Deep Bedrock Wells for Municipal Use: A Series on the Process - #2) Geophysical Logging & Well Design
This is the second installment in the LWS well drilling series. In the first blog we talked about how the borehole for the well is drilled. Once the drilling is complete to the total depth of the borehole, it is geophysically logged. In this blog we will discuss the basics of geophysical logging and the well design obtained from the logs.
Once the drill bit has reached the predesignated total well depth, the drill bit and drill stem must be removed from the borehole. However, before removing the drill stem, the drilling mud in the borehole has to be “conditioned”. The conditioning of the mud is principally to remove most of the cuttings from the borehole. This takes a while since, for this Arapahoe aquifer well, the drill stem and bit are 1,720 feet deep into the ground.
After this prep work is completed, it’s time to start logging. The logging is done by a third party company which is hired by the driller. Logging is a complicated process where many different parameters can be measured, we will focus on four significant parameters:
Borehole diameter;
Single point resistance;
Natural gamma; and
Spontaneous potential.
BOREHOLE DIAMETER
The logging equipment is lowered into the borehole from a large roll of cable until it reaches the bottom, and the logging itself is done as the logging probe is pulled back to the surface. The first measurement we will go over is the caliper measurement. Three arms on the caliper are deployed when the caliper reaches the bottom of the borehole, and the arms lightly press out against the sidewall of the borehole and send a live feed of the results to computer equipment mounted inside the logging truck where the operator and engineers observe the data as it comes in. This measurement tells our team if there are any areas of the borehole where the walls have collapsed and/or created voids, and these data are also used to calibrate the other logs. The caliper data are also very helpful in estimating the volume of gravel and grout which will be needed to fill the area between the well casing and the walls of the borehole, i.e., the annular space.
Drill rig for well in a deep bedrock aquifer.
Example of a geophysical log.
SINGLE POINT RESISTANCE
The second important measurement is the single point electrical resistivity. Basically, this is a measurement of how much electrical current can flow through the aquifer area where the probe is located. Higher electrical resistivity is cause by larger grain size materials like sands, which are good water-producing layers, and lower resistivity values are generated by finer grained materials like silts and clays. While this information is extremely helpful, it is paired with the results of our next test to identify what we believe are the best water-producing layers.
NATURAL GAMMA
While not the last aquifer characteristic logged, the last one we will discuss here is the natural gamma radiation of the aquifer materials. Natural radiation is all around us in different levels. For example, bananas are naturally radioactive due to their high potassium content. About 1/100th of one percent of naturally-occurring potassium in nature is an unstable and slightly radioactive form. The same is true for the potassium and other low level radioactive elements often found in clays. The natural gamma log shows us the clay layers of the aquifer which have higher radiation.
When we see a spike in natural gamma in the same area as a drop in the electrical resistivity, we can be fairly confident that area of the borehole is a clay layer, which assists in our design of the well since we don’t want to place well screens opposite non-water bearing clays. Where the logs exhibit low gamma levels and high electrical resistivity are indicative of larger grain sands, which produce more water, so those areas are where the well screens are placed.
SPONTANEOUS POTENTIAL
Similar to the gamma log, the spontaneous potential log generally produces a straight line in clays that has become known as the “shale baseline” while, when the log encounters water-producing sands, the log trace deviates from the shale baseline. Therefore, interpretation of the resistivity log, in conjunction with the gamma and spontaneous potential logs, provides the basis for designing the well.
WELL DESIGN
Time is of the essence to interpret the geophysical logs and provide the well design to the drilling contractor as the borehole needs to stay open to allow for the casing/screen assembly to be set in the borehole (see detailed information on well design in the Oct. 6, 2020 LWS blog). The longer it takes before the casing string can be run in the borehole after logging, the greater the risk of borehole collapse. So once the logging is completed the well design work immediately begins. For some reason this tends to happen in the middle of the night, so the engineers meet in the wee hours of the morning to decide which sections of the well will be screen or solid casing.
Well casing (solid)
Well screen (perforated)
In the Denver Basin, the well permit issued by the Colorado Division of Water Resources (CDWR) has a presumptive top and bottom of the aquifer to be tapped by the well. As long as the geophysical logs indicate that the actual formation boundaries are within that presumptive range, the well design can proceed, followed by the well installation. However, if the logs indicate that either the top or bottom of the formation, or both, are outside the specified depths on the well permit, a variance has to be obtained from the CDWR prior to proceeding. This is a critical issue if a variance is needed because it delays the ability to move forward with the well construction. In proposed revised Statewide Nontributary Groundwater Rules, Rule 9.8.1 requires the CDWR to respond within 12 hours of the logs being submitted where a variance is being requested, as long as other conditions of the rule are met.
Once the well design has been completed from analysis of the geophysical logs, the engineers immediately return to the site with the final well design to observe the welding of the casing and screen sections. But that is a lesson for another blog, so stay tuned for the next installment in our well drilling series!
And if you have a need for a water resources firm for well drilling, groundwater modeling, water rights cases, or more, please reach out to any of the LWS team members below and we will be happy to put together a plan for your needs. We help with projects big and small.
Bruce Lytle, President of LWS
Chris Fehn, Senior Project Engineer
Anna Elgqvist, Senior Engineer
