H2S and its effects on PDMs - a field hands perspective.
The mechanism starts with lone hydrogen atoms diffusing through the metal. At high temperatures, the elevated solubility of hydrogen allows hydrogen to diffuse into the metal (or the hydrogen can diffuse in at a low temperature, assisted by a concentration gradient). When these hydrogen atoms re-combine in minuscule voids of the metal matrix to form hydrogen molecules, they create pressure from inside the cavity they are in. This pressure can increase to levels where the metal has reduced ductility and tensile strength, up to the point where it cracks open. High-strength and low-alloy steels, nickel and titanium alloys are most susceptible.
The steel from which most motors are made is AISI 4140 which is a standard Chromium-Molybdenum Alloy steel. As such the effects would be no more detrimental to that on the rest of the BHA components.
However, there are two other materials in contact with the circulating medium, and are hence susceptible to H2S effects.
Stator Elastomer.
The standard NBR elastomer is not recommended for a H2S environment as it is open to attack from the free sulphur radicals from the H2S at the unsaturation points (double bonds) within the Nitrile polymer chain network. This would be visible as embitterment and cracking of the elastomer. HNBR (Hydrogenated Nitrile Buna Rubber) elastomer would be recommended where the level of unsaturation is minimal.
Chrome or carbide plated Rotor.
Stainless steels with less than 20% chromium may form less stable chromium sulphide 'scales'. The 1.4845(310) grade (24-26% Cr in BS EN 10095) can be considered for hydrogen sulphide environments, hence Chromium plating is suitable for H2S environments.
A tungsten carbide coating does not have the pore space to allow Hydrogen invasion and is inert to the effects of H2S – hence this coating would be ideal.
Future developments and Comments.
In higher levels of H2S a TFE/P compound would be better from the perspective of avoiding chemical attack, but this would have poor aromatic hydrocarbon resistance as well as poor low temperature flexibility; this coupled with relatively poor physical properties would not suit a PDM application.
The installation of a float valve in the bit box would prevent H2S from percolating into the interior of the motor during periods of inactivity and would shield the elastomer from the H2S environment.
IPM Head of Drilling Engineering at SeaLand
7 年Great explanation Chris
Rental tool coordinator
7 年4140/4145 (120k yield) is what most BHA components are manufactured out of. When working in an H2S environment 100k yield (4130 if I remember correctly) is the material of choice and also grade E drill pipe. The H2S impregnates the metal and over time it becomes brittle. So the effect of H2S is not immediately known, but a failure generally occurs later - on a totally unrelated well. This makes it tough on the owner of the equipment to mitigate. Designate inventory for these conditions and keep great maintenance records as a 1st line of defense. If you see a component with constant micro cracks - replace it. I think Brine water drilling fluids may multiply the effect on rotors and non-mag. I also worry more about the float sub than float valve as a highly respected 3rd party auditor made my team cut all our subs to the wrong spec before being approved for use which over time became the norm for all Machine shop/rental companies. I know it caused MWD failures; I hope no one died from H2S if they relied on this component.
Unemployed
7 年BOP's
Drilling Consultant at EOG
7 年What is your barrier at connection time if you are drilling underbalanced?????
Directional Drilling Engineer at Henderson Directional Drilling.
7 年Gentlemen - I agree completely. The float valve should never be considered a primary well control barrier. I was replying to the gentleman solely on the basis of should a pressure test be requested by client.