With the declining access to ‘easy oil’ and an increase in energy demand, oil and gas producers are increasingly focusing on producing from more challenging unconventional reserves. This can especially be seen from the widespread proliferation of fracturing operations, during which a production well is purposely hydraulically fractured by injecting fracturing fluids at elevated pressures. However, in many operations, especially in deep reservoirs, casing deformations or cement layer integrity failures are observed. Therefore, it is necessary to thoroughly understand wellbore integrity issues for ensuring safe fracturing operation.

A decay in the mechanical properties of steel and cement is expected after exposure to different operations, pressures, temperatures and fluids over time, which can significantly degrade the thickness and burst pressures ratings of the casing string. In this study, the degree of casing wear developed due to routine operations as well as corrosion and erosion in existing conventional wells is calculated using drilling and completion program datasets. Casing wear depends on several factors such as rate of penetration, rotary speed, wellbore orientation, as well as wellbore wear constants. These parameters are used in various models to calculate the wear volume, which is then correlated to an average percentage reduction in thickness of the casing string. This is then used as an input parameter for a finite-element analysis of casing-cement interaction under different scenarios, especially focusing on stresses in the connections.

Analyses show that long term wellbore integrity does indeed depend on the quality of the casing program, cementing job as well as the minimization of human errors. The design of fracture treatments should therefore consider weak points around the casing and the cementing layer, especially those arising due to casing wear. Based on calculations, casing wear losses of over 40% of the initial volume can occur during the operational life cycle of a well, leading to a considerable decrease in thickness and burst ratings of the casing string. Finite element simulations show that varying stress concentrations in the steel and cement components can lead to fatigue and failure through tension and compression. Depending on the extent of casing wear, stresses above the unconfined compressive strength of common cement formulations are observed. If not accounted for, these lapses can lead to cement integrity issues and a critical well integrity failure.

The results of this work can be used as a simpler field evaluation to determine actual casing and cement properties over the operational lifespan of a well and provide a methodology to assess the effects of wear on the casing, connections and the cement layer. This should be correlated to the integrity of the wellbore under different conditions before starting fracturing operations.

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