CALCIUM-CARBONATE PRECIPITATION AND PH VARIATIONS IN OIL-FIELD WATERS- A COMPARISON BETWEEN EXPERIMENTAL-DATA AND MODEL-CALCULATIONS

A Peng-Robinson equation of state (PR EOS) gives reliable data for CO2 fugacities in CO2-CH4 gas mixtures over large composition, temperatur e (0<T/degrees C<200) and pressure (0<P-tot/bar<400) ranges. By introd ucing the PR EOS in a scale prediction model to be used for cil field waters, we are able to calculate the amount of CaCO3 precipitated and pH in water solutions. Experimental data at 25 and 50 degrees C at CO2 pressures of 0.5 and 1 bar showed good agreement with calculated pH ( SD = 0.034 pH units) and precipitated CaCO3 (SDrel%approximate to 5). Literature data on pH measurements and solubility of CO2(g) and CaCO3 also agree well with model predictions. Even at 35 degrees C and below and at very high CO2 pressures (300 bar) experimental CO2 solubilitie s agreed with model calculations within a few %. pH measurements under pressures of up to 40 bar CO2 also compare well with model calculatio ns within 0.1 pH units. Experimental data predicted an increase in pH of about 0.2 pH units when pure CO2 was substituted by a mixture of CO 2 and CH4 at a constant partial pressure of CO2. The calculated pH, ho wever, only showed a very small increase owing to the change in the CO 2 fugacity coefficient.