ABSTRACT

Secondary fabric-selective porosity has been experimentally produced in the Ste Genevieve Limestone (Upper Valmeyeran, Mississippian, Illinois). Jacketed cylindrical specimens of the limestone were placed in a specially designed triaxial apparatus that permits circulation of pore fluid (CO₂-charged water) under constant pressure while subjecting the rock specimen to constant vertical pressure, lateral confining pressure, and temperature. Burial conditions of about 10,000 ft (3,050 m) were simulated by an axial pressure of 10,150 psi (70 MPa), a lateral confuting pressure of 8,700 psi (60 MPa), and a pore pressure of 5,800 psi (40 MPa). All pressures were held constant throughout the tests and the temperature was maintained at 75° F (24° C).

The results obtained indicate that fabric-selective secondary porosity can be developed in carbonate rocks by circulating fluids under conditions characteristic of deep burial, i.e., after pervasive cementation and after mineralogic stability have occurred. In the tested oolitic calcarenite, secondary porosity was developed and permeability was enhanced significantly by the centripetal preferential dissolution of the microporous cortical layers of the ooids, development of pore throats at the point contacts between ooids. and, ultimately, by the removal of nuclei to produce oomoldic porosity.

Differences in CO₂ concentration and flow rate of the fluid and in test specimen configuration had no observable influence on the dissolution effects, only on the solution rates.