Method for calculating the fracture porosity of tight-fracture reservoirs

Abstract

Identifying fractures and evaluating the parameters of tight reservoirs are important problems. Developing methods to accurately interpret logging data for tight sandstone and shale reservoirs is of great significance, especially when only conventional logging data can be obtained. Identifying natural fractures with limited available data is challenging. Comparing and analyzing the log response characteristics of the natural fracture zone of a tight reservoir indicate that acoustic (AC) and density (DEN) data are highly sensitive to fractures in tight reservoirs. First, we have obtained the characteristics and differences of the log response of natural fractures. Second, we established a model that is based on these sensitive log-response characteristics (AC and DEN) to identify fractures (correlation coefficient method with window lengths), and we analyzed the cut-off of the correlation coefficient. Then, we established a model (the revised Wyllie difference method) to characterize the fracture porosity based on the difference in the sensitive log responses of the fracture (the difference in the AC and DEN curves). Finally, we applied this methodology to a case study of a tight reservoir in the Sichuan Basin, China, and the AC-DEN correlation coefficient and fracture porosity are calculated. The AC-DEN correlation coefficient adequately identifies fractures, and the calculated fracture porosity is consistent with the fracture porosity from full borehole microresistivity imaging. Thus, this method is applicable to evaluating fractures in tight-fracture reservoirs.