Fundamentals of Gas Transport in Tight Gas Sandstones and Shales

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.

Physics of Flow in Shales

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.

Structure of Nanopores in Shales

Recent observations of nanopores within carbonaceous material in mudrocks have led to the hypothesis 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 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.

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.

Publications and Presentations

For additional info, please contact Steven L. Bryant (Steven_Bryant@mail.utexas.edu).

See the Unconventional Resources page for information on related research.