Mathematical interpretation of maximum casing pressures received during a gas kick indicates that this should not be a significant problem. Problems appear to arise due to improper handling of the kick or very large influx volumes.
Drilling with minimum differential, or with a differential imposed on the system with choke pressure, should then pose no problems due to surface pressures.
The trend towards using a minimum differential in drilling appears to offer a considerable advantage in both drilling rate and information. Drilling rate as a function of rack behavior is fast becoming an important exploration tool. Jorden and Shirley of Shell Oil Co. have described the effects of the reduction of wellbore to formation differential pressure when entering overpressured zones on the Louisiana-Texas Gulf Coast. They present as a primary conclusion. "Drilling performance data can be used to detect the top of overpressured sediments. A plot of normalized rate of penetration will show a trend of continually decreasing penetration rates with depth and a reversal in this trend as overpressures are penetrated by the bit."
It is generally accepted that to achieve maximum drilling rates differential pressures must be at a minimum.
With this sort of background, it is apparent that drilling with minimum differential is going to become more commonplace. It has, in fact, become a common procedure on the Gulf Coast. Drilling with a back pressure has, in fact, become a common procedure on the is the obvious next step in this direction.
These developments required that a study of both the theoretical and practical aspects of gas, oil and salt-water intrusions into the wellbore be made, and the following are some of the results of that study that pertain to gas intrusion.
The reasons for developing an expression for maximum surface pressure to be expected during a gas kick were twofold: [11 to study the requirements for a blow-out control procedure, and (2) to provide a means of quickly ascertaining what the maximum surface pressure would be on the drilling rig.
With this in mind, it was decided that the solution to the problem was not significant beyond an accuracy of 150 Psi, which was assumed to be the accuracy of drilling rig equipment. This would suffice for any study purpose and the data would also be more than accurate enough for rig use. It follows that a rigorous solution was not necessary and that variables made a rigorous solution questionable, since [1] laminar flow patterns may tend to disrupt the gas bubble; [2] the kick material might not be known if the gauge of the hole is unknown; and [3] low permeability may cause the gas to be dispersed throughout a long column of mud and not rise as a bubble, or as any regular form. A dynamic solution then would appear to be no more accurate than a simpler static solution.
It appeared on the basis of the preceding points that an annular pressure technique was unreliable from an engineering standpoint and that a drill-pipe pressure technique should be used.
P. 81^
