Changes

Because drilling fluid is pumped down the inside of the drill string to the bit, surface-to-bit (s-to-b) calculations are made as follows:

Because drilling fluid is pumped down the inside of the drill string to the bit, surface-to-bit (s-to-b) calculations are made as follows:

:<math>\mbox{S-to-b strokes}  = (\mbox{Total capacity in bbl})/(\mbox{pump output in}</math>

:<math>\mbox{S-to-b strokes}  = (\mbox{Total capacity in bbl})/(\mbox{pump output in bbl/stroke})</math>

:<math>&\quad \ \mbox{bbl/stroke})</math>

[[Drilling fluid]] is then pumped back up the hole to the surface via the annulus, therefore bit-to-surface (b-to-s) or lag calculations are made as follows:

[[Drilling fluid]] is then pumped back up the hole to the surface via the annulus, therefore bit-to-surface (b-to-s) or lag calculations are made as follows:

:<math>\mbox{B-to-s strokes}  = (\mbox{Total annular volume in bbl})/(\mbox{pump}</math>

:<math>\mbox{B-to-s strokes}  = (\mbox{Total annular volume in bbl})/(\mbox{pump output in bbl/stroke})</math>

:<math>&\quad \ \mbox{output in bbl/stroke})\\&\\\mbox{B-to-s min} &= (\mbox{B-to-s strokes})/(\mbox{pump rate in strokes/min})</math>

:<math>\mbox{B-to-s min} = (\mbox{B-to-s strokes})/(\mbox{pump rate in strokes/min})</math>

'''''Example''''': Using the previously calculated data, we can calculate circulating time as follows:

'''''Example''''': Using the previously calculated data, we can calculate circulating time as follows:

:<math>\mbox{S-to-b strokes}  = (155.4 \mbox{ bbl})/(\mbox{0.914-in. bbl/stroke})</math>

:<math>\mbox{S-to-b strokes}  = (155.4 \mbox{ bbl})/(\mbox{0.914-in. bbl/stroke}) = 1700 \mbox{ strokes}</math>

:<math> &= 1700 \mbox{ strokes}\\&\\\mbox{S-to-b min} &= (1700 \mbox{ strokes})/(98 \mbox{ strokes/min}) = 17.35 \mbox{ min}\\&\\\mbox{B-to-s strokes} &= (378.9 \mbox{ bbl})/(\mbox{0.0914-in. bbl/stroke})\\&= 4146 \mbox{ strokes}\\&\\\mbox{B-to-s min} &= (4146 \mbox{ strokes})/(98 \mbox{ strokes/min}) = 42.30 \mbox{ min}</math>

:<math>\mbox{S-to-b min} = (1700 \mbox{ strokes})/(98 \mbox{ strokes/min}) = 17.35 \mbox{ min}</math>

:<math>\mbox{B-to-s strokes} = (378.9 \mbox{ bbl})/(\mbox{0.0914-in. bbl/stroke}) = 4146 \mbox{ strokes}</math>

:<math>\mbox{B-to-s min} = (4146 \mbox{ strokes})/(98 \mbox{ strokes/min}) = 42.30 \mbox{ min}</math>

Lag time can also be measured at the wellsite by placing calcium carbide in the pipe when a connection is made. The carbide reacts with the water in the mud and forms acetylene gas that is readily detected by the chromatograph. The stroke counter is reset when the carbide is “dropped” down the pipe, and the number of strokes needed for the acetylene return is noted. The actual and calculated lags are then compared, and an interpretation is made.

Lag time can also be measured at the wellsite by placing calcium carbide in the pipe when a connection is made. The carbide reacts with the water in the mud and forms acetylene gas that is readily detected by the chromatograph. The stroke counter is reset when the carbide is “dropped” down the pipe, and the number of strokes needed for the acetylene return is noted. The actual and calculated lags are then compared, and an interpretation is made.