Effect of High-Density Water-Based Drilling Fluid on the Mechanical Properties of the Drilled Formation in Horizontal Wells Available to Purchase

During drilling operations, the wellbore stability is affected by many factors. One of the most significant factors that degrade the wellbore stability is the change in the mechanical properties of the drilled formation because of the interaction with the drilling fluid filtrate. This paper studies the effect of high-density water-based drilling fluid on the mechanical properties of the drilled formation in horizontal wells, where the exposure time of the formation to the drilling fluid may reach up to 20 days based on the length of lateral. High-pressure high-temperature (HPHT) filtration test was used to expose real sandstone and limestone core samples to the drilling fluid for different time periods, 30 min, 1 day, and 2 days at 300 psi differential pressure and 250°F. The HPHT cell was modified to be used with real sandstone and limestone core samples (1.5” diameter and 2” length). Mechanical properties were measured, using the standard unconfined uniaxial compression test and ultra-sonic pulse velocity test, before and after filtration tests. The results of this study showed that, drilling fluid after different exposure time had no impact on Poisson's ratio for both sandstone and limestone core samples. Sandstone core samples showed significant increase in Young's modulus and compressive strength with the increase in exposure time that can be attributed to the polymer residuals from fluid filtrate invasion. A reduction in limestone core samples strengths was observed. Young's modulus and compressive strength decreased as the exposure time increased. This study highlights the importance of drilling fluid invasion in drilling operations especially in horizontal wells as the variation in exposure time plays a great role in changing the mechanical properties of the drilled formations that may lead to failure in drilling and production operations.