Research On The Evaluation Method Of Brittleness Of Shale Gas Reservoirs

In recent years,shale gas reservoirs have received active attention from various major energy countries and become the focus of unconventional oil and gas exploration and development.However,due to the low porosity and low permeability physical properties of shale gas reservoirs,exploration and development are difficult.As a common method of shale gas development,hydraulic fracturing has been widely used,but there are many factors that affect the effect of fracturing,such as rock brittleness,formation sensitivity and rock stress direction,making fracturing operations difficult to succeed.Therefore,the shale needs to be transformed.Studies have shown that the prerequisite for shale reservoir fracturing reconstruction is to conduct rock brittleness evaluation.At present,the brittleness index used to evaluate the shale compressibility mostly considers the mechanical properties before or after the peak,and the calculated brittleness index does not conform to the objective law that the brittleness decreases monotonously as the confining pressure increases.Based on the stress-strain curve of the rock,this paper analyzes the energy changes during the rock failure process,and uses the acoustic logging data combined with the mechanical experiment method to propose a new rock brittleness evaluation index.First,the ratio of the actual elastic deformation energy to the ideal elastic energy during rock compression is used to characterize the peak-front characteristics of the rock.The ratio of the energy provided by the outside world after the stress reaches the peak and the fracture energy of the rock fracture characterizes the peak of the rock After the characteristics,using the multiplicative synthesis method to combine the two,a brittleness index B that can comprehensively reflect the energy characteristics before and after the peak is proposed.After verification by examples,the brittleness index calculated using this index is consistent with the brittleness monotonous with the increase of confining pressure The objective law of diminishing.Secondly,the longitudinal wave time difference and the shear wave time difference are obtained from the sonic logging data,the dynamic elastic modulus and Poisson’s ratio are calculated by using theoretical formulas,and the static elastic modulus and Poisson’s ratio are obtained after fitting it.Peak strain and post-peak modulus fitting relationship.Finally,the rock mechanics experiment and logging technology are combined to perform laboratory experiments on seven rock samples to obtain elastic modulus and other parameters,and the brittleness index and logging data obtained directly from the rock mechanics experimental data are combined with the mechanical test method to obtain The comparison of the brittleness index verifies that the sonic logging data combined with the experimental mechanics method has high feasibility,can calculate the rock brittleness index more efficiently,quickly and conveniently,and can ensure the continuity of the obtained data,which can be accurate Effective hydraulic fracturing provides a reference basis for better use in actual projects.