Influence Of High Pressure With Fracturing Fluid On Microstructure Of Low-Medium Rank Coal Reservoir

Hydraulic fracturing is the main measure to increase production in coalbed methane industry in China at present.Under the action of high pressure injection in the fracturing process,the fracturing fluid is bound to cause damage to the coal reservoir near the fracture zone and affect the productivity of coal seam gas wells.Therefore,the study on the damage characteristics and control mechanism of high pressure fracturing fluid on coal reservoir is of great significance for the development of reservoir protection theory of coalbed methane and the development of a new type of low harmful fracturing fluid system.In this paper,low and medium-rank coal samples were selected.The coal sample was first cut into a coal column with a diameter of 25 mm and a height of about 50 mm by a sand line cutter,and then a simulated high-pressure fracturing fluid injection treatment experiment was carried out.and a series of experimental research and control mechanism were discussed for the changes of microstructure characteristics of coal before and after treatment.Using carbon dioxide adsorption,low temperature liquid nitrogen adsorption and other analytical techniques to compare the pore differences between the Hequ coal sample and the Liulin coal sample before and after the fracturing fluid treatment,According to the relevant data in liquid nitrogen adsorption,the fractal dimension of the coal sample is obtained,so as to obtain the influence of the fracturing fluid on the pores such as adsorption pores and seepage holes in the coal.FTIR,XRD,¹³C-NMR and other test methods were used to compare the microstructure changes of functional groups and crystallites in coal molecules before and after fracturing fluid.The fracturing fluid was explained by the influence of fracturing fluid on pores and coal molecular structure.The change of permeability and wettability;the change of pore structure of coal reservoir under high pressure and the change of surface properties of coal reservoir under fluid action to comprehensively discuss the effect of fracturing fluid treatment on coal reservoir adsorption and permeability mechanism.Based on the processing of the data obtained by the above test methods and the analysis of the experimental results,the following conclusions are obtained:（1）After the fracturing fluid,the total adsorption capacity of coal increases in the isotherm adsorption curve of carbon dioxide and liquid nitrogen;the area of the hysteresis loop in the liquid nitrogen adsorption desorption curve decreases,indicating the proportion of the fine bottleneck hole and ink bottle hole in the coal has dropped.The reason is that on the one hand,The high pressure effect makes the coal reservoir more dense,which leads to the decrease of the number of open pores.on the other hand,the high pressure injected water and pulverized coal remaining in the coal micropores block part of the pore channels.In addition,the fractal dimension of the coal sample decreases,indicating that the internal structure of the coal near the seam is more regular,which is consistent with the influence of compaction in the macroscopic sense.（2）From the test results of Fourier infrared:After the fracturing treatment,the substitution mode,oxygen-containing functional groups,aliphatic hydrocarbons and hydrogen bonds of the aromatic rings in the two coals have undergone a certain degree of change.the number of branches of the aliphatic chains in the two coals is reduced to some extent.It can be found from the infrared of the residual liquid that the characteristic absorption peak of the coal sample residual liquid changes greatly compared with the reference liquid distilled water.This is due to the hydrogen bonding between the oxygen-containing functional groups in the coal molecules and the water molecules,which causes the type and size of the hydroge bonds to change.In addition,the content of the hydrocarbon group in the liquid is increased as compared with the infrared spectrum of the remnant of the river and the residual liquid of the willow.（3）It can be seen from the crystallite structure parameters of XRD that the layer spacing d₀₀₂02 of the two coals is slightly reduced after the action of the fracturing fluid,and the diameter L_a of the aroma layer and the stacking height L_c of the layer are slightly increased.This is mainly due to the fact that under the action of high pressure,the coal reservoir is more dense and the structure of the coal molecules tends to be more oriented.From the XRD test results of unashed coal,it is known that the coal mainly contains minerals such as montmorillonite,kaolinite and anhydrite.After the fracturing fluid,the content of minerals in the coal decreases.（4）Through the ¹³C-NMR data processing and analysis of coal samples,the total carbon of sp² hybridization in the Hequ coal increased,and the total carbon of sp² hybridization in Liulin coal decreased.The total carbon content of sp³hybrids in the river is reduced,while the methyl content is unchanged,so the methylene or methine content is reduced.For Liulin coal,the total carbon of sp³hybridization increases,while the content of methyl group does not change.Therefore,the content of methylene or methine increases,which is consistent with the research results of infrared aliphatic hydrocarbons.（5）From the contact angle of the coal sample,it is known that after the action of the fracturing fluid,the contact angle of the Hequ coal is slightly increased,and the contact angle of the Liulin coal is reduced,indicating that the fracturing fluid makes the meander coal hydrophobic and the Liulin coal hydrophilic.This is closely related to the content of carboxyl groups in the coal sample.（6）By testing the gas permeability of coal samples before and after fracturing,it can be found that:after fracturing action,the permeability of the two coal samples decreased,This may be related to the increase of the number of parallel plate-like pores closed at one end of liquid nitrogen adsorption and the decrease of FHH fractal dimension（compacting in the macroscopic sense）.