# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 5 Lost Circulation While Cementing through Naturally Fractured Reservoirs
### Cement Slurry and Set Cement Properties
#### 5.1.1 Cement Slurry Density  
In some cases, it is sufficient to reduce the density of the cement slurry to prevent circulation losses. A reduction in cement slurry density during the cementation of casings is achieved by adding light-weight materials to the cement slurry or by foaming the cement slurry [20; 44; 49; 98; 99; 100; 101; 102; 103]. Critical design factors must be established to minimize dynamic pressure or equivalent circulating density (ECD) which is exerted in the potential thief zones. To reduce the hydrostatic pressure of the fluid column during cement slurry placement, in situations with lost-circulation and weak formations with low fracture gradients, one should use low-density (lightweight) or ultralow-density (ultra-lightweight) cement slurries having a lower density than API G neat cement slurry density (<1800 kg/m\({}^{3}\)), but still high enough to develop compressive strength in the expected time [44; 99]. A variety of materials are available to provide lower density of the oil-well cement slurry as well as lost-circulationprevention agents (see Table 6). Cement slurry density can be reduced by adding conventional lightweight materials such as: bentonite, sodium silicates, diatomaceous earth, fly ash, fume silica, gilsonite, powdered coal, microspheres, and nitrogen. The ultra-lightweight cement slurries with densities less than 1200 kg/m\({}^{3}\) can be prepared using hollow microspheres or nitrogen, and lightweight cement slurries using other lightweight materials. The above listed materials fall into the category of extenders. Their density is lower than the density of cement, and some of them adsorb water, so their addition increases the yield of a cement slurry [44; 99]. Unlike extenders, lost-circulation control agents prevent the loss of a cement slurry into the thief zone by bridging the pores and fractures of exposed rocks or by causing thixotropic behavior of the cement slurry. Common cement additives used to prevent lost circulation include granular additives (such as gilsonite and coal, ground walnut shells), flakes (such as cellophane flakes), or fibers (made from glass or synthetic polymers) as bridging materials. In addition, different materials (gypsum, certain soluble sulfate salts, bentonite, cross-linked cellulosic polymers) can be added to Portland cement to obtain a thixotropic cement slurry (see Table 6) [44]. Cementing operations using advanced, chemically inert fibers mitigate lost-circulation problems without compromising operational efficiency or the quality of the slurry or set cement [49; 103]. Extenders allow for the addition of more water or lightweight materials to a slurry to lighten the mixture and to keep the cement solids from separating. In addition, cement extenders reduce the amount of cement needed for a cementing operation and thus increase slurry yield [44]. Some of the listed extenders (such as hollow glass and ceramic microspheres (also known as cenospheres) with a size of 10 to 100 microns) are crushed under the action of high pressure, so the effect of reducing the density is lost. Therefore, when choosing lightweight materials, one should consider their strength, i.e., the maximum pressure that