Riserless Drilling With Casing: Deepwater Casing Seat Optimization

Abstract

Abstract This technical submission on Deepwater Casing Seat Optimization is a follow up to the submission of Riserless Drilling with Casing: A New Paradigm for Deepwater Well Design OTC-19914, presented in May 2009. This paper had discussed the need for a deepwater well design change for GOM due to the well documented poor performance of drilling complex wells in the GOM, and subsequently proposed a well design concept based upon setting the first two casing strings significantly deeper than the current practice. This submission continues the discussion the rational for setting the riserless casing strings deeper than current practices and reviews the feasibility and challenges of a well design to implement such a significant change in the current deepwater well design. Riserless Casing Seating Optimization is a design method for the installation of riserless casing strings in deepwater drilling environments. The conventional deepwater well design model fails to take advantage of the early and progressive growth of the subsurface fracture gradient immediately below the mud line, due to perceived requirement to set an another casing string above anticipated shallow drilling hazards. This results in the inability to supply sufficient and safe "leak-off" for the drilling of subsequent holes and casing installations, adequately mitigate shallow hazards, and avoid "wasting" casing diameters in the riserless hole sections and thus subsequent sections to total vertical well depth. This method provides for the installation of at the minimum two riserless casing strings, beginning with the structural conductor, which is conventionally installed by "jetting" operations. The proposal is for the dual purpose use of the structural conductor in that it is installed at deeper depth so that it takes advantage of the early growth of the fracture gradient, and then becomes a true structural casing being able to support the axial loads of the BOP’s and subsequent casing strings. Furthermore this ensures sufficient leak-off protection for next string and the safe drilling of subsequent shallow hazards. This technical presentation suggests that drilling with casing could be the mitigating technology that will allow the hole sections depths to be drilled to depths corresponding to the pore pressure / fracture gradient environment. Drilling with casing, actually liner drilling due to the water depth, improves the near wellbore shallow fracture gradient while providing better dynamic ECD control of potential shallow water or gas flows. Furthermore there is a discussion on some of the key well design issues that pertain to this proposal.