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 unsteady-state 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 are more difficult to conduct than single phase measurements. Recently published data 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. Typically, higher irreducible water saturations are seen from capillary pressure curves using vapor desorption data and high-speed centrifuge or high-pressure porous plate data in low permeability rocks.
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 (n) 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.
