Evaluation and Application of Digital Rock Physics (DRP) for Special Core Analysis in Carbonate Formations

Abstract

Abstract A pilot study to evaluate the quality and validity of special core analysis (SCA) data from Digital Rock Physics (DRP) has provided results that are comparable to laboratory measurements. The DRP technique applied in this study employs the Lattice Boltzmann Method (LBM) for computing relative permeability (Kr(Sw)) and capillary pressure (Pc(Sw)) curves from high resolution digital pore structures obtained from micro-CT image data. The DRP processes, results, and comparisons with laboratory measurements on carbonate rock samples from different Saudi Arabian carbonate reservoirs are presented. DRP conventional core analysis (DRP-CCA) computations include porosity, permeability, formation factor, and dynamic elastic properties. DRP special core analysis (DRP-SCA) computations include Kr(Sw) and Pc(Sw). The translation of DRP-CCA and DRP-SCA determinations from imaged 4 mm subsamples to the 38 mm core plug-scale was achieved by upscaling the data for the various flow units and porosity structures in each plug. The number of flow units within each plug varied between one and four. The process of assembling plug-scale DRP-CCA and DRP-SCA properties is discussed. DRP-SCA results and laboratory measurements from similar rock types in the same wells are comparable and show inherent process and inter-lab uncertainties. The dynamic range of the computed relative permeability curves is superior to the laboratory measurements. The comparisons further showed the benefit of the DRP images and computations in capturing the detailed pore structure and fabric of the rock, especially in the capillary pressure responses. The DRP-SCA computations accentuate spontaneous imbibition and the transition to forced imbibition, a region that traditional laboratory methods may not adequately capture. Computations for different wetting conditions provide relative permeability data that cover all possible rock-fluid wettability states. Similar attempts in traditional laboratory experiments would be long, tedious and expensive. This work shows that DRP can provide satisfactory and complementary data for reservoir studies. The images are readily available and can be used for sensitivity studies. The workflow allows users to conduct their own validation tests, just as we have done, to determine the applicability of the method. Introduction In this work we consider the computation of porosity, conductivity, (relative) permeability and capillary pressure. These rock properties are of interest to petroleum engineers for characterizing a reservoir and measure the ability of the rock to transport fluids as well as the fluid pressure and saturation behavior exhibited through various hydraulic processes. These properties can be computed based on the pore space representation from CT or FIB-SEM based acquisition, physical models and their numerical implementation. Since rocks normally exhibit a strong multi-scale behavior, it is necessary to scan and compute properties on different scales. Scales can be differentiated by the resolution of the scans, but it is more useful to differentiate them conceptually. For example, there is a scale where the pore space is not directly visible at a certain resolution, but it is possible to identify different material regions that can be described by averaged properties. Physical processes can be described by equations like Darcy flow. We will call that scale the Darcy scale. On the other hand, there is the pore scale where the pore space is visible and the physical processes are described directly by pore scale physics. There can also be a mixture of both scales at a certain scan resolution. This includes large vugs embedded in one or several Darcy regions.