Frac plug milling is the final step on the journey to completing a shale producer.  Return on the investment that has gone in to drilling the well and pumping the frac is contingent upon completing this step successfully.

 

A Milling Monitoring Tool can be indispensable in execution of this critical operation. Here’s how it can help.

 

To avoid fluid loss to the new perforations, the plugs optimally should be milled while the pressure inside the completion is kept below that of the formation. Whether this underbalance is achieved with a single phase light fluid, or by injecting gas, continual monitoring of downhole pressure delivers assurance that optimal conditions are maintained throughout the milling operation.

 

Deployed on e-coil, the MMT’s wired telemetry system is unaffected by gasified fluids and the high data rate permits second by second control of milling parameters to optimise performance. Both internal and external pressure are monitored, giving the ability to observe lifetime trends in the motor’s performance, empowering the operator to proactively pull an aged motor once readings indicate that it is about to become a dead horse.

 

Milling often takes place along extended reach laterals – monitoring injector weight alone gives the CT operator very poor knowledge of the weight applied at the bit, yet this parameter is critical to milling the plug efficiently. It is necessary to set down enough weight to get the mill to bite.  Too much weight and the motor will stall, too little and the plug will work harden, the mill will polish and penetration will be minimal. Milling with gasified fluids add presents additional problems. It is easier to get into a stall as there is almost no positive torque response as the bit slows down. When the stall happens, it takes much longer to detect at surface, as the pressure response at the gauge is softened by the gas phase. Thirdly, thanks to the compression in the gas phase during the stall, the energy released when the bit becomes unstuck can overspeed the motor and junk it prematurely.

 

The MMT puts the coil operator back in touch with what is happening down hole.  Second by second readout of Weight on bit and Torque means that the mill can be kept optimally engaged with the plug, while real time vibration indication enables stalls to be detected – and most importantly remedied, before damaging amounts of energy accumulate in the CT.

 

To sum up, an MMT can help to maintain the pristine quality of the new frac, can speed up the plug milling time and maximise the life of the milling motor. It’s a good friend to have on the journey.  

Author: Richard Stevens

Published: 5th July 2017

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