Miscible Displacement: Recovery Tests With Nitrogen

Abstract

Summary High-pressure slim-tube tests and phase-behavior studies were performed with reservoir fluid and three component hydrocarbon mixtures. The amount of methane and intermediates in the reservoir fluid were varied while other parameters that affect the oil recovery were kept constant. The effects of displacement velocity, temperature, and length of the packed column on slim-tube recovery were also investigated. The results obtained show that the amount of methane and intermediates in the reservoir fluid has a significant effect on minimum miscibility pressure (MMP). The results suggest very strongly that a reservoir fluid with a low methane content needs a long path length to develop miscibility with N2 (a large fraction of the oil is bypassed in the first part of the column). The slim-tube oil recovery, and hence the average microscopic displacement efficiency of the process, is shown to increase significantly with slim-tube length when injection pressure and other variables are kept constant. The results from simulation tests of phase behavior and fluid flow in the slim tube confirmed some of the conclusions made from the experimental results obtained in this work. An N2 MMP correlation was developed from displacement test data reported in the literature and in the present study. Introduction Two recovery mechanisms are generally considered with N2 injection: miscible and immiscible displacement. In the miscible process, increased recovery is caused by transfer of light components from the oil into the gas phase, where the resulting gas develops miscibility with the oil. This process is characterized by an absence of a discrete fluid/fluid interface between the injected gas and the reservoir fluid. The multiple-contact situation is described best by use of a ternary diagram (Fig. 1). With N2, we will have a vaporizing-gas process. The extractable components are the C1 through C6 fraction of the oil. Multiple contact of gas with oil will continue until a sufficient amount of C1 through C6 has been vaporized into the N2 to beach the critical point on the phase envelope. Miscibility is attained if the composition of the reservoir fluid lies on or to the right of the limiting tie-line for a given reservoir pressure. Thus, if N2 is the injection gas, the concentrations of methane and the intermediate components in the oil are important factors to obtain miscibility between the oil and gas. The phenomenon of miscibility between N2 and a hydrocarbon multicomponent system is complex. Most of the literature data related to this subject are on three-component hydrocarbon systems. Only a few papers are available on high-pressure N2 gasflooding of reservoir fluids. In 1958, Koch and Hutchinson performed laboratory studies on miscible displacement of oil with N2. They found that a high methane concentration in the reservoir fluid improved the attainment of vaporizing-gas-drive miscibility with N2, and speculated that the presence of methane in the reservoir fluid is helpful in volatilizing the C2 through C6 fraction of oil into the gas. Rushing et al. conducted displacement tests of light oils with N2. They concluded that oil from deeper, high-temperature reservoirs may undergo miscible-type displacement with N2 because the crude oil has a substantial quantity of intermediates. In 1983, Alcocer et al conducted displacement tests of a 0.81-g/cm3 [43 API] -gravity crude recombined with natural gas. They studied the effect of temperature and the amount of gas in solution on oil recovery with N2 and showed that the oil recovery increased with temperature. They also concluded that the distance to form a miscibility zone slightly decreases with temperature and gas in solution. This work is a continuation of the investigations made in Refs. 1 through 3. The expected results from the study were (1) criteria to define miscibility of reservoir oil with N2 and (2) an MMP correlation with N2.