In-situ combustion of heavy oil within a vuggy carbonate reservoir: Part II—Mechanism of permeability inversion and rock fabric transformation

Predicting the performance of in-situ combustion (ISC) in carbonate reservoirs is notoriously challenging. This study reveals a paradoxical permeability inversion phenomenon: initially high-permeability grainstones (up to 2600 mD) suffer catastrophic permeability damage, while initially low-permeability packstones (below 250 mD) show marked improvement. We demonstrate that this counter-intuitive behavior is driven by the thermal decomposition of dolomite. This process fundamentally transforms the rock fabric, shifting it from a robust, grain-supported architecture (grainstone/packstone) to a fragile, matrix-supported texture (wackstone-like), which critically reduces pore throat radii in high-quality rocks. At the micro-scale, the key agent of this transformation is identified as the neoformation of needle-shaped magnesium-calcium hydrosilicate crystals, which occlude pores and can occupy up to 50% of the pore volume. Stable isotope analysis confirms that this mineralogical alteration is the dominant process, revealing up to 70% of CO 2 produced originates from dolomite decomposition rather than hydrocarbon combustion. These findings prove that initial reservoir quality is a poor predictor of ISC performance; instead, the geochemical reactivity and thermal stability of the rock fabric are paramount. We therefore propose the rock-fabric number as a more robust metric for predicting reservoir response to thermal EOR processes.