Theoretical Analysis of the Micro Annulus of an Oil-Well Cement Sheath Formed via Cooling under Acid-Fracturing Conditions

Abstract

The plastic deformation and interface micro annulus of oil-well cement during acid fracturing are key reasons for the failure of the wellbore seal and sustained casing pressure. However, most of the existing research ignores the influence of the wellbore cooling effect during acid fracturing, owing to which, the design conditions in the theoretical analysis may be inconsistent with those of the actual wellbore, and the calculation results may be biased. In this study, a novel elastoplastic mechanical model of the cement sheath was established. This model can analyze the yield state of the cement sheath under the influence of three-dimensional principal stress and consider the effect of the differential temperature stress on the interface debonding of the cement sheath from the beginning to the end of acid fracturing. Moreover, the generation mechanism and development law of the interface micro annulus were clarified. The findings indicated that the influence of the intermediate principal stress cannot be ignored; otherwise, the elastoplastic analysis results of the cement sheath may be conservative. During acid fracturing, the casing–cement sheath interface is influenced by the differential temperature stress, and the interface is debonded; however, a micro annulus is not generated. The debonding of the cement sheath–formation interface and micro annulus occurs only when the cement sheath is completely plastic. After acid fracturing, the interface micro annulus is likely to be generated at the casing–cement sheath interface, and the presence of the differential temperature stress may increase the formation risk of the interface micro annulus. The research results can provide theoretical guidance for the prediction of oil-well cement sheath interface seals under acid-fracturing conditions.

Analytical Perspectives on Cement Sheath Integrity: A Comprehensive Review of Theoretical Research

“…Su et al 88 investigated the effects of differential temperature stresses on the debonding of cement sheath interfaces under acid-fracturing pressures and developed a novel elastic-plasticity mechanical model for the cement sheath. The study indicated that under acid-fracturing pressures, the casing-cement sheath interface is influenced by differential temperature stresses, which leads to interface debonding, but does not form micro annulus.…”

Failure Analysis of Cement Sheath Mechanical Integrity Based on the Statistical Damage Variable

“…The forces on the two interfaces can be decomposed as follows: (4) To get the interaction forces p 2 and p 3 of the cement sheath, casing and surrounding rock under subsequent wellbore operations from eq 4, the increments Δp 2 and Δp 3 must be obtained first. The casing-cement sheath-surrounding rock composite model is established for the interfacial force increment (see the right side of the equation in Figure 5).…”

“…Portland cement stone is a brittle material with congenital defects. During the drilling and production of oil and gas wells, under the action of the surrounding rock stress and wellbore stress, , the cement sheath is easily broken to form macroscopic cracks, which become gas channeling paths and cause sustained casing pressure. − Maintaining the sealing integrity of the cement sheath is the key to ensuring the long-term safe production of oil and gas wells, and how effectively evaluating the sealing performance of the cement sheath is a basic problem that needs to be solved urgently . At present, the direct way is to evaluate the damage and failure mode of the cement sheath through annulus isolation simulation experiments.…”

Enhancing Environmental Protection in Oil and Gas Wells through Improved Prediction Method of Cement Slurry Temperature

“…This may create flow paths in the annular space between the casing string and the borehole walls. In petroleum terminology, this phenomenon is referred to as "flow behind cement", "gas migration", "annular migration", or, more recently, "sustained casing pressure (SCF)" [8][9][10]. After this phenomenon occurs in gas wells, the wellhead pressure continues to rise as the gas continues to move up and accumulate at the top of Energies 2023, 16, 4852 2 of 17 the wellbore [11][12][13][14].…”