Evaluation of Caprock Integrity with Laboratory Scale In-Situ CO2 Injection Test
Authors: Adisornsupawat Kanitthorn , Sangnimnuan Anusarn , Rabiabpo Porranut , Govindarajan Sudarshan , Thombare Akshay , Gokaraju Deepak , Aldin Munir 
Published: August 2024

ABSTRACT:
Maximum injection or breakthrough pressure is a key factor to determine storage volume of CO2 and to ensure integrity of caprock. In some studies, the maximum injection pressure is based on 90% of fracture pressure (Solomon, 2007). Seal capacity using Mercury Injection Capillary Pressure (MICP) has mainly been utilized in petroleum industry to obtain maximum injection pressure (Purcell, 1949). Laboratory experiments of injection pressure can be performed using both direct and indirect method. The indirect method that widely used mainly consists of mercury intrusion porosimetry. This can quickly estimate breakthrough pressure. However, conversion factor from mercury-air condition into CO2-brine condition is needed. In addition, most of the tests are conducted at ambient condition where impact of pressure and temperature are excluded. In this study, Super Critical CO2 is injected directly into reservoir samples to obtain maximum injection pressure under reservoir condition. The step rate pressure was then applied at a series of increasing magnitudes with a steady holding period until breakthrough pressure is observed. Representative stress and temperature were maintained using confining pressure and heater in triaxial system. Acoustic and resistivity sensors were recorded in real-time. Results of pressure breakthrough for outcrops and reservoir samples are presented.

1. INTRODUCTION
With the increase of global warming due to excessive release of CO2 emission, Carbon dioxide capture and geological storage or utilization and storage (CCS or CCUS) has become crucial and to be implemented to meet the net zero emission goals (Masson-Delmotte et al., 2018, Metz et al., 2005). In many industries, the policies start to include the net zero emission as one of the main company's goals. Carbon credit will also be obtained as the CO2 emission has been reduced. The implementation of CCS and CCUS has started globally, including the US, EU, and Asia. The policy for each region/country differs but under the same goal to reduce CO2 emission.
The success of a CCUS operation and emissions reduction is dependent on injecting CO2 back to the reservoir after production and keeping the CO2 stored inside the reservoir without migrating to the caprock. Therefore, the caprock sealing ability is crucial and needs to be studied thoroughly. One of the parameters used to characterize the caprock integrity is the breakthrough pressure, which is the point where CO2 starts to leak into the caprock after being injected into the reservoir. The breakthrough or maximum injection pressure is also used to design the CO2 storage volume in reservoir or aquifer. Whether the desired volume can be achieved or not, successful sealing is directly dependent on breakthrough pressure. If the breakthrough pressure used in the study is too low, the storage volume might not be enough to meet the CO2 reduction and carbon credit goal set by the company. This will result in a new site or an additional site to be able to meet the injection target (Smith et al., 2011, DOE-NETL, 2017).