fundamentals of gas transport in tight gas sandstones and shales

8/14/2019 Fundamentals of Gas Transport in Tight Gas Sandstones and Shales

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Fundamentals of Gas Transport in Tight Gas Sandstones andShales

Principal Investigator: Steven L. Bryant (in collaboration with Masa Prodanovic, Peter Eichhubl

(BEG) and Peter Flemings (BEG))

Mechanisms of Porosity Reduction

Several unique characteristics of these rocks are the consequence of post depositional diagenetic

processes including mechanical compaction, quartz and other mineral cementation, and mineral

dissolution. These processes lead to permanent alteration of the initial pore structure causing an

increase in the number of isolated and disconnected pores and thus in the tortuosity. The

objective of this research is to develop a pore scale model of the geological processes that create

tight gas sandstones and to carry out drainage simulations in these models. These models can be

used to understand the flow connections between tight gas sandstone matrix and the hydraulic

fractures needed for commercial production rates.

Mechanistic model of compaction of a mixture of ductile and rigid grains


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Grain-scale compaction model explains experimental observations, including

threshold ductile content for complete loss of porosity

Network models of intergranular pore space cannot explain mercury

intrusion capillary pressure experiments in tight gas sandstones! We have

developed a new class of model to explain this phenomenon.

Physics of Flow in Shales


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Grain scale models of void space in shale, coupled with models of the physics of gas transport in

confined conduits, enable predictions of the effective permeability to gas during production of a

shale reservoir. The model predicts significant increase in permeability as reservoir pressure

decreases, which is consistent with the slower decline in production rates characteristic of shale

gas wells.

The ratio of gas permeability at pressures below initial reservoir pressure,

kg2,insitu, to gas permeability at initial pressure (P=28MPa), kg1,insitu, increases

as production continues and pressure declines accordingly.

Structure of Nanopores in Shales

Recent observations of nanopores within carbonaceous material in mudrocks have led to thehypothesis that such material provides conduits for gas migration within the mudrock matrix.

This hypothesis requires that the carbonaceous material exist not as isolated grains but as

connected clusters of grains within the mudrock. To examine this hypothesis, we develop an

algorithm for the grain-scale modeling of the spatial distribution of grains of carbonaceous matter

in a matrix of non-carbonaceous material (silt, clay). The algorithm produces a grain-scale model

of the sediment which is precursor to a mudrock, then a sequence of models of the grain

arrangement as burial compacts the sediment into mudrock. We determine the size distribution of

clusters of touching carbonaceous grains, focusing particularly upon the approach toward


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percolation (when a cluster spans the entire packing). The model allows estimation of threshold

fraction of carbonaceous material for significantly connected clusters to form. Beyond a

threshold degree of compaction, connected clusters become much more prevalent. The

emergence of large numbers of clusters, or of a few large clusters, increases the probability that

nanoporous conduits within the clusters would intersect a fracture in the mudrock. This should

correlate with greater producibility of gas from the mudrock.

Relationship between number of clusters and sediment porosity with 5 percent

of carbonaceous material in the initial bulk volume for initial sediment

porosity of 70%. The number of clusters having more than 10 grains of

touching carbonaceous material (purple dots) is negligible in the original

sediment (porosity = 70%) but becomes appreciable when compaction

decreases the porosity below 40%.

Influence of Water on Gas Production

In tight gas sandstone the productivity of a well is sometimes quite different from that of a

nearby well. Several mechanisms for this observation have been advanced. Of interest in this

paper is the possibility that a small change in water saturation can change the gas phase

permeability significantly in rocks with small porosity and very small permeability. We quantify

the effect of small saturations of the wetting phase on nonwetting phase relative permeability by

modeling the geometry of the wetting phase. We also show how a porosity-reducing process

relevant in tight gas sandstones magnifies this effect.


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Gas effective permeability for tight sandstones: experimental data and

trends of model predictions. Confining pressure increases the sensitivity of

effective gas permeability to small water saturations. If water saturation

increases exclusively by changing the number and size of pendular

rings/liquid bridges (no pore-filling), the gas permeability (red line)

decreases faster than observed. If water saturation increases exclusively by

pore filling (no rings/bridges), the gas permeability decreases more slowly

than observed. Rings and bridges are thus the main mechanism for watersensitivity at small saturations.

Publications and Presentations

Sakhaee-Pour, A. and Bryant, S. "Producibility of tight gas sandstones," to be submitted,AAPG Bulletin, 2011.


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Sakhaee-Pour, A. and Bryant, S. "Gas Permeability of Shale," SPE 146944 prepared forpresentation at the SPE Annual Technical Conference and Exhibition held in Denver,

Colorado, USA, 30 October-2 November 2011.

Mousavi, M. "Pore scale characterization and modeling of two-phase flow in tight gassandstones," Ph.D. dissertation, The University of Texas at Austin, 2010.

Kumar, A. "Quantitative Geometric Model of Connected Carbonaceous Material inMudrocks," M.S. thesis, The University of Texas at Austin, 2010.

Motealleh, S. and Bryant, S. "Quantitative Mechanism for Permeability Reduction by SmallWater Saturation in Tight Gas Sandstones," Soc. Pet. Eng. J., Volume 14, Number 2, June

2009, pp. 252-258.

Motealleh, S. "Mechanistic study of menisci motion within homogeneously andheterogeneously wet porous media," Ph.D. dissertation, The University of Texas at Austin,

2009.

Mousavi, M. and Bryant, S. "Connectivity of Pore Space: The Primary Control on Two-Phase Flow Properties of Tight-Gas Sands," American Association of Petroleum Geologist

Annual Convention and Exhibition, Denver, CO, 7-10 Jun 2009.

Mousavi, M. and Bryant, S. "Predicting the effect of diagenetic alteration on two phaseflow properties in tight gas sands," American Association of Petroleum Geologists

Southwest Section 2008 Convention, Abilene, Texas, 24-27 Feb. 2008.

Motealleh, S. and Bryant, S.L. "Predictive model for permeability reduction by smallwetting phase saturations," Water Resour. Res., 43, W12S07, doi:10.1029/2006WR005684,

2007.

Mousavi, M. and Bryant, S. "Geometric Models of Porosity Reduction Mechanisms inTight Gas Sands," SPE 107963, 2007 SPE Rocky Mountain Oil & Gas Technology

Symposium, Denver, Colorado, U.S.A., 16-18 April 2007.

Motealleh, S. and Bryant, S. "Predictive Model for Permeability Reduction by SmallWetting Phase Saturations," Proceedings of the Computational Methods in Water

Resources Conference XVI, Copenhagen, June 19-22, 2006.

For additional info, please contact Steven L. Bryant ([email protected]).

See theUnconventional Resources pagefor information on related research.
mailto:[email protected]:[email protected]://www.cpge.utexas.edu/ur/http://www.cpge.utexas.edu/ur/http://www.cpge.utexas.edu/ur/http://www.cpge.utexas.edu/ur/mailto:[email protected]

fundamentals of gas transport in tight gas sandstones and shales

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