)的排采规律以及不同初始含水饱和度、不同CO_(2)注入条件对CH_(4)产量、CO_(2)储存和储层渗透率的影响,构建了竞争吸附、温度变化、煤体变形以及水运移的流−固−热耦合模型,通过与现场数据、已有试验以及现有模型的数值解结果,对比证明了模型有较高的准确性并对模型的优势做出具体阐述,随后利用COMSOL开展了CO_(2)−ECBM数值模拟。结果表明:注CO_(2)可以提升CH_(4)产气速率和产气量,这表明了注CO_(2)增产的可行性。随着CO_(2)持续注入,储层CH_(4)浓度降低,CO_(2)浓度升高,注气井附近温度升高,生产井附近温度降低且从注气井到生产井的温度缓慢下降;注气抽采期间,水相相对渗透率逐渐减小,气相相对渗透率逐渐增大。由于有效应力、基质收缩/膨胀共同作用,储层渗透率呈现“降低—升高—降低”的趋势;煤层初始含水饱和度越大,CH_(4)产量越低,渗透率下降幅度越小。累计CH_(4)产量最大下降15.19%,忽略煤层中水的影响会高估CH_(4)产量,在进行数值模拟时要考虑煤层水的影响;CO_(2)注入温度与注入压力越大,CH_(4)产量越大,渗透率下降幅度越大。累计CH_(4)产量分别增加13.27%、39.77%,渗透率分别下降20.4%、46.14%,提高CO_(2)注入温度和注入压力有利于提高CH_(4)产量。

煤储层含水性及其对煤层气产出的控制机理

煤储层的含水性不仅决定了煤层气吸附成藏理论的应用,影响储层改造措施的选取,也决定了对煤层气产出过程的认识。针对煤层的含水性,煤层气地质学认为煤储层基本饱和水,而瓦斯地质学在瓦斯含量计算和瓦斯渗流中基本没考虑水的影响,相当于认为煤储层饱和气。从煤层水来源、煤储层埋藏及热演化史的角度阐述了煤储层含水性的变化,分析现今煤储层可能存在饱和水、超低饱和含水、不饱和含水和不含水4种状态。煤储层热演化生烃及其造成的储层润湿性的改变,先存气体对后期水侵作用的影响是影响储层现今含水性的关键因素。由于我国构造运动复杂,多发生后期水侵,“大孔裂隙饱和水、小微孔饱和气”可能为多数煤储层气、水赋存状态。这种气、水赋存状态,可以有效回应煤层气地质理论中“煤储层饱和水,兰氏吸附理论不适用”的论点,同时也决定了在煤储层含气性分析中应该考虑毛管力的影响。此外,煤储层为低孔、低渗储层,早期的排采阶段一般为单相水流,煤层气的排采过程需要确定并考虑启动压力梯度的影响。综上所述,开展煤-水-甲烷条件下的煤储层润湿性研究,生烃作用及先存气体对储层含水性的影响研究,重新认识煤储层的含水性,并基于此进一步刻画煤层气的赋存和产出过程,可能是解决我国煤层气,特别是中、高煤阶煤层气低产问题的一把钥匙。

水分作用下无烟煤吸附瓦斯特性的分子模拟研究

为探究水分对煤吸附瓦斯气体的影响规律,基于蒙特卡洛方法和分子模拟仿真技术,研究了3种无烟煤在不同含水率(0~5%)和气体压力(0~10 MPa)条件下的瓦斯吸附特性。结果表明:相比于Langmuir模型和DR模型,DA模型能较好地表征不同含水率条件下无烟煤对甲烷气体的吸附特征;煤的瓦斯气体饱和吸附量、瓦斯气体吸附热均表现出随含水率升高而下降的趋势。进一步提出了一种DA吸附模型与具有指数表达式的水分修正系数相结合的新预测方程,该方程对不同含水率条件下的无烟煤吸附瓦斯气体量具有很好的预测效果。

水化学性质对煤粉聚集沉降的影响实验

煤粉问题制约着煤层气井高效开发,煤粉聚集会导致储层渗透率和产气能力降低。为了查明水矿化度和水化学类型对煤粉聚集的影响,针对水化学类型为NaHCO_(3)、Na_(2)SO_(4)、CaCl_(2)和MgCl_(2)和水矿化度为3000mg/L、6000mg/L、12000mg/L、18000mg/L的水溶液,开展了不同矿化度和水化学类型的水对煤粉聚集影响的实验。从煤粉聚集沉降情况观察、煤粉悬浮液中煤粉质量浓度及煤粉粒度分布,探究了煤粉在不同矿化度和水化学类型的煤粉悬浮液中的聚集沉降特征。结果表明:随着煤粉悬浮液静置时间的增长,各水化学性质煤粉悬浮液的颜色均不同程度的变浅,逐渐出现分层。在矿化度为3000~18000mg/L的煤粉悬浮液中,随着悬浮液矿化度的增加,煤粉在悬浮液中的聚集性增强。在四种水化学类型的悬浮液中,煤粉在CaCl_(2)和MgCl_(2)煤粉悬浮液中聚集最强,在NaHCO_(3)煤粉悬浮液中聚集最差。根据扩展DLVO理论,煤粉悬浮液中Ca^(2+)、Mg^(2+)、Na^(+)和H^(+)等阳离子可中和煤粉颗粒表面的双电层电荷,高价的Ca^(2+)、Mg^(2+)所带的正电荷比低价的Na^(+)和H^(+)所带的正电荷多,煤粉颗粒对Ca^(2+)、Mg^(2+)具有较强的吸附能力,由于Ca^(2+)与Mg^(2+)离子半径更大,压缩双电层的能力较强,因此,高矿化度的CaCl_(2)和MgCl_(2)地下水对煤粉聚集沉降较强。该成果为煤层气煤粉管控措施的制定提供理论依据。

无烟煤储层煤层水的相变传质过程及其煤层气开发的工程启示

科学认识煤层孔裂隙中流体流动与传质机理是实现煤层气高效产出的治本之策。煤层气开发过程中,煤层水的赋存和运移产出特征不仅决定了煤层气井疏水降压效果,同时显著影响了CH4解吸、运移、产出过程,对煤层气井高效排采管控至关重要。以沁水盆地南部樊庄区块为例,基于煤层水赋存形式、产出过程和流动形态的认识,开展了煤层水相变传质机制的研究,并量化了工程背景下无烟煤储层煤层水相变传质的阈值孔径,探讨了其对煤层气开发的工程启示。结果表明:煤层水的赋存状态可动态转变,流体压差是发生动态转变的主要机制。水的单相流动阶段,可动水的运移主要受表面张力作用;工程背景下,大孔(孔径>50 nm)尺度孔裂隙中可动水的运移产出是煤层压力整体释放的前提,并奠定了煤层气井高产稳产的基础。气水两相流条件下,束缚水(毛管水)向可动水的转变受煤岩润湿性以及气水之间的界面性质与相互作用的重要影响,并因毛管效应发生而压力条件要求增加。其中,泡状流和气团流为主的阶段,CH4的解吸产出促进了部分介孔(2~50 nm)尺度孔裂隙中束缚水(毛管水)向可动水的转变,并决定了煤层压力的整体释放效果;指示了见套压后,煤层压力的整体释放是气水两相共同作用的结果,排水阶段向降压采气阶段缓慢过渡对煤层气井高产是必需,持续憋压可能造成介孔尺度孔裂隙中过早形成段塞流,不利于煤层气井高产稳产。随煤层压力降低,等效孔径>40 nm的孔裂隙中的液态水可汽化脱离形成气态水自由水,与吸附态CH4形成竞争吸附/解吸,而等效孔径<40 nm的孔裂隙中的煤层水则以液态形式保存或进行润湿迁移,对气水两相流阶段束缚水(毛管水)向可动水的转变影响较小。

水分对煤层气吸附/解吸微观作用研究进展

水分是制约煤层气吸附/解吸的关键因素之一,受煤储层多元孔隙结构和煤岩组分润湿性差异影响,煤-水-甲烷界面作用导致煤层气产出过程中CH4与H2O相互激励、相互制约。立足于水分对煤层气吸附/解吸作用的研究进展与前沿认识,从煤储层水分赋存状态、煤-水界面微观作用和水分对甲烷吸附/解吸影响3个方面重点分析了水分与煤层气吸附/解吸微观效应之间的内在关系。研究认为煤储层孔隙结构及水分赋存状态复杂。以煤-水界面作用及孔隙结构特征为依据将煤储层水划分为结合水、束缚水和自由水3种主要类型,不同类型水分对甲烷吸附的抑制作用机制存在差异、且对低阶煤的影响程度严重。水分相态变化成为影响甲烷解吸-运移的核心,水蒸汽分子通过竞争吸附置换吸附态甲烷,液态水在润湿性和毛细管力作用下水锁堵孔、抑制气-水运移。在地面煤层气钻采过程中水分的作用机理随储层温度-压力环境动态变化而变化。针对水分对甲烷解吸作用机理不清、影响界限不明的现状,由此提出了量化储层水分含量及分布特征,增强甲烷解吸与气-水运移,完善甲烷吸附/解吸理论与模型,强化水分激励、促进煤层气增产4方面的科学问题及发展方向,进一步深化煤-水界面微观作用在煤层气解吸运移过程中的机理,促进煤层气持续稳定高效开发。