How to perform well testing on offshore platforms?

At-a-Glance: Offshore well testing is a controlled, short-duration operation to quantify well deliverability and fluid properties under safe, permitted conditions. Execute with a clear objective, correctly sized surface spread, disciplined flow–shut-in sequence, rigorous metering/sampling, and robust HSE barriers.

I. Objective Definition and Key KPIs

• I.1 Primary objectives
  • Establish stabilized rates (oil, gas, water) at standard conditions and flowing wellhead pressure (FWHP) across 2–4 choke settings.
  • Determine reservoir pressure, skin, and k·h via buildup or multi-rate testing.
  • Collect representative PVT and water samples (pressurized where needed).
  • Validate MPFM against a reference (test separator) and quantify uncertainties.
• I.2 Operational KPIs
  • Data uptime: =95% of planned test window.
  • Rate measurement uncertainty (1s): liquids =±3%; gas =±5%.
  • Mass balance closure on test separator: 95–105% over stabilized windows.
  • Stabilization criterion: rate and FWHP drift =±5% over =30–60 minutes.
  • Reservoir pressure uncertainty: =±50 psi; skin uncertainty: ±1.
  • HSE: zero recordables; no unplanned ESDs; flare within permit; emissions intensity minimized.
  • Sand production: below facility threshold (e.g., =10–20 mg/L or no sustained sand alarms).

II. Critical Parameters and Target Ranges

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

III.1 Planning and Readiness

• III.1.1 Define scope and envelope
  • List test objectives, expected min/avg/max rates, fluid PVT, H2S/CO2, sand risk.
  • Obtain flare/vent permits, produced water discharge consent, and any third-party approvals.
  • SIMOPS plan: production impacts, helideck, lifting operations, hot work, ESD philosophy.
• III.1.2 Engineering and HAZID/HAZOP
  • Process and instrumentation diagram (P&ID) for test spread: choke manifold, test separator, heaters, MPFM, burner/flare, desander, tanks, reliefs, ESDV/SDVs, drains.
  • Verify MAWP of all spools/hoses; specify double isolation and non-return valves at well test inlet.
  • Thermal radiation and noise contours for burner; swing angles vs wind/current; exclusion zones.
• III.1.3 Equipment and instrumentation
  • Choke manifold (manual/auto), rated to wellhead pressure; erosion-resistant trims if sandy.
  • Test separator with level controls, high-performance internals; dedicated oil/gas meters (Coriolis/turbine/orifice) with flow computer.
  • MPFM on test line; plan cross-check against separator during at least one stabilized point.
  • Burner/flare package with ignition, pilots, flame arrestors, and radiation monitoring; water curtain if required.
  • Chemical injection: methanol/MEG, demulsifier, defoamer, wax inhibitor, corrosion inhibitor.
  • Sand monitoring: acoustic probe; optional wellhead desander/bypass.
  • Gauges: surface pressure/temperature sensors, downhole memory or surface readout gauge; data logger with synchronized time.
• III.1.4 Pre-mobilization QA/QC
  • Calibration certificates for all meters and transmitters; verify K-factors and fluid property settings.
  • Functional test of ESD loops, PSD/ESD cause-and-effect, gas detection, and ignition systems.
  • Pressure test test-spread to 1.3× MAWP with documented hold and leak checks.
  • Crew competency and toolbox talks; roles for choke operator, panel operator, test engineer, safety watch.

III.2 Execution: Flow, Stabilize, Sample, Buildup

• III.2.1 Controlled start-up and cleanup
  • Open upstream SDVs per permit; verify pressure equalization and barrier status.
  • Begin with small choke, ramp slowly; monitor FWHP, line temps, differential across choke, separator levels.
  • Cleanup until sand decays to acceptable levels and produced fluids free of excessive debris/emulsion.
• III.2.2 Stabilized test points (2–4 chokes)
  • At each choke, hold steady separator P/T and levels; tune chemicals to minimize foam/emulsion.
  • Collect stabilized data once drift =±5% for =30–60 minutes: FWHP, WHT, rates (oil/gas/water), GOR, WGR/WOR, sand alarms, engine/burner parameters.
  • Record environmental conditions (wind, sea state) and SIMOPS status.
• III.2.3 Meter cross-check and uncertainty
  • Run MPFM in series with test separator; compare liquids and gas over at least one stable point.
  • Apply correction factors if within allowable procedures; document residual bias.
• III.2.4 Sampling
  • Pressurized recombination samples for oil and gas at conditions above bubble point/dew point.
  • Water sample at separator water draw with corrosion, scale, and bacteria bottles as required.
  • Label with date/time, choke setting, separator conditions; maintain chain of custody.
• III.2.5 Pressure buildup (or drawdown)
  • Shut-in at wellhead (preferred) to minimize wellbore storage; record downhole and surface pressures at high frequency initially (1–5 s), then decimate.
  • Hold until derivative shows radial flow and approach to reservoir pressure (estimated), typically several hours to overnight depending on mobility.
• III.2.6 Optional multi-rate deliverability test
  • Two- or four-point test for gas wells to fit backpressure equation.
  • Vogel curve points for solution-gas drive oil wells (if p_wf below p_b).
• III.2.7 Controlled shutdown and demobilization
  • Depressurize to closed drain per procedure; purge gas lines; flush chemical lines.
  • Post-job meter verification; collect as-left calibration and ESD function records.
  • Backload equipment within crane/weather limits; close-out SIMOPS.

III.3 Data Reduction (onboard)

• III.3.1 Convert rates to standard conditions; close mass balance.
• III.3.2 Generate IPR/deliverability curves and preliminary reservoir parameters (k·h, s, p*).
• III.3.3 Validate against MPFM and historical production; flag anomalies for re-test if time allows.

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

• IV.1 High-pressure hydrocarbons on deck
  • Mitigation: Double isolation, check valves, pressure testing, ESD interlocks, rated hoses/spools, pressure relief to safe location.
• IV.2 Flaring/burning hazards (radiation, smoke, noise)
  • Mitigation: Burner capacity checks, radiation modeling, exclusion zones, pilot/ignition redundancy, wind-direction monitoring.
• IV.3 Hydrates/wax/asphaltenes
  • Mitigation: Maintain temperature, chemical injection (MEG/MeOH, inhibitors), insulation/heat tracing, depressurization strategy.
• IV.4 Sand erosion
  • Mitigation: Conservative choke-up; erosion-resistant trims; desanders; sand monitoring with triggers to step down or shut-in.
• IV.5 H2S/CO2 toxicity and corrosion
  • Mitigation: Area classification, gas detection, escape sets, sour-service metallurgy, corrosion inhibitor dosing, confined space controls.
• IV.6 SIMOPS conflicts and lifting
  • Mitigation: Permit-to-work integration, lifting plans, weather windows, helideck coordination, stop-work authority.
• IV.7 Environmental discharges
  • Mitigation: Produced water routing to treatment; zero overboard for test drains; flare within permit; spill response readiness.
• IV.8 Data loss/quality issues
  • Mitigation: Redundant logging, clock sync, backup power, onboard QA/QC dashboards, spot checks vs manual tank/gauge readings.

V. Optimization Levers (Performance, Cost, Emissions)

• V.1 Use MPFM for most points; validate on 1–2 separator points to minimize flaring time and footprint.
• V.2 Dynamic choke schedule: smaller initial steps, longer hold at highest rate to maximize test value while staying within sand/emulsion limits.
• V.3 Real-time analytics: derivative plots onboard for buildup to decide optimal shut-in duration; stop early when radial flow achieved.
• V.4 Chemical optimization: titrate demulsifier/defoamer with bottle tests to shorten stabilization and improve separator efficiency.
• V.5 Heat management: preheat at separator/burner to reduce smoke and improve combustion, reducing visible emissions and shutdowns.
• V.6 Redundancy: spare transmitters, choke trims, ignition systems; hot-standby data logger; quick-change filter elements.
• V.7 Deck utilization: modular skid layout, vertical stacking where safe, and pre-assembled manifolds to shorten crane time and exposure.

VI. Verification & Monitoring Plan

• VI.1 What to measure
  • Pressures and temperatures: WHP, WHT, upstream/downstream choke, separator P/T, burner header P.
  • Rates: MPFM phase rates, separator oil/gas/water meters, flare/vent rate if metered.
  • Composition/quality: GOR, water cut, sand alarms, basic sediment and water (BS&W), samples.
  • ESD/gas detection status, burner flame status, chemical injection rates.
• VI.2 Frequency
  • Transient phases: 1–5 s logging; stabilized phases: 10–60 s aggregates; hourly rollups for KPIs.
  • Manual rounds every 30–60 minutes for levels, foam, leaks, vibration, radiation readings.
  • Daily calibration drift check on critical transmitters if feasible.
• VI.3 Acceptance and close-out
  • Mass balance within 95–105%; MPFM vs separator within uncertainty bands.
  • Stable IPR points (FWHP and rates steady within ±5%); derivative indicates radial flow during buildup.
  • Lab PVT/water analysis chain of custody complete; reconciliation of on-site vs lab shrinkage factors.
  • Final report: objectives met, KPIs, data plots, uncertainties, recommended operating envelope.

VII. Core Equations and Conversions (Oilfield Units)

Assumptions (estimated): Single-phase properties during flow points unless noted; standard conditions at 14.7 psia and 60 °F; base-10 logarithms where shown.

VII.1 Rates to standard conditions

• VII.1.1 Oil and water standard rates:

  \( q_{o,sc} = \dfrac{q_{o,sep}}{B_o}, \quad q_{w,sc} = \dfrac{q_{w,sep}}{B_w} \)

• VII.1.2 Gas volumetric correction from line to standard:

  \( q_{g,sc} = q_{g,meas}\times \dfrac{P_{meas}}{T_{meas}}\times \dfrac{Z_{meas}}{Z_{sc}}\times \dfrac{T_{sc}}{P_{sc}} \)

• VII.1.3 GOR and water cut:

  \( R_s = \dfrac{q_{g,sc}}{q_{o,sc}}, \qquad f_w = \dfrac{q_{w,sc}}{q_{o,sc}+q_{w,sc}} \)

VII.2 Deliverability and IPR

• VII.2.1 Productivity index (single-phase oil):

  \( J = \dfrac{q}{p_r - p_{wf}} \)

• VII.2.2 Steady radial flow with skin (oil):

  \( q = \dfrac{k\,h}{141.2\,\mu\,B}\cdot \dfrac{(p_r - p_{wf})}{\ln\!\left(\dfrac{r_e}{r_w}\right) + s} \)

• VII.2.3 Gas backpressure deliverability:

  \( q = C\,(p_r^2 - p_{wf}^2)^{n} \) where C and n from two-point test.

• VII.2.4 Vogel (solution gas drive oil, \(p_{wf} \le p_b\)):

  \( \dfrac{q}{q_{max}} = 1 - 0.2\left(\dfrac{p_{wf}}{p_b}\right) - 0.8\left(\dfrac{p_{wf}}{p_b}\right)^2 \)

VII.3 Buildup analysis (Horner method)

• VII.3.1 Horner time function:

  \( H = \dfrac{t_p + \Delta t}{\Delta t} \)

• VII.3.2 Semilog slope and mobility-thickness:

  \( m = \dfrac{162.6\, q\, \mu\, B}{k\, h} \;\Rightarrow\; k\,h = \dfrac{162.6\, q\, \mu\, B}{m} \)

• VII.3.3 Qualitative skin estimation:

  From straight-line intercept and known rock/fluid properties; corroborate with derivative and steady radial-flow diagnostics. Use deconvolution where data support it.

VIII. Practical Tips Specific to Offshore Platforms

• VIII.1 Space/weight: pre-assemble manifolds, use compact separators, and confirm deck load paths; plan crane lifts for slack tides and low wind.
• VIII.2 Burner operation: verify pilots and auto-ignition before flow; keep header pressure steady to avoid flameout/smoke; water curtain only as designed.
• VIII.3 Produced water: route to treatment; if separator not tied in, store in portable tanks and analyze before disposal.
• VIII.4 Weather: set go/no-go for swell, wind, and visibility; rehearse emergency depressurization and mustering.
• VIII.5 Communication: single test director; clear choke commands and data callouts over dedicated radio channel.