Dual-effect Drilling Fluids with Weak Gel and Micro Solids for CBM Drilling

Dual-effect drilling fluids with weak gel and micro solids have been developed to better satisfy the needs of borehole wall protection in coal bed methane (CBM) drilling, to minimize the damage to CBM pay zones by drilling fluid, and to realize safe and efficient drilling operations. Laboratory evaluation of the drilling fluids showed that they had excellent basic properties, good inhibitive capacity, temperature resistance (50℃), salt resistance (NaCl, CaCl₂, MaCl₂) and drilled cutting (coal) tolerance. A 500 mL weak-gel drilling fluid sample and a micro-solid drilling fluid sample treated respectively with 0.1 g cellulose, 0.1 g neutralized α-amylase and 0.1 g compound enzyme had rates of degradation of the two drilling fluid sample were 90.9% and 83.0%, respectively. The weak-gel drilling fluid and the micro-solid drilling fluid had friction coefficient of 0.17 and 0.12, respectively, and percent permeability recovery of 91.3% and 82.3%, respectively, exhibiting good lubricity and reservoir protection capacity. Observation under microscopy showed that contact angles of the two drilling fluids were increased, capable of protecting formations from being damaged by water block. The compound bio-enzyme used in the drilling fluids not only reduced the apparent viscosity of the drilling fluids, it also helped degrade mud cakes, thereby minimizing blocking of fractures in CBM zones, greatly mitigating the negative effects of mud cakes on CBM production.