Trust Your Driller!

Oil/Gas and water well drillers are brought up in their trade/profession with a lot of apprenticing experience.  In addition to operating and maintaining their great or small drilling machines, they use tested and handed-down means of describing formations that they drill.

Today, I want to call attention to water well drillers' descriptions of how fast and in what way their drill bits progress through parts of formations.  My example is from the Willamette Valley of Oregon, USA.

When the driller of the Fairview School Well, a few miles southeast of the State Capitol, drilled soil and rock formations in this well, he used three speed terms; Soft, Moderate and Hard.  Soft means that the drill bit was not resisted greatly and he had fast drilling.  This drilling rate, in Oklahoma Oil/Gas drilling charts would be about 1-2 minutes per foot, or a penetration rate of greater than fifty feet an hour.  Moderate or "Medium" means that drilling is going at an average and the drill bit is meeting some resistance in drilling.  This would be common of a lightly cemented sand and gravel or a rock.  The drilling rate would be 3 to 5 minutes per foot or a penetration rate of 10 to 20 feet per hour.   Hard drilling means that the drill bit is meeting a lot of resistance.  Simply put, the rock is very hard.  The drilling rate is 8 to 12 minutes or more per foot, or less than 10 feet per hour as a penetration rate.  Directional drillers in oil and gas work would consider "tripping the bit" (pulling out of the hole, thousands of feet and changing the drill bit), but water wells are more shallower enterprises and water well drillers wait.

In the oil boom of the 1980s, I learned with Gulf Oil in Oklahoma that plotted drilling rate could mimic the Gamma Ray (GR) Correlation Curve that oil and gas geologists used from downhole geophysical logs.  We combined our sample logs with the GR curve to find how deep the drill was and to evaluate reservoir quality of formations.  Drill Time and the driller's rock descriptions (left column) reveal good qualities below the Shale (Depth 173', Elevation +37 ') that enabled the well to produce 380 gallons per minute with 72 feet of drawdown.  That is a good well.  The "drilling breaks", where the drilling gets "soft" or faster are where the lava flow tops are vesicular. There is "porosity" in the breaks.  With a little tectonically-imparted fracturing the vesicles become interconnected and provide good pathways to the open borehole.