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.
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.