An automated wireline milling solution targeted for removal of wellbore obstructions of a varying type, from scale to metal, with built-in capabilities of autonomous cruise navigation between consecutive obstacles, is presented. This paper highlights design features that made a step change in the efficiency and usability of milling services.
Control challenges are still common in downhole milling technology. Changes in milling target composition, cuttings accumulation around the target, drag forces from production flow, and other variations can reduce system efficiency and result in lost time or failed interventions. In the case of wireline milling technology, inclusion of intelligent on-board electronics in the downhole equipment presents an opportunity to actively control the milling process to optimize rate of penetration and implement additional protections to reduce operational risk. We describe a robotic toolstring that automatically and independently controls a wireline tractor using real-time feedback from a milling cartridge and other on-board sensors. Embedded control algorithms implement intuitive workflows derived from the combined experience of multiple experts in well intervention.
With this automated wireline milling system, the user can initiate the milling process by defining certain milling parameters and then can monitor progress in real time while the downhole robotic tool regulates weight on bit and the milling motor. This new automated downhole control system significantly improves torque-on-bit and weight-on-bit controls yielding superior performance, such as rate of penetration and usability. Dynamic load conditions are handled in a high-speed distributed control loop downhole to get most of bit torque capacity across the entire speed range defined by the motor power curve. Tractor push force is adjusted quasi-instantaneously with changes in cutting conditions. Control responsiveness along with software solutions for tracking of motor stall preconditions and a torque limiter greatly reduce the occurrence of motor stalls arising due to the bit wedging in highly reactive targets. With stall avoidance and an automatic backing-off feature to reengage the bit in case of a sporadic torque spike, direct involvement of an operator is significantly minimized compared to the previous tool generation. Head-voltage stabilization is another factor positively impacting the overall power stability and performance of electromechanical tools downhole. Safety features are also in place to prevent cable twisting and protect assets from overcurrent and overtemperature conditions.
The progressive design of the automated milling tool boosts operational efficiency and autonomy, minimizes human mistakes, and reduces risk of getting stuck during the service. Case histories demonstrate the first field jobs and system integration tests performed with this new tool.