Damage mechanism and water-based drilling fluid protection technology for shale gas reservoir

Key words:  shale gas reservoir  damage mechanism  microemulsion  water-based drilling fluid  isothermal adsorption

Abstract:

Aimed at the features of shale gas reservoir and shale gas exploitation manners, the potential damage factors and sensitivities of shale formation in Shahejie Formation of Shengli Oilfield were analyzed by X-ray diffraction, scanning electron microscope, gas expansion displacement method, pulse decay method, and sensitivity evaluation. The protection measurement of water-based drilling and completing fluid for shale gas reservoir was proposed. The water-based drilling fluid formulation was developed based on the selecting of microemulsion shale gas formation protector YYBH-1and polyamine surface hydration inhibitor BMYZ-1. Then the water-based drilling and completing fluid for shale gas reservoir was prepared. The results show that the clay mineral of shale formation is mainly composed of illites, in which the porosity distribution varies from 0.45％ to 2.50％, the average pore diameter is between 4.54nm and 6.17nm, and the permeability is less than 0.026×10-3 μm². It is classic low porous and ultra-low permeable reservoir and has less medium water sensitivity, alkaline sensitivity, as well as medium stress sensitivity. This system can prevent water sensitivity and improve the flowback of water effectively, decrease the loss ratio of shale permeability and absorptive capacity of methane. It has excellent protection function for shale gas formation.