Abstract: Background, aim, and scope Global warming caused by emission of greenhouse gases dominated by CO2 has become a severe challenge to human being. Many measures have been taken to combat global warming all over the world. What is worth mentioning is the CO2 geological sequestration, it is an effective and burgeoning measure to achieve carbon emission reduction. The CO2 sequestration potential is very huge in Ordos Basin. So, in this paper the formation water characteristics in Ma51 submember of Majiagou formation are systematically studied, which is the precondition for further engineering design of CO2 sequestration in Ordos Basin. Materials and methods Based on the direct observations and experimental tests of deep drilling samplings，the method of “single index classification evaluation and multiple index comprehensive evaluation” is adopted. The physical properties, chemical properties, formation water types and hydrochemical characteristic parameters of the Majiagou formation Ma51 submember are analyzed in detail. Results The results indicate that the pH of the water ranges from 3.5 to 8.6, mainly from 4.5 to 6.5; the water density ranges from 1.0 g/cm3 to 1.5 g/cm3, mostly 1.1～1.3g/cm3; the mineralization degree is generally greater than 50g/L, and average mineralization degree is 165.6g/L; the highest concentration ion is Cl-, followed by Ca2+; the concentration order of the ions is r(Cl-)>r(Ca2+)>r(Na++K+)>r(Mg2+)>r(HCO3- )>r(SO42- ), and the formation water is CaCl2 type. The analysis results of water chemical characteristic parameters show that Na/Cl factor（rNa+/rCl-） ranges from 0.18 to 0.55, 0.34 on average; Cl/Mg factor (rCl-/rMg2+) ranges from 11.04 to 165.16, 32.11 on average; desulfurization factor (100×rSO42- /rCl-) ranges from 0 to 0.45, 0.09 on average; Mg/Ca factor (rMg2+/rCa2+) range from 0.01 to 0.30, 0.17 on average. Discussion Most samples have a pH of 4.5~6.5, which does not meet the drinking water standard (6.5~8.5). The density is generally 1.1~1.3g/cm3, indicating that the salinity may be high. The sample salinity is generally greater than 50 g/L and the average salinity is 165.6 g/L further explains that the water is not suitable for drinking. The ion composition and concentration, the “salinity classification” and “Sulin’s classification” together indicate that the water with high salinity is brine and it belongs to the CaCl2 type. The comprehensive analysis of the Na/Cl factor (rNa+/rCl-), Cl/Mg factor (rCl-/rMg2+), desulfurization factor (100×rSO42- /rCl-) and Mg/Ca factor (rMg2+/rCa2+) show that the content of Cl- and Ca2+ is relatively high, and the content of Na+, Mg2+ and SO42- is low. Overall, the reduction reaction is complete and the formation water is in a closed environment. Conclusions The conclusions that can be drawn from the above results and discussion are as follows: the water can not serve as drinking water and the implement of CO2 sequestration in Ma51 submember will not cause contamination to fresh water we live on; formation water of Ma51 submember is under a very strong reducing environment, which is very good for CO2 sequestration and can greatly reduces the risk of CO2 leakage; the ion with the highest content is Cl-, and the second content is Ca2+, which can weaken the chemical reactions and help to generate CaCO3 precipitation in the end , thus prolonging the CO2 sequestration time and reducing the risk of CO2 leaking. Recommendations and perspectives CO2 sequestration is a comprehensive environment protection issue that involves the combination of geology and engineering. So, only studying the significance of formation water characteristics for CO2 sequestration is far from enough. The industrial CO2 sequestration still has many problems to deal with. For example, laboratory tests of CO2 sequestration mechanisms；analysis on affecting factors of CO2 geologic sequestration; blocks selection and evaluation for CO2 sequestration; evaluation on sequestration potential of a region or layer; leakage risk assessment after CO2 sequestration; improvement in public recognition and research of relevant laws and regulations. The process of CO2 sequestration still has a long and hard way to go.
Keywords: Ma51 submember; mineralization; brine; CO2 sequestration; Ordos Basin