Drilling through total loss zones without returns at surface is a common practice for Operating Companies in the Middle East. This is known as the Mud Cap/Blind Drilling technique and it has been used for a long time in the region. The lessons learned throughout these years have produced well established procedures and "rules of thumb" for estimating mud density, volume and pumping schedule of the mud cap for different situations.

The application of this Mud Cap/Blind Drilling technique becomes more challenging when drilling through oil reservoirs with a high H2S content and combining zones with pressure regimes significantly different in the same hole section. This situation is often the case for some well designs commonly used to re-enter oil producer wells in Saudi Arabia. Under these circumstances, the main well control strategy is to maintain enough of a mud cap to prevent any migration of the hazardous gases and/or liquid hydrocarbons to the surface.

This approach often results in a conservative design of mud cap densities and volumes, which in turn generates huge expenditures in mud material and puts tremendous stress on logistics and transportation.

The Pressurized Mud Cap technique offers a safe alternative to Blind Drilling. This technique uses regular Managed Pressure Drilling (MPD) equipment to monitor the behavior of the reservoir at all times, improving the safety of the operation, by having accurate well control and realizing substantial cost savings, by optimizing the usage of drilling fluid materials and other resources associated with the mixing and pumping of the mud cap.

This paper describes the successful implementation of this technique for re-entering oil wells in an onshore field in Saudi Arabia. The paper also provides details for the equipment and procedures utilized to maintain a full column of fluid under pressure, by keeping the MPD choke fully closed while drilling without any returns at the surface. The results, when compared with the conventional Mud Cap/Blind Drilling Technique, show significant improvements in safety (well control), cost efficiency and logistics.

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