Study on Acidizing Effect of Cationic β-Cyclodextrin Inclusion

After the N-β-CD–HEDP solution (3000 mg/L, 20 mL) had reacted with 3 mL of oil at 30 °C and 120 rpm for 12 h, it was pumped into the separating fu...
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Study on Acidizing Effect of Cationic β‑Cyclodextrin Inclusion Complex with Sandstone for Enhancing Oil Recovery Changjun Zou,*,† Yibie Qin,† Xueling Yan,† Lu Zhou,† and Pingya Luo‡ †

School of Chemistry and Chemical Engineering and ‡State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, People’s Republic of China S Supporting Information *

ABSTRACT: A novel supramolecular inclusion complex was prepared with cationic β-cyclodextrin and organic phosphoric acid. The inclusion action was investigated by UV−vis spectroscopy, and the cationic β-cyclodextrin inclusion complex was structurally evidenced by 1H and 31P NMR and FT-IR. With linear expansion tests, it was found that the novel inclusion complex possessed excellent clay stability. FT-IR, single-factor, and response surface method analyses showed that the inclusion complex acid system could not only retard acid−rock reaction but also intelligently regulate the acid−rock reaction rate to ensure a sufficient amount of acid to react with high-temperature oil-bearing sandstone reservoir in the progress of acid stimulation. On the basis of the core flood tests, an interaction mechanism was suggested that the inclusion complex acid system may open up for a new possibility to exhibit greater acidizing performance in high clay content, high-temperature, and low-permeability oil reservoirs, which contributes to improving core permeability and enhancing oil recovery. For the well-established ability of β-cyclodextrin (β-CD) to bind other molecules, β-CD derivatives with quaternary ammonium salt cations can act as a more efficient clay stabilizer.17,41 In addition to the ability of generating host/guest inclusion complexes, β-CD also can selectively incorporate with other molecules as the guest into its cavity.18−21 The host/guest theory implies that host β-CD molecules can not only recognize molecules but also self-assemble and release guest molecules, which opens a new direction for the research and development of special functional materials.22,23 However, little information has focused on including organic phosphoric acids with cationic β-CD derivatives. In the present work, to overcome many of the drawbacks and combine the advantages of the organic phosphoric acid system, a novel retarded acid composition, 2-O-(hydroxypropyl-N,Ndimethyloctadecylamine)-β-cyclodextrin−(1-hydroxyethylene)diphosphonicacid (N-β-CD−HEDP), was prepared and characterized by UV−vis, 1H NMR, 31P NMR, and FT-IR. In addition, we evaluated the antiswelling efficiency as a clay stabilizer with linear expansion tests, investigated the intelligent regulation and acid retardation during acidizing with FT-IR, univariate analysis, and response surface method (RSM),24,25 and discussed the ability of enhancing oil recovery with core flood experiments.26,27

1. INTRODUCTION Acidization has played a significant role in stimulating oil and gas recovery.1,2 Matrix acidizing generally uses acid solutions, which creates or enlarges the flow channels of formation.3 The right type of acids or acid mixtures must be used to remove damage and improve the permeability of reserviors.4 Therefore, careful acid choice is critical for a successful matrix acidization. Mud acid (mixtures of hydrochloric acid and hydrofluoric acid) has been widely used to acidify sandstone reservoirs.5−7 Hydrofluoric acid (HF) reacts with rock to increase the core permeability. Hydrochloric acid (HCl) always acts as a preflush to dissolve carbonate and is used to prevent precipitation of acid−rock reaction products.8 However, mud acid easily initiates acidizing in the near-wellbore area and results in short penetration depth for the rapid acid−rock reaction rate. A few retarded acid systems, such as self-generating acid,9 fluoroboric, organic-HF acid, and organic phosphoric acid systems, were developed to solve these problems. The selfgenerating acid combines the advantages of acid retardation and corrosion inhibition, but it is highly flammable and cannot effectively restrain precipitation.10 The organic HF, which also has a lower acid−rock reaction rate compared to mud acid, such as acetic acid and formic acid, can strongly chelate metal ions to prevent precipitation,11 but their dissolving power of quartz is quite limited. The HBF4 system combines the advantages of acid retardation and clay stabilization at room temperature, but this desired effect was nearly absent at elevated temperatures near 66 °C.10 Compared with organic HF, the organic phosphoric acid system12,13 not only can delay the acid−rock reaction rate and inhibit the formation of sediments but also has a limited solubility with clays and a higher dissolving power with quartz.14,15 However, these systems are incapable of self-regulating the acid−rock reaction rate or self-inhibiting clay swelling, which works against the maximum penetration depth of the oil reservoir.16 © 2014 American Chemical Society

2. MATERIALS AND METHODS 2.1. Materials. All of the reagents, including β-CD, HCl, HF, dimethylformamide (DMF), methanol, toluene, acetone, sodium hydroxide, ammonium bifluoride (NH4F·HF), epoxypropylstearyldimethylammonium chloride (ESDAC), 1-hyReceived: Revised: Accepted: Published: 12901

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HEDP was from 0:1 to 2:1, respectively, and the mixture was stirred at 120 rpm. After reacting for 24 h at constant temperature (60 °C), the solution was cooled to ambient temperature and refrigerated for 12 h at 10 °C. White powder was collected by vacuum filtration and washed several times with cold distilled water and acetone. The product of N-β-CD− HEDP was not kept in the sealed condition until it was dried at 40 °C for 48 h. The synthesis process of the N-β-CD−HEDP inclusion complex is shown in Figure 1.

droxyethylidene-1,1-diphosphonic acid (HEDP), corrosion inhibitor, clay stabilizer, and nonemulsifier, were of analytical reagent grade and purchased from Chengdu Kelong Chemical Reagent Factory, China. Bentonite with a high percentage of montmorillonite (90 wt %) (provided by National Oil and Gas Exploration and Development Corp., China) was selected as a type of clay to investigate the antiswelling efficiency of different clay stabilizers. The main features of the crude oil supplied by China’s Liaohe Co. are of relatively low density (0.8686 g/mL at 20 °C) and low viscosity (18.67 mPa·s at 50 °C). The group composition of this oil was 62 wt % of saturates, 26.9 wt % of aromatics, 5.7 wt % of resins, and 2.2 wt % of n-C6 asphaltenes. Before the core powder was used in the dissolution rate determination (supplied by Liaohe Oilfield Kangda Industrial Co.), it was extracted with a methanol−toluene mixture and cleaned with reservoir brine to adjust the pH value to be neutral and then dried at 60 °C for 48 h. Besides, the composition of core powder determined by X-ray diffraction analysis (X’pertPRO, PANalytical B.V., The Netherlands) was 16.1% clay, 23.2% feldspar, 41.4% quartz, 10.1% dolomite and 9.5% calcite. The XRD pattern of core powder is shown in Figure S1 in the Supporting Information. Core plugs used in the core flood experiments were cut from Liaohe (cores 1−3) sandstone blocks, and the size of the cores was 3.8 cm × 15 cm. The mineralogy of the Liaohe sandstone cores derived from XRD analysis is given in Table 1. The XRD patterns of these core plugs are shown in Figures S2−S4 in the Supporting Information. Table 1. Mineralogy of the Liaohe Sandstone Cores Derived from XRD Analysis content, wt % mineral

core 1

core 2

core 3

quartz K-feldspar plagioclase calcite dolomite smectite chlorite illite kaolinite mixed-layered iliite/smectite total clay

61.5 6.0 7.8 5.7 6.0 6.2 0.4 0.5 5.0 0.9 12.6

63.0 5.6 6.1 6.1 6.0 6.6 0.0 0.7 5.1 0.8 13.8

60.0 6.1 7.9 6.7 6.2 6.5 0 0.7 5.4 0.5 13.8

Figure 1. Synthesis process of N-β-CD−HEDP inclusion complex.

2.3. Characterization. The UV absorption spectra were recorded with a UV-2102C spectrophotometer (Shanghai Spectrotech Instruments Co., Ltd., China) in a range of 190−400 nm. The FT-IR spectra were recorded on a Nicolet Nexus 470 spectrometer. The chemical was taken in KBr pellets. American NMR measurements were performed on a Bruker AC-E 200 spectrometer (Bruker BioSpin, Switzerland) with 1H frequency of 200 MHz and 31P frequency of 120 MHz at 30 °C. 2.4. Measurements of Antiswelling Rate. First, 14 g of dried bentonite, which was mainly composed of sodium montmorillonite, was pressed into a thin slice (10 MPa pressure for 3 min) and fixed in the chamber of the NP-3 HTHP swelling apparatus (Haian Oil Scientific Instrument Co., Ltd., China). When the chamber is heated to the required temperature, 100 mL of distilled water or N-β-CD−HEDP solution (3000 mg/L), which has been preheated to a specified temperature, enters the chamber. The swelling height was tested by computer automatic recording at 10 min intervals, and the antiswelling rate was calculated using eq 1:

2.2. Preparation of N-β-CD−HEDP Inclusion Complex. Initially, 11.352 g of β-CD was dissolved with 150 g of sodium hydroxide solution (mass fraction, 2.0%) in the shaker (Shanghai Fuma Laboratory Instrument Co. Ltd., China), to which 5.851 g of ESDAC was added. After reacting for 6 h at 50 °C and 150 rpm, the solution was cooled to ambient temperature, the pH was adjusted to neutral by the addition of moderate hydrochloric acid, and the mixture was distilled to nearly dry state. Then moderate DMF was added to dissolve and filter out inorganic salt. After repeatedly washes with acetone and drying at 60 °C for 48 h in the vacuum drying oven, the product of 2-O-(hydroxypropyl-N,N-dimethyloctadecylamine)-β-cyclodextrin (N-β-CD) was kept in the sealed condition. Afterward, quantitative HEDP was dissolved in a certain amount of distilled water, to which the prepared N-βCD was added in the shaker. The molar ratio of N-β-CD and

anti‐swelling rate = (hw − hs)/(hw − h0) × 100% 12902

(1)

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Here hw and hs are the height of bentonite in distilled water and sample solutions under specified time and temperature conditions, respectively. h0 is the initial height of dried bentonite. Under the same operating conditions, the antiswelling rates of N-β-CD−HEDP, HEDP, β-CD−HEDP, and a physical mixture of N-β-CD with HEDP were tested, respectively. 2.5. Tests of the Intelligent Regulation. 2.5.1. FT-IR Analysis. After the N-β-CD−HEDP solution (3000 mg/L, 20 mL) had reacted with 3 mL of oil at 30 °C and 120 rpm for 12 h, it was pumped into the separating funnel. An obvious oil− water stratification phenomenon presented, and a layer of foamy substance appeared at the interface of the oil and water phases; the foam was collected and characterized by FT-IR (Nicolet Nexus 470 spectrometer, USA). 2.5.2. Single-Factor Experiments. To confirm the feasibility that oil phase and high temperature cause the HEDP guest released from the cavity of β-cyclodextrin, the dissolution rates of N-β-CD−HEDP acid system (which was composed of N-βCD−HEDP and NH4F·HF (molar ratio = 1:1), 12 wt % of HCl, and distilled water) with arid sandstone under three different conditions (30 °C and sufficient oil, 180 °C and oilfree, 30 °C and oil-free) were measured. Here, sufficient oil was 40:1 of weight ratio (oil/N-β-CD-HEDP), and the dissolution rate was calculated by weightless method.28 Under the same conditions, the mud acid system (12% HCl−6% HF) and HEDP system (which was also composed of HEDP and NH4F· HF (molar ratio = 1:1), 12 wt % of HCl, and distilled water) replaced the N-β-CD−HEDP system to carry on the same experiment, respectively. 2.5.3. Response Surface Method. A response surface method was undertaken to assess the interactions of the parameters (oil mass and temperature) and their impacts on the responses of the dissolution rate and regulation factor at a certain time through Design Expert software (for experimental details, see the Supporting Information). Here, the regulation factor is the relative change percentage of the dissolution rate and is calculated according to eq 2 regulation factor = (RHEDP − RN‐β ‐CD−HEDP)/RHEDP

Figure 2. Working wireframe of the YSF-1 unit.

pump). When the pressure drop became stable, this value of delta pressure was used to calculate the initial permeability (K0) of the core using Darcy’s law.30 Afterward, preflush, main flush, and postflush were performed in sequence. The compositions of acid systems used in core flood experiments on Liaohe sandstone cores are listed in Table 2. When the pressure drop became stable again, this value of delta pressure was used to calculate the final permeability of the treated core (Ka). Permeability increment (Φ) is calculated using eq 3: Φ = (K a − K 0)/K 0

(3)

The core effluent samples were collected and analyzed to determine the concentrations of Al, Si, Ca, and Mg by inductively coupled plasma (ICP) analysis using an Optima 7000 DV ICP-OES system and WinLab 32 software.

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. UV Analysis. Figure 3a shows the UV−vis spectra of HEDP in the N-β-CD aqueous solution with different mol ratios. The N-β-CD does not demonstrate characteristic bands in the UV range, and the addition of N-βCD causes the intensities of the band near 195 nm to increase, indicating that the possibility of the molecular interaction of HEDP with N-β-CD exists as a result of a perturbation of the chromophore electrons of HEDP.31−33 This might suggest that HEDP is capable of forming an inclusion complex with N-βCD and a chain of HEDP molecules is probably deeply inserted into the cavity of N-β-CD.34,35 Additionally, with the addition of N-β-CD, the solution absorbance at the maximum absorption wavelength (196 nm) increased more slowly and tended to be stable when the mole ratio (N-β-CD: HEDP) increased to 1:1 (see Figure 3b), revealing that the appropriate inclusion molar ratio was 1:1. 3.1.2. NMR Analysis. Next, the inclusion complex with 1:1 mol ratio was characterized by 1H NMR and 31P NMR,36 respectively. By comparing the 1H NMR spectra of N-β-CD− HEDP and HEDP (Figure 4a), the characteristic shift value assigned to the −CH3 of HEDP changed from 1.53 to 1.39 ppm. It is probable that the interaction between N-β-CD and HEDP induced electrons of HEDP to be removed. By comparison of the 1H NMR spectra of N-β-CD−HEDP and N-β-CD (Figure 4a), the chemical shift values at 3.96 and 3.69 ppm assigned to the β-CD internal cavity (e.g., Hc and He) shift to a higher field after the inclusion of HEDP, but the other characteristic peaks (e.g., Ha, Hb, Hd, Hf−Ho) assigned to the βCD external cavity are nearly unchanged, indicating that HEDP has been included into the cavity of the cyclodextrin molecule.37−39

(2)

where RHEDP is the dissolution rate of the core with the HEDP acid system and RN‑β‑CD−HEDP is the dissolution rate of he core with the N-β-CD−HEDP acid system. 2.6. Core Flood Tests. The permeability of rock samples before and after acidizing was measured using a YSF-1 core laboratory unit (Jiangsu Haian Scientific Research Devices Co. Ltd. of China). The pore volume of the core was determined by dividing the weight difference of the saturated core and the dry core by the density of the brine.29 The core plugs have similar composition and were all characterized by the high percentage of quartz with an average value of 61.5% and a high content of clay mineral with an average value of 13.5%. The working wireframe of the YSF-1 unit is shown in Figure 2. After the core was loaded into the core holder, the back pressure was adjusted to 7 MPa to keep CO2 in solution, and the overburden pressure was fixed at 15 MPa. All core flood runs were performed at a temperature of 120 °C. The injection rate was 1 mL/min in all tests to ensure laminar flow and applicability of Darcy’s law, which is used to calculate plug permeability. Pressure transducers were connected to a computer to monitor and record the pressure drop across the core during the experiments. Initially, 5 wt % NH4Cl was injected by a positive displacement pump (ISCO 260D syringe 12903

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Table 2. Compositions of Acid Systems Used in Core Flood Experiments on Liaohe (Sets 1−3) Sandstone Cores acid system mud acid system

HEDP acid system

N-β-CD−HEDP acid system

main flush

preflush 5% HCl corrosion inhibitor (5 mL/L preflush) clay stabilizer (5 mL/L preflush) monemulsifier (3 mL/L preflush) 5% HCl corrosion inhibitor (2 mL/L preflush) clay stabilizer (5 mL/L preflush) nonemulsifier (3 mL/L preflush) 5% NH4C l → 5%HCl corrosion inhibitor (2 mL/L preflush) clay stabilizer (5 mL/L preflush) nonemulsifier (3 mL/L preflush)

postflush

Core 1 4.5% HCl + 0.5% HF corrosion inhibitor (5 mL/L main flush)

5% NH4Cl corrosion inhibitor (5 mL/L overflush) clay stabilizer (7.5 mL/L overflush) nonemulsifier (50 mL/L preflush)

clay stabilizer (5 mL/L main flush) nonemulsifier (3 mL/L preflush) Core 2 4.5% HCl + 0.225 mol % HEDP + 0.225 mol % NH4HF2 corrosion inhibitor (2 mL/L main flush) clay stabilizer (3 mL/L main flush) nonemulsifier (3 mL/L preflush) Core 3 4.5% HC l+ 0.225 mol % N-β-CD−HEDP + 0.225 mol % NH4HF2 corrosion inhibitor (2 mL/L main flush) nonemulsifier (3 mL/L preflush)

5% NH4Cl corrosion inhibitor (2 mL/L overflush) clay stabilizer (3 mL/L overflush) nonemulsifier (3 mL/L preflush) 5% NH4Cl corrosion inhibitor (2 mL/L overflush) nonemulsifier (3 mL/L preflush)

Figure 3. (a) Effect of N-β-CD on UV absorption spectra of HEDP, A−G (N-β-CD/HEDP): 0:1, 0.3:1, 0.5:1, 0.8:1, 1:1, 1.2:1, 2:1. (b) Absorbency of N-β-CD−HEDP solution at different molar ratios at 196 nm. 31

P NMR spectra of HEDP and N-β-CD−HEDP are all single peaks, and their characteristic shift value changes from 19.59 to 19.11 ppm (see Figure 4b). These results are in good agreement with the analysis of UV−vis spectra and also could be proof of the successful synthesis of N-β-CD−HEDP. 3.2. Study of Antiswelling Efficiency. It can be seen from Figure 5a that the antiswelling efficiency of different chemicals was reinforced with the increase of chemical concentration, and N-β-CD−HEDP exhibited excellent antiswelling capacity (>90% of antiswelling ratio at a relatively low concentration). It can be seen from Figure 5b,c that all of these chemicals containing quaternary ammonium cations showed a much better performance of inhibiting clay swelling than HEDP and β-CD. N-β-CD showed a better performance of inhibiting clay swelling than ESDAC. The N-β-CD−HEDP and physical mixture with HEDP and N-β-CD showed only a little better

performance than N-β-CD. The antiswelling efficiency of N-βCD−HEDP was a little higher than the antiswelling rate of Nβ-CD and the physical mixture with N-β-CD. As shown in Figure 5c, N-β-CD−HEDP possesses an excellent temperature resistance, and the antiswelling rate of N-β-CD−HEDP remains nearly stable with temperature increasing from 20 to180 °C. These phenomena could be illustrated by the follow explanations. First, the increase of concentration could result in stronger electrostatic attraction and hydrogen bonding adsorption. Second, for HEDP and β-CD these are limited to weak hydrogen bonds, whereas the ESDAC, having polar and cationic entities, may be bound by strong electrostatic forces and may be adsorbed onto negatively charged clay surfaces.40 The inclusion complex N-β-CD could adsorb onto the negatively charged clay surfaces by virtue of bearing a strong 12904

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Figure 4. (a) 1H NMR spectra of N-β-CD, HEDP, and N-β-CD− HEDP. (b) 31P NMR spectra of HEDP and N-β-CD−HEDP. Figure 6. FT-IR spectra of (a) HEDP, (b) N-β-CD, (c) the equimolar physical mixture of HEDP and N-β-CD, (d) the N-β-CD−HEDP inclusion complex, and (e) the foam at the interface of the oil and water phases.

electrostatic attraction and weak hydrogen-bonding adsorption. Besides, the N-β-CD, which possesses a large number of hydrophobic cavities of β-CD, could form a double protective layer against the ingress of water to prevent clay swelling and migration.41 Moreover, resulting from the weak hydrogen bonds of HEDP, the N-β-CD−HEDP and physical mixture with HEDP and N-β-CD show only a little better performance than N-β-CD. Futhermore, the hydrogen bonding between the guest HEDP and clay surface may further enhance the stable coverage capacity of hydrophobic cavities on the clay surface. 3.3. FT-IR Analysis. The FT-IR spectra of (a) HEDP, (b) N-β-CD, (c) the equimolar physical mixture of HEDP and Nβ-CD, (d) N-β-CD−HEDP inclusion complex, and (e) the foam at the interface of the oil and water phases are presented in Figure 6. The FT-IR spectrum of HEDP (Figure 6a) showed prominent absorption bands at 2958 and 2870 cm−1 (−CH3), 1451 cm−1 (−COH), 1239 cm−1 (for −PO), and 943 cm−1 (−POH). Figure 5b shows prominent absorption bands of N-β-CD at 1458 cm−1 (−+NCH3), 1028 cm−1 (COC stretching vibrations), and 2940 cm−1 (for −CH stretching vibrations), and 1153 cm−1 (for CO stretching vibrations).42 The FT-IR spectrum of the physical mixture (Figure 5c) showed approximate superimposition of the individual patterns of both N-β-CD and HEDP. However, the FT-IR spectrum of the N-β-CD−HEDP inclusion complex showed different features (Figure 5d). The absorption bands located at 2870 cm−1 (−CH3), 1239 cm−1 (for −PO), and 943 cm−1 (−POH) of HEDP were shifted toward lower

frequencies at 2847, 1204, and 925 cm−1, respectively. These changes may be related to the formation of the N-β-CD− HEDP complex. In Figure 5e, the characteristic peaks of the HEDP portion such as 1413 cm−1 (−COH), 1204 cm−1 (−PO), and 925 cm−1 (−POH) cannot be detected, and the characteristic absorption bands of the oil molecule such as 2854 cm−1 (−CH2−) and 1559 cm−1 (CC) can be clearly observed. These changes may illustrate that oil molecules can interact with N-β-CD−HEDP. The interaction will cause the oil molecules to enter the cyclodextrin cavity and make the inclusion complex to release HEDP, leading to the decrease of the water solubility of N-β-CD and the presence of characteristic absorption bands associated with oil molecules. 3.4. Study of the Intelligent Regulation Function by Univariate Analysis. The dissolution rate of various acid systems with sandstone under three different conditions is shown in Figure 7. As shown in Figure 7a, there is a great discrepancy between the N-β-CD−HEDP system and the HEDP acid system within 6 h. However, after treatment with excess oil or high temperature (180 °C), the difference of the dissolution rate between the N-β-CD−HEDP system and the HEDP acid system was so little as to be unnoticeable, indicating that oil molecules could enter the cavity of β-cyclodextrin and

Figure 5. Effects of (a) chemical concentration, (b) time, and (c) temperature on the antiswelling rate. 12905

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Figure 7. Dissolution rate of various acid systems with sandstone: (a) before and after treatment with enough oil; (b) before and after treatment with high temperature.

Figure 8. 3D response surfaces for (a) dissolution rate of the N-β-CD−HEDP system, (b) regulation factor, and (c) desirability function at the specified level (regulation factor > 5%).

dissolution rate of the N-β-CD−HEDP system and the HEDP acid system as a function of the coded factor levels, respectively.

release HEDP molecules. As shown in Figure 7b, after treatment with high temperature (180 °C), the dissolution rate of the N-β-CD−HEDP acid system was similar to that of HEDP. It illustrated that the high testing temperature may cause inclusion complex liberation and release HEDP molecules. Hence, the existence of a recognition function was further confirmed. The existence of a recognition function is good for reducing the acid−rock interaction rate in the nearwellbore area and inhibiting the rapid consumption of acid to ensure a sufficient amount of acid reacts with sandstone in the high-temperature oil reservoir, which contributes to increasing the effective penetration depth of the acidification liquid. 3.5. Study of the Intelligent Regulation Function by RSM. Through the Design Expert software, a model correlating the response with interactive variables was evolved. The correctness verification of the generated models is provided as Supporting Information. The following second-order polynomial model equations (eqs 4 and 5) describe the

dissolution rate(N‐β ‐CD−HEDP) = 21.02 + 9.75A + 1.52B − 1.03A × B + 3.07A2

dissolution rate(HEDP) = 22.38 + 8.80A + 2.70A2

(4) (5)

Here A and B refer to the temperature and the weight ratio of oil/N-β-CD−HEDP, respectively. The magnitude of the coefficient in the above polynomial model equations denotes the intensity of the variable on the response.43 Thus, it could be seen that the dissolution rate of the N-β-CD−HEDP system was controlled by the temperature and oil mass, whereas the dissolution rate of the HEDP system was influenced only by the temperature. Moreover, the model of the N-β-CD−HEDP system consisted of two-factor interaction effects, revealing that there was interaction between temperature and oil mass on the response of the dissolution rate. 12906

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To gain a clearer perspective on the interaction between the variables and how they influence the dissolution rate of the Nβ-CD−HEDP system, 3D diagram of the second-order interactions was depicted for the generated model. Figure 8a shows that the increase of reaction temperature leads to a higher acid−rock reaction rate at any level of weight ratio of oil/N-β-CD−HEDP. However, the dissolution rate of the N-βCD−HEDP system increased greatly with the increase of oil mass at the temperature range of 30−120 °C; by contrast, the effect of increasing the oil mass was so little as to be unnoticeable at the temperature range of 150−180 °C. The above phenomena probably could be illustrated by the following explanations. On the one hand, the HEDP guest can be slowly driven out of the cavity of β-CD under the confluence of increasing reaction temperature and oil mass and then react with NH4F·HF. HF formed from the above process will take part in the acid−rock reaction. That means the increase of temperature and oil mass is beneficial to raise the concentration of HEDP in solution, thus increasing the acid− rock reaction rate of the N-β-CD−HEDP system. On the other hand, on the basis of the Arrhenius empiric equation,44 the rising absolute temperature can lead to an increase of return rate constant k, thereby increasing the dissolution rate of the HEDP and N-β-CD−HEDP systems accordingly. The use of the regulation factor, which is the relative change percentage of dissolution rate, can effectively excise the interference that results from the increase of reaction rate K and helps to explain the regulation intensity of temperature and oil mass on the dissolution rate. Apparently, the larger the regulation factor is, the stronger the regulation intensity is. Figure 8b depicts the interactive effect of temperature and weight ratio of oil/N-βCD−HEDP on the regulation factor. It reveals that the regulation factor decreases from 26.4 to 0% with the increase of temperature and oil mass, and the regulation function of N-βCD−HEDP is obvious in a certain scope of temperature variables. Therefore, a desirability function was created to determine the effective scope of temperature and oil mass by the Design Expert software. The variation limits for each factor varied throughout its original range, and the target for regulation factor was set in the range of 5−30%. The 3D response surface of the desirability function (see Figure 8c) reveals that the desirable scope is attained at the closed red region in the response surface. In other words, the red contour in the coordinate plane is the boundary point set that meets the target for regulation factor. This experimental result was also in agreement with the above theoretical analysis. 3.6. Core Flood. Figure 9 shows the changes in pressure drop as a function of the cumulative volume injected. On the basis of the pressure drop (Figure 9) before and after the core flood experiment, the permeability, which was calculated by Darcy’ law, is listed in Table 3. The concentration change of ions in effluent samples collected during core flood experiments is shown in Figures 10 and 11. As shown in Figure 9, in core flood experiment 1, the pressure drop across the core decreased rapidly within 1 pore volume of mud acid being injected, and the concentrations of Ca and Mg were detected to increase dramatically in the core effluent at the same time that the pressure started to decrease (see Figure 10). However, the delta pressure (pressure drop across the core) of core 1 gradually increased during the rest time of the main acid injection and the overflush stage. Besides, the concentrations of Ca and Mg started to decrease at the same point as the pressure drop started to increase. These phenomena indicated that the rock

Figure 9. Pressure drop across the core as a function of the cumulative volume injected.

Table 3. Core Flood Experimental Datum and Results core

pore volume (cm3)

1 2 3

7.43 7.4 7.41

acid fluid mud acid system HEDP acid system N-β-CD−HEDP acid system

K0 (md)

Ka (md)

Φ (%)

7 6.91 6.8

4.82 12.4 32

−31.1429 79.45007 370.5882

Figure 10. Change of Ca and Mg concentrations in effluent samples collected during core flood experiments.

Figure 11. Change of Si and Al concentrations in effluent samples collected during core flood experiments.

reaction rate of mud acid with clay and feldspar was the fastest among these acid systems and that severe precipitation processes of Ca and Mg may occur during acidizing. 12907

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Figure 12. Acidizing mechanism schematic drawing of N-β-CD−HEDP acid system.

the near-wellbore area but also inhibit rapid consumption of the acid and ensure a sufficient amount of acid reacts with sandstone in the high-temperature oil reservoir. In addition, in a certain scope (temperature < 150 °C and oil mass < 40:1), the N-β-CD−HEDP system would exhibit greater retarding performance than the HEDP system. In a word, the N-βCD−HEDP system would exhibit a greater acidizing effect compared with organic phosphoric acid, which contributes to optimizing acid stimulation in the high-temperature oil reservoir.

It also can be seen from Figure 11 that the Si concentration of core effluent samples 1 and 2 remained very low during the test, whereas the aluminum content increased significantly during the injection of mud acid system or HEDP acid system. The average ratios of Si/Al were 0.4:1 and 0.7:1 for samples 1 and 2, respectively. The actual values were both much lower than the theoretical values,45 which indicates that a severe silicate precipitation process was occurring in cores 1 and 2. By contrast, since the injection of the N-β-CD−HEDP acid system, Si concentrations of core effluent sample 3 increased rapidly and the average Si/Al ratio of sample 3 was 3.9:1. Besides, the concentrations of Ca and Mg for sample 3 were all lower than the other samples throughout the entire main-flush stage, and the pressure drop across core 3 decreased more slowly than in core flood experiment 2 at the beginning of the injection of the main acid systems. The delta pressure throughout the entire treatment of core 3 decreased until the pressure drop became stable. These phenomena indicated that the acid−rock reaction rate was mud acid > HEDP acid system > N-β-CD−HEDP acid system, and the quartz dissolution was the primary N-β-CD−HEDP acid system consumption process. In the end, from pressure drop and concentration measurements, there was no indication that a precipitation of fine particles (which are insoluble in the solution or are not completely swept and some of them remain in the pores) occurred in the process of acidizing treatment with the N-βCD−HEDP acid system. According to the core flood tests, the N-β-CD−HEDP acid system is the optimal fluid. On the one hand, it has a good permeability improvement ratio in the condition of no clay stabilizer; on the other hand, it presents less damaging precipitates. 3.7. Acidizing Mechanism Schematic Drawing. Through the analyses of the above results, an interaction mechanism was suggested (see Figure 12). In the near-wellbore area, the N-β-CD−HEDP system can inhibit the release of organic phosphonic acid molecules and interrupt the formation of HF, thus retarding the acid−rock reaction rate. With the depth of oil or gas reservoir/well increasing, the real reservoir oil mass will increase, thus facilitating the release of HEDP from the cyclodextrin cavity to react with ammonium bifluoride. The HF formed from the above process will primarily be involved in the process of acid−quartz dissolution, and the host N-β-CD will adsorb onto the negatively charged clay surfaces and form a double protective layer against the ingress of water to prevent clay swelling and migration. Meanwhile, by using intelligent regulation, the N-β-CD−HEDP system could not only reduce the acid−rock interaction rate in

4. CONCLUSIONS N-β-CD−HEDP was prepared successfully and was evidenced by UV−vis, 1H NMR, and 31P NMR. With linear expansion tests, it was found that N-β-CD−HEDP possesses excellent clay stability. With FT-IR, single-factor, and RSM analyses, it was found that the N-β-CD−HEDP acid system could not only retard the acid−rock reaction rate but also intelligently regulate the acid−rock reaction rate. The regulation intensity decreased with the increase of temperature and oil mass, and the regulation range was limited in a certain scope of temperature and oil mass. The core flood test results indicated that the N-βCD−HEDP acid system not only owns a superior performance to improve the core permeability of low-permeability reservoirs but also presents less damaging precipitates. Finally, an interaction mechanism suggested the N-β-CD−HEDP system may provide a new possibility to exhibit greater penetration depth in high clay content, high-temperature, and lowpermeability oil reservoirs, which contributes to improving core permeability and enhancing oil recovery. In addition, further investigations are intended, such as the simulation of wormhole formation in sandstone reservoirs, with the N-βCD−HEDP acid system as well as the application of the N-βCD−HEDP acid system in the actual oilfield.

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ASSOCIATED CONTENT

S Supporting Information *

The XRD patterns of core powder (Figure S1) and core plugs (Figures S2−S4), details of response surface experiments (Tables S1 and S2), and correctness verification of the generated models (Table S3 and S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(C.Z.) E-mail: [email protected]. Mail: School of Chemistry and Chemical Engineering, Southwest Petroleum University, No. 8 Xindu Road, Chengdu 610500, People’s 12908

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Republic of China. Tel: +86 02883037327. Fax: +86 02883037305. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China, China National Petroleum Corporation Petrochemical Unite Funded Project (U1262111).

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ABBREVIATIONS DMF = dimethylformamide ESDAC = epoxypropylstearyldimethylammonium chloride HCl = hydrochloric acid HEDP = 1-hydroxyethylidene-1,1-diphosphonic acid HF = hydrofluoric acid k = absolute permeability NH4F·HF = ammonium bifluoride NH4Cl = ammonium chloride N-β-CD = 2-O-(hydroxypropyl-N,N-dimethyloctadecylamine)-β-cyclodextrin N-β-CD−HEDP = 2-O-(hydroxypropyl-N,N-dimethyloctadecylamine)-β-cyclodextrin−(1-hydroxyethylene)-diphosphonic acid RSM = response surface method β-CD = β-cyclodextrin Φ = permeability increment

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