Page 369 - Numerical Analysis and Modelling in Geomechanics

350 RESERVOIR COMPACTION, SUBSIDENCE AND WELL DAMAGE

Figure 11.6 Showing that the South Belridge field is a north westward tending anticline.

However, while subsidence decreased, and in some cases surface elevation was
32, 38
recovered, the annual well failure rate remained constant at about 2% to 5%.
Although low, failure rates of the magnitude had a sufficiently adverse impact on
field economics to warrant implementation of additional well failure mitigation
measures.
Throughout the South Belridge field up to 90% of the failures occur in the
overburden. 39 In Section 33, most casing damage and failures occur at three
discrete depths: at the Tulare-diatomite unconformity from about 500 to 700 feet
(152 to 213 metres), and at locations of about 300 feet and 350 feet (91 to 107
metres) above the unconformity. The most frequent failure locations in the
overburden of the diatomite have been identified with thin shale or mudstone
layers, referred to here as the A1 shale and the D1 shale, with the D1 shale being
the deeper of the two strata. In Section 33 the shale layers are fairly continuous
across the section, though in other sections the shales are not continuous. A study
of well workover records for Section 33 showed that the casing damage and