Nitrogen Miscible Displacement of Light Crude Oil: A Laboratory Study
- David A. Hudgins (Natl. Inst. for Petroleum and Energy Research) | Feliciano M. Llave (Natl. Inst. for Petroleum and Energy Research) | Frank T.H. Chung (Natl. Inst. for Petroleum and Energy Research)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1990
- Document Type
- Journal Paper
- 100 - 106
- 1990. Society of Petroleum Engineers
- 3.1.6 Gas Lift, 2.4.3 Sand/Solids Control, 5.3.2 Multiphase Flow, 1.6.9 Coring, Fishing, 5.2.2 Fluid Modeling, Equations of State, 5.4.2 Gas Injection Methods, 4.1.5 Processing Equipment, 4.2.3 Materials and Corrosion, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 5.4.9 Miscible Methods, 5.4.1 Waterflooding, 5.4.3 Gas Cycling, 4.6 Natural Gas, 5.3.4 Reduction of Residual Oil Saturation, 4.3.4 Scale, 4.1.9 Tanks and storage systems, 5.2 Reservoir Fluid Dynamics
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Summary. A comprehensive laboratory study of N2 miscible flooding for enhanced recovery of light crude oil was performed. The minimum miscibility pressure (MMP) of N2 is a major constraint to its EOR application, so an empirical correlation for MMP estimation was developed and found to be reliable. Supporting work included many in-house slim-tube displacement determinations of MMP and the compilation and analysis of previously published data. The reservoir fluid composition, especially the amounts of the methane and ethane-through-pentane fractions, was found to be the major determining factor for miscibility. High-pressure coreflooding tests with sandstone cores were performed to evaluate the effects of gravity stability and injection mode on the N2 miscible process. N2-gas miscible flooding successfully recovered most of the oil from laboratory cores. Gravity-stable and gravity-unstable displacements gave different oil recoveries, as did secondary and tertiary N2 displacements.
N2 has been successfully used as the injection fluid for EOR and widely used in oilfield operations for gas cycling, reservoir pressure maintenance, and gas lift. The costs and limitations on availability of natural gas and CO2 have made N2 an economical alternative for oil recovery by gas miscible displacement. N2 is usually cheaper than CO2 or hydrocarbon-gas displacement in EOR applications and is not corrosive. Reservoirs in which miscible N2 injection is being used include Jay field, FL (Exxon) and Painter field, WY (Chevron). Successful miscible N2 injection was also performed in East Binger field, OK (Phillips) and Lake Barre field, LA (Texaco). The conditions that favor miscibility of crude oils with N2 include relatively high reservoir pressures and light or volatile oils rich in light and intermediate hydrocarbon (C2 through C5) components. Reservoirs that fit these conditions must be deep enough for the producing formation to withstand the high pressures required to achieve miscibility. This paper presents the results of a comprehensive laboratory study of N2 miscible flooding for enhanced recovery of light oil. Slim-tube displacement tests and coreflooding tests were performed with the oil to determine the displacement mechanisms. An important screening factor for the use of N2 in EOR is the minimum pressure for N2 to achieve miscibility with the crude oil through a multiple-contact process in porous media. Determination of the MMP of N2 with the oil is necessary to ensure operation of a miscible flood. The available literature data on the MMP of N2 with crude oils and synthetic oils are scarce; therefore, systematic slim-tube tests were conducted to determine the MMP for N2 miscible displacement of candidate oils. The tests determined the MMP of the different oils and the effects of temperature, reservoir fluid composition, and pressure on miscibility. The MMP data generated from this study and MMP data published by others were used to correlate these variables. A new empirical correlation for estimating the MMP for N2 with light oils was developed, tested, and found to be reliable. Coreflooding tests of the N2 miscible EOR process were conducted at high pressures in a Berea sandstone core 2 in. [5.08 cm] in diameter and 24 in. [60.0 cm] long, which provided a reservoir-like porous medium for testing the effect of several variables. Few N2 miscible coreflooding experiments have been reported by others, so one objective of this work was to determine the displacement efficiency of the N2 miscible process in experiments with laboratory cores. Other objectives were to test the effects of gravity stability and the differences between secondary and tertiary N2 injection. An oil-saturated slim tube was added before the core to generate a miscible transition zone before the injected N2 entered the cores. The N2/Lake Barre reservoir oil system previously studied in slim-tube MMP determinations and vapor/liquid equilibrium tests was chosen for the coreflood experiments. All floods were conducted at 6,000 psi [41.4 MPa] backpressure at 225F [107C]. By using the same core, fluids, temperature, pressure, and displacement rate for all corefloods, we could determine the effects of different injection mode and gravity stability.
Slim-tube displacement tests are commonly used for determining MMP. No standard has been agreed on for the apparatus and testing procedure. The length and diameter of the slim tube and the packing material vary. Orr et al., reported a variety of characteristics of slim-tube experiments. Nouar and Rock reported that the length and injection rate will affect oil recovery. In previous tests, we found, as they did, that increasing tube length increased oil recovery for miscible displacements but not for immiscible cases. Furthermore, increasing the injection rate decreased the recovery from an immiscible flood without affecting the recovery from a miscible flood. Thus, increasing both tube length and injection rate resulted in a more obvious inflection point on the recovery-vs.-pressure curve. In this research, a 120-ft [36.6-m]-long slim tube with 0.203-in. [0.516cm] ID was used for the MMP determination. This tube, packed with 140/200 mesh silica sand, had a porosity of 39% and absolute permeability of 7 darcies. The system was designed for a maximum operating pressure of 10,000 psi [68.9 MPa] and a temperature of 300F [149C]. Fig. 1 shows the experimental apparatus used for the slim-tube displacement tests. N2 injection rate was 48 cm3/h at the pump (at room temperature). The actual injection rate at the experimental temperature was higher owing to the thermal expansion of the gas as it entered the oven. Because some of the light crude oil used in these experiments was translucent and only slightly yellowish, the interface between the displacing gas and the displaced oil was not clearly visible in the visual cell. Therefore, distinguishing between "miscible" and "immiscible" in the transition zone was not possible by visual-cell observations. The MMP was therefore determined from a plot of recovery vs. pressure like that shown in Fig. 2. The MMP was defined as the pressure at which the recovery-vs.-pressure curve shows a sharp change in slope (the inflection point). Note that the recovery at 1.2 PV injection is above 95% of the original oil in place (OOIP). Displacement tests were conducted in the slim tube with three live oils that were recombined from the stock-tank oil (61.5 API [0.733 g/cm3]) from Lake Barre field and solution gas at GOR's of 84, 247, and 564 scf/bbl [15.1, 44.5, and 101.6 std m3/m3]. Table 1 gives the compositions of the oil and solution gas. Each recombined oil was tested at 225, 279, and 300F [107, 137, and 149C] and at pressures from its bubblepoint to 10,000 psi [68.9 MPa]. Fig. 2 shows the determination of the MMP for each oil at 279F [137C]. The MMP for stock-tank oil without solution gas is extremely high, but the MMP decreases with an increase in GOR. On the other hand, the bubblepoint pressure of oil increases with the increase of GOR. The bubblepoint pressure is the lower boundary of the MMP because oil at pressures below the bubblepoint becomes two-phase. The MMP's for the Lake Barre oil at three different temperatures, as well as the bubblepoint pressures, are plotted vs. solution GOR in Fig. 3. When CO2 is used as the displacing gas, the MMP is strongly temperature dependent.
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