The concept of this topic is to measure the pressure increment in a hour and divided by gradient of mud weight that you have in the well.
Let’s look at the formula:
Actual gas migration rate = Increase in casing pressure ÷ Pressure gradient of drilling mud
Actual gas migration rate in ft/hr
Increase in casing pressure in psi/hr
Pressure gradient of drilling mud in psi/ft
Let’s try to determine the actual gas migration rate with this following information:
Initial shut in casing pressure = 450 psi
Shut in casing pressure after half an hour shut in = 650 psi
Current mud weight = 12.5 ppg
Pressure increase in half an hour = 650 – 450 = 200 psi
It means that pressure increase in an hour is 400 psi. Therefore, you get the increase in casing pressure = 400 psi/hr.
You need to calculate your mud weight in to ppg with this formula:
Mud gradient = 0.052 x mud weight
Mud gradient = 0.052 x 12.5 = 0.65 psi/ft
Note: you can read more about it via this link: how to convert mud weight to pressure gradient.
Actual gas migration rate = 400 (psi/hr) ÷ 0.65 (psi/ft) = 616 ft/hr
Ref book: Formulas and Calculations for Drilling, Production and Workover, Second Edition