Rapid and Simple Capillary-Rise/Vanishing Interfacial Tension Method To Determine Crude Oil Minimum Miscibility Pressure: Pure and Mixed CO₂, Methane, and Ethane

Abstract

We report here a simplification of the capillary-rise/vanishing interfacial tension (IFT) method to measure minimum miscibility pressure (MMP) based on only requiring knowledge of when the interfacial tension approaches zero. Simply measuring the height of the crude oil in a capillary at several pressures from ambient to near the MMP pressure and extrapolating the oil height versus pressure plot to zero oil height yields the MMP without the need of the additional instrumentation and labor required to perform actual IFT measurements. A total of 2–4 MMP values can be determined per day with only one experimental apparatus, and the method greatly reduces the initial cost and complexity of the required instrumentation. The use of three capillaries having different inner diameters allows for triplicate determinations of MMP from each experiment. Because the actual MMP pressure need not be reached during the experiment, MMP values that exceed the pressure ratings of the equipment can be reasonably estimated (e.g., MMPs using pure nitrogen). The method was used to determine the MMP pressure for crude oil samples from a conventional Muddy Formation reservoir in the Powder River Basin [American Petroleum Institute (API) gravity of 35.8°] and an unconventional Bakken Formation reservoir in the Williston Basin (API gravity of 38.7°). The method is reproducible [typically <4% relative standard deviation (RSD)], and the method gave good agreement for a “live” Bakken oil with the results from a slim tube test of a commercial laboratory. Approximately 80 MMP values were measured using pure CO2, methane, and ethane as well as 0–100% mole ratios of methane/CO2 and methane/ethane. For both oil samples, ethane MMPs were ca. one-half those with CO2, while methane MMPs were ca. double or triple those with CO2. MMPs with mixed methane/CO2 showed a linear increase with mole percent methane for both crude oils, while both oils showed an exponential increase in MMP with mole percent methane in ethane, with a little increase in MMP until ca. 20 mol % methane in ethane.

Molecular dynamics simulation to estimate minimum miscibility pressure for oil with pure and impure CO2

“…MMPs of CO 2 -Bakken oil system predicted by MD simulation under different temperature are listed in table 1 (IFTs as a function of pressure are shown in figure S2). The MMP of CO 2 -Bakken live oil system determined by MD modeling agrees well with the slim tube test result at 129°C [7], and MMPs of CO 2 -Bakken dead oil system from this study are consistent with the VIT method data at two different temperatures [7]. Furthermore, it can be seen that the MMP increases with the temperature increasing for CO 2 -crude oil system, which has been reported by previous work [40][41][42].…”

“…Figure 4 displays the effects of injected gas components on the CO 2 -crude oil MMP (as listed in table S1, the crude oil is a mixture of C 6 , C 10 , C 19 and C 30 ). The MMP of CO 2 /C 1 mixture with crude oil at various C 1 mole fractions in injected gas phase at 110°C (Figure 4(a)) shows that the MMP of this system increases nearly linearly with C 1 mole fraction rising, in agreement with the experimental results [7]. Figure 4(b) illustrates the MMP of CO 2 /N 2 mixture with crude oil as a function of N 2 mole fraction at 110°C, and demonstrates an increase in the MMP with increasing N 2 mole fraction.…”

“…Then Bakken oil with multiple components [7] was chosen as crude oil to validate our method for estimating the MMP. Oil components and their number of molecules in MD simulation are listed in table S1.…”

“…Thus, it is very crucial to know the MMP of CO 2 -oil system before employing CO 2 injection technique. Furthermore, nitrogen and methane are common impurities in CO 2 injection; hence, the MMP for oil with CO 2 , nitrogen, methane and their mixture also are important [6][7][8].…”

“…However, the pendant drop approach fails at low IFTs since it is difficult to form pendant drops [12]. The capillary rise approach needs to measure the density of two phases, the contact angle with capillary tube, and the accurate inner diameter, which requires high precision equipment and makes it complex, and this method has difficulty to measure the IFTs of CO 2 -live oil system since it is easy to form bubbles [7]. Besides experimental methods, some researchers [18][19][20][21] used parachor model to calculate IFTs, which requires the equation of state (e.g., Peng-Robinson equation of state [22] for vapor-liquid equilibria calculation).…”