McSpadden_edit_Outsourcing_book_temp.qxd 11/11/2009 11:28 Page 78
Importance of Predicted Cementing Temperatures for
Critical High-pressure/High-temperature Casing Design
a report by
Albert R McSpadden and Simon Glover
Altus Well Experts, Inc.
Standard industry simulation tools that predict wellbore temperatures Importance of Initial Conditions
Engineering & Construction
during drilling and cementing operations and production operations are The severity of the thermal load is determined by both the initial and final
available. Prediction of initial and subsequent wellbore temperatures temperature state of the casing. The final temperature state will vary with
feed directly into tubular stress analysis, which is often critical for casing different well operations. These include drilling of subsequent sections,
and tubing design of high-pressure/high-temperature (HP/HT) wells. production, stimulation, well kill, etc. However, the initial temperature
state of the casing relative to which all subsequent conditions are
The current industry environment presents many challenges to the evaluated is uniquely defined by the moment during waiting on cement
effective use of the simulation tools available. Operator and engineering (WOC) at which the pumped cement in the annulus sets. This may occur
services organisations are all confronted with a shortage of experienced within four to six hours after the end of the pumping operation. The axial
personnel. At the same time, challenging wells that require substantial load, pressure profile and temperatures prevailing at this point in time are
design effort are becoming more prevalent. As with any detailed locked in when the cement sets and become the reference point for all
modelling technique, a great variety of input parameters must be subsequent load conditions.
accurately determined before thermal simulation and stress analysis can
be effectively achieved. The identification, estimation and collection of Accurate determination of cementing temperatures is not trivial. The
correct input data can be a significant organisational cost in itself. temperature at the time cement sets is a transient phenomenon that
Therefore, the time and effort required to gather data or to determine is directly affected by all preceding well-construction operations. These
proper assumptions for detailed well design must be justified. all generate transient thermal disturbances in the wellbore and
surrounding formation. All parameters describing the full sequence of
In this environment, any simplifying assumptions that can streamline well construction activities up to and including the cementing operation
the design process or bypass intensive modelling efforts are quickly itself must be identified and determined. The well design process often
adopted. All things being equal, simple worst-case assumptions that ignores the complexity of cementing temperatures and simply assumes
facilitate quick analysis and decision processes are favoured over more the initial casing temperature is equal to UDT. What are the implications
detailed modelling. More rigorous, detailed modelling will be conducted of this assumption?
on a selective basis only where the costs can be justified.
Figure 1 shows idealised casing temperature plots representative of major
In the case of modelling wellbore thermal conditions, production thermal conditions over the operational life of the well. The plot of UDT
operations are generally characterised by a few significant parameters assumes a constant geothermal gradient from the surface down to the
that are typically estimated with some degree of certainty. On the reservoir. As an extreme worst-case scanario, production is depicted as a
other hand, the drilling process is subject to many operational constant temperature with the entire wellbore heated up to reservoir
parameters that are more difficult to quantify or predict. Also, the temperature. Likewise, the extreme worst case for high-rate stimulation
drilling process is by its very nature unpredictable and subject to or kill is to cool the entire wellbore down to surface ambient
unanticipated contingencies. A common industry rule of thumb is that temperature. The temperature profile when the cement sets (WOC)
conservative well designs for hot production loads result by assuming reflects the thermodynamics of the preceding drilling and cementing
initial wellbore temperature conditions equal to the undisturbed process. During drilling and cementing, surface fluids are pumped down
geothermal condition. In this way, the need to estimate drilling and into the wellbore with the effect of cooling the deeper wellbore zone; at
cementing parameters and simulate the wellbore construction process the same time, these fluids are circulated up the annulus. This transfers
may be avoided. As this is assumed to result in the worst-case thermal- the heat energy absorbed from the lower wellbore to the upper wellbore
induced loads, the well design is accepted if no unacceptable loads zone, thus heating it above the prevailing geothermal.
are identified.
Also shown in Figure 1 is another factor that should be considered, i.e.
However, the assumption of an initial casing temperature equal to the top of cement (TOC) depth. Changes in temperature and other
undisturbed geothermal may not be appropriate. For HP/HT wells, this conditions in the casing below TOC after the cement has set do not
assumption is sometimes overly conservative and the well design can affect the axial load state of the free length of string. All thermal
be justified only by taking the time and effort to model realistic expansion here is assumed to be fully constrained. Hence, the average
cementing conditions. More importantly, it is quite possible that the temperatures (dashed lines) that drive the thermal axial loads along the
initial temperature profile due to drilling and cementing can result free length of casing are calculated only above the TOC freeze point.
in a more severe temperature change than the assumption of
undisturbed temperatures (UDT). In this case with UDT, critically For a relatively shallow TOC, the tendency will be for the average WOC
unacceptable load conditions may not be identified for the design. temperature to be greater than average UDT temperatures, because the
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