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.