Pore Structure Characterization And Connectivity Evaluation Of The Longmaxi Shale Gas Reservoir In Pengshui Area,Southeast Chongqing

Shale pore structure that includes pore size distribution and connectivity determines the storage and seepage characteristics of methane in shale reservoirs,and also is the key in evaluating shale gas reservoirs.This study takes the Lower Silurian Longmaxi Formation shale gas reservoirs in Pengshui area as an example to perform experiments about pore structure and connectivity characterization.The primary purposes of this study are to reveal pore structure and connectivity and their controlling factors in shale gas reservoirs,and to establish an evaluation model of shale connectivity.This study not only improves the present understandings of microscopic pore structure and connectivity in shale reservoirs,but also provides theoretical guidance for shale gas exploration and development to some extent.The experiments of this study mainly consists of conventional physical properties(porosity and permeability),geochemical analyses,field emission scanning electron microscopy,high pressure mercury intrusion,low temperature CO₂ and N₂ adsorption,nuclear magnetic resonance etc.This study first characterizes pore types of shale.Secondly,pore size and distribution at various scales are revealed by integrating advantages of multiply methods.Thirdly,this study uncovers development characteristics of pores and their influencing factors at different pore size intervals using pore classification based on fractal theory.Fourthly,organic and inorganic pores are described on the basis of nuclear magnetic resonance experiments(saturated oil and saturated water,respectively).Finally,this study quantitatively estimates wettability and connectivity of pores based on the results of imbibition experiments and scanning electron microscopy.The results are that the shale gas reservoirs in the Longmaxi Formation contain various types of pores,including organic pores,intraparticle pores,interparticle pores,micro-cracks.The pore size of shale gas reservoirs spans 4-5 orders of magnitude,ranging from 0.4 nm-10?m.Combining fractal results from low temperature N₂ adsorption and high pressure mercury intrusion,the pores can be divided into micropores(<5nm),small-pores(5-25nm),mesopores(25-100nm)and macropores(>100nm),and micropores and small-pores are extremely developed,accounting for more than 70%of total pore volume.Micropores and small-pore are principally composed of organic pores and relatively smaller clay-related pores,and mesopores are made up of organic pores,relatively greater clay-related pores and carbonate-related pores,and macropores are jointly contributed by quartz mineral interparticle pore and clay mineral interparticle pore.The Longmaxi shale has dual-wettability,but exhibits more hydrophilic,and its connectivity can be well characterized by the slope of imbibition water curves.The slopes of imbibition curves of the Longmaxi shale samples are between 0.31 and 0.85,displaying moderate-good connectivity.Organic matter and clay minerals were the main factors that control pore connectivity,and an evaluation model was established through organic matter and clay minerals.The evaluation model shows that siliceous and argillaceous shales have good connectivity,while mixed and silty shales correspond to worse connectivity.These results are evidently correlated with on-site desorption rate,that is,the better the pore connectivity,the faster the shale gas desorption.