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Internal Olefin Sulfonate foam coreflooding in low-permeable limestone at varying salinity Svetlana Rudyk, Sami Al-Khamisi, Yahya Mansoor Al-Wahaibi, and Nayyar Afzal Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01762 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Internal Olefin Sulfonate foam coreflooding in low-permeable limestone at varying salinity

S. Rudyk1, S. Al-Khamisi1, Y. Al-Wahaibi2, N. Afzal2 1

Oil&Gas Research Centre, Sultan Qaboos University, Muscat, Oman

2

Petroleum and Chemical Engineering Department, Sultan Qaboos University, Muscat, Oman

Abstract Effects of salinity, total flow rate and gas flow rate on the foam generation using internal olefin sulfonate (IOS) surfactant were investigated. The compatibility tests performed at 1-11% NaCl at room temperature of 22 oC showed that the volume of foam on the surface of liquid increased with salinity increase from 1% to 11% NaCl indicating that the foamability did not reduce but solubility of surfactant in brine decreased with increasing salinity. The surfactant precipitation on the bottom of test tubes occurred starting from 6% NaCl at 22 oC and from 8% NaCl at 60 oC. However, surfactant emerged to the surface at 9-11% NaCl because of the increased brine density. The foam scans using 0.5% IOS surfactant were carried out at the salinity of 1, 5, and 8% NaCl through an Indiana limestone core sample of 50 mD at 60 oC. Two total flow rates of 0.4 ml/min and 0.3 mL/min, superficial velocities of 4.39 *10-6 m/s and 5.88*10-6 m/s, respectively, were used to simulate flow rates typically applied at the oil fields at 400 psi and 500 psi. Generally, the differential pressure decreased in the order of gas flow rate decrease while apparent viscosity was higher at lower total flow rate. The apparent viscosity at 5% NaCl were higher than at 1% NaCl at 0.3 mL/min and similar or lower at 0.4 mL/min. The shape of apparent viscosity curves changed from typical for shear-thinning fluids at 1% and 5% NaCl to linearly decreasing at 8% NaCl. The

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typical increase of apparent viscosity in a low flow regime observed by many authors was very gentle because of the low total flow rates compared to previously published. The fine foam was observed at the outlet of the system at 1 and 5 % NaCl at all foam qualities (gas fractions). At 8% NaCl, the foam was observed at 0.3 and 0.4 foam qualities while only bubbles were formed at 0.5-0.9. In a low foam quality regime, the foam viscosity at the outlet of the system indicated by the length of foam drop before fall decreased at increasing gas fraction while the apparent viscosity was constant. This showed that the foam mobility and volumes of arrived foam could be underestimated by using apparent viscosity. The shape of apparent viscosity curves at low total flow rates and difference between actual viscosity and apparent viscosity should be considered in practical use and numerical simulations.

Key words: olefin sulfonate, surfactant, foam scan, salinity

1. Introduction

Harsh conditions of oil reservoirs require surfactant candidates with high cloudpoint temperatures in the presence of concentrated brine in order to prevent surfactant precipitation.1 In addition, the foam coarsens, coalesces and decays faster as a response to the reduction of foamability and foam viscosity at high temperatures and salinities.2 Many surfactants have been tested for their tolerance to temperature and salinity. The oil recovery at the temperature of 120 °C in the presence of 22% of total dissolved solids can be improved using such foaming agents as Ethoxylated amine surfactant (Elhag et al.) 3 or Ethomeen C12 (Cui et al.) 4.

Puerto et al.5 reports that the alkoxyglycidylether sulfonates (AGES) with n-octane/NaCl-brine 2 ACS Paragon Plus Environment

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systems can be stable up to 120 °C with optimal salinities up to 20% NaCl with suitable combinations of hydrophone and alkoxy chain type. Sun et al.6 describes that the combined system mainly composed of Polyoxyethylene ethersulfonate and sulfobetaine can create good foaming ability and foam stability at 100 °C and 20.5×104 mg/L. The experiments using Cocamidopropyl hydroxyl sulfobetaine (CHSB) in the nitrogen foam flooding under 120 oC and 22 g/L NaCl shows that high temperature is unfavorable to foam stability but high-salinity can favor its stability.7 Commonly used foaming agents (e.g., SDS, AOS) can hardly meet the high temperature and high salinity reservoir conditions in foam flooding projects.8.9 For example, the apparent viscosity of AOS foam decreases by 50% as the temperature increases from 20 to 80 oC in a sandstone porous medium with co-injection of foam.2 Levitt et al. describes that the presence of sulfonate unit in the compound increases the surfactant long–term stability at higher temperature and makes the Internal Olefin Sulfonate (IOS) to be stable at temperature up to 200 °C.10 The IOS surfactant is also found stable in salinity up to 6000 ppm with the optimum salinity at 4500 ppm.11 Among all the surfactant in the group of sulfonates, C15-C18 has higher resistance towards salinity.10,11 The foam generation is tested in steady-state coreflooding experiments at various gas and liquid flow rates, temperatures, pressures, and salinities. The strength of foam moving through porous media is related to the magnitude of pressure gradient measured along the medium.12 The finer the texture of generated foam and the smaller foam bubbles, the higher is the pressure gradient and apparent viscosity.13 The constant velocity foam scan is performed in order to measure pressure gradient (dP) and calculate apparent viscosity at specific foam quality (gas fraction).14 Foam quality is a gas fractional flowrate of the total flow rate:


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𝑓𝑔(%) = 100 ×

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𝑞𝑔 1.

𝑞𝑔 + 𝑞𝑙

where liquid injection flowrate 𝑞𝑙 and gas injection rate 𝑞𝑔 are calculated at normal pressure. The apparent viscosity is calculated using Darcy formula: 𝜇𝑎𝑝𝑝 =

𝑘 |∆𝑃| 𝑢𝑡 𝐿

2.

where k is the permeability of the core, 𝑢𝑡 is the total Darcy velocity, ∆P is the pressure drop (dP) over the core, and L is the length of the core. The results are presented in the plot of dP and apparent viscosity vs. foam quality. Typically, the apparent viscosity first increases linearly with foam quality and then decreases sharply.4,14 Thus, two distinct flow regimes can be determined in the plot: low-quality regime (low gas fraction) with strong foam and high-quality regime (high gas fraction) with weak foam separated by the transition foam quality (𝑓𝑡𝑟 𝑔 ), which is characterized by the maximum pressure drop and apparent viscosity. In the low-quality regime, the pressure gradient increases as the gas saturation rises because of bubble trapping and the foam volume increase. The bubble size is controlled by the pore size, and the reduction in gas mobility is greatest. In the high-quality regime, the pressure gradient decreases with increase in foam quality mainly due to coalescence of the gas bubbles. Foam collapses at the transition between two regimes which designates the critical capillary pressure close to the rupture pressure of a single foam film.15 If foam quality rises to a very high level, foam presents the morphology of “gas slug” instead of gas bubbles.8 The imposed capillary pressure on foam films residing in pore throats is balanced with the disjoining pressure. The magnitude of the disjoining pressure varies with surfactant type,


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surfactant concentration, salinity, foam flowrate and porous medium permeability.12 When lamellae can withstand a higher capillary pressure, the surfactants have a greater potential for reducing gas-phase mobility. In the present study, the foam scans were carried out at 60 oC using 0.5% Internal Olefin Sulfonate (IOS) surfactant co-injected with nitrogen through a low permeable Indiana limestone core sample (50 mD) in order to investigate the effects of total flow rates and salinity on the pressure gradient and apparent viscosity of foam. The experiments were carried out in the absence of crude oil, in order to avoid its strong influence on the destruction of the foam, to determine the limits of salinity and use these findings for future comparison of the results that will be carried out in the presence of crude oil.

2. Experimental procedure 2.1 Materials and Methods

The core sample of Indiana limestone used in the tests are purchased at Kochurek company. The longer core size of 10.25-inch (26 cm) in comparison to conventional 6-inch core length is to provide for a longer path for the foam generation and observe its changes over the core length. The weight of a core = 1299.9 g, diameter x length = 3.81 cm x 26.035 cm, area=11.4 cm2, porosity = 12%, permeability= 50 mD, core pore volume (PV) = 35.5 cm3. Nitrogen gas of 99.9% purity is bought at Oman Gas Company. The density and dynamic viscosity are 0.0278 g/cm3 and 0.4 cP, respectively, at the experimental temperature of T=60 oC. Synthetic brine solution ranging from 1% to 11% NaCl are used.


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The surfactant Sodium alkyl (C14–C17) sulfonate mixture called Internal Olefin Sulfonate (IOS) is provided by Shell. Concentration of active part of surfactant is 28.3%. The surfactant solutions used for the compatibility tests and core flooding experiments are produced by mixing an appropriate volume of a surfactant in brine solution.

2.2 Apparatus

The high-pressure core flood rig designed and assembled for this study is shown schematically in Figure 1. The pressure tapped core holder is of Hastelloy C material with a maximum operating pressure of up to 10,000 psi and temperature of up to 150 oC. The core is placed in a sleeve to keep the core in position and to prevent fluid from bypassing along the side of the core. A temperaturecontrolled oven is used to house the core holder and connections at constant temperature. The rig has two storage cells containing the injection fluids, crude oil and brine/surfactant solutions. A single-effect piston displacement pump is used to inject liquid (water, surfactant and oil) around the flow system (core and bypass lines) while a hydraulic pressure pump ICS of 500 mL is used to apply confining (overburden) pressure. Nitrogen gas supplied from the gas tank (200-bar cylinder) passes through a Bronkhorst mass flow controller, which regulates the gas flow before injecting into the lines connecting surfactant and coreholder. Four pressure transducers are spaced equally along the core axis from the inlet to the outlet in order to measure differential pressure changes along the core at the distances of 3”, 6”, 9” and 10.25” (inlet-outlet). A backpressure regulator (BPR) is used to maintain the pressure of the core outlet and deliver the core effluent at atmospheric pressure. While running an experiment, the effluent from the BPR is collected in a graduated cylinder at atmospheric pressure and laboratory temperature.


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Figure 1. A schematic diagram of the high-pressure high-temperature core flood rig used in this study.

2.3 Core cleaning, saturation and permeability measurement The core is saturated with brine by immersing it in a filled cup inside a vacuum desiccator for 24 hours, and the pore volume is calculated from the weight difference between the dry and the wet weights. After that the core is mounted in the core holder and a confining pressure of 2000 psi is applied. A leak test is conducted after which a synthetic brine of 1% NaCl is injected through the system at 1 mL/min to fill the lines and saturate the core. The absolute liquid permeability of the core is then calculated from the pressure drop along the core at different flow rates using Darcy’s law by the following equation: 7 ACS Paragon Plus Environment

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𝐾(𝑚𝐷) =

245𝑞𝜇𝐿

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3.

𝐴∆𝑝

where viscosity (μ) is 0.4 cP, length (L) is 26 cm and cross-sectional area (A) is 11.4 cm2. The dP measured at brine flow rate of 1 mL/min is 4.5 psi. The permeability calculated by Eq.3 is 49.73 mD that matches to 50 mD measured by Kochurek Company. Afterwards, the core is saturated with the surfactant solution by injecting copious amount of the solution (10 PV) at a flow rate of 1 mL/min to satisfy the adsorption of the rock. The IOS surfactant solution concentration of 0.5% is used subsequently for all foam quality scan tests.

2.4 Foam quality scan experiment The surfactant solution and N2 gas are co-injected simultaneously at different volume fractions but the same total flow rate 𝑞𝑡. The liquid fraction and gas fraction/foam quality (𝑓𝑔) are calculated by Eq.1. The outlet pressure is controlled by the Back Pressure Regulator (BPR) set at 400 psi. The gas flow rates for foam scan are recalculated from normal to experimental conditions according to gas law using a reference pressure of BPR. Two total flow rates of 0.4 ml/min and 0.3 mL/min, superficial velocities of 4.39 *10-6 m/s and 5.88*10-6 m/s, respectively, are used. The tests are also performed at gas and liquid flow rates calculated for reference pressure of 500 psi in order to investigate the effect of increased gradient of total flow rate. The gas flow rates at specific liquid flow rates or foam qualities decrease in the following order: T=0.4 mL/min 500 psi; T=0.4 mL/min 400 psi; T=0.3 mL/min 500 psi; T=0.3 mL/min 400 psi (Figure 2). The co-injection is continued until a steady state pressure is observed for the selected foam quality

(𝑓𝑔). Most studies report that the stability of the dP curves is achieved beginning from 1.5-2 PV of injected fluid.4,14 A minimum of 4 PV of fluid for each foam quality after the plateau state is achieved, is injected in our study. The next foam quality is selected, the liquid and gas fractional 8 ACS Paragon Plus Environment

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flow rates are set accordingly and the same procedure is repeated until a new steady state is attained. The procedure continues until the spectrum of 𝑓𝑔 is covered. The foam quality is altered randomly in the experiment to minimize the effect of previously attained steady state on the new results. To determine the foam quality of another surfactant solution at different brine salinity, the same experimental procedure is repeated after injecting 5 PV of foam killer (a mixture of 50% methanol and 50% deionized water) at the end of the previous experiment to break the foam and remove the surfactant from the core. Copious amount of brine is then injected to flush out the foam killer. Foam quality (gas fraction) is calculated using Eq.1 and apparent viscosity using Eq.2. The measured differential pressure between inlet and outlet dP and calculated apparent viscosity are plotted versus respective foam quality. In order to observe changes of foam texture at various foam qualities, the flow of foam is recorded by camera. The average length of 15 foam drops (Ld) before fall at the end of pipe is measured in the course of foam displacement. As with droplet length viscometers, the length of a drop of foam is used as an indicator of the foam viscosity at the outlet of the system.16 The maximal drop length was limited by the distance of 19 cm to the surface of table.


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12 10

qg, mL/min

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8 6 4 2 0 0

0.5

1

fg T=0.4 400 psi

T=0.4 500 psi

T=0.3 400 psi

T=0.3 500 psi

Figure 2. Experimental conditions of the performed foam scans. N2 gas flow rate vs. surfactant flow rate; the respective equations are shown in colors.

3. Results and Discussion 3.1 Surfactant Compatibility Test In order to investigate the compatibility of the surfactant with higher salinity brine, the brine solutions of 1-11% NaCl were prepared in the test tubes of 15 mL volume. 0.5% IOS was added in each of the solutions, thoroughly mixed and allowed to stand for 7 days at 22 oC. It was observed that the surfactant solutions with salinity up to 5% NaCl were clear at the beginning of the test and remained transparent at the end of the 30 days period. The cloudy solutions were formed at 6-11% after IOS was added into the brine solution. The surfactant solutions were transparent with no precipitates up to 7% NaCl in the oven at 60 oC. The images of the test tubes with surfactant solutions of 6% and 8-11% NaCl at the beginning of test and after 7 days of standing at room temperature of 22 oC and after 7 days of standing in the 10 ACS Paragon Plus Environment

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oven at 60 oC are shown in Figure 3. The coagulated solid particles of surfactant could be seen in all test tubes. The solutions were more cloudy and with lower volume of foam on the surface at 6, 8, and 9% NaCl. The solutions of 10% and 11% NaCl were clearer but the volumes of foam on the surface were higher. The height of foam on the surface of liquid increased from 0.5 to 2 cm in the range of 6-11% NaCl (Figure 3a) indicating that the foamability did not reduce but solubility of surfactant in brine decreased with increasing salinity. After 7 days of standing, the liquids in the test tubes were clear but suspended semisolid particles, precipitates and accumulations of surfactant were formed on the bottom and on the surface (Figure 3b). At 6% and 8% NaCl, the precipitates could only be observed on the bottom of test tubes. At 9, 10 and 11% NaCl, the surfactant was mainly accumulated on the surface but some amount could also be observed on the bottom of the test tubes. The amount of precipitates on the bottom was negligible at 10% NaCl and 11% NaCl, while most of surfactant was accumulated on the surface. The amount of precipitates on the bottom decreased and amount of surfactant on the surface increased from 6% NaCl to 11% NaCl. This demonstrated that the partition of surfactant in brines occurred due to the increase of brine density. After shaking, the surfactant solutions were placed into the oven at 60 oC. After 7 days in the oven, the solution of 6% NaCl was clear with lower amount of precipitates on the bottom than at 22 oC (Figure 3c). At 8% NaCl and 60 oC, most of surfactant precipitates floated to the surface compared to 22 oC, at which they were on the bottom. Emergence of surfactant to the surface showed that its density decreased at higher temperature. The solutions of 9, 10, and 11% NaCl were clear and the volume of surfactant on the top of test tubes increased at increasing salinity. Lower volume of surfactant floating on the surface indicated that the bigger part of surfactant was dissolved at 60 oC

compared to 22 oC. Based on the above mentioned observations, the foam generation up to 7%


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NaCl and the surfactant precipitation starting from 8% NaCl could be expected at 60 oC. The foam scans were carried out at 1, 5, and 8% salinity. 6%

8%

9%

10%

11%

6%

8%

9%

10%

11%

6%

8%

9%

10%

11%

d) f)

a)

b)

c)

Figure 3. Compatibility test of surfactant solutions in the brine of 6, 8, 9, 10, 11 % NaCl: (a) 0.5% IOS at the beginning of the test at 22 oC; (b) 0.5% IOS after 7 days of standing at 22 oC; c) 0.5% IOS after 7 days of standing at 60 oC.

3.2 Foam scans at 1 % salinity The foam quality scans of the surfactant solutions with brine salinity of 1% and IOS surfactant concentration of 0.5% were carried out at the gas flow rates and liquid flow rates calculated for the total flow rates of 0.3 mL/min and 0.4 mL/min and pressures of 400 psi and 500 psi, as shown in Figure 2. The measured dP and calculated apparent viscosity were plotted at the specific foam quality (Figure 4). The curves were denoted as e.g. 1 400 0.4, which indicated that the curve was obtained for the conditions of 1% salinity, 400 psi of pressure and 0.4 mL/min of total flow rate. Both dP and apparent viscosity did not vary substantially in the range of fg=0.3-0.6 and sharply decreased afterwards. The dP decreased with the decrease in the total flow rate and pressure similar to the gas flow rate decrease in Fig.2: 500 psi 0.4 mL/min; 400 psi 0.4 ml/min; 500 psi 0.3 mL/min; 400 psi 0.3 ml/min (Figure 4a). However, the order of curves changed after conversion into 12 ACS Paragon Plus Environment

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apparent viscosity by Eq.2 (Figure 4b). Both curves of apparent viscosity of 0.3 mL/min shifted upper to higher values and overlaid the curves of 0.4 mL/min. The apparent viscosity of 500 psi 0.3 mL/min was the highest, that of 400 psi 0.4 mL/min was the lowest, and the curves of apparent viscosity of 1 500 0.4 and 1 400 0.3 overlaid in the range of 0.5-0.9.

1% NaCl 0.040

140

0.035

100 80 60 40 20

15

0.030 0.025 0.020

10

0.015 0.010

5

0.005

0 a)

20

viscosity

120

𝑓𝑡𝑟 𝑔

Ld, cm

160 Apparent viscosity, Pa.s

dP, psi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.000 0.3

0.5

fg

0.7

1 500 0.4

1 400 0.4

1 500 0.3

1 400 0.3

0.9

b)

0.3

0.5 1 500 0.4 1 500 0.3

0.7 fg 1 400 0.4 1 400 0.3

0.9

0 0.3

0.5 1 500 0.4 1 500 0.3

0.7

fg 0.9

1 400 0.4 1 400 0.3

Figure 4. Foam quality scan results for surfactant solution of 1% NaCl: a) dP; b) apparent viscosity of foam; c) length of foam drop before fall. Abbreviation e.g. 1 500 0.4 indicates that the data were obtained for 1% NaCl, pressure of 500 psi and total flow rate of 0.4 mL/min.

The length of foam drop before fall was getting longer, texture coarser and bubbles bigger at increasing foam quality at all total flow rates and pressures (Figure 5). The curves of drop length were reciprocal to the apparent viscosity curves implying that shorter drops were formed at higher viscosity and longer drops at lower viscosity (Figure 4c). The slopes of Ld curves were sharp indicating that the drop length increased due to the increase in gas flow rate but the apparent viscosity did not vary noticeably in a low quality regime. The inflection points at fg=0.6 in Ld curves match to the transition foam quality (fgtr) in the apparent viscosity curves. 13 ACS Paragon Plus Environment

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The order of the Ld curves followed the order of dP curves implying longer foam drops of lower viscosity for 0.4 mL/min compared to 0.3 mL/min. However, the Ld curves of 0.3 mL/min shifted upper and overlaid curves of 0.4 mL/min similar to the curves of apparent viscosity. The length of foam drop was longer at 500 psi due to higher gas fraction, implying less viscous, than at 400 psi for both total flow rates. fg: 0.3

0.4

0.5

0.6

0.7

0.8

0.9

Figure 5. Photos of foam drops obtained at specific foam quality (fg) at 1% NaCl for 0.5% IOS surfactant solution. The foam reached the table surface at the distance of 19 cm at 0.8 and 0.9.

3.3 Foam scans at 5 % salinity The procedure of foam scan was followed for the surfactant solutions of 5% NaCl and the results are shown in Fig.6. The dP curve of 5 500 0.4 was very slightly higher than of 5 400 0.4 while the dP of 5 500 0.3 was higher than of 5 400 0.3 by 10 psi on average. Insignificant increase could be observed for both dP curves of 0.4 mL/min in the range of 0.3-0.5, while both dP curves of 0.3


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mL/min did not vary noticeably. The curves of dP overlaid, and 5 500 0.3 curve was the highest at most foam qualities. Hence, the presence of salt affected the right order of dP changes. Because the dP curves did not differ substantially, the apparent viscosity curves of 0.3 mL/min were substantially higher than of 0.4 mL/min, as could be expected from Eq.2 (Figure 6b). The length of foam drops almost linearly increased up to the inflection points. In this range, the Ld were about two times shorter than at 1% NaCl showing reduced foamability at higher salinity. For the exception of 5 500 0.4, the curves of drop length were very close in the range of 0.3-0.7 of foam quality. The length of foam drop was longer at 500 psi than at 400 psi at a specific total flow rate. If the curve of 5 500 0.4 was shifted along horizontal axe, it would overlaid other curves. Such shift between the curves resulted from the close gas flow rates for different foam quality (Figure 2). For example, 4.46 mL/min was at 0.4 for 500 psi and at 0.5 for 400 psi (total flow rate 0.4 mL/min). The inflection points in Ld curves were shifted to 0.7 compared to transition foam quality of 0.6 in the curves of apparent viscosity following the change of foam texture. The foam texture was similar to that of at 1% NaCl in the range of 0.3-0.7 while the bubble size increased to 2 mm at 0.8 and 5 mm at 0.9.


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5% NaCl 140 120 100 80 60 40 20 0 0.3

0.5

a)

0.7

0.045 0.040 0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000

0.9 b)

fg

16

𝑓𝑡𝑟 𝑔

Ld, cm

Apparent viscosity, Pa.s

160

dP, psi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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14 12 10 8 6 4 2 0

0.3

0.5

fg

0.7

0.3

0.9

0.5

c)

0.7 fg

5 500 04

5 400 0.4

5 500 0.4

5 400 0.4

5 500 0.4

5 400 0.4

5 500 0.3

5 400 0.3

5 500 0.3

5 400 0.3

5 500 0.3

5 400 0.3

0.9

Figure 6. Foam quality scan results for surfactant solution of 8% NaCl: a) dP; b) apparent viscosity of foam; c) length of foam drop before fall. Abbreviation e.g. 5 500 0.4 indicates that the data are obtained for 5% NaCl, pressure of 500 psi and total flow rate of 0.4 mL/min.

3.4 Foam scans at 8 % salinity As followed from the compatibility tests, the surfactant precipitation at higher salinities reduced at 60 oC compared to 22 oC. In order to reduce the surfactant precipitation in the surfactant accumulator before surfactant solution entered the coreholder, it was placed in the oven at the salinities above 5% NaCl. At 8% NaCl, the foam scan was carried out firstly for 400 psi 0.4 mL/min, 400 psi 0.3 mL/min and fg=0.3, 0.4, 0.5, 0.8 at 500 psi 0.4 mL/min. After a break of about 20 days, the foam scan was continued for the remaining fg=0.6, 0.7 at 0.9 for 8 500 0.4 and total foam scan for 8 500 0.3. However, the permeability measured after the break was 30 mD instead of 50 mD, compared to the previous tests. At 8% NaCl, the homogeneous foam similar to the foam texture observed at 1% or 5% NaCl (Figure 5) was only formed at 0.3-0.4. The garlands of 2-3 cm length consisting of 2-4 bubbles 16 ACS Paragon Plus Environment

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were observed at fg=0.5-0.7 and single bubbles of bigger size up to 1 cm at fg=0.8, 0.9. Only single bubbles were observed at all foam qualities for 8 500 0.3 performed after the break in testing. At 8% NaCl, the shape of dP curves was close to linear, particularly starting from fg=0.4 or 0.5 (Figure 7a). The change of the curve slopes occurred at the points where the foam texture changed from foam to bubbles. The dP in foam scans measured before break decreased in the order similar to the decrease of gas flow rate: 8 500 0.4; 8 400 0.4; 8 400 0.3. The dP measured after the break for 8 500 0.3 and remaining foam qualities of 8 500 0.4 were by 50 psi on average higher. To convert dP to apparent viscosity, permeability of 30 mD was used for 8 500 0.3 and for fg=0.6, 0.7 and 0.9 at 8 500 0.4. The points of apparent viscosity at fg=0.6, 0.7 and 0.9 fell exactly on the trendline that crossed fg=0.3, 0.4, 0.5 and 0.8 for 8 500 0.4. The dP at 0.4 mL/min was higher than at 0.3 mL/min while it was the opposite for apparent viscosity. A crossover was observed starting from 0.7, where viscosity of 8 500 0.4 was higher than of 8 400 0.3. The foam coalesced leading to a sharp decrease in viscosity. The linearized shapes of the curves indicated that 8% NaCl was a limited salinity for the foam generation. Indeed, the foam was not generated at 9% NaCl and 10% NaCl. Higher salinity could have caused the surfactant partition and precipitation. The horizontal position of the coreholder could have also played role because the surfactant emerge to the surface of liquid at high salinities.


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8% NaCl 0.035


180 160 140 120

dP, psi

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 60 40

0.025 0.020 0.015 0.010 0.005

20

0.000

0

a)

𝑓𝑡𝑟 𝑔

0.030

0.3 8 500 0.4 8 500 0.3

0.5

0.7

fg

0.9

b)

0.3

8 400 0.4 8 400 0.3

0.5

0.7

fg

8 500 0.4

8 400 0.4

8 500 0.3

8 400 0.3

0.9

Figure 7. Foam quality scan results for surfactant solution of 8 % NaCl: a) dP; b) apparent viscosity of foam. Abbreviation e.g. 8 500 0.4 indicates that the data were obtained for 8% NaCl, pressure of 500 psi and total flow rate of 0.4 mL/min.

3.5 Effect of total flow rate on foam strength The shape of the curves of 1% and 5% NaCl differed significantly from the shape of the curves, reported by [2, 3, 13, 14, 18-20]. The reported curves demonstrated shear-thickening behavior with sharp increase of apparent viscosity in low quality regime. In our experiments, the slopes of curves in a low quality regime were either gentle or the curves demonstrated all attributes of shearthinning behavior including the Newtonian plateau. Various factors can affect the shape of apparent viscosity curve that include temperature, surfactant concentration, flow rates and others. At low temperature of 20 oC, the curve demonstrated shearthickening behavior and a triangular shape with maximum viscosity of 300 cP at 0.8 at 4 ft/day. At 120 oC, however, the viscosity fluctuated around 50 cP with no apparent trend over entire range of 0.3-0.8.4 Substantial decrease of a slope with the temperature increase from 20 oC to 80 oC was 18 ACS Paragon Plus Environment

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shown for AOS solution.2 Low surfactant concentrations also caused the shift of the transition foam quality to lower values and a change of the shape from triangular to fold.20 The experimental results of foam scans were reported at different total flow rates or velocities, and units, which makes them difficult to compare. For example, total flow rates of 4.64 ft/day (Kapetas et al.)2, 10–20 ft/day (Chen et al.)17, up to hundreds of feet per day (Chen et al.)18, 1 ml/min (1.4 × 10−5 m/s) (Kahrobaei et al.)14, 2 ml/min (Xu et al.)19,10 ft/day (1.031 cc/min) (Zhou et al.)21, total superficial velocity of 938 ft/day (Elhag et al.)3, (2.4 ± 0.4) × 10−5 m/s (Jonesa et al.)22, 208 and 833 ft/day (Xue et al.)23, 7.9 and 28.3 ft/day (Ma et al.)24. Only one publication reported the fold shape of the apparent viscosity curve in a range of fg=0.1-0.9 with a top at 0.5 at the low total flow rate of 0.4 mL/min (Wei et al.)8. Meanwhile, the foam apparent viscosity may be underestimated or overestimated at high flow rates because of the shear-thinning or shear-thickening effects (Chen et al.).18 Cui et al. has shown that apparent viscosity of 1% Ethomeen solution was close to null at the superficial velocity of 2.5 ft/day and abruptly increased to about 140 cp at 5 ft/day, further decreasing exponentially in the range of superficial velocity of up to 170 ft/day.4 Foam was generated at high gas fractions of 0.7-0.8 at the flow rates above 10 ft/day only. In addition, the rocks with high permeability, and high temperature and salinity required even higher flow rates for the foam generation that do not exist in reservoir operations. Increase of the apparent viscosity with the increase of total superficial velocity starting from 1 ft/day was also shown by Ma et al.24. The slope of the curves was sharper at steeper velocity gradient.18,24 The injection velocities applied at the oil reservoirs typically do not exceed 1-2 ft/day, equivalent of superficial velocity of 3.53*10-6 – 7*10-6 m/s. In the presented study, the foam scans were


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obtained at low total flow rates of 0.4 ml/min and 0.3 mL/min, equivalent of superficial velocities of 4.39 *10-6 m/s and 5.88*10-6 m/s, respectively. For comparison, 1 mL/min is 1.49*10-5 m/s. It is also worth noting that in our experiments with Berea sandstone (250 mD) at 60 oC, the shearthinning behavior was obtained at 0.3 mL/min and 0.4 mL/min and shear-thickening at 1 mL/min. In addition, shear-thinning behavior have been observed in our experiments using Bentheimer sandstone of 1700 mD permeability at 0.3 mL/min and 0.5 mL/min and 0.5% AOS surfactant conducted at Delft Technical University following the procedure described in Kahrobaei et al.14. The sharp slope of dP curve in a low quality regime was only observed at 1.5 mL/min. The low total flow rates also caused the shift of the transition foam quality to the lower value of 0.6 in the present experiments compared to 0.8 at reported high total flow rates.2,4 At low total flow rates, the capillary forces are strong, and, therefore, the shape of the curves is similar to capillary curves.25,26 The increase in the volume of injected fluid (fg) does not lead to the substantial pressure increase at low total flow rates because all volume of fluid passes through the pores. At high total flow rates, the displacement is piston like and the capillary forces are negligible. Big volumes of foam cannot arrange quickly enough to squeeze through the porous channels leading to substantial pressure increase.

3.6 Salinity effect on foam strength The experimental curves obtained at 1, 5 and 8% NaCl were compared in Figure 8. Generally, the dP at all salinities decreased in the same order as gas flow rates at varying total flow rate and pressure (Figure 2): 500 0.4; 400 0.4; 500 0.3; 400 0.3. However, if the measured dP were close, the apparent viscosity curves overlaid or followed the order: 500 0.3; 400 0.3; 500 0.4; 400 0.4.


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The dP of all 5% NaCl were lower than of 1% NaCl at 0.4 mL/min and higher than of 1% NaCl at 0.3 mL/min (Figure 8a). On the contrary, the apparent viscosity of all 1% NaCl were lower than of 5% NaCl at 0.3 mL/min and higher than of 5% NaCl at 0.4 mL/min (Figure 8b). The dP at 8% NaCl was the lowest but the apparent viscosity at 0.3 mL/min was higher than of 5% NaCl at 0.4 mL/min and 1 400 0.4 at fg=0.3-0.4. The salt precipitation could cause the increase of dP. However, we additionally observed that the foam decayed slower at 5% NaCl than at 1% NaCl during foamability tests in a glass column. Upon the decrease of the foam column, the foam bubbles longer remained attached to the walls of glass column and had a crystal look at higher salinity indicating presence of salt. This showed that higher salinity might have strengthened lamella and increased the foam density and viscosity. For comparison, the apparent viscosity of the C12 Ethomeen solution in deionized water was about 2.5 times lower than in brine, and the transition foam quality was 0.7 with deionized water compared to 0.8 with brine. 4 180

0.05

Newtonian Plateau

160


140

dP, pis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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120 100 80 60 40

0.04 0.03 0.02 0.01

20 0

a)

0.3 1 500 0.4 1 400 0.3 5 500 0.3 8 400 0.4

0.5

fg

0.7

1 400 0.4 5 500 0.4 5 400 0.3 8 500 0.3

0.9 1 500 0.3 5 400 0.4 8 500 0.4 8 400 0.3

0 0.3

b)

1 500 0.4 1 400 0.3 5 500 0.3 8 400 0.4


0.5

fg

1 400 0.4 5 500 0.4 5 400 0.3 8 500 0.3

0.7

0.9 1 500 0.3 5 400 0.4 8 500 0.4 8 400 0.3

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Figure 8. Comparison of the results obtained at 1, 5 and 8 % NaCl at 0.3 and 0.4 mL/min and 400 psi and 500 psi.

3.7 Length of foam drop as viscosity indicator Linear relationships exist between drop length and viscosity.16 The droplet length viscometers are used for Newtonian fluids and non-Newtonian fluids such as shear thinning fluids. However, there was no clear dependence between dP or apparent viscosity and drop length in Figure 4c and Figure 6c. The unitless parameters such as foam quality often mask effects of other parameters and can lead to incorrect conclusions. The Ld curves at both salinities of 1% and 5% NaCl were plotted vs. respective gas flow rate in Figure 9. Generally, the Ld curves followed the order reverse to gas flow rate in Figure 2. For the constant gas flow rate, shown as a dashed line, the drop length decreased with increasing total flow rate because it is related to different foam quality: fg=0.3 for 500 0.4, fg=0.4 for 400 0.4 and 500 0.3, and fg=0.5 for 400 0.3. The distribution of Ld curves of 5% was similar to that of 1 % NaCl. The lines of respective gas flow rates for the specific foam quality given in Figure 2 were plotted in Figure 9a. The curves of drop length closely followed the lines of gas flow rates in the range of fg=0.2-0.5 before the achievement of inflection points at fg=0.5. This demonstrated that the length of foam drop strongly depended on the gas flow rate in a low quality regime. The apparent viscosity in low foam quality regime did not vary substantially while the viscosity of foam flow was decreasing at increasing foam quality, as followed from the increasing length of foam drops consisting of bigger bubbles. In order words, different foam textures generated equal dP leading to equal apparent viscosities. However, the volume of viscous foam produced at fg=


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0.3 can be 2-3 times lower than at fg=0.6. This discrepancy between apparent viscosity and viscosity of foam at the outlet of the system can lead to the underestimation of the foam mobility, and volumes of produced foam and arrived gas by using apparent viscosity.

20

Ld, cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15

10

5

0

b)

0

5 5 500 0.4 5 500 0.3

qg, mL/min

10

5 400 0.4 5 400 0.3

Figure 9. Drop length vs. gas flow rate: a) 1% NaCl; b) 5% NaCl. At constant gas flow rate (qg) (dashed line), Ld decreases at increasing total flow rate.

Conclusions 1. The compatibility test at 1-11% NaCl demonstrated that the foam volume on the surface of surfactant solutions immediately after preparation increased with increasing salinity. This indicated that the foamability did not decrease while the surfactant solubility in brine decreased at higher salinity. At 60 oC, the surfactant precipitation occurred starting from 8% NaCl while surfactant emerged to the surface at 9-11% NaCl because of the increased density of brine. 23 ACS Paragon Plus Environment

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2. At all salinities, the differential pressure decreased in the order similar to the gas flow rate decrease: 0.4 mL/min 500 psi; 0.4 mL/min 400 psi; 0.3 mL/min 500 psi; 0.3 mL/min 400 psi. After conversion to apparent viscosity, the curves overlaid or decreased in the order of: 0.3 mL/min 500 psi; 0.3 mL/min 400 psi; 0.4 mL/min 500 psi; 0.4 mL/min 400 psi. 3. Because of low total flow rate in our experiments, the shape of the dP and apparent viscosity curves was typical for shear-thinning fluids in contrast to shear-thickening behavior observed by other authors at higher total flow rates. The linear shape of dP/viscosity curves at 8% NaCl indicated that the respective salinity was limiting for the foam generation. 4. The foam flow was fine and homogeneous at the outlet of the system at 1% and 5% NaCl at all foam qualities. At 8% NaCl, foam was generated at 0.3 and 0.4 foam quality while only bubbles were formed at 0.5-0.9. 5. In a low quality regime, viscosity of foam at the outlet of the system decreased while the apparent viscosity remained close to constant at increasing foam quality. This discrepancy can lead to the underestimation of the foam mobility and produced volumes of foam. Effect of total flow rate, salinity and decrease of foam viscosity at the outlet of the system should be considered in site operation and numerical modeling.

Acknowledgment We gratefully thank the Petroleum Development Oman for sponsoring this project CR/DVC/OGRC/17/01. We are also grateful to Karl-Heinz Wolf, Rouhi Farajzadeh and Siavash Kahrobaei from Delft Technical University for technical help and valuable discussion.


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Nomenclature AGES Alkoxyglycidylether Sulfonates CHSB Socamidopropyl Hydroxyl Sulfobetaine SDS

Sodium Dodecyl Sulfate

AOS

Alpha Olefin Sulphonate

IOS

Internal Olefin Sulfonate

dP

Pressure gradient, psi

fg

Foam quality (gas fraction)

ftr g

Transition foam qaulity

𝑞𝑙

Liquid injection flowrate, mL/min

𝑞𝑔

Gas injection rate, mL/min

ICS

Instrument Contamination System hydraulic pressure pump

BPR

Back Pressure Regulator

Ld

Length of foam drop, cm

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