Well Completion and Testing

Well completion is a systematic engineering process in oilfield development, bridging the gap between drilling the reservoir rock and bringing the well into production. It encompasses all operations from the initial penetration of the pay zone, through running casing and cementing, perforating, installing the production string, and finally flowing the well to initiate production.

A foundational overview from industry source "Petroleum Papers" outlines the core objectives of completion engineering and details the primary completion methods used for vertical, deviated, and horizontal wells.

The completion process is guided by two fundamental principles:

• Maximize Reservoir Potential: Protect the pay zone throughout the entire process-from the moment it is drilled to the start of production-to ensure the well can produce at its maximum capacity.
• Optimize Energy Use: Utilize well completion design to effectively harness the reservoir's natural energy and bring the well into production using the most efficient and economical method.

The choice of completion method defines how the reservoir is connected to the wellbore. The most common methods include open hole, perforated, slotted liner, and gravel pack completions.

1. Open Hole Completion

In this method, the producing zone is left without any casing or liner cemented across it; the reservoir rock is directly exposed to the wellbore. There are two main types: "barefoot" completions where the reservoir is drilled after setting casing at its top, and "later" open hole completions where casing is set after drilling the pay zone (rarely used today).

Advantages:

• Maximum Productivity: Provides the largest possible inflow area, resulting in a "hydrodynamically perfect" well with minimal flow resistance and potentially high production rates.
• Simplicity & Low Cost: Avoids complex operations like perforating or gravel packing, making the process faster and cheaper.

Disadvantages:

• No control over formation sloughing or sand production.
• No zonal isolation, leading to interference between zones with different pressures or fluids.
• Inability to perform selective stimulations (like fracturing or acidizing) or control production from individual layers.

Applicability: Only suitable for single, competent (hard, stable) reservoirs with no shale interbeds, water zones, or gas caps.

2. Perforated Completion

This is the most widely used method globally (over 90% of wells), involving running and cementing casing across the zone and then creating communication holes (perforations). It can be done with full casing strings or with a cemented liner hung from the previous casing.

Advantages:

• Provides excellent borehole support in weak or unconsolidated formations.
• Effectively isolates different zones, preventing cross-flow and allowing for selective production, testing, and stimulation.
• Compatible with advanced techniques like multi-tubing completions.

Disadvantages:

• The reservoir is exposed to drilling and cement fluids for longer periods, increasing the risk of formation damage.
• It is "hydrodynamically imperfect" – flow converges towards the perforations, creating additional near-wellbore pressure drop and reducing efficiency compared to an open hole.
• High-quality cement job and adequate perforation penetration are critical for success.

Applicability: Suitable for the vast majority of reservoirs, especially where zonal isolation or sand control is needed.

3. Slotted Liner (or Screen) Completion

A pipe with pre-cut slots (slotted liner) or a wire-wrapped screen is run across the open hole reservoir section. It is often used to provide borehole support while maintaining high productivity.

Advantages:

• Offers many benefits of an open hole (large inflow area, low cost) while preventing borehole collapse.
• Drilling fluids compatible with the reservoir can be used, as no cement is placed across the pay.
• The liner can often be retrieved for repair or replacement (in "pre-set" installations).

Function: The slots or screen gaps are sized to allow formation fluids to pass while retaining most formation sand. It provides a degree of sand control, particularly for coarser sands.

Applicability: Widely used in competent but unconsolidated formations with moderate sand production, in single, thick reservoirs with no need for zonal isolation, and very commonly in horizontal wells.

4. Gravel Pack Completion

This is the primary method for controlling severe sand production in poorly consolidated formations. A screen is run across the zone, and carefully sized gravel is pumped into the annulus between the screen and the formation (open hole) or casing (cased hole). The gravel acts as a filter, holding formation sand in place.

• Procedure: Gravel packs can be performed in open hole (requiring under-reaming for a thicker pack) or inside cased hole (after perforating with high-density, large-diameter shots). The gravel is carried by a viscous fluid and placed tightly around the screen.
• Advantages: Creates a stable, highly permeable filter that effectively stops sand production while maintaining good productivity.
• Why Screens over Slotted Liners? Wire-wrapped screens are preferred because they offer a precise, uniform gap (as small as 0.12 mm) suitable for fine sands, superior corrosion resistance, and a much larger inflow area compared to slotted liners.
• Pre-Packed Screens: A simpler alternative where gravel is factory-packed between two concentric screens. While easier to install and lower cost, they are generally less effective and durable than a full gravel pack, as they primarily prevent sand from entering the tubing rather than stabilizing the formation face.

The same principles apply to horizontal wells, but the methods are adapted for the long reservoir contact. Common techniques include:

• Open Hole: For stable formations.
• Slotted Liner: The most common method for horizontal wells, providing borehole support.
• Slotted Liner with External Casing Packers (ECPs): To provide segmentation and zonal isolation along the long horizontal section.
• Perforated Liner: Cemented and perforated liner, though less common.
• Gravel Pack: Used in horizontal wells requiring sand control.
• Intelligent Completions: Advanced systems with downhole sensors and remotely operated valves to monitor and control production from multiple zones along the horizontal wellbore.

Selecting the correct completion method is a foundational decision in production engineering. It balances reservoir characteristics, production requirements, and economic factors. From the simplicity of an open hole to the complexity of a gravel pack, each method serves a specific purpose. Understanding these fundamentals is essential for ensuring that the wellbore is optimally connected to the reservoir, protecting the formation's potential, and providing a safe and efficient. For more detailed information, please don't hesitate to contact Vigor team for more detailed product information.