Survival of hollow glass microspheres in drilling fluids applications - Effect of drill bit/formation contact

This study concentrates on materials called hollow glass spheres (HGS), also known as glass bubbles, which are used to reduce the drilling fluid density below the base fluid density and overcome the shortcomings of compressible drilling fluids. The most important physical properties of the microspheres for oil field applications are their low specific gravity and high collapse strength. The lower the specific gravity, the greater their density reduction capabilities, meaning that less product is required to achieve the drilling fluid target density. High crush strength (and/or survivability) is equally important, as maintaining the target density is key to achieve and maintain bottom hole pressure control. The performance of drilling fluids made with HGS was evaluated for different drilling weight on bit (WOB) and rotation speed (RPM) values while controlling the flow rates. Experimental results show that when the HGS particles are subjected to mechanical contact between bit and formation, they will break. However, it was observed that the majority of HGS will not be exposed to contact between the cutter element and formation. Instead, they pass through the nozzle exit, leave the bit face region in a short period of time, and go directly into annulus, with minimum time of expose for contact. A mathematical model was developed to quantify the amount of HGS that become in contact between bit and formation, depending on various operational parameters, including effective blade length, flow rate and RPM. Finally, results from experiments and estimation models were compared, suggesting that the studied HGS successfully reduce the density of the base drilling fluid, and can survive the tested conditions. In summary, reducing the density of liquid based incompressible drilling fluids with glass bubbles is an alternative to available compressible underbalanced drilling fluid options at depleted zones or offshore wells with a very narrow density window.