Shock Sub

What is a shock sub?

A shock sub, also called a shock tool or vibration dampener, is a spring-loaded section of the bottom hole assembly (BHA). It absorbs axial shock between the drill bit and the rest of the drill string. When the bit bounces on hard rock, the spring takes up part of the impact before it reaches the MWD tools and the pipe above.

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How does a shock sub work?

Inside the tool, a spring element sits between two parts that can slide against each other. The spring may be a stack of steel disc springs, an elastomer, or a hydraulic or gas chamber. When an axial load hits the tool, the spring compresses and then returns, which spreads the impact over time.

A shock sub is passive. It does not measure anything or change its behaviour. It reacts to the loads that reach it.

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Where is a shock sub placed?

Usually close to the bit, so it can take up shock before it travels into the rest of the BHA. When it is run with an agitator tool, a shock tool sits just above the agitator and turns its pressure pulses into an axial movement of the string (AADE-11-NTCE-47).

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When is a shock sub used?

• Hard, broken or interbedded rock: where the bit is likely to bounce.
• Protecting electronics: to reduce the axial shock that reaches MWD and LWD tools.
• With an agitator: to turn pressure pulses into axial oscillation that helps the string slide.

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What are the limits of a shock sub?

• It acts on shock after it starts. It does not change what causes bit bounce.
• It is set up for a range of loads, so it works less well when weight on bit and rock change a lot.
• Elastomer springs and seals have temperature limits, which matters in hot geothermal wells.
• It adds length and another tool that can fail to the BHA.

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Can the force on the bit be held steady downhole?

A shock sub reduces the shock that reaches the BHA. NexTitan generates the force at the bit instead. It applies weight on bit from inside the BHA and is designed to hold the force on the bit steady as the rock changes. See drilling dysfunction.

To estimate what steadier weight at the bit is worth on your section, use the drilling cost calculator.