Development and Application of a Plugging-Type Solid-Free Drilling Fluid System

The Zhina coalfield in Guizhou Province possesses substantial coalbed methane (CBM) resources. The Upper Permian Longtan Formation features multiple thin coal seams with well-developed cleats and microfractures, while the overlying coal seams display complex coal-body characteristics. Wellbore instability issues, such as spalling and collapse, are particularly pronounced during drilling operations within the upper coal-bearing formations. A plugging-type solid-free drilling fluid system can enhance the wellbore stability and mitigate coal seam damage. This study employed X-ray diffraction (XRD), scanning electron microscopy (SEM), hydration stability, and mechanical property tests on coal seam and adjacent strata cores from the Zhangjiawan block of the Zhina coalfield to elucidate potential reservoir damage mechanisms in CBM drilling. To minimize fluid loss and enhance sealing, a fluid loss reducer and viscosifier, AMSN, were developed. AMSN, with a relative molecular mass of 4.81 × 10⁶, forms a three-dimensional spatial network structure in solution, characterized by circular or hexagonal configurations. At a concentration of 300 mg/L, the AMSN solution exhibits a significant increase in apparent viscosity accompanied by the formation of numerous structural entities. The AMSN solution demonstrates excellent thermal resistance, effective plugging capability, and lubricity, maintaining an apparent viscosity retention rate above 47% at 95 °C. Sand bed plugging tests indicate a filtration volume of 0 mL at 3 min. The friction coefficient of the 0.3% AMSN solution was reduced to 13.2 N/m². Based on the developed AMSN, the plugging agent and antiwater blocking agent were optimized, and a plugging-type solid-free drilling fluid system was formulated. This system exhibits superior rheological properties, plugging capacity, and reservoir protection performance. Field application results demonstrate that the drilling fluid system effectively reduces wellbore enlargement in collapse-prone upper coal seams, thereby enhancing the wellbore stability and improving the cementing quality of the CBM wells.