Preparation and Performance Evaluation of a Biodegradable Gel for Deep Coal Reservoir Drilling Fluids

Deep coal reservoirs are characterized by abundant microfractures and cleats, resulting in frequent wellbore instability during long horizontal drilling operations. To address this issue, a biodegradable gel plugging agent (XZ) was synthesized from acrylamide (AM) and carboxymethyl cellulose (CMC) via an orthogonal crosslinking strategy. The gelation behavior, swelling capacity, rheological properties, mechanical strength, thermal and salt resistance, plugging performance, self-degradation characteristics, reservoir protection capability, and drilling-fluid compatibility of XZ were systematically evaluated. The results showed that XZ formed a stable three-dimensional crosslinked network with good thermal stability (T0 = 116 °C, T1/2 = 445 °C). The synergistic effects of covalent and ionic crosslinking endowed the gel with high swelling capacity, mechanical strength, and plugging ability. XZ exhibited excellent temperature and salinity tolerance, with a degradation rate exceeding 90% after 5 days. After degradation, the filter cake became significantly thinner and core permeability recovery reached approximately 80%, demonstrating effective reservoir protection. Moreover, XZ significantly increased drilling-fluid viscosity, reduced fluid loss, and enhanced plugging performance. The XZ-based drilling fluid system integrates efficient plugging, self-deplugging, and reservoir protection functions, providing a promising solution for safe and efficient drilling in deep coalbed methane reservoirs.