Foam Performance in Low Permeability Laminated Sandstones

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Foam Performance in Low Permeability Laminated Sandstones Jonas S. Solbakken,*,†,‡ Arne Skauge,†,‡ and Morten Gunnar Aarra† †

Centre for Integrated Petroleum Research, ‡Uni Research, University of Bergen, Allégaten 41, N-5020 Bergen, Norway ABSTRACT: Most studies on foam are related to homogeneous and highly permeable porous media. As the reservoir situation is rather heterogeneous with respect to permeability and layering, foam properties in layered porous media with lower permeabilities are also important to understand. This study investigates foam behavior and performance in naturally laminated sandstone cores. Laminations are common constituents in sandstone petroleum reservoirs where they usually occur as thin deformed layers in the host formation. Evidences of rock heterogeneity were confirmed by several different analyses on laminated material. From image processing of thin sections and 2D X-ray experiments, the laminas present were found to exhibit both lower porosity and permeability than the host rock, and also shown to form barriers to fluid flow. Foam experiments were performed in three low permeability sandstone cores with relative similar permeability but with a different degree of laminated stratifications parallel to flow direction. Foam was generated in all the low permeability laminated cores. However, the degree of lamina in each core influenced foam performance significantly, reflected by large variations in mobility reduction factors (MRF ∼ 20−500) and foam breakthrough times. Increased lamination resulted in weaker foams and earlier foam breakthroughs. One explanation to this could be that the low permeability laminas introduce different degrees of discontinuities and compartmentalization to foam flow. Findings in our study indicate that foam properties and performance can be strongly influenced by local heterogeneities, such as laminations naturally found in many sandstone reservoirs.

2.2. Foam in Heterogeneous Porous Media. Permeability contrasts within the porous media may lead to further intensifying in instabilities of the gas injection front, such as gas channeling and excessive flow through the most permeable regions. Foam has been recognized as a promising method for controlling gas mobility in heterogeneous porous media. A favorable property of foam is that foam generation will occur in the most permeable zones first, diverting flow to the less permeable zones. This property has been confirmed by several researchers when two cores with contrasting permeabilities were arranged in parallel with no capillary communication.18−24 Siddiqui et al.23 found that the permeability difference between two cores played the most important role in foam diversion and that the chances of getting diversion improved when the permeability contrast increased. This result was contradictory compared to earlier findings.24 Others have investigated foam flow performance in heterogeneous systems where capillary contact and flow among layers were allowed, denoted as crossflow.13,25,26 Selective mobility reduction (SMR) and selfregulating foam behaviors have been reported when crossflow was possible.25,26 An ideal SMR or self-regulating behavior implies that the foam displacement front will propagate at equal velocity in each layer, independent of permeability. Bertin and co-workers26 confirmed this effect on a heterogeneous porous system made of consolidated sandstone surrounded with unconsolidated sand in the annular region. Gas breakthrough close to one pore volume injected was used as a measure to indicate how favorable fluid mobility in heterogeneous porous media could be when foam was applied.

2. INTRODUCTION The low gas viscosity relative to water and oil makes the gas very mobile in porous media. A critical factor of a regular gas injection related to oil recovery is therefore early gas breakthrough and, consequently, poor sweep efficiency. Foam can be applied to control gas mobility in porous media. When gas is dispersed in a liquid, the gas phase becomes discontinuous and less mobile by continuous liquid films. A surfactant is normally used to stabilize the gas−liquid surface. Injection of foam can block and divert fluid flow, which may improve the sweep efficiency. Several successful foam field projects have been accomplished. The generated foams have shown stability and robustness to tough test conditions and as such qualified foam as an enhanced oil recovery (EOR) method.1−4 2.1. Foam in Low Permeability Porous Media. The earliest studies on foam identified and emphasized the efficiency of foam to reduce gas mobility in high permeable and homogeneous sand packs.5−7 Experimentally, Lee and Heller8 and Mannhardt and Novosad9 both found the foam strength, defined through the mobility reduction factor (MRF), to decrease with lowering core permeability. Some authors have speculated if a threshold in permeability to foam may exist, often discussed in relation to the limiting capillary pressure theory, where the capillary pressure is thought to control foam properties. The stability of foam in porous media depends on how the limiting capillary pressure varies with permeability, which also is reported to depend on surfactant type and concentration, salinity, rock type, foam quality and gas/water flow rates used.10−13 Nevertheless, several studies have generated strong foams in relative low permeability porous media and, thus, questioned if a threshold in permeability to foam really exist.14−17 © 2014 American Chemical Society

Received: August 13, 2013 Revised: January 19, 2014 Published: January 23, 2014 803

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This study investigates foam behavior and performance in naturally laminated sandstone core material with lower permeability than normally studied. The literature on foam in naturally layered systems is limited. One major challenge with such complex systems is obviously to detect and quantify the heterogeneity. A suite of techniques were therefore used to analyze and better characterize the laminated rock material prior to experimental foam studies. Foam was generated by coinjection of surfactant solution (AOS C14−C16) and N2-gas at elevated experimental conditions. The laboratory results presented show significant different foam behavior and performance in three heterogeneous cores with relative similar permeability. This might have great implications for realistic and accurate simulation and scaling of foam flooding processes. The detection of core heterogeneity through various rock analyses seems important toward improved understanding of foam results in heterogeneous reservoir core material.

Figure 1. Experimental setup for high pressure and high temperature foam experiments: dP = differential pressure, P1 = injection pressure, P2 = production pressure, GP = gas pump (Quizix), SP = surfactant pump (Quizix), BPR = backpressure regulator.

3. EXPERIMENTAL SECTION 3.1. Core Material. Physical properties of the outcrop Berea sandstone core material used in this study are given in Table 1. The cores were named after degree of lamina present in each core: Weakly laminated (B-WL), moderately laminated (B-ML), and strongly laminated (B-SL).

injection rate corresponds to a superficial velocity of 0.9 m/day. Foam quality (i.e., fraction of gas) was always 0.80 ± 0.01 at the inlet end of the core. Two HPHT Quizix pumps located inside the heat cabinet were used to control the injection rates. The pumps are able to be continually refilled at the inlet pressure of the core. This ensures constant inflow foam quality throughout the entire experiment. Confining pressure was kept 45 ± 5 bar over the injection pressure (P1) at all times. Behind the core outlet, a sight-glass was mounted on the line. Through the sight-glass, with inner diameter of 1.5 mm, we were able to observe foam at experimental conditions out from the core. The sight glass was used to confirm foam generation and to determine foam breakthrough times. Fluids that were produced from the core were either collected into production cylinders or produced through a back pressure regulator (BPR) at constant outlet pressure (P2). Production cylinders can be advantageous to use to provide acquisition of pressure data with minimum noise or oscillation often associated with the back pressure regulator. A heat cabinet supported the experimental setup with constant elevated temperature (±1 °C). Several experiments were performed in the same core. Before the next experiment was started, the core was usually depressurized and then flooded with large volumes of brine (3 wt % NaCl) and isopropyl alcohol (a good gas dissolver) to make the core 100% water saturated again. Restored core permeability or permeability as close as possible to the absolute permeability was used as criteria before starting a new set of experiment. Between 85 and 100% of the original permeability was usually restored. Good experimental reproducibility in subsequent experiment with this procedure was obtained in our recent work.27 3.6. Experimental Summary. The following steps include in each foam core flooding experiment at HPHT: (1) Baseline experiment: simultaneous injection of N2-gas and SSW at 80% gas fraction (Qtot = 40 mL/h). (2) Injection of two pore volumes of surfactant prior to foam generation (Q = 8 mL/h). (3) Foam generation: simultaneous injection of N2-gas and surfactant solution at 80% foam quality. (4) Cleaning of the core back to absolute water permeability (Kw). The generated foams were characterized by several parameters such as pressure build-up along the core, mobility reduction factors (MRF), and visual observation of foam breakthrough from the sight-glass mounted at core outlet. Calculation of MRF is a method to determine and to compare the strength of the foams generated. MRF is defined as the ratio of the pressure drop in the presence of foam (during foam generation) to the corresponding pressure drop in the absence of foam (during baseline experiment) using the same flow rates and volume fractions (see eq 1).28 For baseline pressure, we always used the average value of the steady state pressure drop for the last 0.5 pore

Table 1. Physical Properties of the Core Material core ID

length [cm]

diam. [cm]

cross section area [cm2]

pore vol. [mL]

avg. porosity [%]

avg. permeability, Kw [mD]

B-WL B-ML B-SL

29.9 30.0 23.5

3.75 3.76 3.76

11.04 11.10 11.10

57.0 58.5 49.3

17.3 17.6 18.9

66.9 93.1 130.0

Cores were cut and dried in an oven at 70 °C for 24 h. The dry core was surrounded by a Teflon sleeve, wrapped by aluminum foil and then covered with a Viton rubber sleeve before mounted into a core holder. The core was vacuumed and saturated 100% with synthetic seawater (SSW) and absolute permeability to water (Kw) was measured. 3.2. Brine. The composition of the seawater used in this study is listed in Table 2.

Table 2. Synthetic Seawater (SSW) Compositiona salt [wt %] a

NaCl 2.489

Na2SO4 0.406

NaHCO3 0.019

KCl 0.068

MgCl2 0.521

CaCl2 0.131

The SSW was filtered through a 0.45 μm filter before use.

3.3. Gas. Industrial grade nitrogen delivered by Yara International was used as gas phase in this study. 3.4. Surfactant. An anionic alpha olefin sulfonate surfactant, AOS C14−C16 was used in this study. The surfactant concentration was 0.5 wt % in all experiments. The AOS surfactant was delivered as liquid of 37.6% active material with a molecular weight of 324 g/mol. The choice of foamer is based on promising results from earlier work and field studies using AOS surfactants. The surfactant is commercial and available at low price. The surfactant solution in this study was prepared by mixing surfactant with brine (SSW). 3.5. Experimental Procedures. The experimental setup used for high pressure and high temperature (HPHT) N2-foam experiments in this study is shown in Figure 1. In all foam experiments, we used the coinjection method, injecting surfactant solution and N2 from separate reservoirs into porous media. Total injection rate was set to 40 ml/h (i.e., 32 mL/h with N2 and 8 mL/h with surfactant solution). The total 804

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volume of gas and brine injected. All the baseline pressures obtained and used for calculation of MRF in each experiment (i.e., dPwithout foam) are tabulated in Table 3.

Mobility reduction factor (MRF) = =

mobility and improve gas sweep efficiency in laminated core material compared to regular gas injection without surfactant (Figure A.10).

dP(gas + surfactant)

5. MAIN RESULTSFOAM FLOODING IN LAMINATED ROCK MATERIAL Foam flooding experiments were performed in three laminated Berea sandstone cores with low permeability. The cores contained different degree of laminations parallel to flow direction (B-WL, B-ML and B-SL). Foam was generated by coinjection of surfactant solution (AOS C14−C16) and N2-gas at pressures of 30, 120, and 280 bar and 50 °C. 80% foam quality (fraction of gas) was used in all experiments. 5.1. Foam Generation in B-WL (Weakly Laminated Core). Results from N2-foam generation in B-WL at 120 bar and 50 °C are shown in Figure 2.

dP(gas + water)

dP(with foam) dP(without foam)

(1)

Table 3. Baseline Pressures core ID

expt. no. (in same core)

conditions (P, T)

dPwithout foam (mbar)

B-WL B-WL

1 2

120 bar, 50 °C 120 bar, 50 °C

270 273

B-ML B-ML B-ML B-ML B-ML

1 2 3 4 5

217 227 228 177 178

B-ML B-ML

6 7

30 bar, 50 °C 120 bar, 50 °C 280 bar, 50 °C 280 bar, 100 °C 280 bar, 50 °C (Qtot = 8 mL/h) 30 bar, 50 °C 120 bar, 50 °C

B-SL B-SL

1 2

30 bar, 50 °C 30 bar, 50 °C

165 165

250 257

4. RESULTSCORE ANALYSES ON LAMINATED ROCK MATERIAL The different core analyses applied on used rock material in this study are presented in Appendix A. Results from these analyses have been used as a supplement in our interpretation and discussion of the main foam flooding experiments at elevated conditions. Both visual observation and X-ray imaging showed that different degree of laminations parallel to flow direction was present in each core (Figure A.5 in Appendix A). Also, thin section analyses from the inlet end of the cores confirmed varying degree of lamina to be present. The laminas were seen to be through-going in all the cores with various thicknesses and densities along. Dispersion tests (water displacement into initially water saturated core) indicated early tracer breakthrough and tailing of the dispersion profiles in the laminated cores (Figure A.6). Interestingly, the dispersion profile did not reflect any particular difference between the weakly and the moderately laminated core (B-WL and B-ML). The most anomalous dispersion profile was seen for the strongly laminated core (B-SL). Some of the rock heterogeneity present in B-SL was observed as clusters of cementation rather than thin layers of single structures (Figure A.4c). Back scatter electron microscopy (BSE) images were used to estimate permeability and porosity within several laminas using the methodology described by Torabi et al.29 In general, lower permeabilities and porosities were found in the laminas compared to host rock. 2D X-ray scanning was also utilized to obtain more information about foam and fluid flow in laminated rock material. The laminated sandstone slabs used in these experiments illustrated relative high degree of lamina to be present (Figure A.8). Flooding experiments at low pressure (2 bar and 25 °C) demonstrated that the laminas could act as barriers to both foam and fluid flow (Figures A.9 and A.10). Visual observation showed that foam was able to reduce gas

Figure 2. Mobility reduction factors and differential pressures obtained in B-WL during foam generation at 120 bar and 50 °C.

Figure 2 describes the MRFs and differential pressures obtained in B-WL during N2-foam generation. Very strong N2foams were generated in this weakly laminated sandstone core (MRF close to 500). Foam breakthrough was observed in the sight-glass close to one pore volume injected. This was a considerable improvement compared to gas breakthrough during baseline experiment (that is only injecting N2-gas and seawater), which was observed after less than 0.2 pore volume injected. With foam present in the core, gas propagated close to the injection rate in B-WL. Good reproducibility of foam experiment 1 was found both with respect to foam propagation and foam strength (expt. 2 in Figure 2). Even though laminations were present in B-WL, the results indicated that foam was able to effectively control gas mobility in the low permeability laminated sandstone core flooded. The responses obtained in B-WL with effective pressure build-up, significant delayed foam breakthrough and good experimental reproducibility in subsequent foam experiment are similar to our N2-foams generated in recent work in a homogeneous and high permeability Berea sandstone core.27 5.2. Foam Generation in B-ML (Moderately Laminated Core). Results from foam generation in B-ML are shown in Figure 3. Effect of pressure (30, 120, and 280 bar) was systematically tested at 50 °C with respect to foam properties in laminated core material. Foam breakthrough times observed in the sight-glass at core outlet in each experiment are given in Figure 4. Strong N2-foams were generated in all foam experiments at all the different system pressures applied. The foam strength in B-ML was not negatively affected by increased system pressure (30−280 bar). In fact, slightly improved foam strength with pressure was observed. Similar result on the effect of pressure 805

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adsorption. They discussed this to areas of slow contact, due to micropores, secondary porosity, and dead-end pores found in the complex structures of the Berea sandstone used. In our study, we always injected two pore volumes of surfactant prior to each foam generation. Is this enough to cover surfactant adsorption in low permeability laminated rocks? Our interest in the next core flooded, B-SL (strongly laminated), was therefore to investigate if volume of surfactant solution injected prior to foam generation in laminated core material would have any effect on foam performance. A new low permeability laminated sandstone core, B-SL, was used to test and to compare the effect of 2 versus 20 pore volumes of surfactant solution injected prior to foam generation. Both experiments were performed at 30 bar and 50 °C. Results from foam generation in B-SL are given in Figure 5.

Figure 3. Mobility reduction factors obtained in B-ML during foam generation at different experimental conditions.

Figure 5. Mobility reduction factors obtained in B-SL with different amount of surfactant injected. (Both experiments performed at 30 bar and 50 °C.)

Figure 4. Comparison of foam breakthrough times observed in B-ML at different experimental conditions.

on N2-foam strength was also obtained in our recent work in homogeneous and high permeable Berea sandstone.27 Foam generation at high pressure and high temperature conditions using AOS surfactant have also been reported by several others.14,30−35 Interesting responses in B-ML compared to B-WL were seen, attended with (I) no plateau in differential pressure, even after more than five pore volumes of fluids injected; (II) large differences in MRF compared to B-WL; (III) earlier foam breakthroughs than expected for all three foam experiments (0.44−0.58 PV). The consistent early foam breakthroughs observed for all three experiments indicate that the effectiveness of foam to control gas mobility in B-ML was reduced compared to B-WL. Even though the physical properties of the cores listed in Table 1 illustrate that B-WL and B-ML appear as relatively similar (with respect to Kw, porosity, pore volume, core length), large differences in MRF (∼100−500) were obtained in these two cores. One immediate explanation to the different foam performances experienced between B-ML and B-WL could be the different degree of lamination that is present in each core. A higher degree of low permeability laminas in B-ML versus BWL may have reduced the efficiency of foam to control gas mobility. Reduced mobility control in B-ML was reflected by weaker N2-foams, earlier foam breakthroughs, and less effective pressure build-ups. 5.3. N2-Foam Generation in B-SL (Strongly Laminated Core). In the literature, foam generation and foam propagation properties have been discussed with respect to surfactant adsorption.15 Bai et al.36 tested surfactant adsorption of an anionic surfactant in Berea sandstone (224 mD) and showed that it took about a week to reach the final equilibrium in

Again, N2-foams were generated, even in strongly laminated sandstone core material. The generated foams in B-SL were, however, the weakest ones compared to those obtained in BWL and B-ML, reflected by the lowest MRFs. A somewhat faster increase in pressure build-up was observed when 20 instead of 2 pore volumes of surfactant solution had been injected prior to foam generation (expt. 2 in Figure 5). As the generation continued, more than 2 pore volumes of surfactant solution injected indicated to only have a minor effect on the overall foam strength obtained (although a doubling in MRF). Foam breakthrough occurred early in the sight glass for both experiments (∼0.4 PV). Again, the properties of the strongly laminated core material seems to be a more important factor affecting foam performance than number of pore volume surfactant injected. 5.4. Summary. Figure 6 compares and summarizes the MRFs obtained in this study with respect to core material and subsequent experiment in each core. Figure 7 compares average breakthrough times observed in the sight glass at core outlet. 5.5. Reproducibility in Laminated Core Material. In addition to the three foam generations conducted in B-ML (Figure 3), four new foam experiments were performed in this core. The interest was to evaluate if foam properties could be systematically studied and reproduced in laminated rock material. Restored core permeability or permeability as close as possible to the absolute permeability was used as a criterion before a new foam experiment was started. Good reproducibility in subsequent experiment with this procedure was found in our recent work in homogeneous and high permeable Berea sandstone, even after 11 experiments in the same core.27 806

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Figure 6. Comparing mobility reduction factors (MRFs) obtained at the end of each N2-foam generation in different low permeability laminated Berea sandstone core material (30−280 bar and 50 °C). Logarithmic MRF plotted against subsequent experiment in same core.

Figure 7. Comparing average foam breakthrough times in laminated Berea sandstone cores during N2-foam generation. Typical gas breakthrough time during baseline experiment is included for comparison (i.e., coinjection of N2 and SSW − without surfactant).

Figure 9. Comparing mobility reduction factors (MRFs) obtained at the end of subsequent N2-foam experiments in B-WL. Different variables tested.

Figure 8 shows the reproducibility experiments performed in B-ML with respect to system pressure (i.e., 30 and 120 bar at

Results from both Figure 8 and Figure 9 illustrate that successive stronger foams seems to be generated for repeated experiments in B-ML, independent of the variables tested. A possible explanation to this observation is discussed later (see 6.2). Consistent early foam breakthroughs were observed in the sight glass for all seven experiments performed in B-ML (earlier than 0.6 pore volume injected). The poor reproducibility witnessed is an additional proof of the studied foam properties and performances being controlled by the laminated core material.

Figure 8. Experimental reproducibility in B-ML. Comparing expt. 1 with expt. 6 (30 bar and 50 °C) and expt. 2 with expt. 7 (120 bar and 50 °C).

6. DISCUSSION The main findings from this study have shown that in situ N2foams were able to generate within all three low permeability laminated sandstone cores flooded using AOS surfactant. Foam generation was observed at high pressures (30, 120, and 280 bar) and elevated temperatures (50 and 100 °C). The positive response for the AOS surfactant at high pressure and temperature conditions may be attractive to potential foam field applications. Main findings in this study also demonstrated that the foam strength could be strongly dominated by the core material used.

50 °C, respectively). Poor experimental reproducibility in BML was observed with respect to absolute pressure. Figure 9 compares and summarizes the MRFs obtained in all seven foam experiments conducted in B-ML. Different variables such as pressure (30, 120, and 280 bar), temperature (50 and 100 °C), and injection rate (40 vs 8 mL/h) were systematically tested with respect to foam properties in B-WL. 807

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strength to increase with system pressure (10−290 bar) in a Bentheimer sandstone core between ∼1.5-50 bar/m (factor of 30) using an AOS C16 surfactant. Slight improved foam strength with pressure (30, 120, 280 bar) was also observed in this study in B-ML. The results with pressure are similar to the N2-foams in our recent work in homogeneous and high permeable Berea sandstone (also generated at 30 and 280 bar).27 The effect of pressure on foam strength obtained in our present and recent study was, however, not as significant as that reported by Holt and co-workers. Yaghoobi45 provides pressure drop data from CO2-foam experiments in sandstone cores that ranged from ∼1 to 115 bar/m depending on the core, flow rate, surfactant type and concentration used. Chou’s15 N2-foams in Berea sandstone (150−350 mD) were extraordinary strong and ranged from 500 to 4000 in resistance factor (RF) when foam experiments in two Berea cores with different length and flow rates were compared. Interestingly, the huge differences in RF were suggested to possibly result from the internal variations in core permeability, but without further explanation. Most variables in this study were held constant (i.e., similar flow rate, foam quality, salinity, surfactant type, concentration, etc.), only changing core material. Our results demonstrate that pressure gradients could vary from ∼6.5 to 400 bar/m solely by the core material used. Findings in our study indicate that mobility control with foam could be strongly dominated by core heterogeneities, such as laminations. The large variation in MRF obtained between three sandstone cores with different degree of lamination could, in our opinion, be an important recognition for realistic scaling of foam mobility control to field applications. 6.2. Poor Reproducibility in Laminated Core Material. Restored core permeability or permeability as close as possible to the absolute permeability was used as criteria before starting a new set of experiment in the same laminated core. Good reproducibility in subsequent experiment with this procedure was obtained in our recent work.27 In this paper, poor experimental reproducibility was observed in laminated core material for repeated experiments (Figures 8 and 9). We speculate that foam generation reproducibility for repeated experiments in B-ML was poor because the starting point of each foam experiment was not the same. Zhou et al.46 investigated gas trapping in porous media with a high resolution 3D X-ray scanner. The gas trapping was studied in a low permeability Berea sandstone core (Kg ≈ 170 mD) with visible laminations. They found that most of the gas that entered the laminas became trapped, and remained so. Although the original seawater permeability was able to be restored between 85 and 100% after subsequent experiment in our study, it might be reasonable to believe that trapped gas still could be present within the lamina. If gas already was present within the laminas before the next experiment started could have contributed to give a faster “homogenization” of foam in the core during generation. This may have resulted in the subsequent higher MRFs particularly observed for repeated experiments in B-ML. The successive stronger foams and poor experimental reproducibility observed in laminated core material makes it difficult and risky to study foam properties in heterogeneous cores. The laminated Berea sandstone material may be relevant to improve understanding of foam in relation to heterogeneous porous media but is not recommended for systematic studies of variables affecting foam properties.

Although the physical properties of the cores listed in Table 1 indicated that B-WL, B-ML and B-SL appeared to be very similar (with respect to average Kw, average porosity, pore volume and core dimensions), expecting relative equal foam strength, large variations in MRF (20−500) were obtained in these cores during N2-foam generation. An immediate explanation to what affected foam strength in these experiments appeared to be the presence of core heterogeneities, such as laminations. One significant finding from our core analyses presented in Appendix A showed that different amounts of laminated stratifications were present in each core flooded (i.e., number of lamina in B-SL > B-ML > B-WL). The laminas present were found to exhibit both lower porosity and permeability than the host rock and also shown to form barriers to foam and fluid flow. The extent to which laminated configurations affects fluid flow have been reported in the literature to depend on factors such as the lamina thickness, the amount of lamina relative to scale, and the permeability contrast between lamina and host rock.29,37−43 6.1. Foam Behavior in Naturally Laminated Cores. Literature on foam flow in naturally layered systems is limited. Results from our foam experiments in laminated core material indicate that these stratifications affect foam performance. Increased core lamination in this study resulted in weaker N2foams. Reduced mobility control in cores with increased lamination were supported by (I) less effective pressure buildups across the core during foam generation; (II) earlier foam breakthroughs, and; (III) lower MRFs. We speculate if less effective pressure build-up during coinjection of N2-gas and surfactant solution could have happened if foam formation and flow were mainly limited to the more permeable regions of the core. This could have led to rapid foam propagation and poor areal sweep. The low permeability laminas introduce different degrees of discontinuities and compartmentalization to foam flow. With a higher degree of low permeability laminations present in the core could indicate that more time is required for the pressure gradient to build-up an allow gas or foam to enter a successive larger fraction of the pore volume. Early breakthrough may be a consequence of the foam strength build-up rising too slowly to be able to stabilize the displacement front properly within cores with high degree of lamina. In B-WL, with low degree of lamina present, efficient foam generation and foam propagation close to ideal flow behavior were observed. This was also supported by the strongest N2-foams in terms of MRF. The responses obtained from B-WL are similar to those reported in homogeneous core material where foam flow in most of the pore space available is observed to take place.27,44 Strong foam seems important to stabilize the displacement front and prevent early foam breakthrough in low permeability laminated cores. A positive effect of rapid foam propagation at a larger scale, as we see it, is the possibilities of getting improved mobility control deeper into the reservoir. It should also be noted that the core length of laminated systems could be important. A longer core material may allow foam resistance to build-up over distance to a point where the front stability becomes restored again. More experiments are needed to confirm our speculations about foam’s behavior in low permeability laminated core material. Experimental studies on foam flow through porous media have suggested many variables to largely influence foam properties and performance. Holt et al.32 reported CH4-foam 808

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6.3. Foam in Low Permeability Porous Media. It seems evident from the experimental results in this study that the low core permeability alone was not detrimental to foam generation and strength. Some authors have suggested that foams existence could be limited to higher permeabilites,10,12 but this seems not to be the case here. Strong N2-foams in relative low permeability sandstone cores have also been reported elsewhere, even in sandstone cores with permeability as low as 9 mD.15−17 Compared to MRFs obtained in our recent work in Berea sandstone material using 0.5 wt % AOS C14-C16 surfactant,17,27 the steady state MRF in Berea cores with homogeneous flow behavior and good experimental reproducibility seems to increase when lowering core permeability (MRF 70 mD > 300mD > 1000 mD). The same observation was also noticed by Siddiqui et al.16 in Berea sandstone core material. Others have reported the foam strength to be an increasing function of permeability.8,9 In these studies, different kinds of rock material with varying permeability were compared against each other. One of the factors that might be difficult to access when comparing two or more cores with respect to permeability is the degree of core similarity (e.g., lithology, pore geometry, small scale heterogeneities, core dimensions, etc.). The large variations in foam strength and foam performance seen between three laminated heterogeneous cores with relative similar permeability in this study should motivate more research to improve understanding of foam in relation to rock type and rock properties.

considered and not overinterpreted as representative indications to our main experiments at higher pressures might be lacking. Outcrop Berea sandstone core samples have generally been widely recognized by the petroleum industry as one of the best model rocks to use for characterizing oil production and for optimizing enhanced oil recovery (EOR) processes. The most commonly used form of Berea has permeability of about 500 to 1000 mD and is regarded as relatively uniform. The new supply of Berea cores that were ordered at the beginning of this study turned out to be many times lower in permeability than before, typically in the range of 50−200 mD, and with visible laminations as seen in Figure A.1. Permeability measurements

Figure A.1. Visual differences of Berea sandstone samples: (A) low permeability core with visible laminations, (B) high permeability core (1000 mD).

7. CONCLUSION N2-foam performance in naturally laminated sandstone cores with low permeability have been studied at elevated pressure and temperature conditions using AOS surfactant. In-situ N2-foam was generated in all the laminated sandstone cores flooded. Similar to other work on foam in low permeability porous media, our results confirmed that foam was not limited by low permeability. The results, however, show that the effectiveness of foam to reduce and control gas mobility in sandstone cores with low permeability laminations present may be significant. This was reflected by large variations in MRF (20−500) and foam breakthrough times. Case 1: Weakly laminated core. The strongest foams were generated in the low permeability core containing low degree of lamination. Efficient foam generation and foam propagation close to ideal flow behavior were observed. Case 2: Moderate to strongly laminated cores. In two cores with a higher degree of lamination compared to that in case 1, moderate to weak foams were generated. Consistently early foam breakthroughs in these cores indicated faster foam propagation and less areal sweep. However, 2D imaging confirmed that improved areal gas sweep after foam placement is possible even in highly laminated rock material. The in situ foams generated in more laminated cores showed reduced mobility control by foam compared to homogeneous rock material.

perpendicular on the lamina indicated roughly half of the permeability compared to parallel alignment (∼ 90 vs 45 mD). At Berea’s homepage (www.bereasandstonecores.com), a “split rock” name is given to this type of sandstone that usually falls in the lower milliDarcy range. Several techniques were used to analyze the core material with visible laminations. A homogeneous and high permeable (∼1000 mD) Berea sandstone sample was also used in a number of the analyses for comparison. Five types of analyses were conducted on used batch of rock: (1) combined X-ray diffraction (XRD)/X-ray fluorescence (XRF) spectrometry to determine minerals and element compositions in rock material;47 (2) mercury injection to determine the distribution of pore sizes;48 (3) SEM and optical microscopy techniques of thin sections from inlet end of each core to examine lamina and host rock in terms of porosity, permeability, and mineralogy;29,49 (4) dispersion tests to better understand fluid transport in used cores;50,51 and (5) 2D X-ray scanning for visualization of porous media and fluid flow (i.e., water/gas injections into sandstone slabs). The use of several techniques in combination can be useful and valuable toward a better description of the used material. Table A1 shows the results from XRD measurements of one low permeability laminated rock sample compared to a more common Berea (1000 mD). The Berea sandstone is mainly composted of quarts with different amounts of clay minerals. XRF spectrometry was used on the laminated core to see if we could detect specific element compositions in the lamina versus host rock. The main elements present within the lamina were found to be iron, zirconium, silica, and titanium (in this order based on their relative intensity from XRF measurements). Of the minerals listed in Table A1, siderite (FeCO3) is most likely one of the dominating minerals present in the lamina. Host

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APPENDIX A: ROCK ANALYSES This appendix contains results obtained from several different analyses on used rock material. The rock analyses were first of all performed as a supplement to characterize the core material used and to detect possible indications and influences of rock heterogeneity on foam and fluid flow prior to main foam experiments. The analyses should however be carefully 809

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Table A1. XRD Mineralogy of Berea Sandstone Samples (% of 1 cm3 Rock Sample Analyzed) core ID

quartz

kaolinite

feldspar

chlorite

Berea (laminated) Berea (1000 mD)

87.5 82.7

3.2 6.0

1.9 5.1

1.7 1.1

plagioclase

calcite

dolomite

siderite

3.0 2.7 illite/ smectite

0.9 0.4

traces 0.4

0.9 1.5

0.9 0

0 0.2

Berea (laminated) Berea 1000

illite/mica

Figure A.3. Pore size distribution of Berea samples.

• Lowest porosity and permeability were found within the laminas (ϕ = 0−15%, K = 0−50 mD). • Highest porosity and permeability were found within the host rock (ϕ = 17−23%, K = 100−250 mD). • Petrophysical properties found are roughly estimated values and should not be considered as absolute. • Different degree of lamination were observed in used cores (B-SL > B-ML > B-WL). Figure A.4 shows some optical pictures taken of host rock and lamina, respectively. After the foam flooding experiments were performed, the cores used (i.e., B-WL, B-ML, and B-SL) were sliced lengthwise and perpendicular to the lamination. X-ray imaging of the cores was used to confirm that different degree of lamina was present within each core (Figure A.5). The laminas were seen to be through-going in all the cores with various thicknesses and densities along, supporting internal variations in petrophysical properties. Figure A.6 shows dispersion curves for the three laminated cores used in this study. The cores were initially saturated with brine (containing 1 wt % NaCl) and the tracer response is measured by changing the salinity to 2 wt % NaCl. A flowthrough electrical conductivity meter measured the conductivity of the effluent leaving the core outlet. Dispersion test of a high permeable Berea (standard) was also performed for comparison. Injection rate of 6 mL/h was used in all tests. The cores were horizontally oriented. According to the Coats and Smith50 convection-dispersion (capacitance) model, both dispersivity and flowing fraction of the rock is analyzed. The analysis follows a procedure suggested by Salter and Mohanty51 that detects the flowing fractions as well as isolated or dead-end pores. If no isolated and dead-end pores in the porous media exist, half of the injected tracer

rock, in general, contained all the other minerals found from the XRD results in Table A1. A difference in the XRD results between the laminated Berea and the high permeability Berea is the presence of siderite in the former and its absence in the latter. Iron-containing minerals were found by Wang et al.52,53 to contribute high surfactant adsorption and wettability alteration under aerobic conditions compared to reduced conditions of the reservoir environment. To evaluate the possible effect that iron could have on foam, static experiments with different iron ions (both Fe2+ and Fe3+) added to 0.5 wt % AOS surfactant solution were tested and compared (see Figure A.2). A total iron concentration of 60 ppm was used in these experiments. The concentration level was similar to the maximum dissolved iron concentration found in the produced water from a Berea core in Wang and Guidry.53 The static foam experiments in Figure A.2 indicated, however, little to no influence of iron on the bulk foams generated. The surfactant solution produced relative similar foam heights independent of the iron ions added. Figure A.3 illustrates the distribution of pore sizes in the Berea samples found by mercury injection. The average pore throat radius in tested laminated core was found to be approximately 6 μm. A wider pore size distribution and a larger fraction of smaller pores seems to be present in Berea (low perm.) compared to Berea (1000 mD). The fraction of smaller pores may indicate the pore sizes in some of the lamina. Analyses through image processing of thin sections were used to better characterize and differentiate between the degrees of lamina present in each core used. Back scatter electron microscopy images (BSE) were used to estimate porosity and permeability within several laminas using the methodology described by Torabi et al.29 Important findings from studying the lamina through image processing techniques included the following:

Figure A.2. Effect of iron on surfactant foamability and stability (SSW reflects the base case without iron). 810

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sandstone slabs (10 cm (L) × 5 cm (W) × 1.5 cm (H)) were coated with epoxy and mounted into the scanner. Inlet and outlet ends were designed along the sides (Figure A.7C) to ensure that fluids were evenly distributed along the whole slab width when injected. During fluid injections continuous scans were taken. The images were subtracted from an initial image where the slab was 100% saturated with water (Sw = 1). The changes in attenuation therefore illustrate where fluids actually are flowing. All experiments in slabs were conducted at low pressure (i.e., 2 bar backpressure and 25 °C) due to pressure limitation of the coated epoxy. Figure A.8 displays a dry scan of a laminated slab. The darker horizontal regions in the slab represent lamina. Vertical lines in the picture are noises. The figure illustrates high degree of lamina to be present. Interest was now to use the X-ray scanner to investigate closer if and how the lamina would influence fluid flow (i.e., by water and gas flooding in laminated sandstone slabs). Conventional water and gas floods are often influenced by three main factors, namely, rock heterogeneity (leading to water and gas channeling), gravity segregation due to fluid density contrasts (leading to under- or over-riding of the injected fluids), and unfavorable mobility ratio partly due to fluid viscosity contrasts, which aggravates the negative effects of the two first factors. The preceding analyses of the laminated rock material indicated different physical properties in the lamina compared to host rock to be present (i.e., lower porosity and permeability in the lamina). Also, dispersion tests illustrated that parts of the pore volume in the laminated cores contibuted less to flow compared with a standard Berea sandstone. Influence on fluid flow dynamics by rock heterogenity was therefore expected. To detect possible influence of core heterogeneity on fluid flow, gravity-stable water and gas injections in laminated and relative homogeneous Berea slabs at low injection rate were performed and compared (Figure A.9). During these experiments, slabs of similar size were used (10 × 5 × 1.5 cm). The slabs were prepared and coated with epoxy in similar manner, vacuumed, and saturated with water containing 3 wt % NaCl if nothing else is specified. The gravity-stable water injected contained 10 wt % of NaI to enhance X-ray attenuation. Injection rate of tracer water (Figure A.8A) and N2-gas (Figure A.9B) was always set to 3 mL/h (∼0.4−0.6 m/day). Fluid injections were performed parallel to the lamina (i.e., similar to what was done in the main foam experiments). During gravity-stable water injection (Figure A.9A), unstable water displacement was observed for the laminated slab. The doped water developed an uneven water front that seemed to propagate faster through the more permeable streaks compared to the laminated parts. Water breakthrough occurred after approximately 0.7 pore volume of NaI−water injected. Little to no change in attenuation within the lamina was generally observed when tracer was injected. Most of the laminated parts of the slab remained yellow (i.e., unswept) even after several pore volumes of NaI−water were injected. For the standard Berea sample more stable water displacement front was observed compared to the laminated sample. Water breakthrough occurred close to one pore volume injected, indicated a relative homogeneous Berea sandstone slab. The gravity-stable water injections illustrate that sample heterogeneity could influence water flow by water channeling and earlier water breakthrough than expected. As the NaCl− and NaI−water have relative similar viscosity and density

Figure A.4. Optical images showing: (a) host rock without lamina, (b) single structure of lamina, and (c) clusters of cementation typically found in B-SL.

concentration should break through after 1 PV tracer injected, and the tracer profile should be symmetrical around this point. The early tracer breakthroughs and tailing of the tracer profiles as seen for the laminated cores in Figure A.6 indicate lower flowing fractions of these samples compared to the standard Berea. The weakly and the moderately laminated Berea sample have dispersivity (0.2 cm) somewhat larger than literature data (0.13 cm) report on more homogeneous Berea.54 However, there is a lower flowing fraction and increasing asymmetry of the dispersion curves with increase in lamination, indicating dendritic or dead-end pores that exchange brine slow through diffusion. The strongly laminated Berea (B-SL) shows nonconventional dispersion characteristic with slow increase in tracer concentration and extensive tailing of the dispersion curve. X-ray scanning was also used in this study to provide fundamental understanding of fluid flow dynamics in low permeability laminated sandstone slabs. The scanner is equipped with a low energy X-ray source (60 kV and 320 μA) and a NaI crystal scintillation detector, as well as an X-ray camera, all mounted on linear actuators inside a shielded cabinet (Figure A.7). The X-ray camera has pixel resolution of 0.1 × 0.1 mm (x, y coordinates, respectively). Several laminated 811

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Figure A.5. X-ray images showing different degree of laminations in each core used. The same image setup was used for all three cores for a quantitative comparison. Darker regions in the slab represent lamina. Horizontal lines in the pictures are noises.

Figure A.8. X-ray scan showing dry sample of a low permeability laminated sandstone slab.

Figure A.6. Dispersion tests.

Figure A.7. (A) X-ray scanner, (B) experimental setup with core slab placed in the X-ray scanner, (C) low permeability sandstone slab with visible (horizontal) laminations. 812

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injections were performed horizontally, but still parallel to the lamina. To be able to evaluate foams performance one experiment was performed only injecting N2-gas into the same slab saturated with seawater, no surfactant (see Figure A.10). The

Figure A.9. (A) Gravity-stable water injection in laminated (left) and standard Berea (right) sandstone slab. Pictures from the water injection were taken at tracer breakthrough in both cases. (B) Gravitystable N2-gas injection (yellow) in laminated (left) and standard Berea (right) sandstone slab. Pictures from the gas injection were taken close to 1 PV injected. The homogeneous core was saturated with 10 wt % NaI during gas injection causing higher attenuation, thus darker (reddish) color. Horizontal lines in the pictures are noises. All slab experiments were conducted at 2 bar backpressure and 25 °C.

Figure A.10. (Left side of the line) Regular N2-gas injection into water saturated sample after 0.1 and 1.5 pore volumes injected, respectively. (Right side of the line) N2-gas injection into surfactant saturated sample after 0.2 and 1.5 PV injected, respectively. All injections were performed from left to right at 2 bar backpressure and 25 °C.

ranges, this should in theory result in a stable water front if the porous media is uniform. The water channeling that is illustrated for the laminated slab demonstrates that conductivity differences within the slab are present, which makes it easier for water to preferentially flow through the more permeable regions than through the laminated parts of the rock sample. An unstable displacement of gas was also observed in the laminated sample compared to the standard Berea slab (Figure A.9B). During gravity-stable N2-gas injection the gas was observed fingering/channeling through the laminated slab causing early gas breakthrough (after ∼0.15 pore volume of gas injected) and poor sweep of the sample area. This is a typical example of how we expect the gas to flow in porous media with permeability contrasts. For the standard Berea a relative stable gas injection front was observed, also illustrated by gas more uniformly distributed in the slab compared with the laminated sample. Both slabs seem to be influenced by capillary end effect as the water saturation remains higher near the sample outlet. The capillary end effect is consistent with the theory that the nonwetting phase (i.e., gas) displaces the wetting phase (i.e., water), in which the capillary forces near the outlet end are able to counteract the viscous forces acting on the water. This accumulates water at the end of both slabs. Both gravity-stable water and gas injections into laminated slabs demonstrate that sample heterogeneity affects fluid flow. The presence and orientation of the laminas illustrated by X-ray scanning seems to act as barriers and compartmentalization to fluid flow. The fact that fluid communication and conductivity within the laminated sample are somewhat varying and reduced could possible influence the oil recovery from these types of Berea compared to the more standard Berea samples. Observation of foam generation in laminated rock material was also tested by gas injection into a laminated slab saturated with surfactant solution (0.5 wt % AOS surfactant dissolved in 10 wt % NaI water). Due to pressure limitation of the coated epoxy, we were only able to perform experiments at 2 bar backpressure. The SAG (surfactant alternating gas) method was used to prevent large pressure drop that would overpressurize the experimental system. In these experiments, the gas

gas injection rate was 30 mL/h. Again and similar to Figure A.9B, typical gas finger pattern and poor sweep of the sample area were observed for the regular gas injection into the laminated rock sample. When gas was injected into the same slab saturated with surfactant (Figure A.10 at the right side of the line), experimental observations indicated that foam was able to generate and reduce gas mobility. Foam was seen to reduce frontal instabilities and improve stability of the gas injection front to a larger degree than shown without surfactant. Gas or foamed gas was observed diverting into successive new areas of the tested sample with pore volume gas injected. Even though a lack of details down at pore scale exists because of limited resolution and noise, results showed that improved areal gas sweep after foam placement is possible even in highly laminated rock material.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the assistance of Anita Torabi and Eivind Bastesen for their geological contributions, and Per Arne Ormehaug for his guidance during some of the 2D X-ray experiments. The Norwegian Research Council (PETROMAX program) is gratefully acknowledged for financial support.

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NOMENCLATURE AOS alpha olefin sulfonate bar pressure (1 bar =105 Pa) bar/m pressure per meter (1 bar/m = 105 Pa/m) BSE back scatter electron microscopy B-WL Berea-weakly laminated core 813

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Energy & Fuels B-ML B-SL C Co cm cm2 dP EOR H K Kg Kw L mD ml MRF PV RF SEM SMR SSW Sw T ΔP Phi XRD XRF W 2D

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Article

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Berea-moderately laminated core Berea-strongly laminated core concentration initial concentration centimeter (1 cm =1 × 10−2 m) square centimeter (1 cm2 = 1 × 10−4 m3) differential pressure enhanced oil recovery height (i.e., slab thickness) absolute permeability absolute permeability to gas absolute permeability to water length millidarcy (1000 mD = 1 D = 9.869233 × 10−13 m2) milliliter (1 mL = 1 × 10−6 m3) mobility reduction factor pore volume resistance factor scanning electron microscopy selective mobility reduction synthetic seawater water saturation temperature (0 °C = 273.15 K) differential pressure porosity X-ray diffraction X-ray fluorescence width two-dimensional

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