Summary
Displacing drilling mud with clear solids-free completion brine is a
critical step during well completion. As we move into deeper waters and drill
to deeper depths (greater than 25,000-feet MD), conventional methods and
cleaning fluids become a limiting factor in this phase of the operation.
Conventional cleaning fluids use fresh water or seawater treated with
surfactants to remove wellbore solids and water-wet tubulars. Using low-density
cleaning fluids creates a negative differential pressure between the working
kill weight fluid and the formation, casing, and cement liners. In many
situations, the negative differential pressure cannot be tolerated, and the
risk of failure at the liner top, etc., is increased—especially, if the
wellbore has not been pressure-integrity tested. Additionally, with increasing
rig/spread costs, high pump rates are necessary to decrease the time it takes
to perform these operations. The pump rate is indirectly proportional to the
pump pressures required. Weighted spacers decrease the overall pressure
differential, which allows for higher pumping rates.
To overcome the density limitation of these cleaning fluids, conventional
techniques, such as additional hydraulic horsepower, backpressure schedules,
the addition of solids to lighter cleaning fluids (e.g., water, seawater), or
balancing the weight of the low-density cleaning fluid with a matching
higher-density fluid is used. However, each of these “fixes” has inherent
limitations and is accompanied with reduced cleanup efficiency. Furthermore,
conventional surfactants are not active or effective in high-density brines.
New brine-compatible surfactant chemistry and the corresponding
balanced-displacement engineering design were developed to overcome limitation
of conventional displacement technology.
This paper describes the field applications of new brine-based,
high-density, solids-free cleaning fluids in balanced-displacements in
deepwater and offshore shelf wells. The new high-density fluids were based on
new surfactant technology developed to ensure effective wellbore cleaning,
wellbore design parameters, and displacement modeling. In addition, weighted
spacers aid in reducing high pump pressures and wellbore pressure
differentials. In one case history, a maximum pumping pressure of more than
9,000 psi was expected for conventional water-based displacement but was
reduced to a little more than 3,000 psi with the new design. High-density
cleaning fluids, with densities up to and greater than 17.5 ppg, have been
formulated and used successfully without compromising cleanup efficiency and
significantly reducing differential pressures. Results from laboratory
development and field applications are presented.
Introduction
Completing today’s challenging wells demands the utmost care and attention
to the process of displacing drilling mud or drill-in fluid with clear
completion brine. Failure to properly complete the displacement process can
lead to significant complications in subsequent completion and tool operations,
contribute to increased formation damage, and increase the cost of the
completion.
Krause (1986) presented an overview of important requirements for successful
displacements that included the predisplacement mechanical and chemical
conditioning of the mud; mechanical scraping of the casing, spacers, chemical
washes, pipe rotation, and reciprocation during displacement; and various
displacement scenarios. In addition to mechanical scrapers, other tools (Saasen
et al. 2004) currently in use include brushes, magnetic subs, circulation subs,
and junk baskets, in one form or another. All of these tools are a necessary
part of a current displacement toolkit.
Displacement success is complicated by the fact that pipe in the wellbore is
not concentric (Dutra et al. 2005; Frigaard and Pelipenko 2003), but exhibits a
wide range of eccentricities. Pipe movement, reciprocation, and rotation are
key remedies that are always desired during mud conditioning—especially in
highly deviated wells—but may not be possible for certain displacements, such
as deepwater displacements involving a large riser section or for highly
deviated wellbores. Without being able to move pipe under these circumstances,
an extra strain is placed on the capability and efficiency of chemical wash
spacers, which are then required to have enhanced cleaning activity toward the
target mud. Pumping the displacement spacer system in turbulent flow (Dutra et
al. 2005; Brand et al. 2001) throughout the displacement process contributes to
improved spacer system performance.
Another more recent factor in determining displacement success involves the
base oil and chemical makeup of current-day synthetic oil-based muds (SOBM)
(Berry 2005). To comply with environmental constraints and the demands of
deepwater and high-temperature high-pressure (HT-HP) wells, new emulsifier and
viscosifier packages were developed to stabilize these SOBM. In effect, these
enhanced SOBM are significantly more difficult to break and displace from the
wellbore than are traditional muds. As a result, new chemistries for chemical
wash spacers have been developed and implemented in the field (Berry 2005; Berg
et al. 2002).
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
14 April 2006
- Meeting paper published:
21 February 2006
- Revised manuscript received:
29 June 2007
- Manuscript approved:
17 August 2007
- Version of record:
20 March 2008
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