Summary
This paper is an extended work of SPE 90038. An aerated non-Newtonian flow
and an inclined wellbore section were added to the previous model to study the
hole-cleaning problem while drilling an underbalanced well. This new
mechanistic model for cuttings transport was developed by combining two-phase
hydraulic equations, turbulent boundary layer theory, and particle transport
mechanism. It is shown that the model is useful for predicting minimum annular
velocity and cuttings bed thickness in horizontal and inclined wellbore
geometry. Effects of temperature, bottom hole pressure, liquid flow rate, gas
injection rate, cuttings size and density, inclination angle, and rheological
properties of drilling mud on hole cleaning are analyzed using this mechanistic
model. The model is validated by available experimental data. Computer
simulation indicates that cuttings bed thickness is very sensitive to the
liquid phase flow rate. It dominates cuttings transport efficiency. However,
during underbalanced drilling, increase of liquid phase fraction may not always
be feasible while trying to keep a low equivalent circulating density (ECD).
Meanwhile, injection of gas has positive effects on cuttings transportation
depending on the flow patterns and drilling mud viscosity. Elevated temperature
causes a significant increase of bed thickness, and it is important to
recognize this negative effect, especially when drilling
high-pressure/high-temperature (HPHT) wells. The effect of pressure on cuttings
concentration is negative. Larger size and heavier cuttings make hole cleaning
more difficult and require higher pump rates for low-viscosity fluids.
Increases of liquid phase density result in better hole cleaning. The range of
hole angles from approximately 35° to 60° (from vertical) is the most difficult
for cuttings transport. Frictional pressure losses in a deviated wellbore
highly depend on cutting bed thickness. Simulation results are compared to
available experiment data and show good agreement. In summary, this paper
presents a model that is useful for practical hole cleaning during
underbalanced drilling (UBD).
Introduction
In drilling operation, a cuttings bed normally forms in horizontal or
inclined sections when annular mud flow rate cannot prevent the cuttings from
depositing. As bed thickness grows, mean annular velocity and wall shear stress
increase until an equilibrium condition is reached. Further increase of flow
rate erodes the cuttings bed and creates a new equilibrium. Previous study on
cuttings transport with aerated mud in a horizontal wellbore at simulated
downhole conditions is published in SPE90038. In that paper, both experimental
and modeling studies were presented. Experiments were conducted in a unique,
full-scale flow loop at simulated downhole conditions. A mechanistic model with
aerated Newtonian fluids was developed for predicting cuttings concentration in
the annulus. Only very few experimental studies of two-phase non-Newtonian flow
are reported in the literature. Practical prediction methods are also fairly
limited. Eisenberg and Weinberger (1979) derived a model for annular
gas/non-Newtonian flow. A predictive model for stratified gas/non-Newtonian
flow in a horizontal pipe has been given by Heywood and Charles (1979) and
partially validated by Bishop and Deshphande (1983). Shu (1981) presented
models for power-law fluids for several flow regimes. Dziubinski (1986)
correlated a large amount of pressure drop for power-law fluid flow in a
horizontal intermittent flow. In the study done by Kaminsky, he assumed the
pressure drop of two-phase flow is dominated by the liquid contribution, which
is the case expect for very low liquid holdup systems. Larsen (1990) conducted
an experimental study to determine the critical flow velocity for cuttings
transport in an inclined wellbore for single-phase drilling fluids. The effect
of hole angle, mud rheology, annular velocity, cuttings size, mud weight, pipe
eccentricity, and drillpipe rotation were investigated experimentally. Sharma
et al. (2000) presented a steady-state model to study gas-liquid-solid mixtures
flow in conduits. The liquid phase can be comprised of a mixture of Newtonian
and non-Newtonian fluids. Gas can exist in a free state as well as dissolved in
the liquid phase. The solid phase is assumed to be fully suspended in the
multiphase flow mixture. Sunthankar (2002) conducted aerated non- Newtonian
flow experiments in a full-scale flow loop. Water and CMC were used as the
liquid phase and air as the gas phase. Flow patterns and pressure drop
predictions were made and compared with his experimental results. Mendez (2002)
did an experimental study of cuttings transport in horizontal wells with
aerated fluids and drillpipe rotation. Polymeric fluid and water were used as
the liquid phase and air as the gas phase in the test. Cuttings bed height in a
horizontal section was recorded visually and a semiempirical model was
presented.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
22 December 2005
- Meeting paper published:
21 February 2006
- Revised manuscript received:
28 March 2008
- Manuscript approved:
10 April 2008
- Version of record:
15 September 2008
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