STEAM STIMULATION WELLS
Steam or hot water is often used as the heat transfer medium for the application of heat to a reservoir containing highly viscous crude oil. As a consequence, tubing and casing are placed into an environment of extreme temperatures where typically the upper temperature range varies between 400°F and 600 °F. The upper temperature limit is expected to rise to 700 °F in the near future.
When steam is injected into a well, the casing is gradually heated up and tends to elongate in direct proportion to the change in temperature. Inasmuch as most casing is cemented, the tendency to elongate is replaced by a compressive stress in the casing. Casing failure occurs initially when the temperature-induced compressive stresses exceed the yield strength of the casing. Subsequent cooling
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Fig. 4.27: Thermal cyclic loading diagram for elastic perfect plastic material and the failure of casing coupling. |
of the casing while the well is shut-in or producing, relieves the compressive stress although the deformation produced during the steam injection phase creates a tensile stress as the casing temperature returns to the normal levels that existed prior to steam injection. Often, this tensile force buildup results in either joint failure at the last engaged pipe thread, or tensile failure by pin-end jumpout.
Willhite and Dietrich (1966) were the first to present a comprehensive method for assessing pipe failure under cyclic thermal loading. Holliday (1969) and Goetzen (1985) extended this work and presented a complete analytical treatment for the design of casing strings for use in steam stimulation wells.
In the following sections, the mechanism of casing failure is discussed in detail to provide a basis for selecting safe operating temperatures and related material properties. Next, a systematic method for estimating casing temperature during steam injection is presented. Finally, different techniques used to protect casing from severe thermal stresses are discussed.