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Understanding the porosity and pore size distribution of a reservoir facies is essential in assessing the producibility and risk involved with individual deposits. Currently, directly quantifying porosity of a reservoir facies requires utilization of gas intrusion and/or mercury porosimetry methods; both of which occur after intermediate or complete wellbore emplacement. However, due to heterogeneity of target formations, such methods may result in non-optimal well emplacement.

As the exploration of tight, extended, unconventional shale plays continue to emerge, it is imperative that well-site reservoir petrophysical methods evolve to cater industry needs. The Iowa Pore Index (IPI) test is a water intrusion porosimetry (WIP) method that involves a device and a procedure for measuring the volume of macro- and micro-pores of crushed rock via water intrusion. Although first employed to evaluate pore distributions within crushed rocks, this method can be advantageous for understanding petrophysics of reservoir rock during drilling by quantifying the macro- to micropore ratio in cuttings in an inexpensive and non-destructive manner.

We have modified the traditional IPI, where water intrusion at 35 psi (240 kPa) into a sample is measured after 1 and 15 minutes, by measuring the water intruded over shorter time intervals (0.1-2.0 seconds), as well as measuring intrusion at pressures up to 100 psi (689 kPa). Using this new method, 21 carbonate samples (10 dolostones and 11 limestones) were compared to “traditional” IPI measurements, which resulted with more accurate pore volumes. Results were then compared with traditional petrophysical (e.g., helium and mercury porosimetry) and petrographic techniques. Cumulative intruded volume plotted for the first two minutes of intrusion allowed the differentiation of lithology. Plotting incremental intrusion amongst various pressures shows the transition from macro- to micro-pore intrusion is source specific and differs based on petrophysical rock properties (i.e., pore-diameters, connectivity of pores, and pore throat sizes). Through utilizing individualized IPI calculations, porosity values better represent individual lithologies than they did with the old method. This method may be a tool that, when employed in the field, can provide wellsite geologists with estimations of pore sizes in shale and other tight rocks to ensure the most producible wellbore emplacement within a heterogeneous target zone.