Using an agitator can be a huge boon to a drill operation, cutting down on friction issues and boosting results. But improper set-up and operation, especially in conjunction with MWD equipment, can cause unnecessary complications and headache. In our never-ending pursuit of MWD expertise, we sat down with NOV to flesh out the facts, debunk the Top 4 Myths About Agitators, and give you the 6 Essential Best Practices for using them on your next job.
Top 4 Myths about Agitators
1. The Myth: The Further Back the Better.
It’s all too common for the agitator’s placement to be a topic of heated debate at the rig site, and for good reason: the agitator’s position is crucial to ensuring optimal friction reduction. The company man wants it closer, the directional driller wants it closer, but the MWD hand wants it further back. Here’s the truth: As a rule, the agitator loses .5% energy every 10 feet, and placing the agitator 2000 feet behind the BHA can reduce its effectiveness to near zero. Furthermore, placing the agitator in the wrong position can cause the drill string to reach its resonant frequency, or the point at which the oscillation amplitude is at its greatest. In the vast majority of drilling situations, this is far greater than what is desired, and can cause unwanted and unnecessary complications.
What to do about it:
To ensure maximum effectiveness, the placement should be selected using NOVs Vibrascope. Vibrascope uses a mathematical model to place the Agitator in the optimal position. As a best practice, contact NOV ahead of time and have them run the Vibrascope analysis (normally included in tool rental) to determine placement.
2. The Myth: The Agitator produces 100 G's of shock.
Put simply, if your agitator system is set up and placed correctly, it will never create axial movement greater than 2 G's. Can the agitator possibly cause greater axial shock than 2 G's? YES. But only due to the incorrect application of the friction reduction tool. The most common misuse is over-flowing the tool, creating too large of a pressure drop across the tool and greater axial force than is otherwise possible. This can be caused by both incorrect assembly and improper operation in the field. However, when the tool is built and operated properly any MWD system can be run in conjunction without any negative effects caused by extreme axial shock.
3. The Myth: The Agitator moves axially.
In fact, the agitator has no axial moving components. The agitator is just a motor power section connected to a valve assembly. Agitators are run in conjunction with a shock tool that allows for axial movement from the pressure build up. The specialized shock tool is placed above the agitator in the drill string. It contains a series of individual Bellville stack springs that allows for approximately a quarter inch in travel. The shock tool converts pressure pulses into axial movement.
4. The Myth: Agitators contain baffles.
Actually, agitators have an oscillating valve assembly (OVA) and stationary valve assembly (SVA). The OVA and SVA are set up to a specific pressure drop and flow range for each application. This setup is included on each build sheet that accompanies the tool into the field. The OVA is driven by a power section, which creates pressure pulses. Their frequency is directly proportional to the flow rate.
Now that we've cleared up the myths, here are...
The Top 6 Best Practices When Running an Agitator and Shock Tool
As a best practice, contact NOV for proper placement of the Agitator. Vibrascope analysis is usually included in the day rate the operator pays for the tool. When this is the case, be sure to utilize this analysis to ensure correct placement of the friction reduction tool and minimize unwanted complications.
2. Lockdown system.
Whenever possible use a Lock down mule shoe assembly. SMS Precision Tech offers the perfect solution for use with an agitator. The lockdown system will keep the MWD tool stationary and eliminate any possibility of a jackhammer effect on the tool.
3. Tighter configuration.
To overcome the possible reduction in pulse height or signal amplitude, go one configuration tighter than what would normally be run for the same run.
4. Fully stabilized setup.
To help the tool reduce any extra shock and vibration events, make sure fins are full gauge with the ID of the Non-Mag Collar. SMS Precision Tech also provides collets that can be added to the middle of the pressure housing barrels. These can be used as an excellent mitigation tool for shock and vibration.
5. Copy of build sheet.
On any job that an agitator is used a NOV build sheet will accompany it. Ask the company man for a copy of the build sheet and compare the configuration to the parameters that will be used for the next section being drilled.
6. Follow up.
In the event of multiple MWD failures with the same agitator, ask to have it broken down in front of someone from your team. This should be done to ensure that the tool was built to the proper specifications. This has proven to be a very valuable measure taken by several of our clients.