Laboratory research on degradable drilling-in fluid for complex structure wells in coalbed methane reservoirs

In order to address the contradiction between drilling safety and coalbed damage control, a degradable drilling-in fluid (DIF) was developed for complex structure wells in coalbed methane (CBM) reservoirs. Its conventional properties and degradability were analyzed, and the desorption rate and return permeability methods were presented to evaluate its protection on coal desorption and seepage performance. Laboratory experiments were carried out to optimize thickening agent, fluid loss additive and filtrate reducer in DIF which is applicable to complex structure wells, and to determine the DIF formula. The DIF, with good conventional properties, can effectively carry cuttings and protect sidewall. It is also degradable. The experiment results show that DIF degradation rate is up to 85% in three hours when degradation agent is added at a concentration of 2.0% to 4.0%, and there is less solid residue after degradation. The DIF can effectively protect the seepage performance and also coal desorption, similar to clean water. The return permeability is all above 60% at different flow back pressure and the return permeability becomes bigger with increase of the flow back pressure, the maximum is up to 100%.