Preparation and Performance of the Hyperbranched Polyamine as an Effective Shale Inhibitor for Water-Based Drilling Fluid

Seeking effective solutions to control and mitigate the interaction between drilling fluids and clay formations has been a challenge for many years, and various shale inhibitors have shown excellent results in problematic shale formations around the world. Herein, the hyperbranched polyamine (HBPA) inhibitor with a higher ratio of amine groups and obvious tendentiousness in protonation was successfully synthesized from ethylenediamine, acryloyl chloride and aziridine by five steps, in which the metal-organic framework (MOF) was employed as a catalyst for ring-open polycondensation (ROP). The structure and purity were confirmed by nuclear magnetic resonance hydrogen spectroscopy and high-performance liquid chromatography (HPLC) respec-tively. The HBPA displays more excellent performance than EDA and KCl widely applied in the oil field. After aging at 80˚C and 180˚C, the YP of a slurry system containing 25 wt.% bentonite and 2 wt.% HBPA are just 8.5 Pa and 5.5 Pa (wt.%: percentage of mass), respectively. The swelling lengths of 2 wt.% HBPA are estimated to be 1.78 mm, which falls by 70% compared with that of freshwater. Under a hot rolling aging temperature of 180˚C, the HBPA system demonstrates a significant inhibition with more than 85% shale cuttings recovery rate and is superior to conventional EDA and KCl. Mechanism analysis further validates that the HBPA can help to increase the zeta potential.


Introduction
According to the complex and numerous chemical and physical variations present in the formation, normal inhibitors don't deliver satisfactory performances for preventing shale hydration swelling and dispersion in water-based drilling fluids [1] [2], and the existence of water sensitivity would produce a series of problems such as instability and collapse of the wellbore, bit balling, stuck pipe, etc [3] [4].
The linear-chain molecular structure is a much important factor that restricts the performance of the existing inhibitors [5]. Molecules adsorbed on the surface of the electronegative clay particles have absorbing groups with low density at either end and desorption takes place most easily when subjected to every downhole factor, resulting in a sharp decline in inhibitory action [6] [7] [8].
Compared with linear polymer, hyperbranched polymer, as a kind of quasi-spherical shape polymer with dominated branched structure, can introduce numerous absorbing groups at the end of the molecular chains by the functionalization and modification of end-groups, which was propitious for the molecular chain to the adsorption on the surface of clay [5]. Excluding terminal groups, the increase of absorbing groups along the chains could also enhance the adsorption capacity. The resulting molecular is adsorbed onto the negatively charged clay mineral and enters the interlayer space to tighten the crystalline layers of clay minerals, preventing shale hydration swelling [9].
In this study, a kind of hyperbranched polymer with a higher ratio of amine groups was designed and synthesized. Benefiting from this structure, primary amine, secondary amine and amide groups would produce protonated products in serious alkaline drilling fluids, which endow the molecule with strong adsorbability and thus help boost the inhibition performance of HBPA. Attributing to the characterization results of inhibitive properties, the as-synthesized HBPA significantly performances better than currently available EDA and KCl in bentonite inhibition test, linear swelling test and shale cuttings hot-rolling dispersion test. Besides, zeta potential measurements were carried out to investigate the inhibiting mechanism. Compared to EDA and KCl, the HBPA system experiences the greatest drop and no charge reversal appears. Citric acid monohydrate (AR, >99%): organic compound, melting point −94˚C, boiling point 56˚C, relative density 0.791, used in this paper for removing impurities of the product.

Materials
KCl: white crystal powder, melting point 770˚C, boiling point 1420˚C, relative density 1.98, was used as the control experiment of inhibition performance of synthetic hyperbranched polyamine (HBPA) (Figure 1). Aziridine was prepared based on the relevant reference [10].
The Sodium bentonite (Na-bent) was a natural smectic aluminosilicate.

N-(2-Aminoethyl)Acrylamide (3)
TFA (2.5 mL) was added to a solution of 2 (4.5 mmol, 0.963 g) and ethyl acetate (10 mL) at 0˚C -5˚C. The solution was heated in an oil bath (100˚C) for 12 h, and then the excess solvent was removed under vacuum. The remains were diluted with n-butanol (15 mL) and washed with saturated sodium carbonate solution. The yellow solution was concentrated to give 4 (0.36 g, 70%). 1

Characterization 1 H-NMR and HPLC Analysis
The product of each step was investigated by use of nuclear magnetic resonance hydrogen spectroscopy with tetramethylsilane as an internal reference, and the measurement was performed on an advanced −400 spectrometer at 25˚C. In order to further verify the rationality and reliability of these strategies in the synthesis of HBPA, High-Performance Liquid Chromatography (HPLC) was used to characterize the high purity of the target compound.

Bentonite Inhibition Test
The incorporation of highly active drilled solids into a drilling fluid is simulated in the technique, which closely resembles the process of drilling in water-sensitivity Open Journal of Yangtze Gas and Oil soil. A solution of 5 wt.% bentonite (20 g) and a certain concentration of shale inhibitors dissolved in 400 mL freshwater was stirred for about 30 min, then the mixture was hot rolled for 16 h at different temperatures. After cooling to 25˚C, their rheology was measured. Then, added an equal amount of bentonite and the operation was the same as the previous. This process repeated until the data was unreadable resulting from the excessive viscosity of the sample.

Linear Swelling Test
The impact of inhibitor on shale cuttings exposed to the water-based drilling fluid can be assessed by linear swelling measurements which are carried out at 25˚C. A large linear swelling rate corresponds to fairly strong hydration reactivity. Firstly, bentonite powder (10 g) was compressed into bentonite pellets. Then the swelling heights of pellets soaked in a certain concentration of a solution containing various inhibitors were investigated.

Shale Cuttings Hot-Rolling Dispersion Test
This method is designed to evaluate exactly the ability of inhibitors to prevent shale hydration swelling and dispersion by determining the influence of inhibitors on determining the effect of an inhibitor on the structural integrity of sized cuttings at elevated temperature. Added a 350 mL solution containing 50 g shale cuttings and various concentrations of inhibitors to the aging jars for hot rolling aging for 16 h, then, the remaining shale cuttings of a certain size were screened with a 40 mesh sieve. After washing and drying to a constant mass, the recovery of cuttings can be calculated according to the formula below.

Characterization
The polybasic primary amine structure of HBPA was confirmed by 1 H NMR analysis, which spectra were shown in Figure 2

Bentonite Inhibition Test
The ability of the inhibitor to prevent shale hydration swelling can be detected by the changing rules of yield points for bentonite slurries with various concentrations [11] [12] [13]. The evaluation results of the bentonite inhibition test were shown in Figure 3. When increasing the addition of bentonite under the absence of inhibitors, the YP shows a rapid increase, and the value quickly becomes unmeasurable as the additive content of bentonite reaches 15% of the total drilling fluid mass, suggesting the swelling behavior of bentonite [14]. By comparison, the increasing of YP of a slurry system containing different inhibitors develops slowly initially, and the value changes abruptly under higher bentonite concentration. Encouraging, compared with the two conventional inhibitors, the swelling of bentonite is inhibited by HBPA (2 wt.%) to maximum. To further evaluate the inhibition effect of these serious additives at elevated temperatures, trends in yield points for bentonite slurries with various inhibitors were examined under the conditions of 180˚C.

Linear Swelling Test
Linear swelling test can specifically estimate the swelling degree of rock samples with drilling fluids [15], especially for highly reactive shale formation containing abundant swelling clays [16] [17] [18]. The inhibitive durability of the prepared HBPA inhibitor was detected and the results were shown in Figure 4.  Hyperbranched polymer, as a kind of quasi-spherical shape polymer with dominated branched structure, can introduce numerous absorbing groups at the end of the molecular chains by the functionalization and modification of end-groups.

Rolling Recovery Tests
According to the test standards released by ISO 10416: 2008 Petroleum and Natural Gas Industries Industries Drilling Fluids Laboratory Testing, shale recovery is the most used approach for further evaluating the inhibiting perfor- and hydration of shale are strongly accelerated, and the adsorption of inhibitors on the clay surface, on the other hand, will weaken. These defects could cause rock fragments to separate from the wellbore wall during the drilling [27]. From Figure 5 and Table 1, although the recovery rates for all inhibitors decline as

Mechanism Analysis
Zeta Potential Test Electrokinetic properties of clay were performed to reveal the interactions between HBPA and clay particles, and the zeta potential pattern was displayed in Figure 6, with EDA and KCl inhibitor as reference. Commonly, the bentonite particle is a negative charge surface due to isomorphic replacements of ions in layers [29] [30]. The highly negative zeta potential of clay particles represents the enhanced shale hydration swelling and dispersion. As shown in Figure 6  the testing stage. Benefiting from this hyperbranched polymer with a higher ratio of amine groups, the HBPA increased the zeta potential to a higher degree than EDA and KCL, indicating the excellent performance to prevent shale hydration swelling and dispersion.

Conclusion
Hyperbranched polyamine (HBPA) inhibitor with a higher ratio of amine groups and obvious tendentiousness in protonation used as an effective shale inhibitor in water-based drilling fluids has been developed, and a kind of MOFs