Microscopic Distribution Of Water Within Shale Pores And Its Impact On Shale Gas Desorption

Recently,the exploration and development of shale gas have received extensive attention all over the world.Shale gas reservoirs generally contain water,so the research on shale gas desorption cannot ignore the influence of water.Previous works mainly focused on the shale gas desorption characteristics under equilibrium moistures.Although the existence of water has been considered,they did not explore the influence of the occurrence states of water.In addition,there is a lack of the evaluation theory of microscopic occurrence of pore water,and its influence on shale gas desorption is still not clear.Therefore,it is crucial to establish a theory for evaluating the microscopic occurrence of pore water.This thesis has conducted a series of experimental work,including water saturation and centrifugation experiments,methane desorption of shale under different water-bearing conditions,methane isotope fractionation under different water-bearing conditions,low-temperature nitrogen adsorption,nuclear magnetic resonance,total organic carbon(TOC)content test,and X-ray diffraction analysis,on25 shale samples collected from Upper Ordovician Wufeng Formation and Lower Silurian Longmaxi Formation in southeastern Sichuan.Some understandings are as follows:(1)Based on the water saturation and centrifugation experiments,a method for evaluating the adsorbed/free water content in water-saturated shale was established by re-determining the classification scheme of pore water in the shale matrix.Taking the marine shales from Wufeng Formation and Longmaxi Formation in the Sichuan Basin as examples,the adsorbed/free water content in the matrix pores of saturated /unsaturated water shales under experimental conditions(30℃,atmospheric pressure)were analyzed.In water-saturated shale,adsorbed water is mainly distributed in pores with smaller diameters,while free water is mainly stored in pores with larger diameters.As the centrifugal force increases,the amount of free water is continuously reduced,while the amount of adsorbed water remains stable.The distribution of pore water shows that adsorbed water content positively correlated with the TOC content due to the organic materials that develop nanopores provide a large specific surface area for adsorbed water storage.The free water content has a positive correlation with the clay minerals content,which is attributed to the fact that clay minerals are the main contributors to the mesopores and macropores conducive to free water storage.(2)The experimental results of methane desorption under water-bearing conditions show that as the moisture content increases,the cumulative amount of methane desorption decreases,and the methane desorption rate first decreases and then increases.The intrinsic reason is that the pore water reduces the gas storage performance and reservoir permeability.Moreover,with the increase of moisture content,the cumulative desorption gas volume and desorption rate of shale samples with high clay mineral content have apparent changes.Because clay minerals are hydrophilic,when the water content increases,the samples with high clay mineral content expand due to water absorption,which reduces the amount of free gas and increases the water blockage.The isotopic composition test results show that the whole isotope fractionation can be divided into three stages: the free gas seepage stage,transition stage,and adsorbed gas desorption stage.The presence of pore water mainly affects the latter two stages.In addition,the gas production in the free gas seepage stage accounts for about 90% of the total gas production,which means that the production of shale gas is mainly contributed by free gas.