Mechanism of cement sheath sealing degradation in multistage fracturing of deep coalbed methane

Under multi-stage fracturing conditions for deep coalbed methane, the cement sheath of the upper fracturing section is subjected to a complex loading environment. The high pressure within the wellbore, combined with multiple cycles of alternating loads, can induce damage to the cement sheath, thereby degrading its sealing performance. This issue significantly hinders the safe extraction and efficient development of deep coalbed methane. Based on the stress distribution characteristics of the cement sheath in the upper fracturing section of deep coalbed methane, the study developed a mechanical equivalent model. By considering the operational state of the downhole casing - cement sheath - coalbed methane system and integrating the evaluation instrument for cement sheath sealing integrity, an equivalent physical simulation experimental method was established to assess the sealing performance of the cement sheath under multi-stage fracturing conditions. Triaxial loading-unloading cyclic tests, scanning electron microscopy, nuclear magnetic resonance measurements, and stress evaluation of the cement sheath were performed to investigate the sealing degradation mechanism of the cement sheath in the upper fracturing section under multi-stage fracturing conditions of deep coalbed methane. Research indicates that the cement sheath in deep coalbed methane exhibits excellent load-bearing capacity and can endure a 165 MPa internal casing pressure without sustaining damage. Under a cyclic multi-stage fracturing load of 75 MPa over 45 cycles, the cement sheath at the upper part of the deep coalbed methane fracturing section retains its sealing capability. However, multiple non-penetrating axial cracks have developed. The low elastic modulus of deep coalbed influences the stress distribution in the cement sheath. Under multi-stage fracturing conditions, stress concentration at the internal pores of the cement sheath leads to the initiation of damage, which accumulates as the number of fracturing stages increases. This phenomenon is the primary cause of localized cracking in the cement sheath and the elevated risk of sealing degradation in the top fracturing section. The research findings present a novel approach for assessing the sealing performance of cement sheaths in multi-stage fracturing for deep coalbed methane. Additionally, these results provide critical theoretical support for advancing the understanding of the mechanisms underlying the sealing degradation of cement sheaths in such operations.