Operational envelope for planning cold flow production

Abstract

The paper presents a method to determine an operational envelope for the cold flow technology with respect to producing gas–oil ratio (GOR) and water cut (WC). The cold flow technology is a flow assurance method that enables cold production flow in thermal equilibrium with the seabed temperature. Cold flow converts hydrate and wax into dry, non-plugging microparticles inside a subsea cold flow unit downstream the wellhead. The small particles travel along with the liquid in the form of a slurry to the receiving host. The operational envelope shows that if the production flow reaches certain combinations of GOR and Wc, the hydrate volume fraction in liquid and consequently the slurry’s effective viscosity could be too high for pipe transportation. This paper also investigates the case when the slurry is transported with remaining water, where there might be some emulsion phenomena. Results show that this could reduce the operational region of the cold flow envelope, particularly for GOR–Wc values around the inversion point volume fraction. Therefore, water–slurry emulsification should also be taken into consideration when considering the operational envelope. If the hydrate–oil mixture is found to be too viscous to transport it can be resolved by, for example, removing water from the production flow upstream the cold flow unit. GOR–WC profiles from a real field case are over-imposed on the operational envelope to see the development during the field lifetime. Tieback costs for the cold flow technology are found to be 20–30% less than conventional methods for a 100 km tieback. The relative cost reduction increases with increasing tieback distance.