Nanometals are useful for improving the effectiveness of in situ combustion-enhanced oil recovery. The effects of five different transition-metal nanoparticles on Colombian, Venezuelan, and Mexican crude oil samples are investigated. Experiments are used to measure the change in burning characteristics with additives at two different length scales. At the small-scale, it is shown that select nanoparticles decrease the apparent activation energy for combustion, change the gateway reaction (i.e., the reaction with the greatest activation energy), make combustion fuel more reactive, increase the fuel quality, and alter the low-temperature oxidation products. During reactive flow, the improvements as mentioned above are critical to help sustain combustion when excessive fuel deposition causes premature quenching. Copper, chromium, and titanium nanoparticles, on average, decrease the amount of fuel deposited and consumed during the process by 7%, increase the apparent H/C ratio of the coke by 15%, and increase the molar CO2/CO ratio of the combustion gas by 31%. These changes manifest in a decrease in water production (Delta WORavg = -19%) and an average increase in oil production of 20%.