PLANNING FOR SUCCESS EVEN IN FAILURE
Back in May, Stewart talked about maximizing combined torque and tension in the event of differential sticking. This segues neatly into the subject of stuck pipe, and everybody’s least favorite tool, the drilling jar. When jars are mentioned for a BHA, some common responses include, ‘Best jar placement? On the deck!” and “Waste of time, we won’t get stuck”. Though it is guaranteed the first question you will be asked by a senior manager if you don’t run one and get stuck is, “was there a jar in the assembly?” to which there is usually only one acceptable answer.
So why the dislike, even if it is stated in humorously? Cost? Extra BHA equipment? Restrictions on drilling parameters? Or is it:
• Because you didn’t get instantly free last time with a single firing?
• Because it didn’t fire instantly when required?
• Because we didn’t get free at all when we ran one?
Even with all the planning in the world, there are wells out there that have a high probability of encountering the need to jar at some point in the well and, while we work to minimize the probability of these events from occurring, we like to ensure that if the tools are included in the string they are in the position to be successful used.
From investigations and analysis from out engineers and advisors we have identified some of the common reasons for jars not being successful.
Failure to apply the correct procedures for a sufficient time for successful activation:
This could be classed as human error in many cases and is particularly common when vendors (and hence tools) change, or drill crews are inexperienced. To reduce the risk of this failure, operating procedures and tool limitations should be integrated into work instructions and available on the drill floor at all times. If jarring is required, the sooner it starts, the more likely the string will be freed. While failing to fire is often the primary concern, overloading the jar and causing it to prematurely fail is an important secondary. Some jars are designed to reduce this risk, so know what you have in hole before multiple jarring cycles become required.
Mechanical limitations restrict the ability to fire the jar:
In ERD and high angle wells this is a common scenario as drill string buckling limits the ability to apply down weight to the jar. This results in an inability to cock and fire jars downwards. Jarring up can also be limited in due to the rig not being capable of providing the levels of over pull required to cock and fire the jar in this direction. Prevention is always better than cure and knowing the risk prone sections in advance and what to do to avoid problems in the first place is always preferable. However, if the unexpected does occur having a plan to manage it before it happens always leads to a more successful outcome.
The jar is not correctly placed to provide adequate impulse or impact to release the string:
Jar placement is a balance of hammer weights, impulse and impact forces. While placement can be specified on a section-by-section by the vendor or engineer using software, jars position is often based on factors like ease of handling or previous practice. A jar that’s too far away from a likely sticking point won’t allow enough energy to reach the problem area, or it may be preferable to have a higher impulse at the expense of the total impact force when activation is required.
To conclude, Jars are a critical BHA component and the specification and placement should be carefully considered in the BHA design process. If run, they need to be configured correctly, carefully placed where they provide most benefit and not for ease of racking back or as an answer to a hypothetical managerial question.
By Martyn Hayes