Experimental Determination and Modeling of Bottom-Hole Crude Oil Mixture Liquid Phase Density (Lpd) by Material Balance During Well Fluids Depletion Studies

Abstract

The liquid-phase density behavior of petroleum reservoir fluids under prevailing reservoir conditions, methods of evaluation, and factors affecting petroleum fluid density have been examined. A high-pressure and high-temperature PVT Ruska blind cell volume was calibrated in the temperature and pressure ranges of 730F to 2500F and 15 to 6000 psia. The calibrated cell volume equation was (515.214 + 8.80 x 10-3 T + 8.89 x 10-5 P + 2.30 x 10-8PT). 100.310 cm3 (78.663102g) of bottom-hole oil sample was charged into the cell at 5000 psi and 73 °F, while the cell was heated to the study temperature of 1540 °F in a thermostatic silicone oil bath. A pressure density run was conducted immediately after charging the sample for study. Pump manifold compressibility was determined at 730F with a pressure range of 15 to 8000 psia to establish uniform pump manifold compressibility and to eliminate errors in pressure-volume measurements. The cell temperature was monitored using a precision type J. digital thermocouple, and the equilibrium cell temperature cycle was 154.2°F throughout the experiment. At equilibrium cell temperature, the cell opening pressure was determined at a reference pressure of 2000 psi. The open-air pressure was 274 psi at 1540F. A constant composition expansion was conducted on the fluid sample, where the bubble point pressure and volume were measured to be 2870 psi and 106.743 cm3, respectively. Differential liberation expansion was immediately performed below the bubble point. Liquid phase densities above the bubble point pressure were calculated from the relative volume data obtained from pressure volume measurement, while liquid phase densities below the bubble point pressure during differential liberation were calculated using the mass balance of the initial mass of oil in place. The results showed that the density of the liquid phase at pressures above the bubble point increases with pressure and decreases to a minimum as pressure falls below the bubble point during pressure depletion, and gases liberated from the liquid phase are expelled from the system. The liquid phase undergoes continuous volumetric and compositional changes with enrichment with the heavy fractions, respectively. This contributed to the increasing liquid phase densities as the oil remaining at each differentially liberated step became richer with the heavy (Heptanes plus) fractions. This result was also seen from the gas compositional analysis, as C7+ fractions increased with decreasing pressure.