Quantitative Analysis of Restrained Shrinkage Cracking in Oilwell Cement Using Digital Image Correlation

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-mu m 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.