Logging Evaluation Method Of Sandstone Reservoir With Complex Pore Structure In The Mid-Deep-Depth Of Nanpu Sag

Formed by a combination of deposition and diagenesis,the complex rock fabric of sandstone reservoir in the mid-deep-depth of Nanpu Sag controls pore–size distribution,which,in turn,controls porosity,permeability and capillary properties,having a great effect on rock physical properties,conductivity and seepage characteristics.The main concern for complex pore structure reservoir evaluation includes:unclear formation mechanism,difficult characterization of pore structure,and unclear understanding of oil bearing and seepage law.In this paper,the problems are evaluated to categorize typical reservoir petrophysical facies and quantitatively calculate pore structure characteristics,fluid property and seepage features,such as porosity,permeability,water saturation and relative humidity permeability proposed.Numerous field examples using well logging data and core data have been included to illustrate the applications to verify the method’s accuracy.The sedimentary environment and diagenetic features are the first and most important geologic factors to control the development distribution of complex pore structure reservoirs.According to the principle of petrophysical facies,the effects of sedimentary micro-facies and diagenetic facies on pore structure are studied.The result shows that core samples of the study area from different sedimentary micro-facies can be grouped into distinct areas on a cross-plot of porosity versus permeability.Seepage factor f is defined to describe porosity–permeability distribution pattern.In addition,three kinds of diagenetic facies are defined to depict the influence of diagenetic processes on the porosity–permeability distribution.Further,based on the above,five groups of reservoir petrophysical facies RPFI～RPFV are classified and their corresponding pore structure characteristics,pore-throat distributions and pore-permeability distributions reflected by NMR T₂ spectra have been exhibited.In applications,a converting method from T₂spectra to capillary pressure curve is proposed using quantitatively using NMR logging data to characterize pore structure.The application result shows that the capillary pressure curve transformed by this method agrees with experimental capillary pressure curves.The method realizes the quantitative evaluation of continuous formation pore structure parameters（pore throat radius,mean coefficient,structure coefficient）.Pore fluid property prediction is another most important research content of complex pore structure reservoir evaluation.Firstly,to realize the fast identification of pore fluid property,an oil-bearing indication method based on the NMR T₂ spectrum（long TW and short TE）is established.An oil saturation evaluation method is proposed by the relationship between NMR T₂ geometric meantime and oil saturation and corresponding interpretation chart.Secondly,methods proposed to calculate fluid component volume fraction include:ⅠAn oil saturation calculation method derived by the combination of radial saturation inversion of resistivity logging and multi-means normal distribution characteristics of NMR T₂ spectrum.ⅡAn irreducible water saturation and oil saturation calculation method derived from a modified sandstone conductivity model for the effects of pore structure.Ⅲa water saturation calculation method in a gas-bearing reservoir based on petrophysical facies classification with non-electrical logging data.Evaluation of seepage features and prediction of liquid products is also the most basic concern in complex pore structure reservoir evaluation.The proposed method includes:ⅠA permeability calculation method with NMR logging based on seepage factor f and petrophysical facies classification.ⅡA method of water content ratio of liquid production is derived from the relationship model between water content ratio and the resistivity increasing coefficient in two-phase fluid flow according to the similarity between water rate of production and electrical flow through porous media and fully consider pore structure effects.The verification results of rock samples show that the same model can be used for water rate prediction of production in different pore structures reservoirs.The model directly converts resistivity to water rate of production,improving estimates of oil saturation from statically to dynamicly.This paper forms a set of quantitative evaluation technology for sandstone reservoirs with complex pore structures.It has specific guidance and reference significance for the research work of petrophysics,reservoir evaluation of sandstone reservoir with complex pore structure and unconventional reservoir.