# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 1 Introduction
(2014) proposed a new classification based on their appearance and application that considers conventional and new LCM materials (acid/water soluble, high fluid loss squeeze, swellable/hydratable combinations, and nanoparticles) [4]. [PERSON] et al. (2016 and 2020) presented laboratory evaluation methodology, test results, and field application of the combination of a high fluid loss squeeze (HFLS) and reticulated foam LCM that could seal a laboratory-simulated fracture/slot size of 9800 microns [26] and 31,700 microns [18]. They used a permeability plugging apparatus (PPA) with minor modifications to evaluate the LCM combination mixed in different bases (water, diesel, mineral oil, CaCl\({}_{2}\) brine and CaBr\({}_{2}\) brine). The combination of HFLS and reticulated foam LCM can provide a technically efficient alternative to particulate solutions for managing severe to total losses in the case of uncertain fracture sizes in naturally fractured/vugular formations [18]. [PERSON] et al. (2017) presented a comprehensive analytical model that can be used for different drilling fluids (Newtonian fluid, Bingham fluid, dilatant fluid, and pseudoplastic fluid) [6]. The model considers a linear fracture deformation with pressure change and leak-off from the fracture into the formation which leads to a more accurate estimation of the volume of drilling fluid loss than the pre-existing models. They concluded that the pressure differential is the driving force behind the fluids leaking into the fracture. The fluid rheology also dictates the rate of fluid loss into the formation. The drilling fluid loss increases as the value of flow behavior index and consistency index decreases [6]. Mud loss is influenced by many factors that make finding an analytical solution with acceptable accuracy very difficult or impossible, so in recent years, researchers have been trying to find a new method which can be successfully applied for the prediction of lost circulation zones by applying artificial intelligence (Artificial Neural Network (ANN)) [1; 27; 28; 29; 30; 31; 32]. In recent years, more and more wells are being drilled in complex reservoir conditions so there is need for the improvement of drilling mud properties in order to successfully and safely drill wells. Researchers started to investigate the possibility of applying nanotechnology, especially nanoparticles added to drilling mud for drilling through shales. Conventional drilling additives are too large to seal the nano-sized shale, in contact with water-based drilling muds, shale swells which results in wellbore instability. The results showed that the addition of clay/SiO\({}_{2}\) and TiO\({}_{2}\) nanoparticles improved the rheological properties [33; 34]. In addition to rheological modification, nanoparticles (size 1 to 500 nm) added in a concentration up to 1 wt% are shown to be effective for reducing fluid loss [35]. With this review manuscript, we tried to collect and organize all the available information related to problems that arise during drilling through naturally fractured formations. Different authors through their own studies and research sought to cover specific problems related to lost circulations in certain fractured formations. In most cases, these studies and research provide valuable information for problem solving on the same field and on other fields with similar conditions, but do not provide a broad picture. From this point of view, we tried to collect all this relevant information, and give readers one comprehensive review of the lost circulation problems and solutions for drilling through naturally fractured formations. Along with a detailed analysis and quantification of fluid losses in naturally fractured formations, as well as an overview of the available techniques and technology solutions, for the first time, we also endeavored to comprehensively view and compare the loss circulation problems in drilling and well cementing operations.