Cement Voids and Microannuli Influence Collapse Resistance of Pipe/Cement/Pipe Systems

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 227913, “Influence of Cement Voids and Microannuli on the Collapse Resistance of Pipe/Cement/Pipe Systems,” by Bisen Lin, SPE, and David Coe, SPE, US Steel Tubular Products, and Timothy Thomas, Chevron. The paper has not been peer-reviewed. _ The pipe/cement/pipe system, critical to oil and gas well construction, ensures structural integrity under various loading conditions. Imperfections can reduce the system’s collapse-strength enhancement significantly. This paper examines the effects of cement voids and microannuli 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 assessment 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. Nonlinear Finite-Element Analysis (FEA) Modeling of Pipe/Cement/Pipe Collapse A nonlinear finite-element model, incorporating material nonlinearity (elastic-plastic constitutive model with strain hardening) and geometric nonlinearity (finite deformation) was developed and analyzed. Parametric inputs and scripting were employed to streamline the extensive modeling and simulation efforts. Fig. 1 presents a schematic of the FEA model for the pipe/cement/pipe assembly, including a depiction of the cement void space (defined by void angle θ). The model uses eight CPE4 elements (four-node bilinear plane strain elements) through the pipe-wall thickness for both outer and inner casings, with 1,024 elements around the full 360° circumference. For the cement, five elements are used through the cement-wall thickness. The modified Riks method was applied to calculate the collapse pressure of the pipe/cement/pipe system across various configurations and loading conditions. This method treats the load magnitude (external pressure or external plus annular pressure) as an additional unknown, solving for loads and displacements simultaneously, enabling accurate determination of stable pre-collapse response, onset collapse pressure, and unstable post-collapse behavior. The modified Riks method is thus well-suited for analyzing the complex collapse phenomenon of the pipe/cement/pipe system. Three configurations (analysis cases) were evaluated. All casings were assumed to have nominal outer diameter [100%×specified outer diameter (OD)] and nominal wall thickness (100%×specified wall), with 0.2% OD ovality and 5% wall eccentricity. Residual stresses from pipe manufacturing were not considered. For each case, two extreme interaction scenarios between cement and casings were modeled: tie constraints at the cement/casing interfaces to represent fully bonded cement (TIE), and contact interactions to simulate full microannuli (FMA) at the cement/casing interfaces. Cement-void space was modeled with void angles ranging from 0 to 70° at 5° intervals. For each configuration, two loading scenarios were analyzed: combined external plus annular pressure (EAP), where equal pressure was applied to all surfaces except the inner surface of the inner casing (i.e., the external surface of the outer casing and the annular space), and external pressure only (EP), applied solely to the external surface of the outer casing.