Study On The Generalized Symmetrical Effective Medium Resistivity Model For Low Resistivity Reservoir

Producible oil and gas are being found in low resistivity reservoirs in most oil fields with development of oil exploration, reservoirs what may be thought to water zones from their resistivity in error. Consequently the evaluation on low resistivity reservoir still is a problem to solve from log interpretation. Geneses and conductance mechanisms are studied in detail for variety and complexity in geneses of low resistivity reservoirs, and limitation of currently used resistivity models for evaluation on these reservoirs. An improved generalized symmetrical effective medium resistivity model is proposed for predicting water saturation in low resistivity reservoirs based on the assumption of parallel conduction between laminated shale and dispersed shaly sand patterns, and symmetrical anisotropic effective medium theory of resistivity interpretation. The model can be applied in low resistivity reservoirs containing high salinity water, microscopic capillary pores, dispersed clay, conductive matrix grains, and laminated shale. It is pointed out that shale distribution largely affects water saturation predicted by the model. The curvatures of the curves between resistivity index and water saturation are greatly affected by clay conductivity. Also resistivity index decreases as fraction of dispersed clay, or fraction and conductivity of laminated shale, or conductivity of rock matrix grains, or matrix percolation exponentγ1, or percolation rate of capillary bound waterλwcincreases. Resistivity index decreases as percolation exponent of capillary bound waterγ2, or matrix percolation rateλm a, or free water percolation rateλwfdecreases. The proposed model uses percolation exponent and percolation rate to describe the wettability or surface roughness properties of the rock and connection of each component. It can be applied in shaly sands with multi-continuous components. Rock sample data from low resistivity reservoirs with different geneses show that the data calculated from the proposed model are in good agreement with the measured data, and the proposed model can be considered to be a generalized resistivity model for low resistivity reservoir evaluation. The hybrid iteration algorithm combining Newton iteration and bisection can be used to solve the model for water saturation. The interpretation results for log data from four wells show that the water saturation calculated with the model is reasonable, and interpretation accuracy for low resistivity reservoirs is improved.