In unconventional resource plays, pore pressure plays a critical role in the ability to predict both areas of high overpressure and fracture behavior for the exploitation of these plays. Traditional pore pressure prediction focusses exclusively on clay-rich shales and assumes that all shales have a porosity/effective stress relationship that can be used to link the mechanical compaction of the rock to the pore pressure via the vertical stress (overburden). Shales in unconventional plays have variable clay contents, are uplifted, are affected by chemical processes and diagenetic alteration of the elastic properties such that porosity is not relatable to effective stress.
This paper presents a case study showing that traditional pore pressure prediction techniques can be adapted to predict pore pressure in an unconventional play by using a newly defined Pressure Reference Trend (PRT) in-lieu of a Normal Compaction Trend (NCT) as used in conventional, or traditional, pore pressure prediction. The PRT is not linked to the expected compaction behavior 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. Rather than constraining the surface and matrix value for an NCT using sensible geological parameters, the final position of the PRT in velocity-depth space is a function of the measured pressure, to which the trend is calibrated, combined with a lateral shift towards higher velocity/density due to tectonic uplift, secondary compaction, and chemical diagenesis that has occurred over the geological history of the basin.
Traditional pore pressure prediction typically assumes that all the 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 (e.g., Osborne & Swarbrick, 1997). The critical assumption underpinning traditional pore pressure prediction is that the variations observed in specific wireline data (Vp, Vs, Rho, Neutron, Resistivity) are varying solely due to changes in porosity, and that the porosity is controlled by the pore pressure. High pore fluid pressure results in a high porosity rock as the pore fluid was unable to escape during further sedimentation and compaction; hence the fluid is trapped within the rock preserving high porosity. The follow-on assumption is that the wireline log response can be converted into pore pressure using traditional methods (Eaton Ratio method (Eaton, 1975); Equivalent Depth Method (Foster & Whalen, 1966); Vp-Effective Stress method (Bowers, 1994)) into a magnitude of pore pressure via a relationship between vertical stress (overburden), pore pressure, and the vertical effective stress (Terzaghi, 1943).