Stress Failure Assessment and Potential Well Integrity Issues with Different Oilwell Cement Classes and Formulations in Complex Wells
Authors: Abraham J. J. , Carvero A. , Devers C. , Teodoriu C. , Amani M. 
Published: May 2024

Complex operations such as fracturing, and stimulations have become a mainstay in most drilling and completion operations around the world. Safe technologies have been adopted by the industry to mitigate issues in complex wells, HPHT conditions and difficult formations. However, well integrity problems - especially in the cement layer - are still a major concern in a lot of cases when performing workover, fracturing or re-completion operations in existing or older abandoned wells.

Oilwell cement used in drilling and completion comes in several different classes and grades. Geopolymer based cements are also increasingly being considered for cementing operations, owing to their green credentials. Commonly used API Class C, Class H and Class G cements as wells as Geopolymers all have mechanical properties which vary widely, and a decline in these properties are expected after exposure to different downhole conditions over time. Experimental evaluations were performed to measure mechanical properties such as the Uniaxial Compressive Strength (UCS) and acoustic velocities and determine how they vary over time and under different physical environments. Finite element stress modeling was then performed to determine failure mechanisms in downhole conditions. Degradation of the cement layer due to ageing, as well as exposure to different downhole temperatures especially in the cement-casing interface are of particular interest.

Each of the classes of oilwell cements perform differently and thereby have a different impact on the overall integrity of the well. Results from laboratory testing of samples showed significantly different mechanical properties during the mixing, setting and ageing periods for different oilwell cement classes and at different temperatures. Among the different formulations tested, Class G cement showed the highest failure stress with almost all samples showing a consistent peak UCS growth, before stabilizing. Class C cements and Geopolymers had the lowest stress failure resistance, indicating their unsuitability for HPHT operations. Higher temperatures accelerated the setting time, though reduced the UCS for all classes of cement. When stresses experienced during typical fracturing operations were modeled in a downhole scenario with these cements, propagating failure points were observed. Stresses can migrate and concentrate at different points - which in some cases can exceed the failure criteria of these cements leading to the formation of cracks. These can in turn cause integrity issues in the cement sheath and possibly a critical well integrity situation.

Robust testing of oilwell cements and geopolymers is needed to properly understand their properties, as well as the development of stress failure points around the wellbore. Identifying potential well integrity issues for various cement formulations can in turn help in improving the quality and reliability of cementing operations, reduce the risks associated and ensure safe operations over the lifespan of a well.