Basement Reservoir: Understanding the Interplay of Stresses, Fractures, and Mineralogy With Well-Logs

Abstract

Fractures play the most important role in non-sedimentary reservoirs, both for storage and permeability pathways. Globally, the wells drilled into the igneous/ meta-sedimentary rocks present a complex interplay of stresses, mineralogy and fractures and their control on the reservoir behaviour. This work aims at deciphering this relationship and how a workflow through these can help in best characterization and exploitation. Borehole images provide the near wellbore information associated with fractures and texture and stress-manifestation, whereas the advanced acoustic logs of shear radial profile provide away from the borehole information for the fractures. Critically stressed fractures that are interpreted to be open away from the borehole constitute the zone of interest. The mineralogical information from the cutting reveals if basement is varying in composition from igneous to meta-sedimentary. Some hydrothermally altered facies and basement wash could develop better reservoir properties with development of non-fracture porosity, though still secondary in nature. Along the well-trajectory, a change is the stress-regime is observed on acoustic logs, and a marked change is observed on borehole image logs as well. The various image logs facies define the variation in basement facies and that could be captured aptly on the core. Critically stressed fractures are separated from all the fractures picked, and radial profile for fast and slow shear advises the openness of some of those critically stressed fractures away from the borehole. This workflow attempts an understanding of the relation of fractures with stresses, and proposes a workflow where this information can be integrated with the mineralogical analysis to suggest the best zones in a well, and use that knowledge in field development. The workflow developed in this study can be used as standard basement-characterization with well-logs.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018