Explicit tracking of CO2-flow at the core scale using micro-Positron Emission Tomography (μPET)

Safe subsurface sequestration of carbon dioxide (CO2) is becoming increasingly important to meet climate goals and curb atmospheric CO2 concentrations. The world-wide CO2 storage capacity in carbonate formations is significant; within deep, saline aquifers and through several CO2-enhanced oil recovery projects, with associated CO2 storage. Carbonates are complex, both in terms of heterogeneity and reactivity, and improved core scale and sub core-scale analysis of CO2 flow phenomena is necessary input to simulators, aiming to establish large-scale behavior. This paper presents a recent advancement in in-situ imaging of CO2 flow, utilizing high-resolution micro-Positron Emission Tomography and radioactive tracer [11C]arbon dioxide to explicitly track CO2 during dynamic flow and subsequent trapping at the core scale. Unsteady state water injection (imbibition) and CO2 injection (drainage) were performed in a low-permeable chalk core at elevated pressure conditions. Short-lived radioisotopes were used to label water and CO2, respectively, and facilitated explicit tracking of each phase separately during single phase injection. Local flow patterns and dynamic spatial fluid saturations were determined from in-situ imaging during each experimental step. Initial miscible displacement revealed displacement heterogeneities in the chalk core, and dynamic image data was used to disclose and quantify local permeability variations. Radial permeability variations influenced subsequent flow patterns, where CO2 predominantly flooded the higher-permeability outer part of the core, leaving a higher water saturation in the inner core volume. Injection of water after CO2 flooding is proposed to be the most rapid and effective way to ensure safe storage, by promoting capillary trapping of CO2. PET imaging showed that presence of CO2 reduced the flow of water in higher-permeability areas, improving sweep efficiency and promoting a nearly ideal core-scale displacement. Alternate injections of water and gas is also expected to improve sweep efficiency and contribute to improved oil recovery and CO2 storage on larger scales. Sub-core analysis showed that residually trapped CO2 was evenly distributed in the chalk core, occupying 40% of the pore volume after ended water injection. Micro-Positron Emission Tomography yielded excellent small-scale resolution of both water and CO2 flow, and may contribute to unlocking fluid flow dynamics and determining mechanisms on the millimeter scale; presenting a unique opportunity in experimental core-scale evaluations of CO2 storage and security.publishedVersio

New Insight from Visualization of Mobility Control for Enhanced Oil Recovery Using Polymer Gels and Foams

Several enhanced oil recovery (EOR) methods have been designed and developed in the past decades to maintain economic production from mature reservoirs with declining production rates. This chapter discuss mitigation of poor sweep efficiency in layered or naturally fractured reservoirs. EOR methods designed for such reservoirs all aim to reduce flow through highly conductive pathways and delay early breakthrough in production wells. Two approaches within this EOR class, injection of foam and polymer, specifically aim to improve the mobility ratio between the injected EOR fluid and the reservoir crude oil. Reduction in fracture conductivity may be achieved by adding a crosslinking agent to a polymer solution to create polymer gel. This may also be combined with water or chemical chasefloods (e.g. foam) for integrated enhanced oil recovery (iEOR). Polymer gel and foam mobility control for use in fractured reservoirs are discussed in this chapter, and new knowledge from experimental work is presented. The experiments emphasized visualization and in situ imaging techniques: CT, MRI and PET. New insight to dynamic behaviour and local variations in fluid saturations during injections was achieved through the use of complementary visualization techniques

A Review of Polymer Gel Utilization in Carbon Dioxide Flow Control at the Core and Field Scale

Polymer gel has been used for water conformance control for several decades and may have significant potential in remediating unfavorable carbon dioxide (CO2) flow in the subsurface. High-mobility CO2 may channel quickly through sedimentary reservoirs, where unfavorable displacements are worsened in the presence of heterogeneities. Flow diversion technology targeting and withstanding CO2 is therefore essential to improving sweep efficiency and increasing storage potential. Polymer gel treatments have been demonstrated to remediate CO2 channeling in several enhanced oil recovery (EOR) field applications and have been proposed as a means to remediate wellbore and seal leakage during carbon sequestration. The goal of this review is to assess CO2 conformance control by polymer gel in published laboratory work related to both storage and EOR operations. Although field implementation of polymer gel has been successful in reducing CO2 flow, supporting experimental work on the laboratory scale is scattered, with both results and parameters varied. This paper summarizes the available literature and proposes a framework for future experimental work to aid more systematic assessment.publishedVersio

Multi-scale dissolution dynamics for carbon sequestration in carbonate rock samples

Carbon dioxide (CO2) sequestration in porous, sedimentary reservoirs is a key technology to mitigate emissions of anthropogenic CO2 and curb irreversible climate change. The abundance of carbonate formations, both as saline aquifers and hydrocarbon reservoirs, makes future CO2 storage in carbonate formations highly likely. The weak carbonic acid that forms when CO2 dissolves in water will, however, interact with highly reactive carbonate. Preferential flow paths may form during dissolution or calcite precipitation may reduce injectivity - both processes significantly impacting reservoir sweep efficiency. Hence, understanding the dynamics of the dissolution processes and their influence on flow properties is necessary to safely store CO2 in carbonate formations. Darcy and sub-Darcy scale dissolution kinetics were here assessed in carbonate core plugs with and without pre-existing highly permeable pathways, during multiphase flow and under relevant storage conditions. Darcy-scale dissolution and precipitation data (injectivity changes, effluent analysis and mass loss) confirmed that CO2 and brine co-injections altered the carbonate rock structure on Darcy scale, but could not determine the cause of change. Multi-modal imaging was applied to independently quantify structural changes with computed tomography (CT) and aqueous flow characteristics with positron emission tomography (PET), thereby determining injectivity dependence on local flow patterns. Formation of high permeability pathways, which was expected due to rock dissolution, was only observed in cores with pre-existing open fractures, where reactive flow was limited to the fracture plane. A good correlation between the two imaging modules was found: areas of higher porosity yielded a low-density CT signal (i.e. high number of voids present) and a high PET signal density (i.e. large volume of traced fluid present). Loss of injectivity suggested local changes in the flow pattern due to blocking of pore throats by moving particles or secondary precipitation or mineralization of dissolved ions. High-resolution PET imaging revealed cementation, that was also visible using micro-CT, hence determining sub-Darcy local flow obstructions that led to decreased Darcy scale injectivity. Multi-modal imaging, where core characteristics, such as large vugs and cementation, can be independently determined by complementary modalities, may therefore be a useful tool to quantify reactive flow and resulting dissolution in rock samples.publishedVersio

Unlocking multimodal PET-MR synergies for geoscience

The recent combination of positron emission tomography (PET) and magnetic resonance (MR) imaging modalities in one clinical diagnostic tool represents a scientific advancement with high potential impact in geoscientific research; by enabling simultaneous and explicit quantification of up to three distinct fluids in the same porous system. Decoupled information from PET-MR imaging was used here, for the first time, to quantify spatial and temporal porous media fluid flow. Three-dimensional fluid distribution was quantified simultaneously and independently by each imaging modality, and fluid phases were correlated with high reproducibility between modalities and repetitive fluid injections.publishedVersio

A systematic investigation of polymer influence on core scale wettability aided by positron emission tomography imaging

Polymers have been used as viscosifying agents in enhanced oil recovery applications for decades, but their influence on rock surface wettability is rarely discussed relative to its importance: wettability largely controls fluid flow in porous media and changes in wettability may significantly influence subsequent system performance. This paper presents a two-part systematic investigation of wettability alteration during polymer injection into oil-wet limestone. The first part of the paper determines wettability and wetting stability on the core scale. The well-established Amott–Harvey method is used, and five full cycles performed with repeated spontaneous imbibition and forced displacements. Wettability alterations are measured in a polymer/oil system, to determine polymer influence on wettability, and evaluated towards simpler brine/oil and glycerol/oil systems, to determine reproducibility and uncertainty related to the method and fluid/rock system. Polymer injection into oil-wet limestone core plugs is shown to repeatedly and reproducibly reverse the core wettability towards water-wet. Wettability changed both quicker and towards stronger water-wet conditions with polymer solution as the aqueous phase compared to brine and glycerol. The second part of the paper attempts to explain the observed behavior; by utilizing in situ imaging by Positron Emission Tomography, an emerging imaging technology within the geosciences. High resolution imaging provides insight into fluid flow dynamics during water and polymer injections, identifying uneven displacement fronts and significant polymer adsorption.publishedVersio

Microscopic and macroscopic assessment of carbonate dissolution for geologic CO2 storage

Carbon capture and storage in underground formations might be considered as a relevant technology to curb anthropogenic climate gas emissions. However, carbon dioxide (CO2) injection can lead to severe rock-fluid interactions depending on the thermodynamic conditions, rock and fluids composition. The progressive dissolution of CO2 in the formation brine results in mineral dissolution/precipitation processes that may drastically change the properties of the reservoir. This study is an attempt to get a deeper understanding of the dissolution/precipitation processes in a heterogeneous limestone at microscopic and macroscopic levels by a synergy between Isothermal Titration Calorimetry (ITC) and core flooding experiments with in-situ imaging to quantify uneven displacement fronts and understand the influence of reactions on a larger scale. Rock-fluid and fluid-fluid interactions, evaluated by ITC experiments, indicate that the carbonate dissolution is unfavorable with respect to enthalpy change but thermodynamically favorable with respect to entropy change (cations and hydrogen carbonate increase in the brine). Core flooding experiments with in-situ imaging by PET/CT show that complex pore structures cause a variation in the availability and ratio of the reactive fluid throughout the porous medium, hence, non-uniform dissolution was confirmed at core scale. The synergy between microcalorimetry and core flooding provides relevant insights into the dissolution of heterogeneous carbonate rocks at both microscopic and macroscopic scales.publishedVersio

DarSIA: An Open-Source Python Toolbox for Two-Scale Image Processing of Dynamics in Porous Media

Understanding porous media flow is inherently a multi-scale challenge, where at the core lies the aggregation of pore-level processes to a continuum, or Darcy-scale, description. This challenge is directly mirrored in image processing, where pore-scale grains and interfaces may be clearly visible in the image, yet continuous Darcy-scale parameters may be what are desirable to quantify. Classical image processing is poorly adapted to this setting, as most techniques do not explicitly utilize the fact that the image contains explicit physical processes. Here, we extend classical image processing concepts to what we define as “physical images” of porous materials and processes within them. This is realized through the development of a new open-source image analysis toolbox specifically adapted to time-series of images of porous materials.publishedVersio

Water shutoff and conformance improvement: an introduction

This paper provides an introduction to the topic of water shutoff and conformance improvement. After indicating the volumes of water produced during oilfield operations, a strategy is provided for attacking excess water production problems. Problem types are categorized, typical methods of problem diagnosis are mentioned, and the range of solutions is introduced for each problem type. In the third section of the paper, the concept of disproportionate permeability reduction is introduced—where polymers and gels may reduce permeability to water more than to oil or gas. When and where this property is of value is discussed. The fourth section describes the properties of formed gels as they extrude through fractures and how those properties can be of value when treating conformance problems caused by fractures. Section 5 covers the efficiency with which gels block fractures after gel placement—especially, the impact of fluids injected subsequent to the gel treatment.publishedVersio