Summary
Lost circulation has been one of the major challenges that cause much
nonproductive rig time each year. With recent advances, curing lost circulation
has migrated from "plugging a hole" to "borehole strengthening"
that involves more rock mechanics and engineering. These advances have improved
the industry’s understanding of mechanisms that can eventually be translated
into better solutions and higher success rates. This paper provides a review of
the current status of the approaches and a further understanding on some
controversial points.
There are two general approaches to lost circulation solutions: proactive
and corrective, based on whether lost circulation has occurred or not at the
time of the application. This paper provides a review of both approaches and
discusses the pros and cons related to different methods—from an understanding
of rock mechanics and operational challenges.
Introduction
Lost circulation (LC) is defined as the loss of whole mud (e.g., solids and
liquids) into the formation (Messenger 1981). There are two distinguishable
categories of losses derived from its leakoff flowpath: Natural and Artificial.
Natural lost circulation occurs when drilling operations penetrate formations
with large pores, vugs, leaky faults, natural fractures, etc. Artificial lost
circulation occurs when pressure exerted at the wellbore exceeds the maximum
the wellbore can contain. In this case, hydraulic fractures are generally
created.
During the last century, lost circulation presented great challenges to the
petroleum industry, causing significant expenditure of cash and time in
fighting the problem. Trouble costs have continued into this century for mud
losses, wasted rig time, and ineffective remediation materials and techniques.
In worst cases, these losses can also include costs for lost holes, sidetracks,
bypassed reserves, abandoned wells, relief wells, and lost petroleum reserves.
The risk of drilling wells in areas known to contain these problematic
formations is a key factor in decisions to approve or cancel exploration and
development projects.
Background literature (Messenger 1981) on the subject describes many methods
and materials used to remedy lost circulation. Many of these methods worked in
some wells but not in others. Trial and error applications almost always
resulted in a costly learning curve.
A field practices study (API 1991) of cementing wells, published by the
American Petroleum Institute (API) in 1991, compiled drilling and production
surveys and trade journal data for 339 fields worldwide between 1980 and 1989.
The number of fields in each area is presented for general information and may
not represent all wells or fields in that specific area. The North American
fields include fields in Canada, Mexico, and the USA. Listed among the many
types of data sourced in this study is LC information in relevant fields. This
LC data was analyzed for this paper to obtain the LC event frequencies of
occurrence presented in Table 1. The LC data analysis indicates that up to 45%
of all wells in the 339 fields require intermediate casing or drilling liner
strings to isolate LC zones and prevent LC while drilling deeper to total depth
(TD). Even after using these extra pipe strings, LC events still occurred in 18
to 26% of all the hole sections drilled in relevant fields. Some fields had
higher occurrences of LC events ranging from 40 to 80% of wells. In recent
years, these percentages likely increased as the number of shallow,
easy-to-find reservoirs steadily declined and industry operators intensified
their search for deeper reservoirs and drilled through depleted or partially
depleted formations. Conventional lost-circulation materials (LCM), including
pills, squeezes, pretreatments, and drilling procedures often reach their limit
in effectiveness and become unsuccessful in the deeper hole conditions where
some formations are depleted, structurally weak, or naturally fractured and
faulted.
To address these issues, new LC solutions and concepts, such as borehole
strengthening or wellbore pressure containment (WPC), evolved (Alberty and
Mclean 2004; Aziz et al. 1994; Fuh et al. 1992). The mechanisms behind various
means proposed and used to enhance WPC are still debated and are not fully
understood. Proposed mechanisms include sealing incipient fractures at the
wellbore wall; propping open multiple short fractures at the wellbore wall,
thus increasing compressive stresses around the wellbore; and sealing fractures
with various materials using a hesitation-squeeze technique.
Based on the ongoing debate of these emerging new technologies for
controlling lost circulation, this paper intends to provide a comprehensive
review and analysis for a better understanding of both proactive and corrective
borehole strengthening technologies.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
13 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
11 September 2007
- Manuscript approved:
10 November 2007
- Version of record:
20 June 2008
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