At-a-Glance: Well testing follows a tight sequence: define objectives ? plan and permit ? pre-job readiness and HAZOP ? rig-up and function test ? controlled clean-up and stabilization ? multi-rate flow and shut-in/buildup ? fluid sampling and QA/QC ? demob and analysis/reporting. The aim is to quantify productivity/deliverability, reservoir properties, and fluid behavior while maintaining well control and minimizing emissions.
I. Objective Definition and Key KPIs
- I.1 Objectives (estimated)
- Confirm well deliverability and productivity (oil/gas/water rates, PI/J, AOF for gas).
- Characterize reservoir (k·h, skin s, boundaries, connected volume).
- Obtain representative PVT/contaminant samples (GOR, WGR, H2S/CO2, Hg, sand).
- Verify wellbore integrity (sand/scale/hydrate behavior, erosional limits).
- Gather facility integration data (separator sizing, backpressure behavior).
- I.2 KPIs
- Throughput: stabilized q_o, q_g, q_w; multi-rate coverage; gas AOF.
- Uptime: = 95% test schedule adherence; data completeness = 98% (gauge uptime).
- Quality: stabilization criterion = 5% drift in p_wf and rate over = 1–2 hours; PVT contamination = 10 mol% stock tank gas for oil; sample chain-of-custody 100%.
- Reservoir: PI/J, k·h, s; derivative quality (flat late-time plateau); boundary identification.
- HSE/Environment: zero LTI; zero loss of containment; flaring within permit; emissions intensity (kg CO2e/boe) minimized.
- OPEX/Logistics: first-time-right rig-up; equipment NPT = 2%.
II. Critical Parameters and Target Ranges
Set and control these operating windows to ensure test quality and equipment integrity.
| Parameter | Typical Target/Range (estimated) | Notes/KPIs |
|---|---|---|
| Bottomhole pressure (BHP) gauge depth | 3–10 m above top perforation | Minimize phase interference; use memory/quartz; temp rating adequate |
| Stabilization criterion | ?p_wf and ?q = 5% over 60–120 min | Use rolling window; adjust choke to achieve |
| Drawdown fraction | 10–40% of p¯_i for oil; staged for gas | Avoid coning/sanding and non-Darcy dominance early |
| Choke steps | 3–5 rates (e.g., 12/64 to 40/64 in. equivalent) | Hold each rate to stability or fixed time for isochronal |
| Separator pressure/temperature | Oil: 5–15 bar(g), 40–70 °C; Gas: 10–30 bar(g) | Prevent hydrate/wax; match PVT recomb conditions |
| Water cut limit | As per facility; e.g., = 70% for onshore test packages | Ensure test separator and tanks capacity |
| Sand production | = 10–20 mg/L continuous; transient spikes allowed | Monitor acoustic/probe; below erosional velocity limits |
| Erosional velocity (API 14E) | v = C / v?_m (C commonly 122–152) | Keep line velocity below limit; ?_m in kg/m³ equivalent |
| Wellhead/flowline DP | Within MAWP; = 10% margin | Control via choke; verify PSV setpoints |
| PVT sampling pressure | > bubble/dew point; typically = 1.2× sat pressure | Live oil recombination; gas at flowing conditions |
| Shut-in duration (buildup) | t_bu = 1–2× t_flow; or until derivative stabilizes | Prefer downhole shut-in to minimize wellbore storage |
III. Step-by-Step Procedure / Workflow / Checklist
- III.1 Plan and Permit
- Define test type: oil well drawdown/buildup, gas backpressure (single-point, isochronal, modified isochronal), interference test if applicable.
- Simulate expected rates/pressures/temperatures and flare/vent volumes; confirm facility capacity and MAWPs.
- Secure regulatory approvals: flaring consent, sour service plan, waste handling, noise/emissions.
- Complete HAZID/HAZOP and well control planning (kill fluid, barriers, emergency shut-in logic).
- III.2 Pre-Job Readiness
- Validate well status (integrity test, barrier schematic, packer status, SCSSV function).
- Calibrate instruments (quartz gauges, turbine/Coriolis meters, sand probes, H2S/CO2 detectors, chromatograph).
- QA check test package: separator PSV/PAHH, burners/flare tips, ignition, knockout drums, choke manifold, ESDs.
- Confirm chemical availability: methanol/MEG, defoamer, demulsifier, scale/corrosion inhibitors, antifoam.
- Brief team: procedures, rates schedule, stop criteria, emergency response, communications.
- III.3 Rig-Up and Function Test
- Install well test string or hook-up to permanent completion as applicable; pressure test to = 1.1× MAWP.
- Place downhole gauges 3–10 m above perforations; verify memory start; surface DAQ time-synced (UTC).
- Function test all shutdowns (ESD/DHSV/PSV) and flare ignition; verify draining/earthing.
- III.4 Controlled Clean-Up
- Crack choke small; bleed to flare; monitor returns for solids/cuttings/acid byproducts if post-stim.
- Maintain T/P to avoid hydrates; inject methanol/MEG as required.
- Criteria to end clean-up: sand below threshold, water cut stabilized, separator interface stable.
- III.5 Stabilized Flow Periods (Multi-Rate)
- Execute rate schedule (e.g., 4–5 choke settings). Hold each step until stabilization (= 5% drift) or fixed time for isochronal tests.
- Record: rates (oil/gas/water), WHP/WHT, separator P/T, choke DP, sand rate, line acoustics, gas composition.
- Adjust for erosional velocity limits; maintain separator liquid level control.
- III.6 Shut-In and Buildup
- Prefer downhole shut-in (DST valve/SCSSV) to reduce wellbore storage; otherwise fast surface shut-in.
- Record high-frequency BHP (1 Hz–10 Hz initially) and surface pressures; allow sufficient t_bu (= t_flow, often 6–24 h).
- Do Horner or derivative checks in-field to confirm late-time behavior and boundary effects.
- III.7 Specialized Gas Deliverability (if gas well)
- Single-point: one stabilized point to estimate AOF with assumed n; less preferred.
- Isochronal: multiple equal-time flows separated by shut-ins; derive C, n from backpressure relationship.
- Modified isochronal: fixed short flows without stabilization; practical when stabilization is slow.
- III.8 Sampling and PVT
- Collect live oil and gas samples at flowing conditions; maintain pressure above saturation; avoid flashing.
- Take separator liquid/gas samples for compositional balance and contamination screening; document chain-of-custody.
- Capture solids/sand for PSD analysis; note rates and onset conditions.
- III.9 Demobilization
- Depressurize, purge to flare, drain/flush lines; neutralize chemicals; waste management per permit.
- Recovery and download of memory gauges; archive DAQ data with metadata (depths, times, calibrations).
- III.10 Analysis and Reporting
- QA/QC data; correct for gauge drift, temperature, hydrostatic head; build consistent time base.
- Perform PBU/PFO analysis (Horner, MDH derivative); compute k·h, skin s, J; identify boundaries.
- For gas, fit backpressure or pseudopressure deliverability (C, n); compute AOF.
- Integrate PVT results to update GOR, shrinkage, z-factor, viscosities; recompute rates to standard conditions.
- Issue final test report with uncertainties and recommended operating envelope.
IV. Risk & Mitigation (HSE, Reliability, Redundancy)
- IV.1 Well Control and Overpressure
- Mitigation: dual barriers verified; PSV set/tested; staged choke opening; pressure tests before flow; emergency shutdown drills.
- IV.2 Sour/Acid Gas (H2S/CO2)
- Mitigation: sour service materials; fixed/portable gas detection; escape breathing apparatus; exclusion zones; neutralization/Scavenger chemical plan.
- IV.3 Hydrates/Wax/Asphaltenes
- Mitigation: maintain temperature; MEG/methanol injection; insulation/heat tracing; controlled pressure letdown across choke; chemical program.
- IV.4 Sand and Erosion
- Mitigation: sand detection; rate ramping; choke position control; erosional velocity checks; erosion-resistant trim; sand knockout if needed.
- IV.5 Flaring/Emissions/Noise
- Mitigation: burner efficiency checks; steam/air assist; flare radiation modeling; noise barriers; minimize unnecessary flowing time.
- IV.6 Data Loss/Instrument Failure
- Mitigation: redundant gauges; dual data acquisition; synchronized clocks; periodic data offload; spare sensors on site.
- IV.7 Logistics/Access
- Mitigation: spare parts kit; weather windows; lifting plans; night operations policy and lighting; 24/7 coverage plan.
V. Optimization Levers (Execution and Data Quality)
- V.1 Flow and Choke Strategy
- Automate choke steps with ramp rates; target constant sand-free drawdown; adapt steps based on real-time derivative trends.
- V.2 Reduce Wellbore Storage
- Use downhole shut-in; stabilize fluids before shut-in; minimize gas breakout in oil tests.
- V.3 Real-Time Analytics
- Onsite quick-look PI, Horner plots, derivative diagnostics; alarm on stabilization criteria; early boundary detection to set shut-in length.
- V.4 Chemical and Thermal Control
- Predict hydrate temperature; pre-heat separators; dose inhibitors proportionally to gas/water rates; wax control on cool-down.
- V.5 Debottleneck Test Package
- Parallel separators; higher-Cv choke trim; low-?P piping; ensure flare capacity and knockout sizing to avoid backpressure artifacts.
- V.6 Sampling Quality
- Use isokinetic gas sampling; oil recombination by ratio matching; duplicate samples; document stabilization prior to draw.
VI. Verification & Monitoring Plan
- VI.1 Measurements and Frequency
- Downhole pressure/temperature: 1–10 Hz during transients; 0.1–1 Hz steady.
- Surface pressures/temperatures (WHP, flowline, separator): 1 Hz.
- Rates: continuous meters plus tank strapping/gravimetric cross-check every 2–4 hours.
- Gas composition: spot GC every rate change; continuous Wobbe/BTU analyzer if available.
- Sand: continuous probe and bottle tests every 2–4 hours.
- VI.2 Acceptance Criteria
- Stabilization achieved per Section II; derivative flat at late time for radial flow; consistent PI across rates (within 10–15%).
- Material balance on test separator within ±2–5% (oil/gas/water reconciliation).
- VI.3 Documentation
- Daily test log; event list (choke changes, shut-ins); calibration certificates; chain-of-custody for samples; final well test report with uncertainties.
VII. Key Equations and Formulas
- VII.1 Productivity Index (oil)
- \( J = \dfrac{q_o}{\bar{p}_i - p_{wf}} \) where \(J\) in bbl/d/psi, \(q_o\) in bbl/d.
- VII.2 Radial Flow (oil, steady-state approximation)
- \( q_o = \dfrac{k h}{141.2\,\mu_o\,B_o}\;\dfrac{\bar{p}_i - p_{wf}}{\ln(r_e/r_w) + s} \)
- Units: \(k\) in mD, \(h\) ft, \(\mu_o\) cP, \(B_o\) rb/stb, pressures in psi.
- VII.3 Horner Buildup (liquids)
- \( p_{ws}(\Delta t) = p^* + m \log_{10}\!\left(\dfrac{t_p + \Delta t}{\Delta t}\right) \)
- \( k h = \dfrac{162.6\,q_o\,\mu_o\,B_o}{m} \) and skin from straight-line intercept (classic MDH method).
- VII.4 Pressure Derivative (diagnostics)
- \( \frac{d p}{d \ln t} = t \dfrac{d p}{d t} \); radial flow plateau indicates constant derivative; early-time slope indicates wellbore storage.
- VII.5 Gas Deliverability (backpressure method)
- \( p_r^2 - p_{wf}^2 = C\,q_g^{\,n} \) ? fit \(C, n\) from multi-rate data; AOF at \(p_{wf} = 0\): \( \text{AOF} = \left(\dfrac{p_r^2}{C}\right)^{1/n} \).
- VII.6 Gas Pseudopressure (real-gas)
- \( m(p) = \int_0^p \dfrac{2 p'}{\mu_g(p')\,z(p')} \, dp' \) and \( q_g = \dfrac{k h}{1424\,T}\;\dfrac{m(p_r) - m(p_{wf})}{\mu_g\,z\,(\ln(r_e/r_w) + s)} \) in field units.
- VII.7 Erosional Velocity (screening)
- \( v_e = \dfrac{C}{\sqrt{\rho_m}} \) with \(C\) per service (e.g., 122–152) and \(\rho_m\) mixture density (consistent units).
- VII.8 Material Balance Time (liquids, diagnostic)
- \( t_{MBT}(t) = \dfrac{\left(\int_0^t q(\tau)\,d\tau\right)^2}{\int_0^t q^2(\tau)\,d\tau} \) for variable-rate superposition diagnostics.
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