Application Of Multifractal Theory In Effectiveness Evaluation Of Complex Reservoir

In recent years,with the continuous deepening of oil and gas exploration,more and more complex reservoirs have been discovered,and the evaluation of reservoir effectiveness is facing huge challenges.The pore structure of the reservoir determines the ability of the reservoir to accumulate and fluid migration,and its quantitative characterization is the key to evaluating the effectiveness of the reservoir.However,the Sinian Doushantuo Formation shale gas reservoir in Yichang area in western Hubei is a new set of exploration strata.The accurate measurement and quantitative evaluation of shale nanopores is still in the exploratory stage;the Permian in the Shawan Sag on the northwestern margin of the Junggar Basin The development of cracks and dissolved pores in the glutenite of Jiamuhe Formation led to the loss of the existing methods and technologies in the evaluation of pore structure.This paper selected 20 cores from the Doushantuo Formation shale gas reservoir of the Sinian in Yichang area,western Hubei,and carried out an NMR experiment with a low echo interval(0.069ms)in a 100%saturated state.Through the analysis of multifractal characteristics of NMR T₂ spectrum,the parameters that are more sensitive to characterize the pore structure of shale reservoirs are Dmin-Dmax(the difference between the minimum and maximum generalized fractal dimensions)and??(spectrum width).On this basis,the shale reservoirs in the target area are divided into three types using the intersection diagram of Dmin-Dmax and??.The nuclear magnetic T₂ spectrum of type I reservoirs is dominated by bimodal distribution,which is the reservoir type with the best pore structure in the study area,and its shale gas enrichment potential is the best;the nuclear magnetic T₂ spectrum of type III reservoirs is based on the single-peak distribution.Mainly,the spectrum peaks are mainly distributed between 0.01 and 0.3ms,which is the reservoir type with the worst pore structure in the study area,and its shale gas enrichment potential is the worst;Type II reservoirs are somewhere in between.Through multifractal analysis of the T₂ spectrum of 21 cores of the Permian Jiamuhe Formation glutenite reservoir of the Permian Jiamuhe Formation in the Shawan Sag of the northwestern margin of the Gar Basin,the multifractal parameter D0(The generalized fractal dimension with a moment of 0),and a model for predicting the T₂ cutoff value using D0.Through the multi-fractal study on the electrical imaging logging porosity spectrum corresponding to the depth of 51 cores,it is believed that the multi-fractal parameters that are more sensitive to permeability include?(-10)(singular strength with an order moment of-10)and Dmin(Generalized fractal dimension with order moment-10).On this basis,a model for evaluating the permeability of glutenite reservoirs using electrical imaging logging porosity spectrum was established.Using the above model,the measured porosity spectra of nuclear magnetic resonance logging and electrical imaging logging in the study area were processed respectively,and the predicted T₂ cut-off value and permeability have high consistency with the core experiment results.Compared with the conventional permeability evaluation model,the permeability model proposed based on this paper has higher accuracy.Combining oil test,RQ(sum of D0 and?(-10))and permeability,the effective reservoirs in the study area are divided into three types,which can be used to predict reservoir fluid production.Multifractal can be used as an effective method to quantitatively evaluate the pore size distribution of complex reservoirs.This study provides a new understanding for the effectiveness evaluation of complex reservoirs such as shale and glutenite.It can also provide research ideas for the effectiveness evaluation of tight oil,volcanic rock and other types of complex reservoirs.