Experimental study of fluid transport processes in the matrix system of organic rich shales

The matrix permeability of organic-rich shales and its relationship with lithological factors including maturity, TOC (total organic carbon) content, porosity and mineralogy is fundamental to the process of shale oil/gas production from these unconventional reservoirs. The significances of these individual factors controlling matrix permeability are, however, rock-specific and not necessarily of equal importance in all shale gas reservoirs. The matrix permeability of fine-grained sedimentary rocks also depends on the type of permeating fluid, moisture content, pore pressure and effective stress. The effects of these controlling factors on matrix permeability of fine-grained organic-rich shales/mudstones, originating from different potential and producing shale gas plays worldwide (China, Canada, US), have been reviewed and discussed in Chapter 2. The lithotypes occurring in unconventional oil/gas reservoirs have low to extremely-low permeability coefficients which are difficult to measure in the laboratory on a routine basis. The routine experimental procedures applied for high-permeable samples should be verified and tested for these types of rocks. Chapter 3 compares different methodologies (steady state, non-steady state) for the determination of gas permeability coefficients in tight shale samples under controlled confining pressure. For the samples analyzed in this PhD study, the apparent gas permeability coefficients measured using non-steady state techniques were consistently higher than those measured by the steady state method (for dry samples). The Klinkenberg-corrected gas permeability coefficients obtained from steady state and non-steady state techniques were, however, similar but the slip-factors were different, being larger for the non-steady state technique. The Late Mississippian Upper Alum Shale/Chokier Formation (western Germany, Belgium), the Lower Toarcian Posidonia Shale (northern Germany) and the Cambro-Ordovician Scandinavian Alum Shale (Sweden, Denmark) are among the most promising candidates for shale gas production in Western Europe. The fluid transport properties within these potential shale gas reservoirs are, however, still poorly understood. Chapters 4-6 present results from laboratory studies investigating the fluid flow mechanisms/properties in the matrix system of these European organic-rich shales, differing in pore network characteristics (porosity, pore size distribution), mineralogy (calcite, clay, quartz), TOC content (3-14%) and maturity (0.6-2.4% VRr). Gas (He, Ar, CH4) and water flow properties were determined at effective stresses ranging between 5 and 37 MPa and temperatures of 45°C. The effects of different controlling factors/parameters including maturity, porosity, mineralogy, TOC (total organic carbon) content, permeating fluid, pore pressure and effective stress on the conductivity were analysed and discussed. Among the sample suite analyzed, the lowest permeability coefficients parallel and perpendicular to bedding were measured at intermediate maturity levels (oil-window; 0.88-1.05% VRr). The Klinkenberg-corrected gas permeability coefficients increased significantly with porosity (4-16%) ranging between 4•10 -22 and 9.7•10 -17 m². Permeability coefficients measured parallel to bedding were more than one order of magnitude higher than those measured perpendicular to bedding. The permeability anisotropy appeared, furthermore, to be controlled by the mineral composition. Permeability coefficients measured with He were consistently up to five times higher than those measured with Ar and CH4 under similar experimental conditions. A substantial discrepancy between Klinkenberg-corrected gas permeability coefficients and water permeability coefficients (0.5-10•10-21 m²) was observed for both immature and overmature samples. Klinkenberg-corrected gas (He, Ar, CH4) permeability coefficients measured on dry samples were significantly, up to six times, higher than those measured on as-received samples (moisture contents up to 1.9 wt%). The rate of permeability reduction with increasing effective stress depended significantly on orientation and moisture content. The reduction rate was, however, independent of the type of permeating gas (He, Ar, CH4). The most significant permeability reduction with effective stress was observed for as-received samples (moisture contents up to 1.9 wt%), compared to those, which were dried (105 °C, vacuum, overnight) before the tests.

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