Predicting the total porosity of shale gas reservoirs

The favorable zone with organic-rich shale reserve is the “sweet” in the exploration of shale gas, while the logging evaluation of reservoirs, to which the accurate assessment of total porosity is instrumental, is critical to seek this “sweet.” Taking the reservoirs in the Wufeng-Longmaxi Formation in Jiaoshiba region of Fuling, China as an example, the applicability of Herron Model for total porosity was first analyzed based on the logging data of Well #S1 (including element capture spectroscopy (ECS) and conventional logging) as well as the core analysis data (including total porosity and total organic carbon). Results show that the total porosity of target horizons was not accurately calculated with the Herron Model for the following two reasons: (a) in the calculation of apparent density porosity and apparent neutron porosity, rocks are taken as a simple combination of matrix and pore, neglecting the organic matters in reservoirs; (b) the weight coefficients of apparent density porosity and apparent neutron porosity are constant, ignoring their variation in various geological structures. Based on this analysis, an improved method of evaluating the total porosity of shale gas reservoirs was put forward and used to calculate the total porosity of Well #S2 in Wufeng-Longmaxi Formation in Jiaoshiba. Compared with original Herron Model, the new method offers simpler operation and considerably better accuracy, which make it suitable for wide range of applications.