Experimental rock sample: The Carboniferous Benxi Formation No. 8 coal rock in the Ordos Basin is a typical deep coal rock reservoir with a stable distribution in the whole basin and a buried depth of 2 000-3 800 m ^[7]. The No. 8 coal rock reservoir needs hydraulic fracturing to obtain industrial gas flow. In this paper, rock samples taken from a deep coal rock gas well were used to carry out experiments. The vitrinite reflectance ranges from 1.2% to 1.9%, which belongs to medium-high thermal maturity ^[8]. The maceral is mainly vitrinite with high fixed carbon content. Coal rock is weakly hydrophilic and strongly oleophilic. The coal rock samples were crushed, and two kinds of particles with particle diameter of 425-850 μm and 25-75 μm were sieved. Eight irregular lump coal rocks with contour size of 10 mm to 20 mm were prepared to characterize the microscopic morphology of adsorption layer. Six columnar coal rock cores with diameter of 25 mm and length of 50 mm were prepared for permeability damage evaluation. The coal samples were stored in sealed tube or reagent bag to avoid oxidation.

Experimental fluid: (1) Hydroxypropyl guar gum fracturing fluid and polyacrylamide fracturing fluid, which are commonly used for hydraulic fracturing of deep coal rock, were selected. The relative molecular weight of hydroxypropyl guar gum is (1.8-3.0)×10⁶, and the viscosity of the gel breaking liquid with a mass fraction of 0.4% is 5.0 mPa·s. The relative molecular weight of polyacrylamide is about 3.0×10⁶, and the viscosity of the gel breaking liquid with a mass fraction of 0.4% is 4.5 mPa·s. (2) According to the General Technical Specifications of Fracturing Fluids (SY/T 6376-2008) ^[9], fracturing fluid was prepared with water. After preparation, the fracturing fluid residue was removed by vacuum filtration in Buchner flask, and the filtrate was extracted and set aside. (3) According to the ionic composition of formation water in deep coal rock gas reservoir, calcium chloride type formation water with salinity of 80 000 mg/L was prepared for testing cores permeability.

Based on the coal rock particles static adsorption and core dynamic displacement experiments, the adsorption capacity of fracturing fluid thickener in deep coal rock and the permeability damage of adsorption were evaluated. Atomic force microscopy was used to characterize the three-dimensional morphology of adsorption layer, quantify the thickness of the adsorption layer, and explore the response factors of adsorption layer. Zeta potential, infrared spectrum and X-ray photoelectron spectroscopy were comprehensively used to investigate deep coal rock parameters before and after adsorption and reveal the adsorption mechanism.

After the hydroxypropyl guar gum and polyacrylamide fracturing fluid were standing for 48 h, the mass fraction of thickener decreased from the initial value of 0.400%-0.344% and 0.347% due to the polymer self-degradation, condensation and precipitation. After adsorption, the mass fraction of thickener decreased to 0.248% and 0.265%, respectively, and the adsorption proportion were 28.0% and 23.7% of the total. The static adsorption capacity of coal rock for hydroxypropyl guar gum is 3.86 mg/g, which is higher than that for polyacrylamide 3.29 mg/g. Generally, the thickener adsorption capacity of shale is 3.02-7.23 mg/g, and that of sandstone is 2.23-4.21 mg/g ^[5-6]. It can be seen that the thickener adsorption capacity of deep coal rock is the same order of magnitude as that of shale and sandstone.

The permeability of deep coal rock was damaged to varying degrees after adsorption of hydroxypropyl guar gum and polyacrylamide. As shown in Table 2, the coal rock cores permeability damage rate after adsorption of hydroxypropyl guar gum is 35.24%-37.01%, and that of polyacrylamide is 14.31%-21.93%. The permeability damage rates of both thickeners are lower than that of shale 40.2% ^[5]. Hydroxypropyl guar gum damages coal rock at a higher rate than the 25.8% for sandstone ^[6].

Fig. 1 shows the three-dimensional morphology of hydroxypropyl guar gum adsorbed on the surface of deep coal rock. Coupling with Fig. 2a, findings can be obtained in four aspects. First, the hydroxypropyl guar gum of control group is adsorbed on the surface of deep coal rock and accumulated to form irregular lumpy adsorption layers. The adsorption layers with thickness of 5 nm to 25 nm and above 30 nm account for 65% and 20%, respectively. The maximum thickness of adsorption layer reaches 60 nm. The hydroxypropyl guar gum is adsorbed on the surface of coal rock as a monolayer, or by breaking gel to produce groups with shorter molecular chains. Second, the mass fraction of hydroxypropyl guar gum is reduced to 0.2%, and 99% of the adsorption layer thickness is significantly decreased to less than 10 nm. Third, in neutral condition (pH=7), the adsorption morphology becomes irregular agglomerate-columnar continuous distribution. 89% of the adsorption layer thickness decreases to 5-25 nm. Fourth, when the temperature is increased to 80 °C, the adsorption morphology becomes columnar-acicular with 0.2 μm equivalent diameter and 5-20 nm height. In conclusion, the adsorption layer thickness of hydroxypropyl guar gum on the surface of deep coal rock can be reduced by decreasing the thickener mass fraction and pH value and increasing the ambient temperature. The most significant effect is to reduce the mass fraction.

Fig. 3 shows the three-dimensional morphology of polyacrylamide adsorbed on the surface of deep coal rock. Combined with Fig. 2b, the findings can be obtained in four aspects. First, in the control group, polyacrylamide is adsorbed on the surface of coal rock as monolayer or fewer layers, to form multiple columnar-lamellar structures with 0.2-1.0 μm equivalent diameter and 5-40 nm height. Second, when the mass fraction of polyacrylamide is reduced to 0.2%, the adsorption layer thickness decreases significantly. Third, when pH value is increased to 9, the adsorption layer evolves from lamellar adsorption with clear boundary to continuous and irregular overlapped adsorption. The distribution frequency of adsorption layers with thickness of 10-15 nm and more than 25 nm increases by 5.5 and 7.3 percent points, respectively. Fourth, when the temperature is increased to 80 °C, the C-C bond of polyacrylamide breaks and high temperature self-degradation occurs, resulting in lower molecular weight polyacrylamide adsorbed on the coal rock surface. 99% of the adsorption layer thickness decreases to less than 15 nm, and the lamellar adsorption reduces. In conclusion, reducing the mass fraction and increasing temperature can decrease the thickness of polyacrylamide adsorption layer. The distribution frequency of adsorption layer thickness in alkaline environments is discrete.

The surface of deep coal rock is weakly electrically charged (Fig. 4), and the absolute value of Zeta potential in neutral deionized water is less than 1 mV. When pH=2, due to H⁺ and charge transfer, the coal rock surface has a weak positive charge, the Zeta potential is 0.67 mV and the isoelectric pH is 3. In alkaline solution, the rocks attract OH⁻ to form a partially hydroxylated surface with negative electricity, which is more obvious when pH equals 11 to 12, and the Zeta potential drops to −9.34 mV. When deep coal rock comes into contact with hydroxypropyl guar gum fracturing fluid and polyacrylamide fracturing fluid, some of the thickener molecules are adsorbed on the surface of the coal rock, making it more difficult to ionize hydroxyl groups and inorganic minerals. The Zeta potential decreases to -1.88 mV and -2.25 mV when pH is 11-12. The hydroxypropyl guar gum-coal rock isoelectric point pH is 3, and the polyacrylamide-coal rock isoelectric point pH is 4.4. There is no significant change in Zeta potential under neutral and weakly alkaline conditions. Therefore, it can be judged that electrostatic force is not a major factor in the adsorption of thickeners by coal rocks.

The types of functional groups in deep coal rock are complex, and the adsorption peaks of FTIR show multi-peak characteristic (Fig. 5). Since the test atmosphere of WQF520 spectrometer is air, there is no potential peak of the sample near the CO₂ stretching vibration peak, which has no effect on the experiment. The chemical bonds of hydroxypropyl guar gum are mainly C-H and C-C of polysaccharides, and there is obvious O-H stretching vibration and C-O stretching vibration peaks. No new absorption peaks appear in coal sample adsorbed with hydroxypropyl guar gum fracturing fluid, indicating that no new functional groups or chemical bonds are formed after the adsorption of hydroxypropyl guar gum in coal rock. However, the O-H stretching vibration peaks at wave number 3 500-3 700 cm^-1 almost disappear. It suggests that after adsorption, the hydroxyl groups on the surface of deep coal rock may undergo condensation polymerization with hydroxypropyl guar gum to form C-O-C like ether bond. Alternatively, the presence of intermolecular hydrogen bonds attenuates the O-H vibration.

Polyacrylamide functional groups are mainly hydroxyl and amide groups. Typical C-N stretching vibration, N-H bending vibrations, and C-O stretching vibrations were detected by FTIR. Hydrocarbon C-H bending vibration peaks and O-H stretching vibration peaks were present (Fig. 6). These are the characteristics of the amide group. No new absorption peaks appeared in coal rock after adsorption of polyacrylamide fracturing fluid, preserving the complex multi-peak characteristics of the fingerprint region. It indicates that no new functional groups or chemical bonds were produced after the adsorption of polyacrylamide in coal rock, and no condensation polymerization occurred.

Intermolecular forces affect the electron binding energy through electron cloud interactions. There is no significant difference in the material composition of the coal rock before and after adsorption (Fig. 7).

Three spectral peaks corresponding to C-C, C-O, and C═O bonds can be observed in the C1s band (Fig. 8). The adsorption of hydroxypropyl guar gum in coal rock reduces the binding energy of the three spectral peaks by 0.3-0.4 eV. Hydroxypropyl guar gum adsorption attracts C1s electrons through intermolecular forces, thus weakening the bond strength of C1s electrons to the nucleus. After adsorption of polyacrylamide in coal rock, the amide group increases the proportion of C═O bonds in the C1s spectrum from 3.89% to 33.82%. The C═O and C-O binding energies decrease by 0.3 eV and 0.1 eV, respectively. Polyacrylamide likewise attracts the C1s electrons through intermolecular forces, reducing the binding energy.

Two spectral peaks of C-O bonds and C═O bonds can be observed in O1s band (Fig. 9). After adsorption of hydroxypropyl guar gum and polyacrylamide in coal rock, the spectral peaks are hardly shifted, and the adsorption increases the width of the peaks to different degrees. It indicates that the electron binding energy of the oxygen does not change significantly after adsorption of fracturing fluid thickener.

Fig. 10 shows the pore radius distribution measured by nitrogen adsorption of 250-425 μm coal rock particles. It can be seen that the pores with radius of 1.0-7.5 nm account for 71% of the nanopore volume. Fig. 11 shows the nuclear magnetic resonance T₂ spectra of two coal rock cores DC-7 and DC-8 before and after fracturing fluid flowback by gas displacement. The DC-7 core has a porosity of 2.09% and a permeability of 0.87×10⁻³ μm²; the DC-8 core has a porosity of 1.38% and a permeability of 0.24×10⁻³ μm². As can be seen from Fig. 11, deep coal rock mainly develops nanopores and microfractures. After the fracturing fluid flowback by gas displacement, the fracturing fluid is mainly retained in the pores and fractures with radius of 1-30 nm and 0.1-0.74 μm. Reducing the adsorption capacity of thickener and the thickness of the adsorption layer can mitigate nanopore and microfracture damage and facilitate the production of coal rock gas ^[21].

The adsorption of pulverized coal to thickener has potential solid-phase blockage damage. Yang et al. ^[12] suggested that the adsorption of polyacrylamide by coal particles with diameter less than 150 μm can reach 38 mg/g, which is much higher than the experimental result of 3.29 mg/g in this paper. The experiments in this paper used pulverized coal rock with diameter of 425-850 μm. The specific surface area of 150 μm coal particle is about 8-32 times of that of the pulverized coal used in the experiments in this paper. The increase in specific surface area significantly increases the adsorption capacity of polyacrylamide in pulverized coal. In the process of coal rock gas development, pulverized coal sheds and migrates, and then is produced together with coalbed methane. The diameter of the produced pulverized coal is generally 45-150 μm ^[22], the specific surface area is also much larger than the pulverized coal rock in this paper. Therefore, the adsorption capacity of thickener on the surface of pulverized coal in fracturing may be higher than the experimental results.

Microfractures are developed in the deep coal rock (Fig. 12a, 12b), with width generally ranging from 4 μm to 74 μm ^[23], and they are the main gas migration channels. After the fracturing fluid intrusion and flowback, high molecular weight polymers along with the organic components of coal rock will be adsorbed on microfracture wall by functional group polymerization, and intermolecular forces. This will block seepage channels and reduce coal rock permeability (Fig. 12c). In addition, the polymer thickener adsorbs on the surface of pulverized coal and adheres the pulverized coal to the fracture wall. This intensifies the sedimentation of pulverized coal during migration, blocking the seepage channels (Fig. 12d).

Compared with the middle/shallow burial depth coal, the deep coal rock reservoirs have high gas saturation, high formation water salinity, and extraordinarily low porosity and permeability. Compared with shale and tight sandstone reservoir, deep coal rock has high fracture density, high organic fraction, and low mechanical strength. Therefore, nanopores and microfractures are the main channels for gas migration in deep coal rock reservoirs. During development, pulverized coal easily sheds and migrates, with potential permeability damage. Additionally, in the process of hydraulic fracturing to create fractures, the factors such as blockage of gel breaking residue, solid phase blockage of pulverized coal, liquid phase retention, and polymer adsorption, are superimposed on each other, leading to a decline in the permeability of some fractures and making it difficult to recover, restricting the efficient development of deep coal rock gas.

In order to reduce the potential damage to microscale and nanoscale pores and fractures during the development, 4 countermeasures are proposed with full consideration of the adsorption mechanism and the factors affecting the adsorption layer of the fracturing fluids in deep coal rock. (1) Polymer molecules undergo curling and sedimentation when they come into contact with highly mineralized formation water ^[24]. Reasonable reduction of thickener mass fraction can effectively decrease the thickness of adsorption layer and control the damage. (2) Modify the hydroxyl group of thickener to an inactive group, so as to maintain its water solubility and reduce the adsorption capacity of hydroxyl group by condensation polymerization. Alternatively, acidification can be used to break some of the functional groups of coal rock, so as to reduce its adsorption capacity ^[25]. (3) Apply oxidative thermogenesis gel breakers in fracturing fluid. On the one hand, it promotes polymer degradation. The decomposition of aliphatic hydrocarbons and benzene in coal produces a large amount of CO₂ ^[26] and promotes the desorption of adsorbed methane, while increasing the permeability of the coal rock and promoting diameter reduction of pulverized coal ^[27]. On the other hand, it raises temperature, intensifies molecular motion and reduces van der Waals forces to decrease adsorption capacity. (4) Develop low molecular weight desorption agent, which can prevent adsorption of fracturing fluid. Small molecules containing reactive functional groups, like hydroxyl and amino groups, can decrease the adsorption sites for condensation polymerization of oxygen-containing functional groups in coal rock by competitive adsorption. In addition, aromatic hydrocarbons are able to form tight conjugate with the coal rock to prevent the adsorption of polymers ^[28].