Mechanical behavior of carbonate reservoirs with single karst cavities

Abstract

In recent years, operational issues during the exploration of Pre-salt reservoirs have aroused the interest of the Brazilian oil and gas industry in studying karstification and its effect on carbonate reservoirs. Since then, a massive research effort was dedicated to the geology of the Brazilian onshore karstified carbonate formations; however, geomechanical modeling of reservoirs with karstic cavities remains unexplored in the literature. This work analyzes the mechanical behavior of karstified carbonate reservoirs during the hydrocarbon production period. A synthetic case study based on real geologic horizons of a Brazilian basin is carried out to predict the mechanical response of the reservoir and surroundings. The presence of a preexistent cavity developing around the reservoir symmetry axis is postulated. Different scenarios of karstic geometry and filling material are considered. Results focus on reservoir compaction, surface subsidence, and stress paths in regions of interest of the reservoir, caprocks, and cavity wall. Comparative results show that the horizontal extension of the cavity and the compressibility of the filling material play an important role in reservoir integrity. The qualitative results are useful for the oil and gas industry in terms of identifying problematic regions for wells and establishing loading limits. The findings and the preliminary results of this innovative work shall guide further advanced analyses, especially in the area of dolines generation from karstic cavities.