Harold Vance Department of Petroleum Engineering Lesson 14 Air, Gas, and Mist Drilling Read: UDM Chapter 2.1 - 2.4 pages 2.1-2.74 Harold Vance Department of Petroleum Engineering

Air, Gas, and Mist Drilling Circulating Pressures Harold Vance Department of Petroleum Engineering

Circulating Pressures Calculating standpipe pressure starts with predicting the pressure just below the bit, and working your way back to the surface Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering Bit Pressure Drop As air flows through the jets, it expands in response to the decrease in pressure and it’s velocity increases Once the pressure drop exceeds a certain level, the air velocity reaches the prevailing speed of sound. Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering Bit Pressure Drop At this point, the air cannot expand any faster and the upstream pressure becomes independent of the downstream pressure. This implies that under sonic discharge conditions the standpipe pressure is independent of the annular pressure Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering Bit Pressure Drop Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering Bit Pressure Drop If the upstream pressure is more than 1.89 times the annulus pressure beneath the bit, flow through the bit will be sonic. Harold Vance Department of Petroleum Engineering

Upstream bit pressure - sonic flow Harold Vance Department of Petroleum Engineering

Upstream bit pressure - sonic flow Harold Vance Department of Petroleum Engineering

Upstream bit pressure sub-sonic flow Harold Vance Department of Petroleum Engineering

Upstream bit pressure sub-sonic flow Harold Vance Department of Petroleum Engineering

Upstream bit pressure sub-sonic flow Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering Standpipe pressure Harold Vance Department of Petroleum Engineering

Steps to predicting standpipe pressure Assess whether flow through bit is sonic or sub-sonic If sonic, the pressure above the bit is determined with equation 2.21 This value is used in equation 2.25 to predict standpipe pressure Harold Vance Department of Petroleum Engineering

Steps to predicting standpipe pressure If flow is sub-sonic, the annulus pressure below the bit must be first predicted (Angel’s analysis, etc) using equation 2.12. The pressure above the bit is determined by equation 2.23 This value is used in Equation 2.25 to determine standpipe pressure Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering Important point When air drilling, large changes in annulus pressure may result in smaller changes in standpipe pressure, or in the case of sonic flow through the bit, no change in standpipe pressure at all. Hole problems that lead to an increase in annulus pressure may be indicated by small changes in standpipe pressure. Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering Important point It is very important to monitor the standpipe pressure closely and react promptly to unanticipated changes. It is important to know if flow through the bit is sonic or not. If flow is sonic, standpipe pressure will not change with changes in annulus pressure Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering Example 8.1/2” hole at 6000’ drilled with 4 1/2” drillpipe air rate is 1400 scfm. Penetration rate ranges up to 300 ft/hr. Bit has no nozzles in one example and 3- 14’s in the other. Harold Vance Department of Petroleum Engineering

Harold Vance Department of Petroleum Engineering