Displacement Characteristics of Nitrogen Flooding vs. Methane Flooding in Volatile Oil Reservoirs

Abstract

Summary A comparative study has been carried out of methane and nitrogen injection in to a volatile oil reservoir. The study is based on the analysis of the phase behaviour, compositional reservoir phase behaviour, compositional reservoir simulations and slim tube experiments. The main conclusions of the study are:The minimum miscibility pressure for nitrogen and methane injection intovolatile oil reservoirs is the same. This means that, under ideal dispersion-free conditions, the microscopic displacement efficiency for both processes, at pressures above the MMP, is 100 percent.Under non-ideal conditions, displacements above the MMP can not achieve multiple contact-miscibility. Methane injection shows a better displacement efficiency than nitrogen. This is because of the more favourable phase behaviour characteristics of methane-oil systems.The concept of minimum multiple-contact miscibility pressure has little practical value as a screening parameter for EOR processes. Introduction. High-pressure natural gas injection could be an attractive EOR process for deep, high-pressure, volatile oil reservoirs, as found, for example, in the North-Sea area. The displacement of volatile oils by natural gas is usually miscible or near-miscible, which results in excellent microscopic sweep efficiencies. Natural gas, however, is not always available in sufficient quantities. Even if it is, it is usually committed for sales gas, particularly in areas with an extensive natural gas market. An often proposed alternative to natural gas injection is nitrogen injection. Nitrogen can be made available at virtually any location at relatively low costs. The phase behaviour of nitrogen-volatile oil systems, however. is quite different from that of natural gas-volatile oil systems. For example, nitrogen becomes miscible with volatile oils at much higher pressures than natural gas. The question is, therefore, how good an alternative nitrogen injection really is. In this paper we present the results of a comparative study of nitrogen versus methane injection into volatile oil reservoirs. In this study we have restricted ourselves to the effect of phase behaviour on the displacement efficiency. As a prototype oil we have used a simple synthetic three-component oil consisting of methane, butane and tetradecane, with properties very similar to a real volatile oil. properties very similar to a real volatile oil. As for the reservoir conditions, we have assumed typical North-Sea conditions. The study is based on the analysis of the phase behaviour of oil-injection gas systems with phase behaviour of oil-injection gas systems with an equation-of-state (EOS) program, slim tube experiments and compositional reservoir simulations. The results of this study may find application in the screening of EOR processes, particularly high-pressure gas injection, and in particularly high-pressure gas injection, and in the design and interpretation of high pressure gas injection projects. The prototype oil The prototype oil we have created is a simple three-component oil made up of the pure components methane, butane and tetradecane, representing the light, intermediate and heavy fractions of a real oil. The mole fractions of these components have been chosen such that the bubblepoint pressure and gas/oil ratio (GOR) of the synthetic mixture are representative of a typical volatile oil under North Sea conditions.