How do you measure and optimize the rate of penetration (ROP) in rock drilling operations?

An important aspect of the rock properties is what the surrounding rock is as well. Is the formation drilled interbedded in layers? How clean is it when considering nodules of chert or pyrite? With regard to ROP, this will affect the required drilling parameters as you traverse between rock types and the life of the drill bit needs to be accounted for. Remember that when you’re tripping for the bit, ROP is always zero.

Proper planning of a well requires having information at hand as quickly and accurately as possible, even more in a fast-drilling environment where quick response is needed. Extrapolating information from offset to wells and understanding the drilling environment requires time and analysis. Knowing the rock strength (UCS) across the well, based on offset wells data, aids on the selection of bit design and drilling parameters with the main objective of improving drilling performance. To know how the rock strength varies across a delimited area in different depth intervals will aid not only in the selection of the bit design and parameters but also on the specific requirements a well would need based on its geographical location.

With today’s advanced LWD tools and technologies, you may be lucky to have a Sonic tool along with GR-Res, Density-Neutron which could be used in RT to calculate rock properties. Use that to update , recalibrate pre drill models and adjust to ensure optimum Downhole drilling parameters for maximum DOC and ROP for the given bit design you have in the hole

Make sure the rig site personnel is fully aware of specific and relevant but design features and recommended parameters to optimize ROP. Too many times this information is discussed (operator DE’s and bit manufacturer) and just stays in the office and the onsite personnel run the bit with the parameters considering simply the bit type and model without knowing/using the specific design features and characteristics the bit going in the hole has. A bit model and type May have different design features and unique characteristics ( such a as DOC, back-side rake, hydraulic CFD , among others) which may require different parameters

Aside from bit aggressiveness and durability in bit selection, other factors can come into play that affect overall ROP. Steerability of a bit comes into play when any directional work is involved. This is especially true when using a bent housing motor as a bit’s side cutting capability will drive how efficient a slide is. This will affect the average ROP of a bit, as the typically slower sliding ROP can govern the run if many corrections are necessary. Hydraulic design can also be important for maintaining ROP as lack of cleaning of PDC cutters, especially in the very center, can lead to balling of the bit and inefficient drilling. Running CFD software on a PDC is useful to identify design changes to be made.

Do not forget to go back and update your pre drill models using density of rock being drilled, size of the cuttings, actual hole size if you have actual info/data in real time while drilling. Downhole drilling properties especially Temp and Press, more so if HTHP, makes a very significant impact on hydraulic calculation results. Use most recent mud test (6 or more readings) to update the model as you drill

Cutter Speed We developed some charts on cutter speed. It was extremely beneficial when we were trying to compare rotary speeds for different diameter bits. We found out the slower rotating speeds on larger diameter bits still offered excellent ROP compared to the higher rotary speeds in smaller bits without damaging the outer cutters. An example is on a 12-1/4” diameter bit at 100 RPM the outer rows of cutters are traveling about 20% faster than the cutters on a 9-7/8”. We always made sure our recommendations for RPM on the bit ensured optimum performance.

A trained drilling crew plays also an important role within the operating parameters, to implement an efficient and safe connection time.

The best bit / formation match will require just a balanced proportion of drilling parameters, ensuring that all other drilling factors (BHA, fluid, surface equipment) operate at optimum conditions. An excess of either or all parameters, may i.e. stress the flexibility of the BHA drilling tubulars, deflecting the hole trajectory, regardless the amount of stabilizers, which will then become just dragging lateral points to loose force, instead of centralizing it at the bit.