Saturation Dependent Errors in Interpreting Waterflood Diagnostic Plots

摘要

Various diagnostics plots related to examining injection pressure gradient requirements for maintaining injectivity have been used in waterflood management. Among these plots, Hall plots and the derivative of Hall plots are the most common approaches used in the industry. This paper includes an analysis of some errors associated with interpreting Hall plots and the derivative of Hall plots in monitoring waterfloods. Based on observations from several waterfloods in California, in this paper we demonstrate by numerical modeling, that the cause of slope flattening can also be dependent on the state of saturation history around injection wells. We also show that the flattening can be compensated by skin build up resulting in misleading trends. We conducted a series of numerical modeling work to examine these issues. From such modeling experiments, we considered cases of injection well behavior in virgin reservoirs as well as in mature waterfloods. We show that when new injectors are drilled, water saturation buildup tends to flatten the slope observed on the Hall Plot or the derivative of Hall plot. For new floods, drops in the slope observed can be associated with water saturation buildup caused by limited flooding history. When injection wells are drilled in flooded segments of a field, such saturation dependent flattening may not be observed. This is a significant issue as it affects decisions related to restriction limits on injection gradient as well as unnecessary stimulation work. In summary, we conclude that interpretation of Hall plots without consideration of water saturation history can be misleading.