Automated E-Line Milling in Slimhole Well

Abstract

The first time with an automated electric-line (E-line) milling has been deployed in 2-7/8" tubing which is called "Slimhole well", for scale removal in the Gulf of Thailand (GOT). This paper highlights on operation consideration that made a step change of scale removal in 2-7/8" tubing commingles well. Automated E-line milling is robotic tool string assembly, automatically controls tractor force with intercommunication between milling cartridge and tractor, embedded with control algorithms. Automatic control system improves torque-on-bit by minimize margin of torque spikes and it reduces occurrence of motor stalls with having automatic tractor backing-off feature – reversing bit rotation and commanding tractor to move upward. Tool string has temperature limit at 350-degree F. Standard Slimhole well of Chevron Thailand is 3-string commingle well design with cemented tubing and normally run with 2-7/8" tubing. Challenge of production in 2-7/8" tubing is scale built up after production for a while and restrict flow at perforated hole at pay sand and finally, the scale develops to be full bore restriction of production below. To remove wellbore restriction, slickline or wireline cleanout is primary method with success rate in between 19-33%. If the remaining reserve value below restriction is economic for coil tubing clean out, then those wells will be scheduled and cleaned out by coil tubing unit (CTU). In addition, high temperature is another challenge in GOT for tool string design and selection. In 2023, Chevron Thailand has trial E-line milling to clear scale for 5 wells in gas field, resulting to total of 935 ft length of scale milled and cleared. We have experienced challenges of high temperature (maximum wellbore temperature at 378-degree F), cross flow, and cutting handling. "Quenching technique" by pumping seawater is implemented to mitigating high temperature. However, clearance between tool string and tubing I.D. is limitation of quenching effectiveness and causes of high tension regarding to crossflow between pay sands. Challenge of cutting handling occurs when milling full bore scale or milling scale at bottommost perforated pay sand. This lesson learning will be considered when well candidate selection in new campaign.