Core Analysis Issues in Tight Gas Reservoirs
Miller, Michael, Robert Lieber, Eugene Piekenbrock, and Thalbert McGinness
BP Americas Inc, Houston, TX
Increased focus in tight gas reservoirs has stirred a debate
concerning potential uncertainties in determining gas in place and
recoverable gas. There are questions concerning the reliability
(accuracy and reproducibility) and applicability of routine and special
core analysis measurements to the in-situ rock. Small pore volume
and the low flow capacity make these rocks particularly sensitive to
measurement errors and make it difficult to reproduce in-situ
conditions. A survey of some recent literature provides a glimpse at
the state of the art in low permeability core analysis procedures.
Recently, it has been shown that the most commonly used unsteadystate
technique over estimates permeability. The differences are most
significant for permeability less than 0.01 md. Legacy data for rocks
with permeability of less than 0.01 md will be biased high, potentially
by up to an order of magnitude
. Multiphase permeability
measurements show a wide variability of permeability reduction with
changes in wetting phase saturation. Modeled gas recovery varies by
more than 30 percent based on these data. Differences in irreducible
water saturation from capillary pressure curves exist depending on
test method. Uncorrected high-pressure mercury injection data often
inaccurately characterizes capillary pressures at irreducible water
saturation. Formation water salinity can show significant variability
(+/- an order of
magnitude
) when reconstructed from a Dean Stark
analysis. Water resistivity and saturation in core is difficult to
measure in rocks with low total pore volume. Archie saturation
exponent can vary depending on analysis technique. Single point
versus multipoint resistivity index measurements and test duration
can have a large effect on saturation exponent. These tests can take
weeks/months instead of days to become stable. The prudent
evaluation of low permeability rocks worldwide requires the ability to
understand and limit these and other sources of petrophysical
uncertainty.
AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah