Summary
Many of the problems associated with the use of water-based fluids in
drilling and completion operations are caused by incompatibilities between the
fluids and the shales. Such incompatibilities may result in washouts, increased
drilling costs (e.g., solids handling, rig time, dilution fluids), and shale
sloughing during the drilling operation and after displacements to completion
fluids or during gravel packing. One of the most important factors leading to
an undesired result (either a premature screenout, thus a potential
sand-control failure, or a higher skin) in water-packing of open holes is the
presence of reactive shales in the interval to be gravel packed.
Although there is a substantial amount of literature on shale inhibition with
water-based drilling fluids, the importance of shale inhibition and the
problems associated with shale reactivity during gravel packing remain largely
unexplored. Furthermore, shale-inhibitor selection often relies on a comparison
of the results from bottle-roll tests using shale samples in candidate
fluid/inhibitor pairs (drilling or completion fluid) and on tests measuring the
degree of shale swelling. While these tests are highly functional, they can
provide information only on the relative performance of fluids, and their
relevance to gravel packing is questionable because these tests do not simulate
the conditions experienced during such treatments.
This paper presents guidelines on selection methodology of shale inhibitors
for use in gravel-packing applications on the basis of the data available in
our respective companies, including a comparison of results from conventional
bottle-roll tests to those from flow through predrilled holes in shale core
samples. Recommendations are made depending on brine type and density, type of
shale, temperature, fluid exposure history, and environmental
considerations.
Introduction
Openhole-horizontal completions have emerged as a cost-effective means of
exploiting deepwater reservoirs, many of which require sand control. Gravel
packing is the preferred sand-control technique for such environments where
remedial treatment costs are prohibitively high (Price-Smith et al. 2003). Two
techniques have been employed for gravel packing open holes with varying
degrees of success: alternative path and water packing. The focus of this paper
will be to address one of the problems considered to be a key risk factor in
successful implementation of water-pack treatments.
The risks associated with openhole water packing completions can be
summarized as
- Swabbing, which has been addressed through the development of antiswab-tool
systems (Vozniak et al. 2001)
- Exceeding fracturing pressure
--during the beta-wave, which has been addressed with the development of beta
wave attenuators (Coronado and Corbett 2001) or use of low-density gravel,
allowing lower pump rates without the concern for gravel settling in the work
string (Pedroso et al. 2005)
--during the alpha wave in environments with narrow-frac window, that in some
cases may be addressed through the use of low-density gravel (Pedroso et al.
2005)
- Filter-cake erosion, (the conditions under which this becomes a risk remain
to be determined) (Gilchrist et al. 1998)
- Reactive shales that may either collapse/slough or disperse in the carrier
fluid; the former may lead to a premature screenout because of blockage of the
annulus, and the latter may result in a low-permeability gravel pack because of
shale and gravel intermixing (Gilchrist et al. 1998; Corbett and Winton 2002;
Mathis et al. 2000; Murray et al. 2003)
Shales are characterized by high clay content, low quartz content, and low
permeability (a byproduct of the small-clay size). On the basis of numerous
factors, shale can react catastrophically when exposed to some aqueous fluids.
These factors include downhole-stress states, native-fluid composition,
mineralogical composition, and interaction with the completion-fluid chemistry
and properties. It is important to note that these factors also determine the
time a shale will take to fail when exposed to a given completion fluid, and
hence, a shale that has survived the drilling process may still fail during the
post drilling activities leading to the gravel pack (Dickerson et al.
2003).
It is possible to minimize and even eliminate this adverse reaction by
selecting a suitable completion fluid. This selection may involve choosing the
correct brine type and additives to increase the inhibitive qualities of the
completion fluid.
The literature on the subject of shale compatibility with muds is vast. The
reactivity of shales to aqueous muds with various additives has been well
studied (Chenevert 1970; O’Brien and Chenevert 1973; van Oort 1997). However,
the effect of completion fluids has not been studied extensively. The purpose
of ensuring proper shale inhibition with drilling mud is to address shale
reactivity concerns such as cuttings disintegration, wellbore instability
during drilling, and bit balling (van Oort 1997). On the other hand, a
completion fluid must be formulated to inhibit shale to maintain wellbore
stability after drilling (e.g., during mud displacements or gravel packing) in
reactive-shale sections (Gilchrist et al. 1998; Mathis et al. 2000) and to
prevent erosion of weakened shales during gravel packing (Ali et al. 1999).
Various testing techniques have been proposed in the literature to
characterize the inhibitive properties of drilling fluids (Roehl and Hackett
1982; RP 131 2004; Bailey et al. 1994; Mondshine 1973). Because these tests
were designed specifically for drilling applications, their direct
applicability to water packing, subsequent to water-based drilling, is
questionable. Of these testing techniques, the wellbore-simulator tests first
described by Darley (1969) and further developed by Bailey (1994) and Gaylord
(1983) are more useful for evaluating inhibitor effectiveness in gravel-pack
applications, as is also suggested by Corbett and Winton (2002).
By exposing various fluids to boreholes drilled in shale cores, Darley (1969)
showed the different modes of failure and correlated them to the effect of
tectonic stresses, mineral content, age of shales, and flow of mud through the
shale borehole. Gaylord developed this testing to look further at the effect of
fluid-mechanical parameters on borehole erosion and concluded that erosion will
be most pronounced if the particular shale/fluid system is reactive. In
addition, hole erosion increases with increasing shear stress and is
exacerbated under turbulent conditions. The tests done by Bailey (Price-Smith
et al. 2003) look only at the effects of reactivity by shale but corroborate
the mechanism of weakening of the shale and subsequent erosion by flow. This is
evident in their tests through increased wellbore diameter resulting from
erosion. On the basis of this information, a similar test will be used in this
work to evaluate completion-brine inhibition.
It is the objective of this paper to provide guidelines on selection
methodology of shale inhibitors for use in gravel-packing applications. The
paper is organized as follows. First, a brief description of the typical
critical stages in a gravel-packed completion is given. This is followed by a
discussion of the current testing methodology typically employed in the
industry. Next, the experimental techniques and materials used in this study
are presented, followed by the results from hot-roll and drilled-core
experiments. Finally, conclusions are drawn.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
28 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
29 June 2007
- Manuscript approved:
10 November 2007
- Version of record:
20 June 2008
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