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THE EFFECT OF OIL SATURATION ON THE MECHANISM OF COMPACTION IN GRANULAR MATERIALS:
HIGHER OIL SATURATIONS LEAD TO MORE GRAIN FRACTURING AND LESS PRESSURE SOLUTION
SHANVAS SATHAR, RICHARD H. WORDEN, DANIEL R. FAULKNER, AND P. CRAIG SMALLEY
Journal of Sedimentary Research, 2012, v. 82, 571–584


ABSTRACT : Whether addition of oil to sandstones slows, stops, or has no influence upon pressure solution and quartz
cementation has long been disputed, despite having major implications for petroleum exploration and appraisal strategies. To
elucidate the effect of addition of oil to compaction, pressure solution, and cementation processes, this study utilizes an
experimental approach simulating isochemical, volumetric compaction using granular halite in the presence of variable brine–
oil mixtures. Each experiment, at 500 kPa effective stress, lasted 48 hours and involved repeated measurements of volumetric
strain. The lithified products of experiments were examined using SEM techniques. After 30 hours, approximately 4%
volumetric strain occurs when the brine–oil ratio is 100:0 (pure brine). Fractures are rare in samples from experiments
undertaken with pure brine so that pressure solution, and concomitant cementation, must be the compaction mechanism. When
the brine–oil ratio is decreased from 100:0 to 40:60, the volumetric strain remains about 4% but there are more fractures, as
quantified from SEM image analysis. These observations suggest that, although pressure solution has occurred, some of the
volumetric strain is the result of fracturing, implying that pressure solution has been less effective in the presence of some oil
than it is in the presence of pure brine. When the brine–oil ratio is decreased further from 40:60 to 5:95, the volumetric strain
increases up to 7.3%; SEM image analysis reveals that the strain is predominantly due to grain fracturing. This change from
pressure solution to grain fracturing is likely due to heterogeneous pressure solution resulting from heterogeneous distribution
of stresses. Brine-coated grain contacts undergo pressure solution but oil-coated grain contacts experience increasing stress up
to the point of failure, as the surrounding grain pack compacts. When the brine–oil ratio is decreased to less than 5:95,
volumetric strain decreased to about 1% with few induced fractures and little pressure solution. Compaction is negligible when
the brine–oil ratio is 0:100 (only oil in the pores) and fractures are negligible. Halite is more water-soluble and more
hygroscopic than quartz so that water will cling to halite surfaces more tenaciously than to quartz. Since even halite has its
mechanism of volumetric strain affected when the brine–oil ratio is less than 40:60, quartz pressure solution and subsequent
cementation is even more likely to be affected by oil filling. This experimental study suggests that the mechanism and extent of
compaction and subsequent cementation in sandstones is strongly affected by the addition of oil.


During oil filling of a reservoir, formation water will be gradually expelled until the pores are largely filled with oil. In these diagrams, th e size of the arrows is
proportional to the magnitude of the normal stress acting on the grain contacts.A)At the early phase, the stress distribution is homogeneous along all the grain contacts
irrespective of the presence of oil or brine. B) With time, pressure solution along the grain contacts where the aqueous fluid is available may lead to heterogeneous
distribution of stresses at different grain–grain contacts. At the remaining contacts occupied by oil, no pressure solution occurs. C)This heterogeneous pressure solution
results in the accumulation of stress along the grain contacts occupied by oil and decrease in stress along grain contacts occupied by water as a resultof pressure solution.
D)Sufficient stress accumulates along the oil-occupied grain contacts until it responds byE) grain fracturing. This sort of fracturing does not occur when the brine–oil
ratio is either 100:0 or 0:100 because pressure solution tends to be, respectively, homogeneous or totally absent under those conditions.