Injection of CO2 is a highly technical challenge- CO2 must be injected from the surface to hundreds of metres depth through an injection well which typically has a length to width ratio of 10,000:1. The process of injecting CO2 increases the pressure in the reservoir both through the force of injection and the addition of large volumes of CO2. Great care must be taken during injection as this pressure increase may reactivate old faults or create new ones through hydrofracture. Most formations at depth are underpressured- that is the hydrostatic pressure of the pore fluid is less than that of the lithostatic pressure caused by the weight of the rocks above the formation. Over pressure and formation of fractures occurs when the hydrostatic pressure increases above the fracture gradient (the point at which the rock breaks and accepts fluid) of the formation. For depleted oil and gas fields the pressure can be safely raised to near the pressure prior to hydrocarbon production. There is a trickier challenge for injection into saline aquifers as increased fluid pressures may lead to the storage reservoir becoming overpressured depending on the permeability and lateral continuity of the reservoir.