A large amount of it is being produced from gas wells in the Four Corners area, large pipelines send it across the country (in a relatively impure form, maybe 90% CO2) for reinjection into other well fields as CO2 floods. It dissolves into and thins the oil in the formation and allows it to flow through formation cracks easier while still keeping pressure up, enhancing oil recovery from existing older wells.
What you see in the trucks is normally a higher purity liquefied CO2, being trucked from production areas to market areas, for use in industry (beverage/restaurant use, welding gases, etc). It is also occasionally used in the oilfield for stimulation treatments where they inject it into older wells mixed with hydrochloric acid, to open up clogged rock pores in the formation and allow fluids to flow again.