| Abstract: | The development of secondary porosity in many carbonate rocks appears to be associated closely with thermal maturation of hydrocarbons and plays an important role in their migration and accumulation. Unfortunately, experimental studies of carbonate dissolution and the mechanisms by which secondary porosity is created and permeability is subsequently enhanced are sparse. Recent experimental evidence suggests that fabric-selective secondary porosity can be developed in mineralogically homogenous carbonates by texturally-induced preferential dissolution. The objectives of this study are: one, to develop experimentation techniques and equipment that allow for meaningful qualitative analysis of pore character changes associated with permeability changes created by dissolution of a homogenous calcite rock; and two, to use these techniques to examine the effects of variable pore fluid flow rates and effective confining pressures on the dissolution-induced changing microscopic and macroscopic rock properties. The permeability system was developed to provide controlled, continuous, long term flow of degassed HCl solution (initial pH ≈ 1.5) through core samples of variable length while maintaining a constant pore-fluid pressure (Pf), hydrostatic confining pressure (Pc), and temperature (t). In all experiments recorded here, conditions of Pc = 400 psi, Pf = 280 psi, and t = 25°C were maintained on thermally cracked right circular cylindrical Michael Angelo marble cores (primarily crack type porosity) with permeabilities on the order of .3 millidarcies. Flow rates were maintained at less than 1 ml/hr. The kinetics of the HCl/CaCO₃ reaction played a dominant role in determining sample lengths. At the completion of each experiment, dissolution-induced crack widening was noted at the upstream end of the samples. Secondary products produced unexpectedly during the final experiment (when quartzite spacers were used in the sample assembly) were created as a result of pH controlled quartzite dissolution and precipitation as an amorphous silica gel. |
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