ABSTRACT

Maintaining the integrity of the annular cement in the wellbore is paramount in successful hydrocarbon exploitation, subsurface energy storage, geothermal energy production, and geologic carbon sequestration. Debonding at the casing-cement interface can create connected flow paths for fluid leakage along the well leading to loss of zonal isolation. Reliable estimates of potential well leakage rates require large-scale experiments at representative wellbore conditions. We investigated the behavior of the cement microannulus under various loading conditions on two-meter long casing segments cemented against a rock analogue. The results show that once a microannulus forms, it remains open at casing pressures as high as 40 MPa. The normal stiffness of the microannulus at the casing-cement interface ranged between 50 and 900 GPa, while the shear stiffness ranged between 0.15 and 0.22 GPa. Axial displacement of the casing did not lead to a significant change in the aperture. However, axial loading in presence of a casing coupling reduced the hydraulic aperture. The results of this work indicate an agreement between experimental leakage rates, model predictions, and leakage rates measured at (abandoned) well sites reported in the literature. The laboratory results on the large-scale samples provide benchmark data for validating well integrity models.

1. INTRODUCTION

Wellbores are commonly used as a fluid conduit for applications such as water, gas, and oil production, or waste, energy, hydrogen, and CO2 storage. Drilling a well through various strata creates a risk of fluid communication between different formations. This can lead to contamination of the subsurface water resources or leakage of fluids to surface (Viswanathan et al., 2008; Vidic et al., 2013). To mitigate the risk of leaks, steel casings are placed in a wellbore at several depths. A cement slurry is pumped through the casing which subsequently flows through the annular space between the casing and the formation. Once the cement is set, it provides structural support to the casing and acts as a barrier to flow in the annulus. It is critical to ensure the integrity of the annular cement. This includes ensuring appropriate type of cement is used for the downhole pressure, temperature, and chemical environment (Bennett, 2016). Subsequent well operations could also damage the cement by causing expansion or contraction of the casing resulting from a change in temperature or pressure (Bois et al., 2012). This has been of interest for energy storage, geothermal energy, and carbon sequestration projects, particularly when targeting old oil and gas wells (Zhang and Bachu, 2011; Davis et al., 2014; Miyazaki, 2009). A damaged cement sheath can create a permeable zone around the casing that allows fluids to move upwards. This also exposes the casing to chemical reactions with downhole fluids (Gill et al., 2012) Therefore, it is critical to understand the well conditions that can lead to cement damage and the resulting rate of fluid leakage.

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