Evaluation Of The Effectiveness Of Cracks In Tight Sandstone Formations Using Fast And Slow Shear Waves

In recent years,most of the large-scale high-porosity and high-permeability reservoirs have entered the production and development process,so the focus of oil exploration has shifted to unconventional reservoirs,and conventional logging interpretation methods have been very much interpreted in unconventional reservoirs.Large limits do not accurately describe the reservoir characteristics of a reservoir.As an important part of unconventional oil and gas reservoirs,tight sandstone reservoirs have become an important direction for the interpretation of tight sandstone reservoirs.A basin in an oilfield in western China studied as a typical compact sandstone reservoir with deep reservoir burial,high stratum temperature,complex lithology,diverse reservoir structure,poor physical properties and strong heterogeneity..At the same time,because the reservoir matrix permeability is low and the fractures are the main migration channels of oil and gas in the area,the evaluation of crack effectiveness has become a very important part of the logging interpretation process in this area.As we all know,electric imaging logging is an important logging method for the description of crack information.Due to its special geological features,the research area uses oil-based mud drilling in the area,which makes the effect of electric imaging logging.Subject to great restrictions.Although acoustic imaging is not affected by the nature of the mud,considering the resolution of the acoustic imaging log and its effects are susceptible to the borehole wall,it is not possible to obtain crack length,width,density and face rate information as in electrical imaging logs.Therefore,this paper considers the accuracy of crack validity interpretation by adding dipole sonic logging data.At present,many experts and scholars in China have made great progress in crack identification,fracture development and evaluation of fracture reservoir reliability through their research.The method of judging the anisotropy of the formation by using the fast and slow transverse waves to judge whether the crack is effective is already applied in the carbonate stratum,but the logging evaluation of the tight fractured sandstone reservoir is not reported by the fast and slow transverse wave anisotropy.The reason is that the anisotropy of the formation itself in the carbonate formation is not strong,so in the presence of cracks and caves,the value of anisotropy is greatly increased,and the development of the fracture can be judged.In carbonate rocks,the influence of formation matrix porosity on acoustic energy attenuation is relatively small.Acoustic attenuation is mainly affected by factors such as cracks,karst caves and wellbore collapse.The attenuation of acoustic energy can also reflect factors such as cracks.The situation also shows that the energy attenuation of the fast and slow shear waves has certain application value in evaluating the effectiveness of cracks.Therefore,it is feasible to evaluate the effectiveness of cracks in tight sandstone formations by the amplitude attenuation of fast and slow shear waves.Based on the results of the correlation between the shear wave energy attenuation coefficient and the crack width obtained by the core crack experiment,the fast and slow shear waves are taken into account,and the calculation formula of the crack characterization coefficient is obtained.By processing the cross dipole acoustic logging data,Fast and slow shear wave information,and then calculate the crack characterization coefficient.Comparing the results of electric imaging logging interpretation under water-based mud,it is found that the crack characterization coefficient is sensitive to cracks,and at the crack development,it is relatively high value;under oil-based mud conditions Compared with the results of acoustic imaging logging,it is found that the cracking coefficient shows a high value at the crack development.It is considered that the method has certain effect,and the array acoustic signal is deeper than the imaging logging method,which can be a certain degree.The effective crack development of the ground reaction provides a certain reference value for the evaluation of crack development under oil-based mud conditions.