Persistent sustained casing pressure precludes regulatory approval
During a recent project, a dry-gas drilling program in North Africa was
plagued with persistent well integrity issues. After being drilled with a
polymer-based mud, the wells’ 13 3/8- and 9 5/8-in casing sections were
typically cemented with a dual-slurry system that tended to bond poorly, which
was a precursor to SCP development that limited production from the newly
drilled wells and precluded regulatory approval. Some wells experienced hole
instability or lost circulation, but the problems were unpredictable. In
addition, almost the entire intermediate section of these wells included small
uneconomic volumes of gas that challenged zonal isolation.
Initial attempts to remedy the problem included changing weighting
agents and adding microsilica. Engineers also proposed a dual-lead cement
design with the first slurry overretarded. These options increased operational
complexity without significantly improving bond logs. The operator also
considered casing reciprocation and applying backpressure on the setting cement
but dismissed both options because of the increased risk of lost circulation
and limited rig capability. A specialty chemical wash was also proposed but
declined because it did not meet strict North Sea regulations.
Engineers optimize spacer and cement systems for well conditions
After a thorough review of well conditions, bond logs, and TOC issues,
Schlumberger cementing experts discovered the existing spacer system was not
optimal for the polymer-based drilling mud. The experts also determined that
the cement design had not adequately addressed possible cement shrinkage and
debonding in the downhole environment.
After laboratory testing with several hole-cleaning options, the experts
recommended pumping CemPRIME Scrub spacer ahead of the cement based on its
superior performance with the drilling mud.
Next, the experts adjusted the cementing plan to limit gas channeling by
switching to FlexSTONE technology, which limits cement permeability and
includes an optimized expanding agent to counteract cement shrinkage and reduce
the risk of gas channeling while setting. The slurry density was also optimized
at 1.6 g/cm3 [13.35 lbm/galUS].
To ensure slurry delivery as designed, the engineers also called for
batch mixing rather than real-time mixing that was customary in the field. To
further mitigate the risk of SCP, inflatable and swellable annular casing
packers (ACPs) were used in addition to the flexible cement.
Excellent bond log and pressure tests enable gas production
The first field application of the new spacer and cement designs
resulted in excellent cement bonding; however, because of losses in a weak
formation, the cement did not reach the TOC target.
In subsequent wells, the operator used a lost circulation treatment
while drilling to avoid cement losses during placement. Consequentially, TOC
was reached as designed in the second well, which also had excellent cement
bond to the casing and well tests with zero annulus pressure. It was also
verified that the set cement maintains integrity during anticipated load cases
after cement placement. As a result, the operator was granted regulatory
approval for the well.
Based on this success, the operator has incorporated the lessons learned
and best practices into future operations for wells of similar design and
conditions.
Conventional cementing designs resulted in poor bonding and
sustained casing pressure, as demonstrated by a 200-m section of the well log
(left). Cement bond logs improved across the same section in a newer well
(right) after engineers redesigned the cementing program to use the CemPRIME
Scrub spacer and FlexSTONE technology.
