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Overview of PETGAS

Phase I of the PETGAS (Petrophysics of Tight Gas Sandstone Reservoirs) JIP was undertaken by the University of Leeds, and sponsored by Aurelian Oil and Gas plc, BG, BP, EBN, Shell and Wintershall. A key aim of PETGAS is to create An Atlas of the Petrophysical Properties of Tight Gas Sands containing detailed descriptions of the properties of individual samples (e.g. porosity, gas and brine permeability, Hg-injection characteristics, diagenetic history, mineral composition) as well as individual sections outlining key controls on the petrophysical properties.

Each sponsor provided ~25 tight gas sandstone (TGS) samples. Routine core analysis was conducted on all samples and special core analysis (SCAL) was conducted on subset of ~35% of these  samples. Around 50% of the samples can from the Rotliegend of Europe (UK, Netherlands and Poland) the remaining samples were from Jurassic, Triassic and Carboniferous reservoirs from Europe as well Argentina, Oman, North America and Ukraine.

Analyses undertaken


In total, the PETGAS sponsors provided ~25 samples each for routine core analysis (RCA) and around 35% of these were then subject to special core analysis (SCAL). In the original project proposal the RCA analysis was to involve:

  • X-ray CT tomography;
  • Microstructural analysis using secondary electron microscopy (SEM),
  • Quantitative X-ray diffraction analysis (QXRD),
  • He porosity;
  • Hg-injection porosimetry;
  • Gas permeability at a single effective stress
  • Brine permeability at a single effective stress;
  • Ultrasonic velocities (Vp, Vs1, Vs2) at a single effective stress.
  • Electrical resistivity at 100% brine saturation.
At the start up meeting, several sponsors asked that the routine core analysis should be expanded to include:
  • Gas permeability at a range of effective stresses up to the in situ stress
  • Brine permeability at a range of effective stresses up to the in situ stress
  • Ultrasonic velocities (Vp, Vs1, Vs2) at a range of effective stresses up to the in situ stress.
  • NMR T2 distributions.
We therefore expanded the RCA program to meet this request. The SCAL program has included:
  • Capillary pressure analysis using humidity chambers, porous plates and the centrifuge.
  • Electrical resistivity measurements as a function of saturation.
  • Brine permeability and electrical resistivity of 100% brine saturated samples as a function of stress.
  • Gas relative permeability analysis at each capillary pressure.
  • Stress dependence of relative permeability.
  • NMR T2 at each saturation.
  • Pore volume compressibility.
  • Static elastic properties.



The key deliverables from PETGAS are:

Atlas of Petrophysical Properties of Tight Gas Sandstones

This allows the user to search for samples according to depositional environment, porosity and permeability as well as diagenetic rock type. Webpages with sedimentary and wireline log data as well as core photographs from around where individual specimens were taken are given for each well are given for each well. The position of each specimen is then hyperlinked to pages containing details of the petrophysical properties. The atlas also allows the user to download raw data from individual analyses (e.g. Hg injection, NMR etc so that they can process it using in-house techniques.
Sponsor specific reports
These present the results obtained from the analysis of the samples provided by the individual sponsor. The reports identify key controls on individual properties as well as correlations between individual properties. An attempt has also been made to interpret the wireline log data provided based on the results from the PETGAS study. The effective kh values obtained have then been incorporated into a simple analytical model for radial flow in a gas reservoir to estimate potential flow rates that would be expected from each well.

Reports on specific results

We have attempted to write individual reports in specific results or petrophysical properties. These reports take two forms. The first are a series of “PETnotes”, which are 2-4 page summaries highlighting key points related to the subject area. The second are more detailed reports on individual aspects of the project. Key PETnotes include:
  • PN1: Overview of PETGAS
  • PN2: Rapid petrophysical property estimates from microstructure
  • PN3: Controls on Archie cementation exponent
  • PN4: Comparison of gas and brine permeability
  • PN5: Stress dependent permeability
  • PN6: Capillary pressure from Hg injection tests
  • PN7: Klinkenberg corrections
  • PN8: Comparison of stress sensitivity of different petrophysical properties
  • PN9: NMR permeability predictions
  • PN10: Diagenesis vs depositional environment as a control on k-ø trends
  • PN11: Impact of core damage on petrophysical properties
Detailed reports include:
  • PR1: Controls on porosity-permeability relationships
  • PR2: Electrical properties of tight gas sandstones
  • PR3:  Ultrasonic properties of tight gas sandstones.
  • PR4: Static-to-dynamic elastic property conversion.
  • PR5:  Crack density and aspect ratio inversions for tight gas sandstones
Overall, the PETGAS project has collected so much data that we could probably continue to analyse the data and write reports for many years to come. However, as Phase I funding runs out at the end of 2012 we shall continue to add to these reports until that date.