Structural-Casing/Soil Interaction Effects on Wellhead Motion
- Udaya B. Sathuvalli (Blade Energy Partners) | P. V. (Suri) Suryanarayana (Blade Energy Partners)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 2016
- Document Type
- Journal Paper
- 273 - 285
- 2016.Society of Petroleum Engineers
- Soil friction, Wellhead growth, Wellhead Motion, API 2GEO, Formation effects, ratcheting
- 4 in the last 30 days
- 232 since 2007
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During production in prolific high-temperature wells, the forces exerted by the inner strings can cause noticeable wellhead motion (WHM). The thermal forces created by the inner strings are balanced by the structural casing and by the friction between the structural casing and the formation. Depending on the shear-stress profile at the structural-casing/soil interface, the point of fixity (POF) of the structural casing may lie below the mudline, and gross motion in the subterranean section of the structural casing may occur. When the shut-in well cools back to the undisturbed state, the thermal forces disappear. Because of the anisotropic nature of the frictional forces, the wellhead does not always return to its original position, and a fraction of the overall wellhead displacement is locked in at the mudline. This phenomenon recurs during subsequent production and shut-in cycles, and it can lead to ratcheting. Ratcheting becomes a critical issue when the net thermal force on the wellhead is an appreciable fraction of (or exceeds) the pullout capacity of the structural casing.
Current models to assess WHM regard the inner casings as elastic springs that are attached between the wellhead and the POF. The conductor/soil or the casing/cement interactions below the POF are traditionally addressed by finite-element analyses (FEAs). In this paper, we discuss the importance of the anchoring shear stress at the conductor/soil interface and provide methods to assess its orders of magnitude. We present solutions to frequently encountered problems in operational and design situations where sections of casing below the assumed top of cement (TOC) come loose or when the subterranean motion of the conductor is appreciable. These solutions can be used instead of FEA. By coupling the results of this model with the t–z response of pile-driven conductors (API RP 2GEO 2011), we present semianalytical models that are applicable in a variety of wellhead-loading situations.
|File Size||1 MB||Number of Pages||13|
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