Foam has proved to be effective and economical in underbalanced operations
(UBO) and is gaining wider applications in many areas. It provides the desired
flexibility in controlling pressure profile and equivalent circulating density
(ECD). However, the knowledge of rheology and hydraulics of polymer-thickened
foams is still limited. This paper summarizes the significant effects of
polymer on foam rheology and presents a hydraulic model that simulates aqueous
and polymer-based foam flow in directional and horizontal wellbores.
Experimental studies on the rheology of polymer-enhanced foam were conducted
using a specially designed flow-through rotational viscometer and pipe
viscometers with different concentrations of hydroxyethylcellulose (HEC)
polymer. Correlations have been developed for rheological parameters of
aqueous- and polymer-based drilling foams.
On the basis of the experimental results of foam rheology and a steady-state
momentum balance equation, a foam-flow hydraulics model was developed to
predict pressure profile, ECD, foam velocity, and foam quality along a
vertical/inclined/horizontal wellbore. For practical applications, a simulator
has been developed and validated by experimental flow-loop data obtained from
the Advanced Cuttings Transport Facility of Tulsa University Drilling Research
Project. The effects of polymer concentration, backpressure, and wellbore
trajectory on foam hydraulics were studied extensively using the simulator.
Results show significant impact of polymer on foam hydraulics. When 0.5% volume
to volume (v/v) HEC polymer is added to aqueous foam, bottomhole pressure (BHP)
and foam density are significantly increased, while foam quality and velocity
are greatly decreased. The polymer effects are more pronounced in vertical
wells than in horizontal wells.
Simulation results also indicate that it is possible to use foam to create a
pressure profile within the narrow window between continuously changing
pore-pressure and fracture pressure gradients, which is not possible with
conventional fluids. Those responsible for hydraulic optimization and well
control in managed-pressure drilling/UBO where foam is used will find this
paper useful for practical design applications.
During foam- drilling operations, predicting such parameters as BHP, foam
flow velocity, foam density, and foam quality is a major challenge. Unlike
incompressible drilling fluids, foam is a compressible, high-viscosity,
non-Newtonian fluid. Temperature, pressure, foam quality, foam density, flow
velocity, and rheological parameters vary along the wellbore; in addition,
frictional pressure gradient, hydrostatic pressure gradient, and acceleration
pressure gradient are coupled. This becomes more complex when polymer is added
to the liquid phase (Chen 2005; Chen et al. 2007; Chen et al. 2005).
A comprehensive computer program has been developed to better understand the
difference between compressible foam flow and incompressible fluid flow, to
predict the BHP, to study the effect of polymer on foam flow hydraulics, and to
optimize controllable variables during foam drilling. The model incorporates
both aqueous and polymer-based foam rheological parameters that were obtained
using pipe viscometers with different HEC polymer concentrations. Because
polymers have been used in different underbalanced foam drilling operations,
this model will be useful for both aqueous-based and polymer-thickened foam
drilling design and operations.
The main objectives of this study include: (a) measuring the rheological
properties of foam with and without polymer, (b) creating a mathematical model
for simulating foam flow in the wellbore, (c) generating simulation results and
comparisons between foam flow and incompressible-fluid flow, and (d)
quantifying the effect of polymer on foam hydraulics in vertical, directional,
and horizontal wellbores.
© 2009. Society of Petroleum Engineers
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- Original manuscript received:
17 November 2006
- Meeting paper published:
20 February 2007
- Manuscript approved:
7 May 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009