# 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
#### 5.3.4 Foamed Cement Slurry  
A foamed cement slurry is a cement slurry in which gas (air, or even more often, gaseous nitrogen), is incorporated directly into the base cement slurry, as density-reducing medium. In this way, an ultra-lightweight cement slurry is obtained. The first application of a foamed cement slurry in well cementing occurred in 1979 [44]. The required foamed cement slurry density is achieved by changing the concentration of nitrogen injected into the base cement slurry [105, 106, 107]. The conventional cements slurries with densities of 1800 to 1920 kg/m\({}^{3}\) are usually used as base slurries. The generation of a foamed cement slurry in the field requires a source of gas (a liquid nitrogen vaporizing unit) and a surfactant to stabilize the foam. Generally, foam cementing equipment includes standard and special equipment, such a cementing unit, a nitrogen unit with a nitrogen tank, a chemical injection skid, a foam generator, a batch mixer, a bulk trailer, a water tank, and a data acquisition van [105, 106, 107]. At the location of the well, a foam cement slurry is prepared by adding all of the additives except the surfactant to the base slurry. Gas and surfactant for foam stabilizing are injected into the base slurry during its pumping into the wellbore. A foamed cement slurry is less expensive than a cement slurry with microspheres, and in addition to low density and high plastic viscosity, it develops a relatively high compressive strength, so it is often used to solve circulation problems and cement zones of total losses [44, 108]. [PERSON] et al. (2018) described how they experienced serious downhole problems related to total mud losses and successfully completed 61 foam cementing jobs (from 2013 to 2017) in the Volga-Ural region of Russia. The loss rates varied from 2 to 60 m\({}^{3}\)/h, but some cases were more complicated, with no returns to the surface and a static level drop in the well to 500 m or more below the wellhead. The conventional methods of lost circulation control including high-viscosity circulation materials (LCMs) generally failed, so the operators applied new foam cementing technology which was designed to seal disastrous loss zones in highly permeable and fractured rocks and mitigate or eliminate mud losses during well drilling. The high-viscosity foam water spacer with a density of 300 kg/m\({}^{3}\) to 900 kg/m\({}^{3}\) was injected in the thief zone in front of a foamed cement slurry (with a density less than 1200 kg/m\({}^{3}\)). This reduces hydrostatic pressure in the well and creates a viscous but light layer that prevents loss of a foam cement slurry deep into the thief zone. The plastic viscosity of a base cement slurry with a density of 1950 kg/m\({}^{3}\) foamed to a density of 1000 kg/m\({}^{3}\) (foaming quality 48.7%) is 5.5 times higher than that of a base cement slurry. As a result, a foamed cement slurry can cover the entire thief zone and uniformly fill voids and cavities in the near-wellbore area [107]. The analyzed historical data on the successful application of the foam cementing technology to stop mud losses shows that the average efficiency of the foam technology was 76.2% which is much higher than the average efficiency of conventional methods (30 to 40%). In addition, foam cementing reduced non-productive time by an average of 2 days in comparison to conventional cementing [107]. The foam cementing technique was tested in the Volga-Ural field at the end of 2015 and in early 2016 and became the standard solution for cementing the 178/146 mm production casing strings up to 4000 m Measured Depth (MD), so more than 66 foam cementing jobs were performed from 2016 until 2020 [106].