Nanoparticles Improved Sedimentation Stability of Oil-Based Completion Fluids while Drilling Ultra-Deep Wells

Abstract

Abstract The deep and ultra-deep oil and gas resources are important sources to meet the increasing oil and gas demands. But they are difficult to be explored because their buried geological conditions are extremely complex, such as high temperature (HT), high pressure (HP), thick salt and gypsum beds and borehole collapse. Oil-based drilling fluid (OBDF) is the preferred drilling fluid for ultra-deep wells due to its better wellbore stability and higher lubricity than water-based drilling fluid. Meanwhile, the oil-based completion fluid (OBCF) is also the first choice for operators because it can be turned directly by the OBDF, which is convenient and reusable. However, barite sag is often encountered because the completion fluid is required to be static for 7 to 15 days at a high temperature of more than 180°C, which results in serious problems, pipe sticking, density variation and well control problems, in ultra-deep wells. It is urgent to improve the sedimentation stability of OBCF while drilling the ultra-deep wells. In this paper, a modified nano-silica stabilizer was prepared to improve the sedimentation stability of OBCF. Its dispersity in a water-in-oil (W/O) emulsion with an oil-water ratio of 80:20 was observed by a cryo-scanning electron microscope and the viscosity at low shear rates was also tested. Then OBCF formulations with different densities were developed and modified by the prepared nano-silica stabilizer. The rheology and sedimentation stability were evaluated on HTHP rheometer and sedimentometer, respectively. The HTHP sedimentometer can work at a pressure up to 105 MPa and a temperature up to 300 °C. Results indicated that the novel nano-silica stabilizer could disperse stably in the water-oil interface and in the oil phase. The viscosity of the W/O emulsion at low shear rates was significantly increased after adding the nano-stabilizer. Meanwhile, the HTHP rheology and sedimentation stability of the high-density OBCF were improved by the nano-stabilizer due to its surface and size effects, and the HTHP sedimentation stability of the oil-based fluids was positively related to its HTHP rheological stability. The modified OBCF by the new stabilizer was successfully applied in a deep well with a temperature more than 200°C in China. The completion operation was smooth without downhole issues, and the casing was run to the bottom hole at once. The oil-test operation was perfectly completed for about two months, and no barite sag happened. In a word, the modified OBCF by the new nano-silica stabilizer in this work possessed excellent HTHP rheology and sedimentation stability, expecting to be applied widely in deep and ultra-deep wells.