How does the drilling of deviated or horizontal wells impact the stress regime around the borehole in geothermal environments?

The drilling of deviated or horizontal wells significantly alters the stress regime around the borehole in geothermal environments compared to vertical wells. The stress regime around a borehole is influenced by the in-situ stresses (the stresses that exist in the rock before drilling) and the orientation of the borehole relative to these stresses. Drilling a vertical well creates a relatively symmetrical stress concentration around the borehole, with the maximum stress concentration occurring at the borehole wall in the direction of the maximum horizontal stress. However, when a well is deviated or drilled horizontally, the stress distribution becomes more complex. The borehole intersects the in-situ stress field at an angle, which results in an asymmetrical stress concentration around the borehole. The magnitude and orientation of the stress concentration depend on the borehole's inclination and azimuth relative to the principal stress directions. For example, a horizontal well drilled parallel to the minimum horizontal stress will experience a different stress regime than a horizontal well drilled parallel to the maximum horizontal stress. Deviated and horizontal wells can also be more susceptible to borehole instability, such as borehole breakout and shear failure, because the increased stress concentration can exceed the rock's strength. This is particularly important in geothermal environments, where the rocks are often weakened by high temperatures, hydrothermal alteration, and natural fractures. Furthermore, the drilling fluid pressure in deviated and horizontal wells can have a significant impact on the stress regime. Maintaining the correct drilling fluid pressure is essential to prevent wellbore collapse or fracture initiation. Careful planning and wellbore stability analysis are crucial when drilling deviated or horizontal wells in geothermal environments to minimize the risk of borehole instability and ensure successful well completion.