Open access peer-reviewed chapter
Abstract
The cement sheath is a crucial component of the wellbore system, responsible for maintaining structural integrity, and preventing leakage. Over the life cycle of oil and gas wells, load changes can lead to various cement failure modes, such as disking, radial cracks, and debonding fractures. It is vital to locate and evaluate cement sheath failure in the wellbore. This chapter aims to comprehensively and systematically describe recent advances in cement sheath integrity prediction, control, and monitoring techniques. Firstly, we list the underlying reasons for the cement sheath failure. Then, we extensively discuss current advances in cement sheath integrity. Finally, wellbore integrity control and monitoring techniques are also discussed. This chapter serves as a valuable reference for both scientific research and engineering applications of cement sheath integrity in oil and gas wells.
Keywords
- cement sheath
- cement failure
- interface debonding
- integrity prediction
- monitoring
1. Introduction
Wellbore integrity refers to the implementation of a range of technological, operational, and organizational management measures aimed at minimizing the risk of uncontrolled leakage of formation fluids throughout the entire life cycle of oil and gas wells [1]. The primary objective is to establish effective wellbore barriers at each wellbore stage [2]. The API RP 90 defines the wellbore barrier as an “envelope of one or several dependent barrier elements preventing fluids or gases from flowing unintentionally from the formation into another formation or to the surface,” as shown in Figure 1 [3]. The failure of wellbore integrity in oil and gas production can result in various issues, including lost circulation, casing instability, formation pollution, and environmental pollution. This significantly impacts the safety and efficiency of oil and gas exploration and production activities. Therefore, safeguarding wellbore integrity is an essential task in the oil and gas field development [4].
Figure 1.
General principles of well barriers [
The cement sheath, as a pivotal component in the cementing process of oil and gas wells, plays a significant role in establishing a stable and sealed structure between the casing and the formation. Referring to “API RP 90 Annular Casing Pressure Management for Offshore Wells” [5] and “NORSOK STANDARD D-010 Well integrity in drilling and well operations,” [6] cement sheath integrity is defined as the scientific design of the cement slurry system and implementation of effective technical measures to prevent mechanical integrity and hydraulic sealing failures of the cement sheath. This approach helps minimize the risk of uncontrolled flow of formation fluids throughout the entire life cycle of the borehole and ensures the safe drilling and production of oil and gas wells.
The quality and performance of the cement sheath are directly related to the production and recovery of oil and gas wells. Furthermore, it significantly impacts the environmental and social benefits associated with oil and gas production. Statistics show that as many as 6692 wells in the OCS area of the Gulf of Mexico in the United States have sustained casing pressure (SCP), accounting for 43% of the total number of wells [7]. There are 166 production wells in the Fuling shale gas field in Sichuan, China, and 79.524% of the wells have SCP problems [8]. Therefore, the research and optimization of the cement sheath is of great significance.
2. Reasons for cement failure
The cement sheath is subjected to various complex loads downhole, and the failure of sealing integrity can be divided into the following three categories: shear failure, tensile failure, and interface debonding (Figure 2) [9]. The main reasons for the cement sheath failure are mainly described below.
Figure 2.
Schematic of cement sheath failure [
2.1 Poor cementing quality
Cementing quality is crucial for ensuring both safe and efficient drilling to the designed well depth and the secure production of oil and gas. Several factors influence cementing quality, including the geological characteristics of oil and gas reservoirs, geometric conditions of boreholes, properties of formation fluids, well completion production methods, requirements for reservoir protection and stimulation, technical effects of previous operations, performance of cement slurry, and construction technology [10].
Inhomogeneous cement mixtures can lead to uneven density, hardness, and strength of the cement sheath, thereby affecting the cement sheath integrity [11]. For example, excess water or additives in the cement mixture may result in cracking of the cement sheath. The improper rheological properties of the cement slurry can result in a substandard cement sheath quality. For instance, excessively strong or weak fluidity may lead to a failure in the integrity of the cement sheath. Incorrect operation during the cementing process, such as excessively high or low cementing pressure, overly fast or slow cementing rate, extremely high or low cementing slurry temperature, etc., may lead to the failure of the cement sheath integrity [12]. Therefore, to ensure the quality of the cement sheath, it is necessary to adopt appropriate operational measures and monitoring methods during the cementing process to ensure the stability and reliability of the cementing quality.
2.2 Temperature and pressure change
Several special operations in oil and gas wells, such as hydraulic fracturing, CO2 injection, and gas storage cycle injection and production, can significantly change the pressure and temperature of the wellbore. The change in temperature and pressure will affect the stress distribution in the cement sheath, which may cause the seal integrity failure of the cement sheath [13, 14, 15, 16]. Notably, drilling, fracturing stimulation, and production process induce pressure changes in the wellbore, and excessive stress will lead to the failure of the cement sheath structure. Furthermore, the injection of fracturing fluid into the formation, influx of fluids from the formation, and steam injection cause temperature fluctuations within the wellbore. Consequently, it is imperative to investigate the influence of pressure and temperature changes on the integrity of the cement sheath.
2.3 Cement shrinkage
During the hardening of cementing, the cement slurry system is always accompanied by volume changes in liquid, plastic, and solid states, that is. “volume shrinkage.” This shrinkage occurs due to a chemical reaction between the cement clinker and water, leading to an overall reduction in the volume of the cement slurry [17, 18]. The absolute volume shrinkage of Portland cement slurries ranges from 2.3 to 5.1%. The formation of a gelled structure from the solidification of the cement slurry restricts the downward transmission of gravity and causes a drop in effective liquid column pressure. Consequently, the formation fluid pressure surpasses this weakened pressure, resulting in fluid entering the annulus and leading to channeling.
The volume shrinkage of cement can lead to cracks in the cement sheath. The main reason is that the radial tensile stress of the cement sheath gradually increases during the shrinkage process. When the tensile stress exceeds the tensile strength of the cement, the cement sheath will produce radial cracks, as shown in Figure 3 [19]. The volume shrinkage of cement can also lead to the development of micro-annulus between the cement sheath and the formation and the casing, resulting in the development of channels for oil, gas, and water flow. Therefore, controlling the shrinkage of cement volume during and after cement slurry curing is extremely important to guarantee cementing quality.
Figure 3.
Distribution of radial cracks in the cement sheath at different stages [
2.4 Chemical degradation
The cement sheath is essentially a Portland cement-based material, which is extremely vulnerable to acidic medium corrosion, resulting in alteration of material composition, structural damage, and functional degradation or loss [20]. The corrosion of cement stone by CO2 is a complex and prolonged physical and chemical process [21].
The CO2 corrosion of the cement sheath includes the following three processes. The carbonic acid is generated through the dissolution of carbon dioxide in the formation water. Next, this carbonic acid reacts with calcium hydroxide and calcium silicate hydrate in the cement matrix, forming microspherical calcium carbonate crystals. These crystals lead to the formation of compounds known as Ca(OH)2-CaCO3 and C-S-H-SiO2-CaCO3, respectively. These compounds contribute to an increase in both the equivalent elastic modulus and the equivalent hardness of the cement matrix. Additionally, they also lead to a reduction in both the porosity and permeability of the cement matrix. The specific processes can be observed in formulas [22, 23].
As carbon dioxide continues to dissolve in the formation water, the carbonization process continues, and CaCO3 is formed into Ca(HCO3)2. Ca(HCO3)2 is soluble in water and diffuses into formation water, and this phenomenon is known as the leaching of calcium ions.
Due to the dissolution of Ca(OH)2 and CaCO3, the pH value of the cement matrix increases. As a result, the hydrated calcium silicate gel undergoes transformation into amorphous silica gel, disrupting the internal bonding conditions of the cement matrix and increasing its porosity. Meanwhile, the solid phase particles in the cement matrix dissolve and diffuse into the formation water. This process facilitates the interconnection of pores and cracks in the cement matrix, creating a pathway for corrosive fluids to penetrate the interior and initiate corrosion within the matrix (Figure 4).
The interpretation of experimental results of the reaction between CO2 and cement is shown in Figure 4 [24].
Figure 4.
Interpretation of experimental result of CO2 reaction with cement [
2.5 Perforating operation
In the development of unconventional oil and gas resources, the perforation completion method is often employed to establish the connection between the formation and the wellbore [25, 26]. During the perforation explosion, high-temperature/high-pressure metal jets are generated, which rapidly penetrate the casing, cement sheath, and formation at speeds ranging from 3000 to 8000 m/s and 10–30 GPa pressures. This process occurs within a very short period. The completion of perforation often leads to cracks in the cement sheath. Furthermore, the high pressure experienced during fracturing may cause the cement sheath to debond from the casing, resulting in channeling between oil layers, groundwater layers, and gas layers [27]. For example, Daqing Oilfield conducted ultrasonic testing before and after the perforation operation of 19 wells in a block and found that the cement sheaths of many wells had local debonding and cracking problems. Therefore, clarifying the cause of cement sheath rupture in perforated well completion is the key to improving cementing quality and successfully developing unconventional oil and gas.
3. Cement sheath failure research
Cement sheath is a crucial component of oil and gas wells. Issues in cementing quality or complex loads from downhole operations can lead to yielding and damage of the cement sheath, which compromises its sealing ability and deteriorates casing stress. The integrity of the cement sheath is directly linked to the safe extraction of oil and gas from the wellbore and ensuring environmental protection. Therefore, it is crucial and imperative to analyze the integrity of the cement sheath.
3.1 Cement failure
Due to various factors such as harsh downhole conditions, temperature, and pressure variations, the cement sheath is subjected to different levels of damage and failure. This further increases the risk of wellbore fluid leakage, posing potential threats to the environment, and personnel safety. The failure mechanisms of cement sheath are complex and diverse, primarily including tensile failure and shear failure.
Currently, some progress has been made in the research on the failure mechanisms of cement sheath, but there are still some limitations. The existing studies on the failure of the cement sheath mainly use the ideal constitutive model to describe the mechanical behavior of cement. However, these idealized models are difficult to accurately reflect the mechanical behavior of cement in the actual formation environment. Consequently, this could lead to inaccurate predictions of cement sheath failure behavior. In real downhole environments, the cement sheath is subjected to various mechanical and chemical effects, such as temperature changes, stress changes, formation fluid invasion, etc. Nevertheless, current studies often simplify these factors into a single influencing factor, failing to accurately capture the complex mechanical response of the cement sheath in the actual formation environment. Therefore, future research should focus on enhancing the observation and experimental research of the cement sheath in the actual formation environment. This approach could obtain more accurate mechanical behavior data, and develop a constitutive model that can reflect the cement in the actual formation environment. Additionally, the influence of the actual formation environment on the cement sheath failure should be comprehensively analyzed in combination with field observation data and numerical simulation methods. Such an integrated approach would significantly improve our understanding and predictive capabilities regarding this issue. By bridging the gap between theoretical models and real-world conditions, advancements in this field can contribute to safer and more efficient oil and gas well operations.
3.2 Cement interface debonding
According to the cause of interface debonding, interface debonding can be divided into three types. The first is that under the action of cyclic temperature and pressure, the cement sheath produces cumulative plastic strain, which causes the casing and cement sheath to deform uncoordinatedly, resulting in micro-annulus. The second is that the interface is peeled off under the action of formation fluid driving pressure due to the poor cementing quality. The third is that the volume of the cement sheath shrinks continuously during the process of cement sheath solidification and continuous loss of water. When the radial stress at the interface becomes tensile stress and overcomes the tensile strength of the interface bonding, it may also cause interface debonding.
The current research is mainly based on experiments and numerical simulation methods, lacking the observation data of actual well sites. Furthermore, most existing studies are confined to ideal conditions and fail to deeply explore the impact of complex formation conditions and operational changes that occur in actual well environments. As a result, there is a need for more comprehensive investigations in realistic well conditions. Additionally, the understanding of the mechanism and influencing factors of interface failure under thermal-hydrological-mechanical–chemical coupling remains limited. The fracture behavior of the interface and the crack propagation mechanism need to be further studied.
4. Cement integrity control technology
The research and development of new cement aims to overcome the limitations of traditional cementing by enhancing various properties, including higher strength, improved corrosion resistance, better self-healing capabilities, superior expansion, and increased toughness.
4.1 Anticorrosive cement
The presence of acidic gases, such as CO2 and H2S, can lead to the corrosion of cement when exposed to suitable levels of humidity and pressure. This corrosive action has several negative effects on the cement, including a reduction in its alkalinity, a decrease in the compressive strength of the cement sheath, an increase in permeability, and ultimately, a shortened production life for oil and gas wells. Consequently, it becomes crucial to conduct research on corrosion-resistant cementing materials to mitigate these detrimental impacts [28].
Currently, there are two main methods to solve the problem of cement corrosion caused by acidic medium: (1) Reduce the content of alkaline substances in cement. By adding a type of external filler that reacts with Ca(OH)2 inside the cement, the content of alkaline substances in the cement is reduced. (2) Limit the seepage process to inhibit the corrosion chemical reaction. The pore structure of cement determines the speed at which the corrosive medium penetrates into the cement. To counteract this, certain admixtures can be incorporated to enhance the compactness of the cement. Polymer materials, such as latex, asphalt, and resin, are particularly valuable due to their small particle size, deformability, and corrosion resistance. Therefore, these materials are widely used in the preparation of corrosion-resistant cement slurry systems.
4.2 Self-healing cement
Self-healing cement technology refers to the ability of cement to undergo partial or complete self-repair after sustaining damage. This repair process is facilitated by external factors, as well as inherent conditions, which trigger a series of physical, chemical, and biological reactions. Through these reactions, the cement releases filling materials or generate new substances, enabling the restoration of its integrity [28]. Based on different self-healing principles, this technology can be categorized into four main types: original autonomous self-healing technology, external stimulus-responsive self-healing technology, microcapsule/fiber rupture filling self-healing technology, and microbial metabolite self-healing technology.
Since the self-healing cement is used in the special environment of downhole, the self-healing materials used in the cement slurry must meet the following requirements: (1) The cement sheath formed after the addition of self-healing materials should have mechanical properties that meet the requirements for interlayer isolation. (2) It can make the wellbore withstand the alternating stress and downhole temperature gradient changes during injection and production and meet the technical requirements for long-term hydraulic sealing in the downhole. (3) The performance of the cement sheath after adding the self-healing material should be stable for a long time, and the self-healing material should have good durability. After a certain period of time, the self-healing material can still produce a self-healing effect on micro-cracks [29, 30].
4.3 Expansive cement
Expansion cement for well cementing is a particular type of cement that can expand during the cementing process. It is typically composed of crystal-based expansion materials or gas-releasing expansion agents. Crystal-based expansion materials include calcium aluminate-based crystals and alkaline-earth metal oxide-based materials. The main expansion driving force for calcium aluminate-based materials is calcium aluminate, which includes components, such as sulfates and aluminates. Alkaline earth metal oxide-based materials include CaO expansion agents and MgO expansion agents [31]. Expansion cement plays a significant role in oil and gas extraction by improving cementing effectiveness, reducing wellbore leakage rates, and ensuring the safety and efficiency of the extraction process.
4.4 High toughness cement
The incorporation of toughening materials into the cement slurry results in enhanced impact resistance and compressive performance of the cement annulus, while also inhibiting the development of surface cracks to a certain extent. When external forces are applied to the cement sheath, the toughening materials significantly improve the load-bearing capacity of the cement matrix. Initially, stress primarily acts on the cement matrix, leading to the appearance of surface cracks. However, the crisscrossing fibers within these cracks become critical load-bearing elements, strengthening the material’s ability to handle stress. As stress increases further and exceeds the load-bearing capacity of the fibers, they fracture and detach from the cement matrix. Relevant research shows that the traditional cement sheath will be damaged after 2–10 cycles of cyclic stress. In contrast, cement sheaths with toughening materials can withstand tens of thousands of cyclic stress cycles. In summary, the addition of an appropriate amount of fiber materials during the preparation of cement slurry can influence the microstructure of cement sheath, enhance their stress–strain characteristics, and improve their impact resistance.
5. Cement failure monitoring
On-site cement sheath integrity monitoring is a crucial task in petroleum engineering. Its purpose is to assess the quality and integrity of the cement sheath in the wellbore, ensuring the safe operation of oil and gas wells and environmental protection.
Before beginning well test and production operations, cement-casing system integrity is assessed primarily using cement bond logs and variable-density logs (CBL/VDL). The CBL/VDL tool’s working concept is depicted in Figure 5. Signal attenuation, which has a high correlation to the quality of the cement bond at the cement-casing interface, is recorded by the receiver as a result of changes in the wellbore system. As the signal moves through the wellbore system, the VDL captures the acoustic waveform that reaches the distant receiver. The degree of the bonding quality at the cement formation is shown by the signal attenuation. However, it should be noted that the CBL signal can be affected by various factors, such as the rheological characteristics of the cement slurry, the eccentricity of the cement sheath, the modulus of the cement and casing, and the centralization of the measurement tools. These influences may pose challenges in accurately representing the integrity of a well using typical acoustic logging techniques.
Figure 5.
The operation principle of CBL/VDL tool [
Distributed acoustic sensing (DAS) monitoring technology is an effective method for monitoring the integrity of cement sheaths in downhole environments. This innovative approach utilizes optical fiber sensors to continuously assess various parameters, such as acoustic waves, temperature, and pressure, enabling real-time detection of cement sheath integrity, cracks, and fracturing. Figure 6 shows schematic diagrams of the DAS system. To implement DAS technology, optical fiber serves as a sensor and is excited and detected using lasers and detectors. By analyzing the reflected light signals in the optical fiber, changes in the downhole environment’s parameters can be obtained and assessed. Consequently, the condition of the cement sheath, the presence of cracks, and fracturing can be accurately judged. Compared to traditional monitoring technologies, DAS offers distinct advantages, including a wide monitoring range, high resolution, and real-time solid performance. As a result, DAS has found extensive application in monitoring cement sheath integrity during cementing operations.
Figure 6.
A schematic representation of the DAS sensing unit [
6. Conclusion
The cement sheath integrity plays a critical role in the long-term safe and stable operation of the wellbore. This chapter systematically reviews the influencing factors of cement sheath integrity, prediction and evaluation model, cementing quality control technology, and cement sheath integrity monitoring technology.
The failure modes of the cement sheath primarily consist of tensile failure, shear failure, and interface debonding. These failures can be attributed to various factors, including cementing quality problems, temperature and pressure effects, cement shrinkage, corrosion erosion, and other complex factors. Understanding the failure mechanisms of the cement sheath accurately holds significant importance in improving cementing quality. The cement sheath and interface failure model considering multi-field coupling can reasonably predict the cement sheath damage process under complex loading conditions. The development of novel anti-corrosion, self-healing and other special cementing, and the optimization of construction technology are effective ways to ensure the long-term integrity of cement sheath. The application of monitoring technologies, such as CBL, VDL, USI, DAS, and other means, can realize the monitoring and evaluation of the cement sheath integrity.
The future research on the integrity of cement sheath in well cementing lies in establishing an integrated system for well cementing technology that encompasses multiple-field coupling prediction models, intelligent process monitoring, and new types of adaptive cementing materials.
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