Effect of Spreading and Wettability on Immiscible WAG Displacements

Transcription

1 International Energy Agency Collaborative Project on Enhanced Oil Recovery 32 nd Annual Symposium and Workshop October 2011, Vienna, Austria Effect of Spreading and Wettability on Immiscible WAG Displacements A. Edalatnoor, M. Feali, Y. Cinar* The University of New South Wales (UNSW) School of Petroleum Engineering, UNSW, Kensington, NSW, 2033, Australia *corresponding author; unsw.edu.au Abstract This paper presents an experimentalstudy that investigatesthe effect of wettability and spreading characteristics of fluid-rock systemson immiscible water-alternate-gas (WAG) oil recoveries. We use an edged-glass micromodel and glass beads pack for porous media. The micromodel is used for observations of three phase pore-scale distributionswhile the beads pack for accurate oil recovery measurements. We treat both porous media by Coatasil TM to make them oil-wet. We use two different fluid systems composed of water, Soltrol-130 (refined oil), isobutanol, and air, one of which shows a positive spreading of oil (Fluids-1)while the other has a negative spreading coefficient (Fluids-2) for water-wet media. In oil-wet media, both fluid systems have negative spreading which does not allow intermediate phase to create a film between wetting and the most non-wetting fluids. We report on immiscible WAG displacements for four different cases for each fluid-wettability system. The micromodel observations indicate the presence of oil film only for Fluids-1 in water-wet media. Three phase contact lines can be observed for Fluids-2 in both oil- and water-wet media. The glass bead pack results indicate a higher waterflooded residual oil recovery in water-wet media for the non-spreading fluid system. The reason for this higher recovery is attributed to the lower interfacial tensions.the recoveries in oil-wet media indicate Fluids-2 yield higher WAG recovery compared to Fluids-1. This is attributed to the thicker oil filmsand capillary forces.

2 1. Introduction Water alternating gas (WAG) injection is one of the successfully applied enhanced oil recovery methods. The reason for this is mainly the ease of applying it as it usually uses the same facilities as it has been used for gas injection or water flooding.wag can be applied in reservoirs that show a significant drop in oil production and leads to less water or gas cut and higher recovery factor. Gasfloodingis effective but limited. The limitations for gas flooding are the front instability and early breakthrough. Having gas slugs alternated with water slugs controls the gas mobility,leading to lower residual oil saturations. WAG involves the flow of three phases and uncertainties in the prediction of wettability and spreading characteristics that have a significant impact on three-phase relative permeability are the major challenges in the assessment of WAG processes.research on wettability and spreading effects on three-phase flow has been reported (Vizika et al. 1998; Dicarlo, et al. 2000; Sohrabi et al. 2001) but not extended to WAG processes. Oil recovery from a WAG process depends on the relative permeability of each phase, and these, in turn, depend on distributions of the phases in porous media. Two major factors thatcontrolthe distribution of multiple phases in porous media are wettability and spreading coefficient. A broad study on the effect of wettability on multiphase flow has been reported in the literature(agbalaka et al. 2008). The wettability can affect fluids flow and displacement, which eventually control oil recovery(donaldson and Thomas 1971; Skauge and Dale 2007; Salimi and Bruining 2010). Lake (1999) reported that 50 % of sandstone reservoirs and more than 80 % of carbonate reservoirs are oil-wet. Considering this fact, it is essential to study oil-wet reservoirs as well as water-wet reservoirs for WAG injection. Studies that have been published on spreading coefficient show its importance on fluid configurations and distributions in porous media (Oren and Pinczewski 1994; Keller et al. 1997; Mani and Mohanty 1997).Mani and Mohanty (1997)described the three-phase flow in a spreading system using network modelling and presented the benefits of positive spreading coefficient on incremental oil recovery. The combined effect of wettability and spreading coefficient on the three-phase flow in WAG processes has not been well investigated. The objective of the present paper is to study the combined role of wettability and spreading on the oil recovery from WAG processes. In the sections that follow, we first present micromodel visualisation experiments to examine spreading and non-spreading fluid distributions in oil- and water-wet systems. We then report on the repeat experiments in glass bead packs to measure the impact of these combinations on the oil recovery. 2. Experimental Approach 2.1.PorousMedia Glass Micromodel The visualization was carried out in a 2D transparent glass micro-model which contained a regular square network of intersecting capillaries. The inlet side of the micro-model had a channelled system to obtaina uniform distribution of fluids. The outlet, on the other hand, had a short capillary to act as a capillary barrier for non-wetting fluids. The main purpose of using the - 2 -

3 capillary barrier was to avoid an early gas breakthrough and to reach a higher capillary pressure within the model (Oren et al. 1992). To ensure a strong water-wettability, the micro-model was cleaned with chromic acid. Then, the model was flooded with isopropanol and dried in an oven at 60 C. In order to neglect any gravity effects on the floods, the micromodel was kept horizontal during the experiments. Micromodel observations were performed using a stereomicroscope under the transmitted light. The microscope was equipped with a high resolution digital camera and magnification lens (Fig. 1).The micromodel was treated with Coatasil for 3 hoursto make it oil-wet. Glass Beads Pack A glass tube of about 35 cm in length and 2.55 cm in inner diameter wasused to hold uniformsized glass beads. The inlet and outlet of the glass beads pack were connected to a pressure transducer to measurethe pressure dropacross the pack during the experiments. The measured average porosity of packs and absolute permeability wereapproximately 40% and 16 Darcy for the water-wet pack and 39 % and 17 Darcy for the oil-wet pack, respectively. The glass-beads packs properties are tabulated in Table1. For the oil-wet experiments, the glass beads were soaked in Coatasil for 3 hours before packing into the tube. The inside part of the glass tube was also treated with Coatasil. The packing was preparedby pouring the glass beads into a glass column under a continuous vibration. To determine the pore volume of the column with a known bulk volume, the pack was fully saturated using a low rate water injection and the duration was recorded. The subsequent porosity of the pack was determined using this data. 2.2.Fluids We used one spreading and one non-spreading fluidsystem composed of air, water, and oil. Fluids were prepared carefully to avoid any contamination in order to keepthe interfacial tensions unaltered. The spreading fluid system (Fluids-1) consisted of deaired water, Soltrol-130 (refined oil) and air, with a positive spreading coefficient of +9.7 dynes/cm. In order to make the spreading coefficient negative, a small quantity of isobutanol (8% by volume) was added to a mixture of water and Soltrol (Fluids-2). The mixture was rest for 24 hours in order to achieve a chemical equilibrium and gravity segregation. The calculated spreading coefficient was -2.9 dynes/cm. Table2 gives a summary of measured density and interfacial tension values. 2.3.Procedures All the experiments were carried out under strong water-wet and oil-wet conditions and at ambient temperature and pressure Experiments with Micromodel The experiments consisted of the following displacements after a complete initial saturation ofthe micro-model with water: (1) Soltrol was injected at a relatively high flow rate (0.01 ml/min)to obtain a residual water saturation, (2) water was then injected at a relatively low flow rate (0.001 ml/min)to obtain a waterflood residual oil saturation, (3) 1 pore volume (PV) of gas was then drained as the first cycle of WAG, and (4) water was injected to complete a WAG cycle of 1:1 ratio

4 The fluids were injected using a syringe pump. Injection rate was set in a way to inject each fluid with 1 PV/hour. To ensure a better visualization, the oil phase was doped with a red dye. The set-up was then connected to a filter before the micro-model inlet to avoid tiny particles blocking the model channels Experiments with Glass-Bead Packs Fig.2shows the set-up used to carry out the quantification section.the steps below were followed in all experiments with in oil- and water-wet conditions: 1. The pack was mounted and kept in the vertical position 2. The pack was fully saturated with water (the inlet injection placed at bottom) 3. Oil injection to reach a residual water saturation(from the top of the column) 4. Water injection to reach a residual oil saturation(from top of the column) 5. Air/water cycles injections- each slug was 20% of pack s pore volume. The initial conditions before the WAG process were obtained in the following way. The pack was flushed with CO 2 to displace air from the pores. Then the de-aired water was injected from the bottom of the pack to obtain a fully water saturated pack. For the oil phase injection, the pack was inverted for a stable displacement. The oil injection continued until no further water phase was produced. Then primary waterfloodingwas carried out to obtain the initial condition for the WAG experiment. Finally, two cycles of air and water (each at 0.4 PV) were run.after each experiment, the column was washed using isopropanol and air dried for the next experiment. Glass-bead pack experiments were carried out using the same fluid systems (Fluids-1 and Fluids-2) in both strongly water- and oil-wet conditions. During the initial waterflooding, productions including oil and water were measured every 10 minutes. In the WAG process, the productions were measured at the end of each slug injection which included a total of 10 measurements for 5 complete cycles of gas and water with a WAG ratio of 1:1. 3. Results and Discussion A total of 10 experiments were carried out. The experimental results are given in the following sections. 3.1.Micromodel Results Strongly Water-wet Case Fig.3 shows two-and three-phase distributions of Fluids-1 after various displacementsthat took place in the strongly water-wet micromodel. Fig.3a shows the end of the primary waterflooding. As seen, the oil resides in the pore bodies while water surrounds all surfaces of the micromodel and oil phase is continuous in this section.after the first gas floodspreading fluids keep oil s hydraulic conductivity in the model through films (Fig.3b). Gas, as the most non-wetting phase, always tends to occupy the pore bodies while oil prefers to fill the adjacent throats or bodies entrances (Figs.3b and 3c). Fig.3c demonstrates that gas becomes disconnected after the second waterflooding, oil phase remains connected via films. No-direct contact betweenthe wetting (water) and the non-wetting (gas) phases can be used to describe the finger shape interfaces of - 4 -

5 gas and oil at the entrance of throats which are presented in Fig.4a. As lower gas/oil interfacial tension providesa situation whichmakes gas compete withthe throat threshold pressure, therefore interfacial tension balance is fulfilled at the throats entrance. Fig.4b showsthe three-phase distribution of Fluids-2 in the strongly water-wet model. The oil and gas interfaces show smooth curvatures everywhere at the entrance of the pore bodies. This is because the capillary thresholds for throats are relatively high as there are direct contacts between the gas and water phases resided in the throats (Fig. 4b). This prevents gas from formingfingershapedinterfaces at the entrance of the throats. However, the fingering contacts might occur where the throats are filled with oil. Fig.2 also shows oil clusters which are disconnected from the bulk after gas injection and these clusters might coalescence if two bulk clusters contact together. Øren and Pinczewski (1995)presented the conditions required for the existence of a stable threephase contact line for non-spreading systems in strong wetting conditions. Three-phase contact line requires the following inequalityto be fulfilled. W,NW W,IW IW,NW > 1 (1) where σ W,NW, σ W,IW, and σ ΙW,NW are the interfacial tensions between wetting, intermediatewetting and non-wetting phases. Substituting the measured interfacial tensions for Fluids-2 yields: GW GO OW = 4.55>1 which satisfies the inequality, showing that athree-phase contact line is expected. This is consistent with the observation as shown in Fig. 4b. Strongly Oil-wet Case Distributions of threephases of Fluids-2 in the oil-wet micromodel after gas and second water injection are shownin Fig.5. InFig.5a, oil is surrounded by water and gas where the former two occupy mostof the porespace. At the end of second waterflood, oil is produced through oil films, which confirms hydraulic connectivity of oil in the porous medium. This suggests that the main oil recovery mechanism in strongly oil-wet reservoirs during a WAG process is oil films rather than oil bulk recovery.in the same wettability conditions, the interfacial interactions among fluids are crucial in oil recovery analysis during a WAG process. Various interfacial tensions lead to different fluidconfigurations and distributions which, in turn, influence thefinal oil recovery. Fig. 6 compares the configurations of three-phase fluids in the strongly oil-wet micromodel for Fuilds-1 and Fluids-2. As it can be seen, there is absence of intermediate phase films for both fluids systems. This is because of the interfacial tension balance,introducing a negative spreading coefficient. Under strongly oil-wet conditions, oil films even at very low oil saturationsareexpected. In addition, the oil film is thicker and conductive in all glass micromodels. This conductivity results in higher oil recovery during a WAG process. However, different configurations were observed for both fluids systems. Comparing water/oil and gas/oil interfacial tensions, the most non-wetting phase can be either gas or water depending onthe fluid - 5 -

6 system applied. The following inequality determines where the three-phase contact line occurs in the pore space. σ W,NW σ W,IW (2) For Fluids-1,Eq.2 is fulfilled since water is the most non-wetting phase while gas is the intermediate phase. Fig.7a shows the configuration of Fluids-1 where gas is the intermediate phase, filling the throats and water is the non-wetting phaseoccupyingthe pore bodies. The threephase contact line most likely occurs in the pore body rather than pore throat which is shown in Fig.7a. For Fluids-2, the inequality in Eq.2 does not satisfybecausethe intermediate phase changes from gas to water. As a result, the force balance occurs at adifferent position for Fluids-2. Fig.7b shows that the three-phase contact lineoccursat the throats entrance. Unlike the water-wet case, in the oil-wet micromodel, both fluid systems are non-spreading. The presence of three-phase contact line has direct influence on final recovery. The importance of this will be discussed in the next section. Substituting the interfacial tensions of each pair in Eq.1 gives the following results. Fluids-1 Fluids-2 WO WG GO =0.8 < 1 GO =1.2 >1 GW WO This demonstrates that the three-phase contact line is available for Fluids-2 but not for Fluids-1 which is consistent with the observation reported in the literature (Øren and Pinczewski 1995). 3.2 Glass-beadPack Results Strongly Water-wet Case Fig.8shows the experimental results of waterflooding and WAG process oil recovery versus injected pore volume for strongly water-wet glass beads pack. Experimental recoveries were reported for two fluid systems, Fluids-1 (spreading) and Fluids-2 (non-spreading).in order to verify the reproducibility of the experiments, an extra test was carried out using Fluids-1, which confirmed that the experimental results are reproducible within an acceptable experimental error. FromFig.8, the following two observations can be made: 1) There is no considerable difference between the two initial waterflood although the Fluids-2 introduces slightly higher oil recovery (Fluid-2 recovery ~34 %; Fluids-1 recovery~32%). 2) Applying WAGincreases oil production remarkably. However, the higher total recovery is observed where Fluids-2 is used compared to Fluids-1. As the micromodel results in the previous sectionsuggest, displacement mechanisms are mainly controlled by fluid distributions which are determined by capillary forces (Oren et al, 1995). WAG includes double and multiple displacements instead of single drainage or imbibition, as it consists of multiple water and gas injections. For instance, gas injection follows with double - 6 -

7 drainage displacement mechanism as gas displaces the oil which displaces water. Thus, if the displacement process is to be carried out then capillary pressures given by Eqs.3 and 4 must be overcome. Capillary forces with regard to each fluid system are calculated using Eq.3 and Eq.4 for Fluids-1 and Fluids-2 in strongly wetting condition, respectively, which follow directly from Laplace s equation and the geometry of the menisci. P G,O P C G,O O,W (3) O,W (4) where θ is the gas and oil contact angle on water phase, and r 1 and r 2 represent the mean radius curvature of adjacent pore throats. These equations yield higher values of capillary pressure for Fluids-1 compared to Fluids-2. Lower interfacial tensions between each phase set in Fluids-2 result in less resistance for gas and water slugs. This leads to higher final oil recovery which is plotted in Fig.8. Although the presence of oil films isproved forfluids-1 in the water-wet condition, the oil film could not help higher production due to two reasons. Firstly, oil film can be effective only fora sufficiently long time which was not the case in this experiment. Secondly, the very low relative permeability of oil films neglectsits importance in governing the final recovery. Although the oil recovery is dominated by oil film recovery in the oil-wet micromodel, in the water-wet condition, however, the oil film recovery becomes negligible and bulk recovery plays the major role in recovery.this lowers the significance of spreading oil film in a WAG process in a water-wet reservoir. Strongly Oil-wet Case Fig.9 shows the experimental results of waterflood and WAG processes for both fluid systems in strongly oil-wet glass beads packs. An acceptable reproduction of the experimental results was obtained.from Fig.9, the following observations can be made: 1) There is a remarkable difference between the two initial waterflood recoveries (Fluid-2 recovery ~38 %; Fluids-1 recovery~33%). This can be attributed to the different IFT between oil and water for both fluid systems. As Table 2 shows, the water-oil IFT in Fluids-2 is lower than that in Fluids-1, which results in lower capillary resistance for the injected water of Fluids-2 to displace oil from the bead pack. 2) Applying WAG injection after the waterflood increases oil production significantly. However, a higher total recovery was obtained with Fluids-1 compared to Fluids-2. In Fig.9, Fluids-1 oil recovery is slightly higher compared to Fluids-2 even though it has lower initial waterflood recovery. This higher recovery can be attributed to the threephase contact configurations previously discussed. As it is demonstrated before, Fluids-1 has no three phase contact line.instead,the wetting phase exists between both non-wetting phases and coat both of them separately. This means that oil as the wetting phase exists between gas and water. Knowing this fact, capillary pressure can be calculated by: P W,O O,G G,O (5) - 7 -

8 On the other hand, Fluids-2 introduces a three-phase contact line. As a result, Eq.6 must be used for calculating capillary pressure, P C W,G W,O (6) where θ s contact angle between gas and water, and r 1 and r 2 represent mean radius curvatures. Capillary pressure from Eq.6 is slightly higher than from Eq. 5, suggesting that Fluids-2 has higher capillary forces to overcome. This is the reason whythe WAG with Fluids-2 yields a lower recovery compared to Fluids-1 (approximately 7% higher during WAG). An interesting phenomenon occurs in oil-wet media withrespect to alteration of the non-wetting phases. In Fig.9,by comparison of each injected slug of each WAG cycle in Fluids-1 with the similar slug in Fluids-2, higher oil recovery is recorded for the intermediate phase. For instance in cycle 3 in Fluids-1, where gas is the intermediate phase, gas injection led to higher recovery as twice as the similar fluid injection in Fluids- 2. In contrast, water as the non-wetting phase in Fluids-1 in the same cycle led to lower recoveries by one quarter compared to water injection recovery in Fluids-2. This pattern repeats in all the cycles except cycle one, which might be due to high water saturation of the waterflooded pack. The reason behind this pattern might be due to less resistance in front of the intermediate phase to flow through pore throats which reside considerable amount of oil. Comparing Figs.8and 9 forthe ultimaterecoveries shows thatthe oil-wet glass bead pack yieldshigher values for both fluid systems. This is because the conditions are quite different when oil is the wetting phase. Oil may flow in thick films between pore walls and non-wetting phases. In addition, the oil phase remains hydraulically connected until a very low oil saturation which is not the case in water-wet. 4. Conclusions We havepresented an experimental study to investigatethe effect of wettability and spreading characteristics of fluid-rock systemson immiscible WAG oil recoveries. Following conclusions are derived from this study: In WAG processes, pore-scale distribution of three phases is determined by wettability, interfacial tension balance among fluids and capillary pressure. Displacement mechanisms are mainly controlled by fluid distributions which are determined by capillary forces.fluid systems with higher capillary forces result in less oil recovery. In WAG processes in water-wet reservoirs, oil film contribution to theultimate oil recovery is not critical.a higher WAG oil recovery is obtained from a non-spreading system

9 In WAG processes in oil-wet reservoirs, oil films play an important role in ultimate oil recovery. As a result, a higher WAG recovery is obtained from oil-wet porous media. In oil-wet media, it is expected that gas or water both can bethe intermediate phase depending on their interfacial tensions with oil. Itis shown that the intermediate phase regardless water or gas has a higher contribution in oil recovery at each cycle and ultimate oil recovery

10 References Agbalaka, C. C., A. Y. Dandekar, et al. (2008). THE EFFECT OF WETTABILITY ON OIL RECOVERY: A REVIEW. SPE Asia Pacific Oil and Gas Conference and Exhibition. Perth, Australia. Dicarlo, D. A., A. Sahni, et al. (2000). "The Effect of Wettability on Three-Phase Relative Permeability." Transport in Porous Media39(3): Donaldson, E. C. and R. D. Thomas (1971). Microscopic Observations of Oil Displacement in Water-Wet and Oil- Wet Systems. Fall Meeting of the Society of Petroleum Engineers of AIME. New Orleans, Louisiana. Keller, A. A., M. J. Blunt, et al. (1997). "Micromodel Observation of the Role of Oil Layers in Three-Phase Flow." Transport in Porous Media26(3): Mani, V. and K. K. Mohanty (1997). "Effect of the Spreading Coefficient on Three-Phase Flow in Porous Media." Journal of Colloid and Interface Science187(1): Oren, P. E., J. Billiotte, et al. (1992). "Mobilization of Waterflood Residual Oil by Gas Injection for Water-Wet Conditions." SPE Formation Evaluation7(1). Oren, P. E. and W. V. Pinczewski (1994). "The Effect of Wettability and Spreading Coefficients on the Recovery of Waterflood Residual Oil by Miscible Gasflooding." SPE Formation Evaluation9(2). Øren, P. E. and W. V. Pinczewski (1995). "Fluid distribution and pore-scale displacement mechanisms in drainage dominated three-phase flow." Transport in Porous Media20(1): Salimi, H. and J. Bruining (2010). The Influence of Wettability on Oil Recovery From Naturally Fractured Oil Reservoirs Including Non-Equilibrium Effects. SPE Latin American and Caribbean Petroleum Engineering Conference. Lima, Peru. Skauge, A. and E. I. Dale (2007). Progress in Immiscible WAG Modelling. SPE/EAGE Reservoir Characterization and Simulation Conference. Abu Dhabi, UAE. Sohrabi, M., D. H. Tehrani, et al. (2001). Visualisation of Oil Recovery by Water Alternating Gas (WAG) Injection Using High Pressure Micromodels - Oil-Wet & Mixed-Wet Systems. SPE Annual Technical Conference and Exhibition. New Orleans, Louisiana. Vizika, O., E. Rosenberg, et al. (1998). "Study of wettability and spreading impact in three-phase gas injection by cryo-scanning electron microscopy." Journal of Petroleum Science and Engineering20(3-4):

11 Table 1 Average properties of glass bead packs. W-W Pack O-W Pack Permeability (Darcy) Pore Volume (cm 3 ) ~ 64 ~62 Bulk Volume(cm 3 ) Porosity 40% 39% Table 2 Physiochemical properties of the fluid systems. ρ W σ wg σ og K Fluid System ρ O Kg/m 3 ρ g σ ow mn/m mn/m Fluids-1 Water Soltrol 130 Air Fluids-2 Water + isobutanol Soltrol 130+isobutanol Air

12 Fig.1 Micromodel experimental flow apparatus

13 Fig.2 Glass bead pack experimental setup

14 Oil Water Gas (a) Waterflood Oil-Water (b) WAG Gasflood (c) WAG Waterflood Fig. 3 Fluid distribution during oil recovery from strongly water-wet micromodel (Fluids 1)

15 Oil β α δ Oil Gas Gas Water Water (a) Positive spreading distribution (Fluids-1) (b) Non spreading distribution (Fluids-2) Fig. 4 Spreading (Fluids-1) and non-spreading (Fluids-2) fluid distribution in the water-wet micromodel

16 GAS OIL WATE (a) Fluid distribution after initial nitial gasflooding (b) Fluid distribution after second waterflooding w Fig.5.5 Fluids-2 in the strongly oil-wet micromodel

17 Water Gas Fig. 6(a) Fluid distribution in the oil-wet micromodel (Fluids-1) Fig. 6(b) Fluid distribution in the oil-wet micromodel (Fluids-2)

18 Fig. 7(a) Fluid distribution (Fluids-1 in oil-wet et micromodel) Fig. 7 (b) Fluid distribution (Fluids-2 in oil--wet micromodel)

19 Fig. 8 - Oil recovery from the water-wet glass-bead pack

20 Fig. 9 - Oil recovery from the oil-wet glass-bead pack