Theory For Resistivity Of Fractured-vuggy Reservoirs Under High Temperature And Pressure

Fractured-vuggy reservoirs are very important oil reservoirs and high temperature andhigh pressure is the important characteristic in the deep reservoir environment. The researchfor electrical parameters of fractured-vuggy reservoirs can better the accuracy for reservoirevaluation. Based on Warren-Root electric conduction model and the series-parallel theory ofresistance，the resistivity models for the single，double and triple pore reservoirs have beenestablished. The analysis for the effect of pore structure on formation factor and pore structureindex has been carried out. It has been completed that the building of fractured-vuggyresistivity model under high temperature，high pressure by the theory of rock’s thermalexpansion and compression properties，the adjusting for water saturation，which providetheoretical help to reservoir evaluation.On the basis of Warren-Root model the resistivity pore models for fractured-vuggyreservoirs have been found. It has been found through analysis of fractures and non-connectedvugs’ effect on resistivity parameters that，fracture has the major effect on resistivity offractured-vuggy reservoirs, though small， can improve the connection of reservoirs.Non-connected vugs have a little help for increase of resistivity so that the improvement offormation factor is less than10%when the fractures exist.Analysis of effect on resistivity parameters by pore structure has been accomplishedbased on established fractured-vuggy resistivity pore models. The more porosity，the morepore structure index.The pore structure index is smaller when the fracture ratio is bigger，however，bigger along with increase of non-connected vugs ratio. The pore structure index oftriple pore reservoir depends on porosity ratio and should be comprehensive reflection ofreservoir style，pore structure and cementation.On account of rock thermal expansion，the temperature-resistivity parameter modelshave been set up. The results show that formation fact and pore structure index increases withthe increase of temperature，except that the higher temperature，the smaller pore structureindex for fractured reservoirs. The variation trend of formation factor and pore structure indexfollowing temperature is related to porosity and coefficient of expansion. The rocks have been divided into hard part and soft part so that it helps to bring about thefracture-vuggy reservoir pressure-resistivity models， which depends on compressionproperties of rocks and property of fractured-vuggy reservoirs. The formation factor increasesin non-linear curve when pressure increases and the increase rate and size are relate to porecompression coefficient of hard part and bulk modulus of soft part. Pore structure indexmainly decreases following the increase of pressure.With the joint effect of temperature and pressure，pore structure index of dual and triplepore reservoirs decreases as the temperature increases，increases as the pressure increases.