The Causes of Shallow Gas Kicks

Abstract

ABSTRACT Shallow gas blowouts have occurred worldwide. Short casing strings and low fracture gradients prohibit holding back-pressure, and wells can only be diverted. Under these conditions well control procedures are much less certain, and risk of cratering and loss of well is high. This places a great emphasis on kick prevention. Well control schools tend to focus on deeper drilling and the peculiarities of shallow gas are not thoroughly addressed. Due to the low margin of overbalance that exists on shallow sands, a small loss of hydrostatic pressure, easily tolerated in deeper drilling, can initiate flow into the wellbore. These conditions are illustrated and related to loss of circulation and hole filling during trips. Drilling shallow gas sands too fast is examined as a potential mechanism for causing blowouts. Previous work in this area has shown that gas cut mud will seldom cause a blowout from the formation being drilled. While this is generally true, gas cut mud can lower the hydrostatic pressure enough to allow shallower formations to flow. A more comprehensive equation than heretofore published is developed, and several drilling parameters are examined to show their effects on the effective mud density at depths above the interval being drilled. A significant conclusion is that a sand might be drilled safely if casing is set just above the sand, but the same drilling conditions could cause an uncased, shallower sand to blow out. The paper also shows that shallow gas sands which are normally pressured at the bottom of the gas interval can require higher mud weights to control formation pressures at the top of the gas interval. This is due to the difference between the gas gradient in the formation and normal formation gradient.