Completion: Definition, Well Completion Types, and Oil and Gas Operations
What Is a Completion in Oil and Gas?
A completion in oil and gas is the set of operations carried out after a well has been drilled to prepare it for production or injection. It encompasses perforating the casing to connect the wellbore to the reservoir, installing downhole equipment (tubing string, packer, safety valve, and artificial lift system), and stimulating the formation if needed (hydraulic fracturing, acidising) to achieve commercial flow rates. The completion design directly determines the well's initial production rate, its long-term decline behaviour, and whether it will be economic — making completion engineering one of the highest-value disciplines in the upstream industry, particularly in unconventional shale plays where completion optimisation drives the difference between a marginal and a highly profitable well.
Key Takeaways
- A completion transitions a drilled wellbore into a producing or injecting well — it is the bridge between drilling and production.
- Cased-hole completions perforate through casing cement; openhole completions leave the reservoir section uncased — each suits different reservoir types and completion strategies.
- Multistage hydraulic fracturing completions in horizontal wells are the standard for tight oil and shale gas — Permian Basin laterals now routinely have 60–100+ frac stages per well.
- Completion design variables — stage count, cluster spacing, proppant intensity, fluid type — are the primary optimisation levers in unconventional development.
- A subsurface safety valve (SSSV) installed in the tubing string is mandatory in all offshore completions and many onshore jurisdictions under API RP 14B and BSEE regulations.
Cased-Hole vs. Openhole Completions
Cased-hole completions run and cement a casing string through the reservoir, then perforate the casing and cement with shaped explosive charges to create flow paths into the formation. This is the standard completion type for most onshore wells, most offshore wells, and all wells where zone isolation (preventing communication between different reservoir intervals) is required. Perforating parameters — shot density, phasing, charge size, and perforation tunnel depth — are engineered for the specific reservoir and completion design.
Openhole completions leave the reservoir section uncased, with the formation exposed directly to the wellbore. Used primarily in horizontal wells targeting homogeneous, competent carbonate reservoirs (North Sea chalk, Middle East carbonates) and in SAGD heavy oil pairs (Alberta). Openhole completions avoid perforation skin but sacrifice zone control — water and gas influx cannot be isolated without mechanical intervention.
Multistage hydraulic fracturing completions in horizontal wells are the transformative completion technology of the unconventional revolution. Plugs or sliding sleeves isolate sections of the horizontal wellbore into discrete stages, each individually hydraulically fractured. The Permian Basin Wolfcamp, Montney, and Marcellus developments routinely complete 60–100+ stages per lateral with 30–60 m cluster spacing, 1,500–2,500 kg/m of proppant per stage, and 15–25 m³/min pump rates.
- Types: cased-hole, openhole, multistage fracture (plug-and-perf or sliding sleeve)
- Key downhole equipment: production tubing, packer, SSSV, gas lift mandrels or ESP
- Offshore safety requirement: subsurface safety valve (SSSV) — API RP 14B, BSEE 30 CFR 250
- Unconventional stage count: 40–100+ stages per horizontal lateral (Permian, Montney)
- Perforation standard: API RP 19B (evaluation of well perforators)
- Tubing design standard: API 5CT (casing and tubing specification)
- Completion cost as % of well cost: 40–60% in unconventional horizontal wells
- Key completion optimisation metric: production per lateral foot (boe/ft)
In plug-and-perf multistage completions, cluster spacing (distance between perforation clusters within a stage) is as important as stage count. Tighter cluster spacing (6–12 m) creates more fracture initiation points per unit of lateral length and can improve well productivity — but only if all clusters take fluid uniformly. Stress shadowing from adjacent clusters often causes uneven stimulation, with near-wellbore clusters dominating fluid entry and far-field clusters starved. Diverter agents (dissolvable ball sealers or chemical diverters) temporarily plug the dominant clusters mid-stage, forcing fluid into lower-stress perforation clusters and improving fracture complexity and coverage along the lateral.
Completion Synonyms and Related Terminology
Completion is also known as:
- Well completion — full formal term in engineering and regulatory documents
- Completion operations — refers to the suite of activities (perforating, fracking, tubing installation) collectively
- Frac completion — informal term specific to hydraulic fracturing-based completions in unconventional wells
- Production completion — distinguishes producer completions from injection well completions
- Recompletion — a completion operation on a previously completed well targeting a new interval or improving an existing one
Related terms: Perforation, Hydraulic Fracturing, Casing, Packer
Frequently Asked Questions About Completion
What is the difference between a completion and a workover?
A completion is performed on a newly drilled well to make it capable of producing. A workover is a remedial operation on an existing producing well to restore, maintain, or improve production — examples include reperforation to access a new zone, ESP replacement, or sand cleanout. Both may involve similar equipment (wireline, coiled tubing, or a workover rig) but differ in context. In common field usage, "workover" sometimes loosely includes recompletion operations, but technically a completion is a first-time event and a workover is a subsequent intervention.
What is plug-and-perf and why is it the dominant unconventional completion method?
Plug-and-perf is a cased-hole multistage fracturing method where a wireline-conveyed perforating gun shoots perforations through casing in a defined cluster pattern, then a dissolvable or drillable frac plug is set below that cluster to isolate it from the previous stage. The next stage is then perforated and fractured. The process repeats up the wellbore from toe to heel. It dominates unconventional completions because it allows precise placement of perforations, unlimited stage count, and independent optimisation of each stage — advantages over sliding sleeve systems, which commit perforation placement during casing design before the formation is fully characterised.
How does a tubing-conveyed perforating (TCP) completion differ from wireline perforating?
In wireline perforating, the perforating gun is lowered on an electrical cable and fired by surface command — fast and inexpensive but limited to wells with positive overbalance (wellbore pressure exceeds reservoir pressure). TCP involves running the perforating guns on the tubing string itself, simultaneously perforating and completing the well in a single trip. TCP is preferred in underbalanced or highly deviated wells where wireline cannot be conveyed to depth by gravity, and in situations where immediate well control after perforation is required with the tubing string already in place.
Why Completion Matters in Oil and Gas
In the unconventional era, completion engineering has become the dominant technical discipline driving well economics. The difference between a Tier 1 and Tier 3 Permian Basin Wolfcamp completion — in terms of stage count, cluster spacing, proppant intensity, and fluid design — can be a factor of 2–3× in initial production rate and EUR. Companies like ExxonMobil, Chevron, and Canadian Natural Resources invest heavily in completion optimisation programmes specifically because this is where the incremental dollar of capital investment generates the most additional production from already-leased acreage.