Probing pore systems in carbonates: Correlations to petrophysical properties

Abstract

The understanding of petrophysical and multiphase flow properties is essential for the assessment and exploitation of hydrocarbon reserves; these properties in turn are dependent on the geometric and connectivity properties of the pore space. The determination of the pore size distribution in carbonate rocks remains challenging; extreme variability in carbonate depositional environments and susceptibility to a range of post-depositional processes results in complex pore structures comprising length scales from sub-micron to several centimeters. In this paper we illustrate that combining experimental techniques including micro-computed tomography and scanning electron microscopy (SEM) allows one to probe the pore scale structure in carbonates across a continuous range over many decades of length scales (from 10 nm to cm scales). Image data at nanoscales can be incorporated into the prediction of petrophysical properties of 3D images. This is especially important in limestones. We describe the study of the internal structure of over 30 carbonate samples across this range of length scales. The samples include carbonates with different porosity classifications including end members ranging from depositional porosity, diagenetic porosity and fracture porosity and many hybrid samples with mixtures of pore types. Particular emphasis is placed on the quantification of the pore sizes, shapes and interconnectivities in three dimensions. Extreme variability in pore network structures is observed. The pore network statistics (e.g., coordination, aspect ratio, pore size) for the connected samples are often similar despite the clear visual difference in the network structures. This illustrates the danger of attempting to stochastically generate pore networks for different carbonate samples based on matching network statistics alone. Petrophysical properties including resistivity, acoustic response and permeability are calculated on image data and where available, compared to experimental data. Comparisons between simulation and experimental data on the same core material are satisfactory. Flow properties of systems exhibiting significant microporosity require further development.