best practice in understanding and managing lost circulation
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Best Practice in Understanding andManaging Lost Circulation Challenges
Hong (Max) Wang, SPE, Ronald Sweatman, SPE, Bob Engelman, SPE, Halliburton; Wolfgang Deeg (formerlyHalliburton), SPE; Don Whitfill, SPE, Mohamed Soliman, SPE, Halliburton; and Brian F. Towler, SPE, University of Wyoming
SummaryLost circulation has been one of the major challenges that causemuch nonproductive rig time each year. With recent advances,curing lost circulation has migrated from plugging a hole toborehole strengthening that involves more rock mechanics andengineering. These advances have improved the industrys under-standing of mechanisms that can eventually be translated into bet-ter solutions and higher success rates. This paper provides a reviewof the current status of the approaches and a further understandingon some controversial points.
There are two general approaches to lost circulation solutions:proactive and corrective, based on whether lost circulation hasoccurred or not at the time of the application. This paper providesa review of both approaches and discusses the pros and cons re-lated to different methodsfrom an understanding of rock me-chanics and operational challenges.
IntroductionLost circulation (LC) is defined as the loss of whole mud (e.g.,solids and liquids) into the formation (Messenger 1981). There aretwo distinguishable categories of losses derived from its leakoffflowpath: Natural and Artificial. Natural lost circulation occurswhen drilling operations penetrate formations with large pores,vugs, leaky faults, natural fractures, etc. Artificial lost circulationoccurs when pressure exerted at the wellbore exceeds the maxi-mum the wellbore can contain. In this case, hydraulic fractures aregenerally created.
During the last century, lost circulation presented great chal-lenges to the petroleum industry, causing significant expenditureof cash and time in fighting the problem. Trouble costs have con-tinued into this century for mud losses, wasted rig time, and inef-fective remediation materials and techniques. In worst cases, theselosses can also include costs for lost holes, sidetracks, bypassedreserves, abandoned wells, relief wells, and lost petroleum re-serves. The risk of drilling wells in areas known to contain theseproblematic formations is a key factor in decisions to approve orcancel exploration and development projects.
Background literature (Messenger 1981) on the subject de-scribes many methods and materials used to remedy lost circula-tion. Many of these methods worked in some wells but not inothers. Trial and error applications almost always resulted in acostly learning curve.
A field practices study (API 1991) of cementing wells, pub-lished by the American Petroleum Institute (API) in 1991, com-piled drilling and production surveys and trade journal data for 339fields worldwide between 1980 and 1989. The number of fields ineach area is presented for general information and may not repre-sent all wells or fields in that specific area. The North Americanfields include fields in Canada, Mexico, and the USA. Listedamong the many types of data sourced in this study is LC infor-mation in relevant fields. This LC data was analyzed for this paperto obtain the LC event frequencies of occurrence presented inTable 1. The LC data analysis indicates that up to 45% of all wells
in the 339 fields require intermediate casing or drilling liner stringsto isolate LC zones and prevent LC while drilling deeper to totaldepth (TD). Even after using these extra pipe strings, LC eventsstill occurred in 18 to 26% of all the hole sections drilled inrelevant fields. Some fields had higher occurrences of LC eventsranging from 40 to 80% of wells. In recent years, these percentageslikely increased as the number of shallow, easy-to-find reservoirssteadily declined and industry operators intensified their search fordeeper reservoirs and drilled through depleted or partially depletedformations. Conventional lost-circulation materials (LCM), in-cluding pills, squeezes, pretreatments, and drilling procedures of-ten reach their limit in effectiveness and become unsuccessful inthe deeper hole conditions where some formations are depleted,structurally weak, or naturally fractured and faulted.
To address these issues, new LC solutions and concepts, suchas borehole strengthening or wellbore pressure containment(WPC), evolved (Alberty and Mclean 2004; Aziz et al. 1994; Fuhet al. 1992). The mechanisms behind various means proposed andused to enhance WPC are still debated and are not fully under-stood. Proposed mechanisms include sealing incipient fractures atthe wellbore wall; propping open multiple short fractures at thewellbore wall, thus increasing compressive stresses around thewellbore; and sealing fractures with various materials using a hesi-tation-squeeze technique.
Based on the ongoing debate of these emerging new tech-nologies for controlling lost circulation, this paper intends toprovide a comprehensive review and analysis for a better under-standing of both proactive and corrective borehole strengthen-ing technologies.
Proactive Borehole StrengtheningSuccess and Issues. Muds have been pretreated with particulateshaving a broad size-distribution spectrum for years, yielding someclear benefits (Ali et al. 1991; Fuh et al. 1992; Aston et al. 2004).Based on systematic lab studies, this approach was originally as-sumed to work by tip screenout, isolating the fracture tip fromthe wellbore pressure, thus stopping fracture propagation (Fuhet al. 1992). The pressure containment improvement realized bythis approach depends strongly on the actual fracture length anddecreases rapidly with increasing fracture length (Deeg and Wang2004). To help improve the pressure containment using this ap-proach, the fracture should be bridged or sealed as quickly aspossible before it has a chance to extend a significant distance intothe formation.
Recent improvements in this technology, which include use ofparticulate-treated mud as weak zones are penetrated, have shownsignificant success in substantially increasing WPC (Alberty andMclean 2004; Aston et al. 2004). These successes are supported bystrong evidence from pre- and post-treatment pressure tests. Be-cause of their capability to strengthen during drilling, the use ofthese special muds offers an excellent approach for drilling de-pleted formations and has achieved substantial success in the field.
Its theory, often referred to as stress caging, states that theborehole is strengthened by creating microfractures, then pluggingand propping them open with particulates, increasing the hoopstress. The size distribution of the particulates to be added to themud is determined by using the basic hydraulic-fracturing theoryand an assumed fixed-fracture length of 6 inches (in.).
The theory that explains this mechanism is not totally accepted,because finite element fracture simulations show (Abousleimen
Copyright 2008 Society of Petroleum Engineers
This paper (SPE 95895) was first presented at the 2005 SPE Annual Technical Conferenceand Exhibition, Dallas, 912 October, and revised for publication. Original manuscript re-ceived for review 6 October 2006. Revised manuscript received 11 September 2007. Paperpeer approved 10 November 2007.
168 June 2008 SPE Drilling & Completion
et al. 2005) that a stable microfracture that could be plugged byparticulates in the described manner (Alberty et al. 2004) isnot present. Further, it was found that two published borehole-strengthening data points are still within the Kirsch hoop-stress range for an impermeable circular wellbore-boundarycondition (Abousleimen et al. 2005). The Kirsch hoop-stress equa-tion defines the upper bound of fracture-initiation pressure for aperfectly circular wellbore in impermeable rock. It is thereforepossible that the special mud actually satisfied the boundary con-dition of impermeability, sealing pore throats, and keeping fluidfrom leaking off into the formation. The assumed 6-in. fracturelength also lacks support and could easily be exceeded duringfracture initiation.
Sealing Short Fractures. It is widely accepted that the formationbreakdown pressure can be much greater if the wellbore can betreated as an impermeable boundary in depleted formations (Gid-ley et al. 1989). Todays drilling fluids used for drilling depletedformations frequently provide good fluid-loss control, but we havenot seen a particular conventional drill fluid that alone can prevent LC.
Although plugging rock-matrix pore throats can create a nec-essary condition, treating short fractures may also be important. Intectonically active areas, Sh can be much smaller than SH. Undercertain conditions, fractures can initiate regardless of wellborepressure. Joints created by tectonic activities may be open andready to take fluids.
Depletion can also result in fractures and faults. When a res-ervoir is depleted, the pore pressure is decreased and the effectivestress increases accordingly. Depletion can cause subsidence inhigh-porosity, weak formations. It is possible that damage to therock matrix resulting from compaction could lead to the creation offractures throughout the formation. When these fractures are openand can conduct fluid, any wellbore pressure in excess of the leastprincipal stress within the formation can likely cause these frac-tures to extend, resulting in LC events. The presence of thesefractures, in effect, negates the hoop-stress concentration at theborehole wall required for pressure containment. This has beenconfirmed by Onyia (1994), who noted in the laboratory that whennotched or prefractured, wellbores have breakdown pressuressubstantially lower than predicted for intact, unfractured well-bores. Similar results, especially with oil-based mud, have beenobserved by Morita et al. (1996), who explained that