Impact of Solvent-Extraction on Fluid Storage and Transport Properties of Montney Formation

Abstract

Abstract Pore and pore-throat blocking materials may have a negative effect on reservoir quality, as has been recently determined for the low-permeability, hydrocarbon bearing portions of the Montney Formation. Some of these materials, such as salt and bitumen, may be extractable using different solvents combined with the Dean-Stark extraction process. The primary objective of the current study is therefore to investigate the impact of solvent-extraction on various geochemical and petrophysical characteristics of low-permeability intervals. To accomplish this goal, a comprehensive dataset was collected for two sample suites from the Montney Formation (western Alberta, northeastern British Columbia; Canada), before and after sequential solvent-extractions with organic solvents. The samples are analyzed after three different treatments: 1) "as-received", 2) toluene-extracted and dried, and, 3) toluene/methanol-extracted and dried. The methods used for characterization of the samples after each treatment are Rock-Eval pyrolysis (TOC content, S1, S2); helium pycnometry (grain density, porosity); low-pressure gas (N2, CO2) adsorption (surface area, pore volume, pore size distribution); and crushed-rock gas (He) permeability. Importantly, to ensure a proper comparison of the different sample treatments, the solvent-extraction and subsequent geochemical and petrophysical analyses are performed on identical samples; therefore, the effect of sample heterogeneity is mitigated. The impact of solvent-extraction on grain density, pore network attributes (surface area, pore volume, pore size distribution) and permeability of the Montney samples depends on the organic matter content, solvent type and other sample-to-sample variations. For one dataset (batch A), the change in petrophysical properties is variable and not predictable, while for the other (batch B), grain density, pore network attributes (surface area, pore volume, modal pore size distribution) and permeability exhibit an increase after sequential solvent-extraction with toluene and methanol. The variability observed for batch A is possibly attributed to (1) different degrees of salt precipitation, depending on the "in-situ" water/brine content and the salinity of the "in-situ" (formation) water and/or (2) experimental uncertainties/errors. A detailed discussion of the experimental uncertainties/errors is provided to elucidate the impact of these factors on the experimental outcomes. In the current study, it is demonstrated that by applying multiple analysis techniques on two diverse sample suites subject to three different treatments, the variation in pore structure and fluid flow characteristics of fine-grained tight oil/gas reservoirs before and after solvent-extraction can be quantified. The quantification of these effects may have important implications for both shale matrix transport characterization, which usually involves some form of extraction prior to petrophysical evaluation, and stimulation treatments for improving hydrocarbon recovery by removal of pore-blocking materials.