What's your approach to optimizing bottom hole assembly for directional drilling?

BHA design is an extensive field of drilling engineering. Bit being the driver for this, hold special place. DEs need to look more than # of blades, cutter size for bit design. There are comprehensive papers on bit design guidelines (SPE-215007) that highlight that devil lies in details. Details like, cutter density, its fracture toughness, depth of cut elements, bit DOC vs Torque/WOB graphs, WOB limit of bit based on FEA etc. These factors and information makes a huge difference. In one Well , with this knowledge we used 12.25'' bits to max SWOB of 65Klbs, Although brochure said max WOB limit was 45Klbs. Having understanding on how strong matrix/Steel bodies are, we used high WOB to suppress Whirl, get better ROP, and better bit life.

Once you move above the bit, Stabilizer design and placement is another tool that can change BHA dynamics. As Fred Dupriest would say "The lowly stabilizer that we are using wrong" Stabilizers are usually thought to be for inclination management, but they are really to manage your whirl (if you can identify it) and manage your losses. Yes, losses through filter cake management. Papers like SPE-189649-MS and SPE 163565 have laid out the details of stabilizer design that should be known to DEs and discussed with service providers. For BHA design, simulate and identify your BHA Stress points, BM & Shear loads, and strategically place stabilizers to minimize BHA wave (Sine wave) to minimize whirl while allowing the flexibility to build/turn.

Instead of selecting the motor with highest AKO or highest torque. DEs need to understand whether they really need a higher AKO & or a high torque one. Identify through offset analysis whether there was sufficient requirement of motor torque or a high-speed motor can do a better job, Whether it is better to build with longer slides with lower AKO or you want to rotate, slide, rotate for a more tortuous hole with high AKO. We had trouble running 9-5/8''MM on 500+diff on a 1400+psi rated motor in OBM and HT environment. Make sure you have a prior discussion during selection with service hands-on safety factors that are going to factor in. Vendor used 3x SF for that motor & after technical discussion we used 2x SF & able to run 600+/- psi.

More than modelling, understanding and interpretation of simulation results and their tie up with actual Well conditions is what is lacking with drilling engineers. Run comprehensive BHA models with multiple iterations with a realization that simulation can not encompass all the details in BHA design, there are limitations in every simulation software package. Talk to your service hand to share the complete simulation of BHA, ask them to explain each graph, cross-question and be ready to learn, unlearn and relearn. BHA simulations/iterations should have a single point to design the most silent possible BHA for a hole section that can get the job done.

Modeling of the cuttings flow and transport around the BHA to optimize flow area, particularly for ERD wells is recommended.

In my perspective, wherever you are drilling, check the previous successful drilling BHA, every hole on the ground have different behavior, so it is the most important that after doing homework, now we should evaluate the formation tendency, its inclination, drift in certain direction, after reviewing, we need to select our BHA.