Abstract
Controlling lost circulation during drilling operations in a reservoir prone to fluid losses is typically remedied by cement squeezing or plug setting as the last resort. The aim being to minimize or stop drilling fluid losses and to regain full returns at surface, and to maintain wellbore integrity. Different placement methods of cement plugs have been discussed in detail in the literature, except for the ‘level-off’ method, which can be effective for curing complete loss circulation cases. Following modeling and calculations of this cement plug placement method, its design and execution procedures are discussed, together with two successful field cases in highly fractured carbonate reservoirs in the Middle East. Using drill pipe and a Retrievable-Test-Treat-Squeeze (RTTS) packer, set with some spacing from the loss zone, the method entails that the cement slurry is allowed to drop by gravity in order to cure lost circulation. As the column of fluid, mud and slurry in the well exceeds formation pore pressure, i.e., overbalanced conditions, a volume of acid-soluble cement slurry is allowed to slowly drop and freely penetrate the formation, i.e., through its fractures or caverns. During the penetration of this viscous slurry into the loss zone, the cement slurry can set and the fracture or fissure openings are plugged. Presented are detailed design calculations for the level-off placement technique, determination of required cement slurry and displacement volumes, and recommended displacement and RTTS packer setting depths. The expected depth of the top of cement plug is estimated. The design parameters are compared with field cases and explanations are given for possible discrepancies. Success of the operation is discussed in terms of final mud loss after cement plugging and Non-Productive Time mitigation. Detailed field procedures and execution are also presented. The level-off job is already practiced by the industry, but it is not published in the literature, in some cases they have different methods with causing some errors. To the best of authors’ knowledge, this is the first detailed description and stepwise calculation of the level-off cement placement technique in the literature.
Highlights
During drilling and workover operations highpressure-high-temperature (HPHT), some instances of mud loss and lost circulation may generally be encountered (Alrasheed et al 2018)
Some cement slurry is spotted above the loss zone in cased hole and with enough spacing from the packer
WhereVc,f is the final volume of cement slurry above casing shoe [bbl]ΔVc,l is the lost slurry volume to loss zone [bbl]Δhm,d is the dropped mud level in drill pipe (DP) after leveling-off /balanced condition with the formation pressure is reached [ft], andCapDP is the capacity of drill pipe [bbl/ ft]Δhm,d[measured depth (MD)] is equal to Δhm,d[TVD] because it is near the surface where the hole has no deviation
Summary
During drilling and workover operations highpressure-high-temperature (HPHT), some instances of mud loss and lost circulation may generally be encountered (Alrasheed et al 2018). In highly fractured or cavernous and depleted reservoirs, mud losses may not be controllable by standard curing methods In such cases, mud losses may increase further and lead to an almost formation-induced lost circulation or complete loss, e.g., mud losses > 100 bbl/hr. To the best of the authors’ knowledge, there are no publications (except for Ashena et al 2020) in the literature explaining on how to set a level-off cement plug, its required accurate design calculations and comparison with field cases This method has already been applied by some drilling companies in the industry such as Bikmukhametov et al (2014) which provided examples of level-off cement plugging but no details and design calculations were presented.
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