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What is the impact of GeoPressure on Unconventional Plays?

Abstract

Typical unconventional reservoirs are tight-gas sands, coal-bed methane, and gas shales. These plays tend to be regionally pervasive, low permeability gas accumulations where “sweet spots” i.e. sandstones and/or silt layers are targeted. Frequently these accumulations occur in shallow, uplifted older basins. Examples include the Barnett, Marcellus, Bakken and Eagleford accumulations. Bossier and Haynesville do not match this characterisation, that is they are not uplifted and are significantly deeper and younger (Jurassic). It would seem therefore that unconventional accumulations have a wide range of differing characteristics. However, the search for “sweet spots” or enhanced reservoir quality is common to all. Pressure or rather the increase in overpressure will improve reservoir quality in unconventional plays by preserving porosity during burial or by increasing porosity by elastic rebound during uplift. Both processes allow for increased producibility, reducing t the need for artificial hydraulic fracturing. Indeed, some highly overpressured plays have their own natural fracture network due to pore pressure exceeding the fracture strength of the rock. Predicting the pressure regime in plays is notoriously problematic due to lack of permeability to take most direct pressure tests, as well as the presence of gas affecting Vp logs. Some solutions are provided in the literature; however these tend to be focused solely on the Bossier and Haynesville shales. It may well be that the techniques and approaches used to-date in these non-uplifted, Jurassic formations to estimate pore pressure may only be partially transferrable to other types of unconventional plays mentioned above. Thus, in this paper, therefore, we discuss the need to take a basin-scale view to pore pressure, and highlight some approaches that can be taken to understand the pore pressure more accurately, including understanding our shales mineralogically, and the effects of gas expansion upon uplift. If the areas of high pore pressure can be established, these may offer enhanced recovery and zones to be targeted preferentially.