We imaged the pore-scale distribution of air and water within packed columns of glass spheres of different textures using x-ray microcomputed tomography after primary drainage and after secondary imbibition. Postimbibition residual air saturation increases with roughness size. Clusters larger than a critical size of about 15 to 40 pores are distributed according to a power law, with exponents ranging from τ = 2.29 ± 0.04 to 3.00 ± 0.13 and displaying a weak negative correlation with roughness size. The largest cluster constitutes 7 to 20% of the total residual gas saturation, with no clear correlation with roughness size. These results imply that activities that enhance grain roughness by, e.g., creating acidic conditions in the subsurface, will promote capillary trapping of nonwetting phases under capillary-dominated conditions. Enhanced trapping, in turn, may be desirable in some engineering applications such as geological CO 2 storage, but detrimental to others such as groundwater remediation and hydrocarbon recovery.

Albian carbonate reservoirs are prominent in the subsurface of Angola and the Republic of Congo. Equivalent deposits are well exposed in southern Angola (Benguela and Namibe basins), but have received relatively little detailed sedimentological work. In the Namibe Basin, carbonates form m-thick beds interbedded with shallow-marine and continental alluvial fan siliciclastics. Characteristic carbonate mounds or pillars (≤5m high, 1-2m in diameter) rise above a basal carbonate bed, which consists of oncoid-peloidal rud-grainstones with oysters and echinoderms. Thrombolitic microfacies in the mounds include red algae and microbial-algal crusts. Microfacies are marine, but formed in a stressed environment as a result of detrital input and/or brackish conditions. Shallow burial processes dominated diagenesis; meteoric diagenesis is possible, but no conclusive evidence was found in this study. Primary (intergranular) and secondary (microporosity, mouldic, vuggy, fracture) porosity developed in the carbonates as a consequence of important dissolution and only partial cementation.Up to 22% porosity were estimated visually, suggesting good reservoir potential, but contingent on offshore carbonate development and presence of comparable diagenetic processes. Depositional models for these carbonates need to explain the localized occurrence and bioherm nature. A marine ingression into a coastal embayment and formation of shallow-water microbial

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We present a cheap, efficient, and non-hazardous protocol for altering the roughness of hard particles at the nanometer-scale using a stone tumbler, a tool which is normally used for polishing stones. Six different textures were achieved by lining the tumbler with sandpaper of mean grit diameters $$d_{\mathrm{g}}=201$$dg=201, 58.5, 18.3, 12.6, and $$8.4\,\upmu \hbox {m}$$8.4μm. Two textures were created by tumbling a batch of glass spheres for 4 h and for 12 h with the $$12.6\,\upmu \hbox {m}$$12.6μm sandpaper; all other textures were established by tumbling for 12 h. Surface roughness was characterized by the integral length scale, $$\xi$$ξ, evaluated from 7 nm/pix resolution scanning electron microscope images. Roughness size increased from $$\xi = 24$$ξ=24 to 31 nm as the grit size decreased from $$d_{\mathrm{g}} = 201$$dg=201 to $$18.3\,\upmu \hbox {m}$$18.3μm, and then decreased to $$\xi = 6.4\,\hbox {nm}$$ξ=6.4nm at the smallest $$d_{\mathrm{g}}$$dg. The largest $$\xi \,(= 34\,\hbox {nm})$$ξ(=34nm) was achieved using a $$12.6\,\upmu \hbox {m}$$12.6μm sandpaper and the shorter tumbling time of 4 h. The permeability of a packed column of the particles broadly decreased with increasing $$\xi$$ξ, indicating that permeability decreases with increasing roughness size.