Research On The Law Of Initiation And Propagation Of Rock Cracks Based On Acoustic Emission Technology

With the continuous development of oil and gas energy exploration,oil and gas reservoirs which are difficult to exploit,such as tight,low permeability and shale,account for an increasing proportion,and fracturing transformation can improve the development efficiency of oil-gas fields.Research on the law of rock fracturing initiation and propagation is of great significance to improve the effect of hydraulic fracturing.In this paper,the hydraulic fracturing tests of artificial rock samples are carried out by considering different factors,and the law of fracture initiation and propagation is studied by analyzing fracture pressure and fracture propagation time with acoustic emission technology.And with Gaussian Mixture Model(GMM),the influence of different factors on the types and proportion of fracturing fractures is studied.The main conclusions of this paper are as follows:(1)The fracture pressure of rock samples is mainly influenced by the viscosity of fracturing fluid.The higher the viscosity is,the greater the fracture pressure is.If the displacement of fracturing fluid increases,the differential horizontal principal stress decreases,the gravel content increases,the perforation density increases,or the number of perforation clusters decreases,the fracture pressure would increase,but the effect is not as great as that of viscosity.(2)The time spent for rock fracturing is mainly influenced by the number of perforation clusters.The number of perforation clusters is doubled,the fracturing time would be shortened by nearly a half.The increase of gravel content and viscosity of the fracturing fluid can also obviously shorten the fracturing time.The differential horizontal principal stresses and the perforation density have almost no effect on the fracturing time.The fracture pressure and fracturing time of rock samples are mainly influenced by the filtration effect of rock samples.Multiple fractures of rock samples can be realized by increasing the displacement stepwise or using higher viscosity fracturing fluid.(3)The fracture initiation location is only related to the location of perforation.In the early stage of fracturing,if the fracturing fluid displacement is small,the differential horizontal principal stresses are great,the viscosity of the fracturing fluid is low,the gravel content is high,and the perforation density is high,more micro-fissures would be produced.The energy of acoustic emission signal in the early stage of rock fracturing is negatively correlated with the number of acoustic emissions,and positively correlated with the initiation pressure of rock samples.(4)The acoustic emission signal analysis shows that rock fracturing fractures extend along the direction of the maximum horizontal principal stress,and the differential horizontal principal stresses strictly controls the fracture direction and the uncertainty of micro-fissures.When viscosity is low and displacement is small,the differential horizontal principal stress is small,the number of AE at the initiation point is small,the position distribution is concentrated,and the AE signal shows a banded distribution of micro-fissures expansion.The perforation density and the number of perforation clusters have little influence on the initiation and propagation direction of the fracture.(5)With the two-dimensional Gaussian mixture model based on the RA values and AF values of the acoustic emission signal,and taking RA and AF values as criterion,rock fractures can be divided into shear cracks,tensile cracks,as well as mixed cracks.(6)The viscosity and displacement of fracturing fluid have great influence on the number of pure shear cracks and mixed cracks.The gravel content has an obvious influence on the type of fractures.If the gravel content is too large or too small,there are mainly tensile cracks,and if the gravel content is moderate,there are mainly shear cracks.