Multi-scale Digital Rock: Application of a multi-scale multi-phase workflow to a Carbonate reservoir rock

Abstract

In some of the challenging digital rock applications the trade-off between model resolution and representative elemental volume is not captured in a single resolution model satisfying the minimum requirements for both aspects. In the wide range of lithofacies found in carbonate reservoir rocks, some facies fall in this category, where large pores, ooids or vugs, are connected by small scale porous structures that could have orders of magnitude smaller pores. In these cases a multi-scale digital rock approach is needed. We recently developed an extension to a digital rock workflow that includes a way to handle sub-resolution pore structures in single phase and multi-phase flow scenarios in addition to regular resolvable pore structures. Here we present an application of this methodology to a multi-scale limestone carbonate rock. A microCT image captures the large pores for this sample, but does not resolve all the pores smaller than the pixel size. A three phase image segmentation that considers pore, solid and under-resolved pores or porous media (PM) is generated. A high resolution confocal image model is obtained for a representative region of the smaller pores or PM region. A set of constitutive relationships (namely permeability vs. porosity, capillary pressure vs saturation and relative permeability vs saturation) are obtained by simulation from the high resolution confocal model. The low resolution segmented image, a porosity distribution image, and the constitutive relationships for the PM are input in an extended LBM multi-scale multi-phase solver. First we present results for absolute permeability and show a parametric study on PM permeability. The model recovers the expected behaviour when the PM regions are considered pore or solid. A consistent value of permeability with experiments is obtained when we use the PM permeability from the high resolution model. To demonstrate the multi-phase behaviour, we present results for capillary pressure imbibition multi-scale simulations. Here a small model for a dual porosity system is created in order to compare single scale results with the multi-scale solver. Finally, capillary imbibition results for the whole domain are shown and different wettability scenario results are discussed. This application illustrates a novel multi-scale simulation approach that can address a long standing problem in digital rock.

Towards Multiscale Digital Rocks: Application of a Sub-Resolution Production Model to a multiscale Sandstone

“…For these high clay content sandstones, with half of the pore space below the microCT image resolution, the current workflow predicts porosities within 5%, absolute permeability within 10% and relative permeability 𝐾 curves consistent with existing lab results. The current workflow demonstrate that is possible to use simplified models to extend the scope of application of digital rock technology to some challenging rock types, like high clay content sandstones, without having to explicitly simulate fluids movement in the unresolved pore regions using other multiscale multiphase flow methods such as [26][27][28][29][30]. In cases where the main percolating path is capture, but still a significant pore space fraction is unresolved, the method proposed in this paper could be computationally less expensive than full multiscale multiphase fluid flow methods [26][27][28][29][30].…”

“…In this case it is impossible to cover all pore scales in one digital rock analysis. There are several studies on the effect of unresolved pores and how to predict petrophysical properties, such as porosity, capillary pressure, absolute and relative permeabilities [6][7][8][9][10][26][27][28][29][30] but none, to our knowledge, on how to predict relative permeability curves by using multiphase fluid flow from direct numerical simulation in high clay content sandstones, such as the case just described, leading to a model with significant amount of unresolved porosity. This work investigate this case, a high clay content sandstone, by using an extension of current numerical digital rock workflows [11] to simulate fluid flow in rocks where a connected pore-space is resolved and unresolved pores do not provide a significant contribution to single phase flow, however for multiphase flow can provide critical connectivity paths for the wetting phase and impact the overall behavior of 𝑘 and 𝑃 curves.…”

A Simulation Approach Including Underresolved Scales for Two-Component Fluid Flows in Multiscale Porous Structures

“…The simulation cost is saved by a factor of 200 at best. Further applications of this underresolved simulation approach for reservoir rock can be found in [38]. The models and methodology in this study can be extensively applied to various engineering systems of multiscale porous structures, such as the example cases in the introduction.…”

“…i. Conduct a geometrical analysis for the original image and categorize the porous structures [38]. Then, the porous structure in each underresolved region, i.e., the gray region in Fig.…”