$4\mathrm{.}39$ Compute the equivalent density below the casing seat at $4,000ft$ when a mud having a density of $10lb\frac{m}{g}al$ and a gel strength of $70lb\frac{m}{100}$sqftjust begins to flow. The casing has an internal diameter of $7\mathrm{.}825$ in$.,$ and the drillpipe has an external diameter of $5$ in $.$ Discuss how the pressure required to start circulation can be reduced. Answer: $11\mathrm{.}6$ lbm$/$gal$.$