Abstract
Mud cap drilling technology would often be an effective solution in drilling through total loss zones otherwise not drillable using conventional methods, even when a formation fluid influx is encountered during operation. In addition to this hazardous situation, however, coincidentally occurred borehole enlargement due to formation instability may cause complex well control issues as well as severe hole cleaning troubles such as pack-off or pipe sticking which sometimes result in an unintended sidetracking.
The previously developed cuttings transport numerical simulator using a one-dimensional transient two-layer model was modified to enable to reproduce lost circulation situations. The simulator predicts transient behaviors of cuttings transport including the amount of cuttings accumulation and annular pressure. In the simulator, a supposed certain loss rate of drilling fluid can be set at a certain depth of loss zone and the rate of cuttings injected with drilling fluid into the loss zone is set according to the loss rate. The extended model can also deal with borehole enlargement with variable wellbore geometries and subsequent cuttings generation from the enlarged hole area due to formation breakout.
Simulation studies on cuttings transport or hole cleaning in occurrence of partial and total loss of circulation and borehole enlargement due to shale instability were conducted, and feasibility of mud cap drilling operation under total loss situation coincident with borehole enlargement was examined. By use of modified cuttings transport simulator, transient behaviors of cuttings accumulation and equivalent circulating density (ECD) during drilling extended-reach wells in occurrence of lost circulation and borehole enlargement were successfully simulated. From the simulation results, the mechanism and the degree of cuttings accumulation under these severe situations were well demonstrated.
