BAKER HUGHES - Drilling Fluids Reference Manual

CORROSION
• Saturated Salt Fluids – Oxygen solubility is low but conductivity is high; chlorides increase
pitting attack. These fluids may have moderately high oxygen contents due to O 2 entrapment.
• Dispersed – Slightly corrosive; less oxygen entrapment due to lower gel strengths and yield
points. In addition, most dispersants have some oxygen scavenging ability.
• Lignosulfonates – Slightly corrosive due to same conditions indicated in Dispersed.
Lignosulfonates are, to a certain degree, natural oxygen scavengers. In addition, lignosulfonate
fluids usually have a pH high enough (9.0 or greater) to greatly reduce corrosion. However,
where salts are present, conductivity may be increased, thereby increasing corrosivity.
• Synthetic and Oil-Based Fluids – Considered non-corrosive
Note:
Aluminum drillpipe forms an aluminum oxide coating or film that protects it
from corrosion in many environments. This film is quite stable in neutral
conditions but is attacked by alkalinities. Therefore, when using aluminum
drillpipe, the pH of the fluid should be at 9.0 or below.
USE OF CORROSION COUPONS
Ring Coupons
The most effective method of measuring the corrosivity of a drilling fluid involves the use of preweighed
ring coupons. These rings are sized to fit into the relief groove in the tool joint box. These
rings are placed in the drillstring and exposed to the fluid for a period of time during the drilling
operation. After exposure to the system for a minimum of 40 hours, the rings are retrieved, cleaned,
and reweighed in the laboratory to within 1/10 milligram. The difference between the initial and
final weights (or the weight lost) is attributed to corrosion, and the corrosion rate is calculated and
reported as lb m /ft 2 /year or mils/year. The term mils/year refers to the loss of metal in thousandths of
an inch per year. Two types of rings are available, bare uncoated steel and steel rings having a hard
plastic backing. See Table 8 - 2.
BAKER HUGHES DRILLING FLUIDS
REVISION 2006 8-8
REFERENCE MANUAL