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A new method based on CO2-switchable wormlike micelles for controlling CO2 breakthrough in tight fractured oil reservoir Zihao Yang, Xiaochen Li, Danyang Li, Taiheng Yin, Peizhong Zhang, Zhaoxia Dong, Meiqin Lin, and Juan Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00362 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 11, 2019
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Energy & Fuels
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A new method based on CO2-switchable wormlike micelles for controlling CO2
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breakthrough in tight fractured oil reservoir
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Zihao Yang,*,† Xiaochen Li,† Danyang Li,† Taiheng Yin,*,† Peizhong Zhang,† Zhaoxia
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Dong,† Meiqin Lin,†,‡ and Juan Zhang†
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†Unconventional
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Petroleum Research Institute, China University of Petroleum
(Beijing), Beijing, 102249, People’s Republic of China ‡Key
Laboratory of Enhanced Oil Recovery, CNPC, Changping, Beijing, 102249,
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People’s Republic of China
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KEYWORDS: CO2-switchable wormlike micelles; CO2 flooding; tight fractured oil
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reservoir; gas channeling sealing agent; Enhanced oil recovery.
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ABSTRACT
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CO2 is widely utilized for enhancing oil recovery（EOR）due to its high ability of
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washing oil and favorable injectivity, especially for tight oil reservoir. During EOR
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process, the oil recovery is significantly affected by gas channeling and the sweep
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efficiency of CO2 is limited. Herein, we report a CO2-switchable smart wormlike
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micelles (WLMs) based on sodium dodecyl sulfate (SDS) and diethylenetriamine
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(DETA) to prevent gas channeling of CO2 in tight fractured oil reservoir.
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The proof to the microstructure, formation mechanism and plugging performance of
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CO2-switchable WLMs were studied by cryo-TEM, DLS, NMR, rheology and plugging
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property measurement. The results indicated the system can be reversibly circulated
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between spherical micelles and wormlike micelles by repeatedly bubbling and
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removing CO2. When CO2 is introduced to the solution, part of DETA molecules are
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protonated, and “bridge” two SDS molecules to form pseudo-gemini surfactant by
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noncovalent electrostatic attraction, behaving a high viscosity fluid. Upon removal of
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CO2, the protonated DETA molecules return to original state, causing the pseudo-
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gemini structure being destroyed and the viscosity of the fluid is recovered. Moreover,
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based on the results of plugging property measurement, the solution presents
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conspicuous injection and plugging performance. This WLMs viscoelastic fluid might
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have potential application in the enhancement of CO2 flooding in tight fracture reservoir.
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1. Introduction
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Over the past decades, CO2 flooding technology has been widely applied in oil and
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gas fields production and recognized as the most promising application approach in
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tight reservoir due to its favorable injectivity. 1-4 In recent years, tight reservoirs have
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an increasing proportion of oil development, and it account for about 40% of the
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recoverable oil resources in China.5, 6 In order to increase the initial oil production,
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hydraulic fracturing technology is mainly adopted, which forces the induced fractures
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and natural fractures to form complex network in tight reservoirs.6-8 However, this
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fracture network tends to become a gas channel during CO2 flooding, which causes the
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injected CO2 breakthrough and bypasses crude oil, seriously affecting the CO2 2 ACS Paragon Plus Environment
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sweeping volume and the ultimate oil recovery.9-11 Plugging gas channeling is a major preventive method to control CO2 breakthrough.
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It has been widely demonstrated that methods based on gel12-14, foam15,
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alternating-gas17, 18 can effectively inhibit CO2 breakthrough. But these conventional
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methods are limited in tight fractured oil reservoirs because of difficult injection, poor
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stability and low strength, resulting in failure to effectively improve the swept volume
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of CO2. Therefore, it is extremely urgent to develop a novel CO2 channeling sealing
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agent which is suitable for tight fractured oil reservoir to enhance oil recovery.
water-
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Since the advent of environmentally stimuli-responsive smart materials, CO2 stimuli-
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responsive materials have emerged in an endless stream.19-22 These kinds of CO2-
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switchable smart materials provide new ideas and possibilities for development a novel
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approach to control gas channeling during CO2 flooding.
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Li et al.23 created a CO2-triggered gelation for mobility control and channeling
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blocking in CO2 flooding process. This gel is solidified by CO2 triggered, thus
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effectively plugging gas channeling. Due to the solidification in near-wellbore area and
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the viscous effect accumulates, the CO2-triggered gelation system is difficultly injected
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into the formation. Thereby, it is not suitable to be applied for in tight fractured reservoir.
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Liu et al.24 reported a CO2-switchable silica nanohybrids for enhancing CO2 flooding
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in tight sandstones. The nanoparticle can enter in-depth reservoirs with its particle size
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of about 50 nm. However, the viscosity change of the system after CO2 sparging is not
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significant enough to inhibit CO2 channeling in fracture. Li et al.25 synthesized CO2 3 ACS Paragon Plus Environment
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sensitive compound octadecyl dipropylene triamine and prepared CO2 foam. The foam
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shows excellent performance in blocking and mobility control. However, the imperfect
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of this technology is prone to displace the crude oil in high-permeability area. When
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there are fractures in the reservoir, the method is not effective.26
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Considering the characteristics of tight fractured reservoir, CO2-switchable wormlike
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micelles (WLMs) have become a viable option of gas channeling sealing agent. The
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CO2-switchable WLMs can transform between low-viscosity fluid and high-viscosity
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fluid with bubbling and removing CO2, and has shear-thinning property.27 This is to say
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that the injected solution have low viscosity, and maintain excellent fluidity in the
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reservoir. When the solution contacts with CO2, the solution viscosity rises to plug the
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gas channeling, thereby the sweep volume is enlarged and more oil can be displaced.
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More importantly, the transformation from high-viscosity to low viscosity enable this
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sealing agent move to the depths of the reservoir.
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A common feature of compounds with CO2 sensitivity is the possession of
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principally amidine, 21, 28 guanidine,29, 30 or amine groups.31, 32 For amine compounds
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with the same molecular structure, the order of alkalinity from strong to weak is
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guanidine, amidine, primary amines, and tertiary amines. It is easier for substance with
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higher alkaline group to react with CO2, which also indicates that the reversible reaction
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is more difficult to be achieved.33, 34 Furthermore, the tertiary amines group does not
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have any hydrogen bonding as compared to the primary amines group, causing poor
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stability of protonated production. Considering the characteristics of these groups, the 4 ACS Paragon Plus Environment
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advantages of the primary amines group are striking. The alkalinity of primary amines
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group is moderate and quickly to respond to CO2, meanwhile, it can reduce the
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adsorption of surfactants on rocks.35 In addition, its protonated production is stable and
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conducive to the existence of WLMs.
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In this work, we create a new method based on CO2-switchable WLMs for
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controlling CO2 breakthrough in tight fractured oil reservoir. This switchable fluid is
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composed of anionic surfactant sodium dodecyl sulfate (SDS, Scheme 1) and
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diethylenetriamine (DETA, Scheme 1), which can be reversibly circulated between
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“gel-like” and water-like fluid by alternately introducing and removing CO2. Further,
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rheological characterization, cryo-transmission electron microscopy (cryo-TEM),
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dynamic light scattering (DLS), and 1H NMR spectroscopic are employed to
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characterize the structure of the micelles and conclude the formation mechanism.
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Subsequently, the plugging capacity of the fluid is evaluated by using a single-tube sand
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pack model and artificial fracture core. In our study, this CO2-switchable WLMs
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demonstrates excellent injection performance and CO2 plugging ability. We expect this
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work can provide guidance for restraining CO2 channeling and improving CO2 flooding
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efficiency in tight fractured oil reservoir.
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2. Experimental section
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2.1. Materials. Diethylenetriamine (DETA,99%), sodium dodecyl sulfate (SDS,97%)
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and sodium chloride (NaCl,99.5%) were purchased from Shanghai Macklin
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Biochemical Co. LTD. All chemicals were used as received without further treatments. 5 ACS Paragon Plus Environment
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Deionized water (18.0MΩ·cm) was used for all aqueous solutions. The brine was
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prepared by NaCl with a salinity of 5000 mg/L.
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2.2. Sample preparation. The solution was prepared by dissolving 500 mmol of
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SDS and 600 mmol of DETA in 1L of deionized water with magnetic stirring (referred
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to as SDS-DETA). Other concentration samples were obtained by diluting the solutions
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with deionized water.
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CO2 was bubbled into the mixture with the flow rate of 0.1L/min at 25 ℃, leading to
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a fluid (referred to as CO2-SDS-DETA). N2 was bubbled into CO2-SDS-DETA at 75 ℃
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as the same flow rate of CO2. The concentration of the mixed solutions is expressed as
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the concentration of DETA. For example, the mixture by 500 mM SDS and 600 mM
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DETA is given as 600 mM SDS-DETA.
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2.3. Rheology. Steady and dynamic rheological measurements were carried out on a
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HAAKE RheoStress 6000 rheometer with a coaxial cylinder sensor system (Z41 Ti).36
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All the samples were equilibrated at 25 ℃ for no less than 20 min prior to the
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experiments. In steady shear measurements, the shear rate was gradually increased from
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0.001 to 1000 s-1. The rheometer was set to ensure the gradient to be less than 0.5
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(Δτ/τ)/Δt% at each shear rate step. In oscillatory experiments, an amplitude sweep at a
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fixed frequency of 1.0 Hz was performed to determine the linear viscoelastic region.
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After 30 minutes, frequency sweep was performed from 0.01 to 10 Hz. The temperature
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was controlled at 25.0±0.1 ℃ by using a cyclic water bath.
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2.4. Cryo-TEM observation. The wormlike micelle microstructures of specimens 6 ACS Paragon Plus Environment
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were determined by cryo-TEM observations.36 About five microliters of sample
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solution preheated at 25 ℃ was loaded onto a carbon-coated holey film which is blotted
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with two pieces of filter paper to obtain a thin liquid film. Next, the specimen was
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precooled instantly into liquid ethane at -165 ℃. Then the vitrified specimen was
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transferred to a JEM-2200FS TEM instrument equipped with a Gatan cryo Holder 626,
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operating at 80 kV. The entire observation process is maintained at -183 ℃.
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2.5. Dynamic Light Scattering (DLS). Dynamic Light Scattering (DLS) was
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performed with an ALV/DLS/SLS-5022F instrument (HOSIC LIMITED, Germany)
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with a 90° back-scattering angle and an He-Ne laser (λ = 633 nm).
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2.6. Plugging property measurement. This study was operated to investigate the
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plugging performance of the dispersion; a single-tube sand pack model and an artificial
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fracture core were employed.6, 37 The basic parameters of the models are shown in Table
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1.
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The single-tube sand pack model was made of quartz sand with a particle size of 100
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mesh. The fracture core was made of natural outcrop core which was from Changqing
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Oilfield, and formed by fracturing along the injection direction. Three φ 0.3×90 mm
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iron wires were used to support fractures, thus simulating the artificial fracture and
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natural fracture in the reservoir. The section, end face and profile of the fracture core
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with the artificial fracture are shown in Figure 1. The temperature of the thermostat was
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maintained at 25 ℃.
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In sand pack model experiment, the injecting rate was set to 2.0 mL/min. First, the 7 ACS Paragon Plus Environment
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brine water and CO2 were sequentially injected for 4.0 PV, respectively. Then the
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sample solution (600 mM SDS-DETA) was infused into the model for 2.5 PV.
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Afterward, CO2 was injected second time for stimulation the solution. After 30 minutes,
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the brine was injected again.
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In fracture core experiment, the brine was injected into the fracture core for 2.0 PV,
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followed by 4.0 PV CO2 to saturate the brine. Continuing, 2.5 PV sample solution
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(600mM) was injected. Then CO2 was injected again to make the sample switch to high
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viscosity state. During the above flooding process, the injection rate was set at 0.4
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mL/min. After 30 minutes, CO2 was injected into the core at a rate of 4.0 mL/min until
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the gas escaped at the end of the core holder.
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The pressure changing was detected by the sensor. The permeability ( kwb, given in
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units of μm2 ) and the plugging rate ( η, expressed in units of %) of sand pack model
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after the plugging process were calculated using the following equations:38, 39
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𝑘=
𝑄𝜇𝐿 𝐴𝛥𝑃
𝑅𝐾 = 𝑘𝑤𝑏/𝑘𝑤𝑎 𝜂(%) =
𝑘𝑤𝑎 ― 𝑘𝑤𝑏 𝑘𝑤𝑎
× 100%
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where ΔP is the injection pressure difference (given in units of 10-1MPa), Q is the
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injection rate (given in units of mL/s), μ is the viscosity of brine water (given in units
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of mPa s), A and L are respectively the sectional area and length of sand pack model
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(expressed in units of cm2 and cm), and kwa is the initial permeability of sand pack model
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(given in units of μm2). The main equipment is shown in Figure 2. 8 ACS Paragon Plus Environment
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3. Results and discussion
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3.1 Preparation of CO2-switchable wormlike micelles. It is acknowledged that
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SDS is generally difficult to self-assemble into viscoelastic WLMs in aqueous solution.
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As exhibited in Figure 3A, when the SDS was mixed with DETA at 5:6 molar ratio, the
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viscosity of 600 mM SDS-DETA remains 1.2 mPa·s and does not vary with the shear
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rate, presenting a typical Newtonian fluid. In addition, SDS-DETA solution
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consistently exhibits Newtonian fluid behaviors with low viscosity in the concentration
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range, as shown in Figure 3B. This suggests that the dispersions may be composed of
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small spherical micelles, or short rod-like micelles. The relative lower viscosity also
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indicates the flowing resistance is weak during the injection process, which is beneficial
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for the flow and spread of the plugging agent into tight oil reservoirs.
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After CO2 is bubbled, the solution (SDS-DETA) transformed from a water-like fluid
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into viscoelastic “gel-like” fluid (CO2-SDS-DETA). In Figure 3A, the viscosity of 600
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mM solution increases nearly 4000 times, after CO2 stimulation. A common feature of
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these three different concentration solutions is that the steady shear rheological
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measurement behaved a Newtonian plateau within the shear-rate range of 10−2 to 3 s−1,
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accompanied by shear thinning over a critical shear rate. Such shear-thinning behavior
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of surfactant solutions is normally as an evidence for the existence of WLMs.40, 41 It is
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worth mentioning that even though the CO2-responsive solution forms a gel in the near-
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well area, because of shear-thinning property, the viscosity of the solution will be at a
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low level with the rock continuously shearing, which facilitates the gas channeling 9 ACS Paragon Plus Environment
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sealing agent penetrating to the deep formation.
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The zero-shear viscosity (η0) of CO2-SDS-DETA and SDS-DETA as a function of
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surfactant concentration (C) is also presented in Figure 3B. Upon sparging CO2, the η0-
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C curve is divided into two part by an obvious break point. The breakpoint is normally
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defined as the critical overlapping concentration (C*).42 When CC*,
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η0 increases exponentially, following the scaling law η0 ∝ Cn,42 where the power-law
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index is 2.57. In addition, as shown in Figure 3B, the viscosity of CO2-SDS-DETA with
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concentrations of 480 mM, 600 mM and 900 mM exceeds 1000 mPa·s, among which
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the viscosity of 600 mM and 900 mM CO2-SDS-DETA is similar to conventional
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sealing agents. Considering the viscosity-increasing effect and economic cost, the
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solution with a concentration of 600mM CO2-SDS-DETA is selected as a research
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object for CO2 channeling sealing agent.
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From the dynamic rheology data (Figure 3C), the viscous modulus (G′′) and elastic
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modulus (G′) of CO2-SDS-DETA dispersion exhibit a typical viscoelastic response of
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WLMs. It is evident that the solution shows viscous behavior (G′′>G′), at low
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oscillatory frequencies, while at high oscillatory frequencies it switches to elastic
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behavior (G′′
