Impact of Cementing Quality on Casing Strength Safety in Coalbed Methane Wells

Abstract

To enhance the structural safety of casings in coalbed methane (CBM) wells, this study develops a finite element model of the casing-cement sheath-formation assembly using ABAQUS software (ABAQUS 6.14). The model systematically investigates the influence of cement sheath defect thickness, defect angle, and internal pressure on the casing stress distribution. The results reveal that the cement sheath defects significantly elevate the casing stress, particularly when the defect is located at the first cementing interface. Casing stress increases most sharply when the defect angle lies between 20° and 60°. Beyond 60°, the stress on the outer wall approaches the yield strength of the casing material. Furthermore, rising internal pressure intensifies stress concentration. When internal pressure exceeds 60 MPa, the outer wall becomes the most likely location for failure initiation. Optimizing the elastic modulus of the cement sheath and employing heavy-wall casing grades such as TP125V can effectively mitigate the casing stress and enhance wellbore integrity. These findings offer both theoretical insights and practical guidance for optimizing cementing design and hydraulic fracturing operations in CBM wells.

1. Introduction

In both conventional and unconventional hydrocarbon developments, cementing quality is widely recognized as a critical factor influencing casing stress and wellbore integrity. Coalbed methane (CBM) reservoirs, typically characterized by low pore pressure, high porosity, and poor permeability, are particularly sensitive to the mechanical integrity of the cement sheath, which plays a key role in ensuring wellbore stability and production safety. Therefore, a thorough investigation into the impact of cementing quality on casing stress is essential for improving completion design and production performance in CBM wells.

Numerous studies have explored the relationship between cementing quality and casing stress, with a focus on cement sheath properties, casing centralization, and cement sheath defects [1,2,3,4,5]. From the perspective of cement mechanical properties, Xi et al. [6] employed a finite element model (FEM) to investigate the effects of different cement properties (elastic modulus, Poisson’s ratio) and the wall thickness of casing on shear deformation. They found that altering cement properties had a minor impact on casing shear deformation, whereas increasing the thickness of the cement sheath effectively reduced deformation. Li [7] proposed the adoption of “high-strength, low stiffness” cement sheath (elastic modulus ≤ 5 GPa) to reduce the casing stress, which subsequently evolved into the development of flexible cement slurries (e.g., hollow glass microsphere-based cement systems). Chen et al. [8] conducted a finite element analysis (FEA) to study the influence of cement sheath on casing bearing capacity under uniform and non-uniform in-situ stress. Their analysis showed that regardless of whether the formation is soft or hard, an ideal cement should have high strength, low elastic modulus, and a small Poisson’s ratio. From the perspective of casing centralization, Huang et al. [9] simulated the stress distribution of cement sheath under both centralized and eccentric casing conditions. Their results demonstrated that under the same conditions, casing eccentricity was more likely to cause cement sheath damage. Ding et al. [10] established a FEA model of the casing-cement sheath-formation system for eccentric casings under cyclic loading, and analyzed the effects of eccentricity and eccentric angle on the micro-annulus. Jiang et al. [11] identified that the cement sheath defect can induce stress concentration, and when the Von Mises stress exceeds the yield strength, radial deformation would initiate.

From the perspective of cement defects, Yu et al. [12] analyzed the impact of cement sheath in deviated sections on casing stress, and found that cement defects can increase casing stress. Zhang et al. [13] studied the effect of cement sheath defects on casing external load in shale-gas stress fields. They reported that the external load reaches the maximum when the casing wear position coincides with the cement sheath defects. Song et al. [14] derived an analytical model under the action of local loads on the cement sheath and calculated the minimum wall thickness required to prevent yield deformation. Wang et al. [15] analyzed the influence of cement defects on casing stress damage in horizontal wells, and concluded that severe stress concentration would form on the inner wall of the casing when the cement defects occur. He et al. [16] established a FEA model of casing-cement sheath-formation system, and studied the effects of in-situ stress magnitude and in-situ stress difference on casing deformation. They argued that poor cementing quality exacerbates the shrinkage of casing diameter and failure. Zhang et al. [17] conducted a probabilistic reliability analysis on casing failure data of deep gas wells, and found that the probability of casing failure increases significantly under poor cementing quality. Fan et al. [18] and Guo et al. [19]. Considered the impact of the drilling and cementing process on in-situ stress, and modified the casing-cement sheath-formation model. Their findings indicated that cementing quality directly affects the stress state of the casing.

In summary, although existing research has achieved significant progress in the field of conventional wells, shale gas, or tight gas reservoirs, there is relatively little research on the impact of cementing quality on casing stress in coalbed methane (CBM) wells. Compared with shale, coal rock exhibits characteristics of low strength, low stiffness, and high porosity [8,12,15,20,21]. Due to the difference in physical properties between coal and shale rock, it is necessary to study the impact of CBM cementing quality on casing stress. Therefore, this paper presents a FEM model of the casing–cement sheath–formation system for CBM horizontal wells, developed using ABAQUS. The model is used to systematically investigate the effects of defect thickness, defect angle, and internal pressure on casing stress distributions. In addition, the feasibility of improving wellbore integrity through optimization of cement sheath mechanical properties and casing design is explored, with the aim of providing a theoretical foundation for cementing design and hydraulic fracturing operations in CBM wells.

2. Analysis of Cementing Quality Issues

Field data indicate that within the well area where DS-1 is located, a total of five wells have been fractured, three of which experienced casing deformation issues. Well DS-1, a representative CBM well, was drilled with a three-section structure, shown in Figure 1a. The surface section reaches 411 m with a 339.7 mm outer-diameter (OD) casing, the second section reaches 3020 m with a 244.8 mm OD casing, and the third section reaches 4070 m with a 139.7 mm OD casing. Subsequently, hydraulic fracturing with proppant was carried out with a maximum wellhead pressure of 120 MPa and a designed pump rate of 18 m³/min. While running a wireline-deployed plug-setting tool, an obstruction was encountered at 3205 m in the horizontal section (red line in Figure 1a).

Figure 1b shows the lead-impression result of the deformed casing section. It can be observed that the Φ112 mm probe shows no imprint on its end face, and the minimum measured dimension at the bottom was 110 mm. Variable density logging (VDL) analysis indicated poor bonding at the second cementing interface over the interval 3195–3210 m. It is therefore inferred that the observed casing deformation is linked to substandard cementing quality, most likely caused by mud stratification or sedimentation related to formation characteristics or poor mud properties. Localized absence of the cement sheath would have left the casing poorly supported and subject to high localized stresses.

3. FEA Model

In accordance with actual field conditions, a FEM model of the casing-cement sheath-formation assembly was established using ABAQUS software to represent the horizontal section of Well DS-1 under in-situ conditions.

The model is based on three assumptions [22]: (i) all interfaces (casing–cement and cement–formation) are in perfect contact without gaps between them; (ii) both cement sheath and formation behave as linearly elastic, isotropic materials; (iii) the casing, cement sheath, and borehole are perfectly concentric. During model construction, the borehole diameter and casing dimensions were strictly matched to those used in actual drilling operations. The casing has an outer diameter of 139.7 mm with a 10.54 mm wall thickness. According to Saint-Venant’s principle, when the dimensions of the formation model exceed five to six times the borehole radius, the influence of far-field stresses becomes negligible. To sufficiently minimize the boundary effect, a cubic domain of 3 m × 3 m × 3 m—approximately ten times the borehole diameter—was adopted for the formation in this study [23]. Based on field data and laboratory measurements, the formation was assigned an elastic modulus of 7 GPa and a Poisson’s ratio of 0.22. The cement sheath was characterized by an elastic modulus of 5 GPa and a Poisson’s ratio of 0.13. The casing, manufactured from TP140V steel grade, has an elastic modulus of 210 GPa, a Poisson’s ratio of 0.30, a yield strength of 965 MPa, an ultimate tensile strength of 1165 MPa, and an elongation at fracture of 23% [24].

In-situ stress magnitudes were derived from well-log interpretations, yielding maximum horizontal, minimum horizontal, and vertical stresses of 61.07 MPa, 53.66 MPa, and 64.82 MPa, respectively. These values were applied as initial stress conditions within the formation using ABAQUS’s predefined field option [25]. The bottom surface of the formation was fixed, and normal displacements were constrained on the remaining surfaces. During the fracturing simulation, internal pressures ranging from 40 to 120 MPa were incrementally applied to the inner surface of the casing. The surface-to-surface contact was defined between the casing and cement sheath, as well as between the cement sheath and the formation. A friction coefficient of 0.6 was adopted for both interfaces to simulate the realistic contact conditions [16,23,25]. The FEA model was discretized using structured eight-node hexahedral elements (C3D8). A graded meshing strategy was employed, following the principle that the highest mesh density is at the casing and coarser elements gradually grow outward. The FEM model of the formation-cement sheath-casing assembly and its mesh division are shown in Figure 2.

4. Results

4.1. Verification

To verify the correctness of the finite element model, this paper considered the casing-cement sheath-formation (without cement defects). Following the analytical procedures widely reported in the literature [17,26,27,28,29], this paper established a mechanical model of the casing-cement sheath-formation system under the non-uniform stress field. According to the non-uniform stress field and boundary conditions in Section 3, the casing stress under the ideal scenario was calculated.

The circumferential stress distribution on the casing was extracted and compared with the results of the FEA model of this paper. It can be observed from Figure 3 that the variation trends of casing stress calculated by the two methods are generally consistent. The maximum stress calculated by the theoretical method is 103.9 MPa, while that derived from the FEA model is 100.8 MPa, yielding a difference of 3.1 MPa and an error of 3.1%. Therefore, the accuracy of the finite element model in this paper was verified.

4.2. Influence of Defect’s Thickness

Typical defects consist of local cement absence adjacent to either the first-cementing interface (first CIF) or the second-cementing interface (second CIF). Assuming the outer and inner radius of the cement sheath are $R$ and $r$, then the radial defect thickness is $\gamma =∆r/(R-r)$, where $∆r$ is the defect’s thickness. The influence of cement sheath radial defects on casing stress is analyzed when γ equals 0, 1/6, 1/3, 1/2, 2/3, 5/6, and 1, respectively.

Figure 4 and Figure 5 show the casing stress for varying $\gamma$ at the first CIF and second CIF. It can be observed that once the cement sheath defect occurs, there is a stress concentration phenomenon on the outer wall of the casing at the location of the missing cement sheath, while the inner wall exhibits stress concentration both at the location of the defects and at the junction of the missing cement sheath. Furthermore, when γ is relatively small, the maximum stress on the inner wall is located at the defect site. Whereas γ gradually increases, the location of maximum stress on the inner wall gradually shifts toward the junction of the cement sheath defects.

Figure 6 illustrates the variation of the maximum stress in the casing under different defect thicknesses. It can be seen that, compared to the case without defects, the absence of the cement sheath leads to a significant increase in the stresses on both the inner and outer walls of the casing. As γ gradually increases, the casing stress progressively increases both at the first CIF and second CIF. However, there are differences in the stress variation patterns between the two cases. When the cement sheath is missing at the first CIF, the casing stress is less sensitive to the defect thickness, showing a limited increase as the defect thickness increases. For example, when γ increases from 1/6 to 1, the stresses on the outer and inner walls of the casing increase by 54 MPa and 65 MPa, respectively. In contrast, when the cement sheath is missing at the second CIF, the casing stress is more sensitive to the defect thickness, showing a significant increase in casing stress as the thickness increases. When γ increases from 1/6 to 1, the stresses on the outer and inner walls of the casing increase by 241 MPa and 129 MPa, respectively. Overall, under comparative conditions, the casing stress level when the cement sheath is missing at the first CIF is higher than that at the second CIF. It is evident that the absence of the cement sheath at the first CIF has a significant impact on the casing stress. Therefore, the subsequent analysis adopts the condition that the cement sheath defect occurs at the first CIF. These findings are consistent with those reported in the reference [15]. Considering the special characteristics of coal, more attention should be paid to the cementing quality at the first CIF instead of the second CIF.

4.3. Influence of Defect Angle

Based on the field data, the missing angle may vary from 0° to 100°. The stress changes of the inner and outer walls of the casing under different defect angles are shown in Figure 7. Globally, the maximum stress on the outer wall of the casing always occurs at the location where the cement sheath is missing. while for the inner wall, the maximum stress on the inner surface is also located at the defects. But, with the gradual increase of α, the maximum stress of the inner wall is gradually transferred to the junction of the cement sheath defects. At the same time, due to the lack of cement sheath protection, the cross-sectional shape of the casing gradually changed from an oval to a pear shape under the action of in-situ stress and internal pressure.

Figure 8 presents the curves of maximum stress on the inner and outer walls of the casing under different defect angles. As α increases from 0° to 100°, the casing stress initially increases and then tends to stabilize. This indicates that poor cementing quality can lead to defects, which worsen the stress condition of the casing and increase the risk of stress-induced failure. Notably, when α does not exceed 10°, the stress on the inner surface is greater than that on the outer surface, but the stress levels remain relatively close to those under the no-defect condition. Once α exceeds 10°, the stresses on both the inner and outer surfaces increase rapidly, with the outer surface stress surpassing the inner surface stress. When α exceeds 60°, the maximum stress on the outer surface no longer increases significantly and reaches the yield strength of the TP140V casing, 965 MPa. After α exceeds 80°, the maximum stress on the inner surface also does not change significantly, reaching the yield strength. The above trend demonstrates that as α increases, the outer wall of the casing first reaches the yield strength of the casing, and therefore undergoes strength failure earlier.

The above demonstrates that the quality of cementing has a great influence on the stress and deformation failure of casing. Therefore, during cementing operations, attention should be paid to improving the displacement efficiency of the drilling fluid, in order to minimize cement sheath defects. This conclusion is consistent with the findings reported in the reference [27].

4.4. Influence of Internal Pressure

After cementing operations are completed, high-pressure fluid is typically pumped into the formation to perform volumetric fracturing, which correspondingly alters the internal pressure of the casing. According to the limits of internal pressure of field conditions, the internal pressure can change from 40 MPa to 120 MPa. Therefore, this section examines the influence of internal pressure on casing stress under the condition of γ = 1 and α = 30°.

Figure 9 illustrates the distribution of the casing stress under different internal pressures. As the internal pressure increases, the overall casing stress rises, particularly the stress at the junction of the cement sheath defects. When the internal pressure is below 90 MPa, the maximum stress on the inner surface occurs at the defects. As the internal pressure increases further, the location of maximum stress on the inner surface gradually shifts toward the junction of the cement sheath defects. In contrast, the maximum stress on the outer surface consistently remains at the defects.

Figure 10 presents the maximum stress curves of the inner and outer walls under different internal pressures. The results indicate that as internal pressure increases, the stress on the outer wall increases approximately linearly, while the stress on the inner wall initially decreases and then increases. When internal pressure is less than 60 MPa, the stress on the outer wall is lower than the inner-wall stress, indicating that in-situ stress plays a dominant role. When internal pressure exceeds 60 MPa, the outer-wall stress surpasses the inner-wall stress, suggesting that the internal pressure becomes the dominant factor, gradually reducing the effective external load on the casing. Under the current engineering parameters, the maximum stress does not exceed the casing yield strength, indicating that the casing remains safe even in the presence of cement sheath defects.

5. Discussion

Section 4 analyzed the influence of cement sheath defects and internal pressure on casing strength safety. In field operations, the long cemented sections in CBM wells can adversely affect the displacement efficiency of the drilling fluid, and easily creates localized cement defects. Therefore, how to minimize the above impact is a major concern. In the following, we further discuss practical measures for improving cementing quality and their influence on casing strength safety.

5.1. Adjust the Cement Parameters

During cementing, different additives can be used to alter the mechanical properties of the cement, thereby modifying the stress state of the casing. This section examines the influence of cement parameters on the maximum casing stress under both ideal conditions (no cement defects) and defective conditions.

As illustrated in Figure 11, when the cement sheath is perfectly intact, the casing stress decreases progressively as the elastic modulus of the cement (E) increases. This occurs because a higher elastic modulus can enhance the collapse resistance of the cement sheath, allowing it to absorb more stress from the casing. The radial deformation of the cement sheath therefore becomes smaller, its shielding effect on the casing is enhanced, and the assembly can better resist external formation loading. For instance, at a Poisson’s ratio of 0.16, when the elastic modulus E increases from 3 GPa to 30 GPa, the maximum casing stress decreases by 98 MPa. Under the condition of γ = 1, α = 30°, the maximum stress of the casing is similar to that in the intact case. When the Poisson’s ratio is 0.16, the same increase in modulus increment (3 GPa to 30 GPa) now reduces the peak casing stress by 153 MPa—an even larger reduction than in the intact case. Thus, a high-modulus cement is comparatively more beneficial when the cement sheath is present. When the elastic modulus is constant, varying the Poisson’s ratio has a minimal effect on casing stress. For example, when the elastic modulus is 20 GPa, adjusting the Poisson’s ratio from 0.13 to 0.19 can reduce the maximum casing stress by only 3.4 MPa. Therefore, during cementing operations, it is advisable to utilize cement with a relatively low Poisson’s ratio, while excessive emphasis on optimizing the Poisson’s ratio performance is not necessary.

In summary, priority should be given to selecting cement formulations with a higher elastic modulus, as this can effectively improve casing stress distribution and reduce the risk of stress-induced failure in CBM wells. This conclusion is consistent with the findings reported in references [1,3,27], but contradicts the understanding of soft formations described by Li et al. [8] and Chen et al. [9].

5.2. Optimal Casing Selection

With reference to the casing types commonly used in the horizontal sections of CBM wells—namely P110 (Φ139.7 × 7.72 mm), TP125V (Φ139.7 × 12.34 mm), and TP140V (Φ139.7 × 10.54 mm), the evolution of casing stress under the defect condition γ = 1, α = 30° was investigated for different thicknesses. The elastic modulus and Poisson’s ratio of the three casings are all 210 GPa and 0.3.

As shown in Figure 12, the stress change law of the casing is as follows: TP125V < TP140V < P110. Under the existing engineering parameters, the maximum stresses are 831 MPa for P110, 417 MPa for TP125V, and 493 MPa for TP140V. The maximum stress in the TP125V is therefore 414 MPa lower than in P110 and 76 MPa lower than in TP140V. This demonstrates that, for the same material properties, the larger the casing wall thickness, the less stress the casing is subjected to, and the less impact of cementing quality on the casing safety. This can be attributed to the fact that greater wall thickness can enhance casing stiffness and improve resistance to external loads, thereby reducing stress concentrations.

In summary, the choice of casing with a larger wall thickness (such as TP125V) in CBM wells is an effective and economic way to reduce the impact of cementing quality on casing strength and safety.

6. Conclusions

(1) This study establishes a finite element model specific to CBM wells, demonstrating that cement sheath defects—particularly at the first cementing interface—significantly amplify casing stress, with defect angles between 20° and 60° posing the highest risk of casing yielding.

(2) Increasing internal pressure shifts the failure mode, with outer-wall stress dominating beyond 60 MPa, highlighting the critical need for pressure management in CBM stimulation designs.

(3) Utilizing cement with a high elastic modulus and selecting thick-walled casing (e.g., TP125V) are proven effective strategies to mitigate stress concentration and enhance wellbore integrity under defective cement conditions.

(4) The research provides practical, actionable guidance for cementing and completion design in CBM reservoirs, addressing a critical industry need and offering a foundation for further studies on long-term cement-formation interaction in weak coal formations.