Its all part of the general "enhanced oil recovery" EOR processes.
In order to improve extraction a range of different techniques and fluids - very field specific - are engineered to increase production recovery %.
These include waterflood, gas re-injection, CO2 injection, pumping, gas lifting, fracturing / reperforating etc. Each has pros and cons for any oil or gas field based on availability, cost, safety, environment & of course the geology / chemistry of the rocks as well as the production history.