Page 368 - Numerical Analysis and Modelling in Geomechanics

RESERVOIR COMPACTION, SUBSIDENCE AND WELL DAMAGE 349
model and a fully three-dimensional model may, in fact, be more economical and
efficient.

Example computation: the South Belridge field
The Belridge field is located about 45 miles west of Bakersfield, California, as
shown in Figure 11.6.

Reservoir geology
Detailed descriptions of the geology of the Belridge field have been published by
93
Bowersox, and a brief synopsis of the facts relevant to the compaction problem
is included here. A geologic cross section of Section 33 of the field is shown in
Figure 11.1. The South Belridge field is a northwestward trending anticline. The
Belridge diatomite is a massive reservoir, in some locations, more than 1000 feet
(305 metres) thick. The porosity of the diatomite is high, varying from 45% to
75%, but the matrix permeability is low, 0.1 mDa or less. However, this feature
of the Belridge diatomite also makes it highly susceptible to compaction as the
reservoir is depleted. Overlying the Belridge diatomite are the Tulare sands,
which are produced simultaneously with, but independently of, the diatomite. In
fact, to date about 75% of the production in the Belridge field has been from
Tulare sands. Thin layers of laterally continuous shales (i.e., mudstones), 10 to
50 feet thick (3.0515 to 15.24 metres), have been identified throughout the
Tulare, separating major sand intervals, and at the unconformity between the lower
Tulare sand and Belridge diatomite. Unsaturated alluvium, near the surface,
along with the Tulare sands, comprises the overburden above the Belridge
diatomite.

Historical well failures
Aggressive production in the early 1970s was spurred by improvements in
techniques to hydraulically fracture diatomite reservoirs. 94 However, with the
increased production came reservoir compaction, surface subsidence and well
31
damage and failures. Measurements taken from 1987 to 1995 show that surface
31
subsidence reached 12.5 feet (3.8 metres) in Section 12, greater than 9 feet (2.
32
40
74 metres) in Section 19, more than 10 feet (3.05 metres) in Section 33, and
nearly 20 feet in other areas of the field. Surface subsidence in Section 12 was so
severe that surface fissures formed at the edges of the subsidence bowl. 31
Problems servicing wells and casing failures followed the trend of increasing
subsidence. In some areas of the field 20% of the wells were damaged or failed.
In Section 33, the well failure rate averaged about 4% per year between 1985
and 1996, reaching a peak of almost 8% in mid-1988. To stem the rate of
subsidence and mitigate the apparently related trend in well failure rates, water
injection programs were initiated in many areas of the field in the early 1980s.