Time delay effect due to pore pressure changes and existence of cleats on borehole stability in coal seam

When horizontal wells are drilled underbalanced in coal seam, it was found that borehole which kept stable at the beginning would probably collapse in some time. The phenomenon was called as time delay effect. In order to clarify the concept of time delay effect, the influence of pore pressure on stresses of cleats was analyzed based on the characteristic that cleats are abundant in coal seam, indicating that pore pressure influences the normal stresses of cleat surface while it has no impact on the tangential stresses. Using the data from field production in a CBM well, the influence of pore pressure changes on stability of coal rock was analyzed, indicating that contents of coal fines in drained water increases as pore pressure decreases. Pore pressure changes due to seepage in underbalanced drilling or overbalanced drilling were analyzed. This study above demonstrates that the time delay effect exists absolutely in borehole stability in coal seams during underbalanced drilling. The pore pressure changes are the main factor on the time delay effect, while cleats are the internal cause. In order to analyze further time delay effect quantitatively, the fluid–solid coupling during underbalanced drilling was numerically simulated, and the relationship between pore pressure and time was fitted, and the equation for computing the delaying time was posed by means of borehole stability theory in fracture mechanics. The analysis of the time delay effect can guide the field drilling operation and avoid the instability caused by pore pressure changes. The delaying time can be altered through adjusting bottom hole pressure in order to supply sufficient time for drilling operation.

Research Highlights
► Pore pressure has a great influence on the normal stress acting on the cracks surface while has no influence on the tangential stress. In coal seam which is full of a large number of cleats, pore pressure changes cause an increase or decrease in the normal stresses and moreover result in cleats propagation or even failure.
► The influence of pore pressure on borehole stability in coal seam is different from the influence on borehole stability in sandstone. For coal seam which is abundant in cleats, pore pressure reduction can cause an increase of stresses acting on cleats and furthermore make cleats propagate or fail much more easily. However, for sandstone, pore pressure reduction makes borehole more stable.
► Pore pressure is tied up with the contents of coal fines and has a great influence on borehole stability, local shear failure of coal rock, and the closure and opening of the cleats during production by draining water.
► There exists time delay effect in borehole stability in coal seam full of a large number of cleats. Pore pressure changes is the main factor in time delay effect and the cleats in coal seam is the internal cause.
► The analysis of the time delay effect can guide the field drilling operation and avoid the instability caused by pore pressure changes. The delaying time can be altered through adjusting bottom hole pressure in order to supply sufficient time for drilling operation.