An Improvement in Cation Exchange Capacity Estimation and Water Saturation Calculation in Shaly Layers for One of Iranian Oil Fields

Water saturation and cation exchange capacity are the most significant parameters used to calculate a hydrocarbon zone potential. In clean formations, by applying the famous Archie model, which assumes that in the formation the only electric conductor is the formation water, the water saturation can be calculated. Additionally, in shaly sand formations this assumption may not be true as the ions are associated with clay minerals and transport electricity. We attempt to utilize some logs, as well as experimental data, to improve the water saturation calculations and the cation exchange capacity within shaly sand layers for an Iranian oil field. Therefore, several shaly sand models were considered for the calculations of these values for three wells of the Ahvaz oil field. The validity of the different models was controlled through the measured values of cation exchange capacity and water saturation on core samples. The comparative results show that cation exchange capacity and water saturation calculated using Ipek-Bassiouni model are more indicative of zone hydrocarbon potential with correlation coefficients of 0.91 and 0.95 respectively. The results of this study show the requirement for the I-B model in studied reservoir and its superiority compared to other models because it is on the basis of principles reflecting the formation water and clay counter ions conductive behavior.  Concerning to this model, two cement exponents are incorporated to illustrate the electric current path tortuosity in clay bound water and free water. In the current work, cementation exponent in free water (mf) and clay bound water (mc) were estimated from pure shale and clean sand respectively. The results of this study are promising and can be simply extended in other similar neighboring shaly sand reservoirs.