Assessing the representative threshold capillary property of a reservoir formation is very important in the context of reservoir production, basin modeling and geological storage. The long-term safety of CO2 storage is highly dependent on the permanent sealing capacity of the cap rock.

As part of the Northern Lights CO2 storage project, Equinor with partners Shell and TotalEnergies, in Q1 2020 drilled well 31/5-7 in exploitation license EL001 southwest of Troll A in the northern part of the North Sea. The objective of the well was to investigate whether the formations in the Lower Jurassic Dunlin Group (Cook and Johansen storage formations and Drake Formation cap rock) are suitable for storage of CO2.

In this work, laboratory experiments were used to determine the CO2 cap rock threshold pressure for reservoir core material from well 31/5-7. The experiments were performed at reservoir conditions, 260.5 bar and 83.5°C. Prior to the tests, the permeability to water was measured to be in the nano Darcy range. The dynamic method for measuring the threshold capillary pressure relies on establishing a flow of water through the plug before CO2 reaches the plug's surface. Due to the low permeability of the plugs, the amount of water that must be displaced before CO2 contacts the plug's surface was kept low from the inlet. Thus, if CO2 contacts the plug's surface at a pressure below the threshold value, the water production should stop. The onset of water production from the outlet will define the threshold pressure, as CO2 will flow into the plug once the threshold pressure is exceeded. The CO2 pressure was gradually increased stepwise up to a maximum of 360.5 bar over 4 weeks.

The capillary threshold pressure for CO2 was measured for three plugs and was in the range 30 - 50 bar above the pore pressure of the reservoir at reservoir conditions. However, should the aquifer pressure exceed this value, the containment of CO2 below the Drake formation is ensured by its very low permeability (few tens of nD) that will preferentially drive the CO2 horizontally within the storage formations rather than vertically through Drake formation. No values were obtained within the time frame of the experiments for the two remaining plugs. The plugs were exposed to an injection pressure of 360 bar, 100 bar above the pore pressure, and no entry was observed. Thus, the minimum capillary pressure is expected to be greater than 100 bar for these two plugs.

You can access this article if you purchase or spend a download.