# Analysis of Cement Sheath-Rock Damage Mechanism--A Case Study on Water Injection Wells
## 1 Introduction  
In the field of water injection in oilfield development, water breakthrough and casing failure are common challenges. These issues have various consequences, including reduced water injection efficiency and potential contamination of surface water due to suboptimal cementing quality. The aforementioned issue presents inherent safety and environmental hazards within oilfields. It is noteworthy that a substantial proportion (approximately 20%) of well shutdowns and incidents in the Y oilfield are attributed to water breakthroughs,impeding the advancement of water injection development. Academic research on water-oil well communication has reached a consensus that the primary factor contributing to water intrusion is composite rock damage involving the casing cement sheath and the surrounding rock [1]. Contemporary research on water breakthrough beyond the casing of water injection wells primarily concentrates on two fundamental factors: the occurrence of cracking damage on the bonded surface of the rock-cement sheath composite material and the impairment to the cement sheath itself. Research on the damage of cement-rock interfaces encompasses a wide range of aspects, from the development of techniques [2; 3] to the impacts of hydration reactions [4]. It emphasizes the role they play in reducing the strength of composite rock bonding surfaces. Subsequently, stress-induced cracking occurs. Although cracking damage can lead to water infiltration, the causes of such damage are complex. In fact, the damage incurred on composite rocks outside the casing of water injection wells is not limited to cracking; it also includes elastic damage. This elastic damage disrupts the integrity of the composite rock, especially when subjected to axial stresses induced by water pressure. Studies on cement-rock interface damage have focused on investigating the influence of factors such as compressive strength [5], Young's modulus [6], friction angle, and cohesion [7; 8] on the instability and damage of cement sheaths. Some researchers have also noted the impact of filter cake on shear strength [9; 10], leading to further research on the primary causes of cement-rock interface cracking under tension [11]. [PERSON] et al. [12] have drawn attention to the influence of adhesion forces between rock and cement sheaths, while [PERSON] et al. [13] studied the debonding behavior of concrete-rock samples. These studies have contributed to understanding the characteristics of damage development in cement sheaths and the interaction between rock and cement sheaths. However, there is still a need for comprehensive research and analysis, specifically focusing on the characteristics of cement-rock interface damage in water injection wells. Research on the mechanisms of water breakthrough in water injection wells can be categorized into two main aspects: mechanisms related to the injected water and mechanisms occurring outside the casing. While scholars such as [PERSON] et al. [14] and [PERSON] et al. [15] have conducted extensive studies on the water flooding mechanism, particularly during the injection process, their focus has mainly revolved around the formation of water channels between oil and water zones. It is important to note that research on the mechanisms occurring outside the casing is multifaceted, with a primary emphasis on two key areas. One aspect involves analyzing the damage to the cement sheath. [PERSON] et al. [16] have proposed that cement with a high elastic modulus can generate significant tensile stress at the cement-casing interface, leading to integrity damage outside the casing. In contrast, [PERSON] et al. [17] attribute the damage to the cement sheath outside the gas well casing to the differential effect of the elastic modulus between the cement sheath and the formation rock on radial stress. They believe that an increase in the elastic modulus of the cement sheath will significantly elevate the radial stress at the interface.