Research On Microscopic Pore Structures Of Tight Gas Sandstone Reservoirs And Its Controlling Effect To Gas-Water Distribution In The Western Sichuan Depression

The Western Sichuan Depression?WSD?is a significant unconventional resource area in China with its large amount of tight sandstone gas,which shows great value of exploration and development.Study area experimented complex tectonic movement and evolution of sedimentation,which led to high heterogeneity,various microscopic pore structures and complex gas-water distribution.These restricts the progress of exploration.Based on previous studies,this paper used interpretation data of well logging and production of single well from oil fields to analyze gas-water distribution of shallow layer of Jurrasic Penglaizhen Formation and the forth and second member of upper Triassic Xujiahe Formations.We used Optimical Microscopy?OM?,Scanning Electron Microscopy?SEM?,Mercury Intrution Capillary Pressure?MICP?and Constant Rate Murcury Pressure?CRMI?experiments to analyze microscopic pore structures of tight gas sandstone reservoirs of our study area.Finally we found out the controlling effect of microscopic pore structures to gas-water distribution characteristics of tight gas sandstone reservoirs.The results show that the reservoir of upper Penglaizhen formation in Jurassic has three gas-water distribution:gas layer upon the water layer and gas layer below the water layer.The upper section of T₃x⁴ mainly produced gas and some water,while the lower T₃x⁴ had little water layer,water in this strata was characterized by“island-like”,and this layer always produced gas.And the T₃x² reservoirs generally produce gas that has high gas productivity and its water appear“island-like”.The main lithology of study area is lithic sandstone.The main pore types are primarily intergranular pores,secondary intragranular pores and some micro-fractures.While the dominate throat types are reduced-neck throat,slim throat and bending slim throat.The Mercury Injection Capillary Pressure?MICP?experiment showed that the pore throat radius in study area was almost less than 1?m,among them the samples from Penglaizhen formation had slightly larger pore throat,and samples from T₃x²reservoirs had smaller pore throats.The Constant Rate Mercury Injection?CRMI?experiment showed that pore radius from study area was from 100²50?m and throat radius 0¹?m,but few samples 5?m,which was a huge disparity;ratio of pore and pore throat radius?RPTR?data also showed that different samples had different distribution of RPTR,which demonstrated the relation of pore and pore throats was complex.Analysis of sinlge-factor and multi-factors showed that pore and pore throat types,pore and pore throat sizes and pore connectivity had some influence on interpretation and productivity of gas and water of tight gas sandstone reservoirs.Large amount of secondary pores,slim throat and larger pore and pore throat radius were beneficial for free flowing of gas and water in reservoirs,which made more water layer and gas-water layer.We also combined these single characteristics to make a classification of microscopic pore structures,and different types of pore structures linked well to different interpretation of reservoirs:type I corresponded to higher gas and water productivity,contigourous gas or water layers,which always had gas-water relation of gas was upon water or gas water layer;type II corresponded to lower water productivity and higher gas productivity,which reservoirs were gas layer or gas layer with water production;type III corresponded to much lower or none water productivity or much higher or none gas productivity,which reservoirs were gas layers with much higher productivity or dry layers.