Characteristics Of Pore Structure And Its Constrains On Gas Accumulation And Migration In Marine Shale Of South China

The Lower Silurian Longmaxi Formation and the Lower Cambrian Niutitang Formation organic-rich marine shales are most important shales being studied for unconventional development in South China.In this work,Small-angle neutron scattering(SANS),helium pycnometry,field emission-scanning electron microscopy,mercury intrusion capillary pressure(MICP),low-pressure gas sorption isotherm,spontaneous imbibition,as well as saturated diffusion experiments were used to investigate the pore characteristics of two formation marine shales in the northwest of Guizhou and the southeast of Chongqing.The evolution of organic pore and inorganic pore in two marine shales were revealed based on the diagenesis and thermal evolution history.This work is the first microstructural study of Chinese marine shales by neutron scattering technique.By evaluating the difference of porosities determined from these methods,the closed fraction of shale pores are derived and discussed.Moreover,both He porosity and SANS porosity have positive correlations with TOC in shale samples.The content of clay minerals affects the proportion of macropores in shales.The fractal dimension of shale samples was derived,and results indicated that most over-mature shale are mass fractal.A lower mass fractal dimension means less closed poreswith correspondingly more open structures.The main controlling factors on shale methane adsorption capacity were investigated in this paper.Under geologic condition,the changes in methane adsorption capacity of organic-rich shales over geologic time as a function of burial history.Methane adsorption alone cannot provide suffficient methane for over-mature organic-rich shales to be commercial gas reservoirs.Therefore,the preservation condition of free gas in shale is the prerequisite to realize commercial production.Pore connectivity of shale is a critical factor in shale gas migration andproduction.In order to investigate the pore connectivity of shales,we conducted complementary analyses with field emission-scanning electron microscopy(FE-SEM),mercury intrusion capillary pressure(MICP),gas(CO2 and N2)sorption isotherm,contact angle,spontaneous imbibition,as well as saturated diffusion with tracer distribution mapped with laser ablation-inductively coupled plasma-mass spectrometry.The use of water and n-decane fluids enables us to examine the connectivity of hydrophilic and hydrophobic pore networks of the shale.Pore connectivity is mainly controlled by the development of both organic and inorganic pores and the extent of their connection.The migration of different fluids in shale is influenced by the wettability of shale composition of the pore-containing material.Overall,the variable and limited pore connectivity of shale samples will affect hydrocarbon preservation and recovery.