Summary
Cleaning fill from wellbores is the most common coiled tubing (CT)
application. The process is a function of multiple variables including fluid
properties, flow velocities, wellbore geometry and deviation, pipe
eccentricity, particle properties, fill penetration rate and wiper trip speed.
Removing fill from wells with low bottom hole pressures (BHP) can be
challenging especially if wells are completed with smaller diameter production
tubulars which significantly reduce circulation flow cross-section and choke
flow(1). Such challenges are further compounded by high deviation or
horizontal well trajectories especially in large diameter wellbores.
A variety of cleanout methods have been developed in the past, often
incorporating high circulation rates, special fluids, wiper trips, or reverse
circulation to remove solids. Many of these conventional sand cleanout methods
often apply excess hydrostatic pressure on the formation, resulting in lost
circulation in pressure depleted reservoirs. The conventional solution to
overcome excess hydrostatic pressure has been to include nitrogen to reduce
fluid density and thus lessen the hydrostatic head; however, sand vacuuming
technology using a concentric coiled tubing (CCT) with a downhole jet pump is
an alternative technique for removing fill without placing a hydrostatic load
on the reservoir.
This paper reviews the individual sand cleanout systems and discusses the
advantages and limitations related to each method. In recent years, cleaning
sand using the wiper trip method has become the preferred technique. However,
an appropriate pump rate and reservoir pressure are needed to maintain a proper
return flow rate to carry the sands to the surface. For pressure-depleted
reservoirs completed with horizontal wells, a sand vacuuming system can be used
to efficiently remove the debris without circulating nitrogen and without high
pump rates. When the fill cannot be removed from large-diameter deviated
wellbores using conventional low-cost cleanout fluids, then fluids with high
solids suspension capability (under shear conditions) in conjunction with wiper
tripping may be an economical option. The main application for the reverse
circulation technique is cleaning sand from large diameter wellbores when the
necessary pump rates for conventional "forward" circulation are not achievable.
A venturi junk bailer is often used to retrieve larger or heavier material
which cannot be circulated out by traditional methods. Field cases are
provided, demonstrating how to select the proper cleaning method and how to
efficiently remove sand from a wellbore based on both operational and
logistical conditions.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
23 June 2009
- Meeting paper published:
9 June 2008
- Revised manuscript received:
23 April 2010
- Manuscript approved:
29 April 2010
- Published online:
4 August 2010
- Version of record:
3 August 2010
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