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Design and installation of monitoring wells
Published in Neal Wilson, Soil Water and Ground Water Sampling, 2020
After the borehole is drilled, the screen and casing sections are joined together and lowered into the hole, section by section. The sections are joined either by welding (for metal screen and riser pipe) or using threaded couplings (for metal or PVC screen and riser pipe). Sufficient tensile strength is required for the casing sections (as they hang over the hole) so that the topmost sections do not pull apart from the rest of the already-joined casing string. Depending on how casings are joined, the most likely point of failure is at the casing joints. Dividing the tensile strength by the linear weight of the casing will provide the maximum depth that a dry string of casing can be suspended. If PVC casing is used and the borehole is partially filled with water, a buoyant force will partially offset tensile stresses.
Offshore drilling blowout risk model – an integration of basic causes, safety barriers, risk influencing factors and operational performance indicators
Published in Safety and Reliability, 2018
Pedro Perez, Guido Dalu, Natalia Gomez, Henry Tan
The mud is critical for maintaining well integrity through hydrostatic pressure (ph). The casing and cementing phase starts with the installation of a large diameter steel pipe known as a conductor. Typically, a 30–36-inch diameter pipe (conductor) is installed, with the aim of preventing hole collapse and providing a conduit for circulation (fluid return) for the subsequent hole section, avoiding severe wash outs (mud circulation causing unwanted hole enlargement due to excessive flow rate and/or unconsolidated formation) or losses. Oil and gas wells are drilled in different sections, with decreasing diameters as depth increases. As detailed by Thomas (2001), the number of phases depends on the geological characteristics of the formation and the total depth of the well. After having been drilled, each phase is completed with the installation and cementation of a casing string. Casing strings have different functions according to the section they cover, including: preventing borehole collapse, avoiding contamination of underground fresh water layers close to the surface, avoiding migration of fluids from the formation, and sustaining the well head and well control equipment.