Determination of chemical compositions of pore water in clayrich formations using geochemical modeling

Chemical behavior of pore water in clay-rich aquitards plays an important role in the quality and quantity studies of aquitard pore water. It is also of great significance in transport of contaminant in clay formations, in evaluation of a host rock for radioactive waste storage and in cap evaluation of oil and gas reservoirs. However, the traditional methods to extract pore water are quite difficult due to the low permeability of clay. This paper presents an approach for calculating pore water compositions in the aquitards, using the PHREEQC software, based on the laboratory-measured properties of core samples. The Cl- and SO2-4 contents are calculated through leaching experiment by anion-access porosity (50% of the total porosity). Based on the cation exchange capacity and exchange selectivity coefficient, the mineral phase equilibrium is included in the model, and the major ion contents are constrained. The results show that the modeled pore water compositions reclose to the squeeze water (basically in-situ pore water) other than the leaching water. The obvious over-estimate of the pore water compositions is calculated by the leaching water due to the dissolution of minerals and desorption of clay minerals. The cation exchange capacity is between 13.4 and 37.8 meq/100g soil in the Tianjin coastal clay formation, with the major exchangeable ions of Na, Mg and Ca. The pore water is reductive, and becomes much stronger with depth. The mineral phases we choose in the model is shown to be in equilibrium with the contacting pore water. There are also some exceptions that Fe (hydr) oxides and Al-bearing minerals oversaturated are observed, indicating that more accurate Fe-mineral measurement is needed to constrain the Fe content. This method has a great potential in low permeability, consolidated, and low water content clay pore water studies.