Fracture-Enhanced Permeability in Tight Carbonate Reservoir Analogue

Abstract

The exploitation of hydrocarbon reserves in naturally fractured carbonate reservoirs has motivated increasing numbers of investigations into their geomechanical and structural characteristics due to their importance in fluid flow behaviour. The geomechanical characterization of outcrop analogues has been used for improving the understanding of the relative contribution of structural evolution regarding the fracture networks in different burial conditions. In this study, the Lower Cretaceous lacustrine laminites from Crato Formation, Araripe Basin, NE Brazil, have been investigated as an analogue to some of the observed carbonate facies present in the pre-salt reservoir sequence of the Brazilian marginal basins. Samples of laminites were deformed triaxially to evaluate changes of properties induced by experimental deformation, such as porosity, permeability, mineralogy, and distribution of fracture networks. After initial pre test analyses, over 20 samples were deformed triaxially under a range of confining pressures (20MPa - 50MPa), to reach reservoir conditions. Comparison of pre- and post-deformation properties, as well as image analysis techniques indicates a strong link between laminated facies characteristics and deformation response. Even though different stress-strain responses were observed, the overall increase in porosity and permeability due to fracturing of intact laminites samples was not expressive, with an average of 5.94% (±1.66) and below 7.0mD, respectively. However, laminites sampled from a shear-fracture zone showed a more expressive increase in porosity of 17.11% (±4.48), after triaxial deformation. This indicates that deformation behaviour for planar laminites could be predicted based on key properties such as porosity, permeability and mineralogy. However, for faulted samples these properties alone cannot predict deformation behaviour, once they can control the stress trajectory and deformation along pre-existing structures, what will lead to reactivation instead of new fracturing. These results have been used to build more reliable geomechanical models through the controlled reproduction of natural fracture systems that have a significant permeability anisotropy within the tight carbonate reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018