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ABSTRACT
Liquid phase density behavior of petroleum reservoir fluids under prevailing conditions of reservoir, methods of evaluation and factors affecting petroleum fluids density have been examined. A high pressure and temperature PVT Ruska blind cell volume was calibrated in the temperature and pressure range of 730F to 2500F and 15 to 6000 psia. The calibrated cell volume equation was ( 515.214 + 8.80 x 10-3 T + 889 x 10-5 P + 2.30 x 10-8PT). 100.310cm3 (78.663102g) of bottom hole oil sample was charged into the cell at 5000psia and 730F while the cell was heated to the study temperature of 1540F in a thermostat silicone oil bath. pressure – density run was conducted immediately after charging the sample for study. Pump manifold compressibility was determined at 730F with 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 where the equilibrium cell temperature cycle was (154±2)0F throughout the experiment. At equilibrium cell temperature, the cell open up pressure was determined at reference pressure of 2000 psia. The open up pressure was 274 psia at 1540F. Constant composition expansion was conducted on the fluid sample where a bubble point pressure and volume were measured to be 2870 psia and 106.743cm3 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 dfferential liberation were calculated using mass balance of initial mass of oil in place. Results showed that, liquid phase density at pressure above the bubble point increases with increase in pressure and decreases towards the bubble point pressure to a minimum and increases gradually and continuously as pressure falls below the bubble point during pressure depletion and gases liberated from liquid phase were 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 differential liberated step becomes richer with the heavy (Heptanes plus) fractions. This result was also seen from the gas compositional analysis as C7+ fractions increase with decreasing pressure.
