Subsurface Fluid Research Articles

Mg and Sr isotopic evidence for basin wide alteration of early diagenetic dolomite in the Williston Basin by ascending crustal fluids

Dolomite is a predominantly diagenetic mineral that forms by replacing marine calcite and aragonite during burial. Dolomitization requires subsurface fluid flow to deliver Mg and remove product Ca, and only forms abiotically in laboratory experiments at temperatures above ~100°C, equivalent to burial depths of 3 km or greater. Limited sources of Mg at these depths would appear to restrict most dolomite formation to shallow burial depths using seawater as a source of reactive Mg, despite the kinetic limitation imposed by lower temperatures. The Williston Basin is replete with evaporites and dolomites that appear to have formed using Mg from evaporatively concentrated seawater at near surface temperatures. However, two dolomite bodies examined in this study record basin wide gradients in 87Sr/86Sr ratios and δ26Mg values that are inconsistent with dolomitization by seawater. We evaluate the processes that may have created these gradients. We propose a model whereby seepage reflux created early diagenetic protodolomite that was later altered during deep burial by 87Sr and Mg-enriched fluids ascending from the deep structural center of the basin with the degree of alteration decreasing towards the edges. The ascending fluid flow event is tentatively linked to anomalous heating in the Late Devonian / Early Carboniferous. Vertical faults are hypothesized to have behaved as conduits for channeling 87Sr and Mg-enriched crustal fluids upwards into the bottom of the Williston Basin, pressurizing the basal aquifer system and commencing up-dip directed fluid flow through confined aquifers. Clumped isotope and fluid inclusion thermometry indicate that the ascending fluids were hydrothermal with an estimated δ26Mg value of –0.65 ±0.20‰, which is much lower than estimates of early Paleozoic seawater δ26Mg values of +0.09 to +0.23‰. Two sources of crustal Mg are consistent with the low inferred δ26Mg values of the ascending fluid: (1) chlorite, which preferentially releases isotopically light Mg, with literature values in the range of –0.78‰ to –1.8‰, and (2) magnesite, which forms during carbonation of serpentinite with literature values in the range –0.7‰ to –1.1‰. Ultramafic rocks belonging to the Thompson Nickel Belt pass beneath the center of the Williston Basin in North Dakota, indicating that dissolution of both chlorite and magnesite may have contributed reactive Mg to the Williston Basin during the ascending fluid flow event. As dolomite is a mineral that is easily altered during burial, our findings warrant caution in choosing dolomites for reconstructing the 26Mg history of seawater.

Analysis of Seismic Damage Zones: A Case Study of the Ordovician Formation in the Shunbei 5 Fault Zone, Tarim Basin, China

Fault damage zone has an important influence on subsurface fluid flow and petrophysical properties. Therefore, it is of great significance to study the characteristics of fault damage zone for oil and gas development of ultra-deep carbonate formation. This study uses seismic data and the derived variance attribute to identify two types of damage zones and analyze the spatial geometric characteristics of the damage zones. The results show that the type 1 damage zone is wider than the type 2 damage zone. The width of damage zones distributed on both sides of the Shunbei 5 fault core shows obvious asymmetry, and the damage zone width and throw conforms to the typical power-law distribution on the log-log plot. We discuss the factors affecting the width of the damage zone and its formation process. Finally, we discuss the influence of the damage zones on oil and gas exploration. It seems that the seismic variance attribute is a useful technique for characterizing the ultra-deep strike-slip fault damage zones.

The Effects of 180 years of Aging on the Physical and Seismic Properties of Partially Saturated Sands

AbstractConstraining how the physical properties and seismic responses of recently deposited sands change with time is important for understanding earthquake site response, subsurface fluid flow, and early stages of lithification. Currently, however, there is no detailed (cm‐scale) assessment of how sand's physical properties and associated seismic velocities evolve over the first two centuries after deposition. Here, we integrate sedimentation rates with seismic velocity and sediment physical properties data to assess how the vadose zone sands at Port Royal Beach, Jamaica, change within 180 years after deposition. We show that compressional and shear wave velocities increase with sediment age, whereas porosity, grain size, sorting, mineralogy, and cementation fraction remain relatively unchanged during the same period. Rock physics models (constrained by the measured physical properties) predict constant seismic velocities at all sites regardless of sediment age, though misfits between modeled and observed velocities increase with sediment age. We explain these misfits by proposing that shallow sands undergo microstructural grain reorganization that leads to a more uniform distribution of grain contact forces with time. Our results imply that beach sands undergo a previously undocumented lithification process that occurs before compaction.

Adding olivine micro particles to Portland cement based wellbore cement slurry as a sacrificial material: A quest for the solution in mitigating corrosion of wellbore cement

Invasion of Portland cement-based wellbore cement sheath and plugs by acidic subsurface geofluids (pH 2–5) causes compositional change due to leaching of Calcium Hydroxide (CH), Calcium Silicate Hydrate (CSH) and precipitation of Si(Al)-anhydrous gel translating to strength loss and physical failure and olivine is a promising additive for mitigation. This study aims to characterize the pH variation of olivine microparticles (30–90 μm) in simulated subsurface fluids, and reaction in high pH Portland cement, w/c ratio 0.404 cured for 3weeks, all at 90 °C and 95% RH. Results show olivine reacts with HCl aqueous solution between pH 1.0 to 1.92 and no significant reactivity in other fluids. SEM/EDS and Raman spectroscopy all indicate unreacted olivine particles embedded in hydrated cement. Therefore, olivine could serve as a micro-sacrificial material within hydrated Portland cement exposed to low pH fluids to form carbonates, mitigating potential CSH and CH leaching, enhancing wellbore cement resistance to leakage.

Verification and Validation of Transient Body Force in GOTHIC for Spent Fuel Pool Response to Seismic Events and Other Applications

Post Fukushima the U.S. Nuclear Regulatory Commission issued an Order on Spent Fuel Instrumentation (EA-12-051) requiring all U.S. nuclear plants to install spent fuel pool (SFP) water level monitoring instrumentation and ensure the instrument would remain functional following a safe shutdown earthquake (SSE). The structural integrity analysis requires an assessment of the hydrodynamic loads and wave impact forces that the instrument is subjected to during an SSE. Modeling and simulation of the SFP response to an SSE can provide this type of information if the simulation tool is able to capture the important physical phenomena, such as seismic acceleration, surface wave formation, fluid velocities, and multidimensional effects. This paper describes the capabilities of GOTHICTM that can be used to simulate the sloshing surface waves and subsurface fluid motion of an SFP in response to an earthquake. GOTHIC is a versatile, general-purpose, thermal-hydraulic software package for multiphase flow that is a hybrid between traditional system thermal-hydraulic and computational fluid dynamics codes. It includes a transient, variable body force capability to simulate multi-axis acceleration, and is therefore applicable to seismic events; movement experienced on ships, airplanes, or spacecraft; and other events with system acceleration. Also, since the gravitational constant can be adjusted, GOTHIC can model systems placed outside the Earth’s atmosphere (e.g., spacecraft, space station, the Moon, or other extraterrestrial bodies). The variable body force capability makes GOTHIC well suited to model the hydrodynamic response of an SFP to a seismic event. This paper describes the governing equations that are solved by GOTHIC as they pertain to accelerating systems. A series of benchmarks covering a range of experiments for surface wave dynamics, acceleration-induced motion, and other important phenomena are presented to demonstrate the verification and validation of GOTHIC for these types of applications. Finally, results from a sample application of GOTHIC for SFP hydrodynamic response are presented that provide the necessary inputs for a structural integrity analysis.

Regional Subsidence Analysis Through a Multi‐Scale Modeling Framework Based on Breakage Mechanics

AbstractAlthough poroelastic models are often used to explain the delay between subsurface fluid depletion and ground subsidence, inelastic compaction involving permanent changes of rock microstructure may exacerbate hydro‐mechanical coupling, thus influencing the interpretation of measurements and long‐term forecasts. Here, a multi‐scale modeling approach is discussed, which accounts for the inherent connection between rock microstructure, hydraulic conductivity, and pore compaction. A constitutive model built within the framework of breakage mechanics is proposed to link the hydraulic conductivity of granular rocks with inelastic deformations and changes in grading caused by injection‐depletion cycles at stress levels far from yielding. The proposed model has been incorporated into large‐scale simulation frameworks, thus enabling the spatiotemporal mapping of regional subsidence through a hybrid, semi‐analytical approach. Numerical results based on this strategy show that the model allows isolating near‐field and far‐field effects into the computation of land subsidence and can generate forecasts for different modeling scenarios (e.g., elastic and inelastic compaction, constant permeability, and concurrent change of compressibility and permeability). In particular, examples of simulations for the case of the Groningen gas field are discussed, showing the model capabilities to use both field measurements and laboratory tests for the generation of reasonable subsidence maps, without expensive computational costs. Results indicate that ignoring coupled inelastic effects has major consequences on the predicted timescale of subsidence. Specifically, while all the model scenarios produced similar long‐term ground settlements, those ignoring breakage‐dependent permeability changes result in a variation of the temporal window of residual subsidence of the order of several decades.

Seismic expression of miocene carbonate platform and reservoir characterization through geophysical approach: application in central Luconia, offshore Malaysia

A substantial proportion of proven oil and gas reserves of the world is contained in the carbonate reservoir. It is estimated that about 60% of the world’s oil and 40% of gas reserves are confined in carbonate reservoirs. Exploration and development of hydrocarbons in carbonate reservoirs are much more challenging due to poor seismic imaging and reservoir heterogeneity caused by diagenetic changes. Evaluation of carbonate reservoirs has been a high priority for researchers and geoscientists working in the petroleum industry mainly due to the challenges presented by these highly heterogeneous reservoir rocks. It is essential for geoscientists, petrophysicists, and engineers to work together from initial phases of exploration and delineation of the pool through mature stages of production, to extract as much information as possible to produce maximum hydrocarbons from the field for the commercial viability of the project. In the absence of the well-log data, the properties are inferred from the inversion of seismic data alone. In oil and gas exploration and production industries, seismic inversion is proven as a tool for tracing the subsurface reservoir facies and their fluid contents. In this paper, seismic inversion demonstrates the understanding of lithology and includes the full band of frequency in our initial model to incorporate the detailed study about the basin for prospect evaluation. 3D seismic data along with the geological & petrophysical information and electrologs acquired from drilled wells are used for interpretation and inversion of seismic data to understand the reservoir geometry and facies variation including the distribution of intervening tight layers within the Miocene carbonate reservoir in the study area of Central Luconia. The out-come of the seismic post-stack inversion technique shows a better subsurface lithofacies and fluid distribution for delineation and detailed study of the reservoir.

Near-surface real-time seismic imaging using parsimonious interferometry

Results are presented for real-time seismic imaging of subsurface fluid flow by parsimonious refraction and surface-wave interferometry. Each subsurface velocity image inverted from time-lapse seismic data only requires several minutes of recording time, which is less than the time-scale of the fluid-induced changes in the rock properties. In this sense this is real-time imaging. The images are P-velocity tomograms inverted from the first-arrival times and the S-velocity tomograms inverted from dispersion curves. Compared to conventional seismic imaging, parsimonious interferometry reduces the recording time and increases the temporal resolution of time-lapse seismic images by more than an order-of-magnitude. In our seismic experiment, we recorded 90 sparse data sets over 4.5 h while injecting 12-tons of water into a sand dune. Results show that the percolation of water is mostly along layered boundaries down to a depth of a few meters, which is consistent with our 3D computational fluid flow simulations and laboratory experiments. The significance of parsimonious interferometry is that it provides more than an order-of-magnitude increase of temporal resolution in time-lapse seismic imaging. We believe that real-time seismic imaging will have important applications for non-destructive characterization in environmental, biomedical, and subsurface imaging.

Adapting Petroleum Exploration Tools To Identify and Prioritize Potential CCS Sites, With Application to Texas and Louisiana

Optimization of storage resource is a key parameter in siting and financing storage hubs. We invert petroleum exploration approaches at a geologic play level to assess subsurface risks to long-term storage, as a mechanism to lower cost and increase security. Basin-scale screening, play definition and prospect description are core tasks of petroleum exploration however and there is a well-defined suite of tools to deal with the problem. The work described here adapts those tools to the identification of CCS sites, using the U.S. Gulf of Mexico basin as a timely example. The approach is essentially a three-part one. First, we use regional cross-sections to identify and describe potential storage plays. Second, we use maps of layered surface and subsurface elements to describe the suitability of potential sites, based on the areal quality variation of available reservoirs and seals, the locations of potential traps and CO2 sources, and the distribution of surface constraints such as land usage, available infrastructure and accessibility. Third, we use risk matrices and historical analog data from petroleum exploration to describe the subsurface risks to successful project delivery. With respect to the Gulf of Mexico, CO2 source locations, current land use, site access and the density of existing well penetrations all favor offshore storage. Accordingly, we have focused our efforts on the coastal waters of Texas and Louisiana, constructing a series of exploration-inspired cross-sections and maps that serve both to help adapt the tools of exploration for CCS and to identify and high-grade potential storage sites in coastal waters. The depth window for CO2 injection is defined by subsurface temperature and fluid pressure. Specifically, it lies between the minimum depth for supercritical CO2, which lies at roughly 1km depth and the top of hard over-pressure, which varies between 1.5 and 4km depth, based on extensive drill data. Within this window, stratal ages vary from Early Cretaceous at the northern (onshore) basin edge to Plio-Pleistocene in the south (offshore), reflecting the long-term progradation of depositional systems toward the basin. For each of these stratigraphic intervals, play fairways are defined by a regional seal and its underlying reservoirs. Focussing on the coastal region (i.e. the Oligo-Miocene), we construct a series of maps for each play. The first describes the regional variation in seal capacity (essentially the clay fraction, calibrated by known hydrocarbon columns). The second and third describe reservoir presence and quality, respectively. Historically such maps are based on depositional environment and sediment provenance and they tend to show rather blocky distributions. In an effort to more faithfully represent geologic variability and gradational transitions, we experiment with using modern algorithms and large log databases to create a more variable visualization. Site selection will determine maximum injection rate, ultimate storage capacity and risk of future leakage. In short, regional subsurface characterization is critical to planning investment and may spell the difference between storage project success and failure. Together, the maps developed here describe the regional variation in storage capacity. Combined with maps of surface factors such as accessibility, land ownership, existing land use and CO2 point source locations, they clearly prioritize areas of the basin for investment in infrastructure and development of large-scale storage projects. Adding potential local traps to the map defines a series of concrete leads for storage sites that can be evaluated in more detail as needed for specific projects.

Contrasting facies patterns between river-dominated and symmetrical wave-dominated delta deposits

ABSTRACT Process-physics-based, coupled hydrodynamic–morphodynamic delta models are constructed to understand preserved facies heterogeneities that can influence subsurface fluid flow. Two deltaic systems are compared that differ only in the presence of waves: one river dominated and the other strongly influenced by longshore currents. To understand an entire preserved deltaic succession, the growth of multiple laterally adjacent delta lobes is modeled to define delta axial to marginal facies trends through an entire regressive–transgressive depositional succession. The goal is to refine a facies model for symmetrical wave-dominated deltas (where littoral drift diverges from the delta lobe apex). Because many factors change depositional processes on deltas, the description of the river-dominated example is included to provide a direct reference case from which to define the impact of waves on preserved facies patterns. Both systems display strong facies trends from delta axis to margin that continued into inter-deltaic areas. River-dominated delta regression preserved a dendritic branching of compensationally stacked bodies. Transgression, initiated by sea-level rise, backfilled the main channel and deposited levees and splays on the submerging delta top. Wave-dominated deltas developed dual clinoforms: a shoreface clinoform built as littoral drift carried sediment away from the river month and onshore, and a subaqueous delta-front clinoform composed of sediment accumulated below wave base. Although littoral drift in both directions away from the delta axis stabilized the position of the river at the shoreline, distributary-channel avulsions and lateral migration of river flows across the subaqueous delta top produced heterogeneities in both sets of clinoform deposits. Separation of shoreface and subaqueous delta-front clinoforms across a subaqueous delta top eroded to wave base produced a discontinuity in progradational vertical successions that appeared gradual in some locations but abrupt in others. Littoral drift flows away from adjacent deltas converged in inter-deltaic areas, producing shallow water longshore bars cut by wave-return-flow channels with associated terminal mouth bars. Transgression initiated by sea-level rise initially led to vertical aggradation of wave-reworked sheet sands on the subaqueous delta top and then retreating shoreface barrier sands as the subaerial delta top flooded. Pseudo inter-well flow tests responded to local heterogeneities in the preserved deposits. As expected, abandoned channels in the river-dominated case defined shoreline-perpendicular preferential flow paths and wave-dominated delta deposits are more locally homogeneous, but scenarios for development of more pronounced shore-parallel heterogeneity patterns for wave-influenced deltas are discussed. The results highlight the need to consider the dual clinoform nature of wave-dominated delta deposition for facies prediction and subsurface interpretation.

Seismoelectric surface-wave analysis for characterization of formation properties, using dispersive relative spectral amplitudes

We have studied seismoelectric (SE) surface-wave signals and found that they can be used to infer changes in the SE coupling properties at depth. SE surface-wave signals have much higher amplitudes than SE body-wave signals. We measured the seismic and the electrical potential or electromagnetic (EM) field along the surface of the earth. We used dispersive relative spectral amplitudes (DRSAs) that measure the frequency-dependent relative strength of electrical signals versus seismic signals associated with SE surface-wave signals. We find that DRSAs have sensitivity to contrasts in the electrokinetic coupling coefficient and other relevant petrophysical properties at depth. Our discovery can mitigate the major limitation that plagues body-wave-based SE methods: the relative weakness of the converted EM signals from seismic body waves. We envision applications to characterize subsurface rock, fluid, and fluid-flow properties (e.g., porosity, permeability, and dynamic fluid viscosity, salinity) in the near surface, for aquifers, and shallow geothermal reservoirs.

Fast modeling and understanding fluid dynamics systems with encoder–decoder networks

Is a deep learning model capable of representing systems governed by certain first principle physics laws by only observing the system’s output? In an effort to simulate two-dimensional subsurface fluid dynamics in porous media, we found that an accurate deep-learning-based proxy model can be taught efficiently by a computationally expensive finite-volume-based simulator. We pose the problem as an image-to-image regression, running the simulator with different input parameters to furnish a synthetic training dataset upon which we fit the deep learning models. Since the data is spatiotemporal, we compare the performance of three alternative treatments of time; a convolutional LSTM, an autoencoder network that treats time as a direct input and an echo state network. Adversarial methods are adopted to address the sharp spatial gradient in the fluid dynamics problem. Compared to traditional simulation, the proposed deep learning approach enables much faster forward computation, which allows us to explore more scenarios with a much larger parameter space given the same time. It is shown that the improved forward computation efficiency is particularly valuable in solving inversion problems, where the physics model has unknown parameters to be determined by history matching. By computing the pixel-level attention of the trained model, we quantify the sensitivity of the deep learning model to key physical parameters and hence demonstrate that the inverse problem can be solved with great acceleration. We assess the efficacy of the machine learning surrogate in terms of its training speed and accuracy. The network can be trained within minutes using limited training data and achieve accuracy that scales desirably with the amount of training data supplied.

Dynamic modelling of overprinted low-permeability fault cores and surrounding damage zones as lower dimensional interfaces for multiphysics simulations

In the modelling of subsurface fluid flow, faults are dominant features since they can act as fluid pathways, baffles or barriers. Special emphasis is therefore placed in representing them in a numerically efficient manner and the use of lower dimensional domains has become prevalent to simulate higher permeability features like fractures. Such features, however, only represent some of the components of natural fault networks, which can also include rather impermeable fault gouges surrounded by higher permeability damage zones for instance. Here we present a numerical approach to simulate such systems at a large scale, where the thickness of those features makes it advantageous to represent them as discrete rather than continuous domains, using lower dimensional interfaces in a conforming mesh. Benchmarks show excellent agreement with equivalent continuous simulations, regardless of the fault thickness or permeability, both for flow conduits and baffles. This approach can also account for the overprinting of faults with different permeabilities, as well as their dynamic evolution, which we illustrate with an example of trap charging. This work demonstrates the applicability of the approach to simulate fluid flow in faulted environments of various permeabilities and we discuss how those results can easily be extended to account for multi-physical processes.

Wall-modeled large eddy simulation of turbulent heat transfer by the lattice Boltzmann method

A novel implementation route of the wall-function method to the lattice Boltzmann method (LBM) is proposed to extend the applicability of the LBM for high Reynolds number turbulent heat transfer in complex geometries. The proposed immersed virtual wall method assumes the virtual wall layer beneath the wall which satisfies the slip wall conditions allowing the subsurface heat and fluid flows within the solid wall. For the validation tests, the D3Q27 multiple-relaxation-time LBM and D3Q19 regularized LBM are used to simulate flow and scaler fields, respectively, and the standard log-law based wall-function method is used. Validation tests against turbulent flows in a two-dimensional channel, circular pipe, channel with two-dimensional constraints confirm that the developed method can deal with complex curvilinear walls and yield grid independent solution with satisfactory accuracy. In addition, the developed method can be applied from partially to highly underresolved conditions, and has a great potential in predicting high Prandtl number flows.

The development of intermittent multiphase fluid flow pathways through a porous rock

Intermittent fluid flow has recently been identified as an important transport mode for processes involving subsurface multiphase fluids such as CO2 storage and natural gas production. However, due to experimental limitations, it has not been possible to identify why intermittency occurs at subsurface conditions and what the implications are for upscaled flow properties such as relative permeability. We address these questions with observations of nitrogen and brine flowing at steady-state through a carbonate rock. We overcome previous imaging limitations with high-speed (1s resolution), synchrotron-based X-ray micro-computed tomography combined with pressure measurements recorded while controlling fluid flux. We observe that intermittent fluid transport allows the non-wetting phase to flow through a more ramified network of pores, which would not be possible with connected pathway flow alone for the same flow rate. The volume of fluid intermittently fluctuating increases with capillary number, with the corresponding expansion of the flow network minimising the role of inertial forces in controlling flow even as the flow rate increases. Intermittent pathway flow sits energetically between laminar and turbulent through connected pathways. While a more ramified flow network favours lowered relative permeability, intermittency is more dissipative than laminar flow through connected pathways, and the relative permeability remains unchanged for low capillary numbers where the pore geometry controls the location of intermittency. However, as the capillary number increases further, the role of pore structure in controlling intermittency decreases which corresponds to an increase in relative permeability. These observations can serve as the basis of a model for the causal links between intermittent fluid flow, fluid distribution throughout the pore space, and the upscaled manifestation in relative permeability.

Innovative Formation Tester Sampling Procedures for Carbon Dioxide and Other Reactive Components

Three questions must be answered to optimize any openhole sampling program. These questions are where to sample, when to sample, and how to sample. Samples must be acquired from the right locations in order to answer the critical questions posed by the asset evaluation and production teams. Samples must be obtained at the right time to minimize contamination while using valuable rig time efficiently. Samples must be taken in a manner that provides the laboratory with the most representative analysis of the subsurface reservoir fluid. Carbon dioxide is a corrosive acidic gas component often found in petroleum reservoirs. Special and relatively costly completion and production equipment is required to produce reservoir fluids containing carbon dioxide. Carbon dioxide must be scrubbed from petroleum before that petroleum is shipped. Carbon dioxide may cause flow assurance issues with regards to scale of inorganic and organic components, thereby requiring costly mitigation. An accurate estimate of carbon dioxide concentration in the reservoir fluid is required to make an accurate determination of the cost of CO2 remediation. New formation tester carbon dioxide analysis technology has allowed detailed examination of these questions due to a previously unobserved phenomenon with respect to formation tester sampling and carbon dioxide. Specifically, carbon dioxide may bind with caustic components of drilling-fluid filtrate in either a reversible manner or be consumed by caustic components in an irreversible manner. Observation of a reversible reaction has been used to validate the phenomena but also provides a cautionary warning that for different chemical reactions in which carbon dioxide may be consumed, openhole samples may significantly underestimate the amount of carbon dioxide in the reservoir fluid. New sampling procedures applicable to existing formation testing infrastructure mitigate this sampling issue by determining the correct carbon dioxide content in the reservoir fluid. The new sampling procedures are validated with the carbon dioxide monitoring technology but are not strictly dependent on it. In addition, the proposed sampling procedures are applicable to the measurement of all reactive petroleum components, such as hydrogen sulfide.

Feasibility of using the P-Cable high-resolution 3D seismic system in detecting and monitoring CO2 leakage

The P-Cable technology is an acquisition principle for high-resolution and ultra-high-resolution 3D seismic data. Many 3D seismic datasets have been acquired over the last decade, but the application in time-lapse studies for monitoring of CO2 storage is a new and intriguing topic. High-resolution 3D (HR3D) seismic has the potential to detect and monitor CO2 leakage at carbon capture and storage sites with higher accuracy at depths ∼0−2 km below the seafloor compared to more traditional conventional seismic time-lapse data. Here, we synthesize and evaluate research on detection of subsurface CO2 movement using the P-Cable system and address the comparative advantages and disadvantages of conventional and HR3D technologies for subsurface fluid migration monitoring. Studies on P-Cable 4D seismic data show good repeatability (NRMS, 10–40 %), indicating a future monitoring potential. Analysis of detection limits of CO2 data from a CO2 storage site show the ability to detect very small amounts of CO2 (1.3–10.6 t; 3.3–27.4 % gas saturation) in the shallow subsurface. These detection limits are ∼30−300 times smaller than the detection limits of conventional seismic data at similar depths. We conclude that the P-Cable acquisition system can be a valuable monitoring tool in detecting small leakages and can complement conventional seismic data monitoring of the deeper interval.

Numerical assessment of compressed air injection for mitigating seawater intrusion in a coastal unconfined aquifer

Subsurface fluids injection is a viable alternative for controlling seawater intrusion in coastal regions, and freshwater is usually served for valid fluids to generate pressure ridge towards the ocean. But other fluids injection, such as compressed air, which presents clean and readily available, is a more attractive solution for repulsing the intruded seawater where freshwater is insufficient. In this study, the performance of compressed air injection (i.e., air barriers) for mitigating seawater intrusion in a coastal unconfined aquifer was quantificationally assessed using a coupled water-air two-phase flow and saltwater transport model. As compressed air is introduced into the aquifer at the toe of saltwater wedge, the seaward hydraulic gradient adjacent to the coast is produced driven by the generated airflow, thereby causing salt-freshwater interface to retreat to the ocean. The magnitude of air injection rate, related to the operational cost of air barriers, does not increases significantly given the progressively decreasing change rate. Compared to the workability of air barriers in the confined aquifer under similar conditions, the reduction in intruded seawater at 365.0 d in the unconfined aquifer reaches only about 0.45 times that in the confined aquifer, which indicates the significance of the boundary condition of aquifer top. The sensitivity analysis of the overlying unsaturated zone reveals that with its permeability decreasing or its thickness increasing, the performance of air barriers is enhanced owing to the blocking effect of the overlying layer on the escape of generated airflow and tends progressively to be stable. Particularly, the efficacy of air barriers in the confined aquifer, including the reduction in intruded seawater and the air injection rate, also can be achieved in an unconfined aquifer with an overlaying sufficient-semipermeable layer.

Geological surface reconstruction from 3D point clouds

The numerical simulation of phenomena such as subsurface fluid flow or rock deformations are based on geological models, where volumes are typically defined through stratigraphic surfaces and faults, which constitute the geometric constraints, and then discretized into blocks to which relevant petrophysical or stress-strain properties are assigned.This paper illustrates the process by which it is possible to reconstruct the triangulation of 3D geological surfaces assigned as point clouds. These geological surfaces can then be used in codes dedicated to volume discretization to generate models of underground rocks.The method comprises the following:- Characterization of the best fitting plane and identification of the concave hull of the point cloud which is projected on it- Triangulation of the point cloud on the plane, constrained to the Planar Straight Line Graph constituted by the concave hullThe algorithm, implemented in C++, depends exclusively on two parameters (nDig, maxCut) which allow one to easily evaluate the optimal refinement level of the hull on a case by case basis.

Subsurface fluid injection and energy storage

Subsurface fluid injection and energy storage