Study On The Seepage Characteristics Of Pulsing Water Injection Into Coal Seam And Main Effective Factors

Coal seam water injection technology remains one of the most significant technology for caol mine dust prevention and control, coal spontaneous combustion and has been widely used for now. As a new technique,pulse water injection technology has showed its merits to improve water injection efficiency in application compared with conventional method. In order to reveal the mechanism of the technique and improve water injection efficiency, both experimental and simulational methods were conducted to study on the seepage law of pulse water injection into coal seam, above that factors concerning pulse water injection efficiency were in ways of injection parameters and coal seam characteristics.Firstly, depending on the generation of pulsive water, the essential difference between pulse water injection technique and conventional one is whether the flow rate is constant or cyclically changes, based on which pulsating parameters were defined. Then experiments on gas and water seepage values and relationship between effective porosity and injection water pressure were carried out, above that a simulation model of pulse water injection into coal seam with gas was established. With a secondary programming to the FLUENT, outcomes containing dynamic pressure response curves indicated that the process of pulse water injection could divided into four stages, initial pressure stage, slow booster stage, rapid booster stage, and stable pressure stage, the peocess was gradually transformed from a unsteady two-phase flow to a steady single-phase flow.Secondly, according to the results of simulation, the mechanism of pulse water injection was analysed from the viewpoint of pressure and energy, its advantages over conventional one lays on it could generate greater pressure peak with the same flow water rate, and to centralize the discreted energy, enhancing the transient ability to break coal seam. The laws of gas and water migration seemed to be complicated in detail, in general, showed a process of gas taking by water gradually, cracks have a significant influence on the process, matrix surrounded by cracks showed an increasing of gas content.Then pulsating water injection peak pressure and pressure pulse rate were proposed acting as the indicators of injection. While the pulsating frequency increases from 1Hz to 10 Hz, both pulsating water injection peak pressure and pressure pulse rate showed obvious decrease. While average flow rate increases from 0.00002m/s to 0.00014m/s, time required drastically reduced with pulsating water injection peak pressure and pressure pulse rate sharply raise. Free gas pressure mainly affects pressure pulse rate, when gas pressure raises from 0.2MPa to 1.5MPa, the pressure pulse rate goes down from 0.50 to 0.20.Finally, experiments showed that stress state had a significant influence on crack volume, whatever the system was open or closed, the increase in stress state would result in the loss of crack volume, thus increasing the attenuation effect of pulsating pressure wave and decresing water content of coal seam after injection. The seepage effect and stress state have a coupling relationship and stress decreases after the injection,the value is less than the original coal seam stress.To optimize the parameters of pulsating water injection, reduce pulsating water frequency, inprove the displacement of pulsating pump if possible, take measures of preliminary drainage of gas before water injection and reduce the stress state are proposed to be able to improve pulsating water injection efficiency.