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

ISOLATING POTENTIAL FLOW ZONES DURING WELL CONSTRUCTION 43
setting or dehydrated “bridges” of a portion of the cement in the upper parts of the cement column. Another way to
visualize this phenomenon is to think of these early set or bridging points as “artificial annular packers” made from
cement or “annular cement packers.” Included in the field study were cases where logs were used to identify and
measure annular flows between zones after casing and liners were cemented.
Also during the 1960 to 1980 time period other investigators, Carter and Slagle [19] and Christian et al [20] presented
substantial evidence of the cement packer effect also called “hydrostatic-pressure bridging” above potential flow
zones that resulted in costly annular flows. In 1979, Tinsley et al [21] identified continuing annular gas flow problems
and associated costs from several tens to many hundreds of thousands of U.S. dollars per wells. Several years of
research efforts to find solutions with associated field applications were summarized by Tinsley. One major finding
was that compressible cement systems had positive results in reducing the occurrence of annular gas flow. Included
in the field study were several offshore wells in the High Island area of the Gulf of Mexico where annular gas flow
events after cementing surface and intermediate casing strings had caused uncontrolled releases of gas to the
surface and to the atmosphere (called blowouts in the paper). Also land wells in South Texas were studied that had a
history of annular gas flows after cementing production casing and liners causing communications between zones.
Other areas with annular gas flows were identified that in low to moderate flow rate cases cause loss of production to
thief zones above and/or below the production interval and in severe cases (also called underground blowouts) result
in high risk conditions for safe well operations. Tinsley cited the researcher’s consensus of opinion on the “annular
cement packer” phenomenon mentioned above that “once pressure in the annulus has decreased by as little as 0.5
psi less than the formation pressure, gas flow can occur” and “this gas entry tends to form a gas channel in the
cement column.” Lab studies were presented that better defined how cement slurries can develop SGS which
prevents transmission of hydrostatic pressure in cement columns. In addition to having “fluid-loss-controlled” cement
slurry properties, Tinsley said that “free water” control in cement slurries, as identified by Webster [22] , was needed for
compressible cement systems to provide more comprehensive solutions to annular gas flow problems. This
combination of cement performance properties was 90 % successful in preventing annular gas flows in over 200 well
applications.
Martinez et al [23] studied the causes of annular gas flow and LWC incidents in Outer Continental Shelf (OCS) wells
for the U.S. Department of Energy (DOE) and published a report on their work in 1980. This report is available at http:/
/www.mms.gov/tarprojects/027.htm This DOE study report includes case history reports on annular flows after
cementing including “USGS” federal agency (pre-MMS) reports on LWC incidents that had similar causes to those
disclosed herein API Std 65—Part 2. Also contained within API Std 65—Part 2 is more information on annular flow
causes and up to date solutions to this challenge. <strong>The</strong> benefits of developing and/or implementing solutions to these
issues were outlined on pp.6-7 in the DOE report and are still applicable today as follows:
1) safety improved by reduced risk from:
a) underground blowouts,
b) pressurized shallow sands,
c) blowout adjacent conductor and potential loss of platform;
2) environmental protection enhanced by reduced potential for leaks to the seafloor or shallow formations;
3) economics:
a) expensive blowout risk is reduced (as well as public loss of confidence in industry and agencies),
b) reduced well control problems save drilling time and cost,
c) remedial squeeze jobs are reduced.