We will be drilling a horizontal well and the landing section passes through a fault. Due to the experience on a previous campaign in the field, we plan to drill the 12 ¼” landing section in 2 runs.
The first run to terminate 150ft above the expected fault will be a RSS (point-the-bit) with PDC bit. This will be changed to a dumb iron packed assembly with a PBL sub included to allow the flexibility of pumping different kinds of LCM pills to manage losses.
We will then drill circa 300 ft (3 stands) blind. The section planned for blind drilling is a tangent with inclination of 78.5 deg, azimuth 101 deg and DLS 2.5. The dumb iron assembly will be POOH to revert back to the RSS and drilling continued to the 12 ¼” landing depth /section TD.
The fault is 470ft from the heel of the well and the risk / potential for the packed BHA to drop or build inclination and change azimuth in the 270ft tangent is high. If this occurs, it will cause significantly high DLS to catch-up original trajectory and could adversely impact hole usability (casing running, completion, e.t.c.).
1] Does anyone have experience(s) drilling through fault they can share with me, what kind of drilling parameters did you employ if you ran a dumb iron BHA?
2] Any advice on the best way to manage BHA / Formation tendencies / bit walk with the dumb iron assembly with Mill Tooth Bit since the first BHA is RSS with PDC bit to minimize DLS
3] Any rules of thumb for modelling or predicting the dumb iron BHA tendency?
4]The formation is deltaic continuous depositional, and permeability is high. We are drilling with SOBM and have tried every known LCM in the past including cement with minimal success. Does anyone know of any other novel LCM / Total Loss Circulation management strategy?
Your question 1/
Yes experience drilling through faults in deep geothermal application (granite formations) with dumb iron BHA owing to bore-hole instability through faults. BHA included a Rock Bit + NBStab 1/8" under-gauge + DC + S.Stab Reamer 1/8" UG (up & down reaming capability) + DC's. WOB of about 2 to 3 mtf per inch of bit diameter - 80/100 rpm indicative but safe rpm range to be derived from a BHA modal analysis. Avoid indeed critical RPM (resonant BHA operating parameters) which may induce borehole instability through unstable faults. Fault crossing can be identified through MSE real time analysis (MSE / bit ratio patterns). If unstable fault is expected, drill by steps of 3-5m then pick-up backreaming to previous interval 60rpm (more if required) until clear, hence the need for a Reamer S.Stab, clean hole with high flow rate and pills if required, once clear resume drilling by 3-5m steps and repeat. The goal is to mitigate borehole packing off consequences. If faulted area is known to be stable, no need to proceed as outlined above.
Your question 2 & 3/
Drilling through a fault will generally result in a local dogleg (a coupe of degress change in inclination / azimuth), hence the need to ream it and purpose of Reamer S.Stab. Additionally, rock bit steerability is high and will drive BHA directional behaviour towards bit side-force (your previous PDC bit used with Point the Bit RSS, genuine Point the Bit (?) is likely a low steerability bit and if so driving the BHA towards bit tilt rather than bit side-force).
Bit-BHA modelling therefore required to design a hold BHA, playing with NBStab to Reamer SStab spacing, accounting for bit steerability. Use an appropriate Bit-BHA modelling solution (we can help on this should need be - firstname.lastname@example.org).
Bit walk with a rotary BHA is not something you can control while drilling. At best through BHA modelling you can design to mitigate the walk trend, again, playing with Stabilizer spacing and gauge as well as bit steerability (rock bit or PDC bit). Slightly undergauge Stabilizer may help lmtigating rock bit right hand walk, unless formation bedding planes are unfavorable.
Let us know your experience upon completion of your well.
What are you trying to achieve with regards to drilling the fault? Do you have an idea of the reactivation pressure and how that relates to your mud weight / ECD? Are you looking to utilise LCM material to increase the apparent rock strength (well bore strengthening)? You're talking about drilling through the fault with a dumb iron assembly, then pulling out of the hole and picking up the RSS again. What's the plan if you don't get losses drilling with the dumb iron assembly and initiate them whilst drilling with the RSS?
You can limit the drilling parameters whilst drilling through the fault. No tools in the BHA to power so you can drop the flow rate down to minimum levels for hole cleaning (150ft/min AV around the drill pipe) or even lower. Only drilling a short section - drill it with low flow, pull back above the fault zone and then circulate clean prior to tripping out is definitely an option if monitored correctly. Aim for me should be to limit the impact of ECD on the fault zone, if you don’t want to break it down.
As far as BHA design goes, your DD company will have some predictive software to give you an idea of the tendency of a simple 3 stab BHA (primarily 12-1/8" NB Stab, 10ft Pony, U/G String Stab, DC, 12-1/8" String Stab, Pony DC, HWDP.....). The size of the first string will be driven by the modelling - probably 11-3/4" to 11-7/8". But then that is only a mathematical model, and my favourite saying these days is "all models are wrong, some are just less wrong than others..." The "less wrong" is determined by historical data. You probably have no historical data of a pure rotary assembly drilling in this formation at this depth. So, where do you start? Look at the RSS steering data and see how much they are having to fight bit walk tendency whilst drilling the tangent. Is the tool steering more to the left or the right when they are trying to drill ahead? If it is primarily drilling straight – steering GTF within an arc towards the top of the hole then I would lean towards drilling with the same bit. A PDC bit will drill the formation at lower weight bit than a roller cone - the higher a weight you have to use with your dumb iron assembly the more likely you will induce a high build rate. A small walk tendency at high angles may be easier to deal with than a large build rate induced by weight. Do some modelling with regards to target and walk rates and see what you can live with. Aim to be as close to your line as possible when you trip, so you have the most room to play with.Just some basic thoughts. Don't know who you are using as your DD company, but chase after a grey haired DD in there (a dying breed) and utilise his knowledge of drilling directional wells with a rotary assembly.