黔西比德—三塘盆地独立叠置含煤层气系统垂向分布与煤储层吸附—渗流特征

Independent Superposed CBM-bearing Systems Vertical Distribution and Coal Reservoir Adsorption &Seepage Characteristics in Bide-Santang Basin,Western Guizhou

通过系统采集黔西比德—三塘盆地2口典型钻孔的煤与岩石样品,对岩样分别开展压汞、突破压力、扫描电镜和铸体薄片实验,对煤样开展压汞与液氮测试,以探索独立叠置含煤层气系统的分布规律和形成机制,以及多煤层与多含气系统叠置条件下煤储层的吸附与渗流特征,结果表明：层序地层格架控制了煤层围岩渗流能力的垂向变化,进而控制了含气系统的垂向分布,具体表现为最大海泛面位置发育区域稳定分布的海相泥岩,其中黄铁矿、菱铁矿、碳酸盐胶结作用强烈,岩性高度致密,孔隙极不发育且突破压力较高,可构成区域性的隔水阻气层;而层序界面附近岩层由于沉积间断时期的暴露、冲刷、淋滤造成次生孔隙发育,增强了岩层的渗透性能,使得相邻煤层存在含气性联系。基于此认识,以最大海泛面为界,将含煤地层划分为1~5号煤、6~13号煤、14~21号煤、22~32号煤及33~35号煤共5套独立含气系统。2号煤和35号煤具备有利于煤层气渗流的孔裂隙系统,其他煤层的渗流条件较差,6号煤具有较强的煤层气吸附储集能力,2、16、23、35号煤次之,21、30、32号煤吸附储集能力较差。

In order to explore the development rule and formation mechanism of independent superposed CBM-bearing systems and reveal the coal reservoirs adsorption and seepage characteristics under condition of multiple superposed coal seams and CBM-bearing systems,this article collects coal and rock samples from two representative boreholes located in Bide-SantangBasin,western Guizhou.For rock samples,experiments of mercury injection porosimetry,breakthrough pressure,scanning electron microscope and casting lamella are conducted respectively,while mercury injection porosimetry and low temperature nitrogen adsorption tests are conducted on coal samples.The following conclusions are obtained：sequence stratigraphic framework controls the CBM-bearing systems vertical distribution by controlling the variation of rock seepage capacity in vertical.Marine mudstone developed regionally in the maximum flooding surface（MFS）is highly compacted with the cementation of pyrite,siderite and carbonate minerals,as a result,the pores developed very weekly and the breakthrough pressure is high,which made the MFS stratum liable to form regional impermeable barrier.The stratum near the sequence boundary（SB）develops secondary pore on the influence of exposure,washing and leaching during the sedimentary hiatus,which enhanced the rock seepage capacity and promoted the fluid contact between adjacent coal seams.On this basis,the coal-bearing strata are divided into five CBM-bearing systems by MFS：No.1-5coal seams,No.6-13 coal seams,No.14-21 coal seams,No.22-32 coal seams and No.33-35 coal seams.No.2and 35 coal seams have favorable pore systems for CBM seepage,while other coal seams have relatively poor seepage condition.No.6coal seam has better CBM adsorption and storage capacity,then the No.2,16,23 and 35coal seams,and the No.21,30 and 32coal seams have relatively poor adsorption and storage capacity.