# Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs
## 4 Preventive and Corrective Lost Circulation Measures
### Preventive Measures to Avoid Lost Circulation during Drilling
#### 4.1.2 Managed Pressure Drilling (MPD)  
Conventional drilling of fractured or depleted formation as well as deep formation in a high pressure and high temperature environment usually can be very difficult because of the small difference between pore pressure and fracture pressure at a certain depth. In depth offshore conventional drilling activities, this means the installation of more casing strings than necessary in order to reach the total depth [63]. These circumstances impose the need for better optimization and a precise control of annular pressure to prevent problems like well control issues, lost circulation, or a stuck pipe [13; 14]. Furthermore, to ensure safety and trouble-free drilling through a formation with narrow drilling margins, it is necessary to maintain the appropriate annular pressure in static and dynamic conditions. According to the International Association of Drilling Contractors (IADC), Managed Pressure Drilling (MPD) represents an adjustable drilling technique for the precise control and maintenance of the annular pressure profile throughout the wellbore. Due to specific downhole conditions, a managed pressure technique except for drilling also can be utilized for different operations which follow the drilling process (liner running, cementing, wireline operations, tripping, etc.) [64; 65]. This relatively new technique combines the low drilling fluid density with the additional pressure applied in the annular space on \begin{table}
\begin{tabular}{l l} \hline \hline  & **Preventive** **Measures** \\ \hline \hline  & \(\bullet\) & Minimizing the hydrostatic pressure in the wellbore using \\  & \(\bullet\) & drilling fluid which has the minimum safe density. \\  & \(\bullet\) & Maintaining proper mud weight. \\  & \(\bullet\) & Minimizing annular-friction pressure losses during drilling and \\  & \(\bullet\) & Minimizing the equivalent circulating density (ECD) by \\  & & changing the mud properties (plastic viscosity, yield point, and \\  & & gel strengths) within the safe margin. \\  & \(\bullet\) & Adequate hole cleaning. \\  & \(\bullet\) & Avoiding restrictions in the annulus. \\  & \(\bullet\) & Updating information about formation pore pressure and \\  & \(\bullet\) & fracture gradients for better accuracy with log and drilling data. \\  & \(\bullet\) & Applying appropriate drilling technology: underbalanced \\  & & drilling (UBD), managed pressure drilling (MPD), casing while \\  & & drilling (CwD). \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Preventive measures to avoid lost circulation during drilling. the surface and all with the purpose of the maintenance annular pressure profile within a safe drilling window. For drilling through zones with a total loss of the drilling fluid, the application of Mud Cap Drilling (MCD) can be very useful. Mud Cap Drilling represents a special variation of the managed pressure drilling and the includes following methods: Floating Mud Cap Drilling (FMCD), Pressurized Mud Cap Drilling (PMCD), Dynamic Mud Cap Drilling (DMCD), and Controlled Mud Cap Drilling (CMC) [63; 66]. Aside from the mentioned MCD, there are other variations of managed pressure drilling such as Constant Bottomhole Pressure (CBHP), Dual Gradient Drilling (DG), and Return Flow Control (RFC) or the Health, Safety, Environment (HSE) Method. Depending on the selected managed pressure drilling method, a drilling rig must be equipped with additional equipment like a rotating control device (RCD), a non-return valve (NRV), chock gates, sliding plates, shuttles, a continuous circulation system (CCS), a continuous circulation valve (CCV), or a backpressure pump. MPD was applied for the drilling of 7 wells in the carbonate naturally fractured reservoir of East Siberia where conventional drilling of a highly fractured zone was accompanied with intensive mud losses, thus over 50% of drilled wells in the oilfield could not reach the final depth.