The volumetric method requires a simple geometric description of the formation that includes formation height and area, porosity, and some type of factor that reflects the pore volume that CO2 can occupy.
The compressibility method relates the change in pore pressure to the change in water and pore volume.
Pore pressure will increase as CO2 is injected.
This causes a decrease in water volume and an increase in pore volume.
The sum of these changes can accommodate space for CO2 storage.
This is primarily an issue for closed reservoirs.