Brief introduction
Wire line Pressure Control Equipment is used to flowing oil well, gas well, water injection well and coal seams well for cable logging with pressure or paraffin removal operation, could supply pressure buffer and transition zone for string tools. In operation, the wellhead can be sealed when cable is static and dynamic state with pressure, or closed in the course of overflow and other unforeseen circumstances beyond controlling, to prevent blowout accidents. Operation conveniently and safety reliable.
The PCE is mainly consists of cable hydraulic seal head, tool catcher, lubricators, Tool Trap (includes of release assly with pressure), Blow Out Preventer, Top& Bottom Sheave, Pressure testing& releasing Assembly, Wellhead crossover flange (crossover sub), Lifting tool, Grease and Hydraulic Control Module&#;Transport and Storage Frame Skid, Pressure test plugs etc.
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From a cable manufacturer&#;s perspective, this technical bulletin intends to look at best practices on pressure work for longer cable life.
Many energy companies perform wireline operations when the well is &#;live&#; or the wellbore pressure exceeds the atmospheric pressure. There are several reasons for performing live operations, including
» lowering the cost of operations,
» reducing formation damage,
» minimizing lost production due to downtime and
» performing pump-down operations.
By design, armored lines (aka wirelines) have interstices between armor wires and armor layers, which you cannot pack off directly. A grease head and lubricator are employed to maintain well control during &#;live&#; operations. Before the rope socket is attached, the wireline passes through several closely fitting flow tubes (pictured below).
Viscous grease is injected into the assembly at pressures exceeding wellbore pressure and fills the space between the wireline and the inner diameter of the flow tubes. The grease packed in the tight annular space provides significant resistance to wellbore pressure by adhering to the surfaces of the cable and the flow tubes while permitting the wireline cable to travel into and out of the wellbore. Each subsequent flow tube creates a compounding pressure drop, so there is no residual pressure drop as the wireline exits the flow tube. Pressure is maintained higher than wellbore pressure with a grease pump. The tighter the annular space, the higher the force (pressure) required to displace the grease. This principle is the same for both braided wireline and slickline.
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A pack-off is mounted at the top of the lubricator to seal the well in case of losing pressure control through the grease head. It should not be used as a line wiper because the packing rubbers are harder and can cause damage to the wireline. These devices also prevent the wireline from rotating, which causes the armor to loosen and may result in a birdcage below the pack-off. Additionally, wear on the pack-off rubbers can limit their ability to seal the wireline effectively if the flow tubes cannot hold back the wellbore pressure. In reality, many operators use the pack-off to wipe the line, specifically when the flow tubes are oversized, and pressure control with grease is difficult to maintain.
The argument for wiping the line is that the wireline equipment and well site stay clean, providing a better work area and more responsible environmental stewardship. More and more operators working offshore and in environmentally sensitive regions require line wipers. If you require or plan to use a line wiper, we highly suggest using one in addition to the pack-off. In other words, do not use the pack-off for this purpose.
You can purchase supplemental line wipers with softer rubber. A proper setup will allow the pressure on the line wiper to have more sensitive control than a pack-off to minimize the force on the wireline. Never use the line wiper to control pressure; their only use is to clean the line. However, the user should always be aware that there is the possibility of damaging the wireline when using a line wiper.
New Cables
New cables can be risky for pressure work because the cable is subjected to only a few hundred pounds of tension during manufacture, so there is essentially no torque in a new cable as delivered. When installed on a truck, the spooling tensions are significantly higher than during manufacturing. Because the cable is not free to rotate, it will develop significant torque and try to rotate to equalize this built-up torque and support the weight of the tool string when making the first field operations.
To illustrate the magnitude of this problem, consider a new 7/32-in. cable deployed in a straight 20,000-ft. well. The total rotations a new cable end would need to rotate to equalize the torque could be over 400. Until the cable is well &#;seasoned,&#; we recommend that flow tubes be on the high end of the industry standard (0.004 to 0.006 of an inch) to allow for line diameter and cable rotation changes. This &#;seasoning&#; is critical to the life of a wireline cable, and we recommend reviewing Technical Bulletin 2&#;New Cables: Understanding the Issues and Tips for Longevity, which discusses line speeds and operational practices for new cables.
Pressure work also slows the seasoning process by lubricating the cable and preventing grit and corrosion from forming in the armor&#;s spaces. This wellbore fluid and grit, combined with corrosion, helps prevent the cable from spinning, which is important in seasoning. Once seasoned, the lubrications from grease jobs are good for the cable. You will require a substantial amount of extra grease to fill the interstices between the armors the first time you use a cable for grease injection.
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