PHASE BEHAVIOUR AND ASPHALTENE ONSET PRESSURE PREDICTION DURING GAS INJECTION IN OIL RESERVOIRS

Abstract

Asphaltene Precipitation (AP), a major challenge in the petroleum industry, causes flow assurance issues in reservoirs, pipelines and flow lines. It is therefore important to properly characterise petroleum fluids and predict the conditions under which AP could be prevented. Many of the reported models used for predicting AP to achieve best reservoir and production management practices are rather complex for routine applications. This study was designed to develop a simple but accurate model for characterising petroleum fluids and predicting AP during gas injection into oil reservoirs.

A generalised model for predicting phase behaviour of multiphase systems having different energy states was derived using partition function principle. The model obtained was simplified for routine application to a three-parameter cubic Equation of State (EOS), which was used to characterise selected pure gases, binary mixtures, four crude oil samples (F1, F2, F3 and F4) and n-Pentane+Violanthrone-97 simulated oil (F5), to obtain pressure-temperature phase profiles. The Upper Asphaltene Onset Pressures (UAOP) were estimated under natural gas (0, 15, and 30wt %); carbon dioxide (CO2) gas (10 and 20wt %) and nitrogen gas (N2) injections (5wt %). Results were validated using published data, and compared with Soave-Redlich-Kwong (SRK), Peng-Robinson (PR) and the Perturbed Chain Statistical Associating Fluid Theory (PCSAFT) EOS. Data were analysed using ANOVA at α0.05.

The developed EOS accounted for the contributions of attractive, repulsive, and non-physical forces in the fluids. Predicted Pressure-Temperature profiles for pure gases compared well to SRK and PR, and matched experimental data with percentage error of 0.04%, 0.13% and 0.05% for nitrogen, carbon dioxide and butane, respectively. The phase envelopes generated for ethanol-benzene, benzene-hexane, hexane-ethanol and methane-propane also matched experimental data. Predicted pressure-temperature envelopes for the crude oils, gave an average deviation of ±0.03%, while the Pentane+Violanthrone-97 rich phase was stable between 16.62 and 65.95kN/m2 at 298K and 65.86 and 241.15kN/m2 at 338K. These predictions matched experimental data and the results obtained using PCSAFT EOS. Estimated UAOP were 30812.93, 38369.75 and 84947.83kN/m2 for F1 at 0, 15, and 30 wt.% natural gas injection, respectively; 52674.81, 59967.88 and 63829.68kN/m2 for F2 at 0, 10, 20 wt.% CO2 gas injection, respectively and 38783.64 kN/m2 at 5 wt.% nitrogen gas injection. For F3 and F4 at no gas injection, predicted UAOP were 117094.58 and 63326.02 kN/m2, respectively; while 19788.77 and 49051.43kN/m2 were predicted for F5 at 10 and 20 wt. % CO2 gas injections, respectively. Generally, UAOP increased with increasing proportion of injected gases and varied with reservoir fluid and type of injected fluid. There was no significant difference between predicted results and those obtained using the PCSAFT EOS.

A simplified model for characterising petroleum fluids and predicting Asphaltene precipitation was developed. The model achieved faster and better control of Asphaltene precipitation during gas injection operations in oil reservoirs.