Managing Losses in Naturally Fractured Formations: Sometimes Nano is too Small

Abstract Controlling severe to total loss situations in naturally fractured and unconsolidated formations can be critical and, most of the time, these situations make it difficult to continue drilling. Conventional lost circulation materials (LCMs), such as ground marble, graphite, and nutshells might not work. In such situations, the operator generally resorts to using cement plugs or mud-cap drilling. This paper presents novel applications in the form of engineered composite LCM solutions (ECS) successfully used to manage severe to total loss situations in naturally fractured carbonate formations. Case studies are presented from applications in North America, the Middle East, and East Africa. Engineered multimodal (MM) LCMs, which are composite LCMs with MM particle-size distribution (PSD), were developed as remedial solutions for handling severe to total losses. The MM PSD is characterized by a PSD curve with two or more concentration peaks, which help better manage uncertainty in fracture sizes. The particulate ECS was extensively tested in the laboratory for its effectiveness in terms of plugging large slots. For the most severe lost circulation cases, a combination of two ECS materials, one containing a swellable component, is proposed as a potential solution. While drilling a severely fractured limestone formation in North America, the combination of particulate and swellable ECS, along with chopped fibers, was applied after all conventional LCM applications failed. The combination was pumped open ended and losses were totally cured, allowing circulation to be established again. With this experience, similar solutions were included in the plan for drilling a highly fractured carbonate formation for an operator in the Middle East. Most of the formations are highly fractured carbonates and are prone to severe to total losses. The ECS combination was successfully used to mitigate complete losses and continue drilling where, in some cases, as much as 90% circulation was regained. Another application of this solution was also proposed for drilling an offshore well for an operator in East Africa. A formation that was severely fractured was to be encountered and total loss was expected. A circulating sub was recommended in the bottomhole assembly (BHA), and high concentrations of ECS were pumped. Losses were managed in most cases that were otherwise unmanageable with conventional LCMs. This enabled the operator to reach total depth (TD) and obtain necessary exploration information.