Characteristics Of Porosity And Permeability Of Plaeozoic Shales In The Yangtze Block

Shale is characterized by low porosity and low permeability.Shale gas can be adsorbed or dissociated in organic rich shale and its interlayers,and is characterized by self-generating,self-storage and self-capping.The progress in horizontal drilling and hydraulic fracturing made a rapid increase in shale gas production.China has the largest shale gas reservoir in the world.The widespread Paleozoic organic-rich shales in the Yangtze block are the main target for shale gas exploration in China.The pore structure and permeability of shales are the key problems in shale gas exploration.Although the commercial exploitation of shale gas has been succeeded in the Upper Yangtze region,formation and enrichment mechanisms of shale gas are not clear,and the development of productivity evaluation is limited.In contrast,the Lower Yangtze region experienced extensive magmatic activity since the Mesozoic.The influence of magmatic intrusion on modification and preservation of shale gas is still under debate.In order to reveal permeability variation mechanisms under different geological settings,this study selected typical Paleozoic shales in the Yangtze block,and measured composition,porosity,pore structure and microfractures,and permeability.The X-ray diffraction(XRD)and total organic carbon(TOC)measurements were used to determine the modal composition of shale samples.The GRI method and low-temperature nitrogen adsorption experiments were carried out to describe the 3D structure of pores.SEM images coupled with Ar milling technique and optical thin sections were observed to quantitatively analyze the 2D structure of pores.Dependence of permeability of shales on the effective pressure and pore pressure were measured using the pressure pulse decay method and the pore pressure oscillation method.The shale samples from the Wufeng and Longmaxi Formations show great variations in permeability anisotropy.For example,under effective pressure of 6.9 MPa,permeability anisotropy ranges from 1.2 to 1864.4.The development of microfractures in samples has a dominant influence on permeability anisotropy.The gas slippage effect,also known as the Klinkenberg effect,is characterized by increasing permeability with the decreasing pore pressure in shales under low pore pressure.At effective pressure of 6.9 MPa,the gas slippage is observed in four cores,with deviation from the Klinkenberg plot in three cores at high pore pressure.A new equation is proposed to describe the log trend of measured permeability with pore pressure.It allows us to quantify the deviation amount from the Klinkenberg plot and to constrain the pore pressure range where the effective stress law breaks down.In addition,the deviation from the Klinkenberg plot is more significant in shales with relatively high permeability and large pore width.According to the calculated Knudsen number,the dominant transport regime in shales can change from the slip flow to the Darcy flow in fracture-bearing samples due to variations in stress,pore size and fracture development.The results indicate the important influence of permeability anisotropy and gas slippage of shales on final gas production and reservoir management.Contact metamorphism will significantly change composition,geochemical properties and pore structure of shales.In order to investigate the influence of magmatic intrusion on permeability of shales,we selected the Luocun section in Zhejiang Province of the Lower Yangtze region,where the Early Cretaceous granite dike intruded into the Cambrian black shales(i.e.,Hetang Formation).Three phases of fractures in the Luocun shales were recognized and related with granite intrusion.Phase-I fractures are shear fractures formed by the compressive stress during early stage of intrusion.With the continuous magma intrusion,thermal evolution of organic matter produced CH4,CO2 and H2O.These volatiles and siliceous fluids increased pore fluid pressure in surrounding shales and produced Phase-II extensional fractures near the dike.In the late cooling stage,Phase-? shrinkage fractures developed in shales near the dike.The aureole width in the Luocun section is 37%of the dike width.Within this aureole,the fracture density,effective porosity,plug permeability,as well as the equivalent vitrinite reflectance,increase sharply with a decreasing distance from the dike,suggesting coincidence of a narrow contact aureole with the "permeability aureole" due to localized hydrofracturing and hydrothermal circulation.Therefore,granite intrusion lead to overmaturation of organic matters.The Phase-? and Phase-?fractures provided effective pathways for fluid migration and shale gas escape.Combined microstructure observation with laboratory measurements,this study improved our understanding of pore structure and mechanisms of permeability and its anisotropy.The results provide important information for designing the hydrofracturing process,modeling shale gas production,and evaluation the influence of thermal evolution on shale gas potential.