Publications
192 results found
Fraga DM, Korre A, Nie Z, et al., 2024, Multi-period, multi-objective optimisation of the Northern Lights and Stella Maris carbon capture and storage chains, Carbon Capture Science & Technology, Vol: 11, ISSN: 2772-6568
A multi-objective optimisation model for CCS chains, aiming to minimise costs and greenhouse gas emissions, considering various transport options is presented. The model builds upon previous work and covers the CCS chain elements after CO2 is captured, including conditioning, pipeline and batch-wise transportation, intermediate hub storage and injection at CO2 storage fields. A prospective Life Cycle Inventory is integrated to evaluate emissions from batch-wise transportation. The model is parameterised for accurate estimations based on site-specific characteristics and is implemented in two standalone CCS chains, that are analogues to the Northern Lights project with dominant ship transport and the Stella Maris concept with direct injection ship transport, also incorporating distinct emission profiles, intermediate storage hubs and injection sites. A third implementation combining both chain concepts is implemented. By increasing in a step-wise manner the weight of the emissions in the objective function, the model evaluates cost and emission trade-offs. The optimisation selects costlier wells to minimise emissions and shifts from batch-wise ships to cross-continent pipelines. Chain emissions decrease over time with CO2 supply increase. Shipping operation dominates emissions, followed by well construction and infrastructure construction. Across all the implementations, the GHG emission intensity of the chain, after CO2 is captured, ranged from 3.3 to 14.2 %, depending on the concept and transport option adopted and accounting for regional characteristics (i.e., the electricity supply mix per country).
Bellezza C, Barison E, Farina B, et al., 2024, Multi-Sensor Seismic Processing Approach Using Geophones and HWC DAS in the Monitoring of CO2 Storage at the Hellisheiði Geothermal Field in Iceland, Sustainability, Vol: 16, Pages: 877-877
<jats:p>Geothermal power production may result in significant CO2 emissions as part of the produced steam. CO2 capture, utilisation, subsurface storage (CCUS) and developments to exploit geothermal resources are focal points for future clean and renewable energy strategies. The Synergetic Utilisation of CO2 Storage Coupled with Geothermal Energy Deployment (SUCCEED) project aims to demonstrate the feasibility of using produced CO2 for re-injection in the geothermal field to improve geothermal performance, while also storing the CO2 as an action for climate change mitigation. Our study has the aim to develop innovative reservoir-monitoring technologies via active-source seismic data acquisition using a novel electric seismic vibrator source and permanently installed helically wound cable (HWC) fibre-optic distributed acoustic sensing (DAS) system. Implemented together with auxiliary multi-component (3C and 2C) geophone receiver arrays, this approach gave us the opportunity to compare and cross-validate the results using wavefields from different acquisition systems. We present the results of the baseline survey of a time-lapse monitoring project at the Hellisheiði geothermal field in Iceland. We perform tomographic inversion and multichannel seismic processing to investigate both the shallower and the deeper basaltic rocks targets. The wavefield analysis is supported by seismic modelling. The HWC DAS and the geophone-stacked sections show good consistency, highlighting the same reflection zones. The comparison of the new DAS technology with the well-known standard geophone acquisition proves the effectiveness and reliability of using broadside sensitivity HWC DAS in surface monitoring applications.</jats:p>
Hassing SHW, Draganov D, Janssen M, et al., 2024, Imaging CO<sub>2</sub> reinjection into basalts at the CarbFix2 reinjection reservoir (Hellisheiði, Iceland) with body‐wave seismic interferometry, Geophysical Prospecting, ISSN: 0016-8025
<jats:title>Abstract</jats:title><jats:p>As part of the Synergetic Utilisation of CO<jats:sub>2</jats:sub> storage Coupled with geothermal EnErgy Deployment project, investigating CO<jats:sub>2</jats:sub> reinjection with different seismic methods, both passive and active seismic surveys have been conducted at the geothermal power plant at Hellisheiði, Iceland. During the 2021 survey, two geophone lines recorded noise for a week. We process the passive‐source data with seismic interferometry to image the subsurface structure around the CarbFix2 reinjection reservoir. To improve image quality, we perform an illumination analysis to select only noise panels dominated by body‐wave energy. The results show that most noise panels are dominated by air‐wave energy arriving from the direction of the power plant. We use panels with a near‐vertical incidence to create a zero‐offset image and a larger selection of body‐wave‐dominated panels to create virtual common‐shot gathers. We process the gathers with a simple reflection seismology processing workflow to obtain stacked images. The zero‐offset images show a relatively lower signal‐to‐noise ratio and only horizontal reflectors. The stacked images show slightly dipping reflectors and possibly lateral amplitude variations around the expected injection region. This could indicate a region of interest for future research into the reinjection reservoir.</jats:p>
Agrawal H, Durucan S, Cao W, et al., 2024, Analytical estimation of strain energy accumulation in retreating longwall mining and sensitivity analysis using the orthogonal testing method, Rock Mechanics and Rock Engineering, ISSN: 0723-2632
Excessive strain energy accumulation in the coal seam is one of the essential conditions for rockbursts to occur. It is imperative to understand the parameters that affect the strain energy accumulation in retreating longwall mining to optimise the design and minimise rockbursts occurrences. In this paper, new analytical models were developed to calculate the strain energy accumulation considering the current state-of-the-art longwall machinery being used in the industry. Seven parameters, i.e., mining depth, length of the cantilever roof, coal seam thickness, thickness of the immediate roof, Young’s modulus of coal and the roof, and Poisson’s ratio of the coal seam, were identified as the parameters affecting the strain energy accumulation. A detailed statistical analysis of the parameters was conducted using the orthogonal testing method, which revealed that mining depth, Young's modulus of coal and the coal seam thickness significantly influence the strain energy accumulation within a 99% confidence interval.
Cao W, Durucan S, Cai W, et al., 2023, Probabilistic evaluation of susceptibility to fluid injection-induced seismicity based on statistics of fracture criticality, Rock Mechanics and Rock Engineering, Vol: 56, Pages: 7003-7025, ISSN: 0723-2632
Fault reactivation and associated microseismicity pose a potential threat to industrial processes involving fluid injection into the subsurface. In this research, fracture criticality, defined as the gradient of critical fluid pressure change to trigger seismicity (Δpc/h), is proposed as a novel reservoir depth-independent metric of fault slip susceptibility. Based on statistics of the fracture criticality, a probabilistic evaluation framework for susceptibility to injection-induced seismicity was developed by integrating seismic observations and hydrogeological modelling of fluid injection operations for faulted reservoirs. The proposed seismic susceptibility evaluation method considers the injection-driven fluid pressure increase, the variability of fracture criticality, and regional fracture density. Utilising this methodology, the probabilistic distribution of fracture criticality was obtained to evaluate the potential for injection-induced seismicity in both fault and off-fault zones at the Hellisheiði geothermal site, Iceland. It has been found that the fracture criticality within both fault and off-fault zones shows natural variability (mostly ranging between 0.001-2.0 bar/km), and that fault zones tend to be characterised by larger fracture criticality values than the off-faut zones. Fracture criticality values estimated within each zone roughly follow a Gaussian distribution. Fault zones around five geothermal fluid re-injection wells at the site were estimated to have relatively high probability of seismic event occurrence, and these regions experienced high levels of induced seismicity over the microseismic monitoring period. The seismotectonic state estimated for each zone is generally consistent with the forecasted susceptibility to seismicity based on statistics of fracture criticality.
Agrawal H, Durucan S, Cao W, et al., 2023, Rockburst and gas outburst forecasting using a probabilistic risk assessment framework in longwall top coal caving faces, Rock Mechanics and Rock Engineering, Vol: 56, Pages: 6929-6958, ISSN: 0723-2632
A probabilistic risk assessment framework was developed to mathematically represent the complex engineering phenomena of rock bursts and gas outbursts for a heterogeneous coal seam. An innovative object-based non-conditional simulation approach was used to distribute lithological heterogeneity present in the coal seam to respect their geological origin. The changing mining conditions during longwall top coal caving mining (LTCC) were extracted from a coupled numerical model to provide statistically sufficient data for probabilistic analysis. The complex interdependencies among abutment stress, pore pressure, the volume of total gas emission and incremental energy release rate, their stochastic variations and uncertainty were realistically implemented in the GoldSim software, and 100,000 equally likely scenarios were simulated using the Monte Carlo method to determine the probability of rock bursts and gas outbursts. The results obtained from the analysis incorporate the variability in mechanical, elastic and reservoir properties of coal due to lithological heterogeneity and result in the probability of the occurrence of rock bursts, coal and gas outbursts, and safe mining conditions. The framework realistically represents the complex mining environment, is resilient and results are reliable. The framework is generic and can be suitably modified to be used in different underground mining scenarios, overcoming the limitations of earlier empirical indices used.
Cao W, Durucan S, Shi JQ, et al., 2023, Slip tendency evaluation of fracture systems associated with seismicity at the Hellisheiði geothermal field, Iceland
An understanding of fracture slip susceptibility in geothermal reservoirs is central to the control of fluid injection induced seismicity. To investigate the role of regional fracture systems on induced seismicity, a coupled thermo-hydro-mechanical (THM) model containing fracture networks, which features direct coupling between different physics for the rock matrix, fractures, and their interactions, as well as indirect coupling through changes of material properties, such as stress-dependent rock and fracture permeabilities, was developed. The model was applied to simulate the geothermal fluid extraction and re-injection over a 10-year period (2011-2021) at the Hellisheiði geothermal field, utilising field recorded monthly production and re-injection rates. Based on the model results, the slip tendency of regional fracture systems was examined under reservoir conditions before and after the start of fluid re-injection. Results have shown that fracture networks act as preferential fluid flow paths that influence fluid pressure and stress distribution and fracture slip tendency in geothermal reservoirs. NE-SW and N-S trending fractures are susceptible to slippage before the start of fluid re-injection, and the distribution of fractures with enhanced slip tendency shifts from surrounding the re-injection region at the onset of fluid re-injection, to a two-lobed pattern in the fault-normal direction around the re-injection region in the long term.
Cao W, Durucan S, Shi J-Q, et al., 2022, Induced seismicity associated with geothermal fluids re-injection: Poroelastic stressing, thermoelastic stressing, or transient cooling-induced permeability enhancement?, Geothermics, Vol: 102, Pages: 1-18, ISSN: 0375-6505
Both field injectivity and induced seismicity were reported to be inversely correlated with the temperature of re-injected fluids at the Hellisheiði geothermal field in Iceland. This observation has led to a hypothesis that transient cooling-induced permeability enhancement is a novel mechanism for induced seismicity, in addition to elevated fluid pressure, poroelastic stressing, and thermoelastic stressing in geothermal environments. In this work, a 3D calibrated coupled THM model was developed to model the colder fluids re-injection process over a 1-year period and evaluate the potential for induced seismicity in terms of Coulomb stress changes at the Hellisheiði geothermal field. Three modelling scenarios taking into account respectively the poroelastic effect, thermoporoelastic effect, and thermoporoelastic effect with permeability enhancement, were examined and compared to identify the dominant mechanism for the recorded seismicity and examine the contribution from each individual mechanism. Results have shown that, under normal fluid re-injection pressure and temperature conditions, the permeability enhancement effect is the dominant mechanism for induced seismicity at the Hellisheiði geothermal field. Specifically, the contribution to Coulomb stress changes from the permeability enhancement effect is almost twice of that from the thermoelastic stressing, which is in turn two orders of magnitude larger than that from the poroelastic stressing. It has also been noted that, when reducing temperature of re-injected fluids from 120°C to 20°C, the temperature change is increased by 2.1 times at 1,000 m depth, while the amount of mass flow by around 4 times. Thus, the amount of heat transferred can be increased 8.4 times by lowering temperature of the injected fluids, which explains the high sensitivity of induced seismicity to temperature. Outcomes of this work suggest temperature control of injected fluids as a feasible regulation method to mit
Cao W, Durucan S, Shi JQ, et al., 2022, Coupled THM modelling of induced seismicity associated with geothermal fluids re-injection: the role of transient cooling-induced permeability enhancement
Seismicity induced by injection of energy depleted geothermal fluid, as well as field injectivity, has been observed to be inversely correlated with the temperature of re-injected fluids at the Hellisheiði geothermal field, Iceland. Using the Hellisheiði field injection data, transient cooling-induced permeability enhancement as a distinctive mechanism for induced seismicity in geothermal reservoirs was investigated. A 3D coupled THM model was developed to model the colder fluids re-injection process over a 1-year period and evaluate the potential for induced seismicity at Hellisheiði. Three modelling scenarios considering respectively the poroelastic effect, thermoporoelastic effect, and thermoporoelastic effect with permeability enhancement, were examined and compared to identify the dominant mechanism for induced seismicity and examine the contribution from each individual mechanism. Results have shown that, under normal fluid re-injection pressure and temperature conditions, the permeability enhancement effect is the dominant mechanism for induced seismicity at Hellisheiði. In addition, by reducing temperature of re-injected fluids from 120 °C to 20 °C, the amount of heat transferred can be increased by 8.4 times, which explains the high sensitivity of induced seismicity to temperature. The findings of this work suggest that temperature control of injected fluids can be a feasible regulation method to mitigate injection-induced seismic risk.
Fraga DM, Skagestad R, Eldrup NH, et al., 2021, Design of a multi-user CO2 intermediate storage facility in the Grenland region of Norway, INTERNATIONAL JOURNAL OF GREENHOUSE GAS CONTROL, Vol: 112, ISSN: 1750-5836
Parlaktuna M, Durucan S, Parlaktuna B, et al., 2021, Seismic velocity characterisation and survey design to assess CO2 injection performance at Kizildere geothermal field, TURKISH JOURNAL OF EARTH SCIENCES, ISSN: 1300-0985
Jahanbakhsh A, Liu Q, Mosleh MH, et al., 2021, An investigation into co2–brine–cement–reservoir rock interactions for wellbore integrity in co2 geological storage, Energies, Vol: 14, Pages: 1-20, ISSN: 1996-1073
Geological storage of CO2 in saline aquifers and depleted oil and gas reservoirs can help mitigate CO2 emissions. However, CO2 leakage over a long storage period represents a potential concern. Therefore, it is critical to establish a good understanding of the interactions between CO2–brine and cement–caprock/reservoir rock to ascertain the potential for CO2 leakage. Accordingly, in this work, we prepared a unique set of composite samples to resemble the cement–reservoir rock inter-face. A series of experiments simulating deep wellbore environments were performed to investigate changes in chemical, physical, mechanical, and petrophysical properties of the composite samples. Here, we present the characterisation of composite core samples, including porosity, permeability, and mechanical properties, determined before and after long‐term exposure to CO2‐rich brine. Some of the composite samples were further analysed by X‐ray microcomputed tomography (X‐ray μ‐CT), X‐ray diffraction (XRD), and scanning electron microscopy–energy‐dispersive X‐ray (SEM–EDX). Moreover, the variation of ions concentration in brine at different timescales was studied by per-forming inductively coupled plasma (ICP) analysis. Although no significant changes were observed in the porosity, permeability of the treated composite samples increased by an order of magnitude, due mainly to an increase in the permeability of the sandstone component of the composite samples, rather than the cement or the cement/sandstone interface. Mechanical properties, including Young’s modulus and Poisson’s ratio, were also reduced.
Santibanez-Borda E, Korre A, Nie Z, et al., 2021, A multi-objective optimisation model to reduce greenhouse gas emissions and costs in offshore natural gas upstream chains, Journal of Cleaner Production, Vol: 297, Pages: 1-14, ISSN: 0959-6526
The urgency of climate change, while the world economy is projected to depend on fossil fuels for some time, requires substantial reduction of greenhouse gas emissions in the oil and gas industry. This study proposes a methodology for the decarbonisation of offshore natural gas production networks through progressive electrification, either by connecting offshore platforms with nearby renewable energy sources, e.g. offshore wind farms, or by sharing resources so as to improve their energy generation efficiency. In this context, a novel multi-objective mixed-integer linear programming model is proposed to simultaneously minimise greenhouse gas emissions and associated costs from a determined offshore platform network, considering technical constraints, such as maintaining the energy balance of the network, ensuring that cables are installed to enable energy flows, and respecting the maximum generation capacities and minimum operating loads of turbines. For demonstration purposes, the proposed methodology was applied to a UK Southern North Sea network and optimised using the augmented -constraint method. The Pareto front approximation obtained suggests that the studied network’s cumulative greenhouse gas emissions can be reduced by 25% over the next 10 years at an average cost of US$370.9 per tonne CO2e. This study also explores the impact that uncertainties and postponing investment decisions may have in the set context.
Cao W, Shi J-Q, Durucan S, et al., 2021, Evaluation of shear slip stress transfer mechanism for induced microseismicity at In Salah CO2 storage site, International Journal of Greenhouse Gas Control, Vol: 107, Pages: 1-20, ISSN: 1750-5836
Stress transfer caused by injection-induced fault reactivation plays a significant role in triggering induced seismicity. This work aims to investigate to which extent the shear slip stress transfer mechanism might have contributed to a 4-month period of heightened microseismicity around one of the horizontal injection wells (KB-502) at the In Salah CO2 storage site. Building upon previous reservoir modelling and history matching work by the authors, coupled geomechanical and reservoir modelling of CO2 injection at KB-502 was carried out, featuring the explicit simulation of injection-induced fault reactivation and stress transfer, and the implementation of a strain-dependent permeability model to represent the fault hydrological behaviour. This approach allows a much-improved overall match to the field bottomhole pressures at KB-502 over the previous results, where fault zone reactivation and associated dynamic permeability behaviour were not considered, especially over the 4-month period of interest. Based upon the coupled modelling results, Coulomb stress changes were used to evaluate the potential for enhanced microseismicity related to CO2 injection-induced fault reactivation at KB-502. Analyses on the potential for microseismicity have shown that seismic events are likely to take place in both hydraulically connected regions and stress transfer influenced regions. The variation of computed Coulomb stress changes in near-fault areas compares favourably with the heightened field recorded seismicity during the period modelled. The integrated interpretation of microseismic monitoring and coupled geomechanics and reservoir modelling have suggested that the shear slip stress transfer mechanism was active and contributed to the occurrence of induced seismicity at In Salah.
Janssen MTG, Barnhoorn A, Draganov D, et al., 2021, Seismic velocity characterisation of geothermal reservoir rocks for CO2 storage performance assessment, Applied Sciences-Basel, Vol: 11, Pages: 1-30, ISSN: 2076-3417
As part of a seismic monitoring project in a geothermal field, where the feasibility of re-injection and storage of produced CO2 is being investigated, a P- and S-wave seismic velocity characterisation study was carried out. The effect of axial (up to 95 MPa) and radial (up to 60 MPa) stress on the seismic velocity was studied in the laboratory for a broad range of dry sedimentary and metamorphic rocks that make up the Kızıldere geothermal system in Turkey. Thin section texture analyses conducted on the main reservoir formations, i.e., marble and calcschist, confirm the importance of the presence of fractures in the reservoir: 2D permeability increases roughly by a factor 10 when fractures are present. Controlled acoustic-assisted unconfined and confined compressive strength experiments revealed the stress-dependence of seismic velocities related to the several rock formations. For each test performed, a sharp increase in velocity was observed at relatively low absolute stress levels, as a result of the closure of microcracks, yielding an increased mineral-to-mineral contact area, thus velocity. A change in radial stress appeared to have a negligible impact on the resulting P-wave velocity, as long as it exceeds atmospheric pressure. The bulk of the rock formations studied showed reducing P-wave velocities as function of increasing temperature due to thermal expansion of the constituting minerals. This effect was most profound for the marble and calcschist samples investigated.
Durucan S, Korre A, Parlaktuna M, et al., 2021, SUCCEED: A CO2 storage and utilisation project aimed at mitigating against greenhouse gas emissions from geothermal power production, 15th Greenhouse Gas Control Technologies 2021 (GHGT-15)
The non-condensable gases in most geothermal resources include CO2 and smaller amounts of other gases. Currently, the worldwide geothermal power is a small sector within the energy industry, and CO2 emissions related to the utilisation of geothermal resources are consequently small. In some countries, however, geothermal energy production contributes significantly to their energy budget and their CO2 emissions are relatively significant. SUCCEED is a targeted innovation and research project which aims to investigate the reinjection of CO2 produced at geothermal power production sites and develop, test and demonstrate at field scale innovative measurement, monitoring and verification (MMV) technologies that can be used in most CO2 geological storage projects. The project is carried out at two operating geothermal energy production sites, the Kızıldere geothermal field in Turkey and the CarbFix project site at the Hellisheiði geothermal field. Together with a brief description of the seismic monitoring technologies proposed in the project, this paper presents the details of the two field sites and the progress made in installing and testing of the surface fibre-optic cables at the Hellisheiði geothermal field in Iceland.
Skagestad R, Fraga DM, Eldrup NH, et al., 2021, Design of a Multi-user Intermediate Storage Facility in the Grenland Region of Norway
This paper describes a design and pre-feasibility study of a multi-user intermediate CO2 storage facility in the Grenland region of Norway considering upstream and downstream issues. The study focuses on the principles for design and installation of a generic hub facility, so the results can be utilised at other sites. The pre-feasibility study found that design pressures of 7 and 15 bar pressures are feasible transport conditions; moreover, showed that economies of scale might reduce the total cost for a CO2 network. It is recognised that cooperation across the chain is crucial in the management of impurities, due to the likely diverse sources of CO2 stream composition. An intermediate storage facility can support the continuous supply of CO2 via a pipeline system for reservoir injection, therefore improving the integrity of the injection well and equipment and the reservoir performance. A mixed integer linear programming optimisation model has been developed for sizing and costing two intermediate storage HUBs of CO2 in Grenland and Kollsnes and shipping connection between them. The model considers a flow of 2 mtpa of CO2 for 27 years. The 7 bar design pressure has shown lower total costs when compared with the 15 bar scenario. This is due to the higher costs for shipping and intermediate storage when operating at higher pressure, which is larger than the cost reduction from liquefying CO2 to a higher pressure and higher temperature than those required for a 7 bar scenario. The estimated levelised costs were 11.7 €/tonne at 7 bar pressure and 13.2 €/tonne at 15 bar pressure.
Nie Z, Korre A, Durucan S, et al., 2021, CO2 pipeline transport and storage network cost modelling and multi-period multi-scenario stochastic optimisation
Carbon capture and storage stakeholders focusing on transport and storage aspects need to consider a wide range of risks and uncertainties when making investment decisions, including market, regulatory, geological and technical risks and uncertainties. This paper presents a stochastic optimisation based real options valuation framework that can be implemented to model CO2 pipeline transport and storage network costs in which uncertainties and engineering flexibilities are appraised, optimised and factored into the investment decisions, so that investors or regulators can confidently and quantitatively evaluate incentives that can support CCS deployment at large scale. The paper describes the models developed for this purpose and demonstrates the application of the modelling framework for a realistically designed CO2 storage cluster around Rotterdam in the Netherlands. It shown that the CCS network evolution is mainly driven by geological and regional geography constrains, CO2 supply rates, and the engineering designs relying on these. It has also been shown that individual storage sites have significantly different cash flows, dictated by their physical characteristics and the injection concept chosen. The storage capacity uncertainty and CO2 supply uncertainty (or CO2 mitigation target uncertainty) have been identified as the main uncertainties affecting a CCS network cluster.
Cao W, Shi JQ, Durucan S, et al., 2021, Investigation into the hydro-geomechanical mechanisms related to induced microseismicity at In Salah storage site
It has been recognised that stress transferred from injection-induced fault reactivation plays a significant role in triggering induced seismicity. In this study, the mechanism which triggered a 4-month long period of heightened microseismicity around one of the horizontal injection wells (KB-502) at the In Salah CO2 storage site has been investigated. Building upon previous reservoir modelling and history matching work by the authors, a coupled geomechanical and reservoir modelling study of CO2 injection at KB-502 was carried out, featuring the implementation of a strain-dependent permeability model developed to describe hydromechanical response of the fault zone to CO2 injection. This approach allows a much improved overall match to the field bottomhole pressures at KB-502 over the previous results where fault zone reactivation and associated dynamic permeability behaviour were not considered, especially over the 4-month period of interest. Based upon the coupled modelling results, Coulomb stress changes were used to evaluate the potential for enhanced microseismicity related to CO2 injection-induced fault reactivation at KB-502, and to ascertain, to which extent stress shear slip stress transfer mechanism might have contributed to the elevated microseismicity recorded around KB-502. Analyses on the potential for microseismicity have shown that seismic events are likely to take place in both hydraulically connected regions and stress transfer influenced regions. The integrated interpretation of microseismic monitoring and coupled geomechanics and reservoir modelling have suggested that the shear slip stress transfer mechanism was active and contributed to the occurrence of induced seismicity at the In Salah site.
Jahanbakhsh A, Bajwa JH, Farooqui NM, et al., 2021, Multi-scale investigation of caprock-cement integrity for CO<inf>2</inf> storage
CO2 storage in saline aquifers and depleted oil and gas reservoirs is a promising solution to help mitigating CO2 emissions and tackling global warming. However, storage security and CO2 leakage over a long period remain as a major concern. Cement degradation after CO2 injection, especially in poorly cemented wells, can provide pathways for CO2 leakage to groundwater and the atmosphere. Therefore, to assess the potential risks of CO2 leakage, it is critical to understand the interactions between the major components of a storage system including CO2, brine, wellbore cement, and caprock/reservoir rocks. In this work, we performed a series of experiments simulating deep wellbore environments to investigate the chemical, physical, and mechanical alterations in cement and rock as a result of being exposed to CO2-rich brine. X-ray micro-computer tomography (µCT), petrophysical evaluation and geomechanical testing were conducted before and after experiments. It was observed that dissolution and precipitation of some minerals started from early stage of exposure and continued as time passed, and, as a result, cement and cement-caprock interface were affected. The high-permeable hairline cracks in the cement were filled due to mineral precipitation and reduced the composite permeability significantly. This phenomenon may help mitigate against CO2 leakage at conditions similar to those used in this work.
Nie Z, Korre A, Santibanez-Borda E, et al., 2021, A comprehensive and systematic modelling tool for natural gas value chain greenhouse gas emissions quantification
This paper presents a comprehensive and systematic life cycle assessment tool developed at Imperial College, the ICLCA model, that can be used to accurately account for greenhouse gas emissions from any natural gas value chain and quantify their global warming potential. The ICLCA tool covers conventional gas production (onshore, offshore), unconventional gas production (shale gas, tight gas, and coalbed methane), gas processing, pipeline transmission, liquefaction, LNG loading, LNG shipping, LNG unloading, regasification and pipeline transport to city gate. The tool is built at unit processes level. Moreover, engineering design features, operational parameters and GHG emission mitigation options are considered in detail and are fully integrated in the tool. The detailed accounting methodology used allows to reconcile top-down and bottom-up emissions estimation approaches, which is an important step change as compared to currently employed methods. The results demonstrate that the tool successfully identifies and quantifies GHG emissions (CO2, CH4, N2O) at unit process level or equipment level and over time, covering combustion emissions, vent, fugitives, and flares. The dynamics of life cycle CO2 emissions, methane emissions and carbon intensity can be captured accurately and reliably.
Cao W, Durucan S, Cai W, et al., 2021, Combining Microseismic Observations and Reservoir Simulation to Interpret Fracture Criticality in Faults at the Hellisheiði Geothermal Field, Iceland, Pages: 418-427
Fault reactivation and associated microseismicity induced by fluid injection into the subsurface pose a potential threat in geothermal power generation. In this research, the gradient of critical pore pressure change to trigger seismicity (Δpc/h), referred to as the fracture criticality, has been proposed to represent the critical state of subsurface fractures. The fracture criticality is subjected to variability due to heterogeneous fracture attributes and rock properties. The statistics of fracture criticality could be applied to the probabilistic evaluation of fluid injection-induced seismic risk, which considers the injection-driven pore pressure increase, the variability of fracture criticality, and local fracture density. The seismic risk evaluation based on statistics of fracture criticality was applied to the Hellisheiði geothermal site, where microseismic observations and reservoir simulation over a half-year fluid injection period were integrated to achieve the probabilistic distribution of fracture criticality and evaluate the injection-induced seismic risk in both fault and off-fault zones. It has been found that the fracture criticality within both fault and off-fault zones shows natural variability (mostly ranging between 0-1.0 bar/km), and the values estimated roughly follow Gaussian distributions. Relatively high probability of seismic event occurrence was estimated for fault zones around five geothermal fluid re-injection wells at the site, which were consistent with seismically-active areas over the microseismic monitoring period.
Agrawal H, Durucan S, Cao W, et al., 2021, Probabilistic Risk Assessment of Rock Bursts and Excessive Gas Emissions in Longwall Top Coal Caving Mining
Despite many years of research, rock bursts and excessive gas emissions remain a longstanding cause for concern and a major hazard in underground coal mining. There are several traditional approaches to forecast rock bursts in underground mining, but it is difficult to use a single criterion for this purpose due to variations in mining conditions worldwide. This paper presents a generic probabilistic risk assessment (PRA) framework developed to estimate the probability of rock bursts and excessive gas emissions by incorporating the inherent uncertainty involved in a typical longwall coal mining scenario. The vertical stress, total volume of gas emission, and incremental energy release rate (ERR) were determined from a coupled geomechanical and gas flow model to represent longwall top coal caving mining practiced at Coal Mine Velenje. The values were fed into the PRA framework and the probability of rock bursts, gas emissions and safe mining conditions were estimated. The probability of rock burst occurrence was calculated to be 0.138, excessive gas emission to be 0.353 and safe mining conditions to be 0.565. This paper offers a new approach to overcome the limitations of traditional approaches for rock burst and excessive gas emissions forecasting.
Fraga D, Korre A, Nie Z, et al., 2021, Multi-period cost optimisation of multi-mode carbon capture and storage chains
A generally applicable multi-period and multi-mode CCS value chain cost model is developed to account for all individual costs and evaluate different options for CCS value chain implementation. The approach followed involves the configuration of a mixed integer linear programming optimisation model, in which the objective function is to minimise the total cost of the CCS chain, considering constraints of mass balance, injection wells, intermediate storage sizing, flow and routing. The model functionality is illustrated through a case study based on a Norwegian CCS chain which is assessed for a planning horizon of 30 years, assigned in five-year long intervals. The model adopts the methodology developed previously by the authors for pipeline transport and geological storage of CO2 and introduces CO2 ship transport and intermediate storage at two design pressures (7 and 15 bar). It also includes a newly introduced CO2 conditioning step (compression and liquefaction) and a further injection mode, directly from ship. For the CCS chain analysed, it is found that higher CO2 supply provides economy of scale gains, thus lower levelized cost. Pressure design for ship transportation plays a significant role on the cost of the chain with the trade-off between low conditioning costs and higher transport cost. In the Norwegian CCS case analysed, individual pipelines from a single hub to each storage site are more economical than a single trunkline with multiple connections to injection sites. Geological storage costs are largely driven by the availability and the operational costs of the individual sites.
Cai W, Durucan S, Cao W, et al., 2021, Seismic Response to Fluid Injection in Faulted Geothermal Reservoirs: A Case Example from Iceland
Seismic response to fluid injection at the Húsmúli area within the Hellisheiði geothermal field in Iceland was investigated by correlating induced seismicity with fault structures and fluid injection rates. Induced seismicity recorded around five injection wells over a half-year fluid injection period was first analysed using the k-means clustering model, which identified two clusters associated with the injection wells studied. Based on the clustered seismic events, the geometry of the injected fluid flow was estimated using the spatial density of seismic events. A statistical approach, using locations of three seismic events to identify one fracture plane, was utilised to identify the dominant fault structures, which spatially correlate well with local fault structures dominating the fluid flow. In the temporal domain, the fluid injection rate and wellhead pressure for the five wells were separately analysed against the seismic response, which shows that the seismic frequency and the b value increased along with local fluctuations of injection rate and pressure in the controlling system. It can be concluded that the integrated analysis of induced seismicity, fault structures and fluid injection in spatial-temporal domains is crucial to understand the seismic response to fluid injection in faulted/fractured geothermal reservoirs.
Cao W, Durucan S, Cai W, et al., 2020, A physics-based probabilistic forecasting methodology for hazardous microseismicity associated with longwall coal mining, International Journal of Coal Geology, Vol: 232, Pages: 1-14, ISSN: 0166-5162
Mining-induced microseismicity is widely considered as a result of slippage of pre-existing critically stressed fractures caused by stress perturbations around an advancing face. An in-depth analysis of the recorded microseismicity associated with longwall top coal caving mining at Coal Mine Velenje in Slovenia has been previously carried out and reported by the authors. It has been concluded that while microseismic event rate is affected by mining intensity (longwall face daily advance rate) as well as local abundance of pre-existing fractures, spatial and magnitude characteristics of microseismicity are predominantly influenced by the latter. Based upon this improved understanding of fracture-slip seismic-generation mechanism, the current work aimed at establishing a data-driven yet physics-based probabilistic forecasting methodology for hazardous microseismicity using microseismic monitoring data with concurrent face advance records. Through performing statistical analyses and probability distribution fitting for temporal, magnitude and spatial characteristics of microseismicity within a time window, a short-term forecasting model is developed to estimate the probability of potentially hazardous microseismicity over the next time interval in the form of a joint probability. The real time forecasting of hazardous microseismicity during longwall coal mining is realised through regularly updating the statistical model using the most recent microseismic sequence datasets and face advance records. This forecasting methodology is featured by the physical basis which provides a good explicability of forecasting results, and the probabilistic perspective which accounts for the stochastic nature of mining-induced microseismicity. This model has been employed to make time-varying forecasts of hazardous microseismicity around two longwall panels over a one-year coal production period at Coal Mine Velenje, and satisfactory results at both panels were achieved. In addition, t
Cao W, Yildirim B, Durucan S, et al., 2020, Fracture behaviour and seismic response of naturally fractured coal subjected to true triaxial stresses and hydraulic fracturing, Fuel: the science and technology of fuel and energy, Vol: 288, Pages: 1-15, ISSN: 0016-2361
Hydraulic fracturing of coalbed methane wells has been widely practised as an effective method to increase drainage efficiency in low-permeability, low-pressure and low-saturated coal seams. To investigate hydraulic fracture performance and associated seismic response in coal, hydraulic fracturing experiments were carried out on two cubic coal blocks containing a host of natural fractures using a true triaxial rock testing machine equipped with loading, injection and acoustic systems. The acoustic system uses transducers with active sources to repetitively generate and receive ultrasonic P/S wave pulses for characterising mechanical properties of the coal blocks and revealing fracture growth. Silicon oil was injected into the middle of coal blocks to create hydraulic fractures under deviatoric stress conditions, and the stress and displacement, borehole pressure and volume, and seismic response were recorded over the injection process. X-ray computed tomography (CT) was conducted before and after the experiments to identify the location and geometry of hydraulic and natural fractures. Results have shown that the fracturing behaviour, the drawdown period of borehole pressure and the intrusion of fracturing fluid are dominated by the complexity and insulation offered by internal natural fracture networks of coal blocks. In addition, seismic spectrograms captured both fracture initiation and its subsequent interaction with natural fractures, which indicates that the induced fracture and fracturing fluid interfere with the propagation of seismic waves and influence ultrasonic seismic characteristics. Seismic velocity tomography of ultrasonic acoustic signals recorded also provided the spatial information of fractures, such as approximate locations of pre-existing fractures and injection-disturbed regions.
Stork AL, Chalari A, Durucan S, et al., 2020, Fibre-optic monitoring for high-temperature Carbon Capture, Utilization and Storage (CCUS) projects at geothermal energy sites, First Break, Vol: 38, Pages: 61-67, ISSN: 0263-5046
It is often assumed that geothermal energy provides a clean source of renewable energy without emissions of carbon dioxide (CO2) or other greenhouse gases. In fact, most geothermal energy plants emit CO2 and small amounts of other gases, typically up to 5% of by weight. Reinjection of produced CO2 back into the geothermal fields has been proposed by several researchers in the past. The EU funded CarbFix and CarbFix2 projects have successfully demonstrated that CO2 reinjection into basaltic rocks can provide a safe and efficient geological storage method. The ACT Programme funded SUCCEED project is focused on understanding the effects of and developing technologies to enable reinjection of produced CO2 at geothermal plants in different geological settings. Monitoring the process is vital to understand the effects, possibilities and limitations of injection but the availability of suitable sensors is limited in high-temperature and harsh environments. This problem can be overcome with the use of distributed fibre-optic sensors which are able to withstand such harsh environments and record temperature, seismic and strain signals. This article describes actual and planned deployments of Distributed Acoustic Sensing (DAS) technology at the Hellisheidi and Kizildere geothermal fields in Iceland and Turkey, respectively, and outlines the practical considerations for such deployments.
Cao W, Durucan S, Cai W, et al., 2020, The role of mining intensity and pre-existing fracture attributes on spatial, temporal and magnitude characteristics of microseismicity in longwall coal mining, Rock Mechanics and Rock Engineering, Vol: 53, Pages: 4139-4162, ISSN: 0723-2632
Knowledge regarding microseismic characteristics associated with longwall coal mining is crucial in evaluating the potential for underground mining hazards. Although microseismicity is induced by mining activities, it still remains uncertain as to what extent mining activities influence the spatial, temporal, and magnitude characteristics of microseismicity. To establish a thorough understanding of the relationship between microseismic characteristics and mining activities, a 27-month long microseismic monitoring campaign was conducted around a highly stressed coal zone and eight producing longwall panels at Coal Mine Velenje in Slovenia. Each microseismic event was classified to be associated with the producing longwall panel that triggered it, and the microseismic response to multi-panel longwall top coal caving face advance was analysed. Monitoring data have shown that locations of microseismic events coincided with stress concentrated regions. It was established that both seismic count and energy-intensive regions associated with coal mining in different panels are spatially connected, but they do not fully overlap with mined-out or stress concentrated areas. In addition, microseismic event counts frequency was found to be well correlated with mining intensity, while seismic energy magnitude and spatial distribution are poorly correlated with the same. Therefore, microseismic characteristics could not be explained solely by the mining-induced stress transfer and mining intensity, but are believed to be dominated by pre-existing natural fractures throughout the coal seam. Analyses of these observations helped the development of a conceptual seismic-generation model, which provides new insights into the causes of microseismicity in coal mining.
Cao W, Shi J-Q, Durucan S, et al., 2020, Gas-driven rapid fracture propagation under unloading conditions in coal and gas outbursts, International Journal of Rock Mechanics and Mining Sciences, Vol: 130, ISSN: 1365-1609
Coal and gas outbursts have long posed a serious risk to safe and efficient production in coal mines. It is recognised that coal and gas outbursts are triggered by excavation unloading followed by gas-driven rapid propagation of a system of pre-existing or mining-induced fractures. Gas-filled fractures parallel to a working face are likely to experience opening first, then expansion and rapid propagation stages under unloading conditions. The fracture opening is driven by the effective stress inside the fracture, while the fracture expansion and rapid propagation is propelled by the pressure build-up of desorbed gas in the vicinity of the fracture. Based upon this understanding, this research aimed to identify the key factors affecting outburst initiation and its temporal evolution during roadway developments. Specifically, the response of pre-set fractures in a thin coal seam sandwiched between rock layers to roadway development is simulated using a geomechanical model coupled with fracture mechanics for fracture opening and propagation. In addition, kinetic gas desorption and its migration into open fractures is considered. During simulations outburst is deemed to occur when the fracture length exceeds the dimension of a host element. The findings of this research suggest that the simulated coal and gas outburst caused by roadway development may be considered as a dynamic gas desorption-driven fracture propagation process. The occurrence of coal and gas outbursts is found to be influenced mainly by the coal properties, fracture attributes, and initial gas pressure and the in situ stress conditions. Furthermore, the model predictions in terms of dome-shaped erupted-zone and layer-by-layer coal breakage are consistent with the field reports. In addition, the model results suggest that delayed occurrence of coal and gas outbursts, especially after sudden exposure of a coal seam or after blasting disturbance, reported in the literature may be related to the gas desorp
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