Fractal Characteristics Of Hydraulic Fracturing Cracks And Theoretical Analysis Of Wellbore Stability

The extraction of oil and gas reservoirs in low-porosity and low-permeability reservoirs usually relies on hydraulic fracturing technology,and the evolution law of hydraulic fracture seam network and its expansion pattern are crucial to evaluate the effect of hydraulic fracturing.At present,sandstone reservoirs in northeast Sichuan basin contain good gas,and sandstone reservoirs exhibit high density,low porosity and low permeability.It is necessary to predict and evaluate the hydraulic fracturing effect under different pumping rates.In this study,based on the mining demand of tight sandstone reservoir in the Qianfuya Formation in northeastern Sichuan,hydraulic fracturing of tight sandstone under different large pumping rates of 200 m L/min and 300 m L/min is carried out as theoretical guidance and support for actual mining.The following conclusions are obtained:(1)With the increase of pumping rate by 1.5 times,the acoustic emission total absolute energy raises about 1.8×10~9 aj,and the cumulative number of acoustic emission events increases by nearly 10,000,with more dense amplitude signals and wider distribution intervals,indicating that the rupture degree of the 300 m L/min large pumping rate specimen is more intense,resulting in more microfracture events and a more complex network of hydraulic cracks.(2)The acoustic emission RA-AF distribution shows that shear damage occurs in 200m L/min pumping rate specimen.300 m L/min pumping rate specimen is more likely to penetrate the primary and tensile fractures when compressed,which leads to the joint development of tensile microfractures and shear microfractures due to the influence of large pumping rate,resulting in a composite tensile-shear type damage.(3)The results of the box dimension fractal algorithm and the G-P fractal algorithm show that the minimum value of the fractal dimension of the surface hydraulic fracture increases by1.5 times of the displacement,and the maximum value increases by 0.1447,and the overall average value of the fractal dimension doubles,indicating that the morphology of the hydraulic fracture network is more complex with larger pumping rate.At the same time,the fractal dimension of the specimens on the time series of acoustic emission event rate decreases about 90%overall,the self-similarity is worse,and the rock samples show large scale damage and more severe fracture.(4)The numerical analytical model of poroelasticity around the wellbore under the initial reservoir stress is established,and the results show that the analytical model is in good agreement with the actual situation when fluid seepage is considered.The radial stress is equal to the borehole pressure at the well wall.The maximum tangential stress on the rock is in the same direction as the maximum principal stress,indicating that the hydraulic fractures expand mainly along the horizontal direction,which is the direction of the maximum principal stress.The radial displacement is overall one order of magnitude larger than the tangential displacement.Tangential displacements appear only on the diagonal,with the same magnitude but in the opposite direction.The stress and displacement distribution pattern of the overall range model is basically the same as that of the local model,and the results of the overall elastic solution around the well can be regarded as the extension and development of the local elastic solution.(5)The radial stress around the well perimeter in the injection flow stage is minimum at the wellbore and maximum at the boundary,while the tangential compressive stress changes in the opposite pattern to the radial stress.The radial and tangential stresses are symmetrical along the horizontal and vertical directions,and both have corresponding stress extrema distribution areas,the area range of which continues to expand in the direction of the wellbore as the discharge volume increases and remains basically unchanged after the wellbore pressure reaches its peak point until the end of injection flow stage,indicating that large-scale tangential stress mainly occurs around the wellbore during hydraulic fracturing,and the model boundary is more prone to severe rupture due to the radial stresses.