A Novel Naturally Fractured Reservoirs Classification Plot Through the Liaison of Dynamic Data, Depositional Environments and Rock Texture

Abstract

Naturally Fractured Reservoirs (NFR) represent a challenge for petroleum industry because they are characterized by complex dynamics associated to the fluids motion and geological events that originated them million years ago, where diagenetic processes have played a transcendental role. In carbonates, the movement of fluids within the reservoir is highly influenced by the fracture systems present in the formation, however, these are intimately related to rock texture and quality, depositional environments, facies changes, regional and local stresses, tectonism and of course, diagenesis. Regarding the dynamic behavior, we can highlight the importance of the type of fluid present in the system and the acting drive indices, which govern the behavior of pressure and production in this type of reservoirs, whose analysis usually goes further of conventional techniques commonly used for its evaluation. One of the problems faced by reservoir engineers is the classification or categorization of these types of reservoirs to know their true potential and try to estimate the recoverable reserves as accurately as possible, since the complex dynamic behavior of NFR hinders its exploitation when the most important parameters for its correct evaluation are not known. From the above, a novel and practical Naturally Fractured Reservoirs (NFR) classification plot is proposed based on the Nelson's classification (2001) and a full revision of other author's technical reviews. The plot is generated through the information obtained from a full reservoir characterization to acquire petrophysical evaluations and Pressure Transient Analysis (PTA) to find the product of the effective porosity and the average flow capacity of each of the fields tested in order to plot them against the recovery factor; this analysis considered more than 200 carbonate fields from more than 40 countries around the world. When plotting the data involved, it is clear to see that they are grouped in different zones for its reclassification as Naturally Fractured Reservoirs, where we added a subcategorization of type II reservoirs (type II A and type II B) and also the influence of vugs in type I reservoirs and the gas and condensates region; all attributed to the dynamic behavior associated to the type of fluid, the acting drive indices, the depositional environments and the rock texture. The results obtained were fully coupled to a probability distribution and have shown to be consistent with the observed behavior, being a useful tool for determining the actual type of NFR, the expected production rates, the range of possible recovery factors to be achieved and the characterization of reservoirs. Likewise, the proposed plot can be applied to the analysis of sectors in the same reservoir or formation to try to identify the variations regarding the type of NFR by zones, blocks or compartments according to the location of each well in the field, considering their respective recovery factors concerning its cumulative production and original reserves.