As part of a multi-year slim hole drilling project in Venezuela, large scale automization of the drilling/coring process has been achieved. An SCR-based constant feed control system has been implemented and proven to be a distinct advantage over standard weight-on-bit (WOB) oriented controls. The rather abrupt feeding when trying to hold WOB constant resulted in feed-off cycle frequencies of approximately 1 s -1 propagating through the drill string and thus inducing core breakage and jamming in the core barrel.

To avoid these axial peak loads, the system has been modified to feed off at a constant, driller-defined rate, slowing down gradually from this rate when approaching pre-set WOB maximum limits. This results in the drill string smoothly "following" the core head drilling off, allowing the core to continuously travel up the inner core barrel. Even laminated formations with ROP variations between <10 and >40 ft/hr were successfully drilled without core jamming and sustained 100% core recovery. Using this system, the driller can match bit and formation characteristics for a given rotational speed to achieve optimum ROP at reduced bit wear.

The system is completely based on the control of the drill string movement using the drawworks' DC motors and SCR system drive, thus eliminating the need for additional sensors in the derrick. It can be retrofitted to any SCR driven rig at low cost in minimum time. It's application is specially suited for slim hole drilling where the constant, smooth feed limits WOB fluctuations to only +/− 200 lbs, but can be used during any drilling operation, improving drilling performance and extending the service life of downhole equipment. This paper describes the technical realization of the system, its application during 4 1/2 inch coring operations and recommendations how to use it for optimum drilling performance and core recovery

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