Summary
Wellbore strengthening or stress cage implementation has been recognized as
an effective means of dealing with lost circulation during drilling operations.
One of the mechanisms developed for strengthening a wellbore has been to prop
induced and existing fractures with particulate lost circulation materials
(LCM) to effectively increase hoop stress in the near wellbore region.
However, a good understanding of this mechanism is necessary in order to
avoid a potentially flawed design and implementation process which could
adversely affect job success in the field. One of the main issues is fracture
stability under the strengthening conditions. This paper will describe those
factors which are important in designing wellbore strengthening jobs and
address the conditions necessary to help ensure fracture stability, as
determined through rock mechanics analysis.
The strengthening of a wellbore by propping fractures has been discussed in
a previous investigation (Wang et al. 2007a). In this paper, continuing the
previous discussion, various parameters that affect the strengthening of the
wellbore are addressed in detail. In-depth discussion of how each of those
parameters affects the process of wellbore strengthening is presented. This
study was accomplished using a boundary element numerical simulator to
facilitate the analyses.
Introduction
With the continued growth of drilling activities and the cost of performing
them, loss of productive time is more important than ever. Recently published
survey data for Gulf of Mexico (GOM) wells with water depths less than 600 feet
indicate that lost circulation contributes over 12% for non-productive time
(NPT) (Dodson et al. 2004). If taken and applied on a worldwide basis, this
translates into billions of dollars of extra cost each year.
For years the use of LCM has been the preferred way for treating
lost-circulation problems (Messenger 1981). However, the lost circulation
problem was not totally resolved. Recent studies have shed much light on its
cause and potential solutions (Deeg and Wang 2004; Wang et al. 2008; Wang et
al. 2007b; Morita et al. 1990; Morita et al. 1996a; Morita et al. 1996b; Onyia
1991). Among many potential solutions, the stress cage concept is of particular
interest, because it has already shown promise in addressing the problem (Fuh
et al. 1992; Dupriest 2005; Aston et al. 2004; Alberty and McLean 2004;
Sweatman et al. 1997; Whitfill et al. 2007; Witfill et al. 2006).
The stress cage concept is an approach developed to enhance wellbore
pressure containment (WPC). It has been found that a weak wellbore can contain
much higher pressure if the wellbore fluid is treated with particulates (Dudley
at al. 2001). Some of the literature referenced above provides support to this
approach.
Understanding the mechanism of stress caging is recommended to design the
treatments for specific field application and to advance the development of the
technology for application across a wider mud weight window and with a higher
success rate. A series of studies has been performed, and some of the results
have been published (Wang et al. 2007a; Wang et al. 2008; Wang et al.
2007b).
The previous studies have involved a series of investigations and reasoning
to evaluate and determine wellbore weakening factors. Hydraulically conductive
wellbore cracks have been identified as the most important factor that lowers
the WPC to its minimum, which is defined by the least principal stress. An
interesting reference is the ideal WPC, which is defined by the Kirsch hoop
stress equation for a perfectly circular wellbore. Though WPC is defined as the
lower of this near wellbore pressure containment and the far field least
principal stress, for wellbore strengthening, the near wellbore pressure
containment appears to be the key. The studies have shown that there are at
least two ways to strengthen a fractured wellbore: sealing the fracture and
propping the fracture. Though sealing alone can strengthen the wellbore,
propping the fracture can improve the WPC beyond its theoretical or ideal
value. Sealing is still a necessary component for strengthening of the wellbore
by propping the fracture. Table 1 is a summary of the WPC weakening and
strengthening mechanisms previously presented (Wang et al. 2007b). In this
paper, we focus the discussion on wellbore strengthening through propping of
the fractures.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
14 December 2007
- Meeting paper published:
4 March 2008
- Revised manuscript received:
26 August 2008
- Manuscript approved:
11 September 2008
- Published online:
23 July 2009
- Version of record:
28 September 2009
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