Gas porosity is
the fraction of a rock or sediment filled with a gas.
Determining the true porosity of
a gas filled formation has always been a problem in the oil industry.
While natural gas is a hydrocarbon, similar to oil, the physical
properties of the fluids are very different, making it very hard to correctly
quantify the total amount of gas in a formation. Well logging interpretation of
the amount of hydrocarbon in the pore space of a formation, relies on the fluid
being oil. Gas is light compared to oil causing density logging (gamma
ray emitting sensors) based measurements to produce anomalous signals.
Similarly, measurements that rely on
detecting hydrogen (neutron emitting sensors) can miss detecting
or correctly interpreting the presence of gas because of the lower hydrogen
concentration in gas, compared to oil.
By properly combining the two erroneous
answers from density and neutron logging, it is possible to arrive at
a more accurate porosity than would be possible by interpreting each of the measurements
separately.
A popular method of obtaining a formation
porosity estimate is based on the simultaneous use of neutron and density logs.
Under normal logging conditions, the porosity estimates obtained from these tools
agree, when plotted on an appropriate lithology and fluid scale.
However, in the case of a reservoir where there is gas instead of water or oil
in the pore space, the two porosity logs separate, to form what is referred to
as gas crossover. Under these conditions, the true formation porosity lies
between the measured neutron and density values. Log interpreters often find it
difficult to accurately estimate the true formation porosity from these two
curves.
Neutron and density logging tools have
different responses to the presence of gas in the formation because of
differences in the physics of the measurements. A neutron tool response is
sensitive mainly to the number of hydrogen atoms in the formation. During the
calibration process, water-filled formations are used to develop
porosity algorithms, and under these conditions, a lower number of
hydrogen atoms is equivalent to a lower porosity. Consequently, when a
gas-filled formation is logged, which has a lower number of hydrogen atoms than
a water-filled formation of the same porosity, the porosity estimate will be
lower than the true porosity.
The density tool, on the other hand,
measures the total number of formation electrons. Like the neutron tool,
water-filled formations are used in its calibration process. Under these
conditions, a lower number of electrons is equivalent to a lower formation
density, or a higher formation porosity. Therefore, logging a gas-filled
formation, results in a porosity estimate that is higher than the true
porosity. Overlaying the neutron and density curves in a gas-bearing zone
results in the classic crossover separation.
