Cuttings-Transport Problems and Solutions in Coiled-Tubing Drilling
- L.J. Leising (Schlumberger) | I.C. Walton (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2002
- Document Type
- Journal Paper
- 54 - 66
- 2002. Society of Petroleum Engineers
- 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 3.1.6 Gas Lift, 1.8 Formation Damage, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.6.9 Coring, Fishing, 2.2.2 Perforating, 5.3.3 Particle Transportation, 1.11 Drilling Fluids and Materials, 1.6.1 Drilling Operation Management, 5.4.10 Microbial Methods, 1.5.4 Bit hydraulics, 5.3.1 Flow in Porous Media, 1.7.7 Cuttings Transport, 2.2.3 Fluid Loss Control, 2.4.3 Sand/Solids Control, 1.6.2 Technical Limit Drilling, 4.2 Pipelines, Flowlines and Risers, 1.10 Drilling Equipment, 5.3.2 Multiphase Flow, 1.6 Drilling Operations, 3.3.1 Production Logging, 1.7.5 Well Control, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.7.1 Underbalanced Drilling
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In spite of the many technological advances that have accompanied the growth of coiled-tubing (CT) drilling, one significant challenge remains - effective cuttings transport, particularly in deviated wells. This paper presents a summary of cuttings-transport problems and current solutions. It is shown that, in many circumstances, hole cleaning is more efficient if a low-viscosity fluid is pumped in turbulent flow rather than a high-viscosity fluid in laminar flow. Case studies are presented that illustrate both cuttings- transport problems and routine applications without cuttings-transport difficulties. The proposed hole-cleaning models are used to interpret these data and to suggest possible alternative approaches.
Two novel approaches to understanding hole cleaning are introduced. First, for laminar flow, the distance that a particle will travel (downstream) before it falls across the annulus clearance is calculated with Stokes' law and the local viscosity while flowing. This analysis may easily be applied to optimize mud selection and wiper trips. Applying this model to high low-shear-rate-viscosity (LSRV) gels shows that they may perform well inside casing but are expected to do a poor job of hole cleaning in a narrow, openhole, horizontal annulus without rotation. Second, for turbulent flow in horizontal wells, the concept of using annular velocity (AV) as a measure of hole cleaning is shown to be insufficient. A more complete term, annular velocity/root diameter (vARD), is introduced and should be used to compare cuttings transport in turbulent flow in horizontal wells of different cross-sectional areas.
Horizontal wells are a significant application for CT drilling. Cuttings transport in horizontal wells remains a challenge. With rotary drilling, drillstring rotation acts to keep the cuttings in suspension. With CT, tubing rotation is not yet possible; thus, muds and techniques that have been borrowed from rotary drilling are often only marginally effective with CT drilling. Underbalanced drilling can reduce transport problems by providing extra annular flow from the formation, but cuttings beds in the curve can still be a problem.
One major application of CT drilling is through-tubing sidetracks in casing. This provides a special challenge because the small bottomhole assembly (BHA) required to drill through tubing must provide enough flow to adequately clean the much larger casing section usually found between the tailpipe and whipstock.
Many factors are important for mud selection when drilling overbalanced with CT. A few of these are:
High lubricity for maximum horizontal reach.
Good hole stability, which is critical for slimholes.
Minimum formation damage.
Low solids content to increase the rate of penetration (ROP) and reduce friction.
Rapid solids removal for fine cuttings with small mud volumes.
Fluid-loss control to prevent differential sticking with highsolids muds.
Compatibility with elastomers.
Adequate cuttings transport.
Low friction pressure to allow maximum flow rate and minimize CT fatigue.
Of these parameters, all of which must be considered when selecting a drilling fluid, only the last two are considered here. Typically, there is a compromise between flow rate and viscosity, both of which contribute beneficially to cuttings removal and detrimentally to frictional pressure losses in the CT and annulus. As experimentally observed by Zamora and Hanson,1 an increase in annular velocity improves hole cleaning regardless of the flow regime. Although this is evident in turbulent flow, in laminar flow the outcome is not so clear. An increased flow rate results in an increase in the settling velocity (owing to shear thinning) and, consequently, a decrease in the settling time, which is offset by more rapid axial transport.
In this paper, cleaning horizontal wells with turbulent flow whenever possible is recommended. This is consistent with the experimental observations of Brown et al.,2 who reported that water is more efficient than a polymer in cleaning a horizontal concentric annulus. This is attributed to the difference in flow regime, which is turbulent for water and laminar for the polymer. Water also outperformed the polymer in an eccentric annulus.
Theory of Cuttings Transport.
The maximum flow rate available for hole cleaning with CT is limited by the pressure at the surface and the flow-rate limitation of the downhole motor/BHA. Typical 2 7/8-in. positive displacement downhole motors (PDM) have flowrate limitations in the range 2.0 to 3.0 BPM (318 to 477 L/min). Surface pumping pressures (SPP) are typically limited to between 3,500 and 4,000 psi while drilling. CT fatigue is magnified greatly at high pressures and is the main limitation to higher SPP. A complete discussion of these tradeoffs is presented by Leising and Newman.3
The traditional guidelines for hole cleaning with unweighted, unviscosified fluids are a minimum annular velocity of 50 ft/min (0.254 m/sec) in vertical holes and 100 ft/min (0.508 m/sec) in horizontal holes. These values are lower than would normally be applied in conventional drilling because of the high downhole motor revolutions per minute (rpm) and low weight on bit (WOB), resulting in small cuttings with CT drilling. Annular velocities can be reduced further if viscosified fluids are used. Another simple rule of thumb for vertical wells is that the annular fluid velocity should be twice the cuttings' fall velocity. Clearly, lower velocities are required for more viscous fluids and smaller cuttings. These rules of thumb cannot capture the complicated physics that contribute to cuttings transport, particularly in highly deviated wells, and, for that reason, they can never provide more than a rough guide as to favorable conditions for efficient hole cleaning. A clearer estimate is provided by examining the physics of hole cleaning in more detail.
There are two major points of departure that distinguish hole cleaning in CT drilling from hole cleaning in conventional rotary drilling.
In rotary drilling, the drilling fluid must be able to support the cuttings while the pumps are switched off making a connection; in CT drilling, shutting down flow is infrequent.
In rotary drilling, the rotation of the drillpipe contributes to hole cleaning by continually entraining cuttings back into the mainstream from the lower side of the hole; in CT drilling, the pipe does not rotate above the motor.
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