DO CUTTINGS BED IMPELLERS WORK?
There are several tools available which claim to improve hole cleaning in high inclination hole sections, ranging from drill pipe with integral scalloped blades (e.g. Hydroclean Enhanced Performance Drill-Pipe) to discrete subs positioned between tool joints. Those could be sourced from various suppliers (Paradigm, Franks, Halliburton, etc.). These tools all work in a broadly similar fashion – blades or scallops create localized mechanical or hydro-mechanical agitation of the cuttings bed which improves the drill pipe’s ability to lift cuttings from the low-side up into the higher velocity fluid zone in the high-side part of the wellbore. Tools are generally spaced such that cuttings should remain in suspension throughout the high inclination interval (i.e. less than the calculated cuttings “settling” period), typically every 2-3 stands.
The down-side of such tools is the additional time required for make-up / break-out, rental and redress costs (typically premium connections are required on wells likely to require such tools), and in smaller hole sizes the cumulative pressure drop across the tools may be of significance (i.e. increased ECD).
There is no doubt that such tools can be effective in reducing the cuttings bed height while drilling. The recommended approach is that hole cleaning concerns should be addressed at the planning stage by ensuring that each high inclination interval can be drilled with hole cleaning parameters in excess of recommended thresholds for the hole size (i.e. rotary speed, flow-rate and mud low shear rheology (6 rpm)). When above minimum recommended thresholds it should be possible to achieve adequate hole cleaning and in such cases it is generally difficult to justify these tools. Note that the longest wells in the world were drilled without them.
Occasions where it may be prudent to consider such tools include:
- Large hole sizes drilled at high inclinations with motor assemblies.
- Where drilling parameters have had to be reduced to below hole cleaning thresholds (e.g. wellbore instability, losses, equipment issues or exceeding existing rig’s capabilities).
- When back-reaming is planned to ensure as clean hole as possible (e.g. for a casing flotation job). Incorporating a few stands above the BHA may assist in keeping the cuttings bed moving up-hole away from the top of the BHA and reduce the risk of pressure fluctuations / pack-offs should the BHA “catch-up” with the cuttings bed.