A novel method to investigate cement-casing bonding using digital image correlation

The main focus of current industry practices on the cementing of oil and gas wells is short-term cement-design goals such as high strength, adequate pumpability and proper placement of cement in the wellbore. However, the bond between cement and casing is a major factor for long-term integrity of wellbore which requires more attention with the current increasing trend in horizontal drilling and hydraulic fracturing operations in unconventional resources. These wells experience various kinds of forces throughout their life in which the casing and cement bond can play a critical role in the integrity or failure of these wells. In this paper, we apply digital image correlation (DIC) technique to monitor strain development over the surface of a casing filled with cement while it is diametrically compressed. In this technique, the casing/cement sample is painted with random white and black speckles and the surface of sample is recorded by camera. Finally, strain development with time is calculated using image processing technique. This visualization helps to find out the points of weakness for strain accumulation which eventually end up for fracture initiation and propagation. In this laboratory setup, the tensile strain accumulation is the highest on the casing/cement interface 90-degree away from casing/loading point and the strain gradually increases at the casing/cement interface toward the casing/loading point. Then there is fracture generation at the center of cement sample propagating toward casing/loading point. We also review the impact of cement contamination, barite and water-cement ratios on cement/casing bonding and their associated DIC results.