# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 1 Introduction  
Oil and gas companies very often face the phenomenon of fluid loss during the drilling process. The loss of large volumes of whole mud to the formation (lost circulation) has historically been the root cause of well control problems and high mud costs. Many drilling hazards such as hole collapse, stuck pipe because of poor cutting removal, and even blowouts have been the result of lost circulation. In addition, lost circulation may cause extensive formation damage from lost circulation remedial treatments. Based on an extensive review of literature, [PERSON] and co-authors (2020) found that lost circulation costs 10-20% of the cost of drilling high pressure high temperature wells and 90% of these losses occur in fractured formations [1]. The negative consequences are magnified greatly in a deep-water environment [2]. The causes of fluid loss may be the presence of natural or induced fractures, cavernos zones, or high permeability of the matrix [3, 4, 5]. Natural fractures may occur in any type of formation. If drilling is continued and more natural fractures are exposed, a complete loss of returns may be experienced [3]. Naturally fractured reservoirs exist throughout the world and contain significant amounts of hydrocarbon reserves. The presence of natural fractures in a reservoir is a problem during drilling and cementing but is desirable from the aspect of well productivity. Natural fractures improve production by enhancing the effective permeability of rocks greatly by connecting pores together but can complicate drilling and cementing operations as well as reservoir stimulation. Induced fractures areprincipally those produced in the surrounding rocks by the fluid pressure applied within a well bore [5]. Induced fractures during drilling are undesirable because large amounts of the fluid can be lost to the surrounding rocks. However, after drilling and well completion, in some reservoirs, fractures are intentionally induced to increase hydrocarbon flow. A large volume of drilling fluid loss into the formation poses severe threats. Although different solutions have been recommended to avoid any kind of fluid loss, the oil and gas industry is still facing difficulties in determining and detecting these problems [6]. Lost circulation is most commonly divided into four types based on the amount of the loss rate (severity of the losses), as follows, seepage loss, partial loss, severe loss, and total loss [7; 8], and can take place in both static (static loss rate) and dynamic conditions (flowing loss rate) [9; 10; 11]. According to the type of losses the industry proposes various precautions that should be applied such as maintaining Equivalent Circulating Density (ECD) by decreasing the mud density as well as by reducing the values of rheological parameters and mud flow rate, adding lost circulation materials (LCMs) to the mud in order to strengthen the formation or the implementation of advanced drilling techniques like Managed Pressure Drilling (MPD), Underbalanced Drilling (UBD), or Casing while Drilling (CwD) [12; 13; 14; 15]. Compared with conventional drilling techniques, for drilling formations below hydrostatic pressure (UBD) low density media can be used as a drilling fluid, such as air, nitrogen, mist and foam so that in underpressure conditions, it causes an inflow of the formation fluid into the well and reduces mud loss [16; 17; 15]. Controlling severe to total losses in naturally fractured/vugular formations is still a challenge, particularly in carbonate formations in which conventional particulate Lost Circulation Materials (LCMs) might not be effective although they have been widely used to avoid or stop losses [18]. A variety of materials and techniques have been developed to stop lost circulation and stabilize a wellbore [3; 19; 20; 21; 22; 23; 24; 25]. For years, classical LCMs have been classified into four categories (fibrous, flaky, and granular or a mixture of all three), but [PERSON] et al.