The use of water-based mud (WBM) in drilling applications under extreme reservoir temperatures has been increasing rapidly in the drilling industry over the past few years. It is now more crucial than ever to close the technological gap that present challenges towards developing an environmentally friendly and cost effective high performance WBMs. Synthetic polymers such as sodium salt of sulfonated acrylamide and vinyl lactam (DD) and sodium salt of sulfonated acrylamide, acrylic amide, sodium acrylate and vinyl lactam (DT) are commonly used as primary viscosifiers and fluid loss agents in WBM systems for HPHT fluids. Potassium formate brine (KCHO2) is usually used to stabilize the drilling fluid's performance under HPHT conditions as it helps in decreasing the flocculation of rheological properties of WBM under extreme temperature including the degradation of their rheolo s. It is known that WBMs experience many challenges when exposed to extreme temperatures gical properties over a long period of time. This study aims to developing a water-based drilling mud that is stable at elevated temperatures up to 300°F, while maintaining sufficient inhibitive and good rheological performances. Various drilling mud additives such as synthetic polymers (DD and DT), KCHO2 and other fluid loss additives were investigated in the formulation of the stable WBM. A total of sixteen (16) drilling fluid samples with different concentrations of synthetic DD (1 ppb to 4 ppb), DT (1 ppb to 4 ppb), and KCHO2 (0 wt.% to 10 wt.%) were formulated using the Taguchi design of experiment (DOE), prepared and tested before and after dynamic aging. The experimental results obtained showed that the following drilling mud formulations had their rheological property (PV/YP) values stabilised even after dynamic aging at 300°F: (a) 3.2 wt.% KCHO2, 3 ppb DD, and 4 ppb DT, (b) 6.7 wt.% KCHO2, 2 - 4 ppb DD, and 2 - 4 ppb DT, and (c) 10.0 wt.% KCHO2, 1 - 4 ppb DD, and 1 - 4 ppb DT. This study provides an optimisation guide for the formulation of stable synthetic polymers and potassium formate brine WBMs at elevated temperatures up to 300°F, thus, saving time and cost by eliminating various laboratory tests.

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