Study on characteristics of shale pore structure and water seepage evolution laws in northern Guizhou

In order to reveal the characteristic pore structure of shale and the evolution rules of water seepage， studies were carried out for the Niutitang Formation shale in northern Guizhou .First， low temperature liquid nitrogen adsorption test and high pressure mercury injection test were employed to analyze the internal micro-pore structure of the shale.Then， under the condition of constant confining pressure and axial pressure， the triaxial seepage tests were carried out using triaxial seepage device under different effective stresses， and the relationship between effective stress and permeability of shale is discussed.A shale permeability model was constructed by considering the linkage effect of pore structure and compression characteristics， and the study results were verified by comparative analysis with the shale permeability model under the stress state of initial permeability.Study results show that the Niutitang Formation shale has a continuous distribution system with the range from micro pore to macro pore， the adsorption-desorption isothermal adsorption curves of the five types of shale have closest similarity with the Type Ⅳfrom the IUPAC Classification and there is consistency between the mercury forming-mercury withdrawing curves； The dominating type of macro-pore is the one of air-tightening with one end closed， most of the meso-pore are open pores such as capillary with both ends open， the main micro-porous main are those of thin neck wide bore body and the ink bottle；The pore size distribution of shale is rather complex， containing a large number of micro-pores and meso-pores， both of which contribute the most of the pore specific surface area， while the macro pore are the main contributor for pore volume； Under the same confining pressure and axial pressure， shale permeability decreases gradually with the increase of effective stress following a exponential relationship；After taking into account of the linkage effect of pore structure and compression characteristics，the calculated curve from shale permeability model is generally consistent with the experimental data from permeability test， and can well reflect the relationship between effective stress and permeability under constant confining pressure and axial pressure.