鄂尔多斯盆地大宁—吉县区块深部煤储层孔隙结构特征及储气潜力

Pore structure characteristics and gas storage potential of deep coal reservoirs in Daning-Jixian block of Ordos Basin

深部复杂的地质环境导致深部煤储层的孔隙、裂隙结构及气体赋存状态有别于浅部煤储层,多相态气体并存使得深部煤层气兼具"常规"与"非常规"双重地质属性。基于系统采集的鄂尔多斯盆地东南缘大宁—吉县区块深部煤岩样品,通过场发射扫描电镜分析、高压压汞实验、低温N2吸附和CO_(2)吸附联测,对深部煤岩全孔径的孔隙、裂隙结构进行了精细表征,并利用低场核磁共振技术分析了深部煤岩的可动流体空间及游离气可容纳潜力。研究结果表明:大宁—吉县区块深部煤岩的孔隙类型以气孔、粒间孔和压缩变形的植物细胞残留孔为主,裂隙由外生裂隙、内生裂隙和黏土矿物微裂隙组成,其中,在片状和手风琴状高岭石矿物中广泛发育微裂隙;孔隙多为一端封闭的不透气型孔和两端开放的透气型孔,含少量墨水瓶状孔。孔隙结构的跨尺度效应明显且非均质性极强。储集空间以孔径<2 nm的微孔最为发育,其次为孔径在50 nm~1μm的宏孔和孔径>10μm的裂隙,介孔(孔径在2~50 nm)及孔径在1~10μm的宏孔的发育程度最差。微孔具有极大的比表面积和强吸附势能,是吸附气的最主要赋存空间;宏孔和微裂隙中均存在一定的可动流体空间,可动孔隙度为1.42%~3.89%,具备容纳游离气的储层条件。深部煤储层中的气、水饱和关系复杂,含水饱和度变化大,这将直接影响游离气的可容纳空间大小。研究认为,受孔隙、裂隙结构影响的气、水饱和关系是制约深部煤层"高饱和—超饱和"含气靶区精准预测的关键问题。

Due to the complex deep geological environment,the pore,fracture structure and gas occurrence state of deep coal reservoirs are different from those of shallow coal reservoirs.The conventional and unconventional geologic attributes of deep coalbed methane are attributed to the coexistence of multi-phase gas.Using the deep coal and rock samples systematically collected from Daning-Jixian block in the eastern margin of Ordos Basin,the full-diameter pore and fracture structure of deep coal was fine characterized by field emission scanning electron microscopy analysis,high-pressure mercury intrusion experiment,and joint measurement of low-temperature N2 adsorption and CO2 adsorption.Moreover,the movable fluid space and accommodation potential of free gas in deep coal was analyzed by means of low-field nuclear magnetic resonance technology.The research results show that the pore types of deep coal in Daning-Jixian block are dominated by gas pore,intergranular pores as well as compressed and deformed residual pores in plant cells.The fractures are composed of exogenous fractures,endogenous fractures and micro-fractures in clay minerals,and the micro-fractures are widely developed in the sheet-like and accordion-like kaolinite.There are mainly air-tight pores closed at one end and breathable pores open at both ends,and a small amount of ink bottle pores.The pore structure exhibits significant cross-scale effects and strong heterogeneity.Micropores with a pore diameter less than 2 nm are the most developed,followed by macropores with a diameter from 50 nm to 1μm and fractures with a diameter greater than 10μm.Mesopores(with the diameter from 2 nm to 50 nm)and macropores with a diameter from 1μm to 10μm are least developed.Micropores have a large specific surface area and strong adsorption potential energy,and are the most important occurrence space for adsorbed gas.In addition,there is a certain movable fluid space in the macropores and micro-fractures with the movable porosity from 1.42%to 3.89%,providing the reservoir conditions to accommodate free gas.However,the relationship between gas and water saturation is quite complex and the water saturation changes greatly in deep coal reservoirs,thus directly affecting the size of the accommodation space of free gas.The research suggests that the relationship between gas and water saturation affected by the porosity and fracture structure will be a key issue that restricts the accurate prediction of highly saturated to supersaturated gas-bearing target areas in deep coal seams.