Porthos – CO2 Storage in Highly-Depleted Gas Fields

Abstract

In two planned large-scale CCS projects in the Netherlands – Porthos and Aramis – depleted gas fields will be used for CO2 storage. These fields are characterized by low reservoir pressures. For example, the Porthos project is planned to inject into a field with a reservoir pressure below 20 bar. Project design and operational philosophy need to be specifically tailored to the storage reservoir properties in order to avoid excessively low temperatures when injecting into such fields. This paper describes how these challenges were addressed for the Porthos project. In most CCS projects, a CO2 mixture is transported in a surface network at high pressure and ambient temperature and injected into an aquifer. At the high reservoir pressure typical of aquifer storage the CO2 stream remains in dense phase or supercritical conditions in the entire system. This dense phase transport strategy is not feasible for the P18 field since the bottomhole pressure (BHP) is around 25 bar at the required injection rates. At this low pressure, CO2 will exist in two-phase conditions which results in very low temperatures of −10 °C. These low temperatures are unacceptable since they may result in hydrate formation in the reservoir and well integrity issues. A specific operating philosophy and project design was developed to avoid unacceptably low temperatures. At a reservoir pressure below 50 bar, CO2 is injected in gas phase in the pipeline and wells. Once the reservoir reaches a pressure of 50 bar the pipeline pressure is increased to 85 bar to achieve dense phase conditions. The well is operated in two-phase conditions but due to the higher BHP well temperatures are now acceptable. However, if CO2 is transported at ambient temperature the injection flow range per well is very narrow and the required project injection range cannot be met. This is addressed by using the heat of compression to heat the CO2 stream and insulating the pipeline to achieve elevated arrival temperature. Without these specific choices, safe injection into the P18 field would not have been possible.