Abstract
With the high demand for oil and gas, operators are becoming increasingly
interested in unconventional sources of hydrocarbon. One of the major sources
in the world is heavy oil, which has been defined by the API as oil that has
API gravity of less than 22.3 °API at 15.6 °C (60 °F) (extra heavy oil, or
bitumen, has 10 °API or less). Because heavy oil does not flow on its own in
the wellbore, a mechanism must be used to recover the oil by making it mobile.
Among the different experimental and operational methods, steam-assisted
gravity drainage (SAGD) has been the method of choice for the past few decades.
This process uses steam to heat the immobile oil and reduce its viscosity so it
can be extracted, usually by using pump jacks. These heavy oil wellbores need
to be zonally isolated to help ensure that the reserves are produced properly
without environmental or production optimization challenges.
In Alberta and Saskatchewan, many wells are drilled and cemented in designated
heavy oil fields where SAGD is applied to stimulate production. Most of these
wells are successfully cemented, however, some wells experience zonal isolation
failures that result in steam breakout or steam loss into non-targeted
zones.
While the problem may be related to the primary cement job, it may also have
been created by post-cementing operations and/or conditions. Sometimes the
problem may be temporarily solved by conventional methods, however, the
ultimate remedy may have to be applied eventually. The long-term integrity of
the cement sheath behind the casing should be planned with drilling,
completion, production/injection and abandonment stages considered. A properly
designed and engineered cement slurry system can help save a wellbore/field
from additional unplanned expenses. The process is shown below:
- Cement slurries are designed under defined wellbore conditions and exposed
to field conditions in the laboratory.
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- The mechanical behaviour of the cement sheath under downhole conditions is
studied using three-dimensional, finite element analysis software.
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- The endurance of the cement sheath under applied conditions is predicted
and optimized.
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- Operation and placement of the selected slurry is studied to help ensure
optimum execution.
Stresses exerted on the cement sheath from wellbore operations during
construction, injection or production that could damage the cement sheath are
integral to the study. These conditions are analyzed to help engineers
understand the post-cementing operational effects on the sealant system. This
paper presents the design criteria for finding the fit-for-purpose sealant
systems that can successfully protect wellbores under harsh environmental
conditions of SAGD and cyclic steam stimulation (CSS) operations.
Introduction
The long-term integrity of a cement sheath throughout a well's life is the
ultimate factor for determining whether the sealant will withstand the planned
operations, production and injection that are planned for the well. In the case
of heavy oil operations, the primary design considerations for a long-lasting
sealant are the temperature and pressure regimes. The stresses caused by the
extreme changes are exerted on casing, cement and the rock.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
31 March 2008
- Meeting paper published:
17 June 2008
- Revised manuscript received:
30 March 2009
- Manuscript approved:
4 May 2009
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