Petrophysical analysis of unconventional gas reservoirs

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.

Underpinning Research

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. 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 [4].

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 [4]. 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 [4].

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 production [4].

Key Researcher

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: 10.1016/j.marpetgeo.2007.07.005
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.