# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 7 Discussion  
The drilling of an oil and gas well, especially a deep, or extended reach horizontal well can be a technically and economically demanding process. This process is usually fraught with numerous problems as a result of an underground condition on the well site (the existence of high pressure or depleted zones, unstable zones, naturally fractured or lost circulation zones, etc.) as well as the technical limitations of the available drilling technology. These mentioned problems can be related directly to pay zones (reservoirs) or to the cap rock and overlaying formation up to the surface. Therefore, problem-solving approaches to the mentioned problems can be very different. Drilling through a naturally fractured formation is closely related to the fluid lost circulation problems during drilling or well cementing operations. Although the causes of lost circulation have been well known for decades, every new well still represents a new challenge even in a well-known areas. This problem is even more pronounced in case of a wildcat well in a relatively unexplored area due to lack of data about the presence of naturally fractured formations. Figure 7: Sequence of procedures for solving the problem of loss of circulation during casing cementing. Moreover, there is no specific data about fracture properties in this formation which makes the selection of prevention method very difficult. This is the reason why most of the authors represent their experiences with lost circulation problems in their manuscripts in a specific naturally fractured formation on a specific field. Unfortunately, in this way, the database is increasing but without a comprehensive, universal solution for all future cases. The lost circulation problem can be solved through preventive and different corrective actions. The emphasis should be put on preventive action and the implementation of preventive measures in the planning phase of a particular well. The planning of the lost circulation preventive action in this early stage of project development strongly depends on the available data collected on offset wells or through geological and geophysical explorations. Usually, the available data provides information about the existence of naturally fractured formations (depth and type), as well as the rate of loss, and to a lesser extent, information about the fractures themselves. The proper quantification of the existing natural fractures within a specific underground formation represents a key factor for the successful prevention of fluid loss during drilling and well cementing operations. After detailed quantification and determination of the underground fracture system and assessment of possible fluid loss, the appropriate preventive measure should be selected. Today, the drilling industry offers several high quality technical and technological solutions for lost circulation problems during drilling operations. The proper solution most often involves the correct fluid selection, the application of different wellbore strengthening or lost circulation materials, as well as the application of advanced technologies such as managed pressure drilling technology or casing drilling technology. This indicates that the oil and gas industry struggles with the lost circulation problem on daily basis during drilling and cementing operations. Except during drilling, lost circulation can be a problem at well completion and workover operations, especially in depleted or naturally fractured reservoirs, but they are beyond the scope of this paper. Even though the causes of the problem have been identified, and high quality technical and technological solutions for the prevention of lost circulation are available, there is still room for improvement in planning strategies. The application of artificial neural networks, radial basis functions, and support vector machines is certainly welcome, but their accuracy strongly depends on a large amount of data from a specific area.