A pH-Responsive Anionic Polyether Copolymer for Improving the Rheological and Filtration Properties of Drilling Fluid in CO2-Contaminated Environments

Summary In the complex geological environment of deep formations, drilling operations are often confronted with carbon dioxide (CO2) contamination, which results in a reduction in the pH of drilling fluids, as well as an increase in their viscosity and filtration loss. However, existing treatment agents mainly function within a pH range of 8–11, and both excessive alkalinity and acidity can lead to their failure. Therefore, to address this challenge, we synthesized an anionic polyether copolymer (JN-1) using acrylic acid (AA), N-vinylpyrrolidone (NVP), sodium methaallyl sulfonate (SMAS), and isopentenyl polyoxyethylene ether (TPEG) as raw materials. The polymer JN-1 exhibited excellent viscosity-reducing performance under the conditions of CO2 saturation, sodium salt, calcium salt, and low pH value. At a concentration of 1 wt%, JN-1 reduced the apparent viscosity (AV), plastic viscosity (PV), and API filtration loss (FLAPI) of the CO2-contaminated base slurry containing 20% sodium chloride (NaCl) from 8.5 mPa·s, 7 mPa·s, and 150 mL to 6 mPa·s, 4 mPa·s, and 110 mL, respectively. It also maintained the AV, PV, and yield point (YP) of the CO2-contaminated base slurry containing 2% calcium chloride (CaCl2) within a certain range, preventing them from decreasing continuously. Moreover, the performance of JN-1 in drilling fluids aged at 180°C was superior to that of commonly used thinners (XY-27, HW-Thin, and SDZ-II). At the same time, JN-1 increased the absolute value of the zeta-potential for the base slurry from 8.2 mV to 23.8 mV and reduced its particle size from 19.1 μm to 13.69 μm. This was mainly because JN-1 could provide a stable protonation-resistant negative charge (-SO3−), which enabled the electric double layer on the bentonite surface to maintain its original thickness even in a low-pH environment. In summary, the development of JN-1 can provide a novel strategy for the design of drilling fluid materials applicable to complex deep formations.