Feasibility Study On Ultrasonic Enhancement Of Coalbed Methane

The coalbed methane,as an unconventional energy source,has been highly valued in recent years.Our country's high rank coal reservoir is rich in resources.But it has the characteristics of low permeability,low pressure and less saturation,which seriously restricts the single well productivity of coalbed methane wells.Therefore,it is imperative to put forward a reasonable way to increase production in coalbed methane wells.At present,the main ways to increase the production of coalbed methane include repeated fracturing,gas injection,multi-layer mining,negative pressure drainage,deployment and optimization of well network.Ultrasonic stimulation technology is a new physical exploitation technology developed in recent decades.Compared with other ways of increasing production,it has the advantages of no pollution to the reservoir,little constrained by reservoir conditions,simple equipment and low cost.The main mechanism of ultrasonic wave includes mechanical vibration effect,thermal effect and cavitation effect.Studies have shown that the mechanical vibration effect and thermal effect produced by ultrasound can effectively improve the permeability of coal reservoirs,while the thermal effect increases the reservoir temperature,thereby promoting the desorption of adsorbed gas on the coal matrix surface.In the process of the transmission of ultrasonic wave in coal reservoir,the energy will decay.In this paper,the relationship between attenuation coefficient and water saturation of different ultrasonic frequencies in water bearing coal is determined through specific experimental equipment,and a fitting function relation between attenuation coefficient and water saturation is obtained.The action radius of ultrasonic wave in the coal reservoir is obtained by taking the attenuation coefficient of ultrasonic wave at different frequencies into the energy attenuation formula.The flow and output of coalbed methane is a complex process.Combined with the experimental results of existing ultrasonic mechanism,this paper get the dynamic relationship of physical parameters of coal reservoir after ultrasonic action.Based on the mechanism of coalbed methane production and the principle of action of ultrasound,this paper deduces the multifield dynamic coupling mathematical model of gas and water two-phase under the action of ultrasonic wave,and gives the conditions for the solution of each field.Finally,the COMSOL Multiphysics multiphysical field coupling solution software is used to solve the model,and the influence of different parameters on the numerical simulation results is analyzed.The results show that the attenuation coefficient of ultrasonic wave in coal under different water saturation is different.Its value increases firstly and then decreases with the increase of water saturation.The two relations can be fitted to express two times function expression.When the measured attenuation coefficient is introduced into the ultrasonic energy attenuation formula,we can see that the attenuation degree of ultrasonic energy in saturated coal is slower than that in dry coal and the energy can spread farther.The action radius of ultrasonic wave at different frequencies is different in coal,the higher the frequency,the greater the attenuation and the smaller the radius of action.According to the calculation results of the model,it is known that the temperature of coal reservoir after the action of ultrasonic will rise to some extent,and the range of the effect of ultrasonic heating can be obtained.It is known that the action range of ultrasonic is not limited to the reservoir near well area.Comparing the effect of ultrasonic wave under different sound field intensity,we draw out the distribution nephogram of coal reservoir pressure,temperature,permeability and gas saturation under different intensity of sound field,we can see the obvious change after ultrasonic action.Compared with a certain period of time,the production variation rule of coal reservoir gas well after ultrasonic action with different sound field intensity and acoustic frequency can be seen that the daily gas production and cumulative gas production of the gas well have been increased to a certain extent,and the effect is obvious.