Subsurface Fluid Research Articles

Feedbacks between the formation of secondary minerals and the infiltration of fluids into the regolith of granitic rocks in different climatic zones (Chilean Coastal Cordillera)

Abstract. Subsurface fluid pathways and the climate-dependent infiltration of fluids into the subsurface jointly control the intensity and depth of mineral weathering reactions. The products of these weathering reactions (secondary minerals), such as Fe(III) oxyhydroxides and clay minerals, in turn exert a control on the subsurface fluid flow and hence on the development of weathering profiles. We explored the dependence of mineral transformations on climate during the weathering of granitic rocks in two 6 m deep weathering profiles in Mediterranean and humid climate zones along the Chilean Coastal Cordillera. We used geochemical and mineralogical methods such as (micro-) X-ray fluorescence (μ-XRF and XRF), oxalate and dithionite extractions, X-ray diffraction (XRD), and electron microprobe (EMP) mapping to elucidate the transformations involved during weathering. In the profile of the Mediterranean climate zone, we found a low weathering intensity affecting the profile down to 6 m depth. In the profile of the humid climate zone, we found a high weathering intensity. Based on our results, we propose mechanisms that can intensify the progression of weathering to depth. The most important is weathering-induced fracturing (WIF) by Fe(II) oxidation in biotite and precipitation of Fe(III) oxyhydroxides and by the swelling of interstratified smectitic clay minerals that promotes the formation of fluid pathways. We also propose mechanisms that mitigate the development of a deep weathering zone, like the precipitation of secondary minerals (e.g., clay minerals) and amorphous phases that can impede the subsurface fluid flow. We conclude that the depth and intensity of primary mineral weathering in the profile of the Mediterranean climate zone is significantly controlled by WIF. It generates a surface–subsurface connectivity that allows fluid infiltration to great depth and hence promotes a deep weathering zone. Moreover, the water supply to the subsurface is limited in the Mediterranean climate, and thus, most of the weathering profile is generally characterized by a low weathering intensity. The depth and intensity of weathering processes in the profile of the humid climate zone, on the other hand, are controlled by an intense formation of secondary minerals in the upper section of the weathering profile. This intense formation arises from pronounced dissolution of primary minerals due to the high water infiltration (high precipitation rate) into the subsurface. The secondary minerals, in turn, impede the infiltration of fluids to great depth and thus mitigate the intensity of primary mineral weathering at depth. These two settings illustrate that the depth and intensity of primary mineral weathering in the upper regolith are controlled by positive and negative feedbacks between the formation of secondary minerals and the infiltration of fluids.

Basin and depositional control on the initiation and development of fluid-escape pipes in the Canterbury Basin, New Zealand

The influence of basin and depositional context on the formation and growth of fluid escape pipes in sedimentary basins is not fully understood. While seismic reflection data is commonly used to study these structures, direct observation and sampling are scarce. In this study, we investigate the evolution of hundreds of meter-tall fluid escape pipes (100–800 m) in the Canterbury Basin, New Zealand, using open source seismic and borehole data. Our analysis includes seismic interpretation of 19 unconformities within the Neogene-Quaternary successions, characterization of pipes and fluid-related anomalies in the subsurface, modeling of the shale volume (Vsh), lithology characterization, 3D static modeling, and seismic inversion. We identify thirty-one vertical to sub-vertical pipes with three parts consisting of top, main conduit/stem, and root zone on seismic profiles. Our results suggest that lithology plays a crucial role in the initiation and modulation of pipe structures in the Canterbury Basin, with pipes mostly formed post Tarantian (0.113 Ma) and truncated at the U19 unconformity. Furthermore, vertical fluid migration from overpressured units, such as the tops of contourite mounds, was observed in the pipes. Our findings provide new insights into the subsurface fluid flow and sedimentary architecture of the Canterbury Basin and have wider implications for other passive continental margins with similar characteristics.

Deep geothermal resources in China: Potential, distribution, exploitation, and utilization

Deep geothermal resources mainly refer to the thermal energy stored in subsurface rocks and fluids therein at a depth of 3–10 km, which is a kind of renewable and sustainable clean energy unaffected by weather and seasonal changes. Large scale exploitation of the deep geothermal resources is of great significance to ensuring national energy security and achieving the “Carbon Peak and Carbon Neutrality”. Based on the latest terrestrial heat flow data, this paper estimated the potential of deep geothermal resources in the terrestrial areas of China, and the results show that the total amount of geothermal resources within 3–10 km under the Earth's surface in the terrestrial areas of China is 24.6 × 1015 GJ. In line with climate zones categorized, the geothermal resource proportion is 43.81% for severe cold regions, 29.19% for cold regions, 6.92% for mild regions, 13.82% for hot summer and cold winter regions, and 6.26% for hot summer and warm winter regions. Statistics according to the burial depth range reveal that the resources within depth ranges of 3–5 km, 5–7 km and 7–10 km under the Earth's surface are 4.3119 × 1015 GJ, 6.37674 × 1015 GJ and 13.89594 × 1015 GJ respectively, showing an increasing trend of geothermal potential with increasing burial depth. The deep geothermal resources are mainly of medium-to-high temperature reserves, and the energy supply strategy can be optimized by combining the climate conditions and population distribution, as well as considering power generation. In regions of cold or severe cold climate, the geothermal resources may be applied to geothermal power generation and district heating in combination; in regions of hot summer and cold winter or mild climates, the resources can be used for geothermal power generation combined with cooling and heating; in regions of hot summer and warm winter climates, the resources may be applied to geothermal power generation combined with cooling and industrial and agricultural utilization. Exploitation of deep geothermal resources also can be combined with carbon dioxide sequestration, multi-mineral resources extraction and energy storage to realize comprehensive exploitation and utilization of various energy resources. It is suggested that theoretical technology research should be combined with pilot tests and field demonstrations, and large-scale economic exploitation of deep geothermal resources should be arranged in a coordinated manner, following the principles of overall planning and step-by-step implementation.

Diagnosing Hydro-Mechanical Effects in Subsurface Fluid Flow Through Fractures

Hydro-mechanically induced transient changes in fracture volume elude an analysis of pressure and flow rate transients by conventional diffusion-based models. We used a previously developed fully coupled, inherently non-linear numerical simulation model to demonstrate that harmonic hydraulic excitation of fractures leads to systematic overtones in the response spectrum that can thus be used as a diagnostic criterion for hydro-mechanical interaction. The examination of response spectra, obtained from harmonic testing at four different field sites, for the occurrence of overtones confirmed their potential for the hydro-mechanical characterization of tested reservoirs. A non-dimensional analysis identified relative aperture change as the critical system parameter.

The long‐term legacy of subvolcanic intrusions on fluid migration in sedimentary basins: The Cerro Alquitrán case study, northern Neuquén Basin, Argentina

AbstractCooling subvolcanic igneous intrusions are known to have a substantial impact on fluid flow in the shallow Earth's crust, for example activation of geothermal systems, circulation of mineralized fluids in ore deposits, fast maturation of organic matter in sedimentary rocks and the potential release of large volumes of greenhouse gases, which have triggered mass extinctions during the Earth's history. However, the long‐term post‐cooling legacy of subvolcanic intrusions on fluid flow received much less attention. Here we describe a demonstrative geological example in the Andean foothills, Argentina, showing that igneous intrusions have long‐term effects on fluid flow after their emplacement and cooling. The case study is a ca. 11‐million‐year‐old, eroded subvolcanic conduit, at the rims of which large volumes of bitumen are naturally seeping out on the Earth's surface. This contribution highlights that intense syn‐emplacement fracturing of the magma has created high‐permeability pathways that affect the regional fluid circulations, even millions of years after cooling. Our observations reveal how extinct subvolcanic intrusions have long‐term consequences on subsurface fluid circulations, which need to be accounted for in the exploration of geothermal energy, drinkable groundwater, hydrocarbons and CO2 sequestration in volcanic basins and regions hosting ancient shallow magma intrusions.

Prediction of the pore structure by machine learning techniques in the carbonate reservoirs in Iraq H oilfield

Pore structure impacts the capability of seepage pattern of subsurface fluid mineral and mineral exploitation efficiency. Because of the strong heterogeneity in carbonate reservoirs, the pore structure is nonlinearly varying and complex in reservoirs. It is necessary to establish a method for pore structure type (PST) prediction. Machine learning provides an efficient prediction method by finding the relationship between core test data and well‐logging data. In this paper, a reservoir identification method based on Gradient Boost Decision Trees (GBDT) is proposed for the pore structure characteristics of carbonate reservoirs integrating core test and logging data. Core testing data are utilized to form the training set for PST prediction. Then, we adopt the mutual information method to optimize the logging data as the input of the machine learning model. GBDT hyperparameters are optimized using the learning curve to make it have optimal performance. Finally, we compare the predicted result of GBDT with K‐Nearest Neighbor and support vector machines, demonstrating the superior performance of GBDT. The results indicate that the four PS are different in some properties, such as permeability, pore radius and shape of pore and throat. 20%, 10 and 0.15 are recommended as the optimal parameter values for Maximum feature, iteration number and Sub‐sample number to improve GBDT accuracy in carbonate reservoir PST prediction. Evaluation indicators of machine learning, including the Kappa coefficient, Heming distance, Jaccard similarity coefficient and Confusion matrices, indicate that GBDT has better performance. This research proves the high accuracy and applicability of machine learning technology in the carbonate reservoir and also provides a meaningful reference for the identification of the carbonate reservoirs in the central area of Iraq.

Subsurface fluid flow: A case study from the Indo-Gangetic peripheral foreland basin

Subsurface fluid flow involves migration of fluids from source to surface through a wide range of geologic structures, and thus plays a crucial role in unlocking potential hydrocarbon plays. Such studies have been mainly carried out in the marine environment by means of high-resolution seismic data in unravelling the fluid flow system and remains poorly documented for on-land data. This research attempts to explore the fluid flow activity in onshore petroliferous Indo-Gangetic peripheral foreland basin of India by using high-quality 3D seismic reflection data. Several seismic attributes have been utilised for efficiently describing the patterns of subsurface fluid flow structures. The attribute-responses of fluid flow are fused into a single attribute, called the Fluid Cube meta-attribute, by designing a workflow based on artificial neural network, which has enabled to elucidate the subsurface fluid migration routes. The subsurface fluid flow features are imaged as relatively vertical mounded structure with conical vent-like morphology. Internally, these features are associated with moderately distorted reflections. The reflections at the top are vertically stacked with medium to high amplitudes. The Fluid Cube meta-attribute demonstrates that subsurface fluid vertically migrates from the Neoproterozoic strata through minute fracture networks and weaker strata of the overlying Tertiary succession. The analysis of surficial geochemical anomalies corroborates quite reasonably with these observations. Thus, the Ganga peripheral foreland basin could be considered a promising area with potential leads that can be unlocked for hydrocarbon exploration. The analysis presented through this research could be efficiently carried out over other onshore basins worldwide.

Seismic Tomography of Nabro Caldera, Eritrea: Insights Into the Magmatic and Hydrothermal Systems of a Recently Erupted Volcano

AbstractUnderstanding the crustal structure and the storage and movement of fluids beneath a volcano is necessary for characterizing volcanic hazard, geothermal prospects and potential mineral resources. This study uses local earthquake traveltime tomography to image the seismic velocity structure beneath Nabro, an off‐rift volcano located within the central part of the Danakil microplate near the Ethiopia‐Eritrea border. Nabro underwent its first historically documented eruption in June 2011, thereby providing an opportunity to analyze its post‐eruptive state by mapping subsurface fluid distributions. We use a catalog of earthquakes detected on a temporary seismic array using machine learning methods to simultaneously relocate the seismicity and invert for the three‐dimensional P‐ and S‐wave velocity structures (VP, VS) and the ratio between them (VP/VS). Overall, our model shows higher than average P‐ and S‐wave velocities, suggesting the presence of high‐strength, solidified intrusive magmatic rocks in the crust. We identify an aseismic region of low VP, low VS, and high VP/VS ratio at depths of 6–10 km b.s.l., interpreted as the primary melt storage region that fed the 2011 eruption. Above this is a zone of high VS, low VP, and low VP/VS ratio, representing an intrusive complex of fractured rocks partially saturated with over‐pressurized gases. Our observations identify the persistence of magma in the subsurface following the eruption, and track the degassing of this melt through the crust to the surface. The presence of volatiles and high temperatures within the shallow crust indicate that Nabro is a viable candidate for geothermal exploration.

Facies Analysis and Reservoir Characterization Using Petrophysical Methods in the Interest Zone in the 'FAN' Field, Kutai Basin, East Kalimantan

One of the most prospective oil and gas producing basins in Indonesia is Kutai Basin which is located in East Borneo. In Kutai Basin there is a natural resource potential in the form of gas which is quite abundant. This is due to the richness of the source rock aspect in the Kutai Basin which is dominated by coal. Besides coal, in the Kutai Basin there is also claystone with rich organic matter (organic shale) that has an important role as a prospective source rock. Therefore, a final project research was conducted in the Kutai Basin, East Kalimantan. The purpose of the research is to identifying the type and content of subsurface lithology and fluids, calculating petrophysical parameters, and making the facies distribution map at the research site in potential and prospective zone to produce hydrocarbons which is called the zones of interest. The research is focused on reservoir rocks at the research site because the hydrocarbons accumulated in the reservoir rocks itself. By focusing research on reservoir rocks, it will be able to describe the modelling and calculation results of petrophysical parameters using several analytical methods such as qualitative and quantitative analysis methods, electrofacies and parasequence analysis methods, stratigraphic correlation analysis methods, facies analysis methods and depositional environments, and facies map analysis methods. In addition, by focusing research on reservoir rocks, it can develop and increase the level of optimization of exploration and exploitation of drilling wells. Based on the analysis data, the research location is included in the transitional depositional environment, delta in particular with distributary channels and mouth bars facies and also it can be identified some lithology such as sandstone, claystone, coal, limestone, and organic claystone and there are fluids with gas and water types.

Microbial‑induced carbonate precipitation (MICP) technology: a review on the fundamentals and engineering applications.

The microbial‑induced carbonate precipitation (MICP), as an emerging biomineralization technology mediated by specific bacteria, has been a popular research focus for scientists and engineers through the previous two decades as an interdisciplinary approach. It provides cutting-edge solutions for various engineering problems emerging in the context of frequent and intense human activities. This paper is aimed at reviewing the fundaments and engineering applications of the MICP technology through existing studies, covering realistic need in geotechnical engineering, construction materials, hydraulic engineering, geological engineering, and environmental engineering. It adds a new perspective on the feasibility and difficulty for field practice. Analysis and discussion within different parts are generally carried out based on specific considerations in each field. MICP may bring comprehensive improvement of static and dynamic characteristics of geomaterials, thus enhancing their bearing capacity and resisting liquefication. It helps produce eco-friendly and durable building materials. MICP is a promising and cost-efficient technology in preserving water resources and subsurface fluid leakage. Piping, internal erosion and surface erosion could also be addressed by this technology. MICP has been proved suitable for stabilizing soils and shows promise in dealing with problematic soils like bentonite and expansive soils. It is also envisaged that this technology may be used to mitigate against impacts of geological hazards such as liquefaction associated with earthquakes. Moreover, global environment issues including fugitive dust, contaminated soil and climate change problems are assumed to be palliated or even removed via the positive effects of this technology. Bioaugmentation, biostimulation, and enzymatic approach are three feasible paths for MICP. Decision makers should choose a compatible, efficient and economical way among them and develop an on-site solution based on engineering conditions. To further decrease the cost and energy consumption of the MICP technology, it is reasonable to make full use of industrial by-products or wastes and non-sterilized media. The prospective direction of this technology is to make construction more intelligent without human intervention, such as autogenous healing. To reach this destination, MICP could be coupled with other techniques like encapsulation and ductile fibers. MICP is undoubtfully a mainstream engineering technology for the future, while ecological balance, environmental impact and industrial applicability should still be cautiously treated in its real practice.

High-resolution geologic modeling and CO2 flow simulation of a realistic clastic deltaic 3D model derived from a laboratory flume tank experiment

Carbon sequestration in geologic reservoirs is a proven method for reducing carbon dioxide (CO2) emissions from industrial point sources. Subsurface deltaic deposits are attractive candidates for CO2 storage projects due to their common occurrence as host to significant hydrocarbon and hydrologic resources, and are highly prospective for subsurface storage of large volumes of CO2. This research informs subsurface passive margin deltaic reservoir characterization and fluid flow performance for geologic carbon storage. The objective of this research is to create a high-resolution geocellular digital model that honors both the large-scale stratigraphic architecture and local vertical and lateral facies heterogeneity represented in deltaic environments of deposition. We present a novel approach to reservoir model generation by utilizing a previously generated meter-scale physical flume sand tank analog experiment to represent realistic deltaic stratigraphic architectures and facies distributions. The result is an extremely high-resolution geologic model that can be used for a variety of applications, including single and multiphase fluid flow simulation. An application is presented using the example model to simulate buoyant fluid (CO2) migration and resulting saturation distribution. Results emphasize the importance of recognizing sequence stratigraphic architectures in deltaic depositional settings for predicting CO2 migration and retention (storage capacity).

Research on the characteristics of fluid escape near the seabed in Bohai Bay

AbstractSeabed fluid flow happens due to the complex interactionamong the accumulation, migration and escape of hydrocarbon and/or fluids. As shallow gas accumulations are anomalously developed in Bohai Bay, studying fluid flow features provides insights for understanding environment fluid in the Bohai Sea. Therefore, in our study, we aimed to make practical and effective use of single‐channel seismic reflection data to delineate and interpret subsurface fluid flow features by analysing the variance, sweetness, instantaneous frequency, and root mean square amplitude of seismic attributes. Four types of submarine fluid escape features, including pockmarks, mounds, fluid vents, dimmed or chaotic seafloor reflections, and two shallow fluid migration pathways (gas chimneys and faults) are distinguished on the single‐channel seismic reflection profiles. Gas chimneys are divided into strict columnar and irregular columnar chimneys, fluid migration along faults is critically dependent on the activity of faults. Enhanced reflections and bright spots may be good indicators of lateral fluid migration and accumulation of fluids in porous formations. We propose a shallow fluid flow model in the study area that illustrates the dynamic fluid migration process in terms of possible fluid sources, shallow migration pathways, and submarine fluid escape features.

Hydraulic Flow Unit Characterization in Sandstone Reservoirs, Niger Delta, Nigeria

The key factor in a successful oil field development plan is understanding the reservoir. The effectiveness of oil extraction is determined by variations in the reservoir, specifically the distribution of porosity and permeability. To create an accurate model of the reservoir, the vertical distribution of its qualities must be determined and separated into flow units, each with its own unique characteristics that affect fluid flow. By identifying these flow units, the preferred flow zones can be discovered. The aim of this study is to identify the number and distribution of hydraulic units, their important characteristics, and the flow performance in two specific wells in the Niger Delta, Nigeria, using the Modified Lorenz Plot (MLP) method. In the hydrocarbon-rich intervals of the wells, a total of 18 flow units were found, with 12 in Well 1 and 6 in Well 2. The number of flow units indicates the level of heterogeneity in the reservoir. This study showed that the Modified Lorenz Plot (MLP) is an efficient and low-cost method for defining petrophysical flow units. This research study aims to transform the conventional approach to reservoir characterization by conducting a thorough analysis of hydraulic flow units in sandstone reservoirs located in the complex geological setting of the Niger Delta, Nigeria. By examining the hydraulic properties of reservoir rocks, the study seeks to gain a better understanding of subsurface fluid flow behavior and the potential for hydrocarbon accumulation.

Hydrogeochronology: Resetting the timestamp for subsurface groundwaters

Waters recharging the subsurface contain a variety of naturally occurring geochemical components, the concentrations of which are principally a function of the atmosphere and surface environment. Over time, water-rock interactions and radiogenic processes in the subsurface control a complex geochemical evolution, impacting the initial surface-related signatures associated with recharged groundwaters. The rates of these geochemical changes are considerable in many important geologic settings, requiring characterisation and quantification of the dominant subsurface processes and evolving fluid characteristics. Constraining this is critically important for the discipline of fluid ‘age’ dating - hydrogeochronology. Age dating, or quantification of fluid residence times is typically evaluated through two categories of tracers; those that are incorporated at the point of recharge and decrease over time through radioactive decay (e.g. 3H, 14C, 81Kr) and those that increase over time (e.g. radiogenic noble gases) due to subsurface production and accumulation in fluids. As these fluids, elements, and tracers have different provenance, the calculated ‘age’ of any fluid represents the average, or mean residence time, of all components. Likewise, additional mechanisms of subsurface tracer production or loss can have significant bearing on any element-specific calculated residence times. The inevitable temporal discrepancies between tracers can be generated or accentuated in the subsurface which can prove difficult to reconcile in hydrogeochronologic modelling approaches.By incorporating geochemical data into existing neutron flux-based models, this study evaluates specifically how subsurface production of ‘diminishing’ tracers in the fluid phase, via neutron capture of parent elements, can produce complexity in the dating of host fluids. The in situ rates and production routes of 3H, 14C, 36Cl 39Ar, and 81Kr in different subsurface fluids are modelled for a variety of host rock lithologies to evaluate the effect such baseline crustal production in different geochemical settings may have on calculated residence times within naturally occurring systems. In particular this model provides a quantitative evaluation for how site-specific subsurface radiogeochemistry may result in enhanced baselines of 36Cl, 39Ar, (and potentially 14C) relative to canonical assumptions, and substantially impact residence time estimates calculated from these tracers for a range of subsurface environments. Additionally, we evaluate the role geologic environments can have on low levels of baseline production and calculated residence times for 3H and 81Kr. This study quantitatively evaluates the dependence of geologic environments on these subsurface processes and the impacts this may have on hydrogeochronologic studies. The ability to quantify production of tracers in the subsurface and recalibrate associated residence times in a wide range of geologic settings is of direct relevance to areas such as groundwater resource management, Carbon Capture and Storage, hydrogen storage and nuclear waste management strategies, the hydrocarbon and energy industry, and investigations of the timing and evolution of subsurface microbial life.

The temporal evolution of induced seismicity sequences generated by low-pressure, long-term fluid injection

Traffic light schemes (TLSs) are commonly used to mitigate induced seismicity caused by subsurface fluid injection. Verdon and Bommer (J Seismol 25:301–326, 2021a) investigated the validity of the implicit assumptions that underpin the successful functioning of TLSs. In particular, they examined the extent to which magnitude jumps (sudden increases in event magnitudes from any preceding seismicity) and trailing events (continued increases in seismicity levels after the end of injection) took place in hydraulic fracturing (HF) induced seismicity sequences. Other technologies such as carbon capture and storage (CCS), wastewater disposal (WWD) and natural gas storage (NGS) involve the gradual but long-term injection of large fluid volumes at low pressure. Hence, we might expect to see a different spatial and temporal evolution of magnitudes for seismicity induced by low-pressure, long-term (LPLT) injections compared to HF. In this study, we compile cases of LPLT injection-induced seismicity in order to examine their temporal evolution. We examine the occurrence of magnitude jumps, trailing events and onset times for seismicity after the initiation of injection. We find that few LPLT injections have produced trailing events, and that magnitude jumps are typically below 1.5 magnitude units. The timescale of event occurrence (relative to the onset of injection) is highly variable, likely reflecting site-specific conditions. For long-term injection, we observe a trend for the largest events to occur within the earlier part of each sequence, with magnitudes then stabilising, or even reducing, as injection continues. Finally, we evaluate the performance of the next record breaking event (NRBE) model as a method for forecasting induced event magnitudes, finding that this method performs reasonably well in most cases, but that in some cases the largest event significantly exceeds this model.

Deep-learning-generalized data-space inversion and uncertainty quantification framework for accelerating geological CO2 plume migration monitoring

Efficient coupling of high-fidelity simulation models and history-monitored data to predict future migration behavior of CO2 plume is crucial for leakage risk management and assessment during the process of geological CO2 sequestration (GCS). However, the conventional model-based inverse modeling and forecasting procedure generally presents intensive computation-cost due to numerous high-fidelity model simulations and thus hinders its practical applications. We present a novel data-driven direct forecast and uncertainty quantification framework that hybridizes deep convolutional autoencoder and deep residual neural network to represent spatial distribution of subsurface fluid by latent space features and achieve predictions of future fluid dynamic behavior given history-monitored data, respectively. The linear-type data-space inversion approaches based on Bayesian theorem are generalized to non-linear model applications through deep-learning, i.e., referred to as DSI-DL. Compared to the conventional learning-based direct forecast approach (LDFA) that numerous training samples are required to guarantee predictability of the supervised machine-learning models, our proposed DSI-DL employs a data-augmentation strategy to automatically generate sufficient training samples from a relatively small ensemble of high-fidelity model simulations. The performance DSI-DL method is demonstrated on a synthetic 2D models and a practical 3D model for deep saline aquifer GCS applications. The deep-learning surrogate directly predicts the latent features of future spatial CO2 plumes given history-monitored CO2 plumes. Comparisons between LDFA and DSI-DL confirm that the proposed DSI-DL method obtains more robust and better results in predicting future CO2 plume migration, especially when solely a few number of large-scale high-fidelity model simulations is available at hand and large observation errors exist. The proposed deep-learning-based direct forecast approach contributes to an efficient computation framework to manage uncertainty of the long term process of GCS.

Laboratory characterization of induced seismicity

Subsurface fluid injection causes an increase in seismic activity near injection sites. To ensure public safety and improve public acceptance of subsurface fluid injection, it is important to understand the underlying mechanisms giving rise to these behaviors and characterize the factors affecting induced seismicity. The work reported here aims to better understand induced seismicity in faulted/fractured subsurface rock formations. A series of experiments are conducted in core flooding and triaxial configurations on test samples with a preexisting fracture/fault to study frictional slipping at small and large scales respectively. The geometries of both surfaces on either side of the fault are characterized using x-ray CT. The extent of fault gouging in a specimen after slipping is quantified by defining a cumulative gouging parameter using x-ray CT measurements before and after the experiment. Acoustic emission (AE) events emanating from the specimens in both configurations are monitored. The results demonstrate that (1) the orientation of fault relative to the major principal stress direction affects the slip characteristics, (2) frictional slipping at a fault can occur at different length scales, (3) with an increase in the scale of slipping, the spectral ratio of low-frequency bins increases while high-frequency bins reduces, and (4) locations on the fault surface that exhibit a sudden change in surface normal are most susceptible to gouging/damage during frictional slipping.

Seismic characterization of a fluid escape structure in the North Sea: the Scanner Pockmark complex area

SUMMARYSubsurface fluid escape structures are geological features which are commonly observed in sedimentary basins worldwide. Their identification and description have implications for various subsurface fluid flow applications, such as assuring integrity of overburden rocks to geological CO2 storage sites. In this study, we applied 3-D first-arrival traveltime tomography to a densely sampled wide-azimuth and wide-angle ocean bottom seismometer (OBS) data set collected over the Scanner Pockmark complex, a site of active gas venting in the North Sea. Seismic reflection data show a chimney structure underlying the Scanner Pockmark. The objective of this study was to characterize this chimney as a representative fluid escape structure in the North Sea. An area of 6$\times $6 km2 down to a depth of 2 km below sea level was investigated using a regularized tomography algorithm. In total, 182 069 manually picked traveltimes from 24 OBS were used. Our final velocity model contains compressional wave velocity perturbations ranging from −125 to +110 ms−1 relative to its average 1-D model and compares favourably with a coincident seismic reflection data set. The tomographic velocity model reveals that the chimney as observed in seismic reflection data is part of a larger complex fluid escape structure, and discriminates the genuine chimney from seismic artefacts. We find that part of the seeping gas migrates from a deep source, accumulates beneath the Crenulate Reflector unconformity at ∼250 m below seafloor (mbsf) before reaching the porous sediments of the Ling Bank and Coal Pit formation at <100 mbsf. In addition, the model shows that the venting gas at Scanner Pockmark is also being fed laterally through a narrow NW–SE shallow channel. Quantitative velocity analysis suggests a patchy gas saturation within the gas-charged sediments of the Ling Bank and the Coal Pit formations. Confined to the well-resolved regions, we estimate a base case average gas saturation of ∼9 per cent and in-situ gas volume of ∼1.64 $\times {10^6}\ {{\rm{m}}^3}$ across the Ling Bank and Coal Pit Fm. that can sustain the observed methane flux rate at the Scanner Pockmark for about 10 to 17 yr.

Analysis of a travertine system controlled by the transpressional activity of the Alhama de Murcia fault: The Carraclaca site, eastern Betic Cordillera, Spain

Continental carbonates, such as travertines and tufas, formed from CO2-rich groundwater degassing as it emerges at the Earth’s surface, are often associated with major crustal-scale faults. The Carraclaca site, in the Lorca-Totana section of the Alhama de Murcia Fault, Spain, presents a complex geomorphological landscape controlled by active tectonics. The geology here records the interaction between Quaternary alluvial fans, travertines, and a pop-up structure developed in a transpressional section of the fault. The Alhama de Murcia Fault is an 80 km long left-lateral strike-slip fault that is one of the main seismogenic structures in the Iberian Peninsula. In this work, we examined the relation between travertine precipitation in the Carraclaca site and the tectonic activity of this fault zone through morphological and geochemical studies. The δ13C and δ18O isotopic signals indicate that the carbonate deposits are hydrothermal. In addition, the 87Sr/86Sr ratios in the samples suggest subsurface fluid interaction with the Miocene sediments and the Alpujárride basement, located below the alluvial deposits. Tectonic activity in the Alhama de Murcia Fault might generate the opening of deep water circulation in the crust every time a seismic event occurs, giving rise to hydrothermally derived carbonates precipitation. Deep waters rise and reach the surface interacting with meteoric waters, resulting in travertine formation. Therefore, the Carraclaca carbonate deposits study can inform us about the seismogenic cycle of the fault in the Lorca-Totana section.

Tomographic evidences for hydraulic fracturing induced seismicity in the Changning shale gas field, southern Sichuan Basin, China

Seismic activity near the shale gas fields in the Sichuan Basin of China has risen significantly in the past few years. Due to the lack of industrial stimulation data to the public, it is difficult to directly link the surge in seismicity near the shale gas fields with hydraulic fracturing stimulation. Meanwhile, it is also challenging to better understand the mechanisms inducing/triggering seismicity in these areas. In the absence of stimulation data, understanding subsurface fluid distribution can go a long way toward addressing these problems. Therefore, in this study, we have applied the double-difference seismic tomography method and the focal mechanism tomography method to the monitoring data of a local seismic array in the Changning shale gas field to build high-resolution three-dimensional models of seismic velocity structure and pore pressure field, then used these models to characterize fluid distributions beneath the shale gas field. From the inverted models, the correlation of overpressure regions (>12 MPa) with moderate-to-high Vp/Vs (1.75-1.90) is clearly observed. With the aid of the exploration data (including well log, active and passive seismic data), the overpressure regions are interpreted to be created by pore pressure or fluid diffusion along pre-existing faults from the shale gas reservoirs, while variations of the Vp/Vs model partly indicate water- and gas-bearing rocks. By jointly analyzing the geophysical model features and earthquake spatio-temporal characteristics, we infer that the earthquakes are primarily caused by increased pore pressures on the seismogenic faults due to the presence of water and/or gas. Besides, the poroelastic stress perturbation and aseismic slip produced by hydraulic fracturing also contribute to the generation of certain earthquake swarms.