Optimization of mud cap drilling for un-drillable wells due to lost circulation.

摘要

Mud cap drilling, (MCD) technique is one of the managed pressure drilling types. This technique is used when all conventional attempts fail to solve the lost circulation. In this technique, a high density mud column, which is called mud cap, is placed in the annulus and above the loss zone to prevent the flow going to the surface facilities; the drilling fluid will flow into the loss formation instead. The cuttings will be carried by the drilling fluid and injected into loss formation. The conventional drilling mud is replaced by cheap and available fluid, which is called sacrificial fluid, to complete drilling operations.;In this study, a step-by-step procedure to design a MCD was proposed. Based on this procedure, a computer model using visual basic for application (VBA) was developed to design and optimize the MCD. By using this simulator, the optimal parameters such as the mud cap height, mud cap density, flow rate, and hole cleaning effectiveness can be calculated. Monitoring the annular fluid's interfaces is important to have an idea about the bottom-hole pressure while drilling. Therefore, the simulator can predict the interface between the top of the mud cap and the empty column if the operation window is known.;A case study was also carried out for a well in an X-field by using this simulator. This well has a high pressure zone from depths of 4800 to 4900-ft, while the loss zone would span from 5300 to 5450-ft. From drilling parameters data from an offset well in this field, the pore pressure and fracture pressure were calculated. Also, a complete drilling program was proposed for the next wells by using the mud cap technique. Therefore, the optimal mud cap height and density were calculated in order to have good bottom hole pressure control. Lastly, a sensitivity analysis between the mud cap height and other drilling parameters were executed to identify the optimal MCD design. It was found that the mud cap height of 1255-ft and its density of 20-ppg would achieve the optimal drilling solution for the well.