A vertical production hole of 6.0" size was drilled using drillstring consisting of 800 ft of 4.75" drill collars and 3.5" drillpipe by using 13.8 ppg drilling fluids. A kick was encountered at a...

A vertical production hole of 6.0" size was drilled using drillstring consisting of 800 ft of 4.75" drill collars and 3.5" drillpipe by using 13.8 ppg drilling fluids. A kick was encountered at a depth of 18,000 ft when the rig crew observed a pit gain of 18 bbls. Annulus and drillpipe pressures were stabilized at 1,200 psi, and 500 psi respectively. The kick gradient was estimated to be 0.097 psi/ft. Determine the formation gradient in ppg and the old mud weight.