What are the steps in wireline logging for reservoir evaluation?

At-a-Glance: Wireline logging for reservoir evaluation is a tightly sequenced operation: plan, condition the hole, rig-up, calibrate, run, acquire/QC, repeat for depth control, perform quick-look petrophysics, and hand over validated data. The KPIs are data quality, depth accuracy, coverage, operational efficiency, and HSE integrity.

I. Objective & KPIs

• I.1 Objective: Acquire high-fidelity open-hole (and/or cased-hole) logs to quantify lithology, porosity, fluid saturations, permeability indicators, and net pay; support completion decisions and reservoir models.
• I.2 Scope: Open-hole triple combo (GR, resistivity, density–neutron, caliper, sonic), imaging/NMR as applicable, plus stations (formation tester, sidewall cores) as required.
• I.3 Primary KPIs:
  • Data quality: SNR = 20; density standoff = 0.25 in; eccentering = 80% contact; repeat overlay within 1–2% of full-scale; image coverage = 80% circumference.
  • Depth accuracy: MD uncertainty = ±0.1% MD or ±0.5 ft (whichever larger); depth match between passes = 0.5 ft.
  • Coverage: = 99% of open hole logged; repeat sections = 100 ft including all reservoir intervals.
  • Operational efficiency: Logging rate within plan; NPT < 5% of job time; tool uptime = 98%.
  • HSE & integrity: Zero recordables; full source accountability; pressure control compliance; emissions minimized via efficient rig time.

II. Critical Parameters & Target Ranges

III. Step-by-Step Procedure / Workflow

III.1 Planning & Pre-Job Readiness

• 1.1 Define objectives: Petrophysical objectives (lithology, porosity, Sw, permeability indicator, fractures) and zonal priorities.
• 1.2 Input data: Prognoses, mud program, expected pressures/temps, deviation, BHA/bit size, casing tally, coring/MDT plan.
• 1.3 Toolstring design: Sequence triple combo first; add sonic, image, NMR; plan station tools (formation tester, sidewall corer); ensure ratings cover HPHT/deviation.
• 1.4 Conveyance plan: Gravity/wireline; contingency for tractors or pipe-conveyed logging in high deviation, ledges, or heavy mud.
• 1.5 QA plan: Cal/verification schedule, repeat sections, depth correlation strategy, environmental corrections inputs, quick-look templates.
• 1.6 HSE plan: Radiation permits, H2S plan, lifting plan, pressure control (if cased-hole/pressurized), well control barriers verified.

III.2 Wellbore Conditioning & Readiness

• 2.1 Circulate clean: Remove cuttings; condition mud properties (density, viscosity, filtrate); record Rm, Rmf at BH temp.
• 2.2 Trip for gauge hole: Caliper expectations; minimize washouts; wiper trip if needed.
• 2.3 Stabilize well: Allow temperature stabilization; check swab/surge risks; verify no losses.
• 2.4 Survey: Updated deviation/DSA for tool reachability; compute maximum tractor pull if required.

III.3 Rig-Up & Calibrations

• 3.1 Surface prep: Rig-up mast, sheaves, depth wheel; verify head tension calibration; ground/electrical checks.
• 3.2 Tool makeup: Assemble toolstring, centralizers, sinker bars; install/deploy radioactive sources per procedure.
• 3.3 Calibrations: Shop and on-site zero/span for density, neutron, sonic, resistivity; background counts; image pads functional; NMR tuning.
• 3.4 Functional tests: Telemetry, memory, acquisition software; latches; acoustic transducers; pressure/temperature sensors.

III.4 Run-In-Hole (RIH) & Correlation

• 4.1 RIH control: Monitor tension, depth, heave compensator (offshore); maintain slow speed through tight spots; avoid slack line.
• 4.2 Correlate depth: Align GR to prior LWD/offsets; in cased hole use CCL and known collars; set zero depth at kelly bushing or rotary table; record stick-up.
• 4.3 Bottom reach: Confirm TD; if not reaching, apply tractors, rollers, or switch to pipe-conveyed plan per contingency.

III.5 Acquisition Pass Plan

• 5.1 First pass up: Continuous logs from TD to shoe with conservative speed (3 m/min); stabilize tools; avoid speed spikes.
• 5.2 Repeat sections: At least two repeats across each reservoir (= 100 ft) and at representative shale/clean zones for environmental correction anchoring.
• 5.3 Specialty tools:
  • Imaging: Slow speed (1–2 m/min), high centralization; verify button contact and mud conductivity suitability.
  • NMR: Stationary or slow pass depending on tool; per-vendor acquisition timing; wait-times to separate T2 components.
  • Formation testing: Pressure pretests; mobility test; sampling if mobility and contamination allow; seal test criteria defined pre-job.
  • Sidewall cores: Fire in target lithofacies with adequate stand-off; maintain depth control.
• 5.4 Second/third passes: As required for depth match between toolstrings; complete hole coverage.

III.6 Real-Time QC & Adjustments

• 6.1 Monitor KPIs: Noise, standoff, count rates, tool temps, tension trends, image pad contact, sonic semblance/coherence.
• 6.2 Speed tuning: Slow down in washouts, tight hole, and target pay; increase in stable sections to save rig time.
• 6.3 Environmental inputs: Update Rm, Rmf, temperature profile, borehole diameter for real-time corrections.
• 6.4 Decision gates: Trigger extra repeats or stationary re-logs if QC thresholds are not met.

III.7 Pull-Out-of-Hole (POOH) & Post-Job

• 7.1 POOH: Controlled speed; maintain tension; avoid bridging; handle sources per procedure; tally tool temps/pressures.
• 7.2 Onsite quick-look: Depth-shift, splice, and compute Vsh, porosity, Sw; identify net pay; advise completion/MDT decisions.
• 7.3 Deliverables: Final depth-shifted curves, environmental correction report, QC overlays, petrophysical quick-look, operations log, source reconciliation.

IV. Risk & Mitigation (HSE, Reliability)

• IV.1 Stuck tools/differential sticking: Mitigate with good hole cleaning, proper mud weight/filtrate control, centralizers, minimizing stationary time; contingency for jars/weak-point release and fishing plan (estimated).
• IV.2 High deviation/ledges: Plan tractors or pipe-conveyed logging; manage drag and tension; avoid slack line.
• IV.3 HPHT exposure: Use rated tools; time-in-hole management; monitor tool temperature; stagger runs to reduce soak time.
• IV.4 Radiation safety: Locked storage, source shuttling controls, exclusion zones; emergency recovery plan.
• IV.5 H2S/toxics: Gas monitoring, PPE, breathing apparatus; contingency egress plan.
• IV.6 Well control: Barrier verification, lubricator/grease head for pressurized/cased-hole; shut-in protocols practiced.
• IV.7 Data loss/telemetry failure: Redundant memory logging; repeat critical zones; power/telemetry checks.
• IV.8 Emissions/time: Optimize pass plan and speeds; avoid re-runs via strong QC to reduce rig time and fuel burn.

V. Optimization Levers (Performance & Data Quality)

• V.1 Tool sequencing: Run heavy/eccentered density pads in smoother hole first; defer image/NMR until hole stabilizes; combine strings to minimize trips.
• V.2 Speed vs resolution: Adaptive speed control using real-time noise metrics; auto-slowdown in target intervals to lift SNR without excessive rig time.
• V.3 Centralization strategy: Model standoff with caliper; add centralizers in enlarged sections; use bowsprings/ribbed protectors to improve pad contact.
• V.4 Environmental corrections: Capture mud and temperature profiles; calibrate OBM effects on neutron/NMR; anchor density corrections with repeat clean sands.
• V.5 Depth control: Multiple independent anchors (GR vs LWD, CCL, magnetic marks); perform systematic repeats at tops/bases for robust depth-shifts.
• V.6 Real-time analytics: Onsite quick-look crossplots (N–D, M–N, Pickett) to validate lithology/Sw; steer MDT/sidewall targeting dynamically.
• V.7 Conveyance efficiency: Tractors only where modeled pull > gravity margin; avoid unnecessary PCWL; manage line speed/tension to reduce NPT.
• V.8 Post-job feedback loop: Compare against core/MDT and LWD; update tool error models and speed tables for the next well.

VI. Verification & Monitoring Plan

• VI.1 What to measure (real-time): SNR per tool, pad standoff, count rates, sonic semblance, image current/contact, NMR echo stability, cable tension, heave, temperature.
• VI.2 Frequency: Continuous for telemetry/tension; QC snapshots every 500 ft; repeats per zone; tool health checks at bottoms and tops of passes.
• VI.3 Acceptance criteria: Repeat overlays within 1–2% FS; depth correlation = 0.5 ft; density Pe noise within specification; image coverage = 80% with minimal dropouts.
• VI.4 Post-job: Run environmental corrections, depth-shift/splice, merge to MASTER set; compute quick-look petrophysical properties; archive raw and processed data with metadata.

VII. Key Quick-Look Petrophysical Equations from Wireline Logs

VII.1 Shale volume from Gamma Ray (linear):

\[ V_{sh} \;=\; \frac{GR - GR_{min}}{GR_{max} - GR_{min}} \]

VII.2 Density porosity (matrix/f luid densities known):

\[ \phi_D \;=\; \frac{\rho_{ma} - \rho_b}{\rho_{ma} - \rho_f} \]

VII.3 Sonic (Wyllie time-average):

\[ \phi_{\Delta t} \;=\; \frac{\Delta t_{log} - \Delta t_{ma}}{\Delta t_f - \Delta t_{ma}} \]

VII.4 Neutron–Density crossplot lithology check (M–N plot):

\[ M \;=\; \frac{\Delta t - \Delta t_{ma}}{\rho_b - \rho_{ma}}, \quad N \;=\; \frac{\phi_N - \phi_{N,ma}}{\rho_b - \rho_{ma}} \]

VII.5 Archie (clean formations):

\[ F \;=\; \frac{a}{\phi^m}, \quad R_t \;=\; F \, R_w \, S_w^{-n} \;\Rightarrow\; S_w \;=\; \left(\frac{a\,R_w}{\phi^m\,R_t}\right)^{1/n} \]

VII.6 Shaly-sand (Simandoux, one form):

\[ \frac{1}{R_t} \;=\; \frac{V_{sh}}{R_{sh}} \;+\; \frac{\phi^m}{a\,R_w}\,S_w^n \]

VII.7 NMR porosity and permeability indicator (Timur–Coates style):

\[ \phi_{NMR} \;=\; \int A(T_2)\,dT_2, \quad k \propto \left(\frac{\phi_{NMR}}{BVI}\right)^2 \,(FFI)^2 \]

VII.8 Water resistivity from SP (simplified, when applicable):

\[ E_{SP} \approx K \,\log_{10}\!\left(\frac{a\,R_m}{R_w}\right) \;\Rightarrow\; R_w \; \text{from measured} \; E_{SP} \]

Variables: GR = gamma ray; ?b = bulk density; ?ma = matrix density; ?f = fluid density; ?t = slowness; f = porosity; Rw = formation water resistivity; Rt = true formation resistivity; a, m, n = Archie constants; Rsh = shale resistivity; Vsh = shale volume; A(T2) = NMR distribution; BVI/FFI = bound/free fluid index; K, a = empirical constants; Rm = mud resistivity.