Session: 11-06-01 Integrity of Well Barriers

Submission Number: 157620

Investigating the Correlation Between Compressive and Tensile Strength of Well Cement During the Curing Process 

Primary cementing is the process of injecting a cement slurry into the annular space between the steel casing and surrounding geological formation. The cement hydrates to form a solid sheath around the steel casing. Maintaining the mechanical integrity of the well cement is of high importance throughout the entire lifespan of wells. The objective of the primary cement job is to provide both zonal isolation and structural support to the well. Mechanical failure of the cement sheath is influenced and governed by many factors including cement characteristics and its strength and stiffness (elastic). Hence, knowledge of the mechanical properties (e.g., stiffness and strength) of the cement sheath throughout the curing/hydration time, is believed to be essential for failure analysis of the cement sheath during its service life. 

While many studies have explored the Young’s modulus and compressive strength of the cement sheath at early ages, the evolution of tensile strength of the cement sheath over the curing process has rarely been investigated, despite its critical role in crack and micro-annulus formation. Although the International Organization for Standardization (ISO) and the American Petroleum Institute (API) have recommended methods for assessing the compressive strengths of oil well cement, there are currently no equivalent standards for evaluating the tensile strength, nor the Young’s modulus. Frequently used well stimulations such as steam injection or multi-stage hydraulic fracturing all induce significant hoop stress and tensile loads in the cement sheath.  Without knowing tensile strength of well cement accurately, it is challenging to determine if a given cement mixture offers sufficient strength to resist cracking and subsequent well leakage under induced tensile loads not only after cement hydration but during the cementing process.

Researchers have observed an empirical relationship between the tensile strength and compressive strength of cured cement, generally reporting in industry that tensile strength ranges between 7-13% of the compressive strength. While different ratios have also been reported in the literature, there is no clear correlation between these properties during the entire curing process. This lack of a consistent relationship highlights the need for further investigation to understand how these mechanical properties evolve over curing time and under varying curing conditions (e.g., cuing age and curing temperature). 

The objective of this study is to investigate the correlation between compressive strength and tensile strength of cement during the curing process, with a particular focus on the impact of curing age and curing temperature on this relationship. Splitting tensile tests and unconfined compression tests have been conducted for this purpose. By analyzing the experimental results, we assess the relationship between tensile and compressive strength during the curing process and determine how varying curing conditions influence this correlation. The predicted correlation is compared with those reported for set cement in the literature. This investigation offers valuable insights into the evolution of tensile properties of well cement throughout the curing process, and its implications for maintaining well integrity over the cement's service life.

Presenting Author: Zahra Rahmani University of British Columbia

Presenting Author Biography: Third-year PhD student in Mechanical Engineering at University of British Columbia. She is under supervision of Dr. Ian Frigaard and Dr. Sardar Malek.