Geophysical Well Logging Fundamentals:
Why Geophysical Logs ???
In many cases we need to know more about what’s in and around a well! Sometimes a simple camera can be lowered, and actually see in the well, but pictures show only the inner-most well, and often the fluid is not clear enough to see. Geophysical measurements allow us to “see” with electrical conductivity, calipers, gamma rays, thermometers, flowmeters, and acoustics. Applications range from simple monitoring to complex interpretation of geohydrology.
In some circles (oil and gas development) geophysical logs are absolutely routine, and usually required. For shallow wells (<1000ft), the use of logs is rapidly growing, now that compact and economic equipment has been developed.
The World’s First Well Log:
Geophysical well logging has historical roots back to Lord Kelvin, hundreds of years ago, when he lowered a thermometer into a well. Many of us think of the underground world as cool, like a cave. Yet the cooling in the subsurface is only true in the first few meters where recharging groundwater exists. In general, the subsurface warms as one goes down a well. About one to two degrees per hundred feet is very common. Changes in temperature with depth can tell a lot about what’s going on around a well. We record some type of temperature measurement in every well.
Two Types of Wells:
- Cased Holes – a “completed” well, with steel, plastic, or fiberglass pipe installed.
- Open Holes – all wells start this way, we like to run logs before casing, to get pure Geology.
Temperature logs can be run in any well, as long as you account for the time transient behavior, i.e. things heat and cool slowly when we’re looking at the subsurface. Some logs measure only the properties of the casing, while others measure the rocks around the casing. Electric logs are shorted by steel, yet electric induction can measure right through plastic. The type of well is crucial for design and interpretation of the proper logging suite.
Three Ways to “Dig” a Well:
- By Hand – usually just a few feet deep and quite large, we have yet to log one of these.
- Cable Tools – heavy percussion point on a cable repeatedly hits the ground, you occasionally bail out the rubble, and slowly dig a well. Classic techniques that are fading as we go deeper for good water.
- Rotary – drill bit on pipe grinds rock into “cuttings” which are washed up to the surface by pumping fluid (or air) down the center of the pipe and up around the annulus of the hole. Fastest technique, the only choice for deep wells.
Rough Rules of Thumb:
- In an area that is swampy year around, you can dig a water well by hand, but contamination is a big problem!
- Cable tools are best for shallow (100 ft) wells. At depth, the hole starts to cave, and it takes forever (weeks).
- Rotary drilling is taking over the market. Sometimes a 1000 ft well can be drilled in a day. Most of the time is spent in setting up the rig and mud system.
Finally, Why Log A Well?
Basically, you cannot just look down a well and tell what’s down there. With hand digging or cable tool bailing, you can pretty much tell what rocks you’ve gone through by keeping track of what comes out of the hole. Rotary drilling pretty much requires a geophysical well log, after the drilling, to get a record of where and what we drilled. This allows:
- better and proper design of well casing, before it’s sunk into the ground (permanently).
- permanent record of geologic layers and hole conditions, much of which cannot be recorded after casing is set!
- assessment of well at installation, allowing proper diagnostics in the distant future if problems should arise.
- continous monitoring of the subsurface with time, to look for changes in groundwater patterns.
- measurements of physical properties versus depth, not just one sample out of a whole well!
- obtaining physical properties of the subsurface that can only be measured with geophysical logs.