The Impact of Permeability and Porosity on In-Situ Combustion Performance in Bakken Formation

Abstract

Abstract In-situ combustion (ISC) is a thermal enhanced oil recovery (EOR) process traditionally deployed in conventional reservoirs. Its applicability to unconventional, ultra-low-permeability reservoirs remains largely untested, due in part to the challenges of initiating and sustaining combustion within restricted flow environments. This study presents a systematic investigation of ISC dynamics in tight Middle Bakken Formation, emphasizing the effects of initial permeability and porosity on combustion performance, oil recovery, and rock properties alteration. Two laboratory-scale dry in-situ combustion tube experiments were conducted using compacted mixtures of Middle Bakken cuttings, brine, and crude oil. The first experiment, referred to as E1, was loosely packed to achieve a baseline average permeability of 10.72 mD and porosity of 40.65%. The second experiment, E2, was tightly packed to simulate lower permeability conditions, with an initial permeability of 0.70 mD and porosity of 20.21%. Both systems were subjected to an identical air injection pressure of 85 psi and backpressure of 40 psi, and air injection rate of 5.5 SLPM with continuous measurement of temperature propagation, gas composition, and fluid recovery. Despite the significant disparity in initial permeability, both systems maintained a stable and self-sustaining combustion front. The tightly packed E2 system demonstrated a higher average front velocity of 21 ft/day and achieved an oil recovery of 78.9 % of the original oil in place (OOIP). In contrast, E1 showed a slower front velocity at 16 ft/day and recovered 71.5 % of OOIP. Gas analysis revealed high combustion efficiency in both cases, with strong oxygen consumption and substantial carbon dioxide production. E2 additionally exhibited cyclic oxygen–carbon dioxide fluctuations, indicative of packing-induced heterogeneity and evolving flow paths. Post-ISC X-ray computed tomography scans revealed fundamentally different alteration mechanisms. E1 exhibited localized fracturing near the ignition zone, whereas E2 showed pervasive micro-fracturing and distributed matrix decomposition. The results from E2 underscore ISC ability to enhance flow capacity in Middle Bakken rock sample not solely through combustion but also via thermal stress-driven microstructural changes. The confined geometry of the tight packing amplified temperature gradients and induced grain-scale cracking across the matrix, thereby improving connectivity and enabling sustained front propagation. These changes resulted in a twofold increase in porosity and a fifteenfold increase in permeability for the tight system, effectively transforming it into a more conductive medium. Collectively, these results validate ISC as a viable and potentially transformative EOR technique for unconventional reservoirs, demonstrating its ability to simultaneously mobilize hydrocarbons and enhance in-situ rock permeability under tight conditions. The significant porosity and permeability gains observed, particularly in the tight-packed system, underscore ISC's potential to unlock bypassed oil by thermally upgrading the reservoir matrix. These outcomes establish a strong experimental foundation for future ISC deployment in low-permeability, organic-rich formations.

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