New Classification of Naturally Fractured Reservoirs Using Ronald Nelson's Method and Well Test Analysis

Abstract Classical well test analysis is limited in describing naturally fractured reservoirs when compared with the modern reservoir characterization methods, geological modeling and fault identification from new seismic acquisitions and modern logging tools. The early well testing fracture modeling techniques provided a basic fractures identification and description because it has not been updated with modern geo-mechanical understanding based on image logging tools and the detailed numerical grid properties based on new well testing software. Later, Ronald Nelson's developed a geological study that combined the fluid dynamics with the static description of naturally fractured reservoirs based on the combinations of rock quality and fracture conductivity. This method classified naturally fractured reservoirs into four types, and it was helpful in understanding the fluid flow mechanisms, but it still needed to be more descriptive of natural fractures abundance, intensity, and connectivity in order to obtain stronger correlation between geology and reservoir engineering and well test analysis. This paper presents a new classification of naturally fractured reservoirs as a proposal to upgrade Ronald Nelson's method with regard to pressure transient behavior studied from the well test analysis. Six different pressure derivatives shapes proves the presence of fractures and describes their different impact on well production, production sustainability and the general impact on fluid flow between reservoir segments. The well test analysis clearly indicates whether fractures are intersecting with the well bore or they are present at a distance from the well bore and also whether fractures are open conductive or sealing faults. A new graphical cross plot is also introduced to show the relationship between well productivity as a function of rock matrix porosity and fracture transmissibility with six levels of matrix porosity/permeability and three levels of natural fractures conductivity and storativity. These parameters are plotted versus well productivity to give a clear visual method to understand reservoir performance and production sustainability. The new description of reservoir performance under various types of fractures will help geologists and reservoir engineers to understand the structural aspects and fluid flow performance of naturally fractured reservoirs and assist in taking more effective decisions to improve field developments and reservoir management for maximum recovery.