Background/Objectives

The long-term security of carbon storage depends on the capacity of the caprock to safely retain captured CO₂ for extended periods. However, the containment depends on the structural ability of the caprock and its own residual or capillary tarping, which is related to the capillary pressure threshold of the caprock to trap the buoyant gas. Capillary leakage occurs when the phase pressure of CO₂ exceeds a threshold called a "breakthrough pressure" or a "threshold pressure". This threshold pressure represents the lowest pressure required to displace the fluid initially within the caprock and is the maximum allowed storage pressure of CO₂ in the reservoir. Capillary trapping is typically assessed by mercury intrusion porosimeter which doesn't represent in-situ conditions, or core flooding experiments by measuring the CO₂ saturation at residual conditions using standard approaches, however, in such approaches the core has to be saturated separately, which requires additional apparatus and adds an extra layer of complexity and can also change the in-situ condition of the core resulting in deviated threshold pressures. Secondly, these measuring procedures usually rely on the flow rate in the core which can be detected late even if the threshold pressure is achieved due to the very low permeability of the cap rock and this can give an overestimation of the actual pressure. Although there has been considerable interest in the precise and timely analysis of threshold pressures, limited research has been done on designing new devices to improve such measurements. 

Approach/Activities

In this study an innovative measuring tool has been engineered to conduct distinct research focused on saturating the core sample using the same apparatus under conditions involving high pressure, replicating in situ environments, and facilitating temporal and spatial analysis of the phenomena. This apparatus is designed to examine the capillary threshold pressure directly under the influence of confining pressure and assess the outlet pressure in a relatively simple way. It is composed of three integrated systems; a high-pressure system, a gas injection system, and a gas emission system. Three core samples were selected, representing a cap rock from different well sites in Poland. Their porosity and permeability were determined, the core samples were placed and sealed within the apparatus, and the fluid was injected for saturation under in situ conditions. Followed by the injection of CO₂; incrementally increasing the test pressure of the nonwetting phase while monitoring the behavior of the system's downstream pressure at each pressure increment at the sample inlet (upstream pressure). The pressure gradually increased until it rose to the outlet, indicating that the non-wetting liquid had reached the minimum critical entry pressure and penetrated the pores of the rock sample. 

Results/Lessons Learned

For caprock sample 1, inlet pressure was incrementally raised until the flow was stabilized and the threshold pressure for this core was observed between 3-4 bars at the outlet. For sample 2 the inlet pressure also started with 2 bars over some time to see any changes at the outlet. Later the pressure was increased to 10 bars incrementally, and the pressure rise at the outlet was observed between 8-10 bars representing its corresponding capillary threshold pressure. For sample 3 the initial inlet pressure was set at 2 bars, and the system was allowed to equilibrate until no pressure changes were detected at the outlet. Despite gradually increasing the inlet pressure to 60 bars, no corresponding pressure change was observed at the outlet. This suggests that the threshold for this particular rock sample has not been reached, even at the elevated pressure of 60 bars. All these experiments were conducted at a confining pressure of 150 bars and were repeated to verify the reliability of the results.