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The penetration rate is useful while drilling to understand how a particular drilling rig and process are moving through different formation types.
By Christopher S. Johnson, PG, CHg
I’ve said designing a well is like trying to build a jigsaw puzzle without the top of the box, which is why you need as many puzzle pieces to figure out what you’re looking at. Along with the usual geologic information, drilling data should be collected because it’s part of the puzzle.
Collecting data on the interaction of various drilling methods and the subsurface is invaluable to the water well geologist. This information can be correlated to formation samples, geophysical logs, and to other wells in the project area. Furthermore, drilling data is useful when drilling more than one well in an area or trying to interpret reported information from other wells in an area.
The drilling process is complex. Important portions of that process to understand include the configuration of the drill string, the capabilities of the entire drilling fluid circulation system, the sample collection and handling process, and the measurement of specific drilling process characteristics.
Drilling processes have characteristics unique to themselves, but all drilling processes have some shared and similar characteristics. It is a few of those shared characteristics that interest a well site geologist.
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[restrict] So, a good field notebook filled with data on drilling conditions and processes, lithologies, and well performance can be of considerable aid to the well site geologist.Understanding Penetration Rate
The following is a page out of one such notebook regarding the utility of understanding and interpreting the drilling penetration rate.
The rate that the drill bit advances in the subsurface is called the penetration rate or rate of penetration (ROP) and is reported in units of length per time (e.g., feet per minute). It is influenced by many drilling process characteristics, along with the lithology of the formation being drilled through.
Let’s quickly cover a few drilling characteristics that can and will influence the penetration rate.
The weight of the entire drill string (e.g., drill bit, connectors, drill collars, and drilling pipe) is considerable. Generally, only a portion of the entire weight rests on drilling bit, and as such, is referred to as the weight on bit, reported in pounds, kilograms, and sometimes tons.
The weight on bit can be adjusted by the driller. Different lithology may require more pressure at the bit face to drill through. In such cases, the weight on the bit can be increased by reducing the hook weight or how much total drill string weight is suspended by the drilling rig.
The rotation rate, usually reported in revolutions per minute, is the rate at which the drill bit is turning. Again, variations in lithology may lead the driller to alter the rotation rate, which may increase or decrease the penetration rate.
The type of bit, be it a static (drag bit) or dynamic (tri-cone rotary bit), can affect penetration rates. Some bits cannot easily shed accumulated cuttings and become plugged, which will slow down the penetration rate, giving a false interpretation of a harder lithology.
The circulation rate (volume per time) is often reported in gallons per minute that the drilling fluid is circulating throughout the drilling system. Pump output is often the driller’s only known value for circulation rate. Pump output is often estimated by the driller, so sometimes it is worth evaluating the estimation method.
In unconsolidated materials especially, high circulation rates can increase the penetration rate by adding a flushing effect at the borehole face.
Increased depth can alter the formation pressures and compaction enough to cause a reduction in the ROP for similar lithologies. Formation pressures are less pronounced in water well drilling, more so in deeper drilling such as geothermal or oil and gas. Formation compaction does come into play in deep water wells.
The drilling goal is often a straight, fast hole and drilling decisions are often made to maintain this goal over sample integrity and accurately understanding subsurface conditions based on things like the penetration rate. Drilling contractors can and will alter drilling characteristics to achieve the drilling goal. These changing characteristics can alter the penetration rate.
Measuring Penetration Rate
The penetration rate is measured in two general ways. The first uses a tool, which records time and depth, from which the penetration rate can be calculated. One brand of such a device is called a geolograph. This recorder provides a long thin strip of graduated paper from which you can estimate the penetration rate.
The second way is by marking the Kelly on the drill rig in standard gradations (feet) and measuring the time between marks. It’s not as precise and more labor intensive, but it is a useful approach when needed.
Interpreting the penetration rate data requires comparing it to the other drilling characteristics and to the drilling cuttings. Comparing the ROP to cuttings will give an indication of formation lithology, and this in turn can be correlated to a geophysical log, if one is obtained from the borehole.
Different lithology (formation types like sands and clays) will cause the ROP to increase or decrease. In general, finer-textured silts and clays may drill faster at first, but their cohesive properties may ultimately slow down the rate of penetration. Slower rotation, reduced circulation rates, and silts or clays in the cuttings will support a decreased ROP associated with this type of formation.
Sands and gravels often drill faster (higher ROP) when unconsolidated. Loss of drilling fluid to the formation, along with depth-related compaction can slow down the ROP in unconsolidated sands and gravels. The actual size of the formation grains can decrease the ROP, especially as these grain sizes increase. Gravels and cobbles will certainly slow down the ROP as the bit finds it more difficult to gain a purchase on the larger grains.
The penetration rate is useful while drilling to understand how a particular drilling rig and process are moving through different formation types. This will allow the well site geologist to not only log the lithologies but begin selecting formation boundaries (i.e., depth intervals).
Comparing the penetration rate, and the other drilling characteristics, to a geophysical log conducted after total depth is reached is useful for correlating drilling characteristic values, lithologic descriptions, and formation boundaries. This in turn is useful in designing a new well.
In conclusion, the rate of penetration is a useful drilling characteristic to aid in lithologic descriptions and useful when attempting to correlate between different drilling locations in the same area, and finally helpful when attempting to correlate between wells over larger geographic areas.
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 chris@aegisgw.com.
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