Preparation and Evaluation of Ammonium Adipate Solutions as Inhibitors of Shale Rock Swelling

Abstract

This study aimed to evaluate the inhibitory effect of a series of ammonium adipate solutions (AASs) by using the linear expansion test, thermogravimetric analysis (TGA), and particle size distribution analysis, and to examine the underlying inhibitory mechanism. A series of AASs was prepared from adipic acid and amines as small-molecule inhibitors of oil shale rock swelling. They were then evaluated by the bentonite linear expansion test. The best one, namely, AAS-8 (synthesized with adipic acid and tetraethylenepentamine in a ratio of acid group to amine group of 1:2), was evaluated in a water-based drilling fluid. The linear expansion test showed that the linear expansion rate of AAS-8 was the lowest (59.61%) when the concentration was 0.1%. The evaluation of the drilling fluid revealed that AAS-8 had a strong inhibitory effect on the swelling of hydrated bentonite particles in the water-based drilling fluid and was compatible with carboxymethyl cellulose (CMC) and modified starch. The inhibition mechanism of AAS-8 was investigated using TGA and particle size distribution analysis, which demonstrated that AAS-8 might enter the clay layer and bind the clay sheets together by electrostatic adsorption and hydrogen bonding.

1. Introduction

Shale rock oil/gas exploration technology has received considerable attention worldwide in recent years. For example, China successfully conducted the exploration of oil and gas deposits in east Chongqing and established a gas field with an annual production of 100 billion m³ of natural gas in 2018. During the drilling of difficult wells, a number of complicated downhole conditions, such as a low drilling rate, blockage during start-up, and formation damage, can lead to serious downhole complications including hole instability and stuck-up drilling [1,2,3]. Traditionally, these problems can be solved in two ways: using oil-based drilling fluid or synthetic-based drilling fluid. The performance of traditional water-based drilling fluid is inadequate, so drilling in oil shale rock formation has always been a great challenge. Oil-based drilling fluids have the disadvantages of environmental pollution, which increases the need for strict environmental regulations, and high cost, which limits their use to a great extent [4,5,6]. Amines have attracted the attention of many researchers in the field as a result of their good shale inhibition capacity, and they are widely used across the world due to their stable rheology and high lubricity [7,8,9]. Polyamines have been used for a long time in a variety of water-based oilfield working fluids due to their high effectiveness and good compatibility with traditional drilling fluid. However, since they do not meet the current environmental protection requirements, researchers have begun to seek more eco-friendly substitutes [10]. This study aimed to evaluate the inhibitory effect of a series of ammonium adipate solutions (AASs) prepared from adipic acid and amines by using the linear expansion method, thermogravimetric analysis (TGA), and particle size distribution analysis, and to examine the underlying inhibitory mechanism.

2. Materials and Methods

2.1. Materials and Reagents

Adipic acid, diethylenetriamine, triethylenetetramine and tetraethylenepentamine were purchased from Tianjin Kermel Chemical Reagent Development Center (Tianjin, China). Bentonite was obtained from Shaanxi Yanchang Oilfield chemical company (Xi’an, China).

2.2. Synthesis of AASs

Adipic acid and organic amines (diethylenetriamine, triethylenetetramine, and tetraethylenepentamine) at various mole ratios of functional groups were dissolved in water at temperatures at 60 °C and at atmospheric pressure. Inhibitor solutions were prepared using speed magnetic stirring for 4 h. The solution was cooled to room temperature to prepare the oil shale rock inhibitor solutions as shown in Figure 1; the end product was a series of ammonium adipate solutions, referred to in this paper as AASs.

2.3. Inhibitive Ability Evaluation

Oil shale rock contains a large amount of clay minerals, which are prone to hydration expansion and dispersion after contact with water. The hydration swelling of bentonite was tested using an NP-01 shale expansion instrument (Qingdao, Haitongda Special Instrument Co., Ltd., Qingdao, China), according to Chinese Petroleum and Natural Gas Industry Standards SY/T5971-1994 and SY/T6335-1997.

2.4. Performance in Drilling Fluid

The bentonite, at a dosage of 4% (m/m), was dispersed in the corresponding AAS or tap water. After stirring for 30 min, the mixture was aged for 16 h at room temperature. Then, the rheological properties, as well as the filtration of the fluid were evaluated using a viscometer, including apparent viscosity (AV), plastic viscosity (PV), yield point (YP), filtrate loss (FL) [11,12]. The AV, PV and YP were calculated from 300 and 600 rpm readings using the formulas in the Chinese Petroleum and Natural Gas Industry Standards GB/T 16783.1-2014 [13].

2.5. Laser Particle Size Distribution Analysis

Bentonite was dispersed in the corresponding AAS or tap water at the same concentration as described above and stirred for 24 h. The inhibitor was added to the dispersion and stirred for 20 min before the test. Then the particle size distribution of bentonite particles was determined using a LS 13 320 laser particle size analyzer (Beckman Coulter, Inc., Brea, CA, USA) [14,15].

2.6. TGA

The bentonite was dispersed in the corresponding ammonium adipate solution or tap water for 24 h. After centrifugation, the samples were dried at 105 ℃ before being subjected to TGA. The conditions were as follows: a heating rate of 20 ℃/min and the nitrogen flow rate increased from room temperature to 300 ℃. The TGA measurements were performed on a thermogravimetric analysis/differential scanning calorimetry (TGA/DSC) thermal analyzer (1:1600; Mettler Toledo Corp., Greifensee, Switzerland).

2.7. Scanning Electron Microscopy (SEM)

The effects of the inhibitors on the surface morphology of the samples were studied by scanning electron microscopy (SEM) using a Hitachi SU6600 field emission scanning electron microscope (Hitachi, Tokyo, Japan), at 20.0 kV acceleration voltage. The sample was attached to the top of the aluminum plug with conductive tape [16].

3. Results and Discussion

3.1. Effect of the Types of Inhibitors

A series of AASs was prepared using adipic acid and organic amines (diethylenetriamine, triethylenetetramine, and tetraethylenepentamine) as raw materials, as shown in

Table 1. Name and inhibitory activity of AASs.

Materials		Ratio of Acid Group to Amine Group	Names	Swelling Rate/% (180 min)
Water		-	-	72.92
KCl		1.0%	-	60.44
Adipic acid	Diethylenetriamine	1:1	AAS-1	63.63
		1:2	AAS-2	56.35
		1:3	AAS-3	65.63
	Triethylenetetramine	1:1	AAS-4	63.38
		1:2	AAS-5	61.80
		1:3	AAS-6	68.72
	Tetraethylenepentamine	1:1	AAS-7	61.20
		1:2	AAS-8	59.61
		1:3	AAS-9	60.28

. The results of the evaluation of the inhibitory effect of 0.1% AAS on the swelling rate of shale rock are shown in Figure 2 and Table 1, which, for comparison, also include the results obtained with 1.0% KCl and water as positive and negative controls, respectively. These results reveal that the swelling rate of bentonite was affected by the type of amine and mole ratio of the functional groups. In particular, the results in Figure 2 show that the expansion rate increased rapidly within 20 min, and the swelling rate was the fastest in distilled water and the slowest in 0.1% AAS-2 [17,18]. The swelling rate tended to reach a plateau after 40 min and remain constant after that, and the inhibitory effect of these substances was significantly different. After 180 min, 0.1% AAS-8 resulted in the lowest expansion rate. The results in Table 1 also show that the linear swelling rate and swelling rate of bentonite in the AAS-8 solution were much lower than those of the other AASs within 180 min. Therefore, AAS-8 was chosen for subsequent experiments in this study.

3.2. Swelling Inhibition

The inhibitory effect of the AASs on the swelling of bentonite was investigated by the linear expansion rate method. The results in Figure 3 show that bentonite expands rapidly in the first 20 min and then its expansion slows down. Compared with tap water and the 1.0% KCl solution, the AASs showed a strong inhibitory effect on the swelling of bentonite. The swelling rate reached its lowest level at 0.1% AAS-8. The inhibition depended on the AAS entering the clay layer and binding the clay sheets together by electrostatic adsorption and hydrogen bonding. Since the inhibition is multifunctional, there is no linear relationship between the swelling degree and concentration. The results of the comprehensive testing confirmed that 0.1% was the optimal concentration of AAS-8 among the tested concentrations [19].

3.3. Performance of AAS-8 in Water-Based Drilling Fluids

The performance of AAS-8 in water-based drilling fluids was evaluated according to GB/T 16783.1-2014. The compatibility of AAS-8 in the drilling fluid was investigated by measuring the changes in the drilling fluid density (ρ), FL, plastic viscosity (μ), and other parameters using different treatment methods, and the results are shown in

Table 2. Results of the analysis of the drilling fluid rheological properties.

Drilling Fluid	AV mPa·s	PV mPa·s	YP /Pa	YP/PV Pa/mPa·s	ρ g/cm3	FL /mL	μ
Base drilling fluid	4.95	2.8	2.1973	0.7848	1.008	13.7	0.0524
0.1% AAS-8	7.10	3.4	3.7814	1.1122	1.016	16.2	0.0524
0.3% CMC	4.90	2.8	2.1462	0.7665	1.018	11.0	0.0875
0.1% AAS-8 + 0.3% CMC	8.40	3.8	4.7012	1.2372	1.020	12.3	0.0437
0.1% PAM	8.00	4.2	3.8836	0.9247	1.022	12.0	0.0699
0.1% AAS-8 + 0.1% PAM	11.70	5.0	6.8474	1.3695	1.020	19.0	0.0437
0.3% Modified starch	6.00	4.0	2.0440	0.5110	1.022	7.5	0.0699
0.1% AAS-8 + 0.3% modified starch	9.60	4.4	5.3144	1.2078	1.025	6.4	0.0699

. The AV, PV, YP, and YP/PV ratio of the drilling fluid, as well as the FL, clearly increased after adding a certain amount of organic amine. After adding 0.1% AAS-8, the values of the AV and PV of the drilling fluid were 1.4 and 1.2 times those of the base mud, respectively, and the FL increased to 16.2 mL. After adding the screened 0.1% AAS-8 into the 0.3% CMC pulp, poly(acrylamide) (PAM) pulp, and modified starch slurry, the performance parameters of the drilling fluid clearly changed: (1) after adding 0.1% AAS-8 into 0.3% CMC, the AV and YP of the drilling fluid were 1.7 and 1.4 times those of the 0.3% CMC, respectively, and the FL and μ also changed; (2) after adding 0.1% AAS-8 into PAM, the rheological parameters of the drilling fluid changed most noticeably, and AV and YP were 1.5 and 1.2 times those of the 0.1% PAM slurry, respectively; and (3) when 0.1% AAS-8 was added to the modified starch slurry, the changes in the drilling fluid rheological parameters were 1.6, 1.1, 2.6, and 2.4 times those of the starch slurry, and the FL and μ of the drilling fluid also changed. Therefore, AAS-8 had a certain compatibility with CMC, PAM, and modified starch, and the compatibility with modified starch was the best [20].

3.4. Laser Particle Size Distribution Analysis

The effect of AAS-8 on the size distribution of bentonite particles is shown in Figure 4 and

Table 3. Average clay particle size.

Content	Mean (µm)	Median (µm)
Unhydrated	33.26	26.65
Hydrated	11.06	7.64
Unhydrated + 0.1% AAS-8	28.58	24.48
Hydrated + 0.1% AAS-8	27.99	22.56

. A similar analysis was conducted on the control samples. The results showed that the mean size of virgin bentonite particles was 33.26 μm, and the size of the hydrated bentonite decreased to 11.06 μm. Adding AAS-8 to a bentonite suspension before and after hydration of bentonite produces different results. After adding 0.1% AAS-8 before hydration, the average clay particle size was 28.58 μm. Sixteen hours after adding AAS-8 to the clay slurry, previously hydrated for 24 h, the clay particle size clearly increased despite the slightly smaller average particle size of bentonite compared with that of unhydrated bentonite. The increase might be due to the flocculation of AAS-8 onto the hydrated bentonite. Moreover, the AAS-8 inhibitor controlled the hydration and dispersion of unhydrated and hydrated bentonite through different mechanisms [21,22].

3.5. TGA

The relationship between mass loss and temperature was determined by TGA as shown in Figure 5. The TGA curves of bentonite show a significant weight loss both in tap water and the AAS-8 solution, and the thermal degradation of bentonite treated with AAS-8 was significantly different from that of blank bentonite at 25 °C. The weight loss rate of tap water–treated bentonite was 4.10%, while the weight loss rate of AAS-8-modified bentonite was not more than 2.50%. These results indicated that AAS-8 can effectively prevent the water molecules from penetrating into the clay interlayer [23].

3.6. Scanning Electron Microscopy (SEM) Analysis

SEM analysis was performed to evaluate the morphology of bentonite particles modified in various ways. The SEM image of the original sample is displayed in Figure 6a. The SEM image of the sample soaked in water for 24 h (Figure 6b) shows that the size of the particles decreased after being immersed in water. The SEM image of bentonite treated with 0.1% ASS-8 for 24 h (Figure 6c) reveals that the treatment with 0.1% ASS-8 did not seem to affect the bentonite particle size, as they were not much different from the original bentonite, although they were larger than the particles after water treatment. The number of large clay particles increased significantly, leading to an increase in the average particle size. Therefore, AAS-8 exhibited inhibitory effects on the swelling and dispersion of bentonite, which is in agreement with previous research [11].

3.7. Mechanism Analysis

The proposed mechanism of inhibition of clay hydration is mainly based on the following aspects. Binding: low-molecular-weight amine shale inhibitors enter the clay layer and bind the clay layer together by electrostatic adsorption and hydrogen bonding, and then reduce the spacing of the crystalline layer of clay minerals. Adsorption: shale inhibitors adsorb on the surface due to the adsorption of polar groups on the molecular chain and clay, forming an adsorption layer and slowing the permeation of water molecules into the shale rock. Therefore, the low-molecular-weight AAS may be partially dissociated in solution to form ammonium ions, leading to a chemical potential difference with the inorganic cations between the clay layers. Then, driven by the chemical potential difference, small-molecule compounds enter the clay layer and ammonium ions exchange inorganic hydration cations by ion exchange. At the same time, the ammonium salt compound forms a hydrogen bond with the silicon hydroxyl group on the surface of the clay, further strengthening its adsorption onto the surface of the clay, thereby inhibiting the dispersion of the large particles, and promoting the aggregation of the small-particle clay pieces [11,12]. The molecular model for clay is illustrated in Figure 7.

4. Conclusions

In this study, AASs were synthesized from adipic acid and amines. AAS-8 was prepared from adipic acid and tetraethylenepentamine in the ratio of 1:2. In particular, the inhibitory effect of 0.1% AAS-8 on clay swelling was studied in detail. The results indicated that 0.1% AAS-8 exhibited a strong inhibitory effect on the hydration and swelling of clay. The inhibition mechanism was studied by TGA and SEM analysis. The proposed inhibition mechanism of 0.1% AAS-8 in shale involves hydrogen bonding, ion exchange, and the anchoring effect, which help to control the hydration and swelling process. In addition, after adding 0.1% AAS-8 to the drilling fluid, the rheological properties of the drilling fluid were enhanced and its compatibility with the modified starch was found to be the best.