Evaluating LCE's Impact on Casing Integrity

Cross-section of the explosive casing expansion tool

Addressing Zonal Isolation in Aging Wells 

As oil and gas reservoirs approach the end of their productive life, there is a growing requirement for robust plug and abandonment (P&A) strategies. A critical component in these strategies is long-term zonal isolation, the prevention of fluid migration along the wellbore. Inadequate isolation risks contamination of groundwater, environmental leakage, and failure of the well’s structural containment, especially if repurposed for CO₂ sequestration or energy storage. 

One emerging remediation approach is localized casing expansion (LCE), which physically reshapes the well casing to reestablish sealing against the cement sheath. The technology generates radial expansions, typically dent-shaped, at defined axial intervals, compressing the surrounding cement and closing micro-annuli or cracks. While the hydraulic sealing effectiveness of LCE has been demonstrated experimentally and in field trials, its structural implications on the casing’s integrity, specifically burst and collapse resistance, require rigorous evaluation. 

Virtual Testing of Deformation-Induced Failure Modes 

To investigate the mechanical implications of LCE, Shell commissioned 4RealSim to perform detailed finite element simulations evaluating how LCE-induced deformations impact casing burst and collapse strength. The analysis focused on the interaction between initial imperfections, residual stresses, and failure modes, excluding cement and rock constraints for a controlled assessment of casing behavior. 

Simulating the Expansion Tool and Deformation Process 

The Local Expander tool features a set of radially deployed fingers driven outward by an axial cone. This expansion process was modeled using Abaqus with a detailed geometric and material setup. The model incorporated: 

• L80 casing pipes with varying wall thicknesses and pipe weights. 

• Shape imperfections including 0.5% initial ovality and 10% wall thickness eccentricity. 

• A semi-pipe domain with symmetry boundary conditions aligned with the tool’s finger geometry. 

• Pipe lengths equal to seven times the outer diameter, matching test protocol standards. 

Material behavior of L80 steel was derived from physical test data, with inverse calibration for strain-hardening beyond the necking point. The expander tool was assumed linear-elastic, and its radial displacement was driven by controlled axial motion of the cone. The full indentation and retraction process was simulated to capture plastic deformation and the residual stress state. 

Two expansion magnitudes per casing geometry were modeled to evaluate dent severity impacts. 

Comparative simulation results showing pipe deformation before and after casing expansion

Final casing deformation showing strain distribution

Material Modeling Strategy and Post-Necking Behavior 

A central requirement of this study was capturing material response deep into the plastic regime, particularly beyond the onset of necking. Conventional bilinear or multilinear hardening approaches were insufficient to model the combination of: 

• Large, localized plastic strains during radial indentation. 

• Progressive plastic zone growth under internal pressure. 

• Post-yield interaction between residual stress relief and external loading. 

To address this, L80 steel stress-strain curves were extracted from tensile tests and then refined using an inverse FEA calibration. Post-necking behavior, where the engineering stress decreases but true stress continues to increase, was accurately recovered by calibrating plastic strain evolution against experimental load-displacement data. This approach ensured the material law accounted for strain localization, hardening saturation, and eventual softening due to thickness reduction. 

This level of fidelity was essential for capturing the load redistribution after yielding and for determining whether dented zones initiate failure or remain mechanically supported by surrounding material. 

Capturing Burst and Collapse Response Post-Expansion 

Following indentation, each model was subjected to either internal (burst) or external (collapse) pressure loading. These simulations: 

• Used final deformed and strain-hardened geometries from the expansion simulations. 

• Applied pressure incrementally until ultimate capacity was reached. 

• Included capped-end conditions for burst and idealized external loading for collapse. 

Undented reference cases with the same imperfection profiles were used for comparison, allowing quantification of LCE’s structural impact. 

Pipe Deformation During the Expansion Phase 

The deformation pattern under tool-induced expansion showed non-uniform plastic straining due to the discrete nature of the expander fingers and pipe imperfections. 

Axial and Circumferential Strain Localization 

Strain concentrated circumferentially at the crest of the dent, particularly where the fingers pushed directly. Axially, strain gradients developed at the transition zones between dent and straight pipe. These transitions created complex bending and shear stress patterns not supported by tool contact. 

As radial displacement increased, unsupported regions between the fingers experienced reduced plasticity, while contact zones near the finger edges developed stress-relief zones due to local overstrain. 

Residual Stress and Wall Thinning Effects 

The post-expansion stress field revealed significant residual stresses, especially at the transitions from dent to pipe body. Imperfections in wall thickness exaggerated axial gradients in plastic strain. Overall, these zones became the key candidates for local weakening during subsequent loading. 

Collapse Failure Behavior in Dented VS. Undented Pipes 

Collapse strength depends on maintaining pipe roundness under external pressure. The simulations showed that: 

• Dented pipes exhibited reduced local ovality near the dent compared to the original imperfections. 

• Ovality developed more slowly under pressure in the dented cases. 

• The ultimate collapse pressure increased with denting, indicating local stiffening effects. 

This stiffening, however, was effective only over short lengths. For practical casing lengths, collapse behavior remained dominated by the pipe body rather than the dent. Therefore, the presence of dents did not degrade, and in some cases improved, collapse resistance. 

Burst Failure Response Under Internal Pressure 

Burst failure involves wall yielding, plastic strain accumulation, and eventual rupture. The simulations compared dented and undented pipes under increasing internal pressure.

Elastic and Elastoplastic Behavior 

Initially, all pipes exhibited elastic hoop expansion. Yielding began at ~618 bar in the pipe body, consistent across cases. Post-yield, the strain-hardened material allowed for further load capacity before full wall plasticization. 

At ~700 bar, through-wall plasticity developed, followed by ballooning and wall thinning. The ultimate burst capacity was ~778 bar for the dented pipe and ~796 bar for the undented pipe, a reduction of ~2%. 

Effect of Residual Stress Near the Dent 

Residual stresses from the LCE process triggered localized plasticity at the dent at lower pressure levels, but this did not significantly reduce overall capacity. These effects acted as plastic relief rather than stress intensification. 

At the dent crest, earlier wall thinning occurred due to pre-strained material and geometric discontinuity. However, full fracture required simultaneous plasticity across both the dent and pipe body, delaying failure propagation. 

The adjacent undented pipe provided support that energetically discouraged neck localization solely at the dent. This constrained deformation spread and maintained global capacity close to the undeformed reference. 

Pressure Capacity Retention Across Geometries 

Across all simulations, including those with 12.7 mm and 19.1 mm dents, the reduction in burst capacity remained within 10–30 bar. These losses are considered modest, especially given that burst failure is a secondary concern compared to collapse in abandonment operations. 

Engineering Implications for Abandonment Casing Design 

The results of this study carry direct implications for engineering decisions in plug and abandonment scenarios: 

1. Collapse resistance is not compromised by LCE denting. In fact, the local stiffness increases around dents can enhance the casing’s stability under external pressure, especially in short, unsupported sections. 

2. Burst resistance is marginally reduced, but remains within 3–5% of the undented casing capacity. This small reduction is unlikely to affect abandonment operations, where collapse is the dominant failure mode. 

3. Residual stresses from expansion do not propagate failure under operational pressures. Instead, they undergo local plastic relief during reloading, with no impact on net pressure integrity. 

4. Dent zones do not serve as failure initiators unless the entire pipe wall enters unstable plasticity. Even then, dented and undented sections converge toward the same rupture threshold, indicating global failure governs performance. 

From a design standpoint, these findings support incorporating LCE into remediation protocols without derating casing strength below conservative design limits, provided that material behavior and tool-induced strain localization are properly modeled. 

Final Remarks 

LCE introduces high-strain localized deformation zones within well casings. While it causes measurable changes in wall stress distribution and geometry, detailed simulation results show that its structural impact is controlled, localized, and manageable. Collapse strength is maintained or improved; burst strength reduction is limited and only becomes critical under overloading. These insights, supported by parallel experimental studies, confirm the robustness of LCE from a mechanical design perspective. 

Need Expert Insight on LCE Modeling?

Curious how localized casing expansion might affect your casing strength calculations or require adaptations in your modeling workflow? If you're evaluating the effects of localized casing expansion on well integrity or need to adapt your simulation models accordingly, 4RealSim can help. Get in touch through our contact form or email us at sales@4realsim.com to discuss your specific case.