WorldWide Drilling Resource

Tales from the Field by Jeremy C. Wire Geoconsultants, Inc. Two Water Wells and an Earthquake There is a saying that something is “solid as a rock”, and in some cases this may be true, particularly in a hard rock drilling environment such as granite. However, introduce some fractures and the rock may behave differently, particularly during an earthquake, which is not an unusual event in southern California where the subject of this tale took place. This incident involves the effect a distant earthquake had on two deep wells completed in granite. The setting was a ranch property, and to obtain more water to irrigate an orchard, a well was drilled in granitic rock to a depth of about 1400 feet. Drilled by the air rotary method, fractures were noted in the borehole, particularly in the part of the column from about 1200 feet to total depth. They were not extensive, although a good yield was obtained, and the well was completed “open hole” with surface casing extending only to a depth of about 120 feet. Some months passed, and a surface reservoir was built to store the water from the well, which provided enough quantity of water for a while. Months later, a sec- ond well was needed, and it was drilled about 400 feet away from the first well, to about the same depth. Generally, the results were the same as with the first well, but the fractures below a depth of 1200 feet were slightly more numerous. With the two wells producing, continuous water level measuring devices were installed in each so changes in water levels during pumping and recovery periods could be closely monitored. The two wells produced satisfactorily for many months until one day a strong earthquake originating about 80miles away occurred, affecting the water levels in both wells. The instant the earthquake hit, the water level rose slightly in one well, but began decl ining slowly in the other. After four hours, the water level then dropped abruptly below the measuring device and the pump, and the well did not yield any water thereafter. The other well was not affected by the earthquake and pre-earthquake static and pumping water levels were maintained. To investigate this phenomenon, the submersible pump was removed, and a downhole video was run, revealing a large cavity. Apparently, the borehole had col- lapsed below a depth of about 1300 feet. The owner had a professional drilling contractor return to the well to try to drill deeper and achieved some additional depth below the caved section, but the borehole section remained basically “dry”. So, in this case, “solid as a rock?” For the one well, yes; and in the other, not particularly, even in granitic rock when subjected to strong earthquake shaking. Jeremy Jeremy Wire may be contacted via e-mail to michele@ worldwidedrillingresource.com 25 WorldWide Drilling Resource ® NOVEMBER 2018 ENV