Carbonate rocks are diverse and their pore space complex. Large variations in petrophysical properties of carbonates are caused by wide variations in pore type, pore shape and pore interconnectivity. Petrophysical properties such as capillary pressure and Archie m, n values of carbonate rocks are directly related to the amount and type of porosity, the dominant feature size and the interconnectivity of different porosity groups. While the petrophysical properties strongly depend on the interconnectivity of pores and pore shapes, accurately measuring these attributes requires the analysis of pore structure in 3D. Until recently this has not been possible, and traditional descriptions of carbonate pore structure and interconnectivity have been inferred from 2D thin section analysis.
In this paper we describe the imaging of a number of carbonate core samples from the UAE in 3D across a range of scales down to 2.8 microns. The samples include sucrosic dolomites and a complex bioclastic grain/packstone. We calculate drainage capillary pressure and resistivity as a function of saturation directly on the images and correlate the resultant petrophysical properties to the pore structure of the rock. While the dolomite samples exhibit a dominant and strongly interconnected macroporous phase (pore throats ≥ 4 μ m), the bioclastic sample exhibits a significant proportion of meso/microporosity (pore throats ≤ 4 μ m).
Pore connectivity is studied for both sets of samples. The connectivity of the sucrosic dolomite exhibits a strong trend of lower connectivity with decreasing porosity. Other pore structural properties (e.g., pore size, pore-to-throat aspect ratio, pore and throat shape) show little variation. The bioclastic sample has significant proportions of both connected and disconnected (separate) macropores. It is shown that inclusion of larger pores associated with the mesoporous phase results in complete connectivity of the macroporous phase.
The relative interconnectivity of the macropores is systematically related to the resultant Archie cementation exponent. The saturation exponent n is calculated for water-wet and oil-wet conditions. Strong differences in n with wettability are noted for the sucrosic samples. The inclusion of microporosity has a dramatic effect on the behaviour of n for the bioclastic sample.
Three dimensional imaging and analysis of carbonate core material at the pore scale can provide a basis for more accurate petrophysical models, narrow the range of uncertainty in estimates of petrophysical properties and improve the quantification of the resource within carbonate reservoirs.