Reservoir fluid effect on caprock properties in the Horda Platform area, northern North Sea

Abstract

Upper Jurassic organic-rich shales' caprock properties in the Horda Platform area are crucial due to their importance as primary seals for the Middle Jurassic reservoir sandstones. The reservoir sandstones are Sognefjord, Fensfjord, and Krossfjord formations. These reservoir sandstones are overlain by the Upper Jurassic Draupne and Heather Formation Shales. Caprock failure due to injection-related pore pressure change is one of the critical risks for CO2 storage. The thick gas column in Troll East Field, located in the Horda Platform area, can consider an analog for a CO2 post-injection scenario in the area. Evaluating the caprock properties above the thick gas column in the Troll Field helps to understand the area's caprock integrity qualitatively. This study assesses Draupne and Heather Formation Shales' caprock properties using petrophysical and rock physical analyses. Four exploration wells from the Troll East area consider in this study where wells 31/3-1 and 31/6-1 have gas columns of approximately 224 and 222 m, respectively, and two other wells (31/3-3 and 31/6-3) are water wet. The exhumation correction suggests that Draupne and Heather Formations of four studied wells' maximum burial depth experienced only mechanical compaction. Two rock physics templates, such as Vp versus density and Young’s modulus versus Poisson’s ratio, are used to evaluate the caprock behavior due to HC accumulation-related overpressure. The caprock shales in wells with gas columns show a significantly soft nature compared to the shale in the dry well 31/3-3. However, a dry well (31/6-3) also shows remarkably low stiffness. The pressure changes due to the HC accumulation might change the pressure above the reservoir. The caprocks may behave more ductile due to the generation of non-connected micro-fractures. However, fracture connectivity might initiate the caprock's mechanical failure, which poses a significant risk in CO2 post-injection scenario. Moreover, a significantly thick caprock in well 31/6-3 may have influenced the caprock properties differently compared to the other dry well. The caprock mechanical behavior is complex and influenced by several factors.

Keywords: Caprock properties characterization, CO2 storage risk, Mechanical compaction, Seal rock Ductility