Nitrogen Miscible Displacement of Light Crude Oil: A Laboratory Study

Abstract

Summary. A comprehensive laboratory study of N2 miscible flooding forenhanced recovery of light crude oil was performed. The minimum miscibilitypressure (MMP) of N2 is a major constraint to its EOR application, so anempirical correlation for MMP estimation was developed and found to bereliable. Supporting work included many in-house slim-tube displacementdeterminations of MMP and the compilation and analysis of previouslypublished data. The reservoir fluid composition, especially the amounts ofthe methane and ethane-through-pentane fractions, was found to be the majordetermining factor for miscibility. High-pressure coreflooding tests withsandstone cores were performed to evaluate the effects of gravity stabilityand injection mode on the N2 miscible process. N2-gas miscible floodingsuccessfully recovered most of the oil from laboratory cores. Gravity-stable and gravity-unstable displacements gave different oilrecoveries, as did secondary and tertiary N2 displacements. Introduction N2 has been successfully used as the injection fluid for EORand widely used in oilfield operations for gas cycling, reservoirpressure maintenance, and gas lift. The costs and limitations onavailability of natural gas and CO2 have made N2 an economicalalternative for oil recovery by gas miscible displacement.N2 is usually cheaper than CO2 or hydrocarbon-gasdisplacement in EOR applications and is not corrosive. Reservoirs in whichmiscible N2 injection is being used include Jay field, FL (Exxon)and Painter field, WY (Chevron). Successful miscible N2injection was also performed in East Binger field, OK (Phillips) andLake Barre field, LA (Texaco). The conditions that favormiscibility of crude oils with N2 include relatively high reservoirpressures and light or volatile oils rich in light and intermediatehydrocarbon (C2 through C5) components. Reservoirs that fit theseconditions must be deep enough for the producing formation towithstand the high pressures required to achieve miscibility. This paper presents the results of a comprehensive laboratorystudy of N2 miscible flooding for enhanced recovery of light oil. Slim-tube displacement tests and coreflooding tests were performedwith the oil to determine the displacement mechanisms. Animportant screening factor for the use of N2 in EOR is the minimumpressure for N2 to achieve miscibility with the crude oil througha multiple-contact process in porous media. Determination of theMMP of N2 with the oil is necessary to ensure operation of amiscible flood. The available literature data on the MMP of N2 withcrude oils and synthetic oils are scarce; therefore, systematicslim-tube tests were conducted to determine the MMP for N2 miscibledisplacement of candidate oils. The tests determined the MMPof the different oils and the effects of temperature, reservoir fluidcomposition, and pressure on miscibility. The MMP data generated from thisstudy and MMP data published by others were used to correlate thesevariables. A new empirical correlation for estimating the MMP for N2 withlight oils was developed, tested, and found to be reliable. Coreflooding tests of the N2 miscible EOR process were conducted at highpressures 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 testingthe effect of several variables. Few N2 miscible coreflooding experimentshave been reported by others, so one objective of this work was todetermine the displacement efficiency of the N2 miscible process inexperiments with laboratory cores. Other objectives were to test theeffects of gravity stability and the differences between secondary andtertiary N2 injection. An oil-saturated slim tube was added before thecore to generate a miscible transition zone before the injected N2 enteredthe cores. The N2/Lake Barre reservoir oil system previously studied inslim-tube MMP determinations and vapor/liquid equilibrium tests was chosenfor the coreflood experiments. All floods were conducted at 6,000 psi [41.4MPa] backpressure at 225F [107C]. By using the same core, fluids, temperature, pressure, and displacement rate for all corefloods, we coulddetermine the effects of different injection mode and gravity stability. MMP Determination Slim-tube displacement tests are commonly used for determiningMMP. No standard has been agreed on for the apparatus andtesting procedure. The length and diameter of the slim tube and thepacking material vary. Orr et al., reported a variety ofcharacteristics of slim-tube experiments. Nouar and Rock reported thatthe length and injection rate will affect oil recovery. In previoustests, we found, as they did, that increasing tube length increasedoil recovery for miscible displacements but not for immiscible cases. Furthermore, increasing the injection rate decreased the recoveryfrom an immiscible flood without affecting the recovery from amiscible flood. Thus, increasing both tube length and injection rateresulted in a more obvious inflection point on the recovery-vs.-pressurecurve. In this research, a 120-ft [36.6-m]-long slim tubewith 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 absolutepermeability of 7 darcies. The system was designed for a maximum operatingpressure of 10,000 psi [68.9 MPa] and a temperature of 300F [149C]. Fig. 1 shows the experimental apparatus used for the slim-tube displacementtests. N2 injection rate was 48 cm3/h at the pump (at room temperature). The actual injection rate at the experimental temperature was higherowing to the thermal expansion of the gas as it entered the oven. Because some of the light crude oil used in these experimentswas translucent and only slightly yellowish, the interface betweenthe displacing gas and the displaced oil was not clearly visible inthe visual cell. Therefore, distinguishing between "miscible" and"immiscible" in the transition zone was not possible by visual-cellobservations. The MMP was therefore determined from a plot ofrecovery vs. pressure like that shown in Fig. 2. The MMP wasdefined as the pressure at which the recovery-vs.-pressure curveshows a sharp change in slope (the inflection point). Note that therecovery at 1.2 PV injection is above 95% of the original oil inplace (OOIP). Displacement tests were conducted in the slim tube with threelive 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'sof 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. Eachrecombined oil was tested at 225, 279, and 300F [107, 137, and 149C] andat pressures from its bubblepoint to 10,000 psi [68.9MPa]. Fig. 2 shows the determination of the MMP for each oilat 279F [137C]. The MMP for stock-tank oil without solutiongas is extremely high, but the MMP decreases with an increase inGOR. On the other hand, the bubblepoint pressure of oil increaseswith the increase of GOR. The bubblepoint pressure is the lowerboundary of the MMP because oil at pressures below the bubblepointbecomes two-phase. The MMP's for the Lake Barre oil at threedifferent temperatures, as well as the bubblepoint pressures, areplotted vs. solution GOR in Fig. 3. When CO2 is used as the displacing gas, the MMP is stronglytemperature dependent. SPERE P. 100⁁