Abstract
In this paper we highlighted the effect of clay mineralogy and the influence of physico-chemical factors on coagulation- dispersion processes and the rate of pore throat bridging by clays. In particular, we describe the effect of different pH (acidic 2.5 and alkaline 9.5) and the effect of salinity (NaCl) on the stability of clay particles in siliciclastic rocks and their influence on the rock permeability which are often ignored in reservoir engineering literature. We conducted the first experiments on powder (0.05-0.1 mm grain size) samples for tracking mineral reactions and core plugs for permeability. X-ray diffraction (XRD) was run for whole-rock and clay fraction by diagnostic treatments to specify types of clay minerals and to calculate the percentage of each type. The composition of starting material is quartz dominant, with carbonates (dolomite, calcite), feldspars (albite, microcline), chlorite, kaolinite, illite, muscovite, with traces of 14 Å smectite and illite/smectite randomly interstratified structure. In strongly acidic solution, according to XRD, smectite is altered (14 Å peak vanishing), chlorite being also partly reduced, while kaolinite is stable. Carbonates are not involved in reactions. In basic solution, smectite was less affected, but chlorite peaks indicate structural transformation. The shape and broadening of 10 Å peak shows changes in illite and illite/smectite interstratified structures. Altogether, mineralogical changes indicate reactions which lead to fines mobilization. Based on experimental work, in the case injection with pH 2.5 and pH 9.5, there has been a rapid and substantial reduction in permeability as an average 38.54% and 73.72% for acidic and basic solutions respectively. The results confirmed that the reason of the reduction in permeability is due to the non- swelling clay minerals which are dispersed and migrated during the changes in pH, salinity and time. Novelty of these experiments is the tracking of changes in mineralogy by Rietveld refinement on XRD, coupled with diagnostic clay mineral investigation.
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