# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 8 Conclusions and Recommendations  
Based on extensive research of the relevant literature dealing with lost circulation in naturally fractured formations, it is possible to conclude the following:  
* A strategy for managing lost circulation must be an integral part of well planning and should include the best drilling and cementing practice, drilling fluid and cement slurry selection, wellbore strengthening and lost circulation material selection, the appropriate corrective measures, as well as an adequate contingency plan.
* The key to success is proper planning, risk assessment to identify potential high-risk zones, quality materials and equipment, and experienced supervision.
* Drilling fluid and cement slurry losses into naturally fractured formations can be avoided or at least mitigated by managing the equivalent circulating density (ECD).
* In the vast majority of cases, losses were initiated prior to cementing, although cementing ECDs are typically higher than drilling ECDs, which is contrary to what is often perceived, and the reason lies in the fact that a cement slurry has the inherent ability to strengthen a well.  
The general recommendations for preventing or treating fluid loss in naturally fractured reservoirs are:  
* Use artificial intelligence such as artificial neural networks (ANN), radial basis function (RBF) and support vector machine (SVM) to predict the occurrence of mud loss and lost circulation zones as well as to predict the fracture width and determine the particle sizes of lost circulation materials.
* Use available methods and tools to determine the location of the loss zone.
* Control bottom hole pressure (BHP) in static and dynamic conditions by applying new technologies and low-density fluids.
* Apply a wellbore strengthening strategy by adding WSMs to the mud to increase fracture pressure and avoid induced fractures or the re-opening of existing fractures.
* Close naturally fractured formations that are also oil and gas reservoirs, select acid-soluble or time and temperature degradable conventional lost circulation materials (LCMs) and/or special LCM pills/plugs (e.g., HFHS pills, polymer-based and fiber-based pills) to avoid permanent formation damage.
* The available materials and methods for solving the problem of lost circulation in naturally fractured reservoirs are not universal and, therefore, based on the collected well data for each specific situation, select the appropriate material and method that have proven successful in practice.  
N.G.-M.--Conceptualization, preparation of the comprehensive literature overview, writing, review and editing of the whole manuscript, visualization; B.P.--editing of text related to lost circulation while cementing and reviewing of the conclusion section; P.M.--writing and editing of text related to preventive measures of lost circulation and preparation of the figures; I.M.--writing and editing of text related to corrective measures of lost circulation and preparation of tables. All authors have read and agreed to the published version of the manuscript.  
This research received financial support from University of Zagreb for scientific research project \"Stress reduction in drill string during drilling of extended reach wells\".  
Not applicable.  
Data Availability Statement: Data available in a publicly accessible repository.  
The authors declare no conflict of interest.