Multiphase Flow: A Cost-Efficient Solution for Marginal Field Developments

Abstract

Abstract Satellite developments are an attractive solution for producing oil from small marginal fields. However, their application is limited by the distance that the crude oil can travel with the natural pressure of the reservoir. To overcome this limitation the paper proposes to use multiphase pumping to provide energy to the fluid. To implement this technology at low technical risks, the pumping unit could be located at the surface, aboard a small floating support. The production would be exported by means of a reusable flexible pipe to a processing centre. After presenting recent advances in multiphase production, the paper describes the proposed system, designed to produce fields with 20 to 60 millions bbl. of recoverable reserves and located from 10 to 60 km, or beyond, from existing facilities or the shore. The system is characterized by its mobility. After depletion of a field, it can be easily retrieved and moved to another location. In addition to reduced capital expenditures and relocatability, the proposed system may accelerate the oil recovery of a field and the produced gas can be processed on the host platform and exported in an existing pipeline system. Introduction In spite of some discoveries above 200 millions barrels, many oil accumulations recently discovered offshore are small: less than 50 million barrels. Most of the fields are located in shallow and medium water depths. Present economical conditions are a major obstacle to develop these fields which require a cost effective solution. In mature offshore basins, such as in the North Sea, the Gulf of Mexico, South-East Asia, off the coasts of Brazil or West Africa, existing production or transportation infrastructures are often under-utilized. Satellite developments are a possible way of reducing development costs. They consist of a small wellhead platform or a subsea template tied back to a nearby facility by a multiphase pipeline. Over the last years, the number of such developments has grown quickly. For instance, in the U.K. zone of the North Sea, oil production from satellite fields has risen from 3% in 1987 to 8% in 1991, and will probably reach 11% in 1992. Around the world, approximately 12% of the 300 offshore fields under development or scheduled for development, by the end of 1991, make use of subsea trees tied back to an existing field. Fields nearer than 15 km from an existing installation are potential candidates for subsea satellite developments. The average tied back distance of existing satellites developments in the North Sea is 8 km. For oil, the present record is about 17 km (Don field in the North Sea). This distance is limited by the natural pressure of the reservoir which provides the necessary energy to drive the fluid. Many small accumulations are located at a greater distance, up to 80 / 90 km. Beyond 15 to 20 km. no commercial solutions are presently available for small oil fields. Satellite developments involving only natural pressure of the reservoir would create a high back pressure because of pressure drops in the pipeline, and this would affect the well productivity. Conversions of semi-submersibles and tankers may be considered when the recoverable reserves are larger than 60 to 80 MMbbl, but stand-alone developments remain expensive for smaller accumulations. P. 463^