页岩和煤岩的孔隙结构差异及其天然气运移机理

Pore structure differences between shale and coal and their gas migration mechanisms

页岩和煤岩作为非常规天然气储层,其孔隙结构控制着天然气的赋存状态与运移机理。为了定量表征两种储层孔隙结构特征及其差异,借助小角中子散射技术、高压压汞法、气体吸附法和场发射扫描电镜观察对下志留统龙马溪组和上二叠统龙潭组过成熟页岩以及上石炭统本溪组煤岩的储集性和孔隙结构进行对比分析,明确了页岩与煤岩的孔隙结构差异及其成因,并研究了甲烷在其储层中的赋存和运移机理。研究结果表明:①煤岩主要储集空间集中在小于5 nm的孔径范围内,其中甲烷赋存状态以吸附气为主,页岩中大于5 nm的孔径范围是主要的游离气储集空间;②基于小角中子散射与高压压汞实验结果的差异特征可以用于评价孔隙连通性,且本研究中煤岩的孔隙连通性要优于页岩;③重复压汞法显示高压对于过成熟的页岩和煤岩不会导致样品产生新的裂缝。结论认为,煤岩储层微孔中的吸附气解吸成为游离气是煤层气开发的关键,页岩储层中页岩气从基质储集空间运移到天然裂缝或人工裂缝中受孔隙连通性的控制;增加页岩中诱导裂缝,有效连通诱导缝和天然裂缝并且提高有机孔隙网络与外部的连通性是提高页岩气采收率的关键,该认识为后续两类非常规油气储层的科学评价提供了重要理论支撑。

Shale and coal are unconventional natural gas reservoirs,and their pore structures control the occurrence state and migration mechanism of natural gas.To quantitatively characterize their pore structure characteristics and differences,this paper comparatively analyzes the reservoir capacities and pore structures of the over-mature shale in Lower Silurian Longmaxi Formation and Upper Permian Longtan Formation and the coal in Upper Carboniferous Benxi Formation using small-angle neutron scattering technology(SANS),mercury intrusion capillary pressure method(MICP),gas adsorption method and field emission-scanning electron microscopy,so as to clarify the pore structure differences between shale and coal and their causes.In addition,the occurrence and migration mechanisms of methane in the reservoirs are studied.And the following research results are obtained.First,the main reservoir space in coal is less than 5 nm in pore size,in which methane is mainly in the form of adsorbed gas,while the pores with size larger than 5 nm in shale are the main reservoir space of free gas.Second,the differential characteristics of pore structures based on the experimental results of SANS and MICP can be used to evaluate pore connectivity.In this study,the pore connectivity of coal is better than that of shale.Third,the repeated MICP results indicate that high pressure cannot induce new fractures in the samples of over-mature shale and coal.In conclusion,the desorption of adsorbed gas in micropores of coal reservoirs into free gas is the key to coalbed methane exploitation,and the migration of shale gas from matrix reservoir space to natural or artificial fractures in shale reservoirs is controlled by pore connectivity.What’s more,the key to improving the recovery factor of shale gas is to increase induced fractures in shale,effectively connect induced fractures with natural fractures and improve the connectivity between organic pore networks and the external parts.The research results provide an important theoretical support for subsequent scientific evaluation on these two kinds of unconventional oil and gas reservoirs.