Study On The Law And Mechanism Of Seepage Change Of Shale Gas Under Ultrasonic Action

With the decrease of exploitable conventional oil and gas resources,unconventional oil and gas resources such as shale gas take up the important position in North America,recently.Currently,shale gas has become the major source of natural gas in America.In China,there are considerable reserves of unconventional oil and gas resources.However,the reservoir of shale gas is commonly low porosity and low permeability,in which the conventional production techniques are hardly to meet the requirement of economic efficiency.As a major enhanced technique,hydraulic fracturing technique is worldwide applied in the development of shale gas,which not only could increase the total annual output of shale gas effectively,but also could bring huge economic benefits.Conversely,the environment problems causing by hydraulic fracturing technique emerge in endlessly.Thus,the development of a low environmental cost advanced enhanced technique is a necessary way for the sustainable development of unconventional gas reservoirs.Basing on the adsorption-desorption and seepage properties of shale gas,the enhancement of output of shale gas was proposed by using the enhanced effect of ultrasonic wave in this work.The research method mainly combines theory with experiment.On the one hand,self-designed equipment was employed to measure the ultrasonic attenuation effect in shale,the adsorption-desorption properties of shale gas and the seepage properties of shale gas under the effect of ultrasonic wave.On the other hand,based on shale gas quasi-steady state desorption model,the seepage model was established considering the effect of acoustic field,temperature field,seepage pressure field and stress field.The detailed results are listed as following:(1)The attenuation effect of ultrasonic wave in shale was conducted and the results showed that the propagation distance of ultrasonic wave was more than 1.2 m in shale.(2)By a self-designed experiment device,the adsorption and desorption properties of shale gas were measured with or without ultrasonic effect.The experimental results indicated that the absorption of gas under ultrasonic is reduced.The experimental data were fitted out by Langmuir adsorption isotherm equation,where the Langmuir adsorption constant decreases gradually as the increase of ultrasonic wave.Moreover,the desorption amount of shale gas increased under the effect of ultrasonic wave,and the desorption rate was faster obviously.When the power of ultrasonic wave was set on 50 W,the desorption amount of shale gas increased to 24%.When the power of ultrasonic wave was set on 110 W,the desorption amount of shale gas increased to 31%.In general,the adsorption and desorption properties of shale gas under ultrasonic wave can still be described by the Langmuir isothermal adsorption equation.(3)By a self-designed experiment device,the seepage properties of shale gas were measured with or without ultrasonic effect.The experimental results indicated that the permeability of shale gas under ultrasonic is increased.Under the different condition of ultrasonic wave power,the increase of shale gas permeability was at the range of 10～40%.Moreover,the effective stress has a power function relation with permeability.The experimental results were fitted out and the linear relationship between ultrasonic sound intensity and permeability was obtained.Furthermore,the calculation model of shale permeability is deduced.(4)Based on shale gas quasi-steady state desorption model,the multi-field coupled seepage model was established considering the effect of acoustic field,temperature field,seepage pressure field and stress field.Finite difference method was used to treat the model in discretization,the initial and boundary conditions were discussed from the development of shale gas and then MATLAB was employed to solve this model The effect of ultrasonic on the parameters of shale gas seepage was analyzed and the mechanical vibration and thermal effect of ultrasonic wave was analyzed to understand the effect mechanism for enhancing the seepage of shale gas in depth.