Page 20 - Enhanced Oil Recovery in Shale and Tight Reservoirs

10 Enhanced Oil Recovery in Shale and Tight Reservoirs

higher than the initial reservoir pressure of the interest. Even at such a higher
pressure, oil may not enter very small pores. A core cannot be fully saturated
by oil in practice. This partial saturation is justified by the fact that oil in the
very small pores (e.g., a few nanometers) cannot be produced anyway.
Therefore, the oil recovery factor from laboratory may be at a higher side
because of this partial oil saturation. The weight of the saturated oil is
W sat W dry . Our experience shows that this error is not significant, as
we checked the oil weights at different saturation pressures; we also checked
the oil weight in the core compared with the pore volume which was inde-
pendently measured by nitrogen injection or a CT scanner.
When CT is used, the porosity calculation formula can be derived. If the
porosity is known, the pore volume is known and the oil weight in the pore
volume can be compared with the weight difference between the saturated
core and the dry core. If the oil weight is equal to or very close to the weight
difference, the core is fully saturated.
Assume the rock is fully saturated with oil, the total mass of the oil-
saturated rock is equal to the total mass of oil and rock:
V or r ¼ V o r þ V r r r (2.1)
or
o
In the above equation, V or ,V o and V r are the rock bulk volume whose
pores are fully saturated by oil, oil volume, and solid rock volume, respec-
tively, and r or , r o , and r r are the densities for the rock bulk fully saturated by
oil, oil and rock itself, respectively. Divided by V or for each term, the above
equation becomes

r ¼ fr þð1 fÞr r (2.2)
o
or
f is the porosity. Assume that the density of a substance is proportional to
the CT number measured in the substance; the above equation can be
written as

(2.3)
CT or ¼ fCT o þð1 fÞCT r
Similarly, for a dry rock which is saturated by air,

CT ar ¼ fCT a þð1 fÞCT r (2.4)
The subscripts o, r and a represent oil, rock, and air, respectively. From
the above two equations, the porosity can be estimated by

CT or CT ar
f ¼ (2.5)
CT o CT a