Gas release from the formation
Drilling mud gas that circulates in the borehole comprises air and gaseous components that are mechanically released as the drill bit, including components present in the pore space of the crushed rock and gas entering the borehole through permeable strata; either as free gas or, more likely, dissolved in liquids. Continuous inflow of fluids in the borehole along the entire borehole wall is hampered through the rapid formation of mud-cake that covers the borehole wall and acts as a seal.