Theoretical And Experimental Studies Of Membrane Potential In Shaly Sands For Improved Rw Calculation

Formation water resistivity (Rw) is one of the most important parameters in log interpretation. Spontaneous potential or SP logs have been used in Rw calculation, which is especially effective in clean sand. However, when the sand contains clay, the clay interferes with the ion diffusion process and thus affects the membrane potential generated. Since the dominant part of SP electrochemical potential is related to the membrane potential, it is very important to study the membrane potential in shaly sand to correctly determine the resistivity of formation water. Because of the difference in the mobility of positive and negative ions, membrane potential is generated when two electrolyte solutions of different concentration come in contact. This membrane potential also depends on the nature of the membrane or the shale content in the sand. In this paper, we present a systematical study of the membrane potential for oil-bearing shaly sands on the basis of the electrochemical theory and the conductivity properties of the shaly sands. Laboratory experiments are designed and carried out. The experimental results agree very well with the theoretical predictions. It is found that the membrane potential of the shaly sands increases with the cation exchange capacity, the hydrocarbon saturation and the salinity difference of the solutions. In addition, the influence of hydrocarbon on the membrane potential is equivalent to a change of the cation exchange capacity of the formation. The electrochemical potential of the shaly sand increases with the increasing hydrocarbon saturation. The developed membrane theory in shaly sands makes an improved SP log interpretation possible. For practical purpose, interpretation graphs calculated based on the developed equations are also presented. Both theoretical and experimental results show that, to calculate Rw from SP in shaly sand, the influences of hydrocarbon saturation and shale content on the spontaneous logs must be taken into account.