Designing a Well: The Puzzle Box
By Christopher S. Johnson, PG, CHg
Designing a well is often like building a jigsaw puzzle. The easiest way to complete the puzzle is to keep your eye on the picture on the box top.This is your mental map—your guide to what it should look like. It directs your design decisions as you search for edge pieces, a particular portion of the finished picture, or a unique shape. For a geologist tasked with designing a well, we often do not have the picture on the top of the box and must instead accumulate a broad understanding of all the pieces to construct our mental “box top.” To that end we need to gather and assess drilling, geologic, and geophysical information and we can then assemble a more comprehensive picture. This leads to a better well design.We will sometimes have local, reliable geologic information obtained from other academic or professional reports, well logs from nearby wells, or have some local geologic experience on which to begin assembling our mental image.
However, the first onsite information comes from drilling operations. As a geologist, you will have been fortunate if your mentor has encouraged you to learn and record not only geologic conditions in the areas you work, but also to correlate those to the drilling conditions you will have encountered. Reminding the drilling crew just how valuable their efforts are to the outcome of a good design is important. The first indications of the subsurface geology will be cuttings returned to the surface, and the integrity of those samples is important, but that integrity is born out of good drilling conditions.Drilling conditions include the weight on bit (how much weight is on the cutting face of the drill bit), the hook weight (how much weight is “hanging” on derrick), the rate of penetration (feet drilled per unit time), and the rotational speed of the bit. Equally important are drilling fluid conditions, particularly when using a drilling fluid such as air, water, or mud. These can include circulation rate,viscosity, fluid density, and water loss, depending on the drilling fluid. All of these can provide indications of the type of material the drilling bit is penetrating, and this can be potentially correlated to geologic and geophysical data.
The representativeness of the formation samples (i.e., the cuttings) will depend on these drilling and mud conditions, along with how the sample is collected. As the geologist, it is important to understand where in the drilling fluid return line the samples are collected by the drilling crew. Straight off the shaker table? It’s all sand top to bottom! Spaghetti strainer straight out of the return flow line? Clay balls and some coarse sand, but where are the fine sands?
It’s also important to work with the drilling crew to obtain as complete a sample of the returned drilling fluid and cuttings as possible. The completeness of the sample will help in the geologic description, interpreting the geophysical log, and correlating the geology and geophysics back to drilling conditions.Geophysical logs are invaluable and even more so when some simple preparation occurs. A good understanding of the local water quality (fresh, brackish, or saline) and then the drilling fluid characteristics will help the geophysical logger orient the logging more efficiently. Keep in mind the scale of the logs you are obtaining. That is, if all the resistivity logs in the area that you obtained are reported on a scale of 0 to 50 ohms, have the same scale on the new log—for easier comparisons.Building the box top involves comparing the difference in drilling conditions, lithologic descriptions, and geophysical responses throughout the depth of the drilled hole (e.g., penetration rate vs. logged cuttings vs. resistivity) and attempting to reconcile each to the other to obtain as good an indication of our box top as feasible. From this we can select formation samples for sieve analysis, which leads to filter pack and screen design, identifying potential depth ranges of waterbearing formations or fracture zones, and ultimately deciding on how to construct the well.
At the conclusion of the effort, the clarity of the box top is a team effort, and the efficacy of the team effort is based on communicating both expectations and abilities.
Christopher S. Johnson, PG, CHg, is the president and principal hydrogeologist at Aegis Groundwater Consulting LLC in Fresno, California. Johnson works with well owners and operators on a variety groundwater related projects, including locating new water resources, well design and construction management, aquifer testing, and well rehabilitation. He can be reached at email@example.com.