Fracture Formation & Development

Key to developing unconventional shale reservoirs is an understanding of how hydraulic fractures develop and their impact on fluid flow

Hydraulic fracture formation and development will vary in different shales due to distinct compositions, textures and stress regimes. These may control hydraulic fracture initiation plus their geometry and extent, but further research is required to delineate how these controls can be predicted. 
The length of time hydraulic fractures stay open for at formation pressures and temperatures is also presently poorly understood.

Impact of Diagenesis

Diagenesis in mudstones and shales requires more research to fully recognise its impact on reservoir quality

Diagenetic evolution in conventional reservoirs is generally considered to be well understood.  However key differences in shales mean that authigenic processes may develop differently to those in conventional reservoirs.
Mudstones and shales are very reactive with high organic matter contents, large surface areas and poorly crystallised detrital mineral phases.  Small grainsize and correspondingly low porosities and permeabilities, are also likely to severely limit fluid flow and element mobility.  These factors may have a large impact on the reservoir quality of shale reservoirs.

Measuring Shale Properties

Accurate measurement of petrophysical properties at reservoir conditions is essential to understanding the behaviour of mudstones and shales

Standard permeability measurements at surface conditions (GRI method) on core material do not reflect the permeability characteristics of shales at reservoir temperatures and pressures.
Pressure-sensitivity of shale permeability is an important factor in estimation of gas in place and likely yield. Without consideration of effective pressure effects accurate fluid flow models cannot be developed.

Basin-scale Variability

Significant variability in mudstone composition and properties is present within sedimentary basins, but this variability is poorly captured in current models.

Limited outcrop and core material has hampered the generation of fit-for-purpose models, but the increasing availability of core material through shale gas and shale oil exploration now afford the opportunity to address this challenge.

Porosity & Permeability

The nature of porosity in mudstone and shales is increasingly known, but the controls on this porosity are still poorly understood.

Shales have different compositions and textures resulting in distinctive porosity types and distributions.  Knowing the dominant pore types and the variability in the physical parameters of these pores is needed to build accurate reserve estimates and to develop detailed reservoir models.
The distribution and inter-connectivity of pore networks has a major impact on the permeability characteristics of mudstones and shales.  Understanding the scale and connectivity of these networks is crucial to predicting fluid flow and production well decline rates.
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