The design of steam wells is complicated by the fact that the axial stress exceeds the yield strength in compression during heating and exceeds the yield strength in tension during subsequent cooling.
As such, a design is required which incorporates post yield behaviour of both casing and connections.
A brief discussion of the issues involved in steam well design is given below, firstly dealing with the loading of the casing, and then its capacity to withstand these loads.
Casing loading
Axial loads
Casing in high temperature (steam) wells is generally cemented from TD to surface for a number of reasons:
- it prevents loads resulting from the thermal linear expansion and contraction of the casing being transmitted to the wellhead;
- it avoids the problem of annulus pressures resulting from thermal expansion of fluids in a sealed annulus;
- it prevents buckling.
However, the cement prevents elongation of the casing during heating and this causes large axial stresses which yield the casing in compression. Similarly, the cement prevents contraction of the casing during cooling, causing large tensile stresses close to or above yield.
Current steam well casing design models are based on the isotropic hardening plasticity theory. The basic features of this theory are the non-ideal elastic/plastic behaviour and stress relaxation under conditions of constant high temperature and pressure. The behaviour of different grades of steel means that the maximum loads are not constant but are a function of grade. Generally, higher grade materials give higher loads.
Collapse and burst loads
Precise collapse and burst loading conditions will depend on the well type. In general, it is the collapse loading which determines casing wall thickness and grade for steam injection wells.
Biaxial effects
Selection of casing for steam well application is based on a requirement to withstand collapse loads while subject to axial stress near or beyond yield in a high temperature environment.
API Bull 5C3 provides guidance on the biaxial derating of casing collapse capacity in the presence of tension. However, this approach is based on theoretical behaviour in the elastic range only, and the collapse capacity is assumed to be zero when axial tension reaches yield strength. Casing does, in fact, retain some collapse capacity once tensile yield is reached, although the degree to which this occurs depends on the material grade.
At tensile yield, about 65% of the uniaxial collapse strength remains in K55 casing, and at approximately 145% of tensile yield, about 40% of the uniaxial collapse strength remains. For C95, however, only 15% of collapse strength remains at 105% of tensile yield.
Biaxial test data indicate that compression does not decrease collapse strength.
Casing load bearing capacity
Yield strength reduction
At high temperatures, casing materials exhibit a decrease in yield strength. The derated collapse capacity should be obtained by substituting the new yield strength into the appropriate collapse formula and the corresponding regression coefficients.
The reduction in collapse capacity due to temperature is usually small in comparison with the reduction due to tension.
Casing connections
The production casing connection must provide reliable structural strength and gas tight sealing under the high temperatures and loads of the steam well operating cycle. Gas tight sealing is required to seal the steam (in the event of a tubing leak) and also to seal nitrogen sometimes used as a packer blanket. Because of these severe conditions, a connection should not be used unless it has been tested and qualified accordingly.