Decreasing Water Invasion into Atoka Shale Using Non-modified Silica Nanoparticles

摘要

Fluid penetration from water-based muds into shale formations results in swelling and subsequent wellbore instability. Particles in conventional drilling fluids are too large to seal the nano-sized pore throats of shales and to build an effective mud cake on shale surface and reduce fluid invasion. This paper presents laboratory data showing the positive impact of adding commercially available, inexpensive, non-modified silica nanoparticles (particle sizes vary from 5 nm to 22 nm) to water-based drilling muds and their effect on water invasion into shale. Six brands of commercial and non-modified nanoparticles (NP) were tested and screened by running a three-step pressure penetration test (brine - base mud - NP mud). Two types of common water-based muds (a bentonite mud and a low solids mud, or LSM) in contact with Atoka shale were studied with and without the addition of 10 wt% nanoparticles. We found that a large reduction in shale permeability was observed when using the muds to which the non-modified NPs had been added. For the bentonite muds, the permeability of Atoka shale decreased from 57.7% to 99.3% and for the low solid muds, the permeability of Atoka shale decreased from 45.67% to 87.63%. Higher plastic viscosity (PV), lower yield point (YP) and fluid loss (FL) of the NP muds compared with base muds were also observed by the addition of NPs. We also found that nanoparticles with size varying from 7 nm to 15 nm are more effective plugging agents than larger nanoparticles for the Atoka shale. This study shows for the first time that it is possible to formulate water based muds using inexpensive non-modified and commercially available silica nanoparticles and that these muds significantly reduce the invasion of water into the shale. The addition of silica nanoparticles to water based muds may offer a powerful and economical solution when dealing with wellbore stability problems in troublesome shale formations.