Evaluation Of Miscibility From Slim Tube Tests

Abstract

For a technically and economically successful miscible project, it is important that the solvent composition be as lean as possible for a given design pressure or that the operating pressure be as low as possible for a given solvent composition. In applying slim tube testing to assess miscibility, oil recovery by itself has historically been considered a sufficient criterion. This paper emphasizes that analyzing other test data, such as effluent gas compositions and pressure drop, are likely more reliable because there can be significant experimental errors associated with evaluating recovery. It is demonstrated that the ambient laboratory condition effluent gas compositions accurately reflect the solvent/oil mixture phase behaviour. In addition, the slim tube pressure drop data can be used to verify miscibility. Thus, by measuring these parameters, one can readily correlate miscibility conditions. It is also demonstrated that on accurately tuned equation of state together with a reliable prediction technique can significantly reduce the number of slim tube displacement tests required to quantify the miscibility conditions. Introduction Economics and solvent supply dictate the selection of miscible gas as an enhanced oil recovery technique. Miscible solvent design criteria, based mainly on oil recovery levels obtained from slim tube displacement tests, have been described in the literature(1,2) for evaluating miscibility. The purpose of this paper is to emphasize that other measured data such as atmospheric flash condition effluent gas compositions and pressure drops are reliable indicators of miscibility and must be included. There are generally two approaches to miscibility design. The first requires determining solvent compositions for a given reservoir pressure and is typically applied to hydrocarbon-based processes. The second approach is to predict a minimum miscibility pressure for a specific solvent composition. This technique is appropriate when considering carbon dioxide, nitrogen, or methane as solvents. An equation of state (EOS), can be used as a predictive tool for miscibility design(3). To use an EOS, it is required that the equation be tuned to accurately match reservoir gas-liquid phase behaviour. That is, the experimentally measured bubble point pressure, (Pb), gas-oil ratio, (GOR), and atmospheric flashed gas and liquid compositions of either a bottom-hole or a recombined test separator reservoir fluid sample must be accurately represented by an EOS. The EOS can then be readily used as basis to predict miscibility conditions. To verify an EOS prediction, two types of laboratory tests can be conducted: static vapour-liquid equilibria (VLE) measurements and/or dynamic slim tube displacement tests(l). Although interpreting VLE test data is usually straightforward, slim tube displacement tests are often misinterpreted by industry. Hence, it is necessary to review the physics occurring during displacement testing, so as to interpret the data from both the fluid phase behaviour and fluid displacement points of view. Terminology Reference to the ternary phase diagram in Figure 1 enables a review of the terminology. This diagram illustrates the phase behaviour existing at a single temperature and pressure. Components are grouped at the three vertices according to whether they are light ends, intermediates, or heavy ends.

Does Miscibility of In Situ Generated Flue Gases With Light Crude Oils Contribute to Oil Recovery Under High Pressure Air Injection?

“…Similar trend was obtained for flue gas A. The observed C 1 peaks at the leading edge of the transition zone are an indication of immiscibility 28,29 . The displacement front passed through the tube a little earlier at 4028 psi (27.77 MPa) with a higher C 1 peak.…”

“…The profiles thereafter maintained an almost constant concentration level in the transition zone, and then gradually declined behind the front. The banking of C 2 and C 3 observed in the transition zone and the fluid produced from the swept zone is a characteristic of classical vapourizing gas drive process 28 . Similar profiles were obtained for the other intermediate components (C 4 to C 6 ).…”

“…Two-phases were not observed visually in the transition zone, and this is supported by the density profile in this region. The non-observation of a vapour phase in the transition zone could be due to the displacement being dominated by a vapourizing process 28 . The nitrogen displacement experiments were conducted without the benefit of flashed fluid analysis.…”

“…It has been shown that miscibility is not always related to a high microscopic displacement efficiency (oil recovery). When the transition zone needed to obtain miscibility is long compared to the total length of the flow path, the recovery of oil will be relatively low (60 to 80%) 28,33 . Consideration of all aspects of slim-tube performance 28,34 , including the analysis of the effluent gas composition, the flashed liquid composition, as well as the density profile obtained at test conditions, and the pattern of flow observed through the sight glass after gas breakthrough, would go a long way in distinguishing between immiscible, near-miscible, and miscible displacements.…”

Integrating support vector regression with genetic algorithm for CO2-oil minimum miscibility pressure (MMP) in pure and impure CO2 streams

“…Experimental measurements include the use of a slim tube [6], a rising bubble apparatus (RBA) [7], multi-contact mixing-cell experiment [8,9] or the vanishing interfacial tension (VIT) technique [10][11][12][13]. Although necessary, experimental measurements are expensive, time-consuming and cumbersome.…”