Study group report 2007: cuttings transport (Schlumberger)
This is the final report on the problem of cuttings transport incorporating the effects of drillstring rotation, brought to ESGI59 by Schlumberger. Click on the link at the bottom to download the full report as a pdf document.

Report prepared by
David Allwright (Industrial Mathematics KTN), Clare Bailey (Loughborough University), Chris Cawthorn (DAMTP) and Andrew Lacey (Heriot-Watt)

Executive summary
When an oil well is drilled, it is necessary to transport the rock cuttings up to the surface. To do this, fluid is pumped down through the centre of the drillstring, through nozzles in the bit, and back up to the surface in the annular gap between the drillstring and the drilled hole as illustrated in Figure 1.

Figure 1: Schematic of rotation and flow in horizontal drilling.

This fluid is a polymer solution, viscous, shear-thinning, and will typically have a gel strength. The flow in the annular region may be laminar or turbulent. The wellbore may have long sections that are approximately horizontal, in which a bed of cuttings may form at the bottom of the annuals. The drillstring is rotating, and generally off-centre because of its weight, and influences the flow significantly: the rotational velocity of the drillstring and the axial velocity of the fluid are comparable. Schlumberger have a reasonable mechanical model for the transport of rock cuttings when there is no drillstring rotation, and wish to know how this model needs to be modified or replaced to incorporate the effects of rotation. These models are based on work in the published literature, and involve assuming that the cross-section consists of some cuttings in a static bed and some in suspension as illustrated in Figure 2 on the left, with the possibility also of a sliding bed of cuttings as in the centre diagram. However, with rotation we may expect the cross-section of the cuttings bed to be more as shown on the right.

Figure 2: Suspension and rock cuttings bed without rotation (left and centre) and with rotation (right).

Figure 3 comes from a cleaning video made by MI HDD Mining and Waterwell, a company in Houston, Texas. The figure is still from the video showing fluid (in yellow) flowing up an inclined pipe, and carrying in with it some particles (in Black). Inside the fluid is the drill string, which cannot be seen, but which is able to rotate at approximately 150rpm. The video demonstrates that fluids flowing in a channel where the drillpipe is off-centre suspends more of the particles, and in may cases, does not allow a bed to form at all.

Figure 3: Still taken from a video made by MI HDD Mining and Waterwell. It shows fluid (yellow) moving up a pipe, entraning some particles (black) from the bed so they are suspended in the fluid.

Models of rock cuttings transport are important to help avoid operating in regimes where a heavy bed of cutting builds up, since there is then the risk that the drillstring will become stuck. A stuck pipe is very expensive and time-consuming to clear. Some of the existing approaches to modelling cuttings transport are described in the reference. A model for cuttings transport over the length of the pipe requires us to have a model for the rate of transport at each point as a function of the local conditions, and therefore in particular a model of the local flow regime. So the questions adressed by the Study Group were

• Under what rotational conditions does a bed form?
• What is the steady-state bed thickness, and is this small enough not to clog up the pipe?

Click on the link below to view the full report.

Download 'Schlumberger-Cuttings.pdf' (257 Kb).