Abstract
<jats:title>Abstract</jats:title> <jats:p>In-situ combustion (ISC) has re-emerged as a promising enhanced oil recovery (EOR) method for unconventional reservoirs because it generates heat in situ while simultaneously providing gas drive. In hydraulically fractured unconventional reservoirs, however, ISC is unlikely to occur under dry conditions, as significant volumes of injected water are retained within fractures and the surrounding matrix. Despite this practical reality, the impact of wet in-situ combustion on tight pore structure and transport properties has not been previously quantified.</jats:p> <jats:p>This study presents a systematic experimental investigation of pore-space evolution during wet ISC in the Middle Bakken Formation under contrasting initial pore-structure conditions. Two wet ISC combustion-tube experiments were conducted using crushed Middle Bakken rock saturated with light crude oil and high-salinity formation brine in a one-meter stainless-steel combustion tube. Packing density was varied to represent a loosely packed, higher-permeability system (E1) and a tightly packed, lower-permeability system (E2), while identical ignition, injection, and pressure conditions were applied.</jats:p> <jats:p>Both experiments achieved stable, self-sustained wet combustion along the full length of the tube. The tight-packed system sustained a higher average combustion temperature (≈585 °C) and faster front propagation (21 ft/day) compared with the loose-packed system (≈465 °C and 16 ft/day), demonstrating that wet ISC can propagate efficiently even under restricted initial injectivity. X-ray computed tomography (CT) scans acquired before and after combustion reveal fundamentally different pore-structure responses. In E1, average porosity decreased from ~40% to ~30% and permeability declined from 9 mD to 2 mD, consistent with bulk-density increase and pore-space reduction. In contrast, E2 experienced pronounced pore development, with porosity increasing from ~22% to over 40% and permeability increasing from 1 mD to 10.5 mD, without evidence of macroscopic fracturing, indicating distributed micro-scale pore formation.</jats:p> <jats:p>Gas analysis and CT observations show that packing-controlled pore architecture influenced combustion dynamics, producing cyclic oxygen–carbon dioxide behavior in E2. Energy-dispersive X-ray spectroscopy (EDS) of burned rock samples indicates that halite precipitation dominated in E1, contributing to pore plugging, whereas enhanced sulfation and mineral decomposition occurred in E2 under higher-temperature wet combustion conditions.</jats:p> <jats:p>Overall, the results demonstrate that wet in-situ combustion can sustain stable combustion, enhance oil recovery, and fundamentally modify pore structure in tight formations. The strong dependence of pore evolution on initial rock fabric highlights wet ISC as a viable and potentially transformative EOR strategy for hydraulically fractured unconventional reservoirs traditionally considered unsuitable for thermal recovery.</jats:p>