Petrophysical Characterization of the Pore Space in Permian Wolfcamp Rocks

Pore-space characterization is a crucial part of reservoir characterization as the pore space functions both as the storage space for fluids and as a pathway for production. In unconventional reservoirs, natural and hydraulically induced fractures produce conductive pathways for fluid flow into the wellbore and largely control initial production rates; however the matrix storage and permeability strongly influence pressure and flow support and ultimate recovery. Therefore it has become important to identify and characterize the rocks that have the best matrix storage and flow characteristics in the unconventional reservoirs to maximize recovery. The small scale of pore sizes in unconventional rocks makes it difficult to measure and understand how fluids are stored and produced in these tight rocks. While there have been various attempts at modifying the methods for characterizing these small pore spaces, e.g., variations of crushed-rock (i.e., GRI procedures established by Luffel and Guidry, 1992), unsteady-state permeability methods, pressure or pulse decay permeability on plugs, mercury-injection capillary-pressure (MICP) tests on crushed rock, etc., there are inherent limitations to each method and no single method can be used to fully characterize the rock and its pore space. Recent advances in the application of measurements obtained using nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) techniques also offer key insights into these small pore spaces. While each of these methods has its own inherent limitations, each contributes unique pieces of information. The interrogation and integration of these methods leads to a better understanding of the pore-space characteristics. In this paper, we present the results, analysis and integration of MICP, NMR and SEM measurement techniques conducted on the Wolfcamp formation in the Permian basin. Through the analysis of these results we show how these measurements improve our understanding of the rock and its pore space. We also show how we deduce the producible portion of the fluids in the pore space and the type of fluid being produced. We also present a more robust means of characterizing permeability in such heterogeneous rocks than using simple porosity permeability correlations.