Wellbore Stability through Novel Catechol-Chitosan Biopolymer Encapsulator-Based Drilling Mud

The problem of wellbore stability has a marked impact on oil and gas exploration and development in the process of drilling. Marine mussel proteins can adhere and encapsulate firmly on deep-water rocks, providing inspiration for solving borehole stability problem and this ability comes from catechol groups. In this paper, a novel biopolymer was synthesized with chitosan and catechol (named “SDGB”) by Schiff base-reduction reaction, was developed as an encapsulator in water-based drilling fluids (WBDF). In addition, the chemical enhancing wellbore stability performance of different encapsulators were investigated and compared. The results showed that there were aromatic ring structure, amines, and catechol groups in catechol-chitosan biopolymer molecule. The high shale recovery rate demonstrated its strong shale inhibition performance. The rock treated by catechol-chitosan biopolymer had higher tension shear strength and uniaxial compression strength than others, which indicates that it can effectively strengthen the rock and bind loose minerals in micro-pore and micro-fracture of rock samples. The rheological and filtration property of the WBDF containing catechol-chitosan biopolymer is stable before and after 130 °C/16 h hot rolling, demonstrating its good compatibility with other WBDF agents. Moreover, SDGB could chelate with metal ions, forming a stable covalent bond, which plays an important role in adhesiveness, inhibition, and blockage.


Introduction
The problem of wellbore stability in oil and gas drilling engineering has always been a worldwide technical problem. Especially in the complex deep-seated, deep-sea oil and gas, special structural wells, unconventional reservoirs, and other drilling projects, the problem of borehole wall instability is more prominent. According to incomplete statistics, the cost of dealing with borehole instability in drilling construction accounts for 50% of the cost of drilling fluid [1][2][3].
Hydration of shale is a complex process. Shale has low permeability and clay minerals such as montmorillonite are contained in the pores [4]. Na-montmorillonite consists of negatively charged clay sheets, which are similar to pyrophyllite layers, whose negative charge is compensated by interlayer sodium ions to maintain electrical neutrality [5]. The surface hydration first occurs after the montmorillonite meets water. At this stage, the expansion pressure generated by hydration is large and the volume expansion is small. With the hydration, the expansion pressure decreases rapidly [6].
The surface hydration causes crystallization expansion. Ionic hydration begins when the surface is hydrated [7]. Ion hydration brings hydration film to clay and hydration ions compete with water molecules for the connection position of crystal plane of clay. After absorbing water, and the expansion rate can be reduced by about 90% after 8 h, while the water absorption of sodium montmorillonite can be reduced by 30%. It has low toxicity and can effectively seal formation pore under high temperature and strong acid environment.
However, there are some limitations of the above additives, such as limited bearing capacity of the well eye, poor compatibility of drilling fluid, weak effect of high temperature, and unfriendly to the environment. The results show that wellbore instability of shale formation is the result of both physical and chemical factors [38][39][40]. So, it is very important to develop an encapsulator which takes physical plugging and chemical cementing into account to solve the problem of wellbore instability in shale formation.
Mussel can firmly adsorb to rocks under strong winds and waves in the ocean. Its foot silk protein is a powerful adhesive and has been widely used in biological and medical fields. At the same time, the surface becomes denser and stronger after mussels adhere to rocks, hulls, etc., which also shows the potential of physical consolidation [41,42].
The active ingredient of this mussel adhesive protein is a catechol group. Inspired by this characteristic, catechol was grafted onto the main chain of chitosan by Schiff base-reduction reaction method. Finally, a novel environmentally friendly biopolymer encapsulators in WBDF, named SDGB, was developed, and the structural characterization and performance evaluation are conducted.

Orthogonal Experiment of SDGB
The main factors affecting polymer performance include the monomer ratio of catechol to chitosan, reaction time, initiator concentration, and reaction temperature. Orthogonal tests with three levels and four factor designs are shown in Table 1, and the results are shown in Table 2. The rolling recovery of shale rock chips was evaluated in SDGB aqueous solution. The temperature has the most significant effect on the rolling recovery of shale rock chips. The following optimal conditions were determined by rolling recovery of shale rock chips: catechol to chitosan molar ratio of 1:2, initiator concentration of 7 wt%, reaction time of 10 h, and reaction temperature of 10 • C.   The result of FT-IR is shown in Figure 1. It can be seen that the amino group has a stretching vibration absorption peak at 1706 cm −1 , and the peak at 1380 cm −1 belongs to the methyl vibration absorption peak. The typical absorption peaks of O-C stretching vibration and the stretching vibration of aromatic C=C at 1090 cm −1 and 1530 cm −1 existed in chitosan, which proved that catechols had reacted with chitosan successfully. Meanwhile, the IR absorption peak at 1300-1090 cm −1 was the symmetric vibration caused by the covalent bond between anthraquinone and Fe 2+ , which indicated that the structure of catechol in SDGB had strong reactivity and could produce strong reactivity with metal ions [43,44]. The result of FT-IR is shown in Figure 1. It can be seen that the amino grou stretching vibration absorption peak at 1706 cm −1 , and the peak at 1380 cm −1 belong methyl vibration absorption peak. The typical absorption peaks of O-C stretching v and the stretching vibration of aromatic C=C at 1090 cm −1 and 1530 cm −1 existed in c which proved that catechols had reacted with chitosan successfully. Meanwhile, th sorption peak at 1300-1090 cm −1 was the symmetric vibration caused by the covale between anthraquinone and Fe 2+ , which indicated that the structure of catechol in SD strong reactivity and could produce strong reactivity with metal ions [43,44].

NMR Hydrogen Spectroscopy (HNMR) Analysis
The HNMR results of SDGB are shown in Figure 2. It can be seen that there i of protons on the main chain of chitosan at chemical displacement δ of 3.6-4.0. Th ical shift δ at 3.1 corresponds to the C-2 proton peak of chitosan, and the chemica at 1.9 corresponds to the methyl proton peak of acetyl group in chitosan. The chemical shift δ at 6.7 is the proton peak of benzene ring on catechol, and the chemical shift δ at 4.1 is the proton peak of methylene linked to benzene ring, in that the polymer containing catechol structure has successfully reacted with chito

NMR Hydrogen Spectroscopy (HNMR) Analysis
The HNMR results of SDGB are shown in Figure 2. It can be seen that there is a peak of protons on the main chain of chitosan at chemical displacement δ of 3.6-4.0. The chemical shift δ at 3.1 corresponds to the C-2 proton peak of chitosan, and the chemical shift δ at 1.9 corresponds to the methyl proton peak of acetyl group in chitosan. The peak of chemical shift δ at 6.7 is the proton peak of benzene ring on catechol, and the peak of chemical shift δ at 4.1 is the proton peak of methylene linked to benzene ring, indicating that the polymer containing catechol structure has successfully reacted with chitosan.

Thermogravimetric Analysis
The thermogravimetric and differential curves of SDGB are shown in results showed that the weight loss of SDGB could be divided into four stage temperature to about 150 °C, this stage is mainly due to the loss of adsorbed 180 °C to around 245 °C, there is an obvious weight loss at this stage, main degradation of the main polymer chains, and the weight loss is about 30%. to 330 °C, this stage was mainly due to the decomposition of side chains structure. From 330 °C to 360 °C. The chitosan backbone was destroyed an SDGB remained unchanged after decomposition.

Thermogravimetric Analysis
The thermogravimetric and differential curves of SDGB are shown in Figure 3. The results showed that the weight loss of SDGB could be divided into four stages. From room temperature to about 150 • C, this stage is mainly due to the loss of adsorbed water. From 180 • C to around 245 • C, there is an obvious weight loss at this stage, mainly due to the degradation of the main polymer chains, and the weight loss is about 30%. From 280 • C to 330 • C, this stage was mainly due to the decomposition of side chains and catechol structure. From 330 • C to 360 • C. The chitosan backbone was destroyed and the mass of SDGB remained unchanged after decomposition.

Thermogravimetric Analysis
The thermogravimetric and differential curves of SDGB are shown in Figur results showed that the weight loss of SDGB could be divided into four stages. Fro temperature to about 150 °C, this stage is mainly due to the loss of adsorbed wate 180 °C to around 245 °C, there is an obvious weight loss at this stage, mainly du degradation of the main polymer chains, and the weight loss is about 30%. From to 330 °C, this stage was mainly due to the decomposition of side chains and structure. From 330 °C to 360 °C. The chitosan backbone was destroyed and the SDGB remained unchanged after decomposition.

Gel Permeation Chromatography (GPC) Test
The GPC experimental results of SDGB are shown in Table 3. Gel permeatio matography showed that the number average molar weight of SDGB was 25,974 weight average molecular weight was 33,424. The relative molecular weight is m which ensures that it not only has sufficient adsorption strength but also can ent microprobes or cracks. Meanwhile, its polydispersity index was low, indicating molecular weight distribution and good homogeneity.

Gel Permeation Chromatography (GPC) Test
The GPC experimental results of SDGB are shown in Table 3. Gel permeation chromatography showed that the number average molar weight of SDGB was 25,974 and the weight average molecular weight was 33,424. The relative molecular weight is moderate, which ensures that it not only has sufficient adsorption strength but also can enter shale microprobes or cracks. Meanwhile, its polydispersity index was low, indicating narrow molecular weight distribution and good homogeneity. The results of the tensile shear strength test of the lapped samples after treated by amphoteric polymer, SDGB, emulsified asphalt, aluminum humate, polyol, and Na 2 SiO 3 are shown in Figure 4. It can be seen that the lap joint samples treated with SDGB, emulsified asphalt and aluminum humate have higher tensile shear strength in air. The tensile shear strength of 4% SDGB solution in air is about 0.501 MPa, which is the highest among the six samples. The tensile shear strength in water is significantly lower than that in air (0.21 MPa), while the other samples are zero. As can be seen from Figure 5 after treated by SDGB solution, the surface of the metal lap sample is discolored and completely damaged, indicating that SDGB has better adhesion performance. According to Deming TJ et al. [45], catechol group can be oxidized in water to form anthraquinone structure, which can chelate with metal ions or metal oxides on the rock surface to form stable covalent bonds, and then firmly adsorb on the rock surface to prevent rock samples from dispersing in water.  The results of the tensile shear strength test of the lapped sample treated by amphoteric polymer, SDGB, emulsified asphalt, aluminum humat yol, and Na2SiO3 are shown in Figure 4. It can be seen that the lap joint samples with SDGB, emulsified asphalt and aluminum humate have higher tensile shear s in air. The tensile shear strength of 4% SDGB solution in air is about 0.501 MPa, w the highest among the six samples. The tensile shear strength in water is signif lower than that in air (0.21 MPa), while the other samples are zero. As can be see Figure 5 after treated by SDGB solution, the surface of the metal lap sample is disc and completely damaged, indicating that SDGB has better adhesion performance. A ing to Deming TJ et al. [45], catechol group can be oxidized in water to form anthraq structure, which can chelate with metal ions or metal oxides on the rock surface t stable covalent bonds, and then firmly adsorb on the rock surface to prevent rock s from dispersing in water.

Uniaxial Compressive Strength Test
The compressive strength test results of rock samples treated with different wellbore stabilizers are shown in Figure 6. The uniaxial compressive strength of dry rock samples is 12.4 MPa, reduced by about 50% after soaking in clean water. After being treated with different wellbore stabilizers, the compressive strength of rock samples is improved. Among them, 2% of SDGB has the best performance. The compressive strength reaches 8.1 MPa, which is about 25% higher than that of clean water. The results show that SDGB can effectively improve the compressive strength of rock samples.
The compressive strength test results of rock samples treated with differ wellbore stabilizers are shown in Figure 6. The uniaxial compressive strength of dry r samples is 12.4 MPa, reduced by about 50% after soaking in clean water. After be treated with different wellbore stabilizers, the compressive strength of rock sample improved. Among them, 2% of SDGB has the best performance. The compressive stren reaches 8.1 MPa, which is about 25% higher than that of clean water. The results show t SDGB can effectively improve the compressive strength of rock samples.

Inhibitory Shale Hydration Test
The main mineral composition of shale used for inhibitive experiments is shown in Tables 4 and 5. It can be seen that the clay mineral content of the shale selected for the periment is 53%. Illte/smectite minerals are the main clay minerals, indicating that the sh used for this experiment has a high content of active minerals and a high potential hydra ability. 2% SDGB solution can effectively inhibit the dispersion of shale cuttings, and rolling recovery is 91.06% at 70 °C ( Figure 7a). With the increase of temperature, the rol recovery decreases slightly. However, the recovery after 150 °C/16 h hot rolling is stil high as 75.6% (Figure 7b), indicating that SDGB can still effectively inhibit hydration dispersion at high temperature. The swelling rates of rock samples treated at 70 °C and °C for 8 h are 42% and 57% respectively, which is lower than 74% of fresh water, indicat

Inhibitory Shale Hydration Test
The main mineral composition of shale used for inhibitive experiments is shown in the Tables 4 and 5. It can be seen that the clay mineral content of the shale selected for the experiment is 53%. Illte/smectite minerals are the main clay minerals, indicating that the shale used for this experiment has a high content of active minerals and a high potential hydration ability. 2% SDGB solution can effectively inhibit the dispersion of shale cuttings, and the rolling recovery is 91.06% at 70 • C (Figure 7a). With the increase of temperature, the rolling recovery decreases slightly. However, the recovery after 150 • C/16 h hot rolling is still as high as 75.6% (Figure 7b), indicating that SDGB can still effectively inhibit hydration and dispersion at high temperature. The swelling rates of rock samples treated at 70 • C and 150 • C for 8 h are 42% and 57% respectively, which is lower than 74% of fresh water, indicating that the higher the temperature, the worse the inhibition performance.

Hydroscopicity Test
The hydroscopicity test results of shale core treated by different wellbore stabilizers are shown in Figure 8 and Table 6. The rock sample has strong water absorption capacity, which tends to be stable in about 2 h, and the total water absorption reaches 9.1%. After treatment with 2% SDGB solution, the total water absorption of rock samples decreased to 2.75%, nearly 70% lower than that of clean water. The results show that SDGB can firmly adsorb and cement rock samples, so as to inhibit the water absorption capacity of rocks and ensure the stability of wellbore.

Hydroscopicity Test
The hydroscopicity test results of shale core treated by different wellbore stabilizers are shown in Figure 8 and Table 6. The rock sample has strong water absorption capacity, which tends to be stable in about 2 h, and the total water absorption reaches 9.1%. After treatment with 2% SDGB solution, the total water absorption of rock samples decreased to 2.75%, nearly 70% lower than that of clean water. The results show that SDGB can firmly adsorb and cement rock samples, so as to inhibit the water absorption capacity of rocks and ensure the stability of wellbore. are shown in Figure 8 and Table 6. The rock sample has strong water absorptio which tends to be stable in about 2 h, and the total water absorption reaches treatment with 2% SDGB solution, the total water absorption of rock samples to 2.75%, nearly 70% lower than that of clean water. The results show that SDGB adsorb and cement rock samples, so as to inhibit the water absorption capaci and ensure the stability of wellbore.

Compatibility Evaluation in Drilling Fluid
Different amounts of SDGB were added to 4 wt% bentonite water-based drilling fluid, and its apparent viscosity, plastic viscosity and filtration were tested after hot rolling at 130 • C/16 h. The results are shown in Figures 9-11. It is not difficult to find that there is an optimal concentration of SDGB in the drilling fluid, and the addition of SDGB has no adverse effect on the water-based drilling fluid. After hot rolling at 130 • C, it has good rheological and filtration properties, and has the function of increasing viscosity and reducing filtration.

. Compatibility Evaluation in Drilling Fluid
Different amounts of SDGB were added to 4 wt% bentonite water-bas fluid, and its apparent viscosity, plastic viscosity and filtration were tested after at 130 °C/16 h. The results are shown in Figures 9-11. It is not difficult to find th an optimal concentration of SDGB in the drilling fluid, and the addition of SD adverse effect on the water-based drilling fluid. After hot rolling at 130 °C, i rheological and filtration properties, and has the function of increasing viscos ducing filtration.             Figure 12 shows the UV absorption spectrum and adsorption standard curve of SDGB, from which it can be seen that the absorption peak of SDGB is the most obvious at the wavelength of 277.8 nm, and the measured adsorption standard curve is roughly linear. Figure 13 is the curve of sodium adsorption amount of bentonite with different surface temperatures. It shows that the adsorption curves of SDGB at different temperatures are L-shaped, indicating that SDGB mainly adsorbs through a single layer on clay surface. The adsorption capacity of SDGB increases rapidly with increasing concentration. When a certain concentration is reached, the adsorption capacity tends to be flat. The saturated adsorption capacity at 25 • C is 117 mg/g, which shows a strong adsorption capacity. With the increase of temperature, the adsorption capacity of SDGB decreases, and the saturated adsorption capacity can still reach 94 mg/g at 90 • C. SDGB has strong high temperature desorption capacity. The analysis shows that the surface of clay particles is negatively charged and SDGB molecules contain more oxygen-containing functional groups which can be firmly adsorbed on the surface of clay particles by hydrogen bonds. adsorption capacity at 25 °C is 117 mg/g, which shows a strong adsorption capacity. With the increase of temperature, the adsorption capacity of SDGB decreases, and the saturated adsorption capacity can still reach 94 mg/g at 90 °C. SDGB has strong high temperature desorption capacity. The analysis shows that the surface of clay particles is negatively charged and SDGB molecules contain more oxygen-containing functional groups which can be firmly adsorbed on the surface of clay particles by hydrogen bonds.  Shale surfaces are usually rich in metal ions and oxides. Catechol groups can chelate with metal substances to form stable covalent bonds and strongly adsorb on the surface of wellbore rock. In order to further study the adsorption characteristics of SDGB on rock surface, the effects of different concentrations of metal ions (taking ferrous ions as examples) on the adsorption of SDGB were tested. Figure 14 shows the adsorption capacity of SDGB in the presence of Fe 2+ at different concentrations. The results showed that the ad-  Shale surfaces are usually rich in metal ions and oxides. Catechol with metal substances to form stable covalent bonds and strongly ad of wellbore rock. In order to further study the adsorption characteristi surface, the effects of different concentrations of metal ions (taking fe ples) on the adsorption of SDGB were tested. Figure 14  Shale surfaces are usually rich in metal ions and oxides. Catechol groups can chelate with metal substances to form stable covalent bonds and strongly adsorb on the surface of wellbore rock. In order to further study the adsorption characteristics of SDGB on rock surface, the effects of different concentrations of metal ions (taking ferrous ions as examples) on the adsorption of SDGB were tested. Figure 14 shows the adsorption capacity of SDGB in the presence of Fe 2+ at different concentrations. The results showed that the adsorption capacity of catechol-chitosan biopolymer capsules on the surface of montmorillonite particles gradually increased with the increase of Fe 2+ concentration, indicating that the presence of Fe 2+ can effectively improve the adsorption capacity of catechol-chitosan biopolymer capsules on the surface of montmorillonite particles. According to Deming et al. [45], in the presence of Fe 2+ ions or other metal ions, oxygen atoms in the catechol structure of SDGB molecules can interact with metal ions to form relatively stable chelation covalent structures, and some can even be oxidized to anthraquinone structures, thereby enhancing the adsorption strength of catechol structures on rock surfaces. the presence of Fe can effectively improve the adsorption capacity of catechol-chitos biopolymer capsules on the surface of montmorillonite particles. According to Deming al. [45], in the presence of Fe 2+ ions or other metal ions, oxygen atoms in the catechol str ture of SDGB molecules can interact with metal ions to form relatively stable chelati covalent structures, and some can even be oxidized to anthraquinone structures, there enhancing the adsorption strength of catechol structures on rock surfaces.  Figure 15 shows the internal shape of cores before and after different treatments can be seen from (a) that micropore and microfissure are well developed in the core a clay minerals adhere to the grain surface. (b) It can be seen that after SDGB treatment, c minerals originally attached to the grain surface are cemented and filled in the micropo of rock samples, which significantly reduces the micropore in rock samples and mak them more compact. It is found that SDGB not only acts as a seal, but also adsorbs, ceme and solidifies clay minerals in micro-cracks of rock samples, thus improving the compr sive strength of rock.   Figure 15 shows the internal shape of cores before and after different treatments. It can be seen from (a) that micropore and microfissure are well developed in the core and clay minerals adhere to the grain surface. (b) It can be seen that after SDGB treatment, clay minerals originally attached to the grain surface are cemented and filled in the micropore of rock samples, which significantly reduces the micropore in rock samples and makes them more compact. It is found that SDGB not only acts as a seal, but also adsorbs, cements and solidifies clay minerals in micro-cracks of rock samples, thus improving the compressive strength of rock.

Scanning Electron Microscope Test
al. [45], in the presence of Fe 2+ ions or other metal ions, oxygen atoms in the catechol structure of SDGB molecules can interact with metal ions to form relatively stable chelation covalent structures, and some can even be oxidized to anthraquinone structures, thereby enhancing the adsorption strength of catechol structures on rock surfaces.  Figure 15 shows the internal shape of cores before and after different treatments. It can be seen from (a) that micropore and microfissure are well developed in the core and clay minerals adhere to the grain surface. (b) It can be seen that after SDGB treatment, clay minerals originally attached to the grain surface are cemented and filled in the micropore of rock samples, which significantly reduces the micropore in rock samples and makes them more compact. It is found that SDGB not only acts as a seal, but also adsorbs, cements and solidifies clay minerals in micro-cracks of rock samples, thus improving the compressive strength of rock.

FT-IR Test
In order to study the interaction mode and mechanism between SDGB and sodium montmorillonite, the infrared spectra of bentonite, SDGB and SDGB modified bentonite were tested, and the result is shown in Figure 16. In order to study the interaction mode and mechanism between SDGB and sodium montmorillonite, the infrared spectra of bentonite, SDGB and SDGB modified bentonite were tested, and the result is shown in Figure 16. It can be seen from the test results that the main absorption peaks of sodium montmorillonite have not changed significantly before and after modification, indicating that the main molecular structure of sodium montmorillonite has not changed before and after SDGB modification. It can be seen from the infrared spectra of Na-MMT that 3630 cm −1 can be attributed to the Tensile vibration of the hydroxyl group, because bound water molecules still exist in the molecular layer of Na-MMT, which is the result of the conjugation of freely bound water and bound water. 1050 cm −1 is the vibration absorption peak of silica-silicon in sodium montmorillonite molecular layer. 1640 cm −1 can be attributed to the tensile vibration of hydroxyl groups adsorbed in water between different layers of sodium montmorillonite. The results showed that the above main absorption peaks did not change significantly before and after modification, which indicated that the action of SDGB and sodium montmorillonite would not change the main molecular structure of bentonite silicate framework. However, some new infrared absorption peaks can also be found in SDGB-modified sodium montmorillonite. 3430 cm −1 can be attributed to the tensile vibration absorption peak formed by the near oxygen atoms connected with benzene ring. 2930 cm −1 and 2850 cm −1 can be attributed to the symmetric vibration absorption peak of methylene. Meanwhile, the intermolecular hydroxy tensile vibration absorption peak initially appeared at 1640 cm −1 in the modified Na-MMT spectrum appeared at 1570 cm −1 and moved to low frequency to some extent, indicating that SDGB formed hydrogen bond with sodium montmorillonite.

Summary and Analysis of Mechanism
Traditional high-performance amine shale inhibitors often work through higher amine density. In the presence of a large number of amines, a large number of amines in the inhibitor molecules are positively charged, which can electrostatically absorb with negatively charged bentonite particles, thus neutralizing the negative charges of active clays, compressing their diffusion double layer, driving out water molecules and thus inhibiting shale hydration. The schematic diagram of action mechanism of SDGB is shown in Figure 17. SDGB surface does not contain a large number of strong cationic groups to keep the drilling fluid from functioning (most water-based drilling fluid additives are negatively charged). First, the adsorption of SDGB occurs through the presence of trace metal ions on the shale surface. It can be seen from the test results that the main absorption peaks of sodium montmorillonite have not changed significantly before and after modification, indicating that the main molecular structure of sodium montmorillonite has not changed before and after SDGB modification. It can be seen from the infrared spectra of Na-MMT that 3630 cm −1 can be attributed to the Tensile vibration of the hydroxyl group, because bound water molecules still exist in the molecular layer of Na-MMT, which is the result of the conjugation of freely bound water and bound water. 1050 cm −1 is the vibration absorption peak of silica-silicon in sodium montmorillonite molecular layer. 1640 cm −1 can be attributed to the tensile vibration of hydroxyl groups adsorbed in water between different layers of sodium montmorillonite. The results showed that the above main absorption peaks did not change significantly before and after modification, which indicated that the action of SDGB and sodium montmorillonite would not change the main molecular structure of bentonite silicate framework. However, some new infrared absorption peaks can also be found in SDGB-modified sodium montmorillonite. 3430 cm −1 can be attributed to the tensile vibration absorption peak formed by the near oxygen atoms connected with benzene ring. 2930 cm −1 and 2850 cm −1 can be attributed to the symmetric vibration absorption peak of methylene. Meanwhile, the intermolecular hydroxy tensile vibration absorption peak initially appeared at 1640 cm −1 in the modified Na-MMT spectrum appeared at 1570 cm −1 and moved to low frequency to some extent, indicating that SDGB formed hydrogen bond with sodium montmorillonite.

Summary and Analysis of Mechanism
Traditional high-performance amine shale inhibitors often work through higher amine density. In the presence of a large number of amines, a large number of amines in the inhibitor molecules are positively charged, which can electrostatically absorb with negatively charged bentonite particles, thus neutralizing the negative charges of active clays, compressing their diffusion double layer, driving out water molecules and thus inhibiting shale hydration. The schematic diagram of action mechanism of SDGB is shown in Figure 17. SDGB surface does not contain a large number of strong cationic groups to keep the drilling fluid from functioning (most water-based drilling fluid additives are negatively charged). First, the adsorption of SDGB occurs through the presence of trace metal ions on the shale surface. The structure of catechol in SDGB can chelate with metal ions. The active groups such as catechol adsorb rapidly and firmly on the surface of rock particles on the well wall through hydrogen bond and chelation, encapsulate the active groups, and play a "sticky" role in preventing rock particles from dispersing under the action of drilling fluids and improving rock strength. Second, during the adsorption process between chemical wall fixing agent and the surface of rock particles, the polymer chain can also fill and plug the pores of rock, reduce rock permeability, and improve the stability of drilled rock. Third, the molecules of chemical wall fixing agent contain strong adsorptive groups, which can inhibit hydration when strongly adsorbed on rock surface. In short, the combination of "binder curing to increase strength physical blockage to reduce permeability of rock samples to reduce hydration rejection" improves wellbore stability.
The structure of catechol in SDGB can chelate with metal ions. The active groups such catechol adsorb rapidly and firmly on the surface of rock particles on the well wall thro hydrogen bond and chelation, encapsulate the active groups, and play a "sticky" rol preventing rock particles from dispersing under the action of drilling fluids and improv rock strength. Second, during the adsorption process between chemical wall fixing ag and the surface of rock particles, the polymer chain can also fill and plug the pores of ro reduce rock permeability, and improve the stability of drilled rock. Third, the molecule chemical wall fixing agent contain strong adsorptive groups, which can inhibit hydra when strongly adsorbed on rock surface. In short, the combination of "binder curing to crease strength physical blockage to reduce permeability of rock samples to reduce hyd tion rejection" improves wellbore stability.

Conclusions
Inspired by mussels, a novel catechol-conjugated-chitosan biopolymer encapsula SDGB was synthesized. The optimum synthesis conditions are determined in this pa The structure of the products was characterized by IR, NMR, and TG. The results show that the catechol reacted successfully with chitosan, and the decomposition temperat was over 180 °C. The experiment shows that SDGB can effectively improve the sh strength and inhibition of shale. The recovery rate of the shale cuttings was still up to 75 after 150 °C/16 h hot rolling, and the bearing capacity of the rock can be increased by 2 A wellbore strengthening drilling fluid was constructed based on SDGB. The rheolog and filtration performance of this drilling fluid is stable before and after 130 °C/16 h rolling, and shale inhibition behavior is good. Moreover, SDGB could chelate with m ions, forming a stable covalent bond, which plays an important role in adhesiveness, i bition, and blockage.

Conclusions
Inspired by mussels, a novel catechol-conjugated-chitosan biopolymer encapsulators SDGB was synthesized. The optimum synthesis conditions are determined in this paper. The structure of the products was characterized by IR, NMR, and TG. The results showed that the catechol reacted successfully with chitosan, and the decomposition temperature was over 180 • C. The experiment shows that SDGB can effectively improve the shear strength and inhibition of shale. The recovery rate of the shale cuttings was still up to 75.6% after 150 • C/16 h hot rolling, and the bearing capacity of the rock can be increased by 24%. A wellbore strengthening drilling fluid was constructed based on SDGB. The rheological and filtration performance of this drilling fluid is stable before and after 130 • C/16 h hot rolling, and shale inhibition behavior is good. Moreover, SDGB could chelate with metal ions, forming a stable covalent bond, which plays an important role in adhesiveness, inhibition, and blockage.

Synthesis of Catechol-Chitosan Biopolymer Encapsulators
Ferrous chloride was added and fully dissolved in hot ethanol solution (50% wt, 30 mL). A certain amount of polymer containing catechol structure was added into a three-neck flask and stirred with an electric stir bar under an inert N 2 atmosphere. A certain amount of chitosan was dissolved with 20 mL, 1 mol/L hydrochloric acid and them nitrogen is introduced into the reaction system. After the pH was adjusted to a certain value with 5 mol/L NaOH solution, the initiator is dissolved in 10 mL of deionized water and then added dropwise to the reaction solution to trigger the reaction. The mixture was refluxed for a period of time at a certain temperature. The precipitate was filtered and washed with ethanol to get the final product. The molecular structure of SDGB is shown in Figure 18.

Synthesis of Catechol-Chitosan Biopolymer Encapsulators
Ferrous chloride was added and fully dissolved in hot ethanol solution A certain amount of polymer containing catechol structure was added i flask and stirred with an electric stir bar under an inert N2 atmosphere. A of chitosan was dissolved with 20 mL, 1 mol/L hydrochloric acid and t introduced into the reaction system. After the pH was adjusted to a cert mol/L NaOH solution, the initiator is dissolved in 10 mL of deionized added dropwise to the reaction solution to trigger the reaction. The mixtu for a period of time at a certain temperature. The precipitate was filtered a ethanol to get the final product. The molecular structure of SDGB is show Orthogonal tests were used to optimize the four main factors to de formulation and these four factors are the mole ratio between catechol an tion time, initiator concentration, and reaction temperature. The influence tors was analyzed by testing the tensile shear strength and rolling recover

Tensile Shear Strength Test
The mechanical properties of core before and after treated by SDG uniaxial mechanical strength test(Bairoe Test Instrument Co., Ltd. in Sh which can effectively reflect the improvement of shale by strong wall d accordance with the Chinese National standard "methods for determin shear strength of adhesives" (GB7124-1986) and "determination of chem adhesives" (GB/t13353-92), the aqueous solution of SDGB is applied to th face of the sample and the lap joint sample is pressed at 5 MPa for 2 h, then at 50 °C for 24 h. Then longitudinal tensile force is applied to the single of the sample to test the maximum load that the sample can bear on wdw puter controlled triaxial multifunctional machine ( Figure 19). Orthogonal tests were used to optimize the four main factors to determine the best formulation and these four factors are the mole ratio between catechol and chitosan, reaction time, initiator concentration, and reaction temperature. The influence of different factors was analyzed by testing the tensile shear strength and rolling recovery of the product.

Tensile Shear Strength Test
The mechanical properties of core before and after treated by SDGB are tested by uniaxial mechanical strength test(Bairoe Test Instrument Co., Ltd. in Shanghai, China), which can effectively reflect the improvement of shale by strong wall drilling fluids. In accordance with the Chinese National standard "methods for determination of tensile shear strength of adhesives" (GB7124-1986) and "determination of chemical resistance of adhesives" (GB/t13353-92), the aqueous solution of SDGB is applied to the single lap surface of the sample and the lap joint sample is pressed at 5 MPa for 2 h, then placed in water at 50 • C for 24 h. Then longitudinal tensile force is applied to the single lap joint surface of the sample to test the maximum load that the sample can bear on wdw-100 microcomputer controlled triaxial multifunctional machine ( Figure 19). Figure 19. WDW-100 tensile shear strength device.

Cuttings Hot-Rolling Dispersion Test
Shale dispersion recovery experiment is a comm hibition ability of treatment agent. The dispersion pe stability of wellbore. It is one of the important index wellbore macroscopically. The dispersion experiment dration and dispersion performance of rock samples, evaluate the inhibition of drilling fluid on shale. 350 m various concentrations and 50 g of shale cuttings (2-5 of Shahejie formation in Dongying oil field were adde at 77 °C for 16 h, the cuttings were washed with 10% a 40-mesh sieve. The recovered cuttings were dried a of recovery was determined.

Characterization of SDGB
A Bruker AVANCE 400 NMR spectrometer ( Bro was used to measure the proton nuclear magnetic res used for field frequency lock, and the observed 1 H che per million (ppm). Before 1 H NMR analysis, the pH

Cuttings Hot-Rolling Dispersion Test
Shale dispersion recovery experiment is a common method to evaluate the shale inhibition ability of treatment agent. The dispersion performance of shale is related to the stability of wellbore. It is one of the important indexes to evaluate the stability of shale wellbore macroscopically. The dispersion experiment can be used to understand the hydration and dispersion performance of rock samples, and can also be used as a means to evaluate the inhibition of drilling fluid on shale. 350 mL of solution with encapsulators of various concentrations and 50 g of shale cuttings (2-5 mm) obtained from the upper layer of Shahejie formation in Dongying oil field were added into sealed cells. After hot rolling at 77 • C for 16 h, the cuttings were washed with 10% KCl solution and screened through a 40-mesh sieve. The recovered cuttings were dried at 105 • C for 4 h and the percentage of recovery was determined.

Characterization of SDGB
A Bruker AVANCE 400 NMR spectrometer (Brooke Co., Ltd., Zurich, Switzerland) was used to measure the proton nuclear magnetic resonance ( 1 HNMR) spectra. D 2 O was used for field frequency lock, and the observed 1 H chemical shifts were reported in parts per million (ppm). Before 1 H NMR analysis, the pH value of the polymer solution was adjusted to around 9.0 by dilute NaOH/D 2 O solution.
A total of 1 mg dry SDGB powder and 20 mg KBr were mixed fully. The mixture was loaded into the mold and compacted with 50 MPa pressure. FTIR of the compacted tableting were obtained on a NEXUS FT-IR spectrometer(Thermo Nicolet Corporation, Waltham, MA, USA).
The thermogravimetric analysis of SDGB was carried out by mettler-teledothermo gravimetric analyzer (NETZSCH-Gerätebau Co., Ltd., Selb, Germany). The temperature range was from room temperature to 1000 • C, the heating rate was 10 K/min, and the atmosphere was nitrogen, and the gas flow rate was 50 mL/min.
The relative molecular mass of SDGB was determined by the SFD gel permeation chromatograph (GPC) (Schambeck Co., Ltd., Berlin, Germany). The mobile phase was phosphate buffer solution. The column was SHODEX (K-806M chloroform system, and the filler was styrene and two vinyl benzene copolymer).

Core Uniaxial Compressive Strength Test
The rock samples were soaked in different wellbore strengthening agent solutions for 4 h, then taken out carefully and dried at room temperature. Then compressive strength was tested on wdw-100 microcomputer controlled triaxial multifunctional machine (Bairoe Test Instrument Co., Ltd., Shanghai, China).

The Hydroscopicity Test
The hydroscopicity test was conducted by self-made core self-priming experimental device. The water absorption and water absorption rate of typical shale in deionized water were tested. At the same time, the change of water absorption of rock samples treated with different encapsulators with time was tested to comprehensively evaluate the hydration and dispersion performance of rock samples. First, the experimental core is dried in an oven at 105 • C for 12 h, and then placed at 25 • C for 24 h. During the experiment, the room temperature is maintained at 25 • C. Then, the shale standard core is suspended on the hook of the electronic balance, so that its lower end just contacts the solution. The total mass of the rock sample changes after water absorption over time is recorded.

Scanning Electron Microscope Test
Core micro morphology was tested by S-4800 field emission scanning electron microscope (HitaChi, Ltd., Tokyo, Japan) after being treated by SDGB, sodium silicate and aluminum humate.

Adsorption Isotherm Test
In order to further study the action mechanism of SDGB in drilling fluids, the adsorption characteristics of SDGB on bentonite particles were studied.
(1). A sufficient amount of 10% hydrogen peroxide was added to the beaker together with 100 g of bentonite and stirred simultaneously on a heating oven to eliminate the possible influence of the residual organics in the bentonite on the adsorption test results. Deionized water was then added so that the contents in the beaker are well dispersed, and then was placed in a centrifuge and centrifuged at 8000 rpm for 15 min. The upper clear liquid was poured out, and deionized water was continued to be centrifuged repeatedly for three times. The precipitate was collected and then placed into a blast drying oven and dried to constant weight, crushed and sieved.
(2). Some SDGB was added into a beaker containing deionized water, stirred well and dissolved; 2.0 g of purified bentonite was added to another beaker containing deionized water, stirred well and dispersed. After the two beakers had been mixed, they were placed in a magnetic stirrer and stirred thoroughly for 24 h until the adsorption equilibrium was reached. The mixed suspension was placed in a centrifuge and centrifuged at a high speed of 10,000 rpm for 15 min. The absorbance in λ = 200-400 nm of the supernatant was measured by a uv-1750 UV VIS spectrophotometer(Yuanxi Instrument Co., Ltd., Shanghai, China).
(3). In λ = 277.8 nm, the adsorption capacity on the surface of bentonite sodium was calculated from the standard curve of polymer solution concentration and light transmittance (as shown in Figure 12a).

Rheological and Filtration Testing of Drilling Fluids
(1) Drilling fluid preparation and hot rolling A total of 16 g of sodium-based bentonite was added to 400 mL of clear water, and stirred at 8000 rpm in a high-speed blender for 30 min. Then 0.8 g of Na 2 CO 3 was added, stirred for 20 min, and then pre-hydrated for 24 h. A certain amount of polymer was then added to the slurry, which was stirred at 8000 rpm for 30 min in a high-speed blender. The composites were aged at set temperature for 16 h by hot rolling and were cooled to room temperature before stirring at high speed for 20 min. Rheological and filtration properties Gels 2022, 8,307 18 of 20 of drilling fluid before and after rolling at a specific temperature/16 h were tested according to the drilling fluid performance evaluation standard SY/t5621-1993.16g.
(2) API Static Filtration Test The static API filterability of drilling fluid was tested with ZNZ-D3 API medium pressure filter (Qingdao Haitong Instrument Co., Ltd., Qingdao, China). A certain amount of drilling fluid was loaded into the filter kettle and the top was covered with API filter paper and placed under a pressure of 100 psi. The filtered volume (FLAPI) of the drilling fluid is recorded for 30 min, which is recommended by API.
(3). Rheological property test The rheological parameters of the drilling fluid are tested according to the drilling fluid performance evaluation standard SY/T5621-1993. Apparent viscosity, plastic viscosity, and shear force of drilling fluid were measured with ZNP-M7 6-speed rotating viscometer (Haitong Instrument Co., Ltd., Qingdao, China). Then the apparent viscosity, plastic viscosity, and shear force of drilling fluid with ϕ600 and ϕ were measured. The value of 300 is calculated according to the test program recommended by API.
where: AV is the apparent viscosity (mPa·s); PV is the plastic viscosity (mPa·s); ϕ600 is the dial reading of 6-speed rotational viscometer at 600 r/min (dia); ϕ300 is the dial reading of 6-speed rotational viscometer at 300 r/min (dia).