Field experience has shown that inefficient transport of small cuttings is a main factor for excessive drag and torque during extended-reach drilling; however, very little is known about the transport behavior of small cuttings. In this study, extensive experiments with three sizes of cuttings (0.45 to 3.3 mm) were conducted in a field-scale flow loop (8 in.×4.5 in., 100-ft long) to identify the main factors affecting small cuttings transport. The effects of cuttings size, drillpipe rotation, fluid rheology, flow rate, and hole inclination were investigated.
The results show significant differences in cuttings transport based on cuttings size. Smaller cuttings result in a higher cuttings concentration than larger cuttings in a horizontal annulus when tested with water. However, a lower concentration was achieved for smaller cuttings when 0.25-ppb polyanionic cellulose (PAC) solutions were used. Unlike the transport of large cuttings, which is mainly dominated by fluid flow rate, the key factors controlling small cuttings transport were found to be pipe rotation and fluid rheology. Improvement by pipe rotation in the transport efficiency of small cuttings is up to twice as large as the improvement in large cuttings transport. Compared with water, PAC solutions significantly improve smaller cuttings transport, while the transport of larger cuttings is only slightly enhanced.
Mathematical modeling was performed to develop correlations for cuttings concentration and bed height in an annulus for field applications. Predictions from a three-layer model previously developed for larger cuttings were also compared with experimental results. Differences (up to 80%) indicate the need for improving the frequently used three-layer model by including correlations specifically developed for small cuttings to get a better design of extended-reach drilling. This study is also useful for horizontal or high-angle well drilling and completion through sand reservoirs.
Efficient cuttings transport is a major challenge when a long extended-reach well with a horizontal and highly inclined section of more than 20 thousand feet is drilled (Guild et al. 1995; Gao and Young 1995; Schamp et al. 2006). Cuttings can be ground to finer sand while being transported out of the hole, especially when rotary drilling is used. Drilling may not be able to proceed if cuttings transport remains a problem in such a hole. Because of excessive drag and torque caused by small cuttings settled at the lower side of the horizontal or inclined section, it may not be possible to run casing in place even if drilling to the target depth can be achieved. Similar problems exist in horizontal and highly inclined wells drilled through unconsolidated sand reservoirs.
Field practice and experimental observations (Parker 1987; Larsen 1990; Gavignet and Sobey 1989; Ahmed 2001; Bassal 1995) show that smaller cuttings are more difficult to transport under certain conditions. Moreover, smaller particles tend to more easily stick a drillpipe because of their cohesive effects (Ahmed 2001; Gailani et al. 2001). It is even more difficult to release the pipe once it is stuck by small, sand-sized cuttings.
An investigation into previous studies in the area of hole cleaning and sand transport shows that very limited information is available for small cuttings transport under drilling conditions. Though different cuttings have been tested by several investigators (Parker 1987; Larsen 1990; Ahmed 2001; Bassal 1995; Ford et al. 1990; Peden et al. 1990; Martins et al. 1996), no study has been conducted on small cuttings transport in horizontal or high-angle annuli involving drillpipe rotation. Previous conclusions about the cuttings size effects on cuttings transport are quite diverse, and even contradictory in some cases (Parker 1987; Larsen 1990; Gavignet and Sobey 1989; Ahmed 2001; Ford et al. 1990; Peden et al. 1990; Martins et al. 1996). Their experiments, upon which these conclusions are based, were conducted under incomparable conditions. It may not be correct, or at least not safe, to state explicitly that smaller cuttings are harder or easier to transport. The result may depend on various combinations of drilling parameters. This study was undertaken to understand why and under what conditions they are harder or easier to transport.