Publication History: This article was prepared for CPH by E.R. Crain, P.Eng. in 2020. This webpage version is the copyrighted intellectual property of the author.

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PRESSURE CONTROL BASICS
Pressure control during wireline operations, such as perforating and production logging, is critical to the health and safety of workers at the wellsite nearby residents, domestic animals, and wildlife, not to mention the nearby environment and the financial health of the oil company and its executives and shareholders. Government regulations and engineering best-practices keep failures to a minimum, bur a few blowouts and gas leaks hit the evening news often enough to suggest that someone messed up somewhere.

This page describes the basic equipment and terminology related to pressure control at the well.

The pressure control employed during wireline operations is intended to contain pressure originating from the wellbore. During open hole operations, the pressure might result from a well-kick. If serious, the blowout preventer (BOP) would he closed, severing the wireline and dropping the logging tools to bottom of the hole. Heavier mud would be pumped in through the kill-line. With the well stabilized, the BOP would be opened and the wireline and tools fished from the borehole. Logging would resume and the well completed, suspended, or abandoned.

During cased hole wireline work, the well may be capable of producing at high pressure, either during or immediately after the wireline procedure. A fair amount of research and planning is needed to be sure the correct equipment is in place and tested before trying to run the job.

PRESSURE CONTROL EQUIPMENT
This equipment starts with a connection to the top of the casing, which forms the foundation for a semi-permanent fixture called the well head. This is the control system during normal production, and consists of pipes, joints, valves, chokes, and pressure gauges or sensors. Additional equipment is added to the top of the well head during wireline operations in cased holes..

Well head pressure control equipment must be rated to well over the expected well-head pressure. Normal ratings for wireline pressure control equipment is 5,000, 10,000, 15,000, and 20,000 psi.

The well head is also called well head assembly, well head stack, or “Christmas tree”.
 

 
Cut-away view of a typical well head (left) and "Christmas tree" style well head (right). Schlumberger images

The following describes the components of the equipment added to the well head temporarily to allow safe wireline operations in cased holes. They are listed in order from top of well head upward to top of final assembly. This material is from Wikipedia and Schlumberger Glossary, edited for clarity.

Flange
A flange attaches to the top of the well head assembly, usually with some sort of adapter for the rest of the pressure control. A metal gasket is placed between the top of the well head and the flange to seal against well pressure.

Quick-connect sub
A subassembly device that is bolted to the top of the BOP stack that is designed to eliminate traditional bolt-flanges to connect lubricator heads and utilize tapered-wedge and lock ring designs. This allows the same security of traditional pressure control connections

Tool trap
A tool trap has the same purpose as a head catcher in that it prevents the tools from inadvertently dropping down the hole. This device is normally located just above the well control valves, providing protection to these important barriers from a dropped tool. The tool trap has to be open in order to allow the tools to enter the well, and is normally built to allow tools to be recovered through the tool trap even when it is in the closed position.

Wireline valve
A wireline control valve, also called a wireline blowout preventer (BOP), is an enclosed device with one or more rams capable of closing over the wireline in an emergency. A dual wireline valve has two sets of rams and some have the capability of pumping grease in the space between the rams to seal against well pressure.

Lubricator
The lubricator
is a high strength, pressure tested, pipe fitted above the wireline BOP that allows tools to be put into a high pressure well. The top of the lubricator assembly includes a high pressure grease-injection section and sealing elements. The tools are placed in the lubricator and the lubricator pressurized to wellbore pressure. Then the top valves of the well head are opened to enable the tools to fall or be pumped into the wellbore under pressure. To remove the tools, the reverse process is used: the tools are pulled up into the lubricator under wellbore pressure, the well head valves are closed, the lubricator pressure is bled off, and then the lubricator may be opened to remove the tools.

Pump-in sub
Pump-in subs, also called a  flow-tee, allow for the injection of fluid into the pressure control string. Normally these are used for wellsite pressure testing, which is typically performed between every run into the well. They can also be used to bleed off pressure from the string after a run in the well, or to pump in kill fluids to control a wild well.








Grease injector head
The grease injector head is an assembly of components used to contain wellhead fluids and pressure during wireline operations. The wireline passes through a close-tolerance tube assembly as it moves. High-pressure grease is pumped into the surrounding annulus to effect a pressure-tight dynamic seal that is maintained during the operation by injecting more grease as required. A slight leakage of grease is normal, and the addition of fresh grease ensures the seal is maintained.

Pack-off subs
Pack-off subs utilize hydraulic pressure on a two brass fittings which compress a rubber sealing element to create a seal around the wireline. Pack-offs can be hand pumped or compressed through a motorized pumping unit.

Line wiper
A line wiper operates in much the same way as a pack-off sub, except that the rubber element is much softer. Hydraulic pumps exert force on the rubber element until a light pressure is exerted on the wireline, cleaning grease and well fluid off the line as it moves past the wiper.

Ball check valve
If the wireline were to become severed from the tool, a ball check valve can seal the well off from the surface. During wireline operations, a steel ball sits to the side of a confined area within the grease head while the cable runs in and out of the hole. If the wireline exits that confined area under pressure, the pressure will force the steel ball up toward the hole where the wireline had been. The ball's diameter is larger than that of the hole, so the ball effectively seals off pressure to the surface.

Head catcher
A head catcher, also called tool catcher, is a device placed at the top of the lubricator section. Should the wireline tools be forced into the top of the lubricator section, the head catcher, which looks like a small claw, will clamp down on the fishing neck of the tool. This action prevents the tools from falling downhole should the line pull out of the
rope socket. Pressure is applied to the head catcher to release the tools.


 

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