A method for estimation of minimum effective pore throat radius from routine core analysis data in chalk and its applications

A method for estimating minimum effective pore throat radius from Routine Core Analysis porosity and absolute permeability data is introduced for chalk samples. This method can reduce the need for conducting mercury capillary pressure measurement in the lab to obtain minimum effective throat radius of chalk samples and allow sample screening prior to time consuming core flood.The investigation was based on four sets of chalk porosity and permeability data from Rørdal, Stevns Klint and two North Sea wells from Dan and Tyra SE fields. For each porosity and absolute permeability data point, three new parameters are calculated from the entire data set of porosity and absolute permeability values. Using these parameters, the model estimates capillary pressures at very low mercury saturations. First, the capillary entry pressure is calculated from Røgen and Fabricius’ empirical correlation. Hereafter a line is fitted between capillary pressures and mercury saturations at very low mercury saturations including the mercury entry point. The extrapolated line crosses the capillary pressure curve at a high mercury saturation between 0.91 and 0.99, which represents a minimum affective pore throat radius. The validity of the method is verified using a set of mercury capillary pressure measurements of chalk core samples. In the following, a correlation between ultimate laboratory oil recovery achieved from a water flooding process, and an expression consists of minimum effective pore throat radius and specific surface area is presented. Our analysis shows that higher oil recovery by water flooding is related to lower minimum effective pore throat radiuses and higher average pore throat radiuses. Using this simple relationship, one can screen samples and evaluate ultimate oil recovery performance by water flooding before conducting core-flooding experiments. In addition, an empirical index is developed to estimate wettability of chalk samples from calculated Rmin, Rave and Rmax of chalk samples.