Summary Ensuring the zonal isolation of hydrocarbon reservoirs is one of the main tasks of the cement sheath, but the development of cracks and debonding between the interfaces might lead to underground water pollution and leaks of oil and gas to the environment. Restrained shrinkage of the cement sheath is one of the most significant causes of cracking, and an experimental investigation has been conducted to quantify how extensive shrinkage-induced cracking might be due to changes in ambient humidity conditions, confinement levels, and mixture compositions. The presence of lateral confinement due to the presence of shale provided an 80% reduction in the cracked area compared to unconfined specimens, but still radial cracks as wide as 200 µm, microannuli up to 100 µm, and cracked areas of 50 mm2 in a representative well section were observed. Additionally, a reduction in crack widths and potential leak paths was obtained by reinforcing the cement slurry with synthetic fibers. A detailed quantification of shrinkage-induced cracking is reported in this study, providing crack information such as position, width, and orientation as a function of time.
Summary Cracking of oilwell cement sheaths may lead to loss of reservoir isolation and uncontrolled hydrocarbon leakage to the environment. This paper presents a methodology to characterize the crack pattern and quantify individual cracks in cement sheaths formed due to the restrained shrinkage of the cement, focusing on the range of 5 to 200-µm crack widths. For this purpose, high-resolution cameras are used for image acquisition together with a digital image correlation (DIC) method, and a newly developed data analysis process is applied for crack detection and quantification. The methodology is applied in a case study where cracks formed in the top and perimeter surfaces of a cement ring are detected, quantified, and classified according to crack properties such as width and orientation. The obtained information on cracks with a resolution on the micrometer level proves the effectiveness of the methodology to quantify cracks in the target width range. In addition, crack characteristics such as position, length, and orientation are also quantified, and values including spacing between cracks and cracked areas are calculated. This methodology is demonstrated in this paper to detect cracking induced by restrained drying shrinkage deformations but can be applied generally to document cracking in cement sheaths under different loading and boundary conditions.
Summary The loss of well integrity in oil and gas and CO2 injection wells provokes leaks that potentially pollute underground water reservoirs and the surrounding environment. The present publication reviews the existing literature investigating the loss of well integrity due to damage development in the cement sheath, focusing on qualitative and mainly quantitative information regarding cracks, effective permeability, and leak flows. Methods applied for leak detection on-site are reviewed, and the difficulties of these methods in providing quantitative results are highlighted. The outputs of laboratory experiments and computer simulations, considered essential to complement on-site measurements, are also reported. The review of the existing literature shows that for most of the damaged cement sheaths the observed crack widths range between 1 and 500 µm, the permeability ranges from 10−17 to 10−12 m2, and the leak rates range between 10 and 10 000 mL/min for gas leaks and between 1 and 1000 mL/min for oil leaks.
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