| China is rich in tight gas reserves.Many research results show that the main continental facies in Northeast Sichuan has good preservation conditions and oil and gas display is active.This block is tight sandstone reservoirs with small porosity,low permeability and strong heterogeneity.It is difficult to achieve the effect of industrial production,and the economic benefit is very low.Therefore,it is very necessary to use volume fracturing technology to artificially transform the reservoir.Aiming at the continental tight sandstone reservoir in Northeast Sichuan,in order to clarify the formation law of its complex fracture network,this paper carried out hydraulic fracturing tests with different pumping rates by using the indoor large true triaxial rock fracturing microseismic test system,and added acoustic emission positioning monitoring equipment to study the evolution characteristics of rock fractures in the fracturing process.Before and after the fracturing,sound wave test and surface fracture measurement are carried out on sandstone samples to analyze the changes of sound wave characteristics and fracture morphology.Finally,combined with numerical simulation results,the propagation pattern and regularity of hydraulic fractures are determined.The specific research contents and results are as follows:(1)Through the analysis of formation fracture pressure theory and hydraulic fracture initiation and propagation theory,it is predicted that the theoretical value of fracture pressure of tight sandstone will reach 78.55 MPa and the hydraulic fracture stable propagation pressure will reach 80.575 MPa.Through X-ray mineral composition analysis,the brittleness index of tight sandstone is 64.The brittleness grade is “better” by using brittle mineral method.(2)The true triaxial hydraulic fracturing test combined with acoustic emission positioning system of 20 channels to study the fracture of tight sandstone with pumping rate of 100 m L/min 、 200 m L/min and 300 m L/min.The results of wellbore pressure curves,acoustic emission location maps and acoustic emission parameters diagrams show that under the condition of large pumping rate of 300 m L/min,the fracture initiation and peak pressure of wellbore are high,the acoustic emission signals are more,the amplitude is larger,the released energy is the most,and the fracturing effect is the best.(3)Wave velocity test of tight sandstone samples before and after fracturing test is carried out in this paper,and the macro changes of sandstone caused by hydraulic fracturing are explored through the changes of S-wave and P-wave at each point,surface and the whole of the sample.The test results show that the most wave velocities of the samples after fracturing are smaller than that before fracturing.In addition,the rock mechanical parameters such as dynamic elastic modulus and dynamic Poisson’s ratio are obtained through wave velocity.It is found that the dynamic elastic modulus generally decreases and the dynamic Poisson’s ratio generally increases after fracturing,which reflects that the plasticity of rock increases and the elasticity decreases.It is also found that the dynamic elastic modulus increases with the wave velocity.At the same time,using the elastic constant method of rock mechanics to evaluate the brittleness of tight sandstone samples,and the three samples are in the “medium” brittleness level after fracturing,indicating that hydraulic fracturing has caused serious damages to the interior of rocks.(4)Through the fracture plane expansion pictures of tight sandstones,the shape of the main fracture is observed,the length and angle of each new fracture is measured,and the trend and development degree of the fractures are analyzed.It is found that the main fractures of the three samples after fracturing are in the direction of maximum or minimum principal stress.Besides,the new secondary fracture branches appear on the basis of the main fracture and natural fracture.The new fractures mainly develop along the north-south and east-west directions,and show a centrosymmetry law,which is consistent with the acoustic emission positioning information.By calculating the fractured rate,it is found that the samples after fracturing have reached degree of “good development”.(5)The PFC2 D software is used to establish a hydraulic fracturing numerical model to analyze the impact of different pumping rates on the fracturing effect.The simulation results show that the pumping rate is very important to the direction,shape and quantity of fracture propagation.The numerical simulation and experimental analysis results are consistent.The generated fractures are mainly tension fractures,which accounting for more than 80 % of the total fractures.The pumping rate increased from 100 m L/min to 300 m L/min,and the total number of fractures increased nearly 10 times. |
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