The Observation Of Nanometer Pores In Carboniferous Shale In Eastern Qaidam Basin

Shale gas is a kind of clean energy which occurs in shale pores. With the increase of social demand for energy, countries all over the world pay more and more attention to the study and mining of shale gas. China’s research and exploitation for shale lag behind the developed countries, especially for shale microstructure observation and shale gas seepage research. Since microscopic pore accounts for the largest part in shale pore network, finding out its structure characteristics and its effect on shale gas seepage is significantly important for mining shale gas.Qaidam Basin is confirmed to be one of the basins that have potentials for shale gas at present. We selected the shale of Keluke formation of carboniferous system in well zk1-1 and well Shiqian which are located at lime ditch area in the eastern basin. With FESEM secondary imaging technology, we observed fresh sections of shale samples and microscopic structure of argon after polishing in the laboratory. We confirmed that there are five types of pores in the shale in the study area including matrix pores, intergranular pores, dissolution pores, organic matter inner pores and microfracture. Using mercury intrusion method and gas adsorption method, we drew the size distribution of macro pores, mesoporous and microporous. It is concluded that the average shale pore diameter is about 4-6 nm, with the porosity from 0.86% to 5.48%. The porosity distribution ranges from several nanometers to several microns. The pore characteristics are similar to those of Barnett shale in American and the Longmaxi shale formation in Sichuan Basin, which can be used for reservation and migration of shale gas.According to the relevant national standards, we tested samples in the aspects of porosity, mineral composition, TOC, maturity(R0) and kerogen type. The results show that the shale mineral composition in the study area mainly includes detrital minerals, clay minerals and carbonate minerals. Kerogen type is mainly of type II. The maturity ranges from 1.32 to 2.05, which belongs to the medium level. TOC content is low, but meets the minimum requirement for gas production. All these show that the carboniferous system in Qaidam Basin is a set of hydrocarbon source rocks of development value. According to the relationship between pore characteristics and entrance, we find that shale pores are greatly influenced by mineral components and porosity, and that TOC, kerogen type and maturity are closely related to shale gas production capacity.For shale gas seepage, we mainly considered the darcy flow, slippage and Knudsen diffusion. We designed experiments according to the principle of darcy experiment. The experimental data show that seepage ability is positively related to the samples’ pore size. Combined with mass flux model and apparent permeability formula, we calculated slippage factor, diffusion coefficient and darcian permeability. Results show that seepage Knudsen diffusion accounts for a small part. When the pressure is small, slippage comes first followed by darcy flow; when the pressure is high, darcy flow comes first followed by slippage effect.