Experimental study on the effect of drilling fluid wettability on shale wellbore stability

The output of single coalbed methane (CBM) well is rather low,and the comprehensive exploration and de- velopment of unconventional natural gas of coal measures is an important technical approach for improving the develop- ment benefits of CBM. However,this requires that the drilling fluid can solve the wellbore stability problem in coal, tight sandstone and shale strata together. Therefore,the authors proposed a cooperative anti-collapse theory based on positive electricity,neutral wetting,chemical inhibition,nanoparticles plugging and support of reasonable density,and carried out the experimental work on the influence of drilling fluid wettability on shale wellbore stability. Firstly,the authors optimized the compound formulation that could effectively reduce the surface tension of drilling fluid and in- crease the contact angle between drilling fluid and shale. Then,combined with the evaluation of drilling fluid viscosity, filtration,lubrication,water activity and inhibition,and the pressure transmission tests of shale,the effect of drilling fluid wettability on wellbore stability of shale was evaluated. The results show that,compared to optimized water based drilling fluid,the compound surfactants could effectively change its wettability,the surface tension of drilling fluid is reduced by 42. 6% ,and the contact angle to the shale is increased by 162. 1% . In addition,the compound surfactants have a good compatibility with water-based drilling fluid and a certain salt resistance. In contrast with water,the com- pound surfactants could effectively retarded the transmission of shale pore pressure,decrease the degree of the invasion of drilling fluid into shale,and reduce the permeability of the shale with a rate up 99% . Therefore,it is possible to im- prove wellbore stability of shale by controlling drilling fluid’s wettability.