In unconventional resource plays, pore pressure plays a critical role in the ability to predict fracture behaviour, and hence in the exploitation of these plays. Yet it is a parameter that is poorly understood, and little work has been done to understand whether it can be predicted in an unconventional setting. The case study presented here shows how the traditional methods (Eaton Ratio and Equivalent Depth) can be used to predict pore pressure using a Pressure Reference Trend (PRT) in-lieu of a Normal Compaction Trend (NCT) that would be used in the offshore environment. The PRT is not linked to the expected compaction behaviour of the rock (as inferred from an NCT) but it is simply an empirical depth trend from which the pore pressure can be predicted using industry standard formulae.
Traditional pore pressure prediction assumes that all shales are geologically young with low temperatures, are at their maximum burial depth, and have a demonstrable porosity/effective stress relationship where disequilibrium compaction is the mechanism of pressure generation. The critical assumption in traditional pore pressure prediction is that the wireline data are varying due to changes in porosity that can be converted using standard methods (e.g., the Eaton Ratio method) into a magnitude of pore pressure.
In reality, shales in unconventional plays are (or were) at high temperature, are often dramatically uplifted, and have been affected by chemical processes in addition to mechanical compaction such that porosity is not directly relatable to effective stress. Furthermore, the link between pore pressure and log response may be further disrupted by the presence of organic material (high TOC). An increase in TOC has been shown to significantly lower the magnitudes of velocity and density (Passey et al., 1990). Slow velocity (either due to TOC or to free gas in the pores) and low density are typically attributed to an increase in pore pressure so this effect needs to be removed from log data in order to correctly predict pore pressure. The actual presence of higher pore pressure can lead to “sweet spot” preservation and even natural fracturing that can enhance production without the need for artificial stimulation.
