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Efficiency and Sustainability
An Integrated Property-Performance Analysis for CO2-Philic Foam Assisted CO2-EOR Seyedeh Hosna Talebian, Muhammad sagir, and Mudasser Mumtaz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01131 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018
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Energy & Fuels
An Integrated Property-Performance Analysis for CO2-Philic Foam Assisted CO2-EOR Seyedeh Hosna TALEBIAN 1,3*, Muhammad SAGIR2, MudasserMUMTAZ3 1 Production Technology, Centre of Excellence, PETRONAS carigali 2 Department of chemical engineering, university of Gujrat, Gujrat, Pakistan 3 Department of petroleum Engineering, University Teknologi PETRONAS, Malaysia
ABSTRACT
Foam is used in CO2-EOR due to its potential high benefits in mitigating all three causes of CO2 poor sweep efficiency, as it provides a means to lower the effect of permeability heterogeneity, overcome viscous instability, and minimize the occurrence of gravity override. The conventional foaming surfactants are not suitable in contact with oil due to premature lamellae rupture, need of copious amount of water to generate foam, surfactant loss due to adsorption on the rock or partitioning between water and oil, and less tolerance against salinity, pressure and temperature. The surfactant blending and addition of CO2-philic functionalities in surfactant structure is suggested to mitigate the above problems, enhance foam stability, improve mobility control, and accelerate foam propagation. However, there is a lack of general guideline on the evaluation of CO2-philic surfactant properties and applications and the surfactant structure-performance analysis. In the present work, the tailor-made laboratory tests and simulation analysis was
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conducted on CO2-philic surfactants with different structure and chain lengths in conditions close to a Malaysian reservoir case and in the presence of oil. The results from experiments combined with analytical analysis of foam flow parameters are used to provide comprehensive simulation model of CO2-philic surfactant alternating gas process. A meaningful correlation between the CO2-philic surfactant structure and the sensitivity of foam model to different parameters was observed. Based on sensitivity analysis results, optimization of CO2-philic surfactant activity at gas-water and oil-water interfaces can improve the system recovery through macroscopic and microscopic displacement. Keywords: CO2-philic surfactant, Foam Functional Model, Interfacial interaction, Mobility reduction factor 1 INTRODUCTION The microscopic sweep efficiency in carbon dioxide (CO2) injection is potentially very high as a result of miscibility, diffusion and promotion of oil swelling, which makes CO2 injection a favorable enhanced oil recovery (EOR) technique for over 40 years 1-4. Often CO2 is injected into a waterflooded oil reservoir, where the target oil is the water-bypassed oil or the residual oil to waterflood, trapped by capillary forces established during waterflood. In case of strongly waterwet reservoir, residual oil to waterflood is primarily a discontinuous phase of globules of oil trapped by the displacing brine as a result of water shielding, and the CO2 rich phase might not have sufficient capillary pressure to enter all the pores containing the remaining oil.5 In consequence, oil in the smaller pores might not be in contact with CO2 and could remain shielded from contact with CO2.6 Therefore, if CO2 source is available, shifting to CO2 injection in early stages of production without going through lengthy waterflooding process is of interest for water-wet systems, which can also dramatically lower the water injection/treatment cost and 2 ACS Paragon Plus Environment
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injectivity issues.6,
7
However, high CO2 mobility and low density can lead to a number of
conformance and mobility contrast issues such as fingering, gravity segregation and early breakthrough in the production well, which is accentuated in heterogeneous reservoirs.8 Water alternating gas injection (WAG) can be a choice to control the unfavorable CO2 mobility ratio. CO2-WAG however, is strongly influenced by viscous-gravity ratio and reservoir stratification layer If stable enough in presence of oil, CO2-Foam can mitigate all three causes of CO2 poor macroscopic sweep efficiency and improve oil recovery by taking advantage of the synergistic combination of chemical- and gas-EOR methods.1, 9, 10 However, the efficiency and economy of surfactant-generated foam process is not only dependent on the CO2 mobility reduction, but also on the fluid–fluid and rock–fluid interactions involved in the process such as foam-oil interaction, surfactant loss mechanisms and interfacial tension properties.11 Numerous laboratory studies and several field experiences have indicated several limitations involved in conventional aqueous soluble surfactants as foaming agents; Foam dry-out and collapse in the absence of enough water, especially below the critical water saturation 12, 13, the detrimental effect of oil on foam at oil saturations above the so-called critical foaming saturation14 and the lack of a consistent theory establishing a favorable wetting phase of the rock for the formation and propagation of foam15 can be addressed as several challenges of conventional foaming agents. Although recent experimental works showed a reduction in the adsorption of anionic surfactants on formation rock due to the effect of certain sacrificial agents, it was not as effective in sandstone as it was for the carbonates and clays16. Other than adsorption, surfactant loss associated to partitioning to the oil (microemulsion) phase can cause reduced mobility control and slow foam propagation.17, 18
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The addition of CO2-philic moieties along the chains of the hydrocarbon surfactants and surfactant blending have been given importance in recent years to mitigate the issues involved in the conventional foam assisted WAG (FAWAG) applications.19, 20 The typical composition of a foaming agent formulation comprises of foaming surfactant, foam booster, and foam stabilizer. While foaming surfactant and foam booster provide foam generation function, foam stabilizer can provide stability of foam for the surfactant blend. Like the conventional surfactants, CO2philic surfactants have head and tail parts. The sulfonated head groups provide high stability at high temperature and the tail groups have a distinct affinity for CO2 which results in reducing the interfacial tension between CO2 and brine and stabilizing the generated foam. The surfactant tail structure and CO2 philicity has shown to be interdependent, which suggest that a qualitative and quantitative study of structure-property relationship to define the optimum tail length suitable for the maximum W/CO2 microemulsion formation is possible.21, 22 Sanders et al. (2010) reported that a double-tail surfactant has more contact with the interface and consequently offers more stability than a linear CO2-soluble surfactant 19. While branching, low molecular weight, addition of side chains, increase in the methyl group number, lower number of methylene groups and the presence of propylene oxide (PO) have also been identified as factors that favor CO2 solubility of the hydrophobic part of the surfactant.21-23 As demonstrated in the field trial
19
, the injection of
the surfactant in CO2 could allow the surfactant to flow with the CO2-preferred flow path and by reducing the CO2 mobility, increase CO2 diversion into the zones that had previously not seen CO2 and raise sweep efficiency. Ren et. al (2017) compared the performance of novel CO2soluble surfactant with conventional surfactant in both coreflood experiment and field scale simulation and reported better stabilization ability, faster foam propagation and higher sweep efficiency of novel foam, regardless of the injection strategy.
24
They also tested the continuous
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CO2 injection with dissolved CO2-soluble surfactant in consolidated cores for the first time and demonstrated significant improvement in surfactant transportation ability and displacement rate and higher injectivity.24 Lowered surfactant loss due to partitioning in oil (o/w) and less surfactant sensitivity to reservoir conditions (temperature and salinity) was reported by Chen et al. (2014) using hybrid ethoxylated CO2-soluble surfactant.25 The effect of surfactant CO2/W partitioning on foam propagation was studied by Ren et al. (2013) to optimize the injection strategy for CO2-soluble surfactants. Unlike the conventional surfactants, larger cycle of SAG was shown to weaken the foam for all CO2-soluble surfactants tested. In their study, although the effect of partitioning was case dependent due to the hybrid effect of adsorption and surfactant concentration, the CO2 displacement ratio increased with decreasing surfactant partitioning.26 The adsorption of nonionic CO2-soluble surfactants on dolomite rock was also compared with the conventional surfactant, and a significant reduction in adsorption level was observed in CO2soluble surfactants. However, within CO2-soluble surfactants, a gradual increase in adsorption with increased CO2 philicity was also observed.
26
The combined effect of low adsorption, high
CO2/W partition and high cloud points has been achieved in hybrid surfactant blends with suitable combination of tail groups.25,
27
Nevertheless, it is recommended in recent works on
novel CO2-soluble surfactants to further examine the active mechanisms, rock-fluid properties and injection strategies in the presence of CO2-soluble surfactants through tailor-maid laboratory tests and simulations.24 The evaluation of CO2-philic surfactant blends in CO2 mobility control application based on the surfactant structure (tail groups and branches), surfactant properties (fluid/fluid, and fluid/rock interfaces), and the foaming functionality (stability and mobility control) is the main focus of the present work to narrow down some of the technical gaps involved in estimating the
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performance of newly developed blends under the conditions of the reservoir. Coreflood results and all measured properties were used in a commercial reservoir simulator that has adopted the local equilibrium method (LE) to determine fitting parameters of model and active mechanisms involved based on observed characteristics of novel foams. 2 MATERIALS AND METHODS 2.1 Materials The characteristics of Malaysian crude oil, formation brine used in this study are briefly described in Tables 1 and 2. FomaxVII is a blend of two anionic and one amphoteric component with defined weight percent, and FomaxII is a blend of three anionic surfactants. The anionic surfactants contain CO2-philic groups such as twin-tailed methyl group, carbonyl group, and propylene-oxide groups. UTP-Foam is a twin-tail, branched structure surfactant which contains CO2-philic groups such as, carbonyl and propylene oxide groups. Polymers with high distribution of charges and high degree of cross linking are used as foam stabilizer in UTP-Foam surfactant formulation. Properties of the foaming agents are presented in Table 3. The CO2-philic trend of surfactants follows the order of UTP-Foam> FomaxVII> FomaxII> AOS, where the anionic alpha-olefin surfactant (AOS) is CO2-insoluble surfactant. Table 4 presents the properties of Berea cores used for displacement tests. Core C was cut into three equal length cores for mobility reduction factor measurements at the conditions of the reservoir. 2.2 Experimental Methodology The experiments conducted to study the surfactant structure-property analysis are surface and interfacial tension (IFT) measurements, CO2 solubility in surfactant solution, bulk foam stability
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tests, surfactant adsorption onto the rock, mobility reduction factor measurement (MRF) and surfactant alternating gas (SAG) dynamic displacements. Te pendant drop measurement system (Vinci IFT700), the SOLTEQ BP-22 high pressure solubility cell, the dynamic foam analyzer (KRUSS DFA100) and the coreflood apparatus from Sanchez Technologies were used to perform experimental analysis. The extensive details of experimental tools and procedures can be found in previous publications.27-30 To measure the CO2 mobility control associated to surfactants, the RPS-800 coreflood system was used to measure the pressure drop along the core during CO2 flow in formation brine saturated core, and CO2 flow in surfactant saturated core at a constant flow rate of 0.22 cc/min. Berea sandstone core C was cut into three 6 inch cores to evaluate foaming properties of different surfactants at the same porous media conditions. The test was performed at conditions close to the reservoir conditions (124 bar and 90 °C). The MRF value is calculated by using eq 1: =
∆ ( ℎ ) (1) ∆ ( ℎ )
2.3 Foam Assisted WAG Simulation In LE model, the gas mobility is altered due to the presence of foam in the system by a function which captures the effect of different physical parameters. According to Darcy’s law, whether foam decreasing the relative permeability or increasing the viscosity of the gas phase by same factor, will results in the same mathematical expression 31. The Eclipse model is using the relative permeability-based functional form of gas mobility reduction ( ), as described in eq 2, where is the reference mobility reduction factor (MRF in the absence of oil), is the surfactant concentration component, is the water saturation component, is the oil saturation component and accounts for the effect gas velocity.32 As defined in eq 3-6, each of foam 7 ACS Paragon Plus Environment
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function parameters is regulated by two parameters (the property itself and the weighting factor), which needs to be defined base on laboratory tests.
=
1 (2) 1 + ( . . . . )
= ( )
(3)
where ! is the effective surfactant conentration and ! is the reference surfactant concentration, above which the presence of surfactant becomes significant in the creation of foam. " is an exponent which controls the steepness of the change in mobility reduction with concentration.
= 0.5 +
, -) %&%' (( .)*( +*(
(4)
.
1 where S0 is the water saturation, S0 is the limiting water saturation below which the foam ceases
to be effective, f0 is the weighting factor which controls the abruptness of foam collapse, and π is the constant Pi number. = (
*34 +*3 *34
)
3
(5)
where S56 is the maximum tolerable oil saturation for foam to be stable. If S5 >S56 , foam ceases to be effective and F5 equals to 0. e5 is an exponent which controls the steepness of the transition about the point S5 = S56 . 9
= ( : ) 9:
;
(6)
where ; ∆>?
3 @A/(
=
C @3/(
@A/(
= 0.15 @
(7)
3/(
Where D is the radius of oil droplet, DE is the radius of the Plateau border and 0.15 denotes the average ratio between D and DE . As tabulated by Simjoo et al. (2013), the combination of L value below 1 and negative S and E coefficients defines quite stable foam.44 In case of UTP-Foam, the L number is below 1 and spreading coefficient is negative, but the entrance coefficient is positive value of 1.47 at values above CMC. In this case, oil droplets remain as discrete droplets and not spread and foam bridges across the bubbles causing premature lamellae rupture. According to the literature, in hydrocarbon surfactants, chain branching gives a higher CMC than a comparable 13 ACS Paragon Plus Environment
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straight chain surfactant.45 As can be seen form Figure 4a and b, the CO2-philic FomaxVII and UTP-Foam surfactants show higher CMC values at both g/w and o/w interfaces in comparison with linear-structure, anionic AOS surfactant. FomaxVII surfactant with branched-chain structure and lower MW (lower carbon number), has higher CMC values at o/w interface than the twin-tailed UTP-Foam surfactant. Higher CMC of surfactant at o/w interface is reported to have marked influence on the adsorption of surfactant, where higher the CMC value, lower will be the adsorption.28 Figure 6a is the adsorption comparison of UTP-Foam and FomaxVII surfactants with AOS, while Figure 6b is the comparison of CO2 solubility capacity of FomaxII surfactant with AOS and formation brine. As shown in Figure 6a, both CO2-soluble blends demonstrate lower adsorption onto the rock compared to AOS, while adsorption reduced dramatically for UTPFoam surfactant associated to increased number of CO2-philic branching and reduced chain length. According to the literature, increasing the number of side branches in the surfactant structure reduces the adsorption
46
, while favorably increases the surfactant interaction with
CO2.47 The slightly higher CO2 solubility in FomaxII in comparison with AOS at both measured temperatures can be due to higher MW and the long-straight carbon chain in the structure of surfactant. 3.1.3. Effect of surfactant concentration (* ) To estimate the possible range of input parameter values to be used for the effect of surfactant concentration on the gas mobility reduction, the numerical limits on the parameter range can be studied independently using eq 2 and 3, keeping other parameters constant. The variation of gas mobility reduction ( ) and * is plotted as a function of !* at different exponent values ("* ) for two CO2-philic surfactants, in Figure 7a and 7b. The reference mobility reduction factor ( )
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values for each concentration are taken from the experimental data shown in Figure 2, for each surfactant. 3.1.4. Effect of water saturation (F ) , as stated in eq 4, is related to the effect of water saturation on foam function, where GH is the limiting water saturation below which the foam ceases to be effective, is the weighting factor which controls the abruptness of foam collapse. For saturations below limiting water saturation, the foam dies out and its effect is reduced significantly. The value of atan ( . (G − GH )) can be between –π/2 and +π/2 depending on and the saturation difference. The corresponding range of is between 0 and 1.0. To decide on an appropriate value of , the variation of and can be plotted based on eq 2, keeping other parameters and conditions constant. The variations of these parameters are shown in Figure 8a and 8b, for two CO2-philic FomaxVII and UTP-Foam surfactants, at 1 wt.% concentration, and constant reservoir conditions. The foam dry-out saturations for these surfactants can be extracted from dynamic coreflood experiment, which is discussed in the next part. 3.1.5. Effect of Capillary Number ( ) accounts for the effect of gas velocity on gas mobility control via capillary number. The capillary number, as the dimensionless ratio of viscose to capillary forces, in Eclipse model is calculated as eq 8; ‖
(8)
@A(
where K is permeability, P is pressure across the core, PQ is the g/w surface tension, and !9 is a unit conversion factor.
