Page 30 - Origin and Prediction of Abnormal Formation Pressures

INTRODUCTION TO ABNORMALLY PRESSURED FORMATIONS 13

decrease in salinity of water are not clear. In analyzing this problem, one should consider
the following established facts.
(1) Water in shales is much fresher than that in associated sands and sandstones
(Schmidt, 1973; Chilingar and Rieke, 1976).
(2) Influx of fresher shale water into the associated sandstone reservoirs results in
freshening of produced water as production of oil and water progresses (Rieke and
Chilingarian, 1976; Chilingarian et al., 1994).
(3) Salinity of water in well-compacted shales is lower than that in associated
undercompacted shales, but still remains lower than those in associated sands and
sandstones (Chilingar and Rieke, 1976).
(4) In thick sand-shale sequences, with overpressured formations, the salinity of
interstitial water in shales and sandstones often decreases with depth (Rieke and
Chilingarian, 1976).
(5) Water in the center of shale capillaries is more saline than water adjacent to the
capillary walls (Rieke and Chilingarian, 1974).
(6) There is good correlation between the salinity of interstitial water in shales and
sonic data, which is used for prediction of abnormal formation pressure (Vorabutr et al.,
1986).
On considering the above-established facts, one may consider the following sugges-
tions:
(1) In thick shale sequences, as compaction water moves up, it becomes more saline.
Thus, in undercompacted sediments, the salinity of formation waters may decrease with
increasing depth. Also, the more-compacted shales below will contain fresher water
than less-compacted shales above.
(2) In interbedded sands and shales, the variation of salinity with depth is not clear
(see Kucheruk and Shenderey, 1975) and considerable field and laboratory research
work is required to elucidate the problem.

RESERVOIR ENGINEERING CONCEPTS IN ABNORMAL PRESSURE ENVIRONMENTS

Much attention has been focused on the analysis of hydrocarbon reserves, reser-
voir behavior, and possible mechanisms important to the production from abnormally
high-pressured reservoir rocks. Frequently, overpressured gas reservoirs do not be-
have as volumetric reservoirs which complicate gas-in-place estimates. For years it
has been observed that the production curves in many gas reservoirs show a rapid
decline in the early-life history after which they flatten out (Fig. 1-7). The reader is
also referred to the classical book of Poston and Berg (1997) on overpressured gas
reservoirs.
The following characterize overpressured reservoirs: (1) water influx from the
shales into adjacent pay sands (i.e., shale water influx) (Wallace, 1969); (2) rock
compressibility and rock failure (Harville and Hawkins, 1969); (3) water influx into the
reservoirs from limited aquifers.