�?�?t ε l ρ l v l + �?�?ε l ρ l v l v l = ε l �?�?τ l + ε l ρ l g �?ε l �?p �?β v l �?v s The lessen in annular return movement will induce the circulation velocity of drilling fluid from the annulus to minimize, plus the friction involving it and the annulus will lessen, Therefore the BHP and standpipe tension