Summary
Solids cleanouts using coiled tubing (CT) remain a major part of total
activity in the CT industry. Because of the multitude of parameters that
influence solids transport, it can be very challenging to design and execute
solids cleanouts successfully with CT in highly deviated, larger wellbores with
7-in. production tubing, or even larger tubulars, installed.
Numerous papers have been written about the development of wiper-trip
cleanout technology and associated engineering design tools, but this paper is
focused instead on important practical issues that directly impact the
effective implementation of wiper-trip technology in the field. This paper
presents the results and lessons learned based on a database that was compiled
from more than 100 solids cleanout operations using wiper-trip methodologies.
Results will be presented showing how the wiper trip cleanout methodology has
improved cleanout efficiency and success rate. Examples are presented showing
how the effectiveness of cleanout bottomhole assemblies (BHAs) involving
positive-displacement motors (PDMs) and mills has been improved, while
simultaneously reducing stress on surface equipment during the operation.
Circulation rates higher than specified maximum rates for the PDM are being
used without danger of damaging the PDM, while reducing the total volume pumped
through the PDM during the cleanout by 80-90%. Larger outer diameter (OD) items
in the BHA are kept clean of solids while wiper tripping, reducing the risk of
stuck CT and protecting sensitive completion components from undesirable
interactions with the PDM/mill BHA. Multiple wiper trips can be performed in
one run without the use of drop balls, while having the ability to use selected
functions of critical BHA components and full-size drifting of the wellbore for
subsequent operations. Field-proven procedures are explained, allowing solids
loading in the annulus to be controlled and reduced when necessary, and
allowing estimates of solids volumes during the cleanout to be established, on
the basis of feedback from the cleanout BHA before any solids have actually
reached surface.
Background
It is well known that many CT operations start with a cleanout before being
able to conduct other work in the wellbore. A review of all CT operations
performed in the Norwegian sector of the North Sea since 2001 shows that 74% of
the CT operations involve cleanouts of solids from the wellbore. This is reason
enough to concentrate on performing repeatedly successful cleanouts, so that
the process is economical and allows subsequent operations in the wellbore to
continue as planned.
Solids transport is affected by many variables, and the complexity of the
phenomena presents challenges to the field engineer who is trying to determine
how the parameters affect solids transport even as one, or more than one, of
the variables is changing during an operation. Most of the previous
solids-transport studies in the oil industry focused mainly on finding the
minimum critical velocity in the wellbore annulus for conventional rotary
drilling with mud fluids. The studies lack information related to the
prediction of the equilibrium solids-bed height during tripping in, the
wiper-trip speed during tripping out, and the prediction of the hole-cleaning
time. In field operations, people often use outdated "rules of thumb"
(i.e., 2-hole-volume circulation to clean the well, annular fluid velocity two
times the solids-settling velocity, or performing cleanout stages of a certain
length).
In our previous studies (Li and Walker 2001; Walker and Li 2001, 1991; Li et
al. 2002; Li et al. 2005; Li and Wilde 2005), a comprehensive experimental test
of solids transport for both the stationary circulation and the wiper trip was
conducted. The effect of multiphase flow, rate of penetration (ROP), deviation
angle, circulation fluid properties, particle density and size, fluid rheology,
pipe eccentricity, wiper-trip speed and nozzle type on solids transport was
investigated.
On the basis of comprehensive research (Li and Walker 2001; Walker and Li
2000, 2001; Li et al. 2002; Li et al. 2005; Li and Wilde 2005), an effective
solids-cleanout methodology/process using CT (see Fig. 1) has been developed,
patented (Walker et al. 2005), and proved by field operations (Engel and Rae
2002; Ovesen et al. 2003; Hobbs and Liles 2002; Gilmore et al. 2005;
Nasr-El-Din et al. 2006; Li et al. 2006). The developed solids-cleanout
methodology/process includes a specialized downhole cleanout tool and a
solids-transport simulator for CT in vertical, deviated, and horizontal well
conditions. Empirical formulas are applied to predict surface and downhole
pressures, fluid velocities, and solids transport effectiveness. The simulator
is a powerful analytical tool that can characterize wellbore hydraulics and
solids transport considering downhole conditions, especially when applying the
concept of removing solids from wellbores by use of wiper tripping. Its use has
resulted in better-designed and -performed cleanouts (Engel and Rae 2002;
Ovesen et al. 2003; Hobbs and Liles 2002; Gilmore et al. 2005; Nasr-El-Din et
al. 2006; Li et al. 2006).
The specialized downhole cleanout tool offers the option of using
downhole-facing, high-energy jetting nozzles or a PDM, in order to ensure
sufficient penetration energy required for harder solids depositions. Having
penetrated the targeted solids, the specialized downhole cleanout tool allows
the fluid pumped during the cleanout to be redirected to uphole-facing,
low-energy nozzles, simultaneously stopping the fluid stream through the
jetting nozzle or PDM. A surface indication of the
specialized-downhole-cleanout tool position is provided for the CT
operator.
© 2008. Society of Petroleum Engineers
View full textPDF
(
2,580 KB
)
History
- Original manuscript received:
26 January 2007
- Meeting paper published:
20 March 2007
- Revised manuscript received:
23 November 2007
- Manuscript approved:
12 December 2007
- Version of record:
15 August 2008
View Cart
