Different laboratory models were elaborated and applied to fields for HC enhanced recovery. Most of these models performed a 1.6 to 4.7-fold increased reservoir filtration proprieties. In some rare cases, the applied treatment has not improved the recovery, while only in two cases the procedure resulted in production degradation. Undesirable effect was due to the partial dissolution of some rock fabric constituents such as opal and chalcedony by HCl and HF. Acid treatment involves also surface activation, which results in increased amount of the mineral-bound water and, consequently, in decreasing rock permeability. This requires stabilization of swellable clay minerals.
For this purpose, the treatment of reservoir rocks from northern Saharan fields with calcium chloride performed increase of reservoir filtration capacity for two- or even three-fold. The repeated treatment has not produced a substantial increase of the permeability. The favourable effect of CaCl2 treatment is plausibly explained by the change of the film thickness of bound water in the pore space as well as by the replacement of Na+ cations by Ca2+ cations in swellable clay minerals.
This process promotes also the dehydration of sandstone clay cement and desorption of asphaltenes and resins in the stratum. The treatment of the argillaceous cement of sandstone in the reservoir with KCl, HCl and CaCl2 solutions produced decrease in clay swelling by 10 to 50 %. However, the inclusion of solvents, inhibitor agents such as monovalent and polyvalent salts, polymers, emulsion agents and some crude fractions considerably improves protection and stabilization clay minerals in sandstone cement and contribute to minimizing the reservoir plugging. The described approach is at present extended and applied successfully to fields in other basins.