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

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+/CO32-system at different temperatures,comprising the reaction(CO2(g)CO2(aq)),equilibrium between insoluble gas molecule(CO2(g)) and the soluble CO2(aq),the reaction CO2(aq)+H2OH2CO3,equilibrium between soluble CO2(aq) and carbonic acid,the first order ionization reaction(H2CO3H++HCO3-)and the second order ionization reaction(HCO3-H++CO32-).Meanwhile,according to the calcite and dolomite dissolution process under an acidic condition,they got the relationship between fluid pH value(or ) 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.