# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 4 Preventive and Corrective Lost Circulation Measures
### Corrective Measures to Mitigate Lost Circulation during Drilling  
When drilling through fractured and vuggy carbonate, a preventive approach is not always effective, and a corrective approach should be taken. This is done using both conventional lost circulation materials (LCMs) and different types of specially designed plugs based on cement, cross-linked polymers, a mixture of organic products or other materials with or without conventional LCMs as shown in Table 5[1; 3; 4; 22; 82; 83]. In order to seal natural fractures or induced fractures, lost circulation materials are added continuously to the drilling fluid or spotted as a concentrated LCM pill [4]. According to [PERSON] et al. (2019) in Volga-Urals region, many lost circulation control techniques such as a new special-purpose thixotropic cement slurry, foamed cement plugs, cement-bentonite-diesel slurries, fibrous bridging materials, variable-sized bridging agents, shredded polyethylene, detachable plastic tubulars, expandable-profile liners, felted cloth, cord fiber, etc. are currently used [84]. The total loss of circulation is almost always treated with remedial procedures and techniques. Lost circulation materials (LCM) can be added to the mud before losses occur to prevent mud loss, but they can also be added to mud in the form of a pill after losses occur to plug the loss zone. The selected LCM should go through restrictions in Bottom Hole Assembly (BHA). Coarse and extra-coarse grades of LCMs should always be spotted through a bypass circulation tool or open-ended pipe to avoid the potential plugging of downhole tools or bit nozzles. The choice of size, type, and concentration of LCM is extremely important for plugging/bridging pores and fractures, so that the material penetrates deep enough into the rocks and does not remain on the rock surface, because in this case, after re-establishing circulation, mud loss will occur again [5; 10; 46; 83; 85]. High-resiliency graphite bridging agents (median particle size (d\({}_{50}\)): 300-350 microns) provide enhanced bridging and sealing of natural or induced fractures, and can be used to control loss of fluid in partial to severe lost circulation zones. The recommended treatment range in the entire system is 29 to 70 kg/m\({}^{3}\), and for partial losses (1.6 to 16 m\({}^{3}\)/h) is 57 to 143 kg/m\({}^{3}\) in spotted pills. Highly permeable formations such as fractured carbonates and conglomerates may require additional pills in conjunction with lost circulation materials of various sizes. Granular, shear-resistant particulates (medium: 800-1500 microns, coarse: 1500-2280 microns, and extra-coarse: 2050-4300 microns) are useful for lost circulation control and wellbore strengthening when drilling ultrahigh-permeability formations or when fractures up to 4 mm are encountered. They can be used in all types of drilling fluids. The recommended treatment range for spotted pills is 29 to 57 kg/m\({}^{3}\). Typical concentrations include a range of blends (e.g., blends of graphitic material and ground nut shells, sized marble or sized marble and cellulosic material of nominal median particle size (d\({}_{50}\)): 500-600 microns) for continuous particle addition to the circulating drilling fluid when drilling a formation known to have losses is from 35 to 57 kg/m\({}^{3}\), and for remedial lost circulation treatments from 85 to 200 kg/m\({}^{3}\), depending on the severity of the losses. Blends are designed for loss zones with maximum openings of at least 1200 microns [86]. In many cases, the success or failure of a LCM largely depends on the decisions made and methods employed when using the LCM. Because of that, controlling circulation loss