碳酸盐倒退溶解模式的化学热力学基础——与CO_2有关的溶解介质  

Chemical thermodynamics foundation of retrograde solubility for carbonate:Solution media related to CO_2

作　　者：黄思静[1] 黄可可[1] 张雪花[1] 刘丽红[1] 钟倩倩[1] 

机构地区：[1]"油气藏地质及开发工程"国家重点实验室(成都理工大学),成都610059

出　　处：《成都理工大学学报（自然科学版）》2009年第5期457-464,共8页Journal of Chengdu University of Technology: Science & Technology Edition

基　　金：国家自然科学基金资助项目(40839908;40672072)

摘　　要：碳酸盐岩占据了油气储层的"半壁江山",次生孔隙是碳酸盐储集空间的主体,相对浅埋藏条件下碳酸盐矿物更容易溶解的倒退溶解模式在碳酸盐油气勘探中具有非常重要的指导意义。以化学热力学中的吉布斯自由能增量为基础,计算了与CO2(g)/CO2(aq)/H2CO3/HCO3-/H+/CO32-系统有关反应在不同温度下的平衡常数,包括不溶于水的CO2气体分子(即CO2(g))和溶于水中的CO2(即CO2(aq))之间的平衡反应(CO2(g)CO2(aq))、溶于水中的CO2(即CO2(aq))和碳酸(H2CO3)之间的平衡反应(CO2(aq)+H2O H2CO3)、碳酸(H2CO3)的一级电离反应(H2CO H++HCO3-)和碳酸(H2CO3)的二级电离反应(HCO3-H++CO32-)。同时,根据方解石和白云石在酸性条件下的溶解过程,获得了碳酸盐矿物溶解过程中地层中流体的H+离子浓度(或pH值)与pCO2,地层压力和埋藏深度的关系。计算结果表明,在埋藏成岩系统中,地层流体温度的降低、地层压力或pCO2的降低以及埋藏深度的变浅,碳酸盐矿物都可能会从饱和状态进入不饱和状态。该结果支持碳酸盐的倒退溶解模式。Half of the oil-gas reservoir is carbonate rock, and secondary pores serve as the main oil storage spaces for carbonate rock. The retrograde solubility of a carbonate mineral, that is, carbonate more easily dissolves in relative shallowly buried environment, is of great guiding significance for carbonate oil-gas exploration. Based on Gibbs free energy increment, the authors calculate the equilibrium constants of the reactions related to CO2 （g）/CO2 （aq）/H2CO3/HCO3^- /H^＋/CO3^2- system at different tempretures, comprising the reaction （CO2 （g） ←→CO2 （aq））, equilibrium between insoluble gas molecule （CO2 （g）） and the soluble CO2 （aq）, the reaction CO2 （aq） ＋ H2 O←→H2 CO3 , equilibrium between soluble CO2 （aq） and carbonic acid, the first order ionization reaction （H2CO3 ←→H^＋ ＋ HCO3 ）and the second order ionization reaction （HCO3^-←→H^＋ ＋CO3^2- ）. Meanwhile, according to the calcite and dolomite dissolution process under an acidic condition, they got the relationship between fluid pH value （or [H^＋ 1） and pCO2 or formation pressure and buried depth in the calcite and dolomite dissolution process. In the deep buried diagenesis, the decreasing of formation temperature, formation pressure and PCO2 and the shallowing of buried depth will all change the carbonate minerals from saturation state to unsaturated state which future cause the solution of the carbonate minerals and the formation of secondary pores. The chemical calculation supports the retrograde solubility model.

关 键 词：倒退溶解模式 化学热力学 碳酸盐储层 次生孔隙 溶解作用 CO2 

分 类 号：P588.245[天文地球—岩石学] O642[天文地球—地质学]