Experimental Study on Biodegradable Drilling Fluid of Gas Drainage Drilling in Unconsolidated and Soft Coal Seam

To solve the borehole stability problem of gas drainage drilling in unconsolidated and soft coal seam, we propose a novel idea of biodegradable drilling fluid (BDF) which can maintain borehole stability in drilling process and with the addition of enzyme, it can recover permeability of coal methane reservoir at the end of drilling. Firstly, the paper introduced the borehole protection and biodegradation mechanism of BDF. In the following part, rheological tests showed that with the addition of enzyme, rheological parameters such as apparent viscosity (AV), plastic viscosity (PV) and yield point (VP) could decrease significantly with the lapse of time (in 48 hours). Lots of lab tests were also carried out to analyze effects of substrate concentration, pH, temperature, enzyme concentration and action time on viscosity breaking ratio (VBR) in the biodegradation process. We found that appropriate substrate concentration (0.6 wt %), moderate pH (5-7) and temperature (40-60 degrees C), higher enzyme concentration and longer reaction time could increase viscosity breaking efficiency. In the end, the paper presented the permeability recovering tests of BDF and it showed that after degradation, the flow ability of reservoir fluid would recover which could ensure the improvement of gas drainage efficiency. Therefore, BDF has a promising future in gas drainage industry.