LEAKAGES THROUGH RADIAL CRACKS IN CEMENT SHEATHS: EFFECT OF GEOMETRY, VISCOSITY AND APERTURE

Annular cement sheath is considered to be one of the most important barrier elements in the well, both during production and after well abandonment. It is however well-known that mechanical damage to the cement sheath might result in leakage pathways, such as microannuli and radial cracks, and thus loss of zonal isolation. In this paper we have studied the effect of geometry, aperture and viscosity on the resulting pressure driven flow through real radial cracks in cement sheaths using Computational Fluid Dynamics (CFD) simulations. Real radial cracks were created by downscaled laboratory pressure cycling experiments and the resulting geometries were mapped by X-ray Computed Tomography (CT). This gave a unique 3D volume of the degraded cement sheaths which provides detailed information about the morphology, such as the irregular apertures and roughness, as well as locations of the radial cracks. In this study, we have used five experimentally created geometries, varying from barely connected to fully connected and almost uniform cracks. Additionally, theoretical uniform models with homogeneous aperture and a smooth surface were created for comparison. The simulations were performed by importing the experimentally created leak paths into a CFD simulation software, making it possible to determine the actual flowrate as a function of pressure drop. Methane gas, water and oil was used as model fluids. The simulation results show that fluid flow through real cracks in cement sheath is complex with torturous paths, especially around bottlenecks and narrow sections. Additionally, the results show that flow of both methane gas- and water are not linear and hence does not follow Darcy's law. This illustrates that simple models are not able to fully describe fluid flow through such complex geometries.

A Numerical Study on Optimum Material and Design for Dental Trilayer Systems

This study investigated the impacts of geometry, thickness, and material on damage growth in a porcelain-metal restoration structure by utilizing a computational approach. Extended finite element method (XFEM) was used to find the critical loads causing the nucleation of radial cracks at the porcelain undersurface. Plastic deformation also was considered at the metal above the surface as another damage mechanism. The dental system consisted of a brittle outerlayer (porcelain)/metal (Pd/Co/Au alloys)-core/dentin-substrate trilayer system. A tungsten-carbide hemisphere as an indenter was used to apply a compressive loading on the structure. In addition, two different geometries were created to present the dental structure, cylinder, and tapered cylinder. The results showed that a harder and stiffer metal core can resist the initiation of radial cracks. It was also observed that the metal with thinner layers is more vulnerable to radial cracking. In all simulations, the tapered cylinder geometry showed to have higher critical loads in both damage modes. The optimum thickness for the porcelain layer was suggested to be 0.5 mm. The geometry of dental crown-like structures was found to be an important factor in damage initiation. The findings also proposed that the metal layer should not be designed very thin in order to prevent the formation of radial cracks. This numerical investigation also recommended that the stiffness of the metal layer is better to keep higher compared to other layers to hinder the initiation of radial cracks.

Numerical Study of Fracture Characteristics of Deep Granite Induced by Blast Stress Wave

To study the characteristics of rock fracture in deep underground under blast loads, some numerical models were established in AUTODYN code. Weibull distribution was used to characterize the inhomogeneity of rock, and a linear equation of state was applied to describe the relation of pressure and volume of granite elements. A new stress initialization method based on explicit dynamic calculation was developed to get an accurate stress distribution near the borehole. Two types of in situ stress conditions were considered. The effect of heterogeneous characteristics of material on blast-induced granite fracture was investigated. The difference between 2D models and 3D models was discussed. Based on the numerical results, it can be concluded that the increase of the magnitude of initial pressure can change the mechanism of shear failure near the borehole and suppress radial cracks propagation. When initial lateral pressure is invariable, with initial vertical pressure rising, radial cracks along the acting direction of vertical pressure will be promoted, and radial cracks in other directions will be prevented. Heterogeneous characteristics of material have an obvious influence on the shear failure zones around the borehole.

A parametric study for radial cracking in cement under different loading events based on the stress intensity factor

Cement is one of the primary barriers in a wellbore and critical to well integrity. Radial cracking is a pervasive failure mode in cement due to the temperature and pressure variation during drilling, completion, or production. This work presents a comprehensive analysis of radial cracking in cement under various loading events. The proposed model estimates the stress intensity factor and fracture surface displacement as indicators for crack propagation and opening, respectively, through a distributed dislocation technique. Three types of radial cracks, divided by their tips terminating at the casing-cement interface, inside cement, or at the cement-formation interface, are considered. Based on this model, we conduct a parametric study for radial cracking under typical loading events such as steam injection, CO2 injection, and high pressure and high temperature (HPHT) drilling. Results indicate that the crack near the casing-cement interface has an increased risk for steam injection and HPHT drilling, while all three types of radial cracks are destructive during CO2 injection. The thermal expansion coefficient of cement is a significant parameter for steam and CO2 injection wells. The fluid pressure and the cement's thickness are crucial to radial cracking under HPHT conditions. Stiffer cement could promote crack opening for steam injection but prohibit the crack deformation for CO2 injection or HPHT wells. Thicker cement would accelerate radial cracking under the three loading events. These findings are helpful in designing cement to maintain long-term integrity.