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Fluid Flow of Plio-Pleistocene Reefal Carbonates: Assessing Large-Scale Porosity and Permeability Distributions


Accurately predicting fluid flow in carbonate aquifers and reservoirs is a problem that arises due to complex porosity and permeability distribution dependent on both primary matrix porosity related to depositional facies, and secondary porosity resulting from diagenesis. While plug data is informative in assessing the relationship between rock properties and depositional and diagenetic heterogeneities, it may not always provide an accurate characterization of larger scale porosity. Here we use in situ measurements of hydraulic conductivity (K) in a transect of 7 wells from Plio-Pleistocene reefal carbonates in the southern Dominican Republic to evaluate the controlling factors in early porosity evolution and assess the representation of plug data in determining porosity and permeability characteristics. Vertical and lateral variability in hydraulic conductivity was determined from constant-head injection tests using a straddle-packer assemblage with an open interval of 0.3-0.9 m, and performed at 1.0 m intervals. These packer tests provide K data of the combined matrix and larger dissolution features. Based on 2 boreholes with distinct facies and diagenetic characteristics, high flow zones are related to macroporosity produced through the dissolution and leaching of corals, facies changes and exposure surfaces. Values for the upper 15 m of the boreholes range from 0.028 cm/s to 0.18 cm/s, and when converted to permeability (11.778-77.75 darcys), are up to 2 orders of magnitude higher than measurements from plugs (0.001-0.350 darcys). There was not a significant correlation between the permeability from plugs and the in-situ hydraulic conductivity values. This dataset confirms scale limitations of plug data in high-flow carbonate rocks. Results also help to quantify early diagenetic porosity evolution and flow properties at the reservoir scale.