Oil Recovery Performance of Immiscible and Miscible CO2 Huff-and

Jan 22, 2014 - Petroleum Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada...
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Oil Recovery Performance of Immiscible and Miscible CO2 Huff-andPuff Processes Ali Abedini and Farshid Torabi* Petroleum Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada ABSTRACT: The recovery performance of immiscible and miscible CO2 huff-and-puff processes for enhanced oil recovery (EOR) in a light crude oil sample was experimentally investigated. The minimum miscibility pressure (MMP) of the original light crude oil−CO2 system was determined by means of the vanishing interfacial tension technique and found to be MMP = 9.18 MPa. Then, the solubility of the CO2 in the light crude oil and oil swelling factor due to the CO2 dissolution in the oil phase were determined at T = 30 °C and various equilibrium pressures ranging from atmospheric pressure to Peq = 12.55 MPa. Later, series of immiscible and miscible CO2 huff-and-puff tests were designed and carried out at various operating pressures (i.e., Pop = 5.38− 10.34 MPa). The results of the experiments showed that for secondary CO2 huff-and-puff tests performed at the operating pressures below the MMP, the ultimate oil recovery factor is quite low. It was also found that in immiscible CO2 huff-and-puff (i.e., Pop < MMP) scenarios, the oil recovery factor substantially increased as the operating pressure approached near-miscible conditions. The oil recovery factor almost reached its maximum value at operating pressure near MMP (i.e., miscible condition), and further increase of operating pressure beyond MMP did not improve the recovery factor at all. The tertiary mode of miscible CO2 huff-and-puff was also examined, and it was revealed that the oil recovery is significantly improved after a waterflooding process. The oil recovery mechanisms during the CO2 huff-and-puff were mainly recognized to be interfacial tension reduction, oil swelling, and extraction of lighter components by CO2, especially during miscible CO2 injections. In addition, the average asphaltene content of produced oil and the permeability reduction of the porous medium as a result of asphaltene precipitation were measured in each test. It was found that the amount of precipitated asphaltene in the porous medium as well as permeability reduction are considerably higher in near-miscible and miscible CO2 huff-and-puff tests compared to those in immiscible cases. The compositional analysis of remaining oil from CO2 huff-and-puff tests at immiscible and miscible conditions also showed that lighter components of oil are extracted by CO2, leading the remaining oil to become heavier with greater amounts of heavy hydrocarbons (i.e., C30+). However, it was observed that the extraction of lighter components during miscible injection processes is more predominant than that during immiscible injections, resulting in the production of higher quality oil.

1. INTRODUCTION CO2 huff-and-puff process, which is also known as cyclic CO2 injection, has been investigated through experimental and simulation studies as well as field tests as an enhanced oil recovery (EOR) technique for the past three decades. Cyclic CO2 injection was initially proposed as an alternative to cyclic steam stimulation, and it was found that the cyclic CO2 injection process has wider applications in light-oil reservoirs.1 In this technique, after the injection of CO2 into the reservoir, the well would be shut in for a predetermined period of time (i.e., soaking period) depending on the reservoir conditions (i.e., reservoir rock and fluid properties).2,3 Then the oil production initiates by converting the injection well to a producer. The injected CO2 has the ability to change and modify the reservoir rock and fluid properties, ending up enhancing the oil recovery process.4,5 Mechanisms contributing to increased oil recovery in the huff-and-puff process include oil viscosity reduction, oil swelling due to dissolution of gas in crude oil, solution gas drive aided by gravity drainage in thick reservoirs, vaporization of lighter components of oil, interfacial tension reduction, and relative permeability effects.5−8 Black oil simulation of cyclic CO2 stimulation in low-pressure gas solution drive wells shows that relative permeability hysteresis and reservoir pressure increase are the main mechanisms during the CO2 huff-and-puff process.4 Some © 2014 American Chemical Society

studies on performance of CO2 huff-and-puff in shallow light oil depleted reservoirs also indicate that oil swelling and viscosity reduction effects combined with changes in gas−oil relative permeabilities resulted in improvement of oil recovery.5−7 It has also been reported that the CO2 huff-and-puff process benefited from the presence of gas cap, gravity segregation, and higher remaining oil saturation.9−11 In addition, the effect of injected CO2 volume or slug size on the oil recovery during CO2 huffand-puff has been investigated, and it has been observed that higher CO2 volume injected into the reservoir is able to recover more oil.10−14 The performance of CO2 huff-and-puff in fractured systems has also been studied to some extent, and it is found that gravity drainage can significantly increase oil recovery factor after a waterflooding process.15 Moreover, experimental and simulation studies on the miscible and immiscible CO2 huff-and-puff in matrix-fractured systems also show that higher matrix permeability as well as presence of connate water saturation result in substantial oil recovery, particularly in immiscible CO2 injection scenarios.16,17 Received: July 17, 2013 Revised: January 20, 2014 Published: January 22, 2014 774

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Figure 1. Compositional analysis of the original light crude oil sample at atmospheric pressure and T = 21 °C (ρoil = 802 kg/m3; μoil = 2.92 mPa s; MW = 223 g/mol; n-C5 insoluble asphaltene content = 1.23 wt %).

In this study, the oil recovery performance of the CO2 huffand-puff process in a light oil system was investigated through immiscible and miscible CO2 injections. First, a phase behavior study was performed on the crude oil−CO2 system in order to determine the CO2−oil interfacial tension (IFT), CO2 solubility in the original crude oil, and oil swelling factor as a result of CO2 dissolution into the oil phase. Then, a series of CO2 huff-andpuff tests as a secondary and tertiary EOR schemes was designed and carried out at various operating pressures ranging from immiscible conditions (i.e., Pop < minimum miscibility pressure (MMP)) to above miscible conditions (i.e., Pop > MMP) at constant temperature of T = 30 °C. In addition, the amount of asphaltene precipitation in the porous medium as well as subsequent permeability reduction of the system were determined. Stage and cumulative recovery factors, producing gas−oil ratio (GOR), and gas utilization factor (GUF) were also calculated during each CO2 huff-and-puff test. Furthermore, compositional analysis on the remaining crude oil in the core after the huff-and-puff test was conducted to examine and analyze the recovery mechanisms of CO2 huff-and-puff under immiscible and miscible conditions.

Table 1. Measured Viscosity Values of the Crude Oil at Atmospheric Pressure and Various Temperatures temperature (°C)

crude oil viscosity (mPa s)

21 25 30 35 40 45

2.92 2.76 2.64 2.57 2.51 2.39

in-place significantly affects the performance of EOR techniques. Various studies have suggested that low IFT between the injected fluid and reservoir oil can improve sweep efficiency and reduce residual oil saturation.18,19 In CO2-based EOR techniques, at specific thermodynamic conditions (i.e., pressure, temperature, and composition), the IFT of the crude oil−CO2 system decreases to sufficiently low values which leads to a more favorable displacing process.18 In this study, the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case was applied to determine the IFT between the crude oil and CO2.20 Figure 2 shows a schematic diagram of the experimental setup used for measuring the equilibrium IFT of the crude oil−CO2 system at various equilibrium pressures and constant temperature (T = 30 °C). First, the see-through windowed high-pressure IFT cell (Temco Inc.) was heated to the specific experimental temperature of T = 30 °C and then filled with the CO2 at a prespecified equilibrium pressure. Afterward, the crude oil was introduced to the IFT cell through a stainless steel syringe needle which was installed at the top of the IFT cell. Once a well-shaped pendant drop was formed at the tip of the syringe needle, the appropriate sequential digital images of the dynamic pendant oil drop at different times were acquired. Finally, the ADSA program for the pendant drop case was executed to determine the equilibrium IFT between the oil and CO2 at each prespecified pressure and temperature of T = 30 °C. 2.3. CO2 Solubility and Oil Swelling Factor Measurement. Solubility of CO2 in the crude oil is a key parameter affecting the performance of CO2-based EOR processes. Several studies have been conducted and various methods have been proposed to measure and model this parameter for various types of crude oil.21−24 The amount of CO2 solubility in the crude oil does directly have influence on the oil swelling factor, oil viscosity, oil density, and oil−CO2 interfacial tension. In addition, swelling of the oil as a result of dissolution of CO2 is one of the main mechanisms contributing to oil production during

2. EXPERIMENTAL SECTION 2.1. Materials. The crude oil under study was a mixture of various samples from different locations of the Bakken oil field in South Saskatchewan, Canada. The density and viscosity of the original crude oil sample at T = 21 °C and atmospheric pressure were measured to be ρoil = 802 kg/m3 and μoil = 2.92 mPa s, respectively. The n-pentane (nC5) insoluble asphaltene content was determined using the standard ASTM D2007-03 method and found to be 1.23 wt %. The compositional analysis of the original sample crude oil is presented in Figure 1. A DV-II+Viscometer (Can-AM Instruments Ltd.) was used to measure crude oil viscosity at various temperatures (Table 1). A synthetic brine with 2 wt % NaCl concentration, density of ρw = 1001 kg/m3 and viscosity of μw = 0.98 mPa s at T = 21 °C and atmospheric pressure was also used as a representative of reservoir brine in this study. CO2 with a purity of 99.99%, supplied by Praxair, was used as the injected solvent in huff-and-puff tests. 2.2. Crude Oil−CO2 Interfacial Tension Measurement. Interfacial tension (IFT) between an injected phase and reservoir oil 775

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Figure 2. Schematic diagram of the experimental setup used for measuring the equilibrium IFT for crude oil−CO2 system at various equilibrium pressures and temperature of T = 30 °C.

Figure 3. Schematic diagram of the experimental setup used for CO2 solubility and oil swelling factor measurements at various equilibrium pressures and temperature of T = 30 °C. pressure was considered as an equilibrium pressure (Peq) of the system. Lastly, initial and final CO2 volume in visual cell were determined by taking photos and utilizing image analysis technique. Throughout this study, the solubility of CO2 in the oil (χCO2) was defined as the ratio of

CO2-based EOR processes, particularly when applied to light oil reservoirs.25,26 Figure 3 depicts the schematic diagram of experimental apparatus for determining the CO2 solubility in the crude oil and the resulting oil swelling factor at T = 30 °C. The apparatus mainly consisted of a seethrough windowed high-pressure cell (Jerguson Co.), a magnetic stirrer (Fisher Scientific), and a high-pressure CO2 cylinder. A temperature controller (Love Controls Co.) was also used to control the experimental temperature and maintain it at a constant value. The cell was charged with a specific volume of crude oil sample (i.e., Vo,i = 25 cm3). The magnetic stirrer was used to create a consistent turbulence inside the cell. The produced turbulence significantly accelerated the CO2 dissolution into the oil by creating convective mass transfer. During the process, the pressure inside the see-through windowed cell was measured and recorded using a digital pressure gauge (Ashcroft Inc.). Once the visual cell was pressurized with CO2 to a prespecified pressure (Pi), the pressure of the cell was allowed to stabilize while CO2 was dissolving into the crude oil. The test was terminated when the final CO2 pressure (Pf) inside the cell reached a stable value and no further pressure decay was observed. The final

the total mass of dissolved CO2 in 100 gr of the original crude oil sample and was calculated using the mass balance equations as given by the following relationships:

mCO2,dissolved = mCO2,i − mCO2,f ⎛ PV ⎛ Pf VCO2,f MWCO2 ⎞ i CO2 ,i MWCO2 ⎞ =⎜ ⎟−⎜ ⎟ Z iRT Zf RT ⎝ ⎠ ⎝ ⎠ =

moil = (ρoil Voil) 776

⎛ Pf VCO2,f ⎞⎤ MWCO2 ⎡⎛ PV i CO2 ,i ⎞ ⎢⎜ ⎟−⎜ ⎟⎥ RT ⎢⎣⎝ Z i ⎠ ⎝ Zf ⎠⎥⎦

(1) (2)

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Figure 4. Schematic diagram of the experimental setup used for CO2 huff-and-puff tests conducted at various operating pressures and temperature of T = 30 °C.

χCO =

mCO2,dissolved moil

2

=

Table 2. Properties of the Core Sample and Core Holder Used for CO2 Huff-and-Puff Tests

× 100

⎛ Pf VCO2,f ⎞⎤ MWCO2 ⎡⎛ PV i CO2 ,i ⎞ ⎢⎜ ⎟−⎜ ⎟⎥ ρoil VoilRT ⎢⎣⎝ Z i ⎠ ⎝ Zf ⎠⎥⎦

(3)

core sample core holder

The swelling factor (SF) of the oil due to the dissolution of CO2 at any specific operating condition was also determined by the ratio of the final volume of the oil to its initial value at the beginning of the experiment:

SF =

porosity (%)

height (cm)

diameter (cm)

pore volume (cm3)

70.8

18.5

30.21

5.05

111.82

−

−

35.59

6.12

1046.94

connected to the end of the core holder. Prior to each experiment, the core was cleaned, vacuumed, and completely saturated with the brine. During the brine saturating process, the brine injection flow rate was varied in the range of qw‑inj = 0.25−2 cm3/min to determine the absolute permeability of the core sample in each test. The measured absolute permeability for each test was in the range of kabs = 70.6−71.0 mD. Thereafter, the oil sample was injected into the system with constant flow rate of qo‑inj = 0.25 cm3/min to reach the connate water saturation (Swc) and establish the initial oil saturation (Soi). The connate water saturation was found to be Swc = 45.1−45.8%, and the initial oil saturation was in the range of Soi = 54.2−54.9% for all huffand-puff tests. These saturations can be obtained when no more water is produced. Negligible difference in porosity, absolute permeability, connate water, and initial oil saturations showed almost no changes in the characteristics of the core sample during all tests. The initial oil effective permeability (koi) can also be determined using differential pressure between the inlet and outlet of the core holder. After the core was saturated with oil, the core was allowed to remain for 24 h to reach a proper equilibrium condition at constant temperature of T = 30 °C. Because cyclic CO2 injection tests were performed at various operating pressures, the above procedure was repeated for all experiments. For the CO2 huff-and-puff tests, the pressure of the CO2 in the transfer cell was increased to a desired operating pressure for each test

Vo,f Vo,i

permeability (mD)

(4)

2.4. CO2 Huff-and-Puff Tests. Figure 4 shows the experimental setup used for CO2 huff-and-puff tests in this study. It consisted of a high-pressure stainless steel core holder (Hassler, Inc.) with inner and outer diameters of 6.1 and 7.9 cm, respectively. A consolidated Berea core sample with the height of h = 30.21 cm and diameter of d = 5.05 cm was used as a physical porous medium. The core sample had a porosity, absolute permeability, and pore volume of ϕ = 18.5%, kabs = 70.8 mD, and PV = 111.82 cm3, respectively. The core sample was also initially water-wet. Table 2 presents the properties of the core sample and core holder. A strong rubber sleeve (Viton) supported by an overburden pressure of POB = 14 MPa was used to insulate the core in the core holder, allowing fluids to pass through the cross-section of the core and along the horizontal direction, and to prevent flows of fluid around the core. A Teledyne ISCO syringe pump (ISCO Inc., 500D series) was used to inject fluids (i.e., brine, crude oil, and CO2) into the core through high-pressure transfer cells and 1/8 in. i.d. high-pressure stainless steel pipes (Swagelok Company). To maintain desired back pressure in the system, a back pressure regulator (Temco, Inc.) was 777

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Table 3. Experimental Conditions, Total and Stage Oil Recovery Factors, Total Producing GOR, Final GUF, Precipitated Asphaltene Content, and Permeability Reduction of All Five CO2 Huff-and-Puff Tests at Different Operating Pressures test

1

2

3

4

5

operating pressure (MPa) miscibility condition porosity (%) absolute permeability (mD) connate water saturation (%) ultimate recovery factor (%) first stage recovery factor (%) second stage recovery factor (%) total producing GOR (cm3 gas/cm3 oil) final GUF (cm3 oil/cm3 inj. gas) CO2-produced oil asphaltene content (wt %) precipitated asphaltene in the core (wt %) permeability reduction factor to the oil (%)

5.35 immiscible 18.4 70.6 45.4 33.2 7.6 5.8 1610.3 342.8 0.59 0.64 9.81

6.55 immiscible 18.7 70.8 45.9 47.5 12.9 9.8 1560.0 379.4 0.53 0.70 10.72

8.27 near-miscible 18.6 71.0 44.7 55.8 16.8 11.2 2083.1 320.8 0.48 0.75 13.34

9.31 miscible 18.6 70.9 44.9 60.9 23.9 15.1 932.2 574.3 0.46 0.77 13.95

10.34 Miscible 18.5 71.0 44.3 61.5 23.9 16.13 979.6 537.5 0.45 0.78 13.79

determined using the differential pressure across the core in the core holder. Finally, the permeability reduction factor to the oil phase was calculated through the eq 5. There was also no water produced during the reinjection of the crude oil into the system.

and kept for 24 h to equilibrate at the experimental temperature. Then the CO2 was injected into the oil saturated core under constant operating pressure for a definite injection time of tinj = 120 min. After completion of CO2 injection (huff cycle), CO2 was allowed to soak for a soaking period of tsoak = 24 h. The puff cycle was then started by the oil production at the end of the core holder while the back pressure regulator was set at the pressure of PBPR = 3.45 MPa using a nitrogen cylinder. It is noted that during production in each puff cycle of cyclic CO2 injection, no connate water was produced. Because the huff-andpuff process is a single-well injection−production technique, both CO2 injection and oil production in this study were conducted in the same side (i.e., outlet) of the core holder. When the first huff-and-puff cycle was completed, the second cycle was started with a procedure that was the same as that of the first cycle. These cycles were continued until no considerable oil production was obtained. The volume of the produced oil and gas in each puff cycle was measured to calculate the oil recovery factor (RF), producing gas−oil ratio (GOR), and gas utilization factor (GUF). It is worth noting that GUF is defined as the ratio of the produced oil volume to the injected gas volume. Five secondary CO2 huff-and-puff tests were performed at different operating conditions followed by the aforementioned procedure. The experimental conditions (i.e., saturation data and operating pressure) for each test are presented in Table 3. The cyclic CO2 injection tests were carried out at five operating pressures of Pop = 5.38, 6.55, 8.27, 9.31, and 10.34 MPa and constant temperature of T = 30 °C. In addition, a secondary waterflooding test with water injection rate of qw‑inj = 0.75 cm3/min at Pop = 3.45 MPa followed by a miscible CO2 huff-and-puff test at Pop = 9.31 MPa were conducted to investigate the recovery performance of the CO2 huff-and-puff technique as a tertiary oil recovery process. 2.5. Asphaltene Precipitation and Permeability Reduction Measurement. Precipitation and deposition of asphaltene particles in the pore spaces of reservoir rocks result in wettability alteration and permeability reduction in hydrocarbon reservoir which considerably reduce the oil recovery.27−30 Asphaltenes are high-molecular-weight solids which are soluble in aromatic solvents such as benzene and toluene but insoluble in paraffinic solvents (i.e., n-pentane and nheptane).31 In CO2 immiscible and miscible displacement processes, the injected CO2 can induce flocculation and deposition of asphaltenes and other heavy organic particles which consequently cause numerous production problems.32−34 Thus, it is of great importance to determine how much asphaltene precipitates in the porous system during CO2 injection processes. In this study, the cumulative average asphaltene content of the CO2-produced oil in the first and second cycles of each huff-and-puff test was measured using the standard ASTM D2007-03 method and the n-pentane was used as precipitant. Furthermore, to determine the permeability reduction of the system after each CO2 huff-and-puff test, the original light crude oil was reinjected into the core holder with a constant flow rate of qo‑inj = 0.25 cm3/min after the last cycle of production. The final effective oil permeability (kof) was

DFo = 1 −

kof koi

(5)

3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1. IFT and MMP of Crude Oil−CO2 system. Figure 5 depicts the measured IFT values of the crude oil−CO2 system at

Figure 5. Measured CO2−oil IFTs at different equilibrium pressures and multicontact MMP of crude oil−CO2 system obtained from vanishing interfacial tension (VIT) technique at T = 30 °C.

various equilibrium pressures in the range of Peq = 0.66−14.64 MPa and T = 30 °C. Accordingly, it was found that the equilibrium IFT of the crude oil−CO2 system decreases linearly in two distinct ranges. In range I, with the pressure range of Peq = 0.66−6.41 MPa, the IFT reduces linearly because of CO2 dissolution into the oil phase. In range II, with the pressure range of Peq = 7.35−14.64 MPa, the governing mechanism which leads to linear IFT reduction of the crude oil−CO2 system is extraction of lighter hydrocarbon components by CO2. The measured equilibrium IFT decreased from IFTeq = 19.41 mJ/m2 at the equilibrium pressure of Pop = 0.66 MPa to its minimum value of IFTeq = 2.4 mJ/m2 at Peq = 14.64 MPa. 778

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Table 4. Pressure Range, Correlated Equations and Their Accuracy, and Calculated Multicontact and First-Contact MMPs Obtained from VIT Technique at T = 30 °C IFT phase

pressure range (MPa)

correlated equation

accuracy (R2)

calculated MMP (MPa)

range I range II

0.66−6.41 7.35−14.64

IFTeq = −2.3127Peq + 21.1750 IFTeq = −0.3864Peq + 8.0042

0.9983 0.9673

9.18 (multicontact) 20.44 (first-contact)

contact with the surface, more CO2 is dissolved in the crude oil with increased equilibrium pressure. In addition, the volume of the crude oil increases by increasing the equilibrium pressure, which is mainly due to the higher solubility factor of CO2 in the crude oil at higher pressures; as a result of this phenomenon, the crude oil swells in the visual cell. At higher equilibrium pressures, the CO2 phase changes from gas to liquid. Because liquid-phase CO2 has the greater ability to extract hydrocarbon components of crude oil,26 especially the lighter components, compared to the gaseous CO2 phase, the volume of the crude oil in the visual cell is reduced. At the start of the experiment, the solubility of CO2 and swelling factor of crude oil are 0 and 1 respectively. As shown in Figure 6, the solubility of CO2 and swelling factor in the crude oil increase with equilibrium pressure and reach their maximum values at Peq = 6.79 MPa. The maximum CO2 solubility and swelling factor of the crude oil at this pressure are χCO2 = 31.46 wt % and SF = 1.32, respectively. After this point, the extraction phase dominates the swelling phase and leads to a decrease in the volume of the crude oil in the visual cell and a decline in swelling factor because lighter hydrocarbon components are extracted by CO2 and vaporized into the gaseous phase. Results of the swelling tests indicate that extraction phase of crude oil−CO2 system starts at a pressure near Pext = 6.79 MPa, which is in good agreement with the Pext = 6.84 MPa obtained from IFT tests. 3.3. Oil Recovery Factor, Producing GOR, and GUF. In this study, a total of five secondary CO2 huff-and-puff tests were carried out at different operating pressures ranging from Pop = 5.38 to 10.34 MPa and at a temperature of T = 30 °C under immiscible, near-miscible, and miscible conditions. In each cycle, the CO2 was injected into the system for tinj = 120 min, then the system was shut in for the soaking period of tsoak = 24 h; finally, it was allowed to produce. The cycle numbers were continued until no considerable oil production was obtained. It is worth noting that there was no water production during secondary CO2 huff-and-puff tests and the connate water saturation remained constant during the whole process. Figure 7 shows the measured oil recovery factor versus cycle numbers and pore volume of injected CO2 for five huff-and-puff tests at different operating pressures. The oil recovery factor increased with the cycle numbers and pore volume of injected CO2. The results showed that for tests performed at immiscible conditions, specifically test 1 (i.e., Pop = 5.35 MPa) and test 2 (i.e., Pop = 6.55 MPa), the oil recovery factor increases remarkably from RF = 32.2% to 47.5% as the operating pressure increases and reaches RF = 55.83% at the near-miscible condition (i.e., test 3) with Pop = 8.27 MPa. The measured oil recovery factor reached its almost maximum value of RF = 60.9% at operating pressure near MMP, Pop = 9.31 MPa (i.e., test 4). Further increase of operating pressure to Pop = 10.34 MPa (i.e., test 5) did not result in noticeable oil recovery, and a recovery factor of only RF = 61.5% was obtained. However, it was found that in huff-and-puff tests performed at the miscible condition, the ultimate recovery factor was achieved by fewer cycles or pore volume of injected CO2 (i.e., 7 cycles) compared to that in immiscible conditions (i.e., 10−11 cycles). This was

The vanishing interfacial tension (VIT) technique was applied on the measured equilibrium IFTs to determine the minimum miscibility pressure (MMP) of the crude oil−CO2 system. This technique is based on the concept that the interfacial tension (IFT) between a crude oil and CO2 approaches zero when they become miscible.23 Therefore, the MMP can be determined by linearly extrapolating the measured equilibrium IFT values versus equilibrium pressure to the point of zero equilibrium IFT. The measured equilibrium IFTs in the two pressure ranges (Peq = 0.66−6.41 MPa and Peq = 7.35−14.64 MPa) were regressed linearly to correlate with equilibrium pressures as presented in Table 4 and shown in Figure 5. The intersection of the linear equation representing the equilibrium IFTs in range I with the abscissa (i.e., IFTeq = 0) gives the multicontact or minimum CO2 miscibility pressure, which was found to be MMP = 9.18 MPa. The second linear regression intersects with IFTeq = 0 at 20.44 MPa. Because at this pressure almost all intermediate and heavy components become miscible with CO2, this pressure may be interpreted as the first contact CO2 miscibility pressure with the oil. The intersection of the two linear correlations at the extraction pressure of Pext = 6.84 MPa is the point at which the interaction mechanism between the crude oil and CO2 changed from the CO2 solubility to extraction of lighter components by CO2. 3.2. CO2 Solubility and Oil Swelling Factor in the Crude Oil−CO2 System. The experimental results of CO2 solubility in the crude oil sample and oil swelling factor at T = 30 °C are depicted in Figure 6. This figure illustrates that the

Figure 6. Measured CO2 solubility in the crude oil and oil swelling factor of crude oil−CO2 system at various equilibrium pressures and T = 30 °C.

solubility of the CO2 increases as the equilibrium pressure of the system increases. The concentration of dissolved CO2 is proportional to the partial pressure of the CO2. The CO2 partial pressure controls the number of CO2 molecule collisions in contact with the surface of the crude oil sample. Because higher partial pressure (i.e., equilibrium pressure of the system) results in an increase in the number of collisions that occur in 779

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Figure 8. Ultimate, first, and second stage oil recovery factors of five CO2 huff-and-puff tests performed at immiscible, near-miscible, and miscible conditions.

subsequent cycles, resulting in a higher oil recovery factor in the first and second cycles and lower one during preceding cycles of CO2 huff-and-puff process. The producing gas−oil ratio (GOR) and gas utilization factor (GUF) of five cyclic CO2 injection tests performed at different operating pressures are shown in Figure 9. In addition, Figure 10

Figure 7. Cumulative oil recovery factor versus (a) cycle numbers and (b) pore volume of injected CO2 for five CO2 huff-and-puff tests at different operating pressures.

mainly attributed to the faster and stronger CO2 extraction mechanism at miscible conditions, which leads to a significant ultimate oil recovery factor by less pore volume of injected CO2. The ultimate oil recovery factor and first and second stage recovery factors of five secondary CO2 huff-and-puff tests versus operating pressure in three discrete regions of immiscible, nearmiscible, and miscible conditions are plotted in Figure 8 and presented in Table 3. As can be seen from this figure, in the range of immiscible to near-miscible CO2 huff-and-puff processes, the ultimate recovery factor highly depends on operating pressure and increases considerably with pressure. Similarly, the same results were obtained for the first and second stage recovery factors in the aforementioned regions. Moreover, it was found that 40−60% of the recovered oil in huff-and-puff tests was produced in the first and second cycles. During the initial oil saturation of the core sample, because the rock sample was water-wet, the oil phase occupies the porous media starting with larger pore spaces because of lower water−oil capillary pressure. Therefore, the oil saturation is generally higher in larger pores.35 On the other hand, because the capillary pressure of the oil−gas phase is also lower in larger pore spaces of the core, the CO2 molecules begin to occupy and diffuse into the larger pores during initial cycles (i.e., first and second cycles). As a result, the CO2 interacts with a higher volume of the oil inplace during initial cycles and a lower volume during the

Figure 9. (a) Producing gas−oil ratio and (b) gas utilization factor versus pore volume of injected CO2 for five CO2 huff-and-puff tests at different operating pressures. 780

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the conducted tertiary miscible CO 2 huff-and-puff test significantly increases the oil production by an extra recovery factor of RF = 16.3%. The ultimate oil recovery factor of RF = 70.2% was achieved by conducting both secondary waterflooding and tertiary CO2 huff-and-puff tests. The producing water−oil ratio started to decline gradually during the tertiary CO2 huff-and-puff test, while the producing gas−oil ratio was significantly increased. 3.4. Asphaltene Precipitation and Permeability Reduction. The cumulative average asphaltene content of CO2produced oil of the first and second cycles of CO2 huff-and-puff tests as well as precipitated asphaltene in the core are plotted in Figure 12 and presented in Table 3. The initial n-pentane insoluble asphaltene content of original light crude oil was 1.23 wt %, whereas the measured values asphaltene content of CO2produced oil in five CO2 huff-and-puff tests were lower than this value. This is an indication of asphaltene precipitation and deposition phenomena in the pore spaces of the core sample as a result of CO2 injection. As shown in Figure 12, for the CO2 huffand-puff tests carried out at a pressure lower than MMP (i.e., immiscible conditions), specifically test 1 (i.e., Pop = 5.35 MPa) and test 2 (i.e., Pop = 6.55 MPa), the values of cumulative asphaltene content of the CO2-produced oil obtained from the first and second cycles are considerably higher than those of the tests performed at pressures near and above MMP (i.e., nearmiscible and miscible conditions). Conversely, it can be concluded that in the near-miscible and miscible CO2 huffand-puff tests, the amount of precipitated asphaltene in the porous media is drastically higher. It is again due to the stronger light component extraction process by CO2 at pressures near and above MMP that leads to asphaltene particles becoming unstable and a reduction in their association with other hydrocarbon groups, particularly resins. The permeability reduction of the core sample after termination of cyclic CO2 tests at each operating pressure was determined and is illustrated in Figure 12 and tabulated in Table 3. The permeability reduction was calculated using eq 5. The permeability reduction of the core sample is mainly attributed to the rock wettability alteration from water-wet to mixed or oilwet because of the precipitation and deposition of asphaltene

Figure 10. Total producing GOR and final GUF of five CO2 huff-andpuff tests at different operating pressures.

portrays the total producing GOR and final GUF of the tests. It was found that the total producing GOR of miscible CO2 huffand-puff tests is relatively less than that of immiscible and nearmiscible CO2 huff-and-puff tests. This is due to a smaller volume of CO2 (i.e., fewer injection cycles) is required to be injected into the core holder in miscible injection, and more oil was recovered as well. For this same reason, the final GUF of miscible CO2 huff-and-puff tests is higher than that of immiscible tests because a greater volume of the original oil in-place is recovered by injecting a smaller volume of CO2 into the system. Figure 11 depicts the cumulative oil recovery factor, producing GOR, and producing WOR of secondary the waterflooding test followed by a miscible CO2 huff-and-puff (i.e., P op = 9.31 MPa). The results showed that the waterflooding process is able to produce 53.9% of original oil in-place (i.e., ultimate RF = 53.9%). It was also observed that the oil recovery factor at the water breakthrough is RF = 43.2%, showing that most of the produced oil during waterflooding was recovered before the water breakthrough. The producing water−oil ratio (WOR) increased drastically after the water breakthrough and reached WOR = 1.68 at the end of the secondary waterflooding process. The results also indicated that

Figure 11. Cumulative oil recovery factor, producing GOR, and producing WOR during secondary waterflooding and tertiary miscible CO2 huff-andpuff tests. 781

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Figure 12. Asphaltene content of CO2-produced oil, precipitated asphaltene in the core, and permeability reduction of the core sample in all CO2 huffand-puff tests conducted at different operating conditions and under immiscible, near-miscible, and miscible conditions.

Figure 13. Compositional analysis of original crude oil and remaining crude oil of CO2 huff-and-puff tests performed at Pop = 6.55 and 9.31 MPa.

particles on the rock surfaces. The results showed that the permeability reduction of the core sample in near-miscible and miscible CO2 huff-and-puff tests is considerably higher than that in immiscible cases because the remaining and deposited asphaltene particles and heavy components in the porous medium are larger in CO2 huff-and-puff tests carried out at pressures near and above MMP. 3.5. Compositional Analysis of Remaining Oil. Figures 13 and 14 depict the compositional analysis and grouped carbon number distributions of remaining crude oil for CO2 huff-andpuff tests performed at Pop = 6.55 MPa (i.e., immiscible condition) and 9.31 MPa (i.e., miscible condition), respectively. The remaining crude oil was collected at the beginning of the production time during the reinjection of the fresh oil into the core holder. As mentioned earlier, the mechanism of light component extraction by CO2 removed all lighter components ranging C1−C4’s and C1−C5’s from the oil phase at Pop = 6.55 and 9.31 MPa, respectively. Accordingly, the mole percent of

Figure 14. Grouped carbon number distributions of original crude oil and remaining crude oil of CO2 huff-and-puff tests performed at Pop = 6.55 and 9.31 MPa.

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intermediate to heavy hydrocarbons including C10−C19’s, C20− C29’s, C30+, and molecular weight of the remaining oil were relatively higher than those in the original crude oil. On the other hand, comparing the composition of remaining oil at Pop = 6.55 and 9.31 MPa shows that the mechanism of light component extraction is much stronger at Pop = 9.31 MPa, which is a miscible CO2 huff-and-puff test. As a result, the remaining oil in the core after miscible CO2 huff-and-puff tests contained a greater amount of heavy hydrocarbons (C10−C19’s, C20−C29’s, and C30+) and greater molecular weight. This comparison also confirms that the precipitated asphaltene in the core during miscible CO2 huff-and-puff processes was substantially greater than that during immiscible cases.

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CO2 extraction is much stronger at pressures near and above MMP.

AUTHOR INFORMATION

Corresponding Author

*Petroleum Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, SK, S4S 0A2, Canada. E-mail: [email protected]. Tel: 1(306)5855667. Fax: 1(306)585-4673. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Petroleum Technology Research Centre (PTRC) and Faculty of Graduate Studies and Research (FGSR) at the University of Regina for their funding support. The authors also extend their gratitude to Mr. Nader Mosavat for technical assistance during some of the experimental portions, and Dr. Y. Gu and Mr. Y. Gong for their help with the IFT measurement test.

4. CONCLUSIONS In this study, the oil recovery performance of secondary CO2 huff-and-puff process in a light crude oil system under immiscible, near-miscible, and miscible conditions was experimentally investigated. The CO2 solubility, oil swelling factor, and equilibrium interfacial tension of the crude oil−CO2 system was determined at a wide range of equilibrium pressures and a constant temperature of T = 30 °C. In addition, the MMP of CO2 with the oil sample was determined by means of the VIT technique. A total of six CO2 huff-and-puff tests at various operating pressures and temperature of T = 30 °C with the injection time of tinj = 120 min and soaking period of tsoak = 24 h were carried out. According to the obtained experimental results, the following conclusions have been drawn: • The multicontact MMP of the CO2 with the original light crude oil was determined by applying the VIT technique on the measured equilibrium IFTs of the crude oil−CO2 system and found to be MMP = 9.18 MPa at T = 30 °C. This pressure was used as a reference to determine the miscibility condition of CO2 huff-and-puff tests. • The CO2 solubility in the crude oil as well as the oil swelling factor were experimentally determined. On the basis of the results of IFT and swelling tests, it was found that the hydrocarbon extraction mechanism initiates at extraction pressure of Pext = 6.8 MPa. • The ultimate oil recovery factor increased considerably when operating pressure increased and reached the maximum value at miscible condition (i.e., operating pressures near the MMP). Further increase of operating pressure beyond the MMP did not noticeably enhance oil recovery. The results also indicated that the tertiary miscible CO2 huff-and-puff has a great potential to increase the oil recovery from waterflooded reservoirs. • The precipitated asphaltene in the core as a result of CO2 injection into the system in near-miscible and miscible CO2 huff-and-puff tests was substantially higher than that in immiscible tests. Furthermore, because of higher asphaltene precipitation at both near-miscible and miscible conditions, the permeability reduction of the core was drastically higher for both of those scenarios. • Compositional analysis showed that the remaining oil in the core after CO2 huff-and-puff tests contained a greater amount of heavy components and its molecular weight was much higher than that for the initial oil sample because of light component extraction mechanism by CO2. Moreover, it was found that in miscible CO2 huffand-puff tests, the remaining oil is relatively heavier than that in immiscible CO2 huff-and-puff tests because the

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NOMENCLATURE

Symbols

DFo = permeability reduction factor to the oil phase IFTeq = equilibrium IFT (mJ/m2) kabs = absolute permeability (mD) koi = initial oil effective permeability (mD) kof = final oil effective permeability (mD) MW = molecular weight (g/mol) m = mass (gr) n-C5 = normal pentane Pop = operational pressure (MPa) Peq = equilibrium pressure (MPa) Pext = extraction pressure (MPa) PBPR = pressure of back pressure regulator (MPa) POB = over burden pressure (MPa) qw‑inj = water injection flow rate (cm3/min) qo‑inj = oil injection flow rate (cm3/min) R = universal gas constant (J/(mol K)) Swc = connate water saturation Soi = initial oil saturation T = temperature (°C) Texp = experimental temperature (°C) tinj = CO2 injection time (min) tsoak = soaking period (hr) V = volume (cm3) Z = gas compressibility factor Greek Letters

ρoil = oil density (kg/m3) ρw = water density (kg/m3) μoil = oil viscosity (mPa s) μw = water viscosity (mPa s) ϕ = porosity χCO2 = CO2 solubility in crude oil (wt %)

Abbreviations

ADSA = axisymmetric drop shape analysis EOR = enhanced oil recovery GOR = gas−oil ratio GUF = gas utilization factor IFT = interfacial tension MMP = minimum miscibility pressure 783

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VIT = vanishing interfacial tension

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