Development of Thermotransformable Controlled Hydrogel for

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Development of Thermotransformable Controlled Hydrogel for Enhancing Oil Recovery Jingyang Pu,† Jia Zhou,†,‡ Yashu Chen,† and Baojun Bai*,† †

Department of Geosciences and Geological and Petroleum Engineering, Missouri University of Science and Technology, Rolla, Missouri 65409, United States ‡ Baker Hughes, Inc., Houston, Texas 77073, United States S Supporting Information *

ABSTRACT: A novel thermotransformable controlled polymer system (tPPG) is developed that can be injected into fractures or fracturelike features as a millimeter-sized particle gel (100 μm to a few millimeters) and acts as a plugging agent, then dissolves into linear polymer at a designated period (e.g., 6 months), because of the reservoir’s temperature. The dissolved polymer seeps into the depth of the formation and performs as a mobility control agent with high viscosity. Working together with permanent cross-linking the polymer, polyethylene glycol diacrylate 200 (PEG-200) entails the role of controlling dissolution time which has been added into the tPPG as a labile cross-linker. The polymer’s viscosity will not be influenced by the shearing stress during pumping or salinity in the reservoir. The time tPPG requires for transformation is dependent primarily upon the reservoir temperature and labile cross-linker concentration. This strategy offers a facile and economic approach to fabricating a promising dual-functional polymer system. In order to evaluate our proposed approach, main properties of the tPPG polymer are probed, including the swelling ratio, mechanical strength, and thermostability before transformation, viscosity, moving ability, and mobility control ability after transformation.

1. INTRODUCTION Hydrophilic polymer gel, which is also called a superabsorbent polymer (SAP), has found broad interests in recent years. SAPs1 are cross-linked, three-dimensional (3D), hydrophilic networks of polymer chains that can absorb more than 1000 times their mass in water or aqueous solution.2 They belong to the family of hydrogels that are 3D hydrophilic polymers and undergo a volume phase change upon coming into contact with water when there are changes in the surrounding conditions, such as temperature, salinity, or pH.3 However, they will not dissolve as a result of a chemical or physical cross-link. For many decades, hydrogels have been used in cosmetics and medicine,4 drug delivery, hygienic products, horticulture, sealing, and coaldewatering,5−7 and agriculture.8 A review of the literature reveals that the synthesis of SAP is not a novelty. In fact, Marcos et al. presented a superabsorbent hydrogel that has been utilized in agriculture as a soil conditioner.9 The water uptake capacity of the hydrogel was evaluated as a function of time, with results indicating that the hydrogels swelled up to 1000 times their dry weight. The hydrogel can be synthesized by a copolymerization reaction of acrylamide monomers with a chemically modified cashew gum derivative employing cerium(IV) sulfate and nitric acid as crosslinkers. Sakiyama et al. reported a similar product that has been used to retain water in soils and aid in plant growth.10 Kabiri et al. reported a controlled release hydrogel that has been used in agrochemicals.11 Their synthesis involved a solution polymerization of acrylic acid as the monomer and N,N′-methylenebis(acrylamide) and 1,4-butanedioldiacrylate as water and oilsoluble cross-linkers, respectively. In the area of enhanced oil recovery, Bai et al. have reportedly employed several SAP gels as fluid-diverting agents to reduce water production.12 Such gels © XXXX American Chemical Society

were synthesized from a monomer/polymer and cross-linker reaction and were injected into reservoir channels; they swelled up to 20−200 times their original size and plugged these fractures. In existing hydrophilic polymer gel systems, the gels swell, collapse, and even show volumetric phase transition in response or sensitive to changes of surrounding environments, e.g., temperature,13,14 solvent composition,15,16 ion valence and concentration,17 and pH18 of the immersing solution. Among them, temperature causes the phase transition and degradation of the SAP, resulting in reducing the applicability. In some studies, the effects of temperature on the SAP are interpreted.19 Their results show that different extents of influences are found for varying polymers. These types of thermospecific behaviors have also been found in many polymeric fields, e.g., pharmacology,20,21 biochemistry,22 photochemistry,23 and petrochemistry.24,25 The facile synthesis process, low price, and its tunable property has positioned thermospecific polymer or gel as a promising material for various applications, ranging from drug delivery26 to fuel cells.27 In oil or gas production, the discovery of new oil fields has become increasingly difficult and has occurred less frequently.28 Furthermore, with the rapid depletion of proven reserves, and considering that much oil still remains in the reservoirs after conventional methods have been exhausted, a need exists to enhance oil production from existing wells in order to maintain a continuous oil supply that will meet world energy demands.29 However, early water breakthroughs produce excessive unwanted water,30 a frequently reported problem that occurs in Received: October 18, 2017 Revised: November 20, 2017 Published: November 28, 2017 A

DOI: 10.1021/acs.energyfuels.7b03202 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. Chemical structure of tPPG hydrogel.

most mature reservoirs as a result of long-term water flooding that is caused by the geologic structure or injection damage, e.g., unexpected open fracture or fracturelike features underground directly connecting an injector with a producer,31 uneven vertical and horizontal permeability, unusual permeability near-wellbore due to caves, wormholes, fractures, or fracture-like features. Such unwanted water usually leads to increased production costs, the corrosion of downhole equipment, scale formation, and the possibility of early well shut-in and abandonment.32−34 Therefore, a vital need exists to optimize oil production from these existing wells amidst these many limitations, and novel plugging materials are needed to seal the fracturelike area near the wellbore and to propagate to the deep region of the reservoir to reduce fingering problems and divert injected fluids to zones/ areas that have remained unswept because of a reduced mobility ratio. Polymer or gel treatments have been used as chemical conformance improvement methods to increase the sweeping efficiency of mature reservoirs and control high water cut.35 Preformed microspheres (a type of SAP gel) have been successfully and economically applied to control water production and increase oil production.2,36 Over the past decade, the chemistry of PPGs have been explored extensively in order to modify the properties and improve the applications of these gels. Most studies interpreted temperature to these PPG stability influences, but merely mentioned the properties of degraded polymers. Meanwhile, polymer flooding has been proven as a cost-effective technology to improve oil recovery, having been used in significant applications over the last two decades.37−40 However, the original viscosity of the polymer solution at the ground’s surface might decrease by up to 80% at the bottom of the injection well, because of strong shear thinning and strong mechanical/chemical degradation of the polymer chains, both

leading to the dramatic viscosity reduction of the polymer solution in the reservoir.41,42 For an ideal and economic polymer or gel treatment, both near-wellbore and in-depth reservoir treatment should be concerned by a proper thermo transformation, which can be used as cross-linked hydrogel at an early stage and transform to a viscous and sticky polymer after the designed period. It should be dual-functional, which combines the advantages of both gel treatment and polymer flooding process while, to a certain extent, eliminating their respective disadvantages to greatly improve oil production from mature or abandoned reservoirs. The purpose of this research is to develop novel particle gels that can be used in both near-wellbore and deep mobility control agents to enhance oil recovery in mature reservoirs. To reduce the excessive water production, a novel slow thermotransformable particle hydrogel (tPPG) has been developed after employing thermosensitive cross-linking agents.43 The tPPG is initially developed to be strong and approximately a millimeter in size (from 50 μm to a few millimeters). It is introduced into the formation from an injection well and worked instantly by diverting the flow of fluids to low-permeability zones near the wellbore. These dual-functional “smart” particles then spontaneously transform to a viscous polymer solution after a designated time, because of the dissociation of their labile cross-linkers.44 The viscous polymer solution transformed from the tPPG acts as a mobility control agent deep within the reservoir. The novel particle gel prevents polymer shearing degradation during the injection process and creates viscous fluids in the in-depth region of the reservoir, where no significant shearing occurs, because of low velocity, resulting in significant improvements in the efficiency of polymer flooding. B

DOI: 10.1021/acs.energyfuels.7b03202 Energy Fuels XXXX, XXX, XXX−XXX

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terms of the tPPG microstructure, to determine whether tPPG retain their gel form or become linear polymers. The mechanical strength of tPPGs were measured using a THERMO Scientific HAAKE MARS rheometer to investigate the dominance of the gel’s elastic behavior over its viscosity behavior and how the gel’s properties vary with composition. Two tests were conducted for the samples with different cross-linker concentrations to determine the rheological properties, including the gel’s elastic moduli (G′) and viscous moduli (G″) over time with strain under room temperature (23 °C). The measurements were set as an oscillation model, and frequency and stress sweep experiments first were performed in a range of 1−10 Hz and 0.2−100 Pa in order to establish the extent of the linear viscoelastic region. The measurements of the elastic (G′) and viscous (G″) moduli under strain were tested with a fixed frequency of 1 Hz. Instead of been dried, the synthesized bulk gel was directly cut into the small pie with a diameter of 2.5 cm. The sensor used for all measurements was PP35L Ti (Hakker Rheoscope) with a gap of 1 mm. All runs were repeated at least three times. To study the influence of cross-linker concentration, the average molecular weight (M) of the polymer chain segments between two active junctions was estimated from the number of active junctions (n) and the total acrylamide monomer concentration [AM], as follows:

2. EXPERIMENTAL SECTION 2.1. Materials. Acrylamide (AM, 98.5%) was purchased from the Alfa Aesar Company (Ward Hill, MA) and was used without alteration. Potassium persulfate (KPS) and ammonium persulfate (APS), obtained from Sigma−Aldrich, were used as initiators. Acrylic acid (AA), ammonium hydroxide, N,N,N′,N′-tetramethylethylenediamine (TMEDA), urea, sodium carbonate, and sodium formate were obtained from Sigma−Aldrich and used without alteration. Two different crosslinkers, N,N′-methylenebis(acrylamide) (MBAA) (cross-linker A) and polyethylene glycol diacrylate 200 (PEG-200) (cross-linker B) were purchased from commercial companies and used without alteration. Sodium chloride (NaCl, >99.8% purity) was purchased from Fisher Scientific, Inc. and used without alteration. Distilled water was used for the synthesis. 1% NaCl brine was used for swelling and mechanical strength experiments. During the gel synthesis, the term %C was used to describe the mass of the cross-linker, relative to the total mass of the monomer:

%C =

mcrosslinker (g) × 100 mmonomer (g)

(1)

where mmonomer is the mass of monomer and mcross‑linker is the mass of cross-linker. 2.2. Fabrication of tPPG. tPPG is synthesized via free-radical polymerization initiated with the combination of potassium persulfate and TMEDA, because of its high efficiency. Generally, the monomer concentration is in the range of 10%−40%, with a cross-linker concentration between 50 ppm and 5000 ppm, depending on the final field application. The monomers are AM and AA. The labile cross-linker is PEG-200, which can degrade upon hydrolysis at elevated temperatures and/or pH levels. The permanent cross-linker is methylene bis(acrylamide), which does not degrade at elevated temperatures. Redox initiation systems based on APS or KPS and TMEDA were employed to polymerize the monomer solutions of AM and AA in the presence of some other additives, such as ammonium hydrate, urea, sodium formate, etc. The pH of the solution also was adjusted by the addition of ammonium hydrate, and sodium carbonate was used to neutralize the AA monomers. Urea can form strong hydrogen bonds with the forming polymer chains, leading to smaller pore sizes of the formed gels. The addition of sodium formate is believed to increase the solubility of gels in the aqueous solution (a detailed example is listed in the Supporting Information). Figure S1 in the Supporting Information shows a schematic of the gel synthesis and fabrication process. The chemical structure of synthesized tPPG hydrogel is shown in Figure 1. 2.3. Characterization and Evaluation. In order to determine whether tPPG can effective seal fractures and fracturelike features in mature reservoir situations, several evaluations, including mechanical strength, swelling kinetics by tubing test,45 and thermostability test, were established to study the properties of tPPG systems before transformation. Several properties of tPPG were evaluated after their transformation, e.g., morphology study via scanning electron microscopy (SEM) (Hitachi, Model S4700, Tokyo, Japan) operated in high vacuum mode at an accelerating voltage of 5 kV, filtration test, viscosity measurement, and sandpack core flooding test for proving whether the millimeter-sized tPPGs were broken down to viscous fluids and remained high viscosity, which can reduce the permeability of water-flushed zones/areas and reduce mobility ratio. 2.3.1. Structural Study and Mechanical Strength Measurement Method. Samples containing water without drying were mounted on metal stubs at a low vacuum degree (4.6 Torr), and a relatively low temperature (near 0 °C) was observed. The samples first underwent a freeze process in the chamber of an FEI Quanta 600 FEG extended vacuum SEM. To emphasize the gel microstructure, the ESEM imaging protocol was followed: the temperature and pressure were decreased simultaneously from 0 °C and 4.6 Torr to −5 °C and 2−3 Torr, thereby freezing the sample; the temperature then was allowed to rise to 20 °C with a rate of 2 °C/min at a pressure of 2−3 Torr to sublimate water from the sample at a relative humidity (RH) of 12.5%. This treatment method was employed to study the process of this transformation, in

M=

[AM] n

(2)

2.3.2. Swelling Kinetic Measurement and Swollen tPPG Thermostability Test. Swelling studies were performed at room temperature (∼23 °C) by immersing the dry tPPGs (0.2 g) in a 1 wt % sodium chloride (NaCl) solution (49.8 g). These tests assess the tendency of tPPG to swell; such information aids in the selection of the product best suited for a specific field application, with regard to its formation temperature. The swelling ratio of tPPG was calculated from their dry and swollen masses, as eq 3 shows: swelling ratio =

Vt − V0 V0

(3)

where V0 is the initial volume of the dried gels before swelling and Vt is the volume of the swollen gel at different periods. Thermostability studies were carried out with key apparatus, as shown in Figure S2 in the Supporting Information, by monitoring the volume changes of the samples. First, dry tPPG particles (0.12 g) were measured into each ampule, and then brine solution (11.88 g of 1 wt % NaCl) was injected into each ampule, making the gel concentration 10 000 ppm for each ampule. After the ampules were loaded, the vacuum pump continued to run at 25 psi for approximately half an hour, to remove the dissolved gases in the liquid sample, including any trace of dissolved oxygen that might have remained in the sample. Next, the ampules were flame-sealed in place. The sealed ampules were weighed using an analytical balance with an accuracy of 0.0001 g and then placed in an oven and aged at 45, 60, and 80 °C. After the specified aging times, one ampule was taken out of the oven and cooled to room temperature. This ampule was reweighed to confirm that none of the solution had leaked. A loss of ∼0.001 g or more indicated possible leakage from this ampule. In that case, another ampule would be used for a post-aging measurement. 2.3.3. Transformed tPPGs Viscosity Measurement Method. After transformation, the tPPG particle became viscous. The viscosity of the solution was measured under 80 °C by the Brookfield viscometer with No. 18 or No. 34 spindles. The viscosity measured at 6 rpm using a cup acrylic geometry was recorded as the reported value. 2.3.4. Filtration Evaluation Method through a Ceramic Disk. A high-pressure filtration apparatus with thick ceramic disks were used for the tPPG filtration tests to determine if the tPPG particles could propagate through porous media after their transformation. Figure S3 in the Supporting Information shows the subsequent high-temperature and high-pressure filtration apparatus with thick ceramic disks, which came into play because they have a relatively long path of interconnected pores, and large tPPG particles cannot easily pass through them. This filtration test with ceramic disks also can provide a rough idea of particle C

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Figure 2. Macrostructural and microstructural demonstrations of the tPPG: (a) typical dry tPPG particle (0.42−0.5 mm); (b) ESEM observation of a typical dry particle gel at low magnification; (c) swollen tPPG particle with size larger than 2 mm; and (d) ESEM observation of a swollen particle surface with spots and ridges. sizes. The filtration pressure is fixed at 40 psi, the pore size of the ceramic disk can be chosen from 3 μm to 35 μm, and the temperature can be adjusted from room temperature to 177 °C. 2.3.5. tPPG Flooding Test through the Sandpack Model. Sandpack tests46 was introduced to observe the in-depth transportation ability of the gel before transformation. A sandpack model is a cylindrical metal tube (2.54 cm diameter, 53.4 cm length) that has four pressure taps (as shown in Figure S4 in the Supporting Information). The inlet side was connected to an ISCO pump with a 1000 mL accumulator. The outlet side was connected to a graded test tube to collect the fluids. The accumulator has a piston to prevent the injected water from contacting the tPPG samples directly. The pressure pushes the piston and forces 2000 ppm of the transformed tPPG solution to pass through the outlet of the accumulator and into the sandpack. Dried sand manufactured by Paragon Building Products, Inc. (Norco, CA) was packed into the stainless steel tube. The sandpack was evacuated via a vacuum pump and saturated with 1 wt % NaCl. The pore volume (PV) and porosity were determined, as well as the brine permeability of the pack, as measured at three flow rates. The pump was run at constant injection rates of 1, 2, and 3 mL/min. That way, the piston could pump the brine into the model at the same rate. The pump pressure was observed constantly for a pressure drop to detect the movement of brine through the model. For each flow rate, the pressure was monitored at different points of the model body; when a stable pressure was observed, it was recorded as the constant pressure at that particular injection rate. The process was repeated for multiple injection flow rates, and the stable pressures for each of them were recorded. The above procedure was repeated for the initial brine injection (before gel injection), tPPG solution injection, and final brine injection (after gel injection). The pressure drop was recorded by a computer, and the resistance factors and residual resistance factors were calculated using the measured pressure. The resistance factor (Fr) is defined as the effective viscosity of gelant in porous media, relative to that of water, and the residual resistance factor (Frr) shows the reduction in brine

permeability caused by the polymer. Both of them are calculated using the following equations:

Frr =

(k /μ)brine before (k /μ)brine after

(4)

where k is the permeability, μ the viscosity, (k/μ)brine before the brine mobility before polymer placement, and (k/μ)brine after the brine mobility after polymer placement.

3. RESULTS AND DISCUSSION 3.1. Effect of the Structure on tPPG Mechanical Strength. Figure 2 shows the appearance of typical dry and swollen tPPGs. The initial size of the dry tPPG used for the following experiment is ∼0.42−0.50 mm, and the final equilibrium volume of swollen tPPG is more than 2 mm. Figures 2a and 2c illustrate the appearance of the tPPG before and after swelling. Figures 2b and 2d gave ESEM observations of a typical dry particle gel surface morphology both before and after swelling. The dry tPPG surface is smooth with platy structures. After being swollen, several spots and ridges were detected on the surface. These spots and ridges indicated that water molecules enter the gel network structure. Generally, a high cross-linker concentration results in the formation of a hydrogel with higher cross-linking density; hence, the elastic modulus increased. In order to study the tPPG mechanical strength, 0.33%C PEG-200 and 3.33%C PEG-200 are used for elastic and viscous moduli tests, as shown in Figure 3. Different from our expectation, a scan of both rheological parameters over strain confirmed that the 0.33%C PEG-200 sample showed higher elastic modulus (1850 Pa) with low strain (1850 Pa), controllable swelling (7−16 times of the original particle size), and controllable transformation time for tPPG transformation. In addition, the transformed tPPG polymer maintained stable viscosity and high plugging efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b03202. Preparation of the thermodissoluble polymer (tPPG); depictions of the thermostability test apparatus, filtration test apparatus, and sandpack flooding test apparatus (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: 573/341-4016. E-mail: [email protected]. ORCID

Jingyang Pu: 0000-0002-6883-340X Baojun Bai: 0000-0002-3551-4787 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Funding for this study is provided by DOE Project No. DEFE0024558 for CO2 storage. The authors also wish to thank I

DOI: 10.1021/acs.energyfuels.7b03202 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.7b03202 Energy Fuels XXXX, XXX, XXX−XXX