Production-Active Pore Systems — The Pores that Matter

Evensen, Joseph M.

Great effort has been given to characterization of pore systems in shales, often by way of imaging techniques and descriptive interpretation. As a compliment from a different perspective, this study summarizes lab test data that reveal which pore systems physically contribute to fluid recovery.

Recent R&D pursuits derived methods that quantify fluid recovery directly from cuttings, cores or outcrop samples. PVT-based techniques strategically synthesize "production" by physically measuring fluid yield (production potential) at a miniature scale. During runs, a production test profile is produced (the "microPT") and the stylistic manner in which fluids release is tracked. Like fingerprints, styles of curves are diagnostic because they are fundamentally governed by geometries of pores themselves. Pore geometries are known to microscopically control efficiency of hydrocarbon movement (permeability). As a corollary, the manner of hydrocarbon fluid release is genetically indicative of activity by its "parental" pore system(s).

Calibrations establish "end-members" for production-active pore systems. Curve shape attributes cluster into distinct groupings ("Parental Pore Types") that include: Free (open pores), Sorbed, Fracture (superpores), and Fault-Related (pores within fault gauge). Mixed fluid contributions originating from competing pore systems can be distinguished by numerical deconvolution. In some cases triple pore systems (free, fracture, and sorbed) coexist in narrow sections of shale (e.g., Exshaw Fm outcrop, AB).

Distributions of pore populations change dynamically with thermal maturity, evidencing creation and destruction of pore systems during hydrocarbon generation. Investigations on cores of Eagle Ford Formation (TX) reveal that sorbed pores dominate at immature conditions. But hierarchy changes in mature rock where free pores achieve peak populations. At overmature conditions, reign of free pores continues but by lesser margin.

With roots in reservoir performance testing (recovery), this approach uniquely quantifies pore systems on a behavioral basis. Data support new petrophysical logs ("Productive Pores Log") that profile distribution of active pore systems along wellbores. An inherent philosophy of "focus testing" guides in portraying only those pore systems physically capable of hydrocarbon production. Finally, results offer standalone data - originating from the pore scale - for reservoir ranking and sweet spot definition.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013