| Hydraulic fracturing technology is widely used in the development of conventional and unconventional oil and gas resources.Among them,the evaluation method of tight reservoir compressibility is particularly important for the optimization of reservoir fracturing horizon and the improvement of reservoir productivity.The lithology of S block in the central depression area of Songliao Basin is siltstone and argillaceous siltstone.The reservoir property is poor,the permeability is low,and the argillaceous content is high.It is urgent to improve the reservoir productivity by reservoir fracturing technology.Therefore,on the basis of previous studies,taking the tight sandstone samples of Quantou Formation in Well H-2 and H-7 in the study area as an example,the basic mechanical experiments of the rock samples in the study area are carried out to obtain the basic mechanical characteristics of the rock in the study area,and the mineral composition characteristics of the study area are obtained by X-ray diffraction experiments.Then,based on the previous research results of brittleness index evaluation method combined with the experimental data of the study area,the comprehensive brittleness index evaluation method based on energy is selected for evaluation.Then,according to the hydraulic-mechanical-damage coupling numerical simulation method,a field scale numerical model is established to analyze the influence of injection rate and brittleness index on the initiation and propagation of hydraulic fractures.Then,based on the basic mechanical characteristics of rock samples in the study area,combined with the geological survey of tight sandstone reservoirs with high fracture angle in the study area,a large-scale physical model was prepared for true triaxial hydraulic fracturing experiments to analyze the influence of different natural fracture angles and horizontal stress differences on fracture initiation and propagation.Finally,based on the analysis of the influencing factors of tight sandstone reservoir compressibility evaluation,and according to the rock reservoir factors and geological factors,the rock brittleness index,mineral composition,natural fracture index and horizontal stress difference coefficient are selected as the reservoir compressibility evaluation indexes.The weight of each influencing factor is determined by the analytic hierarchy process entropy method,and the compressibility evaluation method of tight sandstone reservoir is established.The reliability of the compressibility evaluation results is verified by the field experiment results,which provides technical support for the subsequent reservoir fracturing development and transformation.The study concluded that :1.Through the uniaxial and triaxial compression experiments,Brazilian splitting and other basic mechanical experiments,it is found that the elastic modulus,Poisson ' s ratio,tensile strength,stress-strain curve and deformation characteristics of the horizontal and vertical directions of the reservoir in the study area are anisotropic.Through X-ray diffraction experiments,it is found that the quartz content of the rock in the two wells of tight sandstone is low,with an average of 23 % and 24.4 %,respectively.From the mineral component content,it can be preliminarily judged that the core brittleness is weak.The calculation results of brittleness index based on energy show that the brittleness of cores from two wells shows a downward trend with the increase of confining pressure,which is consistent with the test results.In the absence of confining pressure,the horizontal brittleness of reservoir rock is higher than that in the vertical direction.However,with the increase of confining pressure,the rock brittleness in the two directions decreases and the difference decreases gradually.2.Based on the previous research results of energy brittleness index and the experimental data of the previous study area,the field-scale numerical model is established by RFPA numerical simulation software on the basis of the hydraulic-mechanical-damage coupling numerical simulation method.The influence of injection rate and brittleness index on the initiation and propagation of hydraulic fractures is studied.The simulation results show that with the increase of injection rate,the fracture morphology after compression is more complex,and the final fracture network is more complex.However,when the injection rate increases to14 m3 / min,the improvement of fracturing effect by increasing displacement is no longer obvious.With the increase of horizontal stress difference,the initiation pressure increases slightly.With the increase of rock brittleness,the initiation pressure of hydraulic fractures will be significantly reduced.Under the condition of low-level stress difference,the hydraulic fracture shows a turning propagation mode in the extension process.Moreover,with the increase of reservoir rock brittleness,the turning extension degree of fracturing fractures is more obvious,and finally a more complex fracture morphology is formed.When the stress difference increases to 5MPa,the degree of fracture extension is weakened,but for the high brittle reservoir,it still forms a more complex fracture morphology.When the stress difference is high,the degree of steering extension of fracturing fractures in low brittle reservoirs is low,and the fracture morphology is relatively simple.Although the fracturing fractures in high brittle reservoirs still have the behavior of steering extension,the complexity of the final fracture is significantly reduced compared with that under low stress difference conditions.3.Based on the basic mechanical characteristics of the study area,combined with the geological survey of tight sandstone reservoir with high fracture angle in the study area,a largescale physical model is prepared for true triaxial hydraulic fracturing experiment to analyze the influence of different natural fracture angles and horizontal stress differences on fracture initiation and propagation.The experimental results show that when the angle between the initiation direction of the hydraulic fracture and the natural fracture is 60°,the fracture extension mode is the most complex,and the hydraulic fracture has multi-directional branch cracks.When the horizontal stress difference is 5MPa,the fracture morphology is more complex and the fracturing effect is better.When the horizontal stress difference is too small or too large,the fracture extension form is relatively single,which is not conducive to the improvement of hydraulic fracturing effect.4.According to the influence law of physical and mechanical characteristics obtained by geological factors and basic mechanical experiments on fracture initiation and propagation,combined with the general situation of the study area,rock brittleness index,mineral composition,natural fractures and horizontal stress difference are selected as reservoir compressibility evaluation indexes.The parameter values of each evaluation parameter are obtained,and the weights of rock brittleness index,mineral composition,natural fracture and horizontal stress difference are calculated as 0.48,0.30,0.15 and 0.07 according to the analytic hierarchy process entropy method,so as to establish the compressibility evaluation model of tight sandstone reservoirs and carry out practical application.The reliability of the compressibility evaluation method is verified. |
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