A New Borehole Wall Stabilizer for Deep Horizontal Drilling of CBM

A borehole wall stabilizer WDJ-1 was developed to address the problem of borehole wall instability and sloughing caused by property differences at the interface of coal-rock and coal-gangue in horizontal wells for deep coalbed methane (CBM). WDJ-1 was synthesized via aqueous solution polymerization using polyvinyl alcohol (PVA) and tannic acid (TA) as functional monomers, and borax and ferric chloride (FeCl₃) as dual crosslinking agents. The synthesis was optimized with a monomer ratio of PVA∶TA = 1∶1, a reaction temperature of 85 ℃, a reaction time of 3 h, a system pH of 7.5, and the concentrations of the crosslinking agents (0.1% borax and 0.25% FeCl₃). The structure of WDJ-1 was characterized by Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA), and its performance was evaluated through rheology, plugging capacity, cementation property, and inhibition property experiments. FT-IR confirms the presence of O—H, C=O, B—O, and Fe—O coordination bonds in the molecules of WDJ-1, indicating sufficient reaction between the monomers and the crosslinking agents. SEM reveals that WDJ-1 exhibits a honeycomb structure, which can improve the compactness of the mud cake. A base fluid treated with 2% WDJ-1 has its API fluid loss reduced from 22.4 mL to 3 mL, the shear strength of the coal rock-coal gangue interface reaches 0.16 MPa, the plugging pressure for a 2 mm slot plate increases by 54.5%, and the 48-hour water absorption rate of mud balls is reduced by 10.5%. TGA measurement proves that WDJ-1 exhibits excellent thermal stability below 240 ℃. WDJ-1 stabilizes wellbore through the four-fold synergistic mechanism of “inhibition-plugging-bonding-high temperature resistance”, and the use of which can provide technical support for the safe and efficient drilling of horizontal wells in deep CBM reservoirs.