THE IMPACT OF ION-EXCHANGE PROCESSES ON SUBSURFACE BRINE TRANSPORT ASOBSERVED ON PIPER DIAGRAMS

An experimental study was conducted to assess how ion exchange process es, which occur during subsurface transport of oilfield brines, manife st themselves on graphical water quality diagrams. Concentrated brine soil column studies indicated sodium in the brine solution is able to exchange for calcium and magnesium on the soil, thereby retarding tran sport of sodium relative to the average ground-water velocity and resu lting in the development of a ''hardness halo'' at the leading edge of the brine plume. In this zone, calcium and magnesium concentrations e xceed the values for both the brine and the fresh ground water. The ha rdness halo manifests itself as a characteristic deviation away from t he mixing lines on the Piper diagram. Initially, the hardness halo cau ses concentrations to plot left of the fresh-water end member in the c ationic triangle. Chloride domination in oilfield brine causes concent rations to plot in the lower right corner of the anionic triangle almo st immediately. After the hardness halo passes through the column, the sampling results show a characteristic pattern of movement parallel t o the mixing line in the cation triangle or along the (decreasing) har dness axis of the upper diamond of the Piper diagram. The laboratory r esults were similar to those exhibited by monitoring well samples take n at a field site involving oilfield brine contamination. The results of this study show the Piper diagram to be potentially useful for earl y detection of brine contamination episodes.