Research And Application Of Shale Pore Structure Based On Fractal Theory

The pore structure of shale affects the enrichment and storage of shale gas,this paper takes the organic-rich shale of the lower Cambrian in Gui Zhou province Songtao area as the research object,combined with domestic shale pore structure research status,using optic microscope,X-Ray Diffraction(XRD),argon ion polishing-scanning electron microscopy(SEM),high-pressure mercury injection,low-pressure nitrogen adsorption experiments combined with fractal theory,analyzing the pore structure and fractal characteristics of the organic-rich shale for Niutitang formation.The results show that the organic-rich shale with organic carbon content between2.77%-5.81%,quartz content between 39%-68.4%and carbonate mineral contents between 0.8%-6.9%and pyrite content between 2.5%-6.1%,clay mineral content between 11.5%-28.2%,has higher mature degree.Macro-pore volume calculated through the high-pressure mercury injection experiment range from 0.0004 to 0.0007cm~3/g,with an average value of 0.0005cm~3/g,porosity range from 0.09%to 0.31%,with an average value of 0.16%.And according to low-pressure nitrogen adsorption experiments,the meso-pore volume range from0.0077 to 0.0119cm~3/g,with an average value of 0.009cm~3/g,the micro-pore volume range from 0.005 to 0.0089cm~3/g,with an average value of 0.006cm~3/g,the specific surface area calculated by the BET method ranges from 11.95 to 21.74m~2/g,with an average value of 14.57m~2/g.The correlational analysis between pore structure parameters and TOC value,quartz and clay mineral content indicates that organic matter and quartz are positively associated with the micro-pores and fine meso-pores,though larger pores can also exist in organic matter,and there is no obvious correlation between clay mineral and pore structure parameters.Based on high-pressure mercury injection experimental data,the fractal dimension D range from 2.05 to 2.84,with an average value of 2.53,used to describe the pore structure characteristics of the macro-pores.Based on the low-pressure nitrogen adsorption experimental data,the fractal dimension can be divided into two parts,the fractal dimension D1 range from 2.78 to 2.85,with an average value of 2.82,and the fractal dimension D2 range from 2.64 to 2.69,with an average value of 2.68,which shows the pore structure of the samples is more complicated,and the heterogeneity is strong.The more micro-pores contained in the shale,the more complex the pore structure will be,and as a result,the fractal dimension will be larger.Therefore,organic matter influence fractal dimension,and there is a negative relationship between organic carbon content and fractal dimension.As a result,there was a negative correlation between fractal dimension and porosity.