Displacement Characteristics of Nitrogen Flooding vs. Methane Flooding in Volatile Oil Reservoirs
- D.M. Boersma (Delft U. of Technology) | J. Hagoort (Delft U. of Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1994
- Document Type
- Journal Paper
- 261 - 265
- 1994. Society of Petroleum Engineers
- 4.6 Natural Gas, 5.2 Reservoir Fluid Dynamics, 5.5 Reservoir Simulation, 5.2.2 Fluid Modeling, Equations of State, 5.4.2 Gas Injection Methods, 5.2.1 Phase Behavior and PVT Measurements
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A comparative study has been carried out of methane and nitrogen injectioninto a volatile oil reservoir.
The study is based on the analysis of the phase behaviour, compositionalreservoir phase behaviour, compositional reservoir simulations and slim tubeexperiments. The main conclusions of the study are:
1. The minimum miscibility pressure for nitrogen and methane injection intovolatile oil reservoirs is the same. This means that, under idealdispersion-free conditions, the microscopic displacement efficiency for bothprocesses, at pressures above the MMP, is 100 percent.
2. Under non-ideal conditions, displacements above the MMP can not achievemultiple contact-miscibility. Methane injection shows a better displacementefficiency than nitrogen. This is because of the more favourable phasebehaviour characteristics of methane-oil systems.
3. The concept of minimum multiple-contact miscibility pressure has littlepractical value as a screening parameter for EOR processes.
High-pressure natural gas injection could be an attractive EOR process fordeep, high-pressure, volatile oil reservoirs, as found, for example, in theNorth-Sea area. The displacement of volatile oils by natural gas is usuallymiscible or near-miscible, which results in excellent microscopic sweepefficiencies. Natural gas, however, is not always available in sufficientquantities. Even if it is, it is usually committed for sales gas, particularlyin areas with an extensive natural gas market.
An often proposed alternative to natural gas injection is nitrogeninjection. Nitrogen can be made available at virtually any location atrelatively low costs. The phase behaviour of nitrogen-volatile oil systems,however. is quite different from that of natural gas-volatile oil systems. Forexample, nitrogen becomes miscible with volatile oils at much higher pressuresthan natural gas. The question is, therefore, how good an alternative nitrogeninjection really is.
In this paper we present the results of a comparative study of nitrogenversus methane injection into volatile oil reservoirs. In this study we haverestricted ourselves to the effect of phase behaviour on the displacementefficiency. As a prototype oil we have used a simple synthetic three-componentoil consisting of methane, butane and tetradecane, with properties very similarto a real volatile oil. properties very similar to a real volatile oil. As forthe reservoir conditions, we have assumed typical North-Sea conditions.
The study is based on the analysis of the phase behaviour of oil-injectiongas systems with phase behaviour of oil-injection gas systems with anequation-of-state (EOS) program, slim tube experiments and compositionalreservoir simulations. The results of this study may find application in thescreening of EOR processes, particularly high-pressure gas injection, and inparticularly high-pressure gas injection, and in the design and interpretationof high pressure gas injection projects.
The prototype oil
The prototype oil we have created is a simple three-component oil made up ofthe pure components methane, butane and tetradecane, representing the light,intermediate and heavy fractions of a real oil. The mole fractions of thesecomponents have been chosen such that the bubblepoint pressure and gas/oilratio (GOR) of the synthetic mixture are representative of a typical volatileoil under North Sea conditions.
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