Multiphase Flow Performance: Operational Lessons from Frontier Developments and Suggestions for the Future
Authors: Leporcher Emile , Saint-Pierre Thierry , Cooley Chris 
Published: October 2008

In the recent years, Total has performed several developments in "frontier environments" in terms of multiphase fluid transfer from downhole to surface treatment facilities. Among them, two have been selected to be presented as field cases. The first one will describe an oil-dominated system implementing a subsea multiphase network by 1500 m of waterdepth, the second one will deal with a longer distance and deeper water subsea gas-dominated system. Both cases will remind the design assumptions and the selection process of the production technology and operational procedures. Then the field experience after two years of operation will be presented. It will focus on the gaps or agreements between predictions and recorded operational parameters. In parallel, several R&D activities are taking place in order to increase the performances of multiphase and fluid-related effects disciplines to produce in deeper and deeper waters over longer and longer distances with more and more difficult fluids. As a conclusion, lessons from these two operations in association with ultimate R&D expectations will be analyzed to suggest optimized guidelines for specifying and operating future deepwater multiphase networks.

Introduction

State functions in thermodynamics are expressed as exact total differentials of state variables. This manifests a very important property: the variation of a state function from an initial state "A" to a final state "B" does not depend on the path from A to B. For a given system, it is appropriate to select the state variables (observable and measurable) best suited for the description of the system and this choice will govern that of state function. In the real life, irreversible and reversible processes take place along the way. Furthermore, because of the length and time scale of our systems, we are never sure to reach the stationary state where the thermodynamics laws applied.

When we speak about fluid transfer in frontier environments like deepwater or/and long distance transport, the pressure and temperature gradients are amplified and create in the multiphase flow accelerated phase changes (vaporization, condensation, solid deposits). The fluid transfer of such complex thermodynamical or unusual routes becomes a challenge for the hydrocarbon production operations. Both following field examples will illustrate most of the parameters to take into consideration to succeed the optimized design of a deepwater multiphase network and upcoming operations.

Overview of the fields

The oil-dominated Girassol development includes a full subsea multiphase production system and an FPSO. The field is located 150 km offshore the coast of Angola by 1,400m of water depth. The reservoir 1,200m under the seabed has a large horizontal extent. The reservoir fluids were encountered at 268 bar and 58-69°C. API gravity is 32, the GOR is 110-130 Sm3/m3. Water flooding injection is the main reservoir drainage mechanism.

Production from the field is gathered through five 8in ID piggable production loops with deviated or horizontal wells connected through "daisy chained" manifolds (see fig 1). The wells are openhole gravel pack or natural sand pack with high productivity up to 40,000b/d. Maximum tieback distance from the FPSO is approximately 6km.