Thermally enhanced shale gas recovery: microstructure characteristics of combusted shale

Recently,thermal recovery technologies such as combustion have been studied for shale gas recovery.Thus,understanding of the microstructure of combusted shale is essential for evaluating the effects of thermal treatment on shale gas transport capacity.In this study,the effect of combustion on shale microstructure changes was investigated.Firstly,different-sized shale samples were combusted at 450℃ for 30 min.Afterward,shale microstructure properties including surface topographies,porosity and permeability of the raw and combusted shale samples were measured and compared.It was found that the pore volume and specific surface area increased after combustion,especially for small pulverized samples.According to surface topography obtained from atomic force microscope,more rough surfaces were obtained for the combusted shale due to larger pores and generation of thermal fractures caused by the removal of organic matter.Based on the mercury intrusion porosimetry measurements,the porosity of the shale samples increased from 2.79% to 5.32% after combustion.In addition,the permeability was greatly improved from 0.0019 to 0.6759 mD,with the effective tortuosity decreased from 1075.40 to49.27.As a result,combustion treatment can significantly improve the gas transport capacity.

A mathematical diffusion model of carbon isotopic reversals inside ultra-tight Longmaxi shale matrixes

Developing mathematical models for high Knudsen number(Kn)flow for isotopic gas fractionation in tight sedimentary rocks is still challenging.In this study,carbon isotopic reversals(δ^(13)C_(1)>δ^(13)C_(2))were found for four Longmaxi shale samples based on gas degassing experiments.Gas in shale with higher gas content exhibits larger reversal.Then,a mathematical model was developed to simulate the carbon isotopic reversals of methane and ethane.This model is based on these hypotheses:(i)diffusion flow is dominating during gas transport process;(ii)diffusion coefficients are nonlinear depending on concentration gradient.Our model not only shows a good agreement with isotopic reversals,but also well predicts gas production rates by selecting appropriate exponents m and m^(*) of gas pressure gradient,where m is for ^(12)C and m^(*)is for ^(13)C.Moreover,the(m−m^(*))value has a positive correlation with fractionation level.(m1−m1^(*))of methane are much higher than that of ethane.Finally,the predicted carbon isotopic reversal magnitude(δ^(13)C_(1)−δ^(13)C_(2))exhibits a positive relationship with total gas content since gas in shale with higher gas content experiences a more extensive high Kn number diffusion flow.As a result,our model demonstrates an impressive agreement with the experimental carbon isotopic reversal data.