Research On The Application And Evaluation Of Cross-dipole Acoustic Logging To Interpretation Of Tight Reservoirs

Along with the deep going study of exploration and development of oil and gasfields, the target of exploration and development shifted from structural reservoirs tolithologic reservoirs.The unconventional oil and gas reservoirs with low porosity, lowpermeability and low saturation have been one of the most important oil and gasresources in the world. In the past ten years, we have made a great development in theexploration and development of tight gas reservoirs.Until now Sugeli gas field was the biggest onshore gas filed with the world-classreserves in China. It’s a typical lithologic trapping reservoir with low porosity, lowpermeability and low saturation. Its complex logging responses caused by featuressuch as thin beds, strong heterogeneity, the large variation of physical property andcomplex relationship among fluids lead to the difficulty of calculating porositypermeability and saturation accurately. The reservoirs couldn’t be evaluatedeffectively with conventional methods.There are tight sandstone and tight conglomerate reservoir existing in the deeplevel of southern Songliao basin.Tight reservoirs with low porosity, low permeability and strong heterogeneityhave complex pore structure which influences the resistivity more than the type offluids in reservoirs. So there is no obvious difference between the gas and water onresistivity log. Therefore it’s hard to recognize the type of fluid with conventionalmethods by analysing resistivity. Fortunately, when the reservoir includes gas，thereare significant changes in the acoustic velocity, acoustic amplitude and somedynamics of rock which make the gas horizon apart from the oil and water throughanalysing acoustics information.In the paper, we took advantage of the array sonicdata to identify the gas reservoir qualitatively and evaluate it quantitatively, whichcontributes to the improvement the accuracy of log interpretation. As a new means of logging, array acoustic logging contains much moreinformation than the old acoustic logging. In this paper, we mainly took thefollowing work:calculating the anisotropy，dividing the shale volume，identifingthe fluid type，quantitatively calculating the gas saturation，replacing the fluids andso on. We found an appropriate method of log interpretation for the tight sandstonereservoir of Sugeli gas field.The completed work included the following parts:1. Processing cross-dipole acoustic logging data and extracting slownessPreprocessing original DSI data includes gain processing, filtering andequalization processing. A mathematical model was set up for extracting the full waveslowness separately in the time domain and the frequency domain.The time-slowness correlation method was adopted in the time domain. Phase method was usedin the frequency domain. We could find that the approach is of correctness andreliability after compared our results with the software processing results.2. Inverting the anisotropy of shear wave with differential evolution algorithmAnisotropic inversion of cross-dipole logging data is an important technique inmodern acoustic logging. We established the differential evolution algorithm on thebasis of waveform inversion objective function.The relative error of results waswithin5%between this algorithm and the simulated annealing treatment.However，the processing speed is faster than the simulated annealing by above20%.3. Summarizing the characteristics of the logging response and the loggingevaluation model in Sugeli gas field，the tight sandstone gas reservoirs.4. Studying the fluid replacement of tight sandstone reservoirsWe studied fluid replacement of tight sandstone reservoir and predicted thechange of elastic parameters caused by the change of fluid. Forward simulation ofcompressional wave velocity and shear wave velocity of rock under different fluidstates lay the foundation for the fluid identification and the AVO effect of seismicexploration.5. Identifying fluid based on the elastic modulus and the wavelet transformWe took advantage of the data of P and S wave from array sonic and conventionallogging curve to recognize the fluid. The fluid was recognized by single parameter elastic modulus，two-dimensional，three-dimensional rendezvous analysis techniquesand wavelet energy spectrum analysis technology. In the wavelet energy spectrumanalysis techniques，we study the ratio of P and S wave. We can process the wholewell continuously by combining with auto-layering technology instead of processinglayer one by one. The matching between energy spectrum matching and depth domaincame to realize. The results could be showed with two-dimensional visual.6. Evaluating laminated shale and dispersed shale quantitativelyWe divided the laminated shale using Tang’s method for the laminated shalewith macro-anisotropic, then, we calibrated it by the results of FMI. Finally， wedistinguished the structural shale from the dispersed shale with the Thomas-Stieberplates.7. Establishing the model for quantitative calculation of gas saturationBased on the Gassmann equation, we took advantage of the P-S slowness and thedensity logging to set up a model for calculating the gas saturation with elasticmodulus. It enriched the methods of evaluating the fluid in reservoirs with loggingdata.In this paper， we evaluated tight sandstone reservoir based on the array soniclogging data, and overcame the difficulties in the current evaluation technology. Anew method without resistivity to evaluate the tight sandstone reservoir was foundand proved well.