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Pore Scale Visualization of Low Salinity Water Flooding as an Enhanced Oil Recovery Method Tammy Amirian, Manouchehr Haghighi, and Peyman Mostaghimi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01702 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017
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Pore Scale Visualization of Low Salinity Water Flooding as an Enhanced Oil Recovery Method Tammy Amirian a, Manouchehr Haghighi a, Peyman Mostaghimi b a
Australian School of Petroleum, Faculty of Engineering, Computer and Mathematical Sciences,
The University of Adelaide, Adelaide, SA 5000, Australia b
School of Petroleum Engineering, The University of New South Wales, Sydney, NSW 2052,
Australia Corresponding Author: Tammy Amirian E-mail address:
[email protected] Abstract The controlling mechanisms behind Low Salinity Water Flooding (LSWF) as an Enhanced oil Recovery (EOR) method are not well understood. So far, limited number of researchers tried to provide visual and direct evidences of underlying mechanisms behind the LS effect. In this paper, to investigate the dynamics of displacement throughout LSWF, clean and clay-coated 2D glass micromodels were used; also the wettability status was set at both water-wet and oil-wet conditions. Hence, pore-scale displacement mechanisms in the presence and absence of clay, as well as in the drainage and imbibition dominated two-phase flow were studied. In water-wet systems, in the absence and presence of clays, LSW hindered ‘snap-off’ perhaps due to the development of a viscoelastic water-oil interface. The wettability alteration towards more water-wetness was observed for oil-wet systems. The observations are discussed in terms of the expansion of Electrical Double Layer (EDL). Fines migration played an insignificant role in our observations.
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Key words: Low salinity water flooding, Enhanced oil recovery, Electrical double layer theory, Glass micromodels, Visualization experiments, Wettability alteration
1. Introduction: The low salinity effect on improved recovery was first reported by Bernard 1, although most of the modern works on this topic began in the 1990s. In 1996 Yildiz and Morrow 2 advanced the research started by Jadhunandan 3, and published their paper on the influence of brine composition on oil recovery. They showed that not only the physical properties of the injection water, i.e. density and viscosity, matter, but also its chemical properties, like brine composition, are of great importance for determining the ultimate level of oil recovery from a reservoir. Since then several researchers have conducted laboratory experiments and field trials to investigate the Low Salinity Water (LSW) effect 4-9. They confirmed that performing a secondary or tertiary LSW, with salinity in the range of 1000-2000 ppm, results in enhanced oil recovery. Based on the experimental observations, several mechanisms of LSWF have been proposed. Among them wettability alteration is the most frequently suggested mechanism in the literature 10-12
. Many other mechanisms could lead to wettability alteration. Fine migration and double
layer effect are among mechanisms which could result in wettability alteration. A number of authors suggested that in complete water-wet system, LSW could lead to the reduction of snapoff and consequently to enhanced oil recovery. There is no general agreement over the main mechanism and none of the suggested mechanisms have so far been generally accepted as the true mechanism
10, 12-16
. For each mechanism or explanation there are several experimental
counter examples published in the literature and this is the challenge of defining the primary mechanism of LSWF. Most likely, different mechanisms work under different conditions. Even though some necessary prerequisites or working conditions for the LSF are known, they are not
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yet sufficient to guarantee the success of LSF recovery. In some cases, where the necessary prerequisites were met, no low salinity effect was observed or vice versa 10, 16. Therefore, more research is needed to identify the true mechanism and sufficient conditions for the low salinity effect to work, so that field performance can be predicted accurately. Most experimental evidence of the improved oil recovery associated with LSWF is indirect and macroscopic, that is, in the form of an increase in the produced oil. Limited numbers of researchers 17-20 took a different approach in order to study LSWF and tried to provide visual and direct evidences underlying mechanisms of LS effect at the pore scale. Berg, et al. 18 and Mahani, et al.
20
in very similar set-up have monitored the effect of low-salinity brine on crude oil
droplets attached to clay particles adhered to a glass substrate that was placed in a transparent flow cell. They monitored the amount of oil recovered as well as the change in the oil droplets contact angel after LS brine was introduced to the system. Song and Kovscek 21 and Barnaji, et al. 17
used clay-coated glass micromodels and measured the oil recovery factor based on the
micromodels images taken before and after LSW injection. They also monitored fines migrations. In a few studies, fluid distribution and micro-mechanism of displacement throughout LSWF have been reported
22-25
. Emadi and Sohrabi
24
used mixed-wet clean micromodels to visualize the
micromechanism of the LS brine effect. They suggested that the formation of water microdispersion in oil phase led to the slight change in the fluid redistribution and resultant improved recovery. Another experimental study
25
identified osmosis as an oil mobilization mechanism
during low salinity water flooding using capillary tubes and micromodels with different wetting properties. Bartels, et al. 23 used single channel clay-functionalized micromodels and a change in oil contact angle was considered as the positive response to LSW. In another study
22
, they
conducted dynamic experiments in natural rock and fluids systems instead of models and
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applied a X-Ray micro-CT technique to image the redistribution of fluids when low salinity brine was injected. They reported the occurrence of additional oil/water structure (micro emulsion) in porous media. However, such observations need to be further assessed to evaluate the existing hypotheses on the influence of LSW on fluid distribution and micro-mechanism of displacement to systems (porous media) with variety of statuses in wettability, fluid properties and lithology. Herein, a microfluidic approach is developed to understand micro-mechanisms of displacement under LSWF at the pore scale level (micrometres to millimetres) using glass micromodels. The micromodels provide direct visualization of the controlling mechanism of LSWF. To investigate the dynamics of displacement in a variety of conditions throughout LSWF, clean and clay-coated micromodels were used; also the wettability status was set at both water-wet and oil-wet. Different micro-mechanisms of displacement were observed for water-wet and oil-wet conditions. These observations have been discussed in term of expansion of Electrical Double Layer (EDL).
2. Materials and Methods 2.1 Test fluid: Deionised water was used to make different concentrations of saline water (NaCl solutions) as the aqueous phases. The saline water properties are listed in Table 1. The crude oil was obtained from the Cooper basin in South Australia and used as the oil phase. The crude oil physical properties are listed in Table 2. The interfacial tension (IFT) of crude oil/water was measured using Kruss spinning drop tensiometer and it is included in the Table 1.
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2.2 Clay minerals: Clay minerals were deposited on the solid surfaces of the glass micromodel to create a visualization platform with more representative surface interactions between the reservoir fluids and the reservoir rock. The clay mineral used in the experiments was kaolinite. Kaolinite was supplied by the clay minerals society (www. clays.org) and was used without any purification or alteration. Kaolinite Al2Si2O5(OH)4, is a typical 1:1-type clay mineral and consists of alternating sheets of silica (SiO4) tetrahedral and aluminium hydroxide (Al(OH)3) octahedral. In the octahedral sheet, each aluminium atom is coordinated by two oxygen atoms of the tetrahedral sheet and shares four hydroxyls with neighbouring aluminium atoms. The hydrogen bonds between the hydroxyl ions of the octahedral sheet of one bilayer and the tetrahedral oxygen atoms of the adjacent silica sheet of the next bilayer hold the kaolinite layers together.
2.3 The clay coating procedure: Clay minerals are always present in sandstone reservoirs from 2% to 15%
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. To coat the
micromodel with clays, slurry made of clay particles and high salinity brine water (30000ppm of NaCl) was made and injected to the system. Prior to injection, to obtain a homogenous solution, the mixture was stirred for 15 mins followed by sonication for half an hour to prevent flocculation form occurring. The slurry was injected into the micromodels using a syringe pump at a very high rate of 20 m per day for several pore volumes to avoid clay settlement and clogging. During several practical tests, the optimized concentration of clay content in the suspension (20 wt. %) also the flow rate were selected to prevent the pore plugging and to ensure quasi-homogeneous distribution of clay in the micromodel.
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For the clay-coating purposes, in the majority of reported experiments in the literature, after clay injection, the micromodels have been heated in ovens for at least 72 hours at 80°C 27-28. As a result, the water of the suspension evaporated and dried clay adhered to the pore walls. Heat drying clay minerals can lead to irreversible clay-coating which means clay left on the surface of throats and grains would be immobile and unresponsive to the change in the salinity. Exposure of micromodels to the extreme heat would induce chemical degradation of clay minerals. To address this issue, the models were air dried while they were placed on a hot (80°c) plate. The micro model was hot-air dried for 2 hours. Very strong interfacial tensions between the air and water phases would also dislodge the blocked throats and the moderate temperature of the hot plate would assist the drying procedure. The whole process was being monitored using the microscope set-up. The images were taken from evenly distributed parts of the micromodel after deposition and analysed to ensure a quasi-homogeneous distribution of clay in the micromodel pores. The clay deposition was characterized mostly as pore lining and for very small parts of the model, the pore bridging structure was observed. A very similar approach was employed by Song and Kovscek
21
but their procedure for clay
deposition has been further optimized to be applicable to the methodology employed in the current study. Song and Kovscek
21
only air dried (no heating was involved) the micromodel to
study the LS brine effect on the clay detachment in single phase flow. Though, to study the wettability alteration throughout LS brine injection for two phase flow analysis; they heated the micromodel up to 120 degrees for 25 mins. Moreover, to acquire an oil-wet system, the pH of the clay suspension was modified. Kaolinite is always (even after drying) covered with a thin water film which originates from the capillary condensation in small pores between clay aggregates also from the air humidity. The wettability
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of kaolinite depends on the stability of the thin brine film which is influenced by the salinity and pH of the liquid phase, as well as the aging time 29. This thin film is increasingly prone to rupture, to allow adhesion and deposition of oil, if the concentration of sodium is decreased and/or brine pH is decreased. Kaolinite was already been saturated in HS brine, so the presence of salt in high concentration is not contributing to the adhesion of oil. Lowering the pH of slurry for almost 20% would contribute to the rupture of the thin film and as a result, the adhesion of oil to the surface during the inadequate time allowed for aging. The pH reduction by only 20% is not considered a drastic modification since the pH is still well above the Zero Point of Charge (ZPC) of kaolinite and the surface remains negative. The stability of the water film would be discussed in more details in section 4.2. Figure 1 shows microscopic images of the micromodel before and after injecting the slurry and air drying. As seen in the figure, grains and throats are evenly coated with a sufficient amount of kaolinite.
2.4 Microfluidic set-up: The microfluidic set-up includes a pump to inject water, a glass micromodel, and a microscope equipped with a camera which is connected to a computer. A schematic of the experimental system has been provided in Figure 2. Flow and transport within the micromodel were visualized using a BH12 fluorescence microscope, Olympus, Tokyo, Japan. The microscope was equipped with a camera, Moticam 580, China. The camera captures 5 megapixel, 2560x1920 images and record videos at 1080P (full HD) using the Motic Images Plus software package.
3. Visualization experiments: The visualization experiments were divided into two categories; 1) LSWF in the absence of clays, 2) LSWF in the presence of clays
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3.1 LSWF in the absence of clay At this stage, a clean micromodel was continuously flushed with several pore volumes of brine with a salinity level close to the initial formation brine (30000 ppm of NaCl). Once the micromodel was fully saturated with brine, crude oil was injected for several pore volumes at a constant rate of 30 m per day. Water saturation was monitored over time and it was continued until the water saturation was invariant. After the microfluidic system was aged (3 weeks), low salinity solutions were injected at a constant rate until no incremental increases in water saturation was observed. The injection rate was set at a rate which corresponds to a domain velocity of 10 m per day. Deionized water was used to make NaCl solutions with different concentrations. Salt concentrations in the reduced salinity saltwater ranged from 6000 ppm to 0 ppm of NaCl (fresh water) in decrements of 2000ppm. pH of brines was neutral.
3.2 LSWF in the presence of clay Following the clay-coating procedure discussed in Section 2.3, high salinity brine (30000 ppm of NaCl) and oil were introduced into the system to replicate the initial reservoir condition. After a delay which took 3 weeks, LS brine was injected at the flow rate of 10 m per day. Similar to LSWF in the absence of clay, LS brines were NaCl solutions at concentrations spanning the range of 6000 ppm to 0 (fresh water) and with pH equals to 7. The LSW injection was continued until the water saturation plateaued. To comprehensively investigate LSW effects and their possible contribution to EOR, the wettability status of microfluidic devices was set at water-wet as well as oil-wet conditions. To make the system oil-wet, the glass micromodel was saturated with oil and then aged for almost 3 weeks. Since the presence of salt in connate water increases the hydrophilicity of clay minerals significantly 30 which leads to an extremely water-wet system, the micromodel was saturated with oil only. In addition, slight pH modification (5
