# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 5 Lost Circulation While Cementing through Naturally Fractured Reservoirs  
The risk of losses is greater in the cementation phase than in the drilling phase. Generally, the values of density and rheological properties of a cement slurry are higher than the values of these parameters for a drilling fluid while the flow area is smaller. Therefore, due to the higher equivalent circulation density (ECD) and the smaller clearance between the casing and the open hole, it can be expected that the dimensions of the existing fractures will increase during cementation, which will lead to the loss of the cement slurry. Extreme circulation losses during cementing operations pose a major threat to successful well cementation.  
The loss of circulation during primary cementing results in reduced annular space coverage, casing corrosion, reduced safety, poor zonal isolation, and often requires the application of some remedial cementing techniques.  
To prevent the loss of a cement slurry, mud loss zones should be sealed by known methods before cementation. If this is not possible, it is recommended to: (1) maintain the pressure during cementation below the maximum equivalent mud density in the circulation (ECD) and (2) place the LCM in the loss zone. Maintaining pressure during cementation below the maximum equivalent mud density in the circulation (ECD) can be achieved by decreasing the cement slurry density, reducing the height of the cement slurry column or limiting the flow resistance during the flow of the cement slurry through the annular  
Figure 6: Procedure of spotting and squeezing cement-based plugs or polymer-based pills.  
space by adjusting its rheological properties. In addition, LCM can be used as a spacer in front of the cement slurry or LCM can be added to the cement slurry. Furthermore, special additives can be used that give the slurry thixotropic properties so that during the penetration of the cement slurry into the loss zone, its speed decreases and the gel structure develops and eventually the zone plugs due to increased resistance to the flow of the thixotropic cement slurry. [PERSON] and [PERSON] (2016) described an engineered cement spacer fluid that can cure moderate to total losses in permeable and fractured formations [98]. This spacer contains hydrophobically modified polysaccharides which form micelles in water. By increasing the differential pressure, the micelles are compressed and absorbed to form a layer of adsorbed micelles that forms an impermeable seal at maximum differential pressure. The layer of adsorbed micelles transforms again into water-dispersed micelles at pressure relaxation or inflow from the formation, so damage to the formation is minimal. [PERSON] et al. (2018) performed a comprehensive field analysis to understand lost circulation initiation during different phases of drilling and primary cementing. They analyzed 40 offshore well sections (18 casing and 22 liners) from four different locations (the Gulf of Mexico, the United Kingdom, Angola, and Azerbaijan) that reported losses prior to, or during, primary cementing operations. The results of the research showed that losses were commonly induced while running casings (in 42% of the cases) or pre-cement job mud circulation (in 48% of the cases), but rarely during cement placement (in 5% of the cases) and suggested that if losses are prevented during drilling, running casings and pre-job circulation, then the probability of having losses during cementing is significantly reduced [48]. The fact that in 95% of the cases losses were initiated prior to cementing, even though cementing ECDs are typically higher than drilling ECDs, is contrary to what is often perceived. Further, the results of the field study showed that the cement slurry was able to reduce the lost circulation rate when losses were severe or total prior to cementing (in more than 59% of cases) but this was not the case when partial or seepage losses occurred. A cement slurry possesses inherent wellbore strengthening capabilities [48].