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

50 API STANDARD 65-2
<strong>The</strong> API Work Group [3] studied 14 of the 19 LWC incidents linked to cementing that occurred from 1996 to 2001 on
the U.S. outer continental shelf (OCS), i.e. annular flow events during or after cementing operations. Conclusions of
the study of the 14 incidents are listed below.
1) Most of the LWC incidents studied took place during or just after cementing surface casing.
2) In more recent years (2003 to 2004), these events involved deep casing strings with no occurrence of LWC
incidents in surface casing cementing operations.
3) Most wells used a mudline hanger/suspension system.
4) Frequently the annulus between surface and conductor casings at the surface was washed out to a point 30 ft
to 50 ft below the mudline after cementing. Washing out this annulus resulted in a small but possibly very
significant reduction in hydrostatic pressure while also impairing the operation of the BOP and diverter (wash
pipes in the annulus prevents sealing).
5) Often, cement slurries were not designed to prevent flows.
6) Effective drilling fluid removal and zonal isolation practices were not followed.
<strong>The</strong> study included reviews of detailed information on the incidents including “lessons learned” presentations by many
of the operators involved. Public documents were available for some of the incidents that reported causes and
proposed preventive measures. <strong>The</strong> studied incident information and the membership’s knowledge of annular flow
events in other areas allowed the Work Group to prepare proven practices contained herein to help prevent future
annular flow incidents and also help reduce the occurrence of SCP.
A.10 Lost Circulation Increases Risk for LWC Incidents
Lost circulation before, during, or just after primary cementing.
a) Can cause a failure to maintain an overbalance across potential flow zones exposed in the wellbore whereby:
1) an inadequately designed cement slurry (density too heavy, etc.) fails to reach the designed depth for the TOC
column;
2) or the drilling fluid column is reduced or “falls back” or “goes on vacuum;”
3) and either one of these shortened columns results in an insufficient hydrostatic head pressure to overbalance
formation(s) pore pressures.
b) Has often been found by investigators as the root cause for many of the LWC incidents experienced in offshore
drilling operations.
c) Can induce LWC incidents at any depth in the well construction process from soon after “spudding” (starting to
drill) the well to drilling the well at total depth when conditions occur such as:
1) structurally weak zones are exposed in the wellbore;
2) naturally occurring leak off flow paths are encountered such as fractures, faults, vugs, caverns, etc.
As mentioned above, lost circulation during primary cementing operations may cause reduced hydrostatic pressure
and underbalanced conditions when losses cause the drilling fluid column to fall to create an underbalance. For
example when heavier density (than the drilling fluid) cement slurries are removed from the annulus by total or partial
lost circulation (cement flows into weak zones), the TOC can be much lower than the designed top of cement depth.