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

Why the hole condition changes the measurement

A logging tool reads a physical response: nuclear, electrical, acoustic, magnetic, pressure, temperature or flow. The path between the sensor and the formation includes borehole fluid, mud cake, invasion, casing, cement and completion hardware when those items exist. Removing or adding those layers changes both the physics and the interpretation model. That is why “run a log” is not a specification. The specification is the hole condition, the measurement family, the temperature-pressure rating, the borehole size, the fluid, and the decision the data must support.

Open-hole acquisition is usually time-critical. Hole stability, differential sticking, lost circulation and rig-time cost all constrain how long tools can stay in the open hole. Cased-hole acquisition can often be planned after the rig has moved or during workover, but the casing and cement become part of the measurement problem.

Open-hole logging: what it is for

Open-hole logs are commonly used to describe lithology, porosity, saturation, structure, geomechanics and fluid contacts before casing decisions are locked. Typical families include resistivity, nuclear porosity and density, neutron, sonic, nuclear magnetic resonance, imaging, and formation-pressure or sampling tools. The exact combination is a well-design choice, not a universal suite.

Benefits include higher sensitivity to formation properties that casing later attenuates, and the ability to influence casing points, perforation strategy and completion design while the well is still open. Constraints include hole quality, rugosity, invasion, mud type, temperature, well control and the mechanical risk of running tools in unsupported hole. A washed-out hole can degrade pad and nuclear measurements even if the tool string is rated for the depth.

Questions that belong in an open-hole logging RFQ

1. What decision will the data change: casing point, completion, reserves, or geomechanics?
2. What is the hole size, deviation, temperature, pressure and mud system?
3. Is wireline, drill-pipe conveyed or logging-while-drilling required, and why?
4. What environmental corrections and quality flags are required in the deliverable?
5. Who owns interpretation: contractor, operator or a named third party?

Cased-hole logging: what it is for

Cased-hole logging is used after casing is set. Common objectives include cement evaluation, casing inspection, saturation behind pipe, production logging, and well-integrity diagnostics. Acoustic cement-bond and ultrasonic tools, pulsed-neutron saturation tools, multi-finger calipers, electromagnetic casing inspection, and production-logging suites (spinner, temperature, pressure, capacitance or gas-holdup sensors) all belong in this family when specified.

Benefits include access after the well is secured, repeatable surveillance during production, and the ability to investigate integrity without returning to open hole. Constraints include attenuation and distortion by steel and cement, reduced vertical resolution for some measurements, and the need to know completion geometry: casing weight and grade, cement, tubing, packers, and perforations. A cased-hole saturation log cannot be interpreted as if it were an open-hole resistivity log.

Open-hole versus cased-hole comparison

Logging while drilling is not a third slogan

Logging-while-drilling (LWD) acquires selected measurements during drilling, still in open hole, but under different time, standoff and trajectory conditions than a wireline trip. It can reduce open-hole exposure and support real-time decisions. It is still open-hole physics with drilling-environment corrections. Treat LWD as a conveyance and timing choice, not as a separate formation type. If both LWD and wireline are proposed, state which measurements are the record copy and which are operational.

Cement, casing and integrity connect the two worlds

The moment casing is cemented, the evaluation problem changes. Cement evaluation is a cased-hole task that checks zonal isolation assumptions used in well design. Poor mud removal, channels, microannulus or lightweight slurry can all change acoustic response. The well-cement types page covers slurry-class language; it does not replace a cement-evaluation procedure. If cement evaluation is required, specify the tool family, logging fluid, pressure on the casing, and the acceptance criteria from the well-design basis.

Casing inspection and well-integrity logs belong with later life-of-well work. They answer different questions from open-hole porosity. Mixing the two in one sentence in an RFQ usually means the decision owner has not been named.

Production logging and later surveillance

Once the well produces, the question often shifts from “what is the rock” to “where is the flow.” Production logging in cased or completed hole uses a different tool mix: spinners, temperature, pressure, holdup sensors and sometimes noise or fibre-based methods. These surveys diagnose contribution, crossflow, leak paths or injection distribution. They do not replace the original open-hole petrophysical program. If both are needed over field life, budget and access for repeat surveys should be in the well plan, not requested after a production surprise.

Quality control is part of the measurement. Depth control, calibration, repeat sections, and environmental corrections decide whether a curve is usable. An RFQ that asks only for “a log” without naming quality rules will receive inconsistent deliverables. State the operator procedure or the equivalent witnessing and acceptance criteria.

How this relates to CNPS equipment pages

CNPS publishes drilling, logging, monitoring and mud-logging related catalogue records under oil and gas solutions and the product catalogue. Those pages list products; they do not select a logging program. A defensible enquiry names the hole condition, measurement objective, temperature-pressure rating, conveyance and deliverable format, then asks which published records, if any, match. If no catalogue record matches, say so and request a technical review rather than forcing a product name.

Offshore and high-cost wells often justify both an open-hole evaluation suite and a later cased-hole surveillance plan. See the offshore drilling explainer for the operating-condition questions that should precede tool lists.

Build a logging program from decisions, not from a catalogue page

Start with the decision: casing point, completion design, reserves booking, production allocation, or integrity. Map each decision to a measurement that can actually change it. Then select conveyance, hole condition, and quality-control rules. Only then compare available tools or services. A “full suite” copied from another well wastes rig time if the lithology, mud system or well objective differ.

Deliverables should include raw curves, environmental corrections, quality flags, depth control method, and the interpretation model. If AI or automated zonation is proposed, it still requires an owner, a review date and a statement of where the model is allowed to fail. Unowned curves are not an evaluation program.

RFQ checklist

1. Hole condition: open hole, cased hole, or both, with sizes and casing details.
2. Objective: formation evaluation, cement evaluation, saturation, production logging, or integrity.
3. Environment: temperature, pressure, deviation, fluid, H2S/CO2 if known.
4. Conveyance: wireline, LWD, tractor, or pipe-conveyed.
5. Deliverable: curves, images, samples, pressures, interpretation and due date.
6. Standards and procedures: operator spec, calibration, depth control and witnessing.

Highly deviated wells may need tractor or pipe-conveyed logging. Conveyance is a mechanical plan covering weak points, fishing, well control and time. Include it so the first technical response can accept or reject the method against the actual trajectory and completion.

Frequently asked questions