Pore Throat Size of Canadian Tight Oil and Liquid-Rich Gas Reservoirs: Implications for Hydrocarbon Transport

Abstract

A variety of techniques such as mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and low-pressure gas (N2, CO₂) adsorption (LPA) analysis can be utilized to characterize the full spectrum of pore sizes in tight rocks. Previous studies indicated the advantage of using a combination of these methods to determine pore size distribution of coals, tight gas sandstones and organic-rich shales. However, the application of these techniques for characterizing the effective (transport) pore throat size is relatively limited. Gas slippage analysis (Klinkenberg approach) is a potential tool for investigating the dominant effective (transport) pore throat size of tight rocks under “in-situ” effective stress condition. So far, only a very few studies have provided a targeted discussion on the use of this technique for characterizing dominant pore throat sizes in tight formations. The number and diversity of the samples used in previous studies have been, nevertheless, comparatively limited. This study presents results from an ongoing laboratory study investigating pore network characteristics of Canadian tight oil and liquid-rich gas reservoirs, differing in total organic carbon (TOC) content and mineralogical composition. A suite of techniques, including gas slippage analysis, LPA and SEM analyses were used to characterize pore networks of Montney and Bakken formations at various reservoir conditions. Parallel to bedding, the effective pore throat sizes estimated from gas slippage analysis range between 30 and 90 nm at effective stress of 15.7 MPa. The estimated effective pore throat sizes are larger for the samples with higher pulse-decay gas (N2) permeability values. The effective pore (throat) sizes estimated from gas slippage analysis are generally larger than those measured/estimated from LPA analysis and SEM images. The effective pore throat sizes obtained from gas slippage analysis compare well with those estimated from a modified Winland-style approach. The observed variations are discussed in the light of differences in mineralogical composition, diagenetic history and the presence/absence of micro-fractures. The dominant pore throat sizes control the hydrocarbon production and capillary processes in tight oil/gas reservoirs. In contrast to routine techniques, this study indicates that the dominant effective pore throat sizes in tight rocks can be quantified under “in-situ” effective stress conditions using Klinkenberg approach.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016