ABSTRACT:

Shallow water flows (SWF) can be a serious problem in some deepwater areas of the world. Encountered just below the sea floor, these flows typically occur while drilling top hole sections or during conductor cementing. Uncontrolled, these over-pressured, unconsolidated, shallow sediments can flow immense volumes of water and carry with it large quantities of sand. This can result in severe erosion of the formation supporting the conductor, considerable hole enlargement and, in a worst case scenario, collapsed casing and loss of the well. These weak, shallow formations will not usually tolerate the hydrostatic head exerted by a fluid-filled riser and the first two top-hole sections are drilled riserless to allow deeper casing setting depths. The pore pressure can be very close to the fracture gradient making well control difficult. In many cases where shallow water flows are encountered, it is necessary to drill riserless with a weighted fluid to maintain hydrostatic control. Flows may also occur during the cementing process, allowing a gas or water influx to migrate up through the slurry. This can cause weak formations to fracture with loss of whole mud or cement to the sea floor. In a few cases, shallow water flows have been known to expose weaknesses in the cement sheath around the conductor during the later stages of well construction, when least expected! Innovative techniques and procedures for dealing with shallow water flows will be discussed. This is based on case histories from wells drilled in the Gulf of Mexico, the North Atlantic Margin and the Eastern Mediterranean

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