Three-phase Mixing Cell Method for Gas Flooding

Open Access
Author:
Li, Liwei
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 20, 2012
Committee Members:
  • Russell Taylor Johns, Thesis Advisor
  • Li Li, Thesis Advisor
  • Luis F Ayala H, Thesis Advisor
Keywords:
  • Three-hydrocarbon-Phase equilibrium
  • Pressure of an efficient displacement
  • mixing cell method
Abstract:
CO2 miscible gas flooding is a major source of oil production in the U.S., particularly in West Texas where natural sources of CO2 can be obtained at reasonable cost. Low-temperature oil displacements by CO2 involve complex phase behavior, where three hydrocarbon-phases can coexist. Reliable design of miscible gas flooding requires knowledge of the minimum miscibility pressure (MMP), which is the required pressure for high displacement efficiency. A recent published study on MMP calculation by a new mixing cell method proved to be robust and reliable for two-phase flash calculations. Unlike previous two-phase “mixing cell” methods, this new method relies entirely on robust P-T EOS flash calculation. Previous computational aspects of MMP estimation do not attempt to estimate the MMP for mixtures exhibiting three hydrocarbon-phases due to lack of robust algorithms for three-phase equilibrium. Further, computation of multicontact miscible gas flooding involves a large number of phase equilibrium calculations in a near-critical region, where the calculations are time-consuming and difficult. Results from experiments and simulation show that more than 95% displacement efficiency can be achieved without the development of complete miscibility for three-phase flow for low-temperature oil displacements by CO2. Recent investigation on the mechanism for high displacement efficiency improved our understanding of the effect of a third hydrocarbon-phase using the robust three-phase equilibrium algorithms developed. The mechanism for high displacement efficiency occurs when critical endpoints (CEP) are encountered at the leading and trailing edges of the three-phase region. How miscibility is developed for three-phase displacements is not well known. A potential solution could be achieved by developing a new mixing cell model by using a similar methodology as the recently published two-phase mixing cell method. We develop a three-phase mixing cell method involving two- and three-phase behavior, which is based on robust phase equilibrium calculation algorithm. By performing cell-to-cell contacts, the mixing-cell model determines the minimum tie-line lengths and minimum tie-triangle lengths developed at a certain pressure. These tie-line lengths are used to determine the MMP by extrapolation to an infinite number of contacts. For three-hydrocarbon-phase displacement, an inaccurate prediction of injection pressure may have significant consequences. For example, if the injection pressure is relatively high, although the displacement efficiency may be satisfactory, leads to an unnecessarily large cost of pressurizing the injected gas. If the injection pressure is low, however, the miscible displacement process may become ineffective and less oil recovery would be achieved. We demonstrate the importance of considering three-hydrocarbon phases, where the MMP assuming a two-phase approximation can result in unnecessary increase of injection pressure without a corresponding increment on oil recovery. The novel three-phase mixing cell method can be used to predict the pressure for an efficient displacement. We further show that there may be no MMP in some displacements where three hydrocarbon-phases exist.