Summary
Pressure buildup, caused by fluid thermal expansion in sealed annuli of
high-presure/high-temperature (HP/HT) wells, can have serious consequences such
as casing failure or tubing collapse. To determine whether mitigation was
required for a HP/HT development, annular pressures in an appraisal well were
studied with a dedicated field test, which consisted of running a
pressure/temperature memory gauge in a casing/casing annulus of a well, and
testing the well several times during a 3-month period, after which the gauge
was retrieved, and the data were read out.
First of all, comparison of the magnitude of the observed annular pressures
with the burst and collapse ratings of the casings shows that annular pressure
buildup is a serious consideration in casing design. Such design is to be based
on theoretical models for annular pressure buildup. The data acquired with the
test serve to validate these models.
The data demonstrate that, in general, the theoretical models overpredicted
pressure buildup in the annulus. This overprediction was more pronounced at
higher temperatures (and pressures) than at lower temperatures, which could not
be explained by mechanical factors such as casing ballooning. The influence of
these factors was quantified by analyzing the transient pressure response of
the annulus. Neither could permanent leakoff of completion fluids explain the
discrepancy between theory and test. Leakoff of the annular fluids, which was
seen to dominate pressure development during a previous test in a well with a
cement shortfall between casings, does not play a significant role in this
fully cemented and sealed annulus. This left (1) the properties of the
completion fluids differing from the properties of the base fluid (water), and
(2) temporary leakoff to near-wellbore fracture systems through the microannuli
between cement sheaths and casings as explanations for the observed
overprediction. Therefore, estimates on the basis of pure water properties,
considering the annulus to be a perfectly pressure-tight vessel, can be
considered a worst-case estimate for pressure buildup and a safe basis for
design.
Introduction
Pressure buildup in tubing-casing or casing-casing annuli is, in general,
undesirable. Although casing design should take into account high pressures at
the casing head (e.g., caused by leakage or thermal expansion of the annular
fluids), high-pressure differences always hold the risk of the casing bursting
or collapsing at weak points, leading to loss of production (Vargo et al. 2002)
or in the worst case, loss of the well (Nelson 2002). For this reason, most
operating companies adhere to annular-pressure-management schemes for onshore
and platform wells, which prescribe bleeding off pressure through the wellhead
once a predetermined pressure level (e.g., 20% of the minimum internal yield
pressure of the affected casing) is reached.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
8 December 2004
- Revised manuscript received:
18 July 2006
- Manuscript approved:
8 September 2006
- Version of record:
20 December 2006
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