Mr. Erik Milito - The House Committee on Natural Resources ...

ISOLATING POTENTIAL FLOW ZONES DURING WELL CONSTRUCTION 45
<strong>The</strong> above 72 year gas migration time period removes cement permeability as a factor or cause for many gas
migration occurrences. MMS statistics on well ages for SCP initiation (see graph in A.15) show that the vast majority
of cases occur in less than 10 years instead of several decades like the 72 year example calculation above.
When cements contain materials that resist unfavorable downhole conditions and allow very low permeabilities to be
achieved, gas migration travel time periods calculated with fractions of a millidarcy (mD) may be several hundreds or
thousands of years depending on cement column lengths and differential pressures. Calculation of time to flow
through permeability does not eliminate permeability as a concern, especially as permeability may act in concert with
other wellbore or cement performance factors, such as communication with channels, high permeability pathways,
etc. Additionally, filtrate water does not have to flow all the way to the surface; it can flow into shallow or shallower
formations. Other factors may restrict annular gas flow and increase gas migration travel times such as a sealed and
fluid filled annulus above the TOC that does not provide a vent for the cement filtrate water pushed out of the cement
top by gas migration.
A.4 Barrier Failure Study
A study of LWC incidents in U.S. areas of the Gulf of Mexico OCS and some of the coastal states from 1960 to 1996
is reported in SPE/IADC 39354 by Skalle and Podio [27] . <strong>The</strong> many types of barrier element failures listed below in
Table A.1 (from Table 6 of SPE/IADC 39354) may be prevented with the updated and proven practices described
within this API publication. Note the higher total of failures for mechanical vs cement types of barrier elements.
Table A.1—Most frequent Primary and Secondary Barriers that Failed in all Phases
(Louisiana + Tx + OCS; 1960 to 1996)
Primary Barrier BO Secondary Barrier BO
Swabbing 158 Failed to close BOP 78
Too low drilling fluid weight 50 Rams not seated 14
Drilling break/unexpectedly high pressure 45 Unloaded too quickly 13
Formation breakdown/lost circulation 43 DC/Kelly/TJ/WL in BOP 5
Wellhead failure 40 BOP failed after closure 66
Trapped/expanding gas 40 BOP not in place 43
Gas cut drilling fluid 33 Fracture at casing shoe 38
Christmas tree failure 23 Failed at stab valve/Kelly/TIW 34
While cement setting 20 Casing leakage 23
Unknown why 19 Diverter—no problem 21
Poor cement 16 String safety valve failed 19
Tubing leak 15 Diverter failed after closures 17
Improper fill up 13 Formation breakdown/ lost circulation 15
Tubing burst 10 String failure 13
Tubing plug failure 9 Casing valve failed 11
Packer leakage 6 Wellhead seal failed 10
Annular losses 6 Failed to operate diverter 7
Uncertain reservoir depth/ pressure 6 Christmas tree failed 7