Analysis of the Mechanism of the Effects of Secondary Porosities on the Cementation Factor and Saturation Index in Reservoir Formation with Vuggs and Fractures

In general,reservoirs with vugs and fractures,such as carbonate reservoirs and volcanic reservoirs,are extremely inhomogeneous,and have various types of pores and complex pore structures.These factors result in the failure of the Archie equation which was founded on experimental data acquired on samples of homogeneous sand with average and high porosity.For the computation of water saturation,the most important parameters including cementation exponent and saturation exponent,demonstrate complex characteristics.The cementation exponent observed in petrophysical experiments is not only related to porosity,but also can be anomalously large or small,at the same time,the saturation exponent is dependent on saturation itself,and it also can be anomalously large and small.Based on the rock model with dualporosity system which holds uniformly distributed intergranular pore as the porous matrix with vugs or fractures as its inclusions,this study has quantitatively and mathematically analyzed the effects of secondary pore system and their wettabilities on cementation and saturation exponents,and recognized the mechanism that how the cementation exponent is dramatically decreased by the fractures and how the cementation exponent is dramatically increased by the vugs.Also the modeling results provide petrophysists and log interpretors a sound understanding of the complexity of the hydrocarbon evaluation of reservoirs with vugs and fractures.And it sets a theoretical foundation for the evaluation of water saturation in reservoirs with vugs and fractures.