Effect of Casing Stand-Off on Cracks Creation Around the Wellbore
Authors: Agofack N. , Larsen I. , Opedal N. , Skorpa R. 
Published: September 2022

ABSTRACT:
Cement sheath plays important role for the integrity of injection/production wells in subsurface activities. During the life of the well, mechanical and thermal loadings are applied to the casing and can lead to cracking the cement sheath and thus the loss of integrity. Additionally, the placement of the cement can also lead to the casing stand-off. To analyze these effects a modified discrete element method (MDEM) is used. Realistic cement and formation properties are used. Isotropic and anisotropic boundary stresses are investigated. The casing pressures considered are relevant for field operations such as during a casing test, XLOT (extended leak-off test) and hydraulic fracturing. The simulation results, show that in addition to the casing stand-off, several other parameters such as the casing pressure, the boundary stress, the cement, and rock properties, affect the cracks behind the casing. It was also observed that the casing stand-off become important in affecting the crack creation when it is around 0%. For values more 50%, no major difference was observed when compared to a centralized casing (100% stand-off).

1. INTRODUCTION
In many subsurface activities including hydrocarbon exploitation, carbon dioxide (CO2) or hydrogen storage, the injection/production wells play important role. These wells are made by drilling a hole in the ground, running a steel tubular (called casing) into that hole, and pumping a cement slurry to fill the space between the outer casing and the rock formation (Figure 1).
The cement slurry hardens to become the cement sheath and play an important role as sealing barrier. However, during the above-mentioned subsurface activities, mechanical and thermal loadings are applied to casing and thus to the cement sheath and rock formation around. These can lead to cracks creation in the cement sheath which can propagate into the rock formation, and therefore constitute potential leakage paths (Bois et al., 2011; Petty et al., 2003). Additionally, the placement of the cement can also lead to the casing stand-off vis-à-vis to the cement sheath (De Andrade and Sangesland, 2016; De Andrade et al., 2014; Khodami et al., 2021). The casing stand-off is evaluated by the following formular (De Andrade and Sangesland, 2016; Mendez Restrepo et al., 2018; Weatherford, 2016):
(Equation)
where A and B are the radius of the hole and the outer radius of the casing, respectively, and C the smallest size between the casing and the hole wall, see Figure 2. A centered casing has stand-off of 100% while the extreme off centered one has a stand-off of 0%. In some regulations, the minimum stand-off of the casing should be 70% (Queensland-Government, 2019). To achieve that, the casing centralizer are used during the running of the casing into the well (Juvkam-Wold and Wu, 1992; Lee et al., 1986; Weatherford, 2016). However, even with the used of centralizers, the stand-off of the casing still be less than 100%. The stand-off may affect the integrity of the well following mechanical and thermal loadings.