Pore Geometry of Carbonate Rocks as Revealed by Pore Casts and Capillary Pressure

The three-dimensional aspects of pores in a selected group of dolomites show that pore connections are commonly sheet-like rather than tubular and that prevailing geometric styles of pores and connections can be described from examination of resin casts of pores with a scanning-electron microscope. As dolomite crystals grow and porosity is reduced, polyhedral pores are transformed to tetrahedral pores and finally to interboundary-sheet pores. In this process of porosity reduction, pore structure becomes geometrically simpler and more regular and pore-size to throat-sizeratios increase. Mercury-ejection efficiency can be defined as the total volume of mercury ejected from a sample on reducing pressure from 100 kg/sq cm to 0.1 kg/sq cm expressed as a percentage of thetotal volume of mercury injected over the same pressure range. Ejection efficiency decreases linearly with decrease in porosity and probably results mainly from decrease in the size of throats relative to the size of pores. Mercury-injection and ejection tests do not provide a close analogue for an oil-water system but may give a relative indication of the efficiency with which various rock types can expel a nonwetting phase when pressure is reduced. Because ejection efficiency correlates positively with porosity, it may be possible from porosity data to provide a factor that will indicate recovery efficiency, insofar as this is dependent on the pore geometry.