CO2 emissions and energy consumption for a subsea compression system compared with a topside compression system

Increasing the recovery rates of oil and gas fields is a common goal between energy companies and policy makers. Through pressure from the public and policy intervention due to changes in the global climate, there is a push to reduce emissions related to the production of hydrocarbons. The installation of gas compressors is often evaluated for gas fields to better the recovery rate and extend the lifetime when the production rates decline. Through development in technology, offshore compression can in some cases be performed both topside and subsea. This thesis analyses the differences in energy consumption and CO2 emissions for a subsea compression system and a topside compression system targeting increased recovery from a gas and condensate reservoir. Two notional generic gas fields were generated and a realistic production system for both reservoirs was defined. The reservoirs were assigned case numbers 1 and 2, where the difference was the condensate to gas ratio which was specified to be 15 and 1 respectively. Integrated Production Modeling was used to perform predictions of the accumulated production volumes, energy consumption and the associated CO2 emissions. The results show that the total energy consumption and CO2 emissions are significantly lower for the subsea system. The average emissions of CO2 per barrel of oil equivalent produced is approximately 53% lower for the subsea system in both cases. This is because of a lower system pressure drop and accelerated production, causing the total energy consumption of the subsea system to be approximately 37% and 38% lower over the lifecycle for Case 1 and Case 2 respectively. In addition to CO2 emissions from the compression work, emissions from production of construction materials and emissions from the support systems for the topside facility were included in the analysis. The results show that the impact of including the emissions from production of the construction materials is dwarfed by the emissions from direct power usage. The additional emissions from the energy consumption of the topside support systems are a significant contribution to overall emissions over the lifecycle. These make up approximately 23% of the total topside emissions in Case 1 and Case 2. This constitutes approximately 44% and 46% of the difference in total emissions between the two systems in Case 1 and Case 2 respectively. The accumulated production volume was higher for the subsea system in both cases, representing significant additional revenue. The additional production volume is a result of a lower system pressure drop for the subsea system, allowing a further drawdown of the reservoir pressure when compared with the topside system