Systematic Evaluation of Sedimentation Stability and Gel Strength of Water-Based Testing Fluid Under Multiple Downhole Factors

Sedimentation stability and gel strength are critical performance metrics of water-based testing fluids, yet their responses to multiple downhole factors have not been systematically compared within a single fluid system. Focusing on the Dengying Formation gas reservoir in the Sichuan Basin, this study systematically investigates the individual effects of temperature, pH, additives (viscosifier and dispersant), solid-phase composition (bentonite, sand, barite), and acid contamination via controlled-variable experiments. Temperature exhibited a pronounced influence on the testing fluid, with viscosity and gel strength decreasing as temperature increased, particularly above 150 °C. While the viscosifier and dispersant both improved sedimentation stability, further improvement became limited at concentrations above 0.6% and 2%, respectively. Notably, noticeable deterioration in sedimentation stability was observed at a sand content of 3%, while the 5:5 API-grade to ultra-fine barite ratio exhibited relatively poorer sedimentation stability among the investigated formulations. In addition, autogenic acid and gelled acid increased the viscosity and gel strength of the testing fluid under the investigated conditions, indicating that acid contamination can substantially alter the rheological behavior. These results provide experimental references for formulation adjustment and stability assessment of water-based testing fluids under laboratory-simulated deep-well conditions.