Characterization of fractures and karst features for the development of discrete models
Carbonate reservoirs; fracture systems; scale laws; karst; connectivity; transmissivity.
The present study presents a multidisciplinary and multiscale approach to build a discrete fracture model taking into account the karstification processes, resulting in a model that we named as DFKN (Discrete Fracture and Karst Network). Using DFKN models it is possible to assess the impact of karstification on geometric fracture properties and in the system transmissivity. The methodology was based on field data collected from scanlines, aerial images (Unmanned Aerial Vehicle - UAV), and Ground Penetrating Radar (GPR), aiming to characterize the fractures and karst features in two outcrops of the Jandaíra Formation. This formation corresponds to an intensely karstified and fractured carbonate platform that emerges in the mid-west and east portions of the Potiguar Basin (Northeastern Brazil). The fractures length distribution, in both outcrops, follow a power law with different exponents, which control the connectivity of the system and the karstification process of each outcrop. The fracture enlargement process due to karstification, modifies the statistical distributions of the apertures that changes from a linear aperture-length relationship in a bilogarithmic graph, to an approximately constant aperture in a very advanced karstification stage. As a result, the enlargement of the fractures promotes an increase of up to eight orders of magnitude in the system transmissivity, which implies that systems with the same connectivity can present very different values of transmissivity. These results can be useful in areas that use geostatistical modeling of carbonate reservoirs and can be used as a reference in future fractured and karstified reservoirs simulation and management.