Experimental study on the shale wellbore stability by optimizing surfactants in drilling

The comprehensive exploration and development of the unconventional natural gas (coalbed methane, tight sandstone gas, and shale gas) of coal measures can effectively improve the output of single coalbed methane, while it also puts forward higher requirements for the performance of the drilling fluid to enhance the stability of the wellbore in the coal seam, tight sandstone formation, and shale formation together. Therefore, the authors carried out the experimental research on the shale wellbore stability by optimizing the compound formula from some single surfactants to reduce the surface tension of the drilling fluid and increase the contact angle between water and the shale. Firstly, the authors optimized the compound formula from 6 single surfactants according to the standards of effectively reducing the surface tension of water by 73.44% and increasing the contact angle between water and the shale by 98.73%. Secondly, combined with the evaluation of the viscosity, filtration, lubrication, water activity, and inhibition of the drilling fluid, we found that the optimized compound formula, 0.2% C-1+0.1% A-1, has a good compatibility with the base mud. What's more, compared with the base mud, the drilling fluid with the optimized compound formula reduces the surface tension of the water-based drilling fluid, base mud, by 53.25% and increases the contact angle between water and the shale by 141.34%.