Geophysical Technology Research On Shale Gas Sweet Spots Prediction Of FuLing Daanzhai Formation In Sichuan Area

As an unconventional resources,shale gas has been regarded as one of substitue resources in China.Now the researches of shale gas mainly focus on its forming geological conditions evaluation and prospective area selection.However,how to predict the "sweet spots" of shale gas,featured as a favorable gas region with higher porosity,higher TOC,fracture zone,as well as better rock brittleness,and well preserved with moderate buried depth in a certain thickness and area,by using geological,petrophysical,geophysical and experiment test data is a brand new task for us to work on.The paper is subjected to the project of Daanzhai shale gas in Fuling area,Sichuan basin.Firstly,we analyzed the mineral components of core in Da2 shale formation,and then built rock solid model.Secondly,we got the porosity both stiff and soft porosity by different-equilibration media,and added which to the above rock solid model.Thirdly,the rock equilibrated elastic modulus with soft porosity are calculated by Kuster-Toksoz model and thus contrasted the obtained Vp of different measured depth with the measured Vp by logging.When the difference of obtained Vp and measured Vp is small enough,the study regarded that the "Dry Rock" is equilibrated to the real rock.Based on above analysis,soft porosity model of Da2 section shale was built objectively for the first time in China.The study compared six rock physics models for Vs prediction,including Xu-White,Krief,Critical Porosity,Vernik,Cement and Soft Porosity Model.The result shows that estimated Vs by soft porosity model is the most close to the measured Vs.Based on known Vp,Vs and density,we can calculate many elastic parameters,for example,P-wave and S-wave impedance,Vp/Vs,Poisson's ratio and Lamer coefficient and so on.Then rock physics analysis had been carried out to try to find the sensitive elastic parameters for shale gas study by scaled with clay volume,TOC and brittleness index.Eventually,the rock physics template of LambdaRho-ModulusRho cross-plotting scaled by TOC had been selected for guiding seismic elastic inversion and "sweet spots" prediction.On seismic research,we obtained shale distribution and its thickness by post-stack seismic impedance inversion,fracture prediction by seismic multi-attributes analysis,shale brittleness by pre-stack elastic inversion and seismic elastic parameters cross-plotting analysis and so on.Guided by the rock physics template of LambdaRho-ModulusRho scaled with TOC,seismic inversions of Young's modulus,Poisson's ratio,LambdaRho and ModulusRho are achieved.By seismic LambdaRho-ModulusRho cross-plotting,the "sweet spots" of Da2 shale of the study area had been predicted,which is helpful for horizontal well planning and hydro-fracturing design in this area.The integrated study indicates that Da2 formation is typical of low porosity and permeability zone.However,the effective porosity,TOC,brittleness as well as gas content of upper zone of Da2 formation are higher than the lower zone.Combined with shale thickness,fracture feature,TOC and brittleness prediction,three "sweet spots" are outlined ultimately at the upper zone of Da2 formation as drilling targets and for horizontal well design.