Experimental and numerical investigation of supercritical CO2 migration in sandstone with multiple clay interlayers

https://doi.org/10.1016/j.ijggc.2020.103194Get rights and content

Highlights

  • Typical reservoir sandstone with multiple clay interlayers is used.

  • A high-resolution X-CT scanner is used to visualize the fluid behavior during the drainage processes.

  • A 2D model based on Darcy flow is developed to analyze fluid migration.

  • The main factors affecting behavior of CO2 during drainage are analyzed.

  • The reasons for the difference between experimental and simulation results are listed.

Abstract

CO2 enhanced water recovery (CO2-EWR) is one of the most promising technologies for geologic carbon sequestration. The migration characteristics and displacement efficiency of supercritical CO2 (SC-CO2) after injection into a saline aquifer directly affect the feasibility and economy of a CO2-EWR project. Multiple clay interlayers often develop in the target sandstone reservoir and combine in different expansive ways, which could influence the SC-CO2 migration. In this study, core flooding experiments were conducted using a special sandstone with multiple thin clay interlayers and the fluid behavior was monitored by an X-ray computed tomography (X-CT) scanner. Based on the porosity matrix obtained from X-CT images, a two-dimension model based on Darcy flow was developed to simulate the fluid behavior under the experiment conditions. The experimental results show that SC-CO2 pathways were developed in the sand part of the sample, while the clay interlayers were fluid-proof. Comparing the experimental and simulation results, it is evident that the porosity distribution, clay interlayers directionality/extent and flooding direction were the main factors that affected the fluid behavior in the sample. There were positive correlations between porosity and SC-CO2 saturation after drainage processes. Flooding directions and clay interlayers were combined to affect the fluid migration. The clay interlayers had functions of barrier and collection when the flooding direction pointed towards the opening of a wedge-shaped semi-open area formed by clay interlayers. While this area was almost free of SC-CO2 when the injection direction reversed, resulting a smaller SC-CO2 saturation in sample during the backward drainage process.

Introduction

Geologic carbon sequestration is considered one of the most effective technologies to reduce carbon dioxide (CO2) level in the atmosphere at a large scale (Jiang et al., 2020; Norhasyima and Mahlia, 2018). The suitable storage sites include oil and gas reservoirs, coal formations, saline aquifers, basalt formations and deep ocean (Kelektsoglou, 2018; Liu et al., 2017). Among these approaches, the technology named CO2 enhance water recovery (CO2-EWR) has received attention from around the world. It has potential to regulate reservoir pressure through a reasonable layout of injection/extraction wells where the produced water can be used for industrial or other uses after purification (e.g. agriculture) (Li et al., 2016, 2015). CO2 is injected into the saline aquifer through the injection well where the pressure and temperature conditions result in a supercritical CO2 (SC-CO2) state and the injected SC-CO2 displaces the brine into the extraction (or production) well. The SC-CO2 migration characteristics and displacement efficiency will directly affect the feasibility and economic benefits of the CO2-EWR project.

Many core-scale flooding experiments and corresponding numerical simulations have been used to study SC-CO2 migration in the subsurface. Jiang et al. (2013) have conducted flooding experiments in a porous media (packed bed of glass beads) under sequestration conditions. Magnetic resonance imaging (MRI) was used to monitor the fluid front during CO2 injection. They found that both the CO2 front and the residual saturation are affected by porosity and permeability. Zhang et al. (2014) conducted drainage and imbibition experiments with a Berea sandstone containing both high- and low-porosity layers and used a medical X-ray computed tomography (X-CT) scanner to visualize the distribution of SC-CO2. The SC-CO2 distribution map obtained from the X-CT images shows that the SC-CO2 preferentially flows into the parts with high porosity due to the smaller capillary entry pressure during the drainage process. They pointed out that the porosity, the permeability and especially the capillary pressures are the main factors which affect SC-CO2 migration. Park et al. (2017) conducted core flooding experiments in Sarukawa sandstone to study the influence of sedimentation heterogeneity on SC-CO2 distribution. X-CT imaging showed that the sedimentation heterogeneity also has a significant impact on SC-CO2 flooding behavior. Zhang et al. (2019) conducted core flooding experiments which were monitored by X-CT and distributed fiber optic strain sensing. They found that the anisotropy within the rock samples studies could affect the evolution of the SC-CO2 plume. Wei et al. (2014) researched the effect of sub-core scale structure heterogeneity on the CO2 flooding properties in Liujiagou sandstone. Core flooding experiments monitored by an X-CT scanner and corresponding numerical simulations based on Darcy’s law were conducted. Experimental and simulation results displayed SC-CO2 flow pattern and the main factors which affect the SC-CO2 migration in Liujiagou sandstone were summarized. Krishnamurthy et al. (2017) performed liquid CO2 drainage experiments in a Boise sandstone core. An invasion percolation model and a Darcy-based flow model were compared with the X-CT images results. They pointed out that the two simulation methods have their own advantages and both provided result similar to the experimental results. The invasion percolation model was more suitable for flows which are dominated by capillary and buoyancy. Although the numerical dispersion would be significant with the Darcy-based flow model when the rock sample is strongly heterogeneous, the Darcy-based flow model can still capture the general fluid migration characteristics. To date, methods (including experimental methods and simulation methods) for studying SC-CO2 migration in core-scale samples are well established.

For rocks formed naturally, heterogeneity structures often develop within rocks due to different endogenic and exogenic geological processes. Clay interlayers with low porosity and permeability have a non-negligible influence on fluid migration (Liu et al., 2020, 2015). Oh et al. (2019) conducted flooding experiments in a reservoir sample which is composed of semi-consolidated mudstone and sandstone interbedded between mudstones. A Darcy-based flow model was developed and compared with the experimental results. They found that the interlayer structure has a significant influence on the CO2 distribution. However, in this case, the thickness of the sand and silt layers are approximately equal while the porosity of the two layers differs an order of magnitude. There are some very thin (less than 1 mm) clay interlayers developed in the sandstone samples and the porosity of the interlayer are quite different from the nearby part. Xu et al. (2020) conducted SC-CO2/brine flooding experiments in two samples which contain different orientations of a single clay interlayer. They showed that a single clay interlayer acts like a shunt to separate the SC-CO2 flow when the injection direction is parallel to the expansion of the clay interlayer while that single clay interlayer acts like a barrier to prevent SC-CO2 moving ahead when the injection direction is perpendicular to the expansion of the clay interlayer. However, in some cases, there are often multiple clay interlayers that develop and even intersect in rock samples. The effect of this phenomenon on SC-CO2 migration should be studied in order to better assess the drainage efficiency.

In this study, a sandstone sample which contains multiple clay interlayers was used to conduct SC-CO2/brine flooding experiments. SC-CO2 was injected from one end of the sample into the brine-saturated sample in the first experiment (which is called forward drainage). Following this, SC-CO2 was injected from the other end into the same brine-saturated sample in the second experiment (which is called backward drainage). A high-resolution X-CT scanner was used to image the SC-CO2 flow pattern inside the sample during two drainage processes. Based on the porosity matrix obtained from the CT images, a two-dimension (2D) model based on Darcy flow was used to simulate the SC-CO2 flow pattern. Through the analysis and comparison of experimental and simulation results, the effect of the multiple clay interlayers on fluid migration were further understood. The purpose of this study is to provide a deep understanding of the SC-CO2 migration in sandstone with distinct multiple clay interlayers that form the Penglaizhen formation. Furthermore, this study aims to provide reasonable geological parameters for the subsequent simulations of CO2-EWR project implementations in this formation.

Section snippets

Rocks and liquids

The sandstone sample (36.41 mm diameter, 59.67 mm length) used in this core flooding experiment was cored from an outcrop in the Penglaizhen formation in Sichuan Basin, China (Fig. 1a–c). As shown in Fig. 1d, X-CT images show three distinct clay interlayers develop inside the sample. The main clay interlayer (layer-I) crosses the whole sample with an extension direction 15 degrees from the axial direction. Two secondary clay interlayers whose extension direction is nearly parallel to the axial

Differential pressure response

The differential pressure was measured during the forward and backward drainage processes (see Fig. 4). It can be used to judge whether surface leakage is occurring and roughly understand the behavior of SC-CO2 inside the sample. The measured differential pressure during the forward drainage can be divided into two stages: forward drainage’s stage I (F-SI) and then F-SII. During the F-SI phase, the differential pressure rapidly increased to 10.55 kPa due to SC-CO2 accumulating at the inlet

Simulation methodology

Core-scale numerical simulations are very useful for researching the flow pattern of SC-CO2 and can provide some new insights. In this work, the simulations of SC-CO2 flow model were based on the established principles derived from previous studies (Shi et al., 2011; Wei et al., 2014). COMSOL Multiphysics™ is a finite element analysis and solver software package for physics and engineering applications. Furthermore, it can be used to simulate saturated and unsaturated flow characteristics in

Main factors effect on the SC-CO2 flow pattern

In the numerical model, the porosity matrix was obtained from CT images using Formula (1) and other parameters were obtained from empirical formulas. The clay interlayers and surrounding sand part were separated by a selected porosity threshold and were given different capillary pressure curves in the numerical simulations. By comparing and analyzing the experimental and simulation results during forward and backward drainage processes, it is evident that the porosity distribution, sedimentary

Conclusion

In this paper, to research the influence of multiple clay interlayers and their geometry on the SC-CO2 migration characteristics and assess the feasibility of these types of reservoirs for CO2-EWR technology, core flooding experiments were conducted and a high-resolution X-CT scanner was used to monitor SC-CO2 flow pattern. A 2D model was built up and COMSOL™ was used to simulate the forward and backward drainage processes under the experiment conditions. The experimental results show that

Author contributions

Qi and Matt designed the research. Liang, Matt and Cameron conducted the X-CT flooding experiments. Liang, Qi and Yongsheng conducted the numerical simulations. Liang and Qi wrote the draft manuscript. Matt and Cameron polished language of the manuscript. All authors performed the research and analyzed the results.

Declaration of competing interest

The authors wish to confirm that there are no known conflict of interests associated with this publication and there has been no significant financial support for this work that could have influenced its outcome

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 41872210 and 41274111), and Open Research Fund of State Key Laboratory of Geomechanics and Geotechnical Engineering, IRSM, CAS (Grant No. Z017002).

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