New methods of petrophysical rock classification based on MICP and grain-size distribution measurements with applications in carbonates and tight-gas sandstones
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Rocks that exhibit bimodal throat-size distributions cannot be reliably appraised and classified using classical techniques such as the Winland R35 method (Pittman, 1992) or Amaefule’s flow zone indicator (Amaefule et al., 1993). These popular classification procedures/protocols are based on the assumption that rocks exhibit a single dominant throat size, and they fail to account for the bimodal nature of pore throat-size distributions. Carbonates and tight-gas sandstones are notorious for exhibiting non-unimodal throat-size distributions and extreme spatial variability. In addition, wide variations of throat sizes are often observed in rocks that have been subject to extreme diagenesis and recrystallization. This thesis introduces new methods for classifying and grouping rocks based on mercury-intrusion capillary pressure (MICP) and grain-size distribution measurements. I use various canonical basis functions to reproduce logarithmic throat-size and grain-size distributions derived from MICP data and grain-size distribution measurements. A bimodal logarithmic Lorentzian distribution is introduced to model the rock’s throat and grain-size distributions. The functions’ free parameters are used to establish correlations with permeability and irreducible water saturation. In cases where grain size data are available, I show how to estimate irreducible water saturation based on the effective surface-to-volume ratio of the rock. Results are then compared with bimodal logarithmic Gaussian distributions (Xu et al., 2013) and logarithmic Thomeer hyperbolas (Thomeer, 1960) to assess when each canonical description may or may not be more accurate than conventional methods for modeling the distribution of throat sizes and the corresponding flow properties. Finally, I introduce two new rock classification methods, which account for the rock’s pore and throat-geometry parameters, that can be used to quantify storage and flow properties. The new petrophysical description and classification methods are verified with several field examples. In the case of Bossier tight-gas sandstones, the methods are capable of identifying outlier permeability measurements and reliably classifying their storage and flow properties. Further, I demonstrate the importance of identifying the bimodal nature of the throat sizes with an example from the Panoma carbonate field. In this example a more accurate rock classification is obtained using the proposed method compared to the flow-zone indicator method. I also introduce an example from a siliciclastic sedimentary sequence located offshore Trinidad, where additional petrophysical data confirms that the bimodal nature of the grain-size distribution yields reliable estimates of irreducible water saturation. Finally, a second rock classification method that integrates grain topology, capillary pressure, and permeability is introduced and tested with data acquired in a tight-gas sandstone formation. The new petrophysical rock classification methods yield improved descriptions of throat and grain-size distributions, which result in more accurate calculations of permeability for rocks exhibiting complex pore-throat geometries.