Storing Co2 in a Reservoir Under Continuous Pressure Depletion; a Simulation Study

Abstract

Norwegian Ministry of Petroleum and Energy commissioned a feasibility study in 2016 to qualify an offshore reservoir for full-scale CCS in Norway. The Smeaheia fault block, a saline aquifer east of the Troll field, was selected as the best alternative among three studied candidates. The Smeaheia CO2 storage prospect is located on the Horda Platform, mainly in Blocks 32/4 and 32/1 on the Norwegian Continental Shelf, east for Troll field. The fault block is shallower than Troll, eliminating the risk of CO2 migration and contaminating the Troll East gas reservoir. The target reservoir for CO2 storage is the Viking group, a 400-meter thick reservoir consisting of the Sognefjord-, Fensfjord- and Krossfjord Formations from top to bottom. Depth of Top Sognefjord varies from around 900 meters in east to around 1300 meters in west close to the Vette fault. Reservoir temperature is expected to vary between 35 to 45 oC depending on the depth. Based on the experience from hydrocarbon production in the Troll field, the Sognefjord and Fensfjord formations generally consist of high permeability and high porosity sands with very good lateral and vertical communication. Although the reservoir quality deteriorates to some extend moving from west to east, the reservoir properties and hence the communication is expected to be as good as in the Troll field. There are two relay ramps cutting the Vette fault with probable sand-sand communication putting the Smeaheia fault block in expected pressure communication with the Troll field. Due to large volumetric production of hydrocarbons from the Troll field, pressure depletion is also expected in the Viking group in Smeaheia fault block in case of pressure communication. In a most likely scenario, Smeaheia is depleted below hydrostatic pressure when CO2 injection starts and pressure will continue to deplete whilst CO2 injection is ongoing. The Smeaheia fault block is, in parts, already on the border of the recommended depth of 800 meters for CO2 storage reservoirs where CO2 is stored as a dense phase. Pressure depletion could cause CO2 stored in shallower parts of the reservoir to convert to gas, resulting in further expansion of gaseous CO2 and hence reducing the ultimate storage capacity of the identified structural traps. A simulation model is developed to study the CO2 movement in storage reservoir and estimate the storage capacity of the Smeaheia fault block under depletion. The structural model is based on the latest seismic interpretation for Smeaheia and the static properties are modelled based on log data from exploration wells on Blocks 32/4 and 32/1. In the absence of pressure measurements, the extent of depletion in Smeaheia as a result of Troll production is inferred from a regional aquifer simulation model. Dummy water producers are used to deplete the model as predicted by the regional model. This paper presents the studies carried out using the developed simulation model to address effects of pressure depletion on CO2 plume movement in Smeaheia. Important decision parameters such as well location, depth of injection, design of the injection well, and number of injectors to increase storage capacity are all affected by the depletion situation in the reservoir. These should be studied thoroughly in order to achieve a cost-effective and robust injection system that ensures the safe storage of CO2 in Smeaheia independent of depletion effects due to Troll production.