The operational use of nanoparticles (NPs) in drilling and completion fluids is still limited at the present time, in part due to lack of consistent evidence for - and clarification of - NP interactions with rock formations, formation fluid, and other fluid additives. For instance, previous fluids research has emphasized that NPs bring about "pore plugging" that reduces pressure transmission, and in turn fluid inflow, into the shale pore matrix which ultimately helps stabilize the borehole. However, it is difficult to understand how pore plugging might be accomplished in the absence of any considerable filtration in shales considering the very low permeability of shales does not allow for any appreciable Darcy flow. This paper addresses the crucial question: "how, when, why do nanoparticles plug up shale pore throats?"

Zeta Potential (ZP) measurements were carried out on the aqueous dispersions (NPs) and on intact shale thin sections exposed to the nanofluid in order to determine the degree of interaction behavior between NPs and shales. The experimental data was then used to calculate DLVO curves (describes the force between charged surfaces interacting through a liquid medium) in order to determine if the total potential energy was sufficient for NP's to diffuse through the repulsive barrier and attract (or overcome repulsion) to the shale surface. Estimated DLVO curves are used to demonstrate the NP's ability to contribute to borehole stability but are not directly correlated, and therefore, NP effects on shale stability were studied in detail using pore pressure transmission tests (PTT), which measure fluid pressure penetration in shales, and modified Thick Wall Collapse (TWC) tests, which explore the influence of NPs on the collapse pressure of shale samples.

Our investigation shows that NPs can reduce fluid pressure penetration and delay borehole collapse in shales, but only under certain conditions. Electrostatic and electrodynamic interaction between NP's and shale surfaces, governed by DLVO forces, is the main mechanism that will lead to pore throat plugging, reducing pressure transmission, which in turn benefits borehole stability by slowing down near-wellbore pore-pressure elevation and effective stress reduction. For Mancos shale, it was shown that 20 nm nanosilica (anionic) are effective in partially plugging the pore throat system, depending on the pH of the nanofluid, which affects the surface potential and ZP of both NPs and shale. Furthermore, the positively charged nanosilica (cationic) showed better results for pore-plugging capabilities than the anionic nanosilica.

The findings lead to some interesting challenges for the practical field application of NP-based drilling fluids for borehole stability, given that efficacy will depend on the specific type of shale, the specific type, size and concentration of NP, the interaction between NP-shale, and external factors such as pH, salinity, temperature etc. NP use for practical shale stabilization therefore requires a dedicated, thoroughly engineered solution for each particular field application, and is unlikely to be "one size fits all".

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