Geological Controls on the CBM Productivity of No.15 Coal Seam of Carboniferous–Permian Taiyuan Formation in Southern Qinshui Basin and Prediction for CBM High-yield Potential Regions

Coalbed methane(CBM) resources in No.15 coal seam of Taiyuan Formation account for 55% of the total CBM resources in southern Qinshui Basin(SQB), and have a great production potential. This study aims at investigating the CBM production in No.15 coal seam and its influence factors. Based on a series of laboratory experiments and latest exploration and development data from local coal mines and CBM companies, the spatial characteristics of gas production of No.15 coal seam were analyzed and then the influences of seven factors on the gas productivity of this coal seam were discussed, including coal thickness, burial depth, gas content, ratio of critical desorption pressure to original coal reservoir pressure(RCPOP), porosity, permeability, and hydrogeological condition. The influences of hydrological condition on CBM production were analyzed based on the discussions of four aspects: hydrogeochemistry, roof lithology and its distribution, hydrodynamic field of groundwater, and recharge rate of groundwater. Finally, a three-level analytic hierarchy process(AHP) evaluation model was proposed for predicting the CBM potentials of the No.15 coal seam in the SQB. The best prospective target area for CBM production of the No.15 coal seam is predicted to be in the districts of Panzhuang, Chengzhuang and south of Hudi.

Structure and production fluid flow pattern of post-fracturing high-rank coal reservoir in Southern Qinshui Basin

Field geological work, field engineering monitoring, laboratory experiments and numerical simulation were used to study the development characteristics of pore-fracture system and hydraulic fracture of No.3 coal reservoir in Southern Qinshui Basin. Flow patterns of methane and water in pore-fracture system and hydraulic fracture were discussed by using limit method and average method. Based on the structure model and flow pattern of post-fracturing high-rank coal reservoir, flow patterns of methane and water were established. Results show that seepage pattern of methane in pore-fracture system is linked with pore diameter, fracture width, coal bed pressure and flow velocity. While in hydraulic fracture, it is controlled by fracture height, pressure and flow velocity. Seepage pattern of water in pore-fracture system is linked with pore diameter, fracture width and flow velocity. While in hydraulic fracture, it is controlled by fracture height and flow velocity. Pores and fractures in different sizes are linked up by ultramicroscopic fissures, micro-fissures and hydraulic fracture. In post-fracturing high-rank coal reservoir, methane has level-three flow and gets through triple medium to the wellbore; and water passes mainly through double medium to the wellbore which is level-two flow.

Lithofacies palaeogeography of the Carboniferous and Permian in the Qinshui Basin, Shanxi Province, China

The Qinshui Basin in the southeastern Shanxi Province is an important area for coalbed methane（CBM） exploration and production in China, and recent exploration has revealed the presence of other unconventional types of gas such as shale gas and tight sandstone gas. The reservoirs for these unconventional types of gas in this basin are mainly the coals, mudstones, and sandstones of the Carboniferous and Permian; the reservoir thicknesses are controlled by the depositional environments and palaeogeography. This paper presents the results of sedimentological investigations based on data from outcrop and borehole sections, and basin-wide palaeogeographical maps of each formation were reconstructed on the basis of the contours of a variety of lithological parameters. The palaeogeographic units include the depositional environments of the fluvial channel, flood basin（lake）, upper delta plain, lower delta plain, delta front, lagoon, tidal flat, barrier bar, and carbonate platform.The Benxi and Taiyuan Formations are composed mainly of limestones, bauxitic mudstones,siltstones, silty mudstones, sandstones, and economically exploitable coal seams, which were formed in delta, tidal flat, lagoon, and carbonate platform environments. The Shanxi Formation consists of sandstones, siltstones, mudstones, and coals; during the deposition of the formation, the northern part of the Qinshui Basin was occupied mainly by an upper delta plain environment, while the central and southern parts were mainly occupied by a lower delta plain environment and the southeastern part by a delta front environment. Thick coal zones occur in the central and southern parts, where the main depositional environment was a lower delta plain. The thick coal zones of the Taiyuan Formation evidently occur in the sandstone-rich belts, located mainly in the lower delta plain environment in the northern part of the basin and the barrier bar environments in the southeastern part of the basin. In contrast, the thick coal zones of the Shanxi Formation extend over the mudstone-rich belts, located in the areas of the lower delta plain environments of the central and southern parts of the Basin.The Xiashihezi, Shangshihezi, and Shiqianfeng Formations consist mainly of red mudstones with thick-interbedded sandstones. During the deposition of these formations, most areas of the basin were occupied by a fluvial channel, resulting in palaeogeographic units that include fluvial channel zones and flood basins. The fluvial channel deposits consist mainly of relatively-thick sandstones, which could have potential for exploration of tight sandstone gas.

Triple Medium Physical Model of Post Fracturing High-Rank Coal Reservoir in Southern Qinshui Basin

In this paper, influences on the reservoir permeability, the reservoir architecture and the fluid flow pattern caused by hydraulic fracturing are analyzed. Based on the structure and production fluid flow model of post fracturing high-rank coal reservoir, Warren-Root Model is improved. A new physical model that is more suitable for post fracturing high-rank coal reservoir is established. The results show that the width, the flow conductivity and the permeability of hydraulic fractures are much larger than natural fractures in coal bed reservoir. Hydraulic fracture changes the flow pattern of gas and flow channel to wellbore, thus should be treated as an independent medium. Warrant-Root Model has some limitations and can’t give a comprehensive interpretation of seepage mechanism in post fracturing high-rank coal reservoir. Modified Warrant-Root Model simplifies coal bed reservoir to an ideal system with hydraulic fracture, orthogonal macroscopic fracture and cuboid matrix. Hydraulic fracture is double wing, vertical and symmetric to wellbore. Coal bed reservoir is divided into cuboids by hydraulic fracture and further by macroscopic fractures. Flow behaviors in coal bed reservoir are simplified to three step flows of gas and two step flows of water. The swap mode of methane between coal matrix and macroscopic fractures is pseudo steady fluid channeling. The flow behaviors of methane to wellbore no longer follow Darcy’s Law and are mainly affected by inertia force. The flow pattern of water follows Darcy’s Law. The new physical model is more suitable for post fracturing high-rank coal reservoir.

Porosity model and pore evolution of transitional shales:an example from the Southern North China Basin

The evolution of shale reservoirs is mainly related to two functions:mechanical compaction controlled by ground stress and chemical compaction controlled by thermal effect.Thermal simulation experiments were conducted to simulate the chemical compaction of marine-continental transitional shale,and X-ray diffraction(XRD),CO2 adsorption,N2 adsorption and high-pressure mercury injection(MIP)were then used to characterize shale diagenesis and porosity.Moreover,simulations of mechanical compaction adhering to mathematical models were performed,and a shale compaction model was proposed considering clay content and kaolinite proportions.The advantage of this model is that the change in shale compressibility,which is caused by the transformation of clay minerals during thermal evolution,may be considered.The combination of the thermal simulation and compaction model may depict the interactions between chemical and mechanical compaction.Such interactions may then express the pore evolution of shale in actual conditions of formation.Accordingly,the obtained results demonstrated that shales having low kaolinite possess higher porosity at the same burial depth and clay mineral content,proving that other clay minerals such as illite-smectite mixed layers(I/S)and illite are conducive to the development of pores.Shales possessing a high clay mineral content have a higher porosity in shallow layers(<3500 m)and a lower porosity in deep layers(>3500 m).Both the amount and location of the increase in porosity differ at different geothermal gradients.High geothermal gradients favor the preservation of high porosity in shale at an appropriate Ro.The pore evolution of the marine-continental transitional shale is divided into five stages.Stage 2 possesses an Ro of 1.0%-1.6%and has high porosity along with a high specific surface area.Stage 3 has an Ro of 1.6%-2.0%and contains a higher porosity with a low specific surface area.Finally,Stage 4 has an Ro of 2.0%-2.9%with a low porosity and high specific surface area.

沁水盆地南部中深部煤层气储层特征及开发技术对策

为了实现沁水盆地南部中深部煤层气高效开发,以郑庄北-沁南西区块为研究对象,基于参数井取心分析测试、注入/压降测试、地应力循环测试结果和大量动静态数据,通过与浅部对比,阐述了中深部煤储层特征,分析了从浅部到中深部煤层直井压裂和水平井分段压裂两种开发技术的改进,进而提出了中深部煤层气主体开发技术。结果表明,郑庄北-沁南西区块3号煤平均埋深1200 m左右,为中深部煤层气储层。随着埋深增加,研究区含气量和吸附时间均先增加后降低,含气量和吸附时间峰值分别位于埋深1100~1200 m和800~1000 m;随着埋深增加,研究区地应力场类型发生了2次转换,埋深小于600 m时,为逆断层型地应力场类型,水力压裂易形成水平缝,利于造长缝;埋深大于1000 m时为走滑断层型地应力场类型,水力压裂易形成垂直缝,裂缝延伸较短;埋深为600~1000 m时,地应力场由逆断层型向走滑断层型转换阶段,水力压裂形成的裂缝系统较为复杂。与浅层相比,中深部储层含气量、解吸效率和应力场发生明显转变。随着埋深增加,无论是直井(定向井)还是水平井,均应采用更大的压裂规模才能获得较好的效果。对于直井,埋深大于800 m后,压裂液量达到1500 m^(3)以上、排量12~15 m^(3)/min以上、砂比10%~14%以上,单井日产气量可以达到1000 m^(3)以上;对于水平井,埋深大于800 m后,压裂段间距控制在70~90 m以下,单段液量、砂量分别达到2000、150 m^(3)以上,排量达到15 m^(3)/min以上开发效果较好,单井产量突破18000 m^(3)。随着埋深增加,水平井开发方式明显优于直井,以二开全通径水平井井型结构、优质层段识别技术和大规模、大排量缝网压裂为核心的水平井开发方式是适用于沁水盆地南部中深部煤层气高效开发的主体工艺技术。

沁水盆地左权地区山西组米氏旋回识别及其天文年代标尺

依据地层序列的旋回信号进行地球轨道参数周期的识别,是确定深时时间尺度的有效方法之一。以沁水盆地左权地区山西组为例,使用时间序列分析法与相关系数法对钻孔剖面自然伽马测井曲线进行旋回性分析,并据此建立山西组年代地层标尺。使用频谱分析,在区内山西组识别出7.13 m、1.62~2.23 m的优势沉积旋回,分别对应405 kyr的长偏心率周期和91.9~126.4 kyr的短偏心率周期,经过天文检验,证实沁水盆地山西组受到天文周期驱动。通过高斯带通滤波,建立沁水盆地山西组“浮动”天文年代标尺,确定出9个405 kyr长偏心率旋回,据此获得山西组沉积时限为3.82 Myr。建立的天文年代标尺可为沁水盆地山西组古环境、古气候和各种地质事件的演化提供年代学依据。

沁水盆地南部深层高阶煤层气成藏特征

沁水盆地南部中浅层已经建成年产能力达26×10^(8)m^(3)的煤层气田,但深层煤层气勘探程度较低,地质认识不足。为了深化区域深层煤层气成藏特征研究,根据研究区探井钻探、分析化验及试采资料,从煤储层特征、热演化及含气特征、保存条件、温压特征等4个方面开展了深层煤层气成藏特征研究。结果表明:(1)3号煤沉积稳定,总厚度为4.0~7.3m,具有镜质组含量高、灰分低、裂隙发育的有利条件;(2)3号煤R_(o)为2.41%~3.03%,为高煤阶,吸附能力强,吸附含气量大于20m^(3)/t;(3)3号煤排采水矿化度大于4000mg/L,水型为NaHCO_(3),处于弱径流环境。煤层气藏具有偏低温低压的特征,表明深层煤层气藏遭受一定的破坏,未明显影响深层吸附气,但是对游离气成藏不利。研究区深层煤层气相对于已评价建产的斜坡带中浅层煤层气成藏条件更加有利,深层3号煤资源量估算1200×10^(8)m^(3),勘探开发潜力好。

煤层气水平井随钻仪器工程方案研究与应用——以沁水盆地安泽区块为例

文章以沁南安泽区块为例,对工程地质因素、地质导向参数、防塌钻进等内容进行研究,从随钻仪器方面助力煤层气安全高效钻井。安泽区块3^(#)煤层具有夹矸层多、煤岩碎裂结构发育等特点,以工程技术结合地质要求开展导向方案与措施的优化,制定出适合该区域的随钻仪器作业模板。本文采用近钻头方位伽马地质导向技术进行现场应用,对A13井实钻作业轨迹进行校正与控制。现场施工顺利,实际水平煤层段累计进尺882 m,钻遇率94.89%,安泽区块随钻仪器作业模板切实可行。

沁南煤层气井产能影响因素分析及开发建议

通过比较山西沁水盆地南部57口煤层气井在1.5 a时间内的产气产水特征,分析了影响该区煤层气井产能变化的地质及工程特征因素,并提出相应的开发建议。结果显示:煤层埋深及地下水动力条件、含气量以及气井所处的构造部位是影响沁南煤层气井产能的主控地质因素;开发前的煤储层压裂改造规模、井底流压下降速度以及排采速度是重要工程因素。同时,提出了相应的参数指标:500～700 m的埋深,大于15 m3/t的含气量;早期排水期,采取比较大的降压幅度和比较大的排采冲次,分别为0.022 MPa/d和3.0次/min;出现产气高峰后,开始缓慢降压和降低冲次,分别为0.002 MPa/d和0.4次/min。

沁水盆地南部山西组煤储层产出水的化学特征

为研究沁水盆地南部柿庄南煤层气开发区块山西组3号主采煤层的地下水径流与水化学特征,从该区采集了煤层气井排出水样,对主要离子浓度进行测定,并分析离子浓度展布特征。研究结果表明:研究区水质类型以Na-HCO_3-Cl或Na-HCO_3型为主;由东向西呈现出水文径流区(氧化环境)到滞流区(还原环境)的过渡特征,受控于矿物溶解反应,KDS、Na^+、K^+、Cl^-、Ca^(2+)、Mg^(2+)、HCO_3^-等离子浓度由东向西逐渐增加,SO_4^(2-)由于脱硫酸效应呈相反变化趋势,高产井多分布于高矿化度、高离子浓度的区域,离子浓度与矿化度指示了研究区的地下水径流特征,可以作为判断煤储层水动力条件与开发有利区的参考因素。

沁水盆地南部15号煤层顶板灰岩特征对煤层气开采的影响

煤层顶板的含水性对煤层气的开采有重要影响。沁水盆地南部上石炭统太原组15号煤层直接或间接顶板多为灰岩,其中以K2灰岩为主,连续分布。顶板泥岩较少,呈零散分布。灰岩的富水性对煤层气的排水降压有影响。因此,主要从灰岩的厚度展布、裂隙发育、与煤层的接触关系以及区域水文地质条件讨论其含水性对煤层气产能的影响。研究结果表明:(1)灰岩的含水性一般较弱,但当遇到断层或岩溶陷落柱较发育的部位,可能与其他含水层沟通,富水性较强。(2)15号煤层顶板灰岩的厚度与煤层气井的产水量并无直接关系,其裂隙较发育,但大多被方解石充填,导水和储水性能较差。(3)灰岩与15号煤层的接触关系有两种:一种是直接接触型,灰岩直接覆于15号煤层之上;另一种是间接接触型,灰岩与15号煤之间夹有泥岩、砂岩或14号煤层。直接接触型煤层气井的产水量、产气量比间接接触型高。间接接触型15号煤层直接顶板的岩性、厚度对产气、产水都没有太大影响。

山西沁水盆地南部太原组煤储层产出水氢氧同位素特征

为了研究沁水盆地南部太原组15号煤储层及其顶板灰岩的含水特征及水动力条件,从沁水盆地南部柿庄地区采集了煤层气井排出水、矿井下的煤层水与煤层顶板灰岩水、地表水共51个水样进行氢氧同位素及主要离子浓度测定。结果表明:目前排采15号煤的煤层气井排出水是煤层水和煤层顶板灰岩水的混合水。15号煤储层和顶板灰岩裂隙含水层之间存在较强的水力联系,煤层在排水过程中接受灰岩水的大量补给。煤层顶板灰岩裂隙含水层封闭性较差,水在灰岩裂隙中径流速度较快。煤层顶板灰岩水表现出^(18)O漂移的特点,排采15号煤的煤层气井排出水既表现出^(18)O漂移特点,也表现出D漂移特点,而排采3号煤的煤层气井排出水则主要表现出D漂移特点。煤层气并排出水的δD和δ^(18)O值都与矿化度TDS呈现出一定的正相关性,δD和δ^(18)O值也可以作为判断煤层水径流条件的参考因素。

沁南盆地樊庄煤层气田早期生产特征及主控因素

在对樊庄区块煤层气井早期产能空间和时间分布特征分析的基础上,重点探讨埋深、煤厚、含气量、孔隙度和渗透率、压裂效果对气井早期产能的影响关系,同时采用灰色关联分析方法,定量确定影响高煤阶煤层气井(田)生产的主控因素。研究表明:压裂效果对煤层气井早期产能的影响最大,并决定气井早期高产的峰值大小,但随着排采时间的推移,其对气井产能的影响程度逐渐降低;而煤层含气量和初始渗透率影响气井整体产能及其变化趋势;含气量越高或初始渗透率越大,气井产能越好,早期高产后其下降速度也越平缓。

山西沁水盆地热史演化特征

沁水盆地是华北克拉通内的构造盆地，是天然气勘探的潜在重要区域，盆地的热史研究是天然气储层评价的重要基础。重点应用磷灰石裂变径迹的分析与模拟，配合镜质体反射率的分析与模拟以及区域构造地质背景分析，恢复了沁水盆地的古地温梯度和地热演化模型：早古生代地温梯度稳定，为3℃／100m，晚二叠世至三叠纪地温梯度较前期略有降低，约为2．5～3．0℃／100m；早、中侏罗世地温梯度开始上升，约为3．0～4．0℃／100m；晚侏罗世-早白垩世地温梯度大幅度上升，为4．5～6．5℃／100m；晚白垩世至古近纪早、中期为高地温场的延续时期，地温梯度为5．5～6．5℃／100m；古近纪晚期-新近纪早期地温梯度大幅度降低，从6．0℃／100m骤降至4．2℃／100m左右；中新世以来地温场逐渐趋于稳定，地温梯度由4℃／100m演变到接近现代地温场的3℃／100m左右。

沁水盆地南部煤层气井排采储层应力敏感研究

为分析煤层气排采不同阶段煤储层应力敏感性及渗透率变化的影响因素,采集沁水盆地南部煤样,开展了不同实验条件的应力敏感实验。结果表明:有效应力从0增加到10 MPa时,煤样渗透率减少了50%~70%;有效应力从10 MPa增加到20 MPa时,损失量仅约占初始渗透率的10%;有效应力低于2.5 MPa时,应力敏感性较强;有效应力增加到3.5 MPa的过程中,渗透率损害系数急剧上升,渗透率损耗为20%~30%;有效应力从2.5 MPa增加到9 MPa时,应力敏感性最强,有效应力从3.5 MPa上升至9 MPa时,渗透率损害系数快速下降,渗透率损耗约60%;有效应力自9MPa之后,渗透率损害系数缓慢下降,渗透率损耗约10%;渗透率损害率介于30%~65%,临界应力为7~11 MPa。有效应力较低且不变时,煤样渗透率随孔隙压力增加而增加。围压不变时,随有效应力下降和孔隙压力增加,煤样渗透率下降,这与有效应力和孔隙压力变化引起的煤储层渗透率变化量有关。

沁水盆地山西组煤储层含气性及控制因素分析

依据实测资料，在统计分析研究区３号煤层含气性特征的基础之上，对控气地质机理进行了探讨．总体上来看，研究区甲烷含量较高及甲烷浓度较大，而含气饱和度偏低．在埋深１０００ｍ以浅，向盆地内部方向以及随煤层埋深加大，甲烷含量、甲烷浓度和含气饱和度具有增大趋势，显示出埋深控气以及次级向斜相对富气的特点，这种特征可能起源于地质历史时期中煤层抬升卸压过程所导致的煤层气解吸扩散作用．因此，进一步查明次级构造的发育展布特征，精细分析其与煤层含气性特征之间的相互关系。

沁水盆地南部含气饱和度特征及控制因素分析

为了查明沁水盆地南部地区含气饱和度的空间分布规律及其控制因素,采用类比法、内插法和综合分析等方法对该地区3煤层和15煤层进行了研究。结果表明:3煤层含气饱和度一般为20.60%～128.01%,平均70.53%,变化范围较大;15煤层含气饱和度一般为11.09%～132.42%,平均59.47%,分布较为集中;3煤层含气饱和度总体上高于15煤层;以此将沁水盆地南部划分为3个区,其中,位于研究区尉迟以北的Ⅰ区含气饱和度最高,位于寺头断层以东、樊庄以西和Ⅰ区以北的Ⅱ区块次之,而位于研究区东部和南部的Ⅲ区块最差。指出含气饱和度与煤层气含量呈正相关关系,与煤储层埋深具有良好的负相关关系,煤厚、煤阶及煤储层所处的水文地质条件亦对含气饱和度有一定的影响。

沁水盆地南部煤层气井生产历史拟合与井网优化研究

以煤储层地质特征和煤层气井生产数据为依据,以沁水盆地南部3口典型压裂直井为例,利用COMET3数值模拟软件对3口煤层气井排采数据进行了反演,确定了煤层气储层的渗透率与含气量等重要的储层参数和工程参数,探讨了煤层气井产能影响因素的敏感性,提供了研究区单井低产能部位煤层气井网部署及优化方案。研究表明:绝对渗透率变化对煤层气单井产能的影响最为显著,相对渗透率曲线形态、含气饱和度与Langumir常数对单井产能的影响依次减弱;井网井距的合理布置可以有效形成区域压降、提高单井产能;研究区低产井周围宜采用煤储层裂隙发育方向控制的矩形井网方式排采,最佳井控面积以0.011 km2为宜,优化井网为150 m×75 m,预测20年后优化井网排采煤层气采收率达75.36%。

沁水盆地安泽区块煤层气藏水文地质特征及其控气作用

在我国华北地区,煤层气藏的水文地质条件与煤层气的运移、散失、保存、富集等关系密切,但过去很少有学者采取动态监测地下水特征的方式来分析其对煤层气藏的影响。为此,以山西沁水盆地安泽区块煤层气藏为研究对象,在动态监测煤层气产出水离子浓度、水质水型及矿化度变化的基础上,结合该区煤层气井开发实际与地下水动力场分布特征,讨论了不同水文地质单元的产气、产水情况,并利用微量元素检测结果分析了合层排采的井间干扰。最后,结合构造、煤质特征探讨了水文地质条件对煤层气富集与产出的控制作用。结果表明:①该区主力煤层为下二叠统山西组3号煤和上石炭统太原组15号煤,煤层顶底板大部分为砂岩和泥质砂岩:②煤层气产出水离子浓度和矿化度随排采时间的增加不断降低,水型以NaCl型和NaHCO_3型为主;③将该区划分为弱径流区、径流区和滞流区3个水文地质单元,其中径流区产气量最高,滞流区产气量最低,合层排采井受15号煤的干扰较大。结论认为:该区煤层气的富集主要受断层及水动力条件的控制,下一步应加大对煤层气优势富集区的开发力度。