HTHP sedimentation stability of water-based completion fluid weighed by ultrafine barite

Abstract

Deep well completion operations are complicated, and the solid phase in the completion fluid tends to settle standing for a long time under high-temperature conditions; thus, sediment or slug would easily form, resulting in downhole accidents. In this paper, a new method for evaluating the settlement stability of completion fluid under high temperature and high pressure (HTHP) was established, which could simulate the temperature and pressure conditions of 300°C and 70MPa, and quantitatively evaluate settlement stability through resistance value. A water-based completion fluid (MWCF) was prepared by modifying the surface of micro powder barite, optimizing stabilizer and dispersant. The rheological properties and sedimentation stability of MWCF with different densities under high temperature, normal pressure, and HTHP conditions were studied. The experimental results showed that its rheology was slightly affected by pressure, and the sedimentation stability was determined by the particle size of the weighted material and the temperature resistance of the dispersant. Based on data analysis, the safety limit chart under HTHP was formulated, which provided a reference for field application. MWCF was successfully used in Well Keshen 21 (185°C, 8098 m) in the Kuche piedmont structure. The settlement factor and resistance value were within safe limits with no barite sag. MWCF has good long-term settlement stability under high temperature and high-density conditions; its composition is simple and can be reused. MWCF meets the needs of HTHP deep well completion and has good application prospects.