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As producing zones mature, drilling can become increasingly difficult as the difference between pore pressure and fracture pressure decreases. Mud densities required to maintain shale stability can be very close to the pressure required to fracture depleted reservoirs. Such situations require special drilling fluids with low equivalent circulating densities (ECD), since the effective density increase due to the additional circulating pressure can, in conventional fluids, be higher than the margin between pore and fracture pressures. Furthermore, drilling in mature fields is often severely cost-constrained, causing some operators to try small holes and techniques such as coiled tubing drilling (CTD) and through-tubing rotary drilling (TTRD). All of these techniques increase the importance of drilling fluid design, as the small borehole sizes employed are likely to further increase ECD-related problems. Thinner fluids are more prone to material sag, and sag itself can be more critical in small hole sizes due to very narrow annular clearances.
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