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TIGHT GAS SANDSTONE: Is it Truly an Unconventional Reservoir?| By Vinay K. Sahay1 and Staffan Kristian Van Dyke2

1 Maheshwari Mining Pvt. Ltd.; India (email: geo_vinay@yahoo.co.in) 2 Nexen Petroleum U.S.A., Inc.; Dallas, U.S.A.

ABSTRACTThe objective of this paper is to evaluate tight gas sandstones in relation to conventional reservoirs (sandstones/carbonates) as well as unconventional reservoirs (coalbed methane / shale gas), with reference to its constituent petroleum system parameters: source, trap, seal, reservoir properties (porosity and permeability), and time factors (timing of charge and migration). Our study indicates significant differences between tight gas sandstones as compared to coalbed methane and / or gas shales. Evaluation of geological evidences indicates that tight gas sandstones, as a reservoir, are closer to conventional type reservoirs than unconventional type reservoirs, like coalbed methane and shale gas. Utilizing the framework described in this paper, tight gas sandstone reservoirs should then be considered as a sub-type category within the overall conventional reservoir definition, as the majority of its geological properties fall within this definition, and not that of an unconventional reservoir. Characterizing tight gas sandstones as an unconventional reservoir is not appropriate, as the geological setting / petroleum system is different in comparison to coalbed methane and shale gas. Tight sandstone is only a reservoir rock whereas coal and shale is source as well as reservoir rock.

INTRODUCTIONUnconventional reservoirs are ones that

cannot be produced at economic flow rates or that do not produce economic volumes of oil and gas without assistance from massive stimulation treatments, such as hydraulic fracturing (frac’ing) or special recovery processes and technologies, such as steam injection. Typically unconventional reservoirs have been described as tight-gas sands, coalbed methane, and gas shales (Holditch, 2003; 2006). However it is an economic definition and does not take into account geological processes. It is also important to understand that a conventional (sandstone/carbonate) reservoir with low natural pressure that depletes very quickly (in the order of weeks to months) and requires artificial hydrocarbon recovery techniques to maintain or increase its economic viability is very similar to the economic definition of an unconventional reservoir given above. But such reservoirs are still categorized as conventional. On the other hand, since tight gas sandstones must be artificially stimulated (frac’ed) in order to produce its gas, it would then seem that this is the only criterion in place required to categorize it as an unconventional reservoir. The objective of this paper is to evaluate tight gas sandstones in relation to conventional reservoirs (sandstones/carbonates) as well as unconventional reservoirs (coalbed methane / shale gas), with reference to its constituent petroleum system parameters: source, trap, seal, reservoir properties (porosity and permeability), and time factors (timing of charge and migration).

COMPARISON OF CONVENTIONAL AND UNCONVENTIONAL RESERVOIRSIn the United States, the tight gas sandstone definition is applied to reservoirs with less than 0.1 mD of permeability (Meckel and Thomasson, 2008). Our investigation indicates that tight gas sandstones have significantly different characteristics in comparison to coal bed methane and shale gas. They are:

1. Tight gas sandstones act purely as a reservoir, whereas coal and shale act as a source rock, as well as a reservoir.

2. Shanley and others (2004) found that the low permeability reservoirs in the Greater Green River Basin of Southwest Wyoming were not part of a continuous-type gas accumulation but were low-permeability rocks in conventional structural, stratigraphic, or combination traps. Earlier, Berry (1959) and Hill and others (1961) proposed that in the San Juan Basin, the gas within the sandstone reservoir was localized in a potentiometric sink associated with down-dip flow of water. In other words, it is a hydrodynamic type trap, thus like conventional trap settings.

3. Gas migrates into tight sandstones from the nearby source rock and the charged gas may be housed within the reservoir due to high capillary pressure

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32 RESERVOIR ISSUE 8 • OCTOBER 2010

Petroleum system and other parameters

Conventional reservoirs (sandstone/carbonate)

Tight gas sandstone reservoirs

Coal bed methane reservoirs Shale gas reservoirs

Source Present Present Coal acts as a source rock and reservoir rock

Shale acts as a source rock as well as reservoir rock

Trap Present Present Entrapment by adsorption in coal matrix (trap not necessary)

Entrapment by adsorption in matrix of organic matter (trap not necessary)

Seal Present Present Entrapment by adsorption in coal matrix (seal not necessary)

Entrapment by adsorption in matrix of organic matter (seal not necessary)

Time Factor(Timing and Migration)

Essential for generation and migration from source and entrapment of oil

and gas by trap and/or seal

Essential for generation and migration from source and entrapment of oil

and gas by trap and/or seal

Time not essential. Time is significant only in context of maturity and

generation of gas from organic matterSimilar to coal bed methane

Reservoir Porosity High: > 10% Low: < 10% Low: < 10% Low: < 10%

Reservoir Permeability High: > 100 mD Low: < 0.1 mD Low: < 0.1 mD Low: < 0.1 mD

Production

Initially due to natural reservoir pressure (primary drive

mechanism); secondary/tertiary recovery methods to follow

Fracturing, flooding (water, steam), acidization

Dewatering and fracturing to decrease water pressure in coal seams to release and flow gas

Hydraulic fracturing

Table 1. A comparison of petroleum system parameters of: tight gas sandstones, coal bed methane, shale gas, and conventional reservoirs.

conditions by virtue of low porosity and permeability, and up-dip presence of water due to regional or local hydrodynamic conditions, whereas in coal and shale gas, it is adsorbed into the matrix of organic matter (Bustin and others, 2009).

4. Many conventional reservoirs are porous and permeable but do not have enough primary energy to support hydrocarbon production unaided at an economic level, but are still categorized as conventional reservoirs. According to the unconventional reservoir definition given above, this quality should then define these reservoirs as unconventional, primarily

because enhanced recovery techniques are required for them to be economically producible. Similarly, tight gas sandstone reservoirs need enhanced recovery techniques like fracturing, flooding, and acidization to make them economically viable. However, instead of categorizing these low primary-energy conventional reservoirs as unconventional, it is the

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RESERVOIR ISSUE 8 • OCTOBER 2010 33

authors’ opinion that they should remain classified as conventional reservoirs, and that tight gas sandstones should be classified as a sub-type within the overall conventional reservoir petroleum system.

5. The only correlatable property of tight sandstones to coal and shale is their low porosity and permeability similarity, unlike the higher porosities and permeabilities typically seen in conventional sandstone / carbonate reservoirs.

The geological aspects discussed above suggest that tight gas sandstone as a reservoir is closer to conventional reservoirs (sandstone / carbonates) than to coalbed methane and shale gas reservoirs. Table 1 summarizes the petroleum system and other parameters with respect to tight gas sandstones, coalbed methane, shale gas, and conventional reservoirs to elucidate the similarities between these reservoir types.

CONCLUSION Evaluation of the above geological aspects suggests that tight gas sandstones, as a reservoir, are closer to conventional type reservoirs than to unconventional type reservoirs, like coalbed methane and shale gas. It is clear that tight gas sandstones

act simply as a reservoir, whereas coal and shale act as a source rock as well as a reservoir for the gas. Tight sandstones may become a hydrocarbon reservoir only when a potential source rock is available within the basin, or a nearby region, capable of charging the reservoir. Utilizing the framework described in this paper, tight gas sandstone reservoirs should be considered as a sub-type conventional reservoir, as the majority of its geological and petroleum system parameters fall within this definition, and not that of an unconventional reservoir.

REFERENCESBerry, F. A. F. 1959. Hydrodynamics and chemistry of the Jurassic and Cretaceous systems in the San Juan Basin, northwestern New Mexico and southwestern Colorado. Stanford University, Ph.D. thesis, 269 p.

Bustin, R. M., Bustin, A., Ross, D., Chalmers, G., Murthy, V., Laxmi, C., and Cui, X. 2009. Shale gas opportunities and challenges. Search and Discovery Article 40382. http://www.searchanddiscovery.net/documents/2009/40382bustin/ndx_bustin.pdf

Hill, G. A., Colburn, W. A., and Knight, J. W. 1961. Reducing oil f inding costs by use of hydrodynamic evaluations, in petroleum

exploration, gambling game or business venture. Institute of Economic Petroleum Exploration, Development, and Property Evaluation. Englewood, New Jersey, Prentice-Hall, p. 38-69

Holditch, S. A. 2003. The increasing role of unconventional reservoirs in the future of the oil and gas business. Society of Petroleum Engineers. http://www.spe.org/jpt/print/archives/2003/11/JPT2003_11_management.pdf

Holditch, S. A. 2006. Tight gas sands (SPE Paper 103356). Journal of Petroleum Technology, v. 58, no. 6, p. 86-94.

Meckel, L. D. and Thomasson, M. R. 2008. Pervasive tight-gas sandstone reservoirs: An overview. In: S. P. Cumella, K. W. Shanley, and W. K. Camp (eds.). Understanding, exploring, and developing tight-gas sands. 2005 Vail Hedberg Conference. American Association of Petroleum Geologists. Hedberg Series, no. 3, p. 13-27.

Shanley, K. W., Cluff, R. M., and Robinson, J. W. 2004. Factors controlling prolif ic gas production from low-permeability sandstone reservoirs: Implications for resource assessment, prospect development, and risk analysis. American Association of Petroleum Geologist Bulletin, v. 88, p. 1083-1121.