Drilling operations

 
The design of oil and gas wells is influenced by various factors, including local conditions, regulations, and the expertise and preferences of the operators involved. Each well design is tailored to accommodate these elements, making it unique to its context.

In general, the fundamentals of drilling can be applied across the oil and gas sector, though there are notable variations, particularly regarding automation levels, environmental considerations, and safety protocols. These factors can lead to different practices being adopted throughout the global oil and gas market.

The drilling sequence begins with the creation of large-diameter holes, followed by the installation of significant casing sizes, and progressively moving to smaller diameters as the project advances. This method incorporates stronger casings and liners to withstand increasing downhole pressures.

A simplified succession of a conventional oil and gas drilling project includes the following steps:

1. Setting the Structural Casing (in offshore drilling conducted from floating rigs)
2. Installation of the conductor casing
3. Setting of the surface casing
4. Installation of intermediate casing
5. Placement of the production casing
6. Setting the production string with production tubing

The drilling process initiates with the placement of large-diameter casings, followed by progressively drilling more minor holes while cementing in smaller, more robust casing strings and liners to handle the geological pressures encountered.

1. The initial drilling phase starts with a large hole using a jet bit; sediments are cleared using high-pressure seawater or conventional drilling bits if hard rock is present. The structural casing is then installed at depths of approximately 75 meters (250 ft), after which the jet bit is substituted with a drill bit, continuing the operation while seawater circulates to remove drill cuttings.
2. After drilling to an adequate depth determined by local conditions, the conductor casing is installed, completed with a float collar near the bottom. Cementing occurs through a tool attached to the conductor’s top; high-pressure cement forces seawater out, flowing through the float collar and into the annular space between the borehole and conductor.
3. Post-curing of the cement allows drilling to progress through the float collar into the target formation rock. Upon reaching the desired depth, the drill string is withdrawn to install the surface conductor, connecting the wellhead, BOP, and marine riser for offshore operations.
4. Drilling continues with the riser in place, switching from seawater to water-based or oil-based drilling mud. During this phase, new casing strings are lowered, cemented, and samples of drilling mud and geophysical logs obtained to assess potential oil and gas presence.
5. Evaluation of analysis results determines further action: plugging and abandoning the well if oil and gas yield appears low, temporarily suspending it for later consideration, or preparing it for production. Should production be deemed feasible, the production casing is set at the chosen depth.
6. Finally, the production string and tubing are installed, perforating the production casing to facilitate the flow of oil and gas to the surface.

Drilling operations employ rotary mechanisms either powered by surface motors (like a rotation table or top drive) or downhole mud motors. The operation injects high-pressure drilling fluids down the drill string, enabling the movement of cuttings through the casing annulus back to the surface.

Well completion transforms a borehole into a functional system designed for the controlled extraction of underground oil and gas, incorporating final casing installations to isolate various fluids.

Casing

The cemented casing serves several critical purposes, such as:

• Preventing contamination of freshwater zones.
• Stabilizing upper formations to prevent cave-ins.
• Providing a sturdy upper foundation.
• Isolating pressure zones or varying fluids (Zonal isolation).
• Sealing high-pressure zones from the surface, minimizing blowout potential.
• Preventing fluid loss or contamination in production zones.
• Facilitating a smooth internal bore.

The specific casing types and depths are established by assessing subsurface rock stresses, pore pressures, casing strengths, potential hole issues, predetermined hole sizes, and anticipated completion methods.

Standard casing types differ in specifications based on local conditions:

Structural casing (OD: 16” – 60”, usual size: 30”)

Conductor Casing (OD: 16” – 48”, usual size: 20”)

Surface Casing (OD: 8 5/8” – 20”, usual size: 13 3/8”)

Intermediate Casing (OD: 7 5/8” – 13 3/8”, usual size: 9 5/8”)

Production Casing (OD: 4 3/8” – 9 5/8”, usual size: 7”)

Production string

Production tubing

For further details, please visit Down-The-Hole Drilling Tools.

Liners

Structural Casing (for floating unit offshore drilling) serves as the outer string of reinforced, heavy-wall pipe used in wells to counteract bending moments from the marine riser and support wellhead installations.

Conductor casing serves as the initial casing string set beneath the structural casing, providing vital structural support and facilitating drilling fluid circulation. This casing is not primarily for pressure containment but may end up containing low annular pressures after well completion.

Surface casing runs inside the conductor casing to isolate water sands while preventing drilling fluid loss. This casing string also withstands pressure during transitions into high-pressure zones and typically serves as the connection point for casing head installation and BOP stacks. Upon cementing, it often reaches the surface or, within offshore drilling, the seabed.

Intermediate casing is cemented in place between the surface casing and production casing/liner. It is primarily utilized in deeper wells to isolate formations with abnormal pressures or conditions requiring special considerations, thereby allowing for deeper drilling.

Production casing is the innermost casing string extending from the wellhead, through which production fluids move into the surface sections.

Production String typically consists of production tubing, which may also include components like subsurface safety valves (SCSSV) and injection ports, used for conveying produced fluids to the surface.

Production Tubing runs within the production casing, channeling fluids from hydrocarbon formations to the surface and facilitating potential injection.

Directional drilling involves guiding wells off a vertical path to reach targeted locations using controlled deviations.

Directional drilling is conducted for a variety of reasons, such as:

• Maximizing reservoir exposure by drilling at angles.
• Accessing reservoirs where vertical drilling is impractical.
• Grouping wellheads at a single surface site to reduce rig relocation and surface disruption, as well as to streamline completion and production.
• Managing uncontrolled well pressure, or “blowouts”, by injecting heavy fluid to stabilize original pressures in the wellbore.

Directional drilling tools encompass whipstocks, bottomhole assembly (BHA) configurations, mud motors, and specialized drill bits.

A common technique involves using mechanical devices to adjust drilling direction near the bit and utilizing steerable mud motors to control the well’s deviation without rotating the entire drill string initially. Once the desired angle is achieved, the full drill string rotates to proceed in the targeted direction.

Coiled Tubing Drilling

This approach employs coiled tubing instead of traditional drill pipe, offering advantages such as ease of maneuverability and reduced effort for inserting and removing the tubing during drilling.

This method allows the tubing to enter the borehole through a stripper mounted on an injector, creating a hydraulic seal. Such setup enables drilling underbalanced, maintaining lower pressures within the wellbore compared to the fluid pressures of the surrounding formations, which enhances drilling speed and minimizes formation damage.

The drill bit operates via a downhole mud motor powered by surface-pumped drilling fluids (slide drilling).

The mud motor is a crucial component of the coiled tubing drilling BHA and facilitates operational logging during the drilling process.