Mineral oil-based drilling fluid formulation using biosurfactant and nanoparticles with good rheological behavior and excellent H2S scavenging capability

There are two commonly used drilling fluids, namely water-based muds (WBMs) and oil-based muds (OBMs); however, the latter type is more desirable for drilling unconventional oilfield reserves. To account for the potential encounter of hydrogen sulfide (H₂S) while drilling, the utilized OBMs should contain scavenger(s) with an effective H₂S mitigation capability in order to in-situ capture this very lethal and corrosive gas. To the best of our knowledge, studies on incorporating H₂S scavengers in OBMs and their testing are still greatly lacking in open literature. Thus, this study contributes into the filling of this gap by preparing a mineral oil-based drilling mud (MOBM) containing potassium permanganate as a promising, widely available, safe, and cheap H₂S scavenger. The MOBM also comprised other ingredients including rhamnolipid biosurfactant as an emulsifier and octadecanethiol-modified (i.e., hydrophobized) zinc nanoparticles (serving as weighting agent). These materials have not been widely utilized so far in open literature for the preparation of MOBM. The results obtained from this study demonstrated that this mud could fully scavenge H₂S for up to 22.7 h (i.e., breakthrough time), and it took about 63 h for the MOBM to become fully saturated with H₂S. The scavenged amounts of H₂S at these times reached 324.4 and 485.8 g/barrel MOBM, respectively. The formulated MOBM also displayed an appropriate non-Newtonian shear thinning behavior, where the apparent viscosity dropped sharply from about 1.96 to 0.71 Pa.s upon increasing the shear rate to from 1 to 10 s⁻¹, followed by a gradual decrease down to 0.31 Pa.s at a shear rate of 1000 s⁻¹. Additionally, the formulated mud is able to dissipate a significant amount of thermal energy as inferred from its estimated high activation energy of 34.93 kJ/mol, suggesting a good thermal stability of the MOBM. The present study reveals the possibility of formulating mineral OBMs with effective H₂S for safely drilling sour oil and gas reservoirs.