| Coal seam water injection can increase the wettability of mining coal seam,which is an effective measure to solve the problem of gas outburst and improve the extraction rate of coal seam.However,the field investigation found that the degree of wettability improvement by water injection in some coal mines is far from expectations.One of the important reasons is that the geological and physical conditions of coal mines are complex.Simply through coal seam water injection can not solve the problem of gas outburst in some coal mining processes,which requires surfactants to improve the wettability of coal seams.But most coal mines in the western plateau,Huanghuai Plain and coastal areas of East China show high salinity characteristics while using surfactants to improve the wettability of coal seams,it is urgent to clarify the influence mechanism of surfactants on coal seam gas desorption under high salinity conditions.In this thesis,the OP-10 nonionic surfactant and related inorganic salt configuration of mineralized water are selected.,and taking the long flame coal of Liangjia Coal Mine in Yantai as the test carrier.The influence mechanism of coal seam salinity on the wetting effect of surfactant to improve coal seam and the characteristics of gas desorption in the process of coal seam extraction are studied.The main conclusions are as follows:(1)The fundamental mechanism of coal seam water injection and surfactant modification is expounded.Van der Waals force and hydrogen bond are the key factors for the stable adsorption of water molecules and the surface of coal matrix.After the water is immersed in the coal body,it gradually penetrates into the capillary with the pore of the coal seam from large to small.If the pore pressure cannot overcome the capillary force,it shows the water lock phenomenon of plugging the pore of the coal seam.The mechanism is mainly manifested as coal adsorption water,liquid phase retention and Jamin effect.The fundamental reason for the surfactant to improve the wettability of coal reservoirs is its special arrangement of bimolecular layers on the surface of coal matrix.(2)The basic physical parameters of coal samples were measured based on the coupling of salinity and surfactant.It was found that the adsorption constant of coal samples treated by mineralized water decreased by about 1.2693 cm~3/g,and the adsorption capacity of coal samples at high salinity decreased slightly.The initial velocity of gas emission from coal samples under coupling conditions was measured.It was found that the initial velocity of gas emission showed an overall downward trend with the increase of salinity,and there was no obvious law with the change of OP-10concentration,and it showed a trend of decreasing more at low surfactant concentration.(3)From the contact angle of coal sample,surface tension of reagent,functional group and phase composition under coupling condition,the change rule of wettability of coal seam under coupling condition is revealed.The contact angle measurement results show that the original contact angle of coal sample is 42.22°,and the improvement effect of OP-10 is between 69.35%and 91.78%.After adding mineralized water treatment,it is only 14.05%～34.37%,the salinity significantly reduces the wettability of the coal seam.The surface tension of OP-10 decreases with the increase of concentration,the lowest is 31.240 m N/m,and the surface tension of mineralized water changes slightly,the maximum is 73.913 m N/m.Fourier transform infrared spectroscopy was used to determine the functional groups of coal samples under coupling.The results showed that the hydrophilic functional groups disappeared and the hydrophobic functional groups were generated with the increase of salinity at the same OP-10 concentration.X-ray diffraction was used to determine the physical composition of coal samples under the coupling.The results showed that there was no significant change in the physical composition with the increase of salinity under the same OP-10 concentration,but the proportion of hydrophilic kaolinite components under pure OP-10 showed an increasing trend.The mechanism of the influence of salinity on the wettability of coal samples was explained from a microscopic perspective.(4)Using the high pressure-water injection-desorption instrument,the gas desorption characteristics under coupling were analyzed from the aspects of gas displacement,gas desorption,gas desorption rate and gas desorption rate.It is found that the process of water injection gas replacement is mainly concentrated within 15min after water injection,and the amount of gas replacement decreases with the increase of salinity at the same concentration of OP-10.By analyzing the gas desorption curve,it is found that although surfactants improve the wettability of coal seams,there are still different degrees of desorption hysteresis caused by water lock under normal pressure desorption.The desorption amount decreases with the increase of salinity,and the change rate is also decreasing,indicating that the inhibition effect of salinity on coal seam gas desorption has a limit value.The influence of salinity on gas desorption rate is very small,and there is an obvious convex peak in the rate curve under high gas pressure.This is due to the fact that coal seam gas and added water are more likely to enter the pores of coal matrix under OP-10 to improve wetting and high confining pressure,resulting in obvious liquid phase lag effect.The maximum gas desorption rate is introduced to characterize the gas desorption capacity of the coal seam.It is found that the gas pressure is inversely proportional to the maximum gas desorption rate,reaching the maximum at 2 MPa,indicating that the gas desorption is the most sufficient under the coupling condition of about 2 MPa pressure.OP-10 surfactant has the best improvement on the wettability of coal seam gas.(5)The influence mechanism of surfactant on coal seam gas desorption characteristics under high salinity is obtained.Firstly,the basic physical conditions of coal seam are changed by inhibiting the adsorption capacity and initial velocity of coal seam gas.At the reagent level,the contact angle and surface tension are increased to reduce the wetting effect of water injection.From the micro level,the disappearance of hydrophilic functional groups and the generation of hydrophobic functional groups are analyzed,but the mechanism of affecting the physical composition is subtle.Finally,based on the macroscopic characteristics of coal sample desorption,under the condition of high salinity coupling,it has the desorption characteristics of reducing the amount of water injection gas replacement,inhibiting the total amount of gas desorption,aggravating the water lock effect to a certain extent,and inhibiting the gas desorption rate as a whole.This thesis has 51 figures,17 tables and 110 references. |
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