Four-Phase Flash Calculation Algorithm Based on the Free-Water Assumption

Sirui Li

doi:10.1021/acs.iecr.1c04912

2022-03-07

PRE

DOI：10.1021/acs.iecr.1c04912

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摘要

En 中文

An aqueous phase, which exists due to the presence of the original water saturation or appears due to the use of secondary and tertiary recovery methods (such as water flooding and water-alternating-gas process), is inevitable in the pore spaces of an oil reservoir. With the existence of an aqueous phase and multiple hydrocarbon phases, up to a total of four phases can coexist at equilibrium under reservoir conditions. To reduce the computational complexity of the four-phase equilibrium calculation problem, we propose a four-phase flash calculation algorithm based on the free-water assumption (i.e., assuming the aqueous phase contains pure water only). The free-water algorithm uses the successive substitution iteration (SSI) method. It contains an outer loop and an inner loop. The outer loop is used to update the equilibrium ratios based on the fugacity equations, while the inner loop is used to calculate the phase fractions. Special measures are adopted to enhance the robustness of the developed algorithm. Case studies involving two fluid mixtures, i.e., one n-butane/heavy-oil/water mixture under high-temperature conditions and one CO2/hydrocarbons/water mixture under low-temperature conditions, are conducted to examine the performance of the four-phase free-water flash calculation algorithm. The algorithm works well for both mixtures, without encountering any convergence issues. It is also shown that the proposed algorithm can help reduce the computational cost (up to 41.51%) of four-phase flash calculations. For the n-butane/heavy-oil/water mixture, the four-phase free-water flash algorithm yields an acceptable deviation from the results given by the conventional full flash calculation algorithm. But it is not applicable to the CO2/hydrocarbons/water mixture since CO2 can have a large solubility in the aqueous phase under high pressures.

Keyword:

RACHFORD-RICE

EQUILIBRIUM CALCULATIONS

PHASE-BEHAVIOR

STABILITY

CO2

SIMULATION

OIL