Summary
In the past, casing-shift problems were thought to be one of the causes of
production decline. This motivated field engineers to investigate whether or
not casing shift occurred by shooting radioactive bullets in productive layers
and measuring the relative movement of casing and formation (Allen 1984).
However, the results were ambiguous because of the uncertainty of
wireline-logging depth measurements during a long reservoir-compaction period.
In this paper, the following will be investigated:
- How the bonding of casing and formation breaks under confining
stresses
- How a casing slides after the bonding breaks
- The magnitude of shear force required to induce casing shift
- The magnitudes of casing shift for typical reservoir conditions
To achieve the above objectives, the following laboratory measurements were
performed:
- Construct test samples simulating casings bonded to formation with cement
and mudcake.
- Conduct experiments measuring shear force required to induce casing shift
under confining stress.
- Simulate the correlation of depth and shear and normal stresses around a
cased hole using a finite-element model for compacting reservoirs.
- Compare the results of experiments with simulations to judge the magnitude
of casing shift.
The laboratory experiments showed that a casing does not simply slip when a
shear load is applied. It accompanies shear failure of cement sheath. Hence,
when casing shift occurs, shear fractures are also induced in the cement, which
may cause gas or liquid migration through the fracture network. Calculations
are performed to estimate the shear stress induced at casing/cement and
formation/cement interfaces during reservoir compaction using a
finite-element-simulation model. Comparing the laboratory-measured sliding
shear stress with the calculated shear stress, it is concluded that a small
casing slippage may occur at casing/cement and formation/cement interfaces. The
amount and distribution of slippage along casing/cement and formation/cement
are also evaluated for typical reservoir conditions. The small slippage may be
serious enough to reduce production. Hence, reperforation at the expected shift
intervals would recover productivity. In addition, the small slippage creates a
shear fracture network that may cause gas or liquid migration.
Several standard cement tests are described in Nelson and Guillout (2007).
This work complements two previous works (Evans and Carter 1963; Ladva et al.
2004) for casing/cement and formation/cement bond strengths using similar
cement. Previous work determines cement shear and hydraulic-bond strengths for
those with small reservoir compaction, while this work focuses on cement-bond
strength for more-severe compactions where casing slippage or failure are
initiated. Unlike the standard hydraulic and shear cement bond tests, the
current test method closely simulates the in-situ conditions, clarifying the
phenomena occurring during casing slippage. In addition, a rule of thumb and a
practical method to evaluate the magnitude of casing shift are proposed.
© 2009. Society of Petroleum Engineers
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History
- Original manuscript received:
4 December 2007
- Meeting paper published:
4 March 2008
- Revised manuscript received:
14 May 2008
- Manuscript approved:
16 May 2008
- Published online:
1 June 2009
- Version of record:
1 June 2009
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