Study On The Effective Medium Resistivity Model For Low Porosity And Permeability Shaly Sand Reservoirs In Bohai Sea

With the development of exploration and exploitation in oil field, reservoirs with low porosity and permeability has become the focus, and evaluation on low porosity and permeability reservoirs has been a primary problem for log interpretation. In this paper, conductive law of low porosity and permeability reservoirs in Bohai sea is studied detailedly with experimental data of high-temperature and high-pressure rock resistivity. The result shows that m is smaller, a is larger than those of high porosity and permeability reservoirs, and n and b are similar to those of high porosity and permeability reservoirs. Also we give methods to determinate the values of m, n, a, b in the areas. Based on symmetrical effective medium conductance theory, a new conductivity model is proposed for low porosity and permeability shaly sand reservoirs with continuous clay distribution and high capillary bound water in pores in Bohai sea. By analyzing effects of some factors on the proposed model, we find out the some rules. Resistivity index decreases and formation conductivity increases as conductivity of clay increases. Resistivity index decreases and formation conductivity increases as percolation rate of capillary bound water increases. Resistivity index increases and formation conductivity decreases as percolation exponent of capillary bound water or percolation exponent of clay increases. Also, we focus on the effect of clay percolation rate related with continuity of clay distribution, and find out that the conductivity of formation increases and resistivity index decreases as clay distribution in pores occurs from discontinuity to continuity. Then, the accuracy of the proposed model is validated with experimental data of rock samples, and it shows that the model can describe the conductive law of low porosity and permeability reservoir in Bohai sea. Empirical formulas to calculate shale content, porosity, permeability, capillary bound water saturation, and cation exchange capacity are established with experimental data of rock samples and logs in Bohai oilfield, and the methods are given to calculate the values of percolation rate and percolation exponent. On the basis of that the program is developed to interpret the log data. Validation with experimental measurements of rock samples and interpretation results for log data shows that the proposed model can improve the accuracy of the evaluation for shaly sand reservoirs with low porosity and permeability in Bohai sea, and meet the needs of exploration and development for low porosity and permeability shaly sand reservoirs in Bohai oil field.