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

作　　者：Bao-Jian Shen Zhi-Liang He Cheng Tao Jin-Cai Shen Zong-Quan Hu Zhi-Ming Li Yuan-Hao Cao Wei Chen 

机构地区：[1]State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development,SINOPEC,Beijing,100083,China [2]Wuxi Petroleum Geology Institute,SINOPEC Petroleum Exploration and Production Research Institute,Wuxi,Jiangsu 214126,China [3]China Petroleum&Chemical Corporation,Beijing,100728,China [4]Chongqing Fuling Shale Gas Exploration and Development Co.,Ltd.,SINOPEC,Chongqing,408014,China [5]Petroleum Exploration and Production Research Institute,SINOPEC,Beijing,100083,China [6]School of Energy,Soochow University,Suzhou,Jiangsu 215006,China

出　　处：《Petroleum Science》2022年第5期2014-2026,共13页石油科学（英文版）

基　　金：support from Enterprise Innovation and Development Joint Fund of National Natural Science Foundation of China"Enrichment regularity and development mechanism of deep marine shale gas(U19B600303)";SINOPEC Science and Technology Department Project"Research on Precision Characterization of Shale Pore and Fluid Dynamic Monitoring Technology(P20059-8)"。

摘　　要：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.

关 键 词：DIFFUSION Isotopic fractionation Carbon isotopic reversal Modeling 

分 类 号：P618.13[天文地球—矿床学]