What are the best practices for well testing on FPSOs?

At-a-Glance: Robust FPSO well testing hinges on safe, low-flare data acquisition that captures stabilized rate–pressure behavior and fluid PVT while managing motion, limited test capacity, and SIMOPS. Key KPIs: test data quality (uncertainty), test duration, flaring intensity, uptime, and HSE compliance.

I. Objective Definition and Key KPIs

• I.1 Objective: Execute repeatable, low-emission well tests on an FPSO to derive reliable inflow, product quality, and allocation factors without upsetting host production or violating permits.
• I.2 Test outcomes: stabilized multiphase rates, GOR/CGR, water cut, sand rate, PVT samples, wellhead/tubing/dh pressures, temperature, and IPR/PI/skin/reservoir deliverability.
• I.3 KPIs:
  • Data quality: oil rate uncertainty = ±1.0–1.5%; gas = ±2–3%; water cut ±2% abs; pressure drift = 0.5 psi/min at stabilization.
  • Throughput/uptime: = 95% test runtime within target envelope; < 1 unplanned shutdown per test.
  • Emissions: flared gas per test = permit; flaring intensity = 1–3 kg CO2e/boe (estimated).
  • Operability: no HP/LP trips or KO drum carryover; zero burner flameouts; no PSVs lift.
  • HSE: zero recordables; ALARP verification of flare radiation; SIMOPS conflicts zero.

II. Critical Parameters and Target Ranges

III. Step-by-Step Procedure / Workflow / Checklist

III.A Planning and SIMOPS

• III.A.1 Define test matrix: 2–4 choke steps per well (low, mid, high, facility limit) + final buildup; schedule high-rate points within flare/permit windows.
• III.A.2 Permit and risk review: HAZID/HAZOP, flare radiation and smoke modeling, noise assessment, and environmental consent; finalize Maximum Credible Rate for flare and separators.
• III.A.3 SIMOPS plan: No cargo offloading, heavy lifts, or helideck ops during high-rate test; compressions/MEG/utility availability confirmed; weather limits set (e.g., Hs, wind).
• III.A.4 Allocation strategy: Decide test separator vs MPFM reference. If MPFM used, plan bracketing tests to update calibration coefficients.
• III.A.5 Data plan: Tag historian points; 1–5 s scan for pressures/temperatures/flow; 1-min event log; sampling plan and custody seals.

III.B Pre-Job Readiness

• III.B.1 Mechanical checks: Line-up test manifold; pressure test to MAWP; verify SSV/SCSSV/ESD functionality; check choke trim condition (erosion); confirm separator internals and LCV tuning.
• III.B.2 Metering: Prove oil meter; verify gas meter orifice/ultrasonic configuration and gas composition; water cut analyzer calibration with standards; sand detector zero/bump test.
• III.B.3 Burner/flare: Inspect burner tips, pilots, flame scanners, KO drum level, seals; verify pilots auto-ignition; set wind/heading alarm limits; confirm heat shields.
• III.B.4 Chemicals and utilities: Stock demulsifier/antifoam/corrosion inhibitor; verify MEG/LDHI; ensure heater duty available; confirm instrument air/N2 backup.
• III.B.5 Communication: Toolbox talk, control room briefing, test log sheets, roles and ESD actions; isolate non-essential alarms to avoid nuisance trips.

III.C Execution – Flow and Stabilization

• III.C.1 Warm-up and purge: Crack open choke to minimum; divert to flare initially; purge riser/flowline. Purge time estimate: \( t_{\text{purge}} = N \cdot \dfrac{V_{\text{line}}}{Q} \) with N = 3–5 (estimated).
• III.C.2 Route to test separator: Bring to test separator once clean and stable; ramp choke in small steps; hold each rate until criteria met (Section II).
• III.C.3 Liquid management: Adjust level controls to dampen vessel motion; use antifoam to avoid KO drum carryover; maintain heater to target temp; watch interface levels for emulsion.
• III.C.4 Gas handling: Prefer compression to HP flare; if flaring, ensure smoke suppression per spec; monitor compressor anti-surge; hold suction pressure within target.
• III.C.5 Sampling: At each stabilized point, take pressurized PVT (oil/gas) and water samples; document choke, rates, P/T, WC, sand.
• III.C.6 Data capture: Snapshot all pressures (WH, THP, casing), separator P/T/L, meter totals/factors, vibration, and chemical rates.

III.D Rate Steps and Build-Up

• III.D.1 Multi-rate program: Capture at least three distinct stabilized rates to define IPR; avoid exceeding erosion or flare limits.
• III.D.2 Shut-in: For buildup, close surface SSV rapidly and safely; if SCSSV closure is required for downhole isolation, coordinate pressure transients to avoid hydrates.
• III.D.3 Pressures: Record downhole gauge if available; otherwise high-frequency WHP/THP; maintain thermal stability to reduce drift.

III.E De-Rig and Handover

• III.E.1 Flush and isolate: Displace test lines with treated fluid/inert; return well to production routing; remove temporary blinds/taps with PTW closure.
• III.E.2 Data QA/QC: Validate against material balance; reconcile MPFM vs test separator; lock test report with sign-offs.

IV. Risk and Mitigation (HSE, Reliability, Redundancy)

• IV.1 Motion-induced upsets: Vessel pitch/roll can cause level swings and foam-over.
  • Mitigation: retune LCV/PID for slower response; use anti-foam; maintain adequate surge volumes; set sea-state limits; ensure KO drum capacity and high-level trips are functional.
• IV.2 Flare/burner hazards: Radiation, smoke, flameout, and liquid carryover.
  • Mitigation: verify pilots/scanners; smoke suppression; burner tilt within wind envelope; enforce exclusion zones; double-block and bleed on drains to flare KO drum.
• IV.3 Compression trips/backpressure: Upsets degrade test stability.
  • Mitigation: hold suction P above min; stagger rate steps; tie-in spare compressor if available; auto-switch to flare with soft landing.
• IV.4 Hydrates/wax/asphaltenes: Cool risers and pressure drops create risk.
  • Mitigation: pre-heat fluids; inject MEG/LDHI; maintain depressurization plan; avoid long cold shut-ins; insulate riser sections as feasible.
• IV.5 Sand/erosion: High-velocity chokes and elbows at risk.
  • Mitigation: limit dP across choke; use hardened trim; real-time sand monitoring; cap rate if erosion alarms trend upward.
• IV.6 H2S/CO2/VOC exposure: Toxicity and corrosion.
  • Mitigation: fixed/portable gas detection; breathing apparatus at burner deck; use corrosion inhibitor; verify metallurgy and PSV setpoints.
• IV.7 SIMOPS conflicts: Offloading/DP/winch ops can change heading/backpressure.
  • Mitigation: SIMOPS matrix; lock-out offtake; weather-vaning constraints; bridge watch; ESD drills.
• IV.8 Metering integrity: Drift and bias.
  • Mitigation: pre/post proving; spot checks with portable references; uncertainty budget tracking.
• IV.9 Electrical/instrument: Power dips cause trips.
  • Mitigation: UPS on critical instruments; redundant transmitters; permissive interlocks reviewed and tested.

V. Optimization Levers (Data, Maintenance, Debottlenecking)

• V.1 Test design: Use design of experiments (DOE) for rate steps to maximize information per flare unit; combine low-flare extended stabilization with one short high-rate point.
• V.2 MPFM strategy: Run frequent short validation tests to maintain MPFM coefficients; trend drift and update allocation factors.
• V.3 Digital analytics: Real-time stabilization detector using thresholds on dGOR/dWC/dP; automate step-hold-release; filter slugging with moving medians.
• V.4 Slug management: Use choke modulation, active gas-lift tuning, and separator anti-surge volumes; consider inline cyclonic deslugging upstream of test separator.
• V.5 Emission minimization: Prefer closed test to production where possible; recycle gas to compression; schedule tests during low ambient winds to avoid smoke constraints; quantify CO2 saved per hour deferred.
• V.6 Chemical optimization: Dose sweeping to minimize BS&W and foam while reducing OPEX; maintain chemical KPIs: demulsifier dose/benefit curve and foam index.
• V.7 Maintenance windows: Align tests post-meter proving and post-compressor overhaul to reduce downtime; keep a spare choke trim and burner ignitor kits onboard.
• V.8 Debottlenecking: If repeated flare-limited, consider temporary portable compression or additional burner booms (subject to permit) and test at night within radiation envelope.
• V.9 Data integration: Link test results with RAM model to forecast uptime impact; feed IPR updates into production optimization and lift settings.

VI. Verification & Monitoring Plan

• VI.1 During test:
  • Track stabilization metrics every minute: ?P/?t, ?GOR, ?WC, KO drum level variance, compressor anti-surge proximity.
  • Alarm thresholds: dp/dt > 1 psi/min, KO drum level > 80%, burner flameout detection, separator DP high.
• VI.2 Post-test QA/QC:
  • Material balance across test separator: oil + gas + water vs inlet; investigate if imbalance > ±2% of total.
  • Compare MPFM to test separator: bias = ±3% oil, ±5% gas. Update coefficients if persistent.
  • Uncertainty roll-up: \( u_c = \sqrt{\sum u_i^2} \) across meters; report 95% confidence intervals.
• VI.3 Engineering analysis:
  • Productivity index: \( J = \dfrac{q_o}{p_r - p_{wf}} \). For gas, use pseudo-pressure if applicable.
  • Multi-rate IPR fitting: Vogel (solution-gas drive) or PI linear where valid.
  • Build-up (Horner) diagnostic: \( p_{ws}(t) = p^* + m \ln \left(\dfrac{t_p + \Delta t}{\Delta t}\right) \); confirm straight-line late-time to estimate \(k h\) and skin.
• VI.4 Frequency:
  • New wells: weekly during cleanup then monthly to quarterly.
  • Mature wells: quarterly or upon significant change (WC, GOR, sand, lift change).
  • After interventions: baseline test within 24–72 hours.
• VI.5 Reporting: Structured report with raw data, stabilized points, uncertainty, PVT results, IPR/PI, emissions summary, HSE deviations, and recommendations to lift settings and allocation.

Key FPSO-Specific Best Practices (Highlights)

• Motion-proof separation: Slow level control, anti-foam, and adequate surge to prevent foam-over and carryover.
• Flare discipline: Pre-define a flare budget per test step; prioritize closed testing or gas recompression; monitor smoke index continuously.
• Short, informative steps: Fewer, well-chosen stabilized points with solid PVT/samples beat long, flare-heavy programs.
• Slug awareness: Start with gentle choke ramps; coordinate gas-lift; use deslugging logic to avoid compressor trips.
• SIMOPS control: Freeze offloading/cranes/heading changes during high-rate points; weather window enforcement.
• Meter integrity: Prove meters before/after; reconcile with material balance; update MPFM coefficients promptly.