Influence of Cement Voids and Micro-Annuli on the Collapse Resistance of Pipe-Cement-Pipe Systems

Abstract The pipe-cement-pipe system, critical to oil and gas well construction, ensures structural integrity under various loading conditions, including external pressure and combined external plus annular pressure. Imperfections, such as cement voids and micro-annuli between the cement and inner or outer casings, can significantly reduce the system's collapse strength enhancement. This paper examines the effects of (i) cement voids and (ii) micro-annuli on the collapse resistance of the pipe-cement-pipe system, with void angles ranging from 0° (no voids) to 70°. The objective is to provide comprehensive technical understanding of these factors, offering valuable insights for risk assessments of the structural integrity of the pipe-cement-pipe system under external pressure alone or with annular pressure buildup between the inner and outer casings. To evaluate the effects of cement voids and micro-annuli on the collapse pressure resistance of the pipe-cement-pipe system, nonlinear elastic-plastic FEA was performed. The modified Riks method was employed to determine the collapse pressure of both the pipe-cement-pipe system and the corresponding standalone casings. Three configurations were analyzed: (1) 13.625" × 0.625" Q125 outer casing with 11.750" × 0.534" P110 inner casing; (2) 16.000" × 0.575" Gr135 outer casing with 13.750" × 0.707" Gr135 inner casing; and (3) 13.750" × 0.707" Q125 outer casing with 10.750" × 0.595" P110 inner casing. Two loading scenarios were considered: external pressure alone (EP) and combined external plus annular pressure (EAP). Void angles from 0° to 70°, encompassing most practical scenarios, were evaluated. Automated tools, including scripting and parametric inputs, were used to manage the extensive FEA simulations efficiently. Based on the FEA results, the following key observations and conclusions can be drawn: (i) Cement Without Voids: For a perfect cement job (0° void angle), the pipe-cement-pipe system exhibits substantial collapse resistance enhancement, over 200% compared to the collapse strength of the corresponding standalone casings. This holds true for external pressure alone (EP) scenarios, regardless of whether the cement is perfectly bonded (TIE) or a full micro-annulus exists (FMA), as well as for combined external plus annular pressure (EAP) scenarios with perfect bonding (TIE). However, the enhancement in collapse resistance is significantly reduced when both EAP and FMA are present. (ii) Effect of Void Angle and Micro-Annulus: The collapse pressure resistance of the system generally decreases as the cement void angle increases. For EP, the difference in collapse resistance between fully bonded cement (TIE) and full micro-annulus (FMA) is minimal. In contrast, for EAP, the difference between TIE and FMA is substantial, highlighting the significant impact of micro-annuli under combined external plus annular pressure condition. The results presented in this paper offer a comprehensive quantitative analysis of the effects of cement void angles and bonding conditions (fully bonded or full micro-annulus) on the collapse resistance enhancement of the pipe-cement-pipe system. These findings provide valuable insights for risk assessments in well casing design.