A 200°C‐Tolerant Water‐Based Drilling Fluid for Ultra‐Deep Wells Using an Organic–Inorganic Multi‐Metal Hydroxide Modifier

A water‐based drilling‐fluid (WBDF) formulation was developed for moderate‐density ultra‐deep intervals requiring thermal stability at 200°C. The optimized formulation contained 5 wt% APEG/sodium‐citrate‐modified quaternary Mg‐Al‐Fe‐Zn multi‐metal hydroxide (ASMH), 0.3 wt% CMC‐HV, 1.5 wt% humic acid, 1 wt% LUBE, and 3 wt% ultrafine CaCO 3 . After thermal aging at 200°C for 16 h, the formulation retained 72.73% of its initial apparent viscosity and exhibited YP/PV = 1.35 and an API filtration loss of 14.2 mL. Using post‐aging YP/PV ≥ 1.0 and API filtration loss ≤ 30 mL as formulation‐level acceptance criteria, workable properties were maintained at 20 wt% NaCl and 1 wt% CaCl 2 . The 120‐min linear‐swelling values were 26.66% at room temperature and 32.33% at 200°C, and the lubrication coefficient decreased from 0.174 for the aged base fluid to 0.132 for the optimized WBDF. Dynamic core‐flow tests yielded permeability recovery of 80.93%–88.46%, corresponding to permeability damage of 11.54%–19.07%. The complete fluid had EC 50 = 54,200 mg·L −1 and BOD 5 /COD Cr = 19.2%. Barite‐weighting tests showed controlled filtration up to 1.5 g·cm −3 , although YP/PV decreased to 0.46 at this density and performance deteriorated sharply at 1.6 g·cm −3 .