A novel oil-in-water nanoemulsion as a high-temperature plugging agent for wellbore wettability alteration and strengthening

Wellbore instability associated with shale hydration and micro-nano pores is one of the most crucial concerns in deep oil and gas drilling operations. In this study, we present a nanoemulsion containing silicates and organosilanes to plug microfractures and mitigate shale hydration by forming a superhydrophobic film in situ on the shale surface via emulsion polymerization and sol–gel process, thereby improving the wellbore stability. At the temperature above 150 °C, the nanoemulsion forms a dense, superhydrophobic film in the pores and on the surface of ceramic sheets, reducing filtration loss by 84.3%. The core showed an increase in strength of 2.86% over the original shale after soaking in the nanoemulsion at 180 °C for 48 h. The shale cuttings covered with film reduced water uptake by 91.56%, thus improving water resistance. The film facilitates to maintain or even improve the rock strength by preventing water absorption and hydration. It also exhibits excellent thermal stability and dynamic water repellency properties at high temperature. We determined that the film consisted of tightly stacked nanofilaments of 5–20 nm in diameter and well-distributed nanospheres of 25–50 nm in diameter, which were identified as crystalline and amorphous symbiotic silica modified by long alkyl chains, respectively. Therefore, in order to overcome the wellbore instability encountered in deep well drilling, we propose a strategy in which the downhole temperature triggers the nanoemulsion to polymerize and seal the pores while simultaneously forming a hydrophobic film on the shale surface. Moreover, it is expected to have practical applications in drilling engineering due to its convenient fabrication, eco-friendly and cost-effective characteristics.