Study On Mechanism Of Shale Erosion By High Pressure Abrasive Water Jet

As an unconventional energy with abundant reserves,shale gas plays an important role in meeting the growing global demand for fossil energy.Shale gas is contained in tight and ultralow permeability shale reservoirs.Hydraulic fracturing is the key to increase production and development.Among them,the abrasive water jet erosion of shale to form a channel is very important to form a shale gas seepage channel between the shale reservoir and the hydraulic fracturing production casing.However,the fragmentation mechanism of shale under highpressure abrasive water jet erosion is not clear.In order to explore the crushing mechanism,the abrasive water jet erosion tests were carried out on three types of typical shales(siliceous shale,calcareous shale and carbonaceous shale)containing different mineral components,revealing the influence laws of jet parameters,abrasive parameters and rock properties on rock crushing,and clarifying the main controlling factors.In terms of research methods,acoustic emission is used to monitor the internal information of shale under abrasive water jet erosion in real time,and the damage characteristics of shale in different erosion periods are defined.At the same time,combined with the numerical simulation of liquid-solid two-phase flow,the coupled motion law of particles and liquid in the microporous channel is deeply analyzed.The specific research contents of this paper are as follows:(1)Using the control variable method,the abrasive water jet erosion test was carried out by changing the jet parameters,abrasive parameters and rock types respectively.It was clear that the shale breaking speed was the most sensitive to the abrasive size,followed by the jet distance,jet pressure and abrasive concentration.The efficiency of shale erosion was positively correlated with the jet pressure,abrasive size and abrasive concentration,and negatively correlated with the jet distance;Compared with quartz sand,garnet has obvious erosion and crushing advantages.With the increase of the content of brittle minerals in shale,the performance of abrasive water jet eroding broken rock is gradually improved.In addition,the abrasive water jet erosion models vp,l,da established by statistical analysis method can comprehensively reflect the impact of jet pressure,jet distance and abrasive particle size on erosion efficiency.(2)Combined with the acoustic signal waveform during abrasive water jet erosion and the micro morphology of shale profile after erosion,it is found that the acoustic signal generated by abrasive water jet impact can reflect the rock fragmentation mechanism,identify the lithologic differences of shale,and explore the relationship between shale mineral content and rock failure characteristics.In the process of abrasive water jet erosion,the siliceous shale and calcareous shale with high content of brittle minerals show the characteristics of tensile failure.The rock fragmentation is mainly particle peeling off,resulting in intergranular fracture,and the acoustic signal is continuous;The carbonaceous shale with high clay mineral content shows the characteristics of shear failure.The rock fracture mainly cuts the shale matrix,resulting in transcrystalline fracture,and the acoustic signal is abrupt.(3)By observing the profile morphology and end face characteristics of abrasive water jet cutting,this paper illustrates the difference of shale fragmentation between abrasive water jet perforation and cutting.It is found that the notch taper decreases with the increase of shale brittle minerals,while the roughness of cutting profile increases with the increase of shale brittle minerals.(4)Based on the experimental conditions,the numerical simulation method of liquid-solid two-phase flow was used to compare the flow field and particle distribution of abrasive water jet erosion channels under different jet parameters,abrasive parameters and channel parameters.The results show that the velocity of abrasive particles is positively correlated with the jet parameters,while changing the particle parameters has little effect on the fluid state,but increasing the diameter and density of abrasive particles can improve the erosion efficiency.In addition,with the increase of slot length and width,the distribution of particles in the slot will gradually become uniform,which will be conducive to cutting a stable slot.