The Use of Subsea Gas-Lift in Deepwater Applications Subash Jayawardena, George Zabaras, and Leonid Dykhno Shell Global Solutions (US) Inc.
Contents Why Gas-Lift is Needed Gas Lift Delivery System Design Dedicated GL riser vs. Shared GL riser GL injection location Insulation Operating Envelope Flow Assurance Concerns Case Studies Systems Suitable for Gas-lift Conclusions
Why Gas-Lift is Needed - Production enhancement Effect of gas lift on flowline pressure 25 2 low water cut manifold pressure, psi 15 1 5 1 2 3 4 5 6 7 8 liquid rate, BLPD % WC, no GL % WC, 2MMSCFD GL
Why Gas-Lift is Needed - Production enhancement Effect of gas lift on flowline pressure 25 2 high water cut manifold pressure, psi 15 1 5 1 2 3 4 5 6 7 8 liquid rate, BLPD 75% WC, no GL 75% WC, 2MMSCFD GL
Why Gas-Lift is Needed - Production enhancement Effect of gas lift on flowline pressure 25 manifold pressure, psi 2 15 1 5 GL FWHP 1 2 3 4 5 6 7 8 liquid rate, BLPD 75% WC, no GL 75% WC, 2MMSCFD GL FWHP
Why Gas-Lift is Needed - Flow stabilization Gas Lift for Slug Suppression 8 7 Severe Slugging 45 4 OLGA results Flow rate to host & flowline pressure pressure, bar 6 5 4 3 With Gas Lift 35 3 25 2 temp, C 15 No GL With GL 2 1 No Gas Lift 1 5 1 9 2 19 18 liquid rate to FPSO, bbl/d f lowline pressure, psi 24-Dec-5 17: 24-Dec-5 18: 24-Dec-5 19: 24-Dec-5 2: Flowline Pressure Boarding Pressure Gas Lift Temp 17 8 16 15 bbl/d 7 6 5 4 3 psia 14 13 12 11 1 9 8 7 6 5 Field Data pressures w/ & w/o gas lift 2 4 3 1 2 1 2 3 4 5 Time [h] 6 7 8 9
Why Gas-Lift is Needed - Flowline depressurization Trend data 2 18 16 production Shut in FL pressure - with gas lif t assist FL pressure - without gas lift assist 14 blowdown 12 psia 1 8 w/o gas lift 6 4 2 2 HDP 21 22 23 24 25 26 27 28 Time [h] with gas lift 29 3 31 32 33 34 35 36
Gas Lift Delivery System Design Shared Gas Lift Risers Uses of gas lift Flowline operating conditions Host limitations Gas Lift Injection Location Geometry (FL, well location) Uncertainty of reservoir performance Blowdown Insulation Cooldown time Cost Considerations production production Subsea Routing or Subsea Splitting (metering/control) gas lift gas gas lift gas
Operating Envelope GL Rate Needed (how much and when) Production enhancement Flow stabilization Depressurization Source of lift gas Available pressure and temperature Heating & compression Flowline Pressure With and without gas lift Dead oil filled Cooldown time considerations Consider all credible scenarios Design Operation
Flow Assurance Concerns Hydrates in GL system Prevention Remediation Asphaltene Destabilization Thermal Considerations Arrival Temperature Cooldown time Slugging Erosion in flowline Available Gas - quality Low Temperature Concerns J-T cooling Engineering and procedural solutions
Case Studies case status WD ft FL Geometry water inj Gas-Reinj 1a producing 6 uphill no no 1b producing 6 downhill no no 2 producing 3 uphill (with dip) yes no 3 design 3 uphill yes no 4 design 3 mixed yes no 5 producing 35 uphill yes no 6 design 45 downhill yes yes
Selected Solutions case FL Geometry water inj Gas-Reinj location shared insulated when 1a uphill no no RB yes *? when needed 1b downhill no no RB/sled no yes/no first oil 2 uphill (with dip) yes no RB no yes first oil 3 uphill yes no manifold yes yes first oil 4 mixed yes no RB no? yes first oil 5 uphill yes no manifold? yes when needed 6 downhill yes yes manifold? yes when needed Location Well & host locations, flowline size and uncertainties about production rates Shared vs. dedicated Host limitations, flowline geometry Insulation Arrival temperature and cooldown time, reservoir temperatures Timing Uncertainty about gas-to-liquid ratio, uses of gas lift
Systems Suitable for Gas-lift Oil systems High water cut and/or low GOR GLR < 5 scf/stb High water depth 3 to 6+ ft Watch for J-T cooling Downhill flowlines Some Uphill flowlines Low to moderate viscosities
Conclusions There is no one size fits all design of gas lift system. Need for integrated subsurface flowline modeling over the field life Accurate modeling of multiphase flow in large diameter, deepwater risers Understand the uncertainties about the reservoir performance Defining the operating envelope of gas lift system include all credible scenarios. Shared gas lift risers are possible, but not always.