Cased Hole: Definition, Completion Types, and Well Operations in Oil and Gas
What Is Cased Hole in Oil and Gas?
Cased hole refers to any section of a wellbore where steel casing has been run and cemented in place, as distinguished from "open hole" — the uncased, exposed formation section. Most wellbore operations after drilling are conducted in cased hole: perforating the casing to access the reservoir, running wireline logs to evaluate cement or production profiles, and performing workovers to isolate zones or replace downhole equipment. The term is also used to describe a specific category of logging and testing services — "cased hole logging" — that evaluate formation and completion performance through the existing casing rather than in the bare formation.
Key Takeaways
- Cased hole means steel casing has been run and cemented through the interval — the formation is hydraulically isolated from the wellbore except through deliberate perforations.
- Cased hole completions require perforating to create flow paths between the reservoir and the wellbore — perforation design directly determines well productivity.
- Cased hole logging tools (PNC, C/O, cement bond logs, production logging) evaluate reservoir and completion performance without requiring open-hole access.
- Cased hole enables selective zone isolation — producing from one zone while another is mechanically isolated with a packer or cement squeeze.
- Most well interventions (stimulation, zone recompletion, sand cleanout, plug setting) are performed as cased-hole operations using wireline, coiled tubing, or a workover rig through the cemented casing.
Cased Hole vs. Open Hole Operations
In open hole, the formation is exposed directly to the wellbore fluid — logged with wireline tools that contact the formation surface, and in some completion styles (openhole gravel packs, barefoot completions) produced without casing. Open hole logging provides the highest-quality formation evaluation data because measurement tools are in direct contact with undisturbed formation.
In cased hole, all formation contact is mediated by the casing and cement. Cased hole logging tools must penetrate the casing and cement to reach the formation — they use nuclear, acoustic, or electromagnetic physics that read through steel. The trade-off is that cased hole enables zone isolation (each producing zone can be independently accessed, stimulated, or abandoned through selectively placed perforations and packers), workover access over the life of the well, and mechanical protection of the wellbore from formation collapse.
Cased Hole Logging Services
Cement bond log (CBL/VDL): Acoustic tool assessing cement quality behind casing — critical for confirming zonal isolation before perforating. Pulsed neutron capture (PNC) and carbon-oxygen (C/O) logs: Distinguish oil, gas, and water saturation behind casing in producing wells without removing tubing — essential for monitoring waterflood fronts and identifying remaining oil. Production logging (PLT): Measures flow rate and fluid type (spinners, capacitance, temperature) at each perforated interval to identify which zones are producing and which are watered out. Casing inspection logs: Ultrasonic or electromagnetic tools detect corrosion, wall thickness loss, and casing deformation — mandatory for integrity management in aging wells.
- Opposite: open hole (uncased, exposed formation section)
- Flow path: through perforations in casing and cement — no natural contact with formation
- Key logging services: CBL, PNC, C/O, PLT, casing inspection
- Perforation standard: API RP 19B (perforation performance evaluation)
- Zone isolation tool: packer (mechanical), bridge plug, cement squeeze
- Intervention access: wireline, coiled tubing, or workover rig through the cased wellbore
- Primary advantage over open hole: long-term mechanical stability and zone selectivity
- Primary disadvantage vs. open hole: additional cost of casing + cementing; perforation skin
Before perforating any cased hole interval, always run a cement bond log (CBL) and a gamma ray/collar log to confirm: (1) the cement has adequate bond behind the casing across the target interval; (2) the perforated interval is correctly identified relative to formation top picks from the open hole log. A poor cement bond opposite the perforated zone allows communication between adjacent zones through the annulus — the well will produce (or inject) into an unintended zone. In wells where the CBL indicates poor cement, squeeze cementing to restore isolation is required before perforating, at significant cost. This investment is almost always cheaper than the consequences of unintended zone communication.
Cased Hole Synonyms and Related Terminology
Cased hole is also known as:
- Cased wellbore — the complete well section with casing in place
- Through-casing logging — used when describing cased hole logging operations specifically
- Behind-pipe evaluation — evaluating formation properties through the existing casing without open-hole access
- CH — abbreviation used in completion design documents and logging service catalogues
Related terms: Casing, Perforation, Completion, Cement
Frequently Asked Questions About Cased Hole
Why is most well logging done in open hole if cased hole logging is also possible?
Open hole logging is performed immediately after drilling, before casing distorts the measurement environment. The formation is fresh, undisturbed, and directly accessible to wireline tools. Open hole logging tools — particularly resistivity arrays and neutron-density combinations — provide the highest-resolution, most accurate formation evaluation data for reserve estimation. Cased hole logging is done later in the well's life when open hole access is no longer available and specific post-completion questions need answering — which zones are producing, where the water-oil contact has moved, is the casing corroding. It supplements, but does not replace, open hole logging.
What is a cased hole completion vs. an openhole completion?
A cased hole completion runs casing to total depth (or through the reservoir), cements it in place, and perforates selectively to access the reservoir. An openhole completion leaves the reservoir section uncased — the well is drilled to the top of the reservoir, cased to that point, then drilled on with a smaller bit and left uncemented through the producing interval. Openhole completions avoid perforation skin and are common in hard, naturally fractured carbonates (horizontal wells in the Middle East and North Sea chalk) and in SAGD heavy oil pairs in Alberta. Cased hole completions dominate in soft sands requiring sand control, multi-zone producers, and wells requiring selective zone management over time.
How long does casing last in a producing well?
Steel casing is designed for the well's full producing life — 20–50 years — but actual integrity depends on corrosion rate, mechanical loading, and wellbore stability. CO2 and H2S in produced fluids accelerate internal corrosion; high chloride formation water attacks both internally and externally through cement gaps. Cathodic protection systems and inhibitor injection slow corrosion but cannot eliminate it. Regulatory requirements (AER Directive 019, BSEE 30 CFR 250) mandate periodic casing inspection through caliper and electromagnetic logging and require remediation or abandonment when wall thickness drops below minimum API standards. In Alberta, more than 20% of wells over 30 years old show measurable casing damage on inspection logs.
Why Cased Hole Matters in Oil and Gas
Cased hole is the standard wellbore configuration for the overwhelming majority of oil and gas production worldwide. Every perforating decision, every stimulation treatment, every production log interpretation, and every workover plan is executed in the cased hole environment. Understanding the capabilities and limitations of operating through casing — from perforating physics to through-casing logging measurement principles — is fundamental to completion engineering, production engineering, and well integrity management across the full life cycle of any well.