Page 58 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 49

The boundary between “sweet” and “sour” service is defined by
d
NACE as 0.05 psi (0.3 kPa) H 2 S partial pressure. Partial pressure can be
defined as the pressure exerted by a single gas in a mixture of gasses, and
is the pressure that would be exerted by a single gas if it alone occupied
the volume originally occupied by the mixture. To establish the partial
pressure of H 2 S from mole % in the gas phase:
(Mole % H S 4 100) 3 bubble point pressure.
2
To convert ppm to percent:
ppm H 2 S
1% 5 10; 000 ppm therefore %H 2 S 5
10; 000
Alternatively, to establish partial pressure directly from ppm
Partial pressure of H S 5 (PPM H S 4 1,000,000) 3 bubble point
2 2
pressure.

1.1.10.3 Carbon dioxide (sweet) corrosion
Carbon dioxide (CO 2 ) or “sweet” corrosion is far more common than
H 2 S corrosion in most producing wells. Carbon dioxide attacks metal due
to the acidic nature of dissolved carbon dioxide (carbonic acid). The acid-
ity (pH) is largely dependent on the partial pressure of CO 2 in the pro-
duced fluids. To calculate the partial pressure of CO 2 (Mole % CO 2 4
100) x bubble point pressure.
Chromium is added to steel to reduce CO 2 corrosion, with 13%
chrome tubing being widely used for completions. Pressure control
equipment that is only exposed to static well fluids is not normally con-
structed using corrosion resistant alloys, since CO 2 corrosion rates are
negligible when the produced fluid is static.

1.1.11 Roles and responsibilities
Completion and Well Intervention Operations often involve many indivi-
duals from a range of disciplines, all coming together to perform a task
that is complex and potentially hazardous. Each individual must have the
necessary training and skills to enable them to perform their assigned task
safely and efficiently. Crucially, they must have the competence, experi-
ence, and confidence to know what to do in the event of an emergency.
d
Petroleum and natural gas industries. Materials for use in H 2 S-containing Environments
in oil and gas production. Part 1: General principles for selection of cracking-resistant
Materials. NACE MR0175/ISO 15156-1:2001 ISBN 1-57590-1765, 2001.