Effects of hydro-mechanical and chemical degradation of carbonate rocks : implications on fracture transmissivity

Abstract

Natural fractures are highly stress-sensitive, significantly impacting hydraulic conductivity and flow behavior. This highlights the need for further investigation, particularly concerning the interplay of fracture roughness and mineral composition. Laboratory fluid-flow experiments through hydraulically transmissive fractures in carbonate rocks subjected to mechanical loading, acidizing, and torsion displacement provided insights into how dissolution processes and shear stress induce changes in fracture transmissivity on smooth and rough fracture surfaces with varying mineral compositions and rock textures. This study aims to further investigate these phenomena. The formations studied in this work, including the Maria Farinha and Gramame Formations, are compositional analogues of Brazilian post-salt reservoirs such as the Santos and Campos Basins, consisting of marly calcareous and marl materials. Additionally, the Jandaíra Formation serves as a textural analogue of Brazilian pre-salt reservoirs like the Barra Velha Formation due to its similar sedimentary textures. The different lithofacies studied-Mudstone, Wackstone, Packstone, Grainstone, and Crystalline rock-were characterized through petrographic analysis, and their elemental compositions were analyzed using X-ray diffraction and X-ray fluorescence tests. The fracture plane topography was scanned before and after acidification to observe changes in roughness due to the presence of the acid. The impact of normal stress on fracture deformation was studied by applying axial UPSCIVE chanyes II TOUYNNiess due to the preSENICE ON tic aciu. The impact of normal stress on fracture deformation was studied by applying axial loading and unloading cycles while maintaining a constant flow rate of water injection into the fracture plane. The dissolution effect was produced by injecting 5 cm3 (at 1 cm3/min) of HCl solution (0.01 mol/l and pH = 2), following the same testing protocol described above. Results showed that fracture plane roughness and the size of unmated (i.e., non-contact) regions are critically important. Rough surfaces promote transmissivity enhancement, while smoother surfaces with higher levels of matedness and contact areas between the opposing fracture planes are less affected by chemical degradation. Such surfaces do not present relevant mechanical variations. Grain- supported calcareous lithofacies with high calcium contents and rough surfaces show the most significant increase in fracture transmissivity after acidizing. Conversely, fracture transmissivity after acid treatment in matrix-supported rocks with high calcium contents is low, even in samples with rough fractures and unmated surfaces. Shear stress and strain were studied by applying constant axial loading while injecting water into the fracture plane at a constant flow rate. Shear displacements resulted from torsion induced by rotating 1/4 pitch of the two horizontal screws. The results show that shear strain due to torsion displacement induces variations in fracture transmissivity, which follows asymptotic trend models. Changes in fracture transmissivity due to shear stress are influenced by surface roughness, initial mechanical aperture, and rock texture. Our findings highlight that fracture surface roughness, mineral composition, and rock texture impact fracture slip during shear failure and chemical dissolution during acidification. The results of both experiments can be used to improve reservoir hydromechanical and geochemical models and provide more robust numerical simulations. Moreover, these results provide valuable insights into understanding the interaction of carbonate rocks with other types of reactive fluids (e.g., carbon dioxide, CO2) and the behavior of fluid flow in fractured reservoirs subjected to normal and shear stresses. Although the impact of CO2 injection is not as intense or significant as acidification using hydrochloric acid, a similar trend of dissolution effects in carbonate rocks is expected.