Estimation of total porosity in shale formations from element capture logging and conventional logging data

The Herron model does not consider the effect of organic matter and gas on total porosity. In this paper, the shale rock is equivalent to three parts: skeleton, organic matter, and pore. According to the volume model, a new total porosity model is established. The new model is applied to estimate the total porosity of a shale gas well in South Sichuan area from element capture logging and conventional logging data. The results show that in the formations with high gas content and organic matter, the total porosity calculated by the new model is smaller than that calculated by the Herron model. To verify the accuracy of the new model in evaluating the total porosity of shale, the advantages and disadvantages of Gas Research Institute (GRI) measurement method and helium injection measurement method for determining porosity are discussed in terms of theoretical analysis and experimental measurement. It is considered that the GRI porosity is closer to the total porosity of shale and it can be used to test the total porosity of logging evaluation. The total porosity of the ten shale samples calculated by the new model and the Herron model is compared with the GRI porosity. Results show that the average relative errors calculated by the Herron model and the new model are 61 and 10%, respectively. It indicates that the accuracy of the total porosity calculated by the new model is higher.