Induced losses occur when the drilling fluid density, required for well control and to maintain wellbore stability, exceeds the fracture resistance of the formations. Depleted reservoirs present a particular challenge. As the reserves decline, the pore pressure decreases, which weakens hydrocarbon-bearing rocks. Nearby or interbedded low permeability rocks, however, may maintain their pore pressure. This scenario can make the drilling of certain depleted zones extremely difficult because the fluid density necessary to support the shale exceeds the fracture resistance of the sands and silts. This situation worsens in offshore operations where logistics impose additional limitations to the capacity of combating mud losses. The potential benefit is clear if a drill-in fluid system can be designed with a density that meets the low pore pressure and fracture gradient levels necessary to drill the reservoir and/or if the fluid includes specially designed lost circulation material to effectively bridge or seal the formation. In both cases, these methods should provide the possibility for removal at a later stage in order to minimize productivity impairments.

This paper presents an extensive review of the various technologies, both proven and potential, in drill-in fluids for depleted reservoirs.

This paper organizes the discussed technologies separately by the density range that they can cover. It also describes the state of the art advantages, limitations, and suggested specific applications, including the required development to expand the application and status of the technologies. Discussions include specific information about safety considerations, cost, and environmental issues associated with each technology. Such technologies include drilling blind, well flowing drilling, managed pressure drilling, aerated mud, foam drilling, air or mist drilling, structured microbubbles of air (aphrons), emulsions (both direct and invert), and the addition of hollow spheres (glass spheres, cenospheres, and plastic spheres).

This review compiles crucial information necessary to design and formulate the proper low density drill-in fluid to address depleted reservoir challenges.

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