Fractal characteristics of shale reservoir pores based on nitrogen adsorption

The fractal model for shale reservoir pores can indirectly reflect the characteristics that is difficult to observe directly.Using samples taken from Quanqian1 well from Sanquan profile in Nanchuan and Guanyinqiao profile in Qijiang,the based structural features were studied by low-temperature nitrogen adsorption experiments and the FHH fractal model was established.The relationship between fractal dimension values and pore structure parameters,TOC content were discussed.The results shows that the shale pores were mainly distributed in micropore under 40 nm and have obvious fractal characteristics.The fractal dimension lies between 2.76~2.85 with correlation coefficient higher than 99%reflecting the complex pore structures and strong heterogeneity.Some samples shows dual fractal feature and the pores can be divided into seepage pore and absorption pore bounded by methane molecular free path.The fractal dimension D1 for absorption pore(2.881~2.917)is higher than D2 for seepage pore(2.791~2.823).The relationship between pore volume,average pore radius and fractal dimension were negatively correlated and the relationship between specific surface area and fractal dimension were positively correlated which shows that the smaller the average pore size,pore volume is and the bigger the specific surface area is,the fractal dimension is more close to 3.There is a positive correlation between the fractal dimension values and TOC content which shows that the TOC content is an important influencing factors of fractal dimension values.The fractal dimension value can be used to evaluate thecomplexity and heterogeneity of reservoir pore quantitatively.It provides support for new reservoir evaluation methods and shale gas occurrence and migration mechanism in nano-pore research.