Single borehole reflection imaging of array acoustic logging has increasingly attracted the attention of geoscientists due to its ability to analyze the reservoir
around a borehole in depth.However,the conventional method mainly focuses on using the reflection events in an image to evaluate the formation,but the distribution of fluid and lithology cannot be revealed by the data.Therefore,a novel approach is needed for providing more detailed imaging information of reservoirs.Accordingly,in this study,we designed a new borehole reflection imaging method providing deep investigation and high resolution (BRIDI) based on the array acoustic logging data,which can be used to evaluate reservoirs far from the borehole.The BRIDI method uses the Shearlet transform to extract
the reflected wave from the logging data by eliminating the slide wave.We proved that this method of extracting reflections can considerably reduce energy loss compared with common methods such as Radon transform,wavelet transform,and the median filtering algorithm.During drilling,the fluid invasion and the induced crack may change the wave velocity of the formation around a borehole.The velocity profile near a borehole can be obtained through tomography and the arrival time of waves.With the reflections and velocity profile,we applied the optimized migration method to enhance the quality of imaging results.These images directly showed the geo-interface,fractures,vugs,distribution of fluids,and lithology changing.Based on the results,we were able to analyze the mesoscale geostructure near a borehole and observe the reservoir and fractures.By applying BRIDI for processing real logging data from different oil fields in China,we found that its investigation depth is about 30m and its resolution is approximately 20 cm.In the images,we observed that the features of the reflection events were different with different reflectors.We could directly observe the distribution of fluids and changes in the reservoir due to hydrofracturing so that the seismic events could be more accurately understood with the analysis of the united logging-seismic imaging data.We verified the validity of the index of gas productivity through a test with a single cluster perforation,which demonstrated the value of applying BRIDI for reservoir evaluation.However,three aspects require further development to improve the performance of the proposed method in real-life applications.The first is obtaining more key information that can be used to image the distribution of the fluids and azimuth of fractures and vugs in the reservoir,which will help to determine the direction of hydrofracturing in petroleum exploitation.The second is studying the three-dimensional imaging techniques used for reflectors
and lithology through the comprehensive use of the reflection waves recorded in the azimuthal receivers,to design a powerful tool for the exploration of oil and gas in the deep underground.The third is developing the adaptive filtering algorithm based on the velocity profile to eliminate the reflections transported along the wall of boreholes:because these waves are reflected by the fractures and geo-interfaces crossing the borehole,they contain no reservoir in formation and seriously disturb the imaging results.
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