A New Method for Determining Fluid Compositions in the H2O-NaCl-CaCl2 System with Cryogenic Raman Spectroscopy

Raman peaks of various hydrates in the H2O-NaCl-CaCl2 system have been previously identified, but a quantitative relationship between the Raman peaks and XNaCl (i.e., NaCl/(NaCl+CaCl2)) has not been established, mainly due to the difficulty to freeze the solutions. This problem was solved by adding alumina powder to the solutions to facilitate nucleation of crystals. Cryogenic (-185 degrees C) Raman spectroscopic studies of alumina-spiced solutions indicate that XNaCl is linearly correlated with the total peak area fraction of hydrohalite. Capsules of solutions made from silica capillary were prepared to simulate fluid inclusions. Most of these artificial fluid inclusions could not be totally frozen even at temperatures as low as -185 degrees C, and the total peak area fraction of hydrohalite is not correlated linearly with XNaCl. However, the degree of deviation (XNacl) from the linear correlation established earlier is related to the amount of residual solution, which is reflected by the ratio (r) of the baseline bump area, resulting from the interstitial unfrozen brine near 3435 cm-1, and the total hydrate peak area between 3350 and 3600 cm-1. A linear correlation between XNaCl and r is established to estimate XNaCl from cryogenic Raman spectroscopic analysis for fluid inclusions.