As widely known, wellbore tortuosity or spiraling can lead to the trapping of a cuttings bed in a trough of a tortuous hole, thereby leading to poor hole cleaning in extended-reach drilling. The objectives of this study included quantitatively evaluating the influences of wellbore tortuosity on hole cleaning and cuttings transport behavior in extended-reach drilling. Additionally, the study provided a recommendation of effective drilling practices. The study involved performing hole cleaning optimization studies for an extended-reach well with a long horizontal section aiming for a maximum reservoir contact, using a transient cuttings transport simulator. The planned trajectory of the well was assumed to have a certain degree of wellbore tortuosity in the horizontal section. The pump rate and bottoms-up circulation operation were optimized based on parameter studies and additional transient simulations by considering the effects of penetration rate and variation in cuttings size.
Simulation results indicated the formation of a considerably high cuttings bed, particularly in the down-dip intervals (up dip in mud flow direction) at an insufficient pump rate. The height of 1/3 to 1/2 of the drill pipe diameter could be potentially buried in a cuttings deposit bed, and this results in a hole packed-off or a stuck pipe. A higher rate of penetration can cause insufficient hole cleaning. In this case, control drilling that maintains a reasonable low penetration rate may be effective. Furthermore, borehole breakout may enlarge hole diameter and generate large-sized cuttings. Both of these have negative impacts on hole cleaning, and thus, borehole stability and smooth wellbore trajectory controls should be carefully considered. To clean these holes, frequent bottoms-up circulations were effective at each stand of drilling even if the optimizations of other drilling parameters were limited. The findings also revealed that accumulated cuttings in a tortuous wellbore were trapped in the trough of the hole and that the bed height of locally trapped cuttings in the down-dip intervals could be much higher than that indicated by previous studies.