Shallow Water Flows: A Problem Solved or a Problem Emerging

Abstract

Abstract In the 15 years since the first shallow water flow (SWF) was observed in thedeep water Gulf of Mexico, operators have developed a suite of strategies andproducts to combat SWF's. Shallow water flows are geopressured sands that are drilled before the well design will allow the drilling system to typically behydraulically closed to provide an overbalanced environment. Early efforts in combating SWF's were focused upon the hydraulic isolationof the encountered pressure in the sands when a marine riser was not present toprovide a hydrostaticly over-balanced environment to run and cement casing. Tools and strategies developed or employed to combat this problem included SWFcement formulations, external casing packers, annular shut-off seals andvalves. As fields moved from discovery to development, the effects of the erosion ofsand associated with the under-balanced drilling of shallow geopressured sandsbecame a paramount concern. A new set of tools and strategies were develop tocombat this new manifestation of shallow water flows which included the use ofoversized marine risers, drilling with weighted mud and returns to theseafloor, and, yet to be deployed, rotating subsea BOP stacks and dual gradientdrilling systems. Subtlety hidden amongst these major developments are indications of problemsthat have the potential to widen the difficulties that burden exploration anddevelopment in shallow water flow environments. The need to drill to deepertargets will increase the need to set 20" casing deeper which makesformerly benign SWF areas potentially high risk areas. The need to set casingdeeper places geopressured sands above the tail cement where hydraulicisolation is much less reliable due to cement contamination and channeling. Theerosion of sand at isolated well locations leaves cavities unfilled with cementthat provide no lateral support to casing, reducing the capacity of casing to bear vertical strain. The erosion of sand and release of geopressured brineproduces compaction which disturbs the strength of overly soils and increasesthe potential of narrowing the pore pressure fracture gradient window.