Density profiles of brines and other well fluids are critical to determining true downhole static pressures during completion, workover, and testing operations. Low-temperature conditions in deepwater applications and elevated temperatures and pressures in high-temperature/high-pressure (HTHP) wells, in particular, can distort predictions if pressure-volume-temperature (PVT) characteristics are not properly considered. The simple, yet useful, PVT method widely used in the industry for heavy completion brines typically works well; however, non-linear behavior diminishes accuracy for lower-salinity brines. In addition, the current industry database is relatively small and does not cover the glycol blended fluids used for thermodynamic hydrate protection in deepwater wells.
Presented in this paper are new PVT measurements made on a range of selected completion brines with nominal densities from 8.345 to 14.4 lbm/gal. Also included is an improved model that addresses non-linear behavior and permits more accurate determination of density profiles under downhole conditions.
Test fluids include deionized water, seawater, calcium chloride, sodium chloride, calcium bromide, ethylene glycol, propylene glycol, and fresh-water/glycol mixtures. Tests were conducted using a commercial HTHP pycnometer at pressures to 30,000 psig and temperatures from 35°F to 500°F. Regression analyses generated a polynomial equation that provides excellent fits over the wide range of fluids and test conditions, including those exhibiting non-linearity. The complete list of correlation coefficients for the test fluids is included. The new model also is suitable for base oils and synthetics.