8. Fast petro-physical analysis of unconventional gas reservoirs to assist in improving drilling strategies.
Submitting InstitutionUniversity of Leeds
Unit of AssessmentEarth Systems and Environmental Sciences
Summary Impact TypeEconomic
Research Subject Area(s)
Earth Sciences: Geophysics
Engineering: Resources Engineering and Extractive Metallurgy, Interdisciplinary Engineering
Summary of the impact
Research performed at the University of Leeds allows the petroleum industry to reduce radically
the amount of time that taken to estimate the key properties of tight sandstones containing natural
gas. These properties largely determine whether gas fields are economically viable. Tests used in
the past have taken between six months and two years to complete; with the Leeds research,
results can now be obtained in less than one day — a radical improvement. Industry has used the
results to justify drilling new prospects and to improve understanding of the controls on gas and
water production in existing fields, which has shaped appraisal and production strategies.
As conventional (high permeability) petroleum resources are becoming depleted, the petroleum
industry is now increasingly turning to unconventional reservoirs, such as tight gas sandstones and
shale reservoirs, to meet global energy demands. These reservoirs are difficult and only marginally
economic to produce. It is therefore essential to dramatically reduce costs in the exploration,
appraisal and development of these reservoirs.
The University of Leeds has established an excellent reputation in industry and academia for
conducting high-quality research on the single and multiphase flow properties of low permeability
rocks [1,2,3]. In response to an industry-led call for future research into unconventional gas
reservoirs, Quentin Fisher established a joint industry project entitled PEtro-physics of Tight Gas
Reservoirs (PETGAS). The initial phase of the project took place from 2009 to 2012 and was
sponsored by Aurelian Oil and Gas, BG, BP, EBN, Shell and Wintershall to a cost of £900,000. A
second phase of the project started in July 2012. The aim of the project was to improve
understanding of the petro-physical properties (porosity, permeability, relative permeability,
capillary pressure, elastic moduli, and electrical resistivity) of tight gas sandstone reservoirs to
improve predictions of reservoir performance and to reduce costs associated with reservoir
characterization in these marginally economic developments.
Fisher conducted a wide range of tests on the petro-physical properties of around 150 tight gas
sandstone samples provided by sponsors. Analyses included: microstructural and mineralogical
assessment, porosity, gas and brine permeability, as well as compressional and shear wave
velocity as a function of stress, electrical resistivity, NMR T2 distribution and Hg-injection
porosimetry. After rock typing, 40% of the samples were subject to special core analysis including
capillary pressure, resistivity and relative permeability as a function of stress, cation exchange
capacity, surface area analysis and NMR cryoporometry. In addition, experiments were conducted
to address specific issues such as the use of restricted rate practice to enhance production from
tight gas reservoirs. These parameters were matched with the microstructure of the samples when
viewed in a microscope. The results showed that the key petro-physical properties of the tight gas
sandstone can be predicted by a sample's microstructure .
A blind test was conducted at the end of the project which attempted to predict a wide range of
properties based simply on their microstructure obtained by scanning electron microscopy. The
estimates provided proved to be incredibly close to the measured values . In other words, simply
by inspecting a sample of the sandstone it was possible to gauge its properties and the likelihood
that it will be potentially productive or otherwise. This in turn means that it is now possible to
provide accurate estimates of reservoir petro-physical properties within a day of them being cored
as opposed to waiting six months to two years for the results from laboratory tests. This provides
the operators with an early indication of the likely reservoir performance, allowing them to optimise
future appraisal and development programmes or in some cases relinquish the asset to avoid more
fruitless expense .
PETGAS has also conducted specific experiments that should lead to changes in practice within
the industry. For example, experiments were conducted to address the impact of stress on
absolute and relative permeability of tight gas sandstones. The results suggest that these
properties are extremely stress-dependent and that it would be worthwhile implementing restricted
rate practice in which lower pressure drawdowns/production rates are used to improve longer-term
Professor Quentin Fisher, Researcher, University of Leeds spin-out company RDR (1992-2008);
Principal Researcher (2003-2007) and Professor of Petroleum Geoengineering (2008-present) in
the School of Earth and Environment, University of Leeds.
References to the research
The initial foundations for the PETGAS project stemmed from research conducted at the University
of Leeds on fault rocks.
1. Fisher, Q.J. and Knipe, R.J. (1998) Fault sealing processes in siliciclastic sediments,
Geological Society Special Publication, 147, 117-134. DOI: 10.1144/GSL.SP.1998.147.01.08.
2. Fisher, Q.J. and Knipe, R.J. (2001) The permeability of faults within siliciclastic petroleum
reservoirs of the North Sea and Norwegian Continental Shelf, Marine and Petroleum Geology,
18, 1063-1081. DOI: 10.1016/S0264-8172(01)00042-3.
3. Al-Hinai, S., Fisher, Q.J., Al-Busafi, B., Guise, P., and Grattoni, C.A. (2008) Laboratory
Measurements of the Relative Permeability of Cataclastic Faults: An Important Consideration
for Production Simulation Modelling, Marine and Petroleum Geology, 25, 473-485. DOI:
4. Fisher, Q.J. (2011) PETGAS Report.
This research and report have led directly to the impact outline within this case study and are
still confidential and only available to sponsors. A username and password will be provided to
the reviewers of the case study so they can view the project website
(www.see.leeds.ac.uk/petgas), which contains all of the results and reports from the project.
Details of the impact
PETGAS created an impact from a very early stage and was viewed as being so successful by the
sponsors that it they provided a further £930,000 to fund a second stage of the project. It also led
to a £400,000 spinout project, SHAle PErmeability (SHAPE) that is applying the same technology
to characterize the properties of shale gas resource plays.
PETGAS proved particularly useful for Aurelian Oil and Gas (now part of San Leon) during their
appraisal of the Siekierki project, which is a large tight gas reservoir in Poland. In 2011-2012,
Aurelian spent £35 million drilling and hydraulically fracturing two horizontal wells within the
Siekierki Field. The well performance was not as good as was hoped for, in that low gas production
rates were accompanied by high water production rates. Aurelian and several of their consultants
initially thought that the poor performance resulted from the hydraulic fractures intersecting natural
fractures that drained the underlying aquifer. A plan was therefore made to drill additional wells in
an area less likely to contain natural fractures. However, results from Leeds as part of the PETGAS
project provided an alternative explanation for the production behaviour. In particular, based on
microstructural analysis of the reservoir rock and comparing these to other rocks on the PETGAS
database it was possible to rapidly estimate the key petro-physical properties (permeability,
capillary pressure and relative permeability) that controlled gas flow in the reservoir. These
properties were then incorporated into a production simulation model and suggested that gas
expansion in the reservoir was responsible for the high rates of water production instead of the
presence of open fractures. The results from the simulation model indicated that it was possible
that the brine production rates would decrease with time. Overall, these results had a very rapid
impact on the field development plan. So instead of drilling more wells with the aim of avoiding
natural fracture networks, the company decided to conduct longer term well tests to assess how
brine and gas production rates evolved with time.
A senior executive of San Leon confirms: "PETGAS input on our Siekierki core significantly helped
in the understanding of water production mechanisms, explaining water production to be from the
rock matrix rather than the initial assumption of flow along natural fractures. This informed the
technical re-evaluation and dramatically changed the proposed reservoir development plan" [A].
The executive adds that "The study showed that by integrating microstructural information with a
database of petro-physical properties of tight gas reservoirs, it may be possible to provide
reasonably accurate estimates of reservoir properties...within a few days rather than waiting
months to years for the results from core analysis" [A].The results of the PETGAS research have
been placed on a web-based database along with wireline log data from the wells analysed [B].
The results can be used easily by the sponsors to identify analogues and improve reservoir
characterisation. The results have also been divided into rock types so that sponsors can readily
obtain the most appropriate properties and relationships for their particular reservoir.
Sources to corroborate the impact
A. Letter to corroborate impact that PETGAS research has had on appraisal of Siekierki project
from Asset Manager, San Leon Energy (dated 20/02/2013).
B. PETGAS website.
This database is still confidential and only available to sponsors. A username and password
will be provided to the reviewers of the case study so they can view the project website
(www.see.leeds.ac.uk/petgas), which contains all of the results and reports from the project.