Performance Evaluation of an Octadecyltrimethyl Ammonium Chloride-Dimethyl Silicone Oil as a Waterproof Locking Agent in Low-Permeability Reservoirs.

Low porosity and permeability of oil and gas reservoirs make it difficult to develop these resources. To address these problems, we developed and evaluated a novel, environmentally friendly waterproof locking agent, which was prepared using dimethyl silicone oil and octadecyltrimethylammonium chloride and characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and particle size measurement. The waterproof locking performance of the agent was evaluated in a low-permeability reservoir using surface tension and contact angle measurements, and thermodynamic calculations were performed. The average particle size or median diameter (D 50) of a 1% mother liquor was 325 nm at 20 °C and 470.8 nm at 70 °C. The contact angle of clean water on the core surface increased from 10 to 110°. At 70 °C, the surface tension of water was reduced to 24 mN·m-1, indicating good waterproof locking performance. The interaction parameters were calculated in accordance with the Langmuir adsorption theory. The increase in temperature from 20 to 70 °C reduced Γmax from 4.59 × 10-6 to 1.36 × 10-6 mol·m-2 and ΔG θ from -40.93 to -56.54 kJ·mol-1. Thus, the adsorption behavior of the developed locking agent is believed to improve the productivity of oil wells.

Introduction

Developing low-permeability reservoirs with low porosity and permeability remains a significant challenge owing to the high reservoir stresses and poor water injection effects; both factors are sensitive to water and lead to a water-locking effect.[1,2] This in turn reduces the water injection capacity and quantity of reservoirs near wells and oil-water transition zones, thus reducing the productivity of oil wells.[3]

The addition of a waterproof locking agent to the drilling fluid effectively overcomes the water-locking effect and reduces the wettability of reservoir cores.[4−6] Waterproof locking agents are typically surfactants with a hydrophilic head and a hydrophobic tail.[7] The positive charge on cationic surfactants can affect the adsorption of the head at the oil–water interface of a low-permeability reservoir core. In contrast, a negative surface charge on the surfactant suppresses the hydration expansion of altered clay[8] and simultaneously induces the wettability reversal effect in the reservoir core. When the core contact angle is greater than 90°, the resulting tension tends to outwardly repel the external body in the pore throat.[9] This significantly weakens the self-imbibition effect of reservoir cores, reducing both the capillary force imparted on the solution and the self-imbibition of the pore throat, thereby alleviating the water-locking effect.[10−12] Fluorinated surfactants are effective waterproof locking agents owing to their ability to reduce the surface tension of water from 70 mN·m–1[13] to less than 20 mN·m–1,[14] thereby minimizing formation damage and extending the reservoir lifetime in low-permeability reservoirs.[15] Aminnaji et al.[16] studied the wettability of carbonate and sandstone cores treated with fluorinated chemicals, using contact angle measurement, scanning electron microscopy (SEM), and energy dispersive X-ray analysis, and observed altered liquid wetting by increasing surface roughness and decreasing surface free energy. However, fluorinated surfactants are expensive, exhibit poor biodegradability, and readily accumulate in the environment. It is therefore important to find inexpensive, environmentally friendly alternatives to fluorinated surfactants.[17] The use of a single surfactant in low-permeability reservoirs is restricted by high costs stemming from its high adsorption and injection characteristics. Meanwhile, complex binary systems have good application prospects owing to their superior emulsification ability, wettability, and low cost.[18−21]

In this study, we developed a new waterproof locking agent based on a dimethyl silicone oil–octadecyltrimethylammonium chloride binary compound. The solution surface tension, contact angle, and reverse wetting ability of this agent on the core surface were investigated using SEM, an integrated analysis method. The calculated thermodynamic properties of the binary compound system elucidated the adsorption behavior of the aqueous solution and explained the performance of the waterproof locking agent.

Materials

Preparation of Cationic Waterproof Locking Agent

All reagents were of analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd. Deionized water was prepared in the laboratory.

The mother liquor of the cationic waterproof locking agent, which is used in low-permeability reservoirs, was obtained by adding dimethyl silicone oil (4.5 g; molecular weight = 9400 Da, viscosity = 200 cSt, relative density = 0.963 g·cm–3) and octadecyltrimethyl ammonium chloride (0.5 g solid; relative density = 0.884 g·cm–3) (Figure ) to deionized water (495 mL) and stirred for 10 min in a high shear emulsifying dispersion machine (3000 r/min) to ensure complete dispersion.

Figure 1

Molecular structure of raw materials: (a) dimethyl silicone oil and (b) octadecyltrimethyl ammonium chloride.

The prepared waterproof locking agent was stirred at 70 °C for 12 h in a 250 mL three-necked flask using a heat-collecting, thermostatic heating magnetic stirrer, for aging.

Analytical Methods

FTIR Spectroscopy

The cationic waterproof locking agent (10 mg) and potassium bromide (100 mg) were pressed into a tablet, to which a drop of dimethyl silicone oil was added with a glass rod. The infrared (IR) spectra was recorded using Fourier transform IR (FTIR) spectroscopy, which was performed using a TP-FTIR spectrometer (Nicolli, USA) at wavenumbers between 400 and 4000 cm–1.

Particle Size Analysis

The particle size distribution was measured using a Winner2000ZD laser particle sizer (Jinan Micro-nano Particle Technology Co. Ltd) before and after aging. The analysis ranged from 0.1 to 40 μm at 25 °C.

XPS Characterization

The natural core was pulverized, and the powder (1 g) was added to a 1% mother liquor (50 ml), in a conical flask, and soaked for 8 h. Afterward, the powder was transferred to a Petri dish and dried in a vacuum drying oven at 50 °C for 12 h. The dried powder was characterized via XPS using an ESCALAB 250Xi XPS photoelectron spectrometer (Thermo Fisher Scientific) under the energy resolution limit of 0.43 eV (the energy analysis range is 0–5000 eV; the pass energy range is 1–400 eV).

Surface Tension Measurement

A portion of the waterproof locking agent was diluted to a concentration of 0.01–1% and heated at 70 °C for 15 min before being quickly transferred into the tensiometer sample cup for surface tension measurements. After cooling to room temperature, the surface tension was measured using the ring method. The surface tension was measured using a JK99B automatic tension meter (Shanghai Wang Xu Electric Co. Ltd); measuring range: 0–500 mN·m–1; sensitivity: 0.01 mN·m–1; accuracy: 0.01 mN·m–1; resolution: 0.01 mN·m–1.

Pretreatment of Natural Core and the Measurement of Contact Angle

The natural core provided by Sinopec Henan Oilfield (mudstone; average porosity = 3.085–14.12%, average permeability = (0.5–24.94) × 10–3 μm2, and average pore throat diameter = 2–30 μm) was broken into 1 cm3. Both sides of the core were smoothed with 1200-grit sandpaper before being soaked in different concentrations of the waterproofing agent for 8 h in a beaker covered with plastic wrap. The stones were gently removed using tweezers and left in a shaded area for 48 h before contact angle measurements were taken.

The contact angle was measured using an OCA15EC contact angle surface performance tester (Data Physics Corporation, Germany) (the contact angle measurement range: 0–180°, measurement accuracy: ± 0.1°, resolution: ± 0.01°).

Results and Discussion

FTIR Spectral Analysis of Waterproof Locking Agent

The FTIR spectrum of the treated core surface (Figure ) exhibits peaks in the range of 2800–3000 cm–1, which are attributed to the CH2 and CH3 stretching vibrations. The peaks at 1472 and 1257 cm–1 are due to the C–N stretching and Si–CH3 deformation vibrations, respectively, while those at 1000–1100 and 800 cm–1 are attributed to the Si–O–Si stretching and Si–O–Si bending vibrations, respectively.

Figure 2

Infrared spectra of the treated core surface.

Particle Size Analysis

The particle size of the waterproof locking agent needs to be small enough to enter the core pores and exert its effect. Therefore, it is necessary to investigate the particle size and distribution of the waterproof locking agent. The size distribution of the micellar particles of the waterproof locking agent is narrow and unimodal and reaches the nano level (Figure ). The D50 values at 20 and 70 °C were 325 and 470.8 nm, respectively, owing to an increase in the Brownian motion of the oil-in-water droplets in the waterproof locking agent; as a result of increased temperature, there is a consequent increase in the number and probability of collisions between droplets, leading to an increase in particle size. The average diameter of the low-permeability pore throat is larger than that of the waterproof locking agent at 0.15–4.18 μm,[19] allowing the waterproof locking agent to enter the pore throat of the low-permeability reservoir and reduce water lock damage.

Figure 3

Particle size distribution for 1% emulsion at (a) 20 °C and (b) 70 °C.

XPS Analysis of Elemental Carbon on the Core Surface

By examining the changes in surface characteristic elements, or groups before and after core treatment, a judgement can be made as to whether the waterproof locking agent is adsorbed onto the core surface or not. The XPS profiles of the core powder before and after treatment with the waterproof locking agent are shown in Figure . The binding energy of the C 1s orbital of the core powder is split into two signal peaks at 284.8 and 286.4 eV, attributable to carbon pollution and the C atom (C–N), respectively. The untreated core powder also contains C–N bonds, confirming that the core contains organic matter.[20] The C 1s content increased from 84.19% to 91.69% after the core powder was processed (Table ), indicating that the ionic surfactant successfully adsorbed onto the surface of the core powder.

Figure 4

XPS spectra of the core (a) before and (b) after treatment.

Table 1

XPS Data of Surface Elements

name	untreated atomic %	treated atomic %
C 1s	84.19	91.69
N 1s	12.79	6.86

Surface Tension Evaluation

Effect of Temperature on Surface Tension at the Surfactant–Polymer Interface

Waterproof locking agents can effectively reduce the surface tension of the wellbore working fluid, weaken the hydration expansion, reduce the diameter of the pore throat, and finally reduce the capillary force of the external fluid invading the pore throat of the reservoir. Figure shows that the waterproof locking agent has high surface activity at low concentrations. However, the surface tension decreases with increasing locking agent concentration, reaching a minimum of 24 mN·m–1 at 70 °C (the formation temperature at the core collected in Henan Oilfield). The surface tension of the waterproof locking agent decreases with increasing temperature because the mobility of the molecules increases with temperature, and the cationic surfactants reach the micellar concentration. The newly formed micelles tend to migrate to the surface of the solution, thereby reducing the surface tension.

Figure 5

Surface tension of the waterproof locking agent at 20 and 70 °C.

The linear fit (Figure ) for C/Γ – C was produced according to the fitting equations: Y = 43.41710.653 ln(C + 0.05) at 20 °C and Y = 30.274 – 3.872 ln(C + 0.005) at 70 °C. The maximum adsorption capacity, Γmax, and thermodynamic constant, K, were calculated by eq (21,22) and are listed in Table .

Figure 6

C/Γ – C linear fit of waterproof locking agent at 20 and 70 °C.

Table 2

Calculated Adsorption Properties of the Waterproof Locking Agent

temperature/°C	Γmax/(10–6 mol·m–2)	Amin/100 nm2	ΔGθ/kJ·mol–1	δ/nm
20	4.59	36.19	–40.93	74.54
70	1.36	122.14	–56.54	22.08

Contact Angle Evaluation

Through the investigation of the contact angle, we can qualitatively understand the change of the contact angle between the hydrophilic fluid and the shale, after the water-blocking agent is adsorbed on the core, and analyze its effect on the protection of oil and gas reservoirs. Contact changes on the core surface before and after treatment with the waterproof locking agent are shown Figure . The surface of the untreated cores appeared granular and contained numerous micropores. After the core was treated with the waterproof locking agent, dimethyl silicone oil was adsorbed onto the surface of the core particles. Notably, increasing the concentration of the waterproof locking agent resulted in the gradual adsorption of the oil onto the surface of the flake-like polymer core in a net-like fashion. This reduced the surface energy of the core and increased the water contact angle on the core surface. However, at a waterproof locking agent concentration of 0.7%, multimolecular adsorption occurred, thus reducing the contact angle.

Figure 7

Effect of the waterproof locking agent concentration on the water contact angle of the core: (a) untreated, (b) 0.1%, (c) 0.3%, (d) 0.5%, and (e) 0.7%.

The data in Table show that the contact angle of the hydrophilic core surface increases to 114° with increasing waterproof locking agent concentration, indicating an excellent wetting reversal ability.[23] The contact angle first increases and then decreases because the adsorption density of the cationic waterproof locking agent emulsion increases, which changes the spatial arrangement of molecules from a flat single-layer to a horizontal double-layer (Figures and 9).[24,25] The waterproof locking agent also enhanced the hydrophilicity and reduced the contact angle of the surface.

Table 3

Contact Angles before and after Treatment of the Core with the Waterproof Locking Agent

	static contact angle/°			dynamic contact angle/°
mass/%	left	right	average	left	right	advancing contact angle	receding contact angle	contact angle hysteresis
0	10.80	10.80	10.80
0.1	90.60	88.80	89.70	84.73	85.15	1.58	0.02	1.56
0.3	114.50	113.90	114.20	109.21	107.31	1.64	0.47	1.17
0.5	114.30	113.20	113.75	116.08	117.50	0.35	0.03	0.32
0.7	96.80	98.90	97.85	109.01	106.57	0.967	0.90	0.06

Figure 8

Schematic of a low-concentration waterproof locking agent adsorbed onto the core (B is a low interfacial energy polymer).

Figure 9

Schematic of a high-concentration waterproof locking agent adsorbed onto the core (B is a low interfacial energy polymer).

Evaluation of Performance after Aging

Evaluation of Surface Tension after Aging

On application in the field, the waterproof locking agent had a long working time in the underground reservoir. Therefore, the surface tension of the waterproof locking agent, at different concentrations and temperatures, was tested after stirring and aging for 12 h. It can be seen from Figure that the surface tension of the surfactant is around 30 mN·m–1 at room temperature and then drops to around 23 mN·m–1 at 70 and 80 °C. The surface tension of the aqueous surfactant solution is almost identical, indicating that the high temperature has little effect on the system, and waterproof locking retains good surface activity after aging.

Figure 10

Surface tension test of the surfactant at different temperatures after aging.

Evaluation of the Contact Angle after Aging

Figure and Table show the static contact angle after aging at 70 °C for 12 h. The contact angle of the hydrophilic core surface increased from 10° to 117° with increasing waterproof locking agent concentration. The effect is similar before aging, demonstrating the high wetting inversion performance of the waterproof locking agent and thus aging resistance.

Figure 11

Influence of the waterproof locking agent concentration on the core contact angle after aging: (a) 0.1%, (b) 0.3%, (c) 0.5%, and (d) 0.7%.

Table 4

Contact Angles before and after the Core was Treated with the Waterproof Locking Agent, after Aging

	static contact angle/°
mass/%	left	right	average
0	10.80	10.80	10.80
0.1	92.80	91.30	92.05
0.3	114.70	113.70	114.20
0.5	117.30	117.20	117.25
0.7	100.10	100.10	100.10

Conclusions

A novel waterproof locking agent was prepared without the use of fluorine compounds. The agent reached the nano level in water with a D50 value of 470.8 nm at 70 °C. The surface tension reached a minimum of 24 mN·m–1 at 70 °C. Treatment of the surface of the hydrophilic core with a surfactant increased the contact angle to more than 90°, demonstrating excellent wettability and reversal abilities.

The surface adsorption of the cationic waterproof locking agent emulsion follows a Langmuir-type adsorption isotherm. According to thermodynamic calculations, an increase in temperature reduces the amount of adsorption onto the surface of the emulsion, the area occupied by the emulsion surface molecules, and the thickness of the adsorption layer. The Gibbs free energy values were negative, showing that the adsorption of the waterproof locking agent on the surface is spontaneous under standard conditions.Amin is the smallest area occupied by surfactant molecules at the gas–liquid interface, nm2. C is the surfactant bulk concentration, gL–1. ΔGθ is the standard free energy of adsorption, kJmol–1. K is the adsorption equilibrium constant. M is the molecular weight, 16 239 gmol–1. N0 is Avogadro’s constant, 6.02 × 1023. R is the gas constant, 8.314 J mol–1·K–1. T is the temperature, K. Γ is the surface tension, mN·m–1. ρ is the liquid density, kg m–3. δ is the saturated adsorption layer thickness, nm. Γmax is the saturated adsorption capacity, mol m–2. Constants related to a, b. a, b empirical constants in Siskowski’s empirical formula.