Oil recovery processes from carbonate or sandstone reservoirs. With a carbonate reservoir, the reservoir is initially flooded with a fluid such as sea water. Then the same fluid containing a plurality of citric acid-filled microcapsules is injected into the reservoir. These microcapsules are left to incubate in the reservoir, which will then infiltrate the rock formations, degrade by heat and release the encapsulated citric acid. The released citric acid reacts with the carbonate rocks to produce CO2 in situ, which causes oil trapped in the rock formations to swell, reduce in viscosity and move towards to a nearby production well. For a sandstone reservoir, calcium carbonate can be also encapsulated with the citric acid for CO2 generation at the reservoir.