# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 5 Lost Circulation While Cementing through Naturally Fractured Reservoirs
### Cement Slurries for Primary Cementing through Fractured Formations
#### 5.3.1 Cement Slurry with Microspheres  
Microspheres are small gas-filled beads with specific gravities ranging from 0.4 to 0.6 [44]. Two types of microspheres are available: ceramic and glass. Ceramic microspheres are derived from fly ash (the shell is aluminosilicate) and they are heavier than glass microspheres with a specific gravity of 0.7; thus, a higher concentration is necessary to achieve low slurry densities. The use of ceramic microspheres is recommended at bottom hole pressures below 31 MPa. The glass microspheres can withstand higher pressures than ceramic microspheres (up to 69 MPa); the particle-size distribution may vary from 20 to 200 microns with a wall thickness from 0.5 to 2.0 microns [44; 105]. [PERSON] et al. (2015) described the results of field implementation of the engineered highly crush-resistant cement slurry (lightweight cement slurry with engineered glass bubble extenders) for casing cementation over a weak formation in zones with low fracture gradients in the three gas fields located in Delta Mahakan in Indonesia instead of a cement slurry with conventional cengospheres as the extender that was previously used [100]. The field implementation results of the engineered highly crush-resistant cement slurry, compared to the previously used slurry, showed: a reduction in losses from 48% to 27%, a decrease in the percentages of zero cement slurry return to the surface from 13% to 3%, the elimination of the necessity of performing second-stage cementing (with a cencosphere-based cement slurry, 1 s-stage cement job out of every 10 cementing jobs), and a reduction in the occurrence of sustained casing pressure from 24% to 11%.  
[PERSON] et al. (2016) also described a lightweight material with a high crush resistant property that was applied to reduce the cement slurry density used in the cementation phase of the 88.9 mm (3 \(\frac{1}{2}\) in) tubing and long column of cement (>2134 m MD) at the Sejadi field offshore of East Kalimantan, Indonesia. These engineered particles resemble a soda-lime borosilicate glass in the form of a white powder. Its specific gravity ranges from 0.39 to 0.45, its particle size is approximately 50 microns, and its isostatic crush strength is 55.16 MPa. At the Sejadi-X well, 116 m\({}^{3}\) of SBM had been lost into the hole when a loss rate of >6.4 m\({}^{3}\)/h was encountered while drilling 215.9 mm (8 \(\frac{1}{2}\)-in) for an open hole section. The loss rate could not be reduced even after spotting 4.8 m\({}^{3}\) conventional lost circulation material (LCM) pill and cost significant volume of SBM and LCM amount lost to the formation three times, as well as 2 rig days spent in an attempt to cure the loss during drilling and cementing through the depleted sandstone formation. The cementation with this highly-crush resistant lightweight cement system was satisfactory with no losses occurring and the targeted intervals were covered with good cement ([PERSON] et al., 2014).