Drilling Operations Explained: A Practical Guide to Well Construction

Drilling an oil or gas well is not a single continuous operation. It is a sequence of distinct, highly specialised phases, each with its own equipment, crew, contract structure, and risk profile, that together take a geological prospect from an untouched formation thousands of feet underground to a producing well delivering hydrocarbons to the surface. Understanding this sequence is fundamental for anyone working in upstream oil and gas, whether in a technical drilling role or in the commercial, project management, or safety functions that support drilling operations.

This guide explains the core phases of well construction, the major drilling technologies in use today, and why the distinction between drilling and completion has become one of the most closely watched operational and commercial metrics in the modern oil and gas industry.

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Key Takeaways

2 phases Drilling and completion are distinct operational phases with separate crews, equipment, and contract structures, and a well can sit drilled but uncompleted for months or years

Horizontal Drilling, combined with hydraulic fracturing, has become the predominant well construction method in most modern onshore developments, exposing far more producing rock per well than vertical drilling

18,000 ft Average lateral length of horizontal wells in major US shale plays by 2019, nearly double the length typical in the early 2000s, according to EIA data

DUC Drilled but uncompleted wells are a key industry inventory metric, reflecting producers’ ability to defer completion spending in response to price signals without losing the drilling investment already made

• Well construction consists of two distinct phases, drilling (creating the wellbore) and completion (casing, cementing, perforating, and in many modern wells, hydraulic fracturing), typically performed by separate crews under separate contracts.
• Horizontal drilling, bending the wellbore from vertical to horizontal once it reaches the target formation, has become the dominant technique in modern unconventional development because it exposes significantly more producing rock than a vertical well.
• The inventory of drilled but uncompleted wells (DUCs) provides operators with flexibility to time completion spending according to price conditions, since drilling and completion can be decoupled by months or years.
• Offshore drilling introduces additional complexity, requiring purpose-built platforms and, in deepwater developments, floating rigs anchored in water depths that can exceed 10,000 feet.

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The Two Phases: Drilling and Completion

The drilling phase begins with spudding, the initial penetration of the ground by the drill bit, and proceeds with a rig and crew drilling the wellbore to its target depth, in stages, using progressively smaller drill bits and installing steel casing at intervals to stabilise the borehole and protect groundwater. According to the US Energy Information Administration, the drilling phase involves dispatching a rig and crew who drill one or more wells on a pad site, often working through several wells in sequence to maximise efficiency and minimise idle time moving equipment between locations.

Completion is a separate phase, typically performed by a different specialist crew, involving casing, cementing, perforating the casing to allow hydrocarbons to flow in, and, in most modern unconventional wells, hydraulic fracturing to stimulate flow from low-permeability rock. Because drilling and completion are contracted and resourced separately, a well can be drilled and then left uncompleted for an extended period, a state the industry tracks closely as an inventory of drilled but uncompleted wells, or DUCs, which rises when operators defer completion spending in a lower price environment and falls when completion activity outpaces new drilling.

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Horizontal Drilling: The Technology That Reshaped Modern Production

Horizontal drilling involves drilling a well vertically to a predetermined depth and then progressively bending the wellbore path until it extends horizontally through the target formation. Because geological formations typically extend much further horizontally than they are thick vertically, a horizontal well exposes dramatically more producing rock to the wellbore than a vertical well drilled through the same formation, translating directly into higher potential recovery per well.

Horizontal drilling, first achieving commercial success in the 1980s, became the predominant method of new well construction in the United States during the 2010s as combined with hydraulic fracturing it unlocked economic production from unconventional shale formations that vertical drilling could not access efficiently. The average lateral length of horizontal wells has grown substantially as drilling technology has improved, nearly doubling between the early 2000s and 2019 as operators pushed laterals further to maximise the producing rock contacted by each well.

Directional Drilling: Precision Beyond a Straight Line

Directional drilling is the broader discipline of steering a wellbore along a deliberately non-vertical path, of which horizontal drilling is the most extreme application. Directional techniques allow operators to reach reservoir targets that are not directly beneath the surface location, critical for offshore platforms drilling multiple wells from a single fixed structure, for avoiding surface obstacles or environmentally sensitive areas, and for intersecting fractured or faulted reservoirs at the optimal angle for production. Modern directional drilling relies on downhole motors, measurement-while-drilling (MWD) tools that provide real-time positional data to the surface, and increasingly automated steering systems that allow drillers to hit subsurface targets with remarkable precision across distances of several kilometres.

The commercial and project management discipline required to plan and execute a drilling campaign, sequencing wells, managing rig contracts, and controlling the significant capital at stake, connects directly to the broader project management principles covered in our article on how to build a project risk register that actually gets used, adapted for the specific technical and geological uncertainties that drilling projects carry.

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Offshore Drilling: An Additional Layer of Complexity

Offshore wells are drilled from platforms that hold the drilling equipment, storage, and crew housing, ranging from fixed platforms in shallow water to floating, dynamically positioned rigs capable of drilling in water depths exceeding 10,000 feet. Offshore drilling is significantly more expensive than onshore drilling, reflecting the cost of the platform infrastructure, the logistical complexity of operating far from shore, and the additional engineering margins required for safety in a marine environment. Once a well is drilled, production equipment is lowered to the seabed, and rigorous environmental and safety regulations govern the entire operation from drilling through eventual well abandonment when production is no longer economical.

Well Design and Casing: Engineering for Decades of Service

A well is not a simple hole in the ground; it is an engineered structure designed to remain safely sealed and operational for decades. As drilling progresses, the wellbore is lined with concentric strings of steel casing, cemented in place, each string smaller in diameter than the last as the well gets deeper. This casing design serves multiple purposes simultaneously: protecting groundwater aquifers from contamination, providing structural integrity against the surrounding rock pressure, and creating the sealed pathway through which hydrocarbons will eventually flow to surface once the well is completed.

Casing and cementing design must account for the specific pressures, temperatures, and rock conditions expected at each depth interval, and errors in this design are among the most serious risks in well construction, capable of causing loss of well control if not properly engineered and executed. This is why casing design calculations are treated as a specialised engineering discipline within drilling engineering, requiring rigorous review before any well is spudded.

Hydraulic Fracturing: Stimulating Flow From Tight Rock

Many modern horizontal wells, particularly those targeting unconventional shale formations, require hydraulic fracturing to produce economically. The technique involves pumping a high-pressure mixture of water, sand, and chemical additives into the completed wellbore, creating small fractures in the surrounding rock that the sand holds open once pressure is released, providing pathways for oil and gas to flow into the well that would not otherwise exist in these low-permeability formations. Fracturing is typically performed in multiple discrete stages along the length of a horizontal lateral, each stage isolated and treated separately to maximise the exposed and stimulated rock volume along the well’s full length. The combination of horizontal drilling and multi-stage hydraulic fracturing is what transformed previously uneconomical shale resources into some of the most productive oil and gas plays in the world.

Frequently Asked Questions

What is the difference between drilling and completion?

Drilling creates the physical wellbore, from spudding to reaching target depth. Completion prepares the well to produce, including casing, cementing, perforating, and often hydraulic fracturing. They are performed by different crews under different contracts, and a well can be drilled but left uncompleted for an extended period.

Why is horizontal drilling more common than vertical drilling today?

Horizontal wells expose significantly more producing rock to the wellbore than vertical wells, because geological formations are typically far more extensive horizontally than they are thick vertically. Combined with hydraulic fracturing, this makes horizontal drilling substantially more productive for unconventional shale developments, despite being more expensive and technically complex to drill.

What does DUC mean in the oil and gas industry?

DUC stands for drilled but uncompleted well, referring to a well where drilling has finished but completion activities have not yet begun. Operators track DUC inventory closely as an indicator of near-term production potential and as a lever for managing capital spending in response to price conditions.

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Conclusion: A Technical Discipline With Major Commercial Consequences

Drilling operations sit at the technical and commercial heart of the upstream oil and gas industry. The choices made in well design, drilling technique, and completion timing have direct consequences for production volumes, capital efficiency, and safety, making a solid understanding of the drilling process valuable not only for drilling engineers but for anyone in a commercial, project management, or operational role connected to upstream activity.

Related reading: Drilling is the first major operational phase of the upstream segment. Our article on oil and gas industry overview: upstream, midstream, and downstream explained covers how drilling and production connect to the rest of the value chain, from exploration through to the midstream infrastructure that moves what drilling ultimately produces.

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