An Alternative Economic Method to Riserbase Gas Lift for Deep Water Subsea Oil/Gas Field Developments
Authors: Johal K.S. , Teh C.E. , Cousins A.R. 
Published: December 2007

An Alternative Economic Method to Riserbase Gas Lift for Deep Water Subsea Oil/Gas Field Developments.

This paper presents the results of a new concept that can be utilized to replace a conventional riser base gas lift system with considerable added benefits in both capex and opex savings with simplified operational procedures and is particularly suitable for deep water oil/gas field developments. A riser base gas lift system is normally incorporated within a subsea field development to (1) minimise unstable pressure and flow fluctuations resulting from normal and or low flowrate production conditions (2) minimise severe slugging due to a combination of riser configuration and production flow hydrodynamics (3) minimise back pressure on wells through the reduction of static pressure head and start-up instability.

The utilization of a riser base gas lift system for deep water fields poses a number of technical problems such as low temperature effects resulting from Joule-Thomson cooling of the gas across the control valves/chokes in the system and require hydrate inhibition. These requirements add to the complexity and operational risk of the riser base injection and control system.

The concept presented overcomes these technical issues through operational procedures developed through the application and understanding of multiphase transient behaviour and control with an added benefit of considerable cost reduction. This paper presents the application of this concept and the associated technical analysis based on modeling of transient multi-phase production operational modes. The proposed concept system can be employed in deepwater field developments.

The proposed system removes the need for a dedicated and complex riser base gas lift system of flexibles and risers, gas transport tubing and associated equipment by using a multiphase riser base lift system method. Comparative economic analysis has shown the potential for significant CAPEX savings (circa 5,000,000) with additional saving from a reduced chemical injection and heating system.

Introduction

Oil and Gas field developments especially in deeper waters that tie back to a receiving facility can experience unstable production pressure and flows. The use of a riser base gas lift system is one method of providing production stabilization. In general, riser base gas lift is beneficial for (1) stabilizing production conditions during severe slugging operating under both normal and system start-up in the pipeline/riser system, (2) minimizing slug sizes arriving on topsides process vessels i.e. slug catcher/separator, (3) reducing the static head in the riser and hence back pressure on the well/reservoir to improve productivity and enable stable start-up after a hot oil flushing operation e.g. when the flowline riser system is completely full of liquid and (4) minimizing pressure fluctuations at the wellhead (and hence downhole) resulting from flowline/riser instability. The main problems, however, associated with operating riser base gas lift systems in deep water developments are the resulting low temperatures at the injection point and the impact on the pipeline material and produced fluids (e.g. hydrates formation and wax deposition). To avoid low temperatures, heating of the lift gas is required, which in turn puts additional demand on the vessel's power supply and requires additional equipment allocation.

Riser base lift gas requirement studies conducted during the parallel engineering phase of BP's Foinaven fast track deep water oil/gas field development project showed little benefit of riser gas in reducing slug sizes and associated gas surge rates. P. 487^