Advancements towards DFKN modelling: Incorporating fracture enlargement resulting from karstic dissolution in discrete fracture networks

Abstract

We present a multiscale approach to develop a deterministic Discrete Fracture Network (DFN) model including fracture enlargement due to karstic dissolution in low-porosity carbonate rocks. The result is a DFKN (Discrete Fracture and Karst Network) model from which it is possible to quantify, in a practical manner, the impact of fracture enlargement due to karstic dissolution on the fracture network transmissivity. We provide a case study using two large outcrops in the Jandaíra Formation, an intensely karstified carbonate platform of Turonian-Campanian age in the Potiguar Basin, Brazil. The study is based on scanlines collected both in field and from Unmanned Aerial Vehicle (UAV) imagery, allowing to cover three orders of magnitude in length and aperture of fractures. We found that if karstic dissolution is low, aperture-length data plotted on bi-logarithmic graphs can be fitted by straight lines, thus following the aperture-length power law typical of mechanical apertures. However, advanced stages of karstification modifies the aperture-length relationship due to the fact that dissolution causes similar fracture enlargement regardless of fracture length. As a result, the aperture-length curve gradually degenerates to a constant value as the degree of dissolution increases. This behavior is the key to quantitatively taking into account fracture enlargement, by simply adapting an aperture-length curve according to the known stage of karstic dissolution of the modeled carbonate rocks. We show that fracture enlargement can increase the transmissivity of a karstified fracture network by up to five orders of magnitude without changing its connectivity. Our approach can be useful to better understand the behavior of fluid flow in carbonate reservoirs with fracture networks affected by karstic dissolution.