Cuttings-Transport Problems and Solutions in Coiled-Tubing Drilling

Abstract

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 cuttingstransport 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 (v ARD ), is introduced and should be used to compare cuttings transport in turbulent flow in horizontal wells of different cross-sectional areas.

Improving Post-Stimulation Coiled Tubing Drillout

“…Moreover, when pumping highly viscous gel sweeps during drillout operations, it has been commonly observed that the solids do not return with the sweeps but, instead, are carried within the Newtonian fluids pumped behind the sweeps. This observation is consistent with that of Leising and Walton (2002).…”

“…Various unsuccessful attempts have been made to use consistent high viscous fluids for CT drillouts, but the high circulating pressure and associated pipe charge has made it uneconomical. Low viscosity Newtonian fluids, either weighted or un-weighted, are generally better suited for solids transport in the horizontal or inclined sections of the coiled tubing annulus (Leising and Walton, 2002). Without misconstruing fluids suspension capability and hydraulic solids transport, experience has shown that the use of shear-thinning high viscous sweeps with good suspension characteristics can be effective for solids removal in the inclined sections of the annulus.…”

“…In the absence of pipe rotation, previous studies have shown that the flow regime and fluid viscosity dictates the effectiveness of solids transport in a highly deviated wellbore annulus (Walton, 1995;Leising and Walton, 2002;Cho et al, 2002). Flow regimes are quantified with Reynolds number which is a function of annular diameter, annular velocity (AV), fluid viscosity, and density (Equation 1 and 2).…”

“…Theoretically, chaotic flow with turbulent eddy drag forces creates the necessary energy to lift and keep solids moving in the flow stream through a process known as saltation. The analytical study conducted by Leising and Walton (2002) demonstrated that the solids are transported a certain length, described as the transport length, after which they settle to the low-side of the wellbore during the saltation process. Thus, the key to achieving good hole cleaning is to maximize the solids transport length.…”

“…Laminar slurry flow of both Newtonian and non-Newtonian fluids is more prone to the formation of solid beds, and the solids at the topmost layer of the bed essentially roll along the horizontal section of the wellbore. In the entire wellbore, the curved section with inclination between 30 and 65 deg is the most difficult to clean (Leising and Walton, 2002). The continual build up of solids in this section can eventually lead to sliding of the solid bed layers back towards the heel of the horizontal in a situation known as an avalanche.…”