Abstract

Hydrocarbon bearing shaly formations can be detected using cation exchange capacity (CEC) shaly sand models. Most CEC shaly sand models still depend on the laboratory measurement of CEC value. In addition, these models use one value of formation resistivity factor, which is a function of the rocks's cementation exponent. Using one formation resistivity factor in shaly sand reservoir can result in overestimation of the water saturation which in turn results in overlooking formations with hydrocarbon potential.

This paper introduces a new CEC shaly sand model, lpek- Bassiouni (I-B) model that improve the definition of the formation resistivity factor used in shaly sand formations. This model can also calculate the CEC value directly from the well log data.

Ipek-Bassiouni (I-B) Shaly Sand Model considers that electric current follows two type of path in shaly sand. One path represents current flow in free water and another path in bound water. The differentiation between these two paths is accomplished by using two different formation resistivity factors in free water and in bound water. The two formation resistivity factors are expressed using two cementation exponents for free water and bound water as well.

The validity of the model was checked using the cation exchange capacity measured from core samples and drill cuttings. Calculated CEC values display a good agreement with the measured CEC values. The estimated water saturations from the model indicate a better hydrocarbon potential in the zone of interest.

Introduction

Water saturation of hydrocarbon bearing shaly formations can be determined using available CEC shaly sand models. Current CEC models are based on cation exchange capacity and ionic double layer concept. However, the use of these models is impractical because most of the time CEC data is not usually available to the log analyst, hence a laboratory measurements of CEC is required. Different laboratory techniques to measure this parameter are found to yield different CEC values for same core sample.

Previous researchers at LSU1–7 have developed a shaly sand interpretations technique, referred herein as LSU model based on log data such as resistivity, spontaneous potential, neutron and density logs. This model is based on the Waxman and Smits8 concept of supplementing water conductivity with clay counterions conductivity. It also utilizes the dual water theory9, which relates each conductivity term to particular type of water, free and bound, each occupying a specific volume of the total pore space. The main assumption of the LSU model is that the counterion conductivity is represented by a hypothetical sodium chlorite solution. The LSU model is a practical approach that represents the conductivity behavior of shaly sand. However, same as all previous models, LSU model also assumes that the electric current follows the same path in both free and bound water area. This leads to use the same formation resistivity factor to evaluate the shaly formations. This assumption can cause hydrocarbon bearing shaly formations to be overlooked due to overestimation of water saturation in the zone of interest.

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