Snubbing Units in Oil & Gas Operations
There is a common misconception that snubbing or hydraulic workover units are exclusively used for uncontrollable wells such as blowouts, which is inaccurate. The ability to run tubulars under pressure has allowed many wells to be controlled effectively, saving considerable costs for the industry. Moreover, these units enable routine workovers and well servicing tasks on live wells, reducing or eliminating the need for kill fluid. This approach protects the reservoir from damage and enhances the efficiency and cost-effectiveness of workover operations.
Hydraulic Workover Units (HWO)
A hydraulic workover (HWO) unit can fulfill the functions of workover and/or drilling rigs but offers distinct advantages in certain scenarios:
– It has a compact footprint, which is ideal for operations in limited space.
– It minimizes environmental impact compared to conventional rigs.
– It can be rapidly mobilized to offshore platforms without a drilling rig.
– It facilitates quicker mobilization to remote land wells than traditional drilling or workover rigs.
– Operating costs are lower than those associated with conventional rigs.
– It requires less extensive platform support due to its ability to be partially or fully supported by the well.
Disadvantages of HWO units include:
– Slower operation due to trip times, especially when making up joints.
– Additional equipment such as mud pumps and solids control is required, integral to workover or drilling rigs.
Snubbing Units
Traditionally used mainly for critical well control and high-risk operations, snubbing units have proven beneficial for enhancing well performance and reducing overall costs. They are particularly useful in operations where killing the well is undesirable or impractical, such as:
– Running and pulling completion strings.
– Performing through-tubing operations like gravel packing and cleaning out obstructions.
– Conducting well control operations during drilling and workovers.
– Addressing well abandonment and perforation needs.
Advantages of snubbing units over coiled tubing or wireline include:
– High load rating suitable for heavy-duty tasks like fishing and milling.
– It is capable of pipe rotation, ensuring effective hole-cleaning properties.
– Ability to handle live well operations without compromising pipe strength.
– Flexibility to operate under various wellhead pressures.
Disadvantages when used as a snubbing unit:
– Slower operation compared to coiled tubing or rigs due to trip times and single-joint handling.
– Higher operational costs relative to coiled tubing are partly due to longer mobilization and rig-up times.
Hydraulic Workover Operations (HWO)
HWO operations encompass a wide range of tasks including full workovers, clean-outs, re-perforating, and running or pulling completion equipment. These operations are executed similar to conventional rig operations with proper pressure control equipment (PCE) and adherence to standard well control procedures.
Snubbing Unit Operations
Snubbing involves inserting a plugged pipe into a live well using blowout preventers (BOPs) to create an external seal around the pipe. This method is preferred for tasks where maintaining well pressure is crucial, such as:
– Running or retrieving completion equipment.
– Milling and washing below production tubing.
– Performing through-tubing operations like gravel packing and acid/cement spotting.
– Addressing well control issues in deviated or high-pressure wells.
BOPs in Snubbing Units
Snubbing operations utilize stripping BOPs for primary well control, with backup BOPs employed on high-pressure wells. The design and configuration of snubbing units vary based on specific job requirements, typically categorized into short stroke, long stroke, and rig-assist (conventional) units.
Components of HWO/Snubbing Units
These units include essential components such as hydraulic jack assemblies, workbaskets, control panels, and safety systems. The hydraulic jack assembly provides the necessary push, pull, and torsional forces on the work string, which is essential for various downhole tasks. Key components include guide tubes, windows, traveling and stationary slips, rotary tables, and power tongs.
Long Stroke, Short Stroke, and Rig Assist Snubbing Units
Additional backup BOPs may be deployed on wells with very high pressure. For tapered workstrings, specific BOPs may be initially provided for each pipe size, resulting in potentially up to 10 BOPs being used.
Snubbing unit configurations are adaptable and customized based on job requirements, typically falling into three main designs:
1. Multiple Ram Configuration
2. Short Stroke unit, which is predominant due to its ability to manage higher snubbing forces, supported by tubing guidance (further descriptions of snubbing equipment primarily refer to Short Stroke units).
3. Long Stroke Unit
4. “Rig assist” or conventional units powered by drilling rigs.
High-pressure wells necessitate additional Well Control equipment, although this advantage is limited by the positioning of the short-stroke snubbing unit jack above all pressure containment devices. BOPs must be rated for specific tasks, such as 345 bar (5,000 psi), 689 bar (10,000 psi), 1034 bar (15,000 psi), etc.
HWO & Snubbing Units Rig-Up
HWO/Snubbing units are rigged directly onto the Xmas tree for through-tubing operations or onto the wellhead after removing the Xmas tree if pulling or running completion components. For washing out, they can be rigged onto a drill pipe.
Typical HWO/Short Stroke Snubbing Unit & Dimensions
Equipment is rigged as individual lifts or sub-assemblies and directly connected to the Xmas tree/Wellhead or the preceding component. Maximum lift usually involves loads up to approximately 6 tonnes with larger jacks, which typically do not pose issues. In cases requiring very tall rig-ups due to multiple BOPs, limitations may arise from the crane’s maximum reach. To address this, a portable rig-up crane may sometimes be necessary despite increasing rig-up time and costs.
All equipment is transported in baskets or on skids. As rig operations proceed, scaffolding is erected around the BOP and the window/jack for access. This scaffolding is standard for one-off jobs, but for multi-well campaigns, transport frames around the BOPs can prove cost-effective, serving as scaffolding post-rig-up.
Similarly, frames can be attached to the window and jack, functioning as platforms for individual platforms. Frame designs depend on rig-up heights and platform layout. Short-stroke units can even be rigged within a drilling rig’s derrick.
The snubbing system is categorized into these main components:
– Basic snubbing unit
– Well Control system
– Well barriers
– Auxiliary equipment
Basic HWO/Snubbing Unit Components
The basic HWO unit is a mechanical or hydraulic machine that generates push, pull, and torsional forces on the workstring for specific well tasks. The two common types are “standalone” and “rig-assist,” which share identical components.
HWO/Snubbing operations necessitate specialized support equipment, including a pipe handling system, transport containers, toolbox, pumping equipment, fluid storage/handling systems, and related support gear.
Equipment Description:
A HWO unit comprises:
– Hydraulic jack assembly featuring guide tube, window, traveling and stationary slips, rotary table, and power tongs.
– Workbasket, control panels, and safety systems.
– Hydraulic power pack, accessories, and hydraulic hoses.
– Circulating system with swivel, Kelly hose, tanks, and pumps.
– The BOP stack includes the control system and hoses.
– Guy wire/support system.
Hydraulic Jack Assembly in Snubbing Unit
The hydraulic jack assembly comprises cylinders configured to move the pipe in or out of the wellbore. Snubbing and pulling capacities depend on cylinder number, size, and hydraulic pressure. Some jacks use a regenerative hydraulic circuit to enhance pulling speed by circulating fluid from the snub side to the lift side.
Key components include:
– Guide tube: prevents work string buckling.
– Window: facilitates tool installation and stripper element changes.
– Traveling slips: grip the pipe during snubbing or lifting.
– Stationary slips: hold the pipe in place.
– Rotary table: for milling and fishing operations.
– Power tongs: hydraulic tools for making up connections.
Snubbing Unit Workbasket, Control Panels, and Safety Systems
The workbasket atop the jack serves as the crew’s platform for operations. It houses controls for jack and slips, BOPs, and counterbalance winches. Safety features include stairs for access and an emergency exit system like the “safety slide.”
Hydraulic Power Pack and Accessories
Ground-based power packs supply hydraulic pressure for jack, BOPs, rotary, winches, and tongs. Pressure regulation ensures safe operation.
Hydraulic Hoses in Snubbing Unit
These hoses transport hydraulic fluid to and from various unit components, tested to maximum working pressure after rig-up.
Circulating System Including Swivel, Kelly Hose, Tanks, and Pumps
The system includes necessary valves and must withstand high pressures for fluid circulation during operations.
Guy Wire Support System
Calculations are essential for supporting the unit’s height and determining cable dimensions, tension, and necessary weights to manage additional wellhead loads.
HWO and snubbing units offer specialized capabilities that enhance operational flexibility and efficiency in well intervention and servicing operations across diverse well environments.
Content Ref: Snubbing Units In Oil & Gas Field – Drilling Manual