What’s the Difference Between Open-Hole and Cased-Hole Logging?

At-a-Glance

Open-hole logging measures untouched formations before casing to quantify rock and fluid properties at highest fidelity. Cased-hole logging evaluates reservoirs and well integrity after casing to diagnose production, saturation changes, and cement quality.

I. Objective Definition and Key KPIs

I.1 Open-Hole (OH) Logging Objective: Quantify original formation properties and net pay prior to completion.

• KPIs: Porosity accuracy (±2 p.u.), water saturation error (±5–10%), net pay (m/ft), permeability estimate confidence (NMR), lithology discrimination, borehole image quality, tool uptime (%), NPT (hours), logging speed (m/hr), depth accuracy (±0.1 m).

I.2 Cased-Hole (CH) Logging Objective: Evaluate behind-pipe saturations, production allocation and well integrity, and guide workovers/recompletions.

• KPIs: Cement bond index (0–1), top-of-cement accuracy (±1–2 m), behind-casing saturation change (?S_w ±5–10%), PLT flow allocation (% per zone), spinner slip factor stability, well uptime (%), intervention time (hours), depth correlation accuracy (±0.3 m GR match).

I.3 Scope note (estimated): Conventional clastic/carbonate reservoirs, salinity >10,000 ppm for resistivity-based Sw, inclination =60° unless stated.

II. Critical Parameters and Target Ranges

II.1 Relevant Equations (Open Hole)

• Archie (clean formations): \[ S_w^n = \frac{a\,R_w}{\phi^m\,R_t} \quad \Rightarrow \quad S_w = \left(\frac{a\,R_w}{\phi^m\,R_t}\right)^{1/n} \]
• Formation factor: \[ F = \frac{R_t}{R_w} = \frac{a}{\phi^m} \]
• Density porosity (single mineral matrix): \[ \phi_d = \frac{\rho_{ma} - \rho_b}{\rho_{ma} - \rho_f} \]
• Sonic (Wyllie time-average, clean): \[ \phi_s = \frac{\Delta t_{log} - \Delta t_{ma}}{\Delta t_f - \Delta t_{ma}} \]
• NMR Timur–Coates permeability (indicative): \[ k \approx a\,\phi^4\,T_{2,LM}^2 \]

II.2 Relevant Equations (Cased Hole)

• Pulsed-neutron sigma-based Sw (two-component approximation): \[ S_w \approx \frac{\Sigma_t - \Sigma_{hc}}{\Sigma_w - \Sigma_{hc}} \]
• C/O qualitative relation (oil vs water): \[ \text{Higher } \frac{C}{O} \Rightarrow \text{oil prone}; \quad \text{Lower } \frac{C}{O} \Rightarrow \text{water prone} \]
• Acoustic impedance (cement/formation): \[ Z = \rho\,V \] and bond reductions correlate with lower CBL amplitude and VDL energy in the casing annulus.
• Spinner volumetric rate (single-phase section): \[ q = A\,v = A\,\frac{N-N_0}{K_s} \] where \(N\) is spinner rpm, \(N_0\) threshold, \(K_s\) slip-corrected constant.

III. Step-by-Step Procedure / Workflow

III.1 Decide Open-Hole vs Cased-Hole

• Choose Open-Hole when you need original-state petrophysics for completion design: porosity, Sw, net pay, stress, image-based fracture analysis.
• Choose Cased-Hole when the well is already cased or producing, to diagnose cement integrity, allocate flow, assess bypassed pay, monitor time-lapse saturation, or plan recompletions.

III.2 Open-Hole Logging Execution

1. Pre-job prep: Condition mud (PV/YP per program; low solids), achieve stable hole; wiper trip; circulate =1.5–2.0 annular volumes; verify temperature/pressure window.
2. Program design: Sequence triple-combo (GR–Res–Density–Neutron–Caliper) + sonic; add image and NMR in net pay. Plan station logs and repeat sections. Target logging speed 300–600 m/hr; slower across thin beds.
3. Conveyance: Wireline if hole stable; switch to drillpipe or pipe-conveyed if differential sticking risk (overbalance >1,500 psi, long open intervals).
4. QC while logging: Monitor density correction (=0.05 g/cc), neutron–density crossover, resistivity deep vs shallow separation (invasion), caliper vs bit size; repeat-pass overlay tolerance within tool spec.
5. Onsite petrophysics: Compute f (density/sonic/NMR), Vsh (GR), Sw (Archie/shaly-sand), flag pay (cutoffs). Export depth match and zonation for completion.

III.3 Cased-Hole Logging Execution

1. Pre-job prep: Confirm cement cure time, casing tally/ID/weight, deviation; pressure-test wellbore; select conveyance (e-line, tractor, CT) for deviation/friction.
2. Depth correlation: Correlate to OH GR or casing collar locator; set master depth reference; target depth match ±0.3 m.
3. Cement evaluation: Run CBL/VDL or ultrasonic; identify TOC and channels; accept if BI =0.7 in isolation intervals; investigate anomalies with VDL waveform.
4. Pulsed neutron baseline: Acquire sigma or C/O log pre-production across reservoir; store as baseline for time-lapse ?Sw tracking.
5. Production logging (PLT): Stabilize rates; run P/T/spinner/holdup; acquire multi-rate passes to calibrate slip and phase holdups; allocate zone rates; flag crossflow.
6. Post-job: Recalibrate with in-situ checks, apply environmental corrections (casing size/weight, borehole fluid), integrate with OH petrophysics.

IV. Risk & Mitigation

IV.1 Open-Hole

• Stuck tool/differential sticking: Maintain ECD and mud cake quality; use non-stick pills; consider drillpipe conveyance; minimize stationary time in permeable overbalance.
• Rugosity/washouts affecting nuclear logs: Centralize density tool; control ROP and mud properties; validate with caliper; apply density corrections conservatively.
• Invasion impacting resistivity: Use multi-lateral resistivity and invasion models; rely on deep-reading tools for Rt; corroborate with NMR/formation tester.
• HSE (radiation, pressure, H₂S): Radiation handling per permit; monitor gas; barrier verification before logging.

IV.2 Cased-Hole

• Conveyance in high deviation: Use tractors or coiled tubing; plan friction and tension windows; set weak-point above toolstring.
• CBL misinterpretation (microannulus): Run pressure/temperature preconditioning; integrate ultrasonic and VDL, not amplitude alone.
• PLT spinner slip/phase segregation: Multi-rate passes; cross-check with holdup, temperature, noise; apply slip corrections.
• Pulsed neutron bias (salinity/rock): Calibrate with OH petrophysics; use C/O in low-salinity or gas; use sigma in high-salinity water.
• HSE (pressure, explosives, radiation): Barrier management; bleed-off/control; certified handling of radioactive and explosive components.

V. Optimization Levers

• Data integration: Combine OH petrophysics (Archie, NMR) with CH pulsed-neutron baselines for robust time-lapse ?Sw and bypassed pay identification.
• Run-plan efficiency: In OH, bundle triple-combo + sonic + image in one descent with careful speed management; in CH, pair CBL with PN to avoid extra rig time.
• Baseline strategy: Acquire early-life CH baselines (post-cement, pre-production) to improve future surveillance discrimination.
• Advanced surveillance: Consider distributed sensing (temperature/acoustics) to complement PLT in horizontal wells; use tractors to reach toe sections.
• QA/QC analytics: Real-time density correction alarms, spinner linearity checks, PN count-rate statistics; reject passes outside QC thresholds.
• Debottleneck conveyance: Pre-model tension/drag; use low-friction cables, roller stems, or wiper runs to ensure depth to target in highly deviated wells.

VI. Verification & Monitoring Plan

VI.1 What to Measure and How Often

• Open-Hole: Repeat sections every 500–1,000 m MD and across key pay; caliper vs density correction continuously; formation tester points in each flow unit.
• Cased-Hole: CBL/UT across barrier intervals once per casing; PN baseline once; PN time-lapse annually or after major rate/water cut changes; PLT on rate changes, after workovers, or annually in complex commingled completions.

VI.2 Acceptance Criteria

• OH Petrophysics: Density correction =0.05 g/cc; neutron–density crossplot consistent with lithology; Archie fit within ±10% Sw against core or testers.
• CH Integrity: BI =0.7 and clear VDL formation arrivals in isolation zones; TOC within ±1–2 m of design; PN count-rate uncertainty =2–3%.
• PLT: Spinner linearity R² =0.95 across multi-rates; mass balance closure within ±10% of surface rate; stable temperature gradient signatures.

Key Practical Differences Summarized

• When: OH before casing; CH after casing/production.
• What: OH measures intrinsic formation properties; CH diagnoses behind-pipe saturation, flow, and cement/pipe condition.
• Resolution: OH higher vertical and property resolution; CH coarser, but operationally flexible and repeatable over well life.
• Risk profile: OH exposed to hole stability and sticking; CH exposed to conveyance and interpretation complexities behind pipe.
• Decisions enabled: OH guides completion and initial reserves; CH guides workovers, zonal control, water shutoff, and surveillance.