Production of hydrocarbons will, in time, lead to a reduction in reservoir pore pressure if pressure is not maintained by a drive mechanism. The resulting increase in effective stress leads to reservoir compaction and deformation of the overburden.
The vertical strain caused by this compaction of the producing interval is transferred to a certain extent to the casing string(s) set across that interval. This casing will thus undergo axial deformation and, in the case of deviated wells, lateral deformation, such as bending, ovalisation or crushing. These lateral loads are comparable in type to lateral loads in squeezing salt formations but are thought to be significantly less in magnitude. Excessive overburden deformation can lead to localised slip across faults and bedding planes. This results in shearing of casing.
These effects are briefly dealt with below, followed by guidance on how compaction loads can be allowed for in the detailed casing design, and possible operational techniques to minimise their impact. Casing capacity is not affected by this environment.
Axial deformation
Axial compression will take place over compacting intervals, while axial tension will be induced over any decompacting intervals, i.e. formations overlying and underlying the reservoir. The transfer of strain from the formation to the casing will depend upon the behaviour of the formation/cement/casing interface. Generally, the casing strain can be assumed of similar magnitude as the formation strain.
Axial compression will tend to initiate buckling. The extent and type of buckling will depend on the amount of lateral support the casing receives. Column-like buckling will only occur in zones where support is small or non-existent e.g. long, badly cemented intervals or zones with cavities due to sand production. After the onset of buckling, deformation of the casing will depend on annular clearance. The local mode of buckling or bulging occurs where the cementation is locally bad, e.g. geometrical irregularities such as casing collars and casing transitions. Also a weak, low strain resistant, box/pin design could lead to these failure modes.
Axial extension may occur if surface subsidence is not equal to the amount of compaction in the producing intervals. The resulting tension may reduce the collapse strength of the casing or even cause tensile failure.
Bending
Bending of the casing in deviated wells occurs due to different compaction strains above and below the reservoir boundary. Excessive bending may lead to loss of roundness or, at an extreme stage, tensile failure on the convex side. The transition interval length determines the dogleg severity.
Ovalisation
Lateral loads resulting from reservoir compaction will depend on the rock type, in-situ stresses, and the tectonic setting. Although this type of loading is less severe than experienced in squeezing salt formations, any non-uniformity of the radial load distribution will result in a stress concentration that may lead to failure. Experiments show the diameter reduction to be in the order of the compaction strain if well cemented.
Shearing
Due to slip of formations in the overburden or reservoir, significant shear stresses can develop in the casing wall. Experience shows that the risk for shear failure increases within thick and relatively shallow reservoir sections.
Detailed casing design
Analysis of the stresses and strains in casing set across compacting and decompacting intervals is complex. Computer program have been developed for this purpose and can for a given casing scheme, a given compaction profile, and a given set of mechanical and geometrical data, determine when and where casing buckling will be initiated.
Operational techniques
It can be shown that at typical casing depths and stress conditions, axial deformation of the casing cannot be avoided. Therefore, solutions must be designed to accommodate it with a minimisation of associated damage. The concept common to most of these techniques is to localise axial casing deformation in given zones, which will by design not undergo severe damage. The deformation of the remaining length of casing is therefore limited. Proposed techniques involve sliding couplings, partly corrugated casing joints, and external sleeves sliding along the casing. These solutions add operational constraints and no field evidence of successful implementation has yet been reported.
With regard to bending, careful selection of well trajectory offers the best solution. Software can be used to quantify the ability to run the next work string or casing, taking into account bending stiffness.
Ovalisation can either be accommodated by more flexible casing and larger clearance or resisted by stronger casing. In the latter case the casing should be designed for full overburden load, as in the case for squeezing salts.