This is getting friction in ND. The geological formation they want to inject the CO2 into isn't oil bearing: it is not a secondary or tertiary recovery scheme to extend the life of oil fields.
Specific concern is present over the Lake Nyos disaster, which, although natural, illustrates what can happen if large amounts of CO2 are released to atmosphere in a populated area. You can't burn it off, it accumulates in low areas, is odorless, colorless, and will not allow you to run any conventionally fueled vehicle (not to mention will asphyxiate you).
North Dakotans, especially considering the route was near enough to the state capital to provide a foreseeable limit on further development in the direction of the pipeline route, have some valid concerns and objections to a project many feel isn't necessary in the first place.