Study on Borehole Wall Real-time Stability of Coal Seam With Coal Cleat When Underbalanced Drilling

Abstract

Coal seam as the gas productive reservoir and gas-bearing reservoir, it is different from the conventional sandstone reservoir. On the one hand, coal cleat is well-developed in coal seam reservoir. Its characteristics are low porosity, small permeability, large specific surface area, low mechanical strength, strong heterogeneity, low reservoir pressure and so forth. These characteristics determine that drilling has more influence on coal seam reservoir than on conventional sandstone reservoir. In the process of drilling, therefore, in order to reduce or avoid the pollution to coal seam, usually adopt underbalanced drilling way to keep negative differential pressure and to reduce the damage of fluid in borehole flowing into the reservoir. At the same time, when underbalanced drilling, formation fluid flows into the borehole, leading to the formation pressure near the borehole to change. On the other hand, due to coal seam with low mechanical strength, great brittleness, and well-developed coal cleat, in the process of drilling especially underbalanced drilling, borehole wall is prone to collapse. Coal cleat exists in the coal seam and affects its mechanical property. When studying coal seam borehole wall stability, coal cleat must be considered. Considering time effect, the paper established the borehole wall stability model of coal seam with coal cleat when underbalanced drilling, obtained the collapse pressure distribution, and analyzed influence factors of coal seam borehole wall stability, providing theoretical guidance to prevent borehole wall instability. Key words : Coal seam; Underbalanced drilling; Negative differential pressure; Borehole wall stability; Coal cleat