Aquifer Pressure Research Articles

Pore-scale binary diffusion behavior of Hydrogen-Cushion gas in saline aquifers for underground hydrogen Storage: Optimization of cushion gas type

Cushion gas is required to prevent hydrogen (H2) trapping and interactions between H2 and brine for underground hydrogen storage (UHS) in saline aquifers. However, selecting an appropriate type of cushion gas is not a straightforward task, as it affects the H2 diffusion characteristic in subsurface porous media, thereby impacting purity, recoverability, etc. In this paper, we developed a modified pore network model (PNM) for binary diffusion that incorporates multi-scale diffusion mechanisms to forecast the H2 effective diffusion coefficient (DH2e) and evaluate the impacts of the cushion gas on DH2e. Additionally, the model accounts for the water-lock effects of residual brine, focusing exclusively on the connected pore-throats occupied by the gas phase, which are identified using the invasion percolation algorithm with trapping. Our key findings are as follows: 1) CO2 and N2 demonstrate superior containment effects on H2 compared to CH4 under the specific conditions of the target aquifer, reducing the H2 contamination caused by binary diffusion; 2) The size and topology of porous media exert minimal impacts on the selection of cushion gas, yet they markedly affect the DH2e; 3) The pressure and temperature conditions of the saline aquifer are critical factors in cushion gas selection, with temperature being the more influential parameter. This study provides valuable insights for the implementation of industry-scale UHS in saline aquifers.

Water-induced physicochemical and pore changes in limestone for surrounding rock across pressure aquifers

Water-induced physicochemical and pore changes in limestone for surrounding rock across pressure aquifers

Global sensitivity analysis of water level response to harmonic aquifer disturbances through a Monte-Carlo based surrogate model with random forest algorithm

Oscillatory water levels are often observed in monitoring wells, when aquifers are disturbed by periodical pressure diffusion (PD) sources, such as oscillatory pumping and ocean tide, and by volumetric strain (VS) sources including the far-field seismic wave and earth tide. The measured water level responses are extensively utilized to interpret aquifer properties and seismic hydrological phenomena. However, owing to wellbore effects, water levels are not always truly representative of pore pressure in aquifers, and can be significantly attenuated or even unobservable in wells. To clarify how water level responds to these disturbances, this study analyzes a global parameter sensitivity of the coupled well-aquifer model based on the assumption of a confined homogeneous aquifer. Parameter uncertainties concerning aquifer properties, source characteristics, and wellbore construction are first analyzed separately for the PD and VS sources. Subsequently, Monte-Carlo based surrogate models linking parameters and response indices are developed using the random forest algorithm. For the PD source, sensitivity results indicate that hydraulic conductivity is the most influential parameter for the yes/no responses. When hydraulic conductivity exceeds 10-4 m/s, water level observations basically represent pore pressure in aquifers. Significant amplitude changes and phase delay occur when the period of disturbances is less than 50 s, majorly due to water column inertia. For VS sources, water level responses are typically observed in aquifers with hydraulic conductivity larger than 10-7 m/s and when disturbed by a source with amplitude greater than 10-3 MPa. However, pronounced amplitude changes and phase shifts occur when the source period varies within 100 s. Consideration of the impact of wellbore effects is recommended, when interpreting water level responses induced by all periodic aquifer disturbances, especially in low-conductivity aquifers with conductivities less than 10-4 m/s.

Extended Aquifer System Pressure Behavior under Carbon Storage

Summary Most reported carbon storage projects have involved inexpensive carbon dioxide (CO2) capture from gas processing plants or ethanol refineries. However, widespread carbon capture and storage (CCS) application must avoid any risk that high capital investment cost for carbon capture from stationary point sources leads to unanticipated issues related to the aquifer storage. This paper reviews successful and unsuccessful carbon storage projects and explains simple extended aquifer system fundamentals that must be considered in selecting a storage aquifer. This study begins by evaluating reported carbon storage projects in the context of an extended aquifer system with specific attention to initial formation pore pressure and potential or known hydraulic vertical or lateral communication with hydrocarbon accumulations and/or fresh water. Further study focuses on how the contrast between injection well and aquifer pressure evolution enables understanding of the overall aquifer material balance. Finally, we consider implications of brine migration during and after long-term CO2 injection in unconfined aquifers. Experience in the petroleum industry with aquifer behavior includes presence or lack of water influx and production from hydrocarbon reservoirs that share a common aquifer. Of particular importance is the observation that hydrostatic initial formation pressure indicates the possibility that a petroleum system, or an extended aquifer system without hydrocarbon accumulation(s), connects to atmospheric pressure through an unconfined aquifer. In such cases, indefinite injection will never increase the regional aquifer pressure. Furthermore, initial formation pressure that exceeds hydrostatic pressure implies a petroleum system or an extended aquifer system that is volumetrically limited. In such cases, injection will increase the system pressure, and pressure monitoring can detect leakage from the system. Finally, CO2 injection into an aquifer will displace brine in the direction of lower pressure that could relate to distant production from the same aquifer or from hydrocarbon reservoirs with which it communicates. Reasons for known carbon storage project interruptions have included unexpected lateral plume migration or aquifer pressure increase during CO2 injection that might have been anticipated with attention to straightforward consideration of aquifer-enabled hydraulic communication. Such extended aquifer dynamics must be included in long-term models for permanent CO2 storage during and after injection.

Basin‐scale responses of groundwater‐resource quality to drought and recovery, San Joaquin Valley, California

AbstractGroundwater‐resource quality is assumed to be less responsive to drought compared to that of surface water due to relatively long transit times of recharge to drinking‐supply wells. Here, we evidence dynamic perturbations in aquifer pressure dynamics during drought and subsequent recovery periods cause dramatic shifts in groundwater quality on a basin scale. We used a novel application of time‐series clustering on annual nitrate anomalies at >450 public‐supply wells (PSWs) across California's San Joaquin Valley during 2000–22 to group sub‐populations of wells with similar water‐quality responses to drought. Additionally, we statistically evaluated the direction and magnitude of multi‐constituent water‐quality changes across the San Joaquin Valley using a broader dataset of >3000 PSWs with data during two select hydrologic stress periods representing an extreme drought (2012–16) and subsequent recovery (2016–19). Results of time‐series clustering and stress‐period change analyses corroborate a predominant regional response to pumping stress characterized by increased concentrations of anthropogenic constituents (nitrate, total dissolved solids) and decreased concentrations of geogenic constituents (arsenic, fluoride), which largely reversed during recovery. Cluster analysis also identified a secondary, less commonly occurring group of PSWs where nitrate decreased during drought, but explanatory factor analysis was not able to discern hydrogeologic drivers for these two divergent response patterns. Long‐term tracer data support the hypothesis that the predominant regional signal of nitrate increase during drought is caused by enhanced capture of modern‐aged groundwater by PSWs during periods of pumping stress, which can drive rapid changes in water quality on seasonal and multiannual timescales. Pumping‐induced migration of modern, oxic groundwater to depth during drought may affect geochemical conditions in deeper portions of regional aquifers controlling the mobility of geogenic contaminants over the long term.

Impact of CO2 sequestration on pressure systems in saline aquifer: Simulation of differential pressure evolution in the caprock-reservoir

CO2 sequestration in the saline aquifer is one of the most promising means of geological sequestration, and the realization of safe CO2 sequestration without leakage is the common goal of sequestration projects. Mature continental sedimentary basins typically have a long drilling and production history, and several wellbores inevitably penetrate the saline aquifer. These wellbores have been identified as risky locations for CO2 leakage. Scale-up CO2 injection can cause large-scale perturbations to the saline aquifer pressure system, affecting the sealing integrity of wells within the sequestration site. Considering that differential pressure is a key driver of wellbore leakage, this paper establishes and validates a three-dimensional cap-reservoir model to simulate the evolution of the differential pressure between the cap-reservoir due to CO2 sequestration. The simulation results show significant differences in the magnitude and rate of pressure buildup in the cap and reservoir during injection and in the rate of overpressure dissipation after stopping injection. These differences caused the dynamic evolution of the interlayer pressure difference. They formed peaks in the injection and stopping stages, and the peak in the injection stage was higher than in the stopping stage. The peak decreases as an exponential function with increasing distance from the injection well. In the constant rate injection mode, the peak rises linearly with increasing injection rate, and the effect of injection temperature on the peak can be neglected. A sensitivity analysis of reservoir hydrogeological parameters was done. From the perspective of controlling differential pressure, an ideal saline aquifer should be characterized by deep burial, low salinity, high permeability, and large effective pore space. This study is significant for CO2 sequestration site optimization and CO2 leakage risk control.

A conceptual CO2 fill-and-spill mega-fairway in the UK Southern North Sea: A new approach to identify and optimise large-scale underground carbon storage (CCS)

This work explores the subsurface CO2 storage potential in UKCS Quadrants 43–44, by integrating 3D seismic interpretation, regional wellbore data and CO2 migration modelling. We show 11 Bunter Sandstone domes linked together in a fill-and-spill “Bunter Mega-Fairway”. These traps host a theoretical storage capacity of 8.4 Gt CO2 (≈3–4 times the filled-to-spill theoretical capacity of the proposed Endurance storage site), while lessening CO2 leakage risks.Within the mega-fairway, the Triassic Bunter Sandstone reservoir shows a P10-P90 thickness of 78–219 m, along with moderate-high porosities (11–28 %) and permeabilities (9–669 mD). A regionally-connected reservoir is suggested from spatially consistent, near-hydrostatic aquifer pressure gradients (≈0.51 psi/ft). The top-seal is laterally effective and structural closures are lower than the CO2 columns necessary to breach the seal.At the mega-fairway storage depths, CO2 would inhabit the reservoir as a supercritical fluid, enabling it to naturally migrate buoyantly from one trap to another, away from strategically-placed injector sites. Through the integration of migration spill-point modelling, multi-physics seabed monitoring nodes and fill-and-spill injection management, the maximum storage capacity could be achieved, while alleviating CO2 leakage hazards and potentially decreasing the number of injector wells.Exploiting fill-and-spill fairways for CCS is a new concept with vast worldwide applicability.

Investigating the enigma of an irregular groundwater age pattern in a confined, presumed “fossil” complex aquifer through mixing cell flow modeling

Significant fluctuations in the groundwater (GW) age along the eastern flow path of the Nubian Sandstone Aquifer's (NSA), as derived from Krypton-81 groundwater dating, have suggested that this aquifer, located in Israel's Negev Desert and previously presumed to be a fossil, is not entirely isolated but mixes with younger and even recent water. The intermittent rejuvenation and drastic increases in the GW age across short distances most likely imply hydraulic connectivity with the surrounding aquifers, which contribute both younger and more ancient water to the NSA. The current study aims at modeling the GW flow system to locate and quantify its water sources despite the aquifer's hydrogeological complexity and the scarcity of hydrological data. We implemented the Mixing Cell Modeling (MCM) approach, understanding that the alternating rejuvenations and increases in the GW age downstream of the NSA's eastern flow trajectory reflect the mixing of the NSA's groundwater with young and old GW bodies, respectively. Thus, prompted by the 81Kr water age distribution, yet independent of the Kr radioisotope data, a multi-tracer mixing cell flow model was adopted based on a set of balance equations of water, dissolved minerals, and stable environmental isotopes. The findings indicate that (1) there is a small, yet substantial, intrusion of old brackish GW from a deep-seated, highly pressurized aquifer into the NSA in the northeastern Negev; (2) the rejuvenation of GW in the NSA is due to significant mixing with water from nearby overlying carbonate and chert aquifers, and (3) the NSA is substantially replenished through the Nubian Sandstone (NS) outcrops along the Negev Desert anticlines. Most GW intrusions into the NSA occur near the intersections of the eastern flow path with some of the Negev's major faults and synclines, such as the Paran and Ramon Fault zones and the Zin Syncline. In light of the relatively young GW age at the end of the NSA's western flow path in the northern Negev, and based on the similarities in the hydrogeological structures in the Negev and northern Sinai Deserts, we propose that similar mixing processes with GW from the overlying carbonate aquifers and direct GW recharge through the NS outcrops also occur in the northern Sinai Peninsula. The approach presented in this study might apply to examining recharge processes and hydraulic connectivity in other aquifers that were formerly classified as “fossil,” such as the immense NSA found in the Arabian (Jordan & Saudi Arabia) and the Western (Egypt) Deserts.

Widespread aquifer depressurization after a century of intensive groundwater use in USA.

Water supplies for household use and irrigated agriculture rely on groundwater wells. When wells are drilled into a highly pressurized aquifer, groundwater may flow up the well and onto the land surface without pumping. These flowing artesian wells were common in the early 1900s in the United States before intensive groundwater withdrawals began, but their present-day prevalence remains unknown. Here, we compile and analyze ten thousand well water observations made more than a century ago. We show that flowing artesian conditions characterized ~61% of wells tapping confined aquifers before 1910, but only ~4% of wells tapping confined aquifers today. This pervasive loss of flowing artesian conditions evidences a widespread depressurization of confined aquifers after a century of intensive groundwater use in the United States. We conclude that this depressurization of confined aquifers has profoundly changed groundwater storage and flow, increasing the vulnerability of deep aquifers to pollutants and contributing to land subsidence.

Coupled processes of groundwater dynamics and land subsidence in the context of active human intervention, a case in Tianjin, China

To address the crisis of water shortage in the North China Plain, the Chinese government implemented the South-to-North Water Transfer Project (SNWTP). In this context, Tianjin, one of the main beneficiaries of this project, has been relieved from water shortages and begun to implement Groundwater Management Plans (GMP) since 2018, which undoubtedly have a significant effect on the groundwater recovery. Meanwhile, this provides a good case for studying the coupled process of ground settlement and groundwater dynamics, especially the soil deformation pattern driven by groundwater level (GWL) rebound. To analyze these issues in detail, field well data was collected to depict groundwater flow field. Moreover, geodetic data was also collated, including leveling, GPS, and InSAR, so that a vertical deformation field with high spatiotemporal resolution could be generated. The results reveal that the GWL of the third confined aquifer which is the main exploitation layer in Tianjin recovered significantly since 2018 with a rate of 2.1 m/yr. The dynamic deformation patterns indicate that the area of land subsidence cones in Tianjin has reduced significantly, accompanied by a sharply declining subsidence rate (decreased from −32.2 mm/yr to −4.5 mm/yr.). Particularly, a significant poroelastic rebound has occurred in the Wuqing and Beichen districts since 2020. Furthermore, due to the delayed pore pressure dissipation in the aquitard, we find a time delay of 0.3–5.5 years between land subsidence and GWL time series, which is far less than that estimated by hydrogeological parameters, as the latter ignored the recharge and recovery capacity of the aquifer system. Finally, an evolution models in Tianjin was presented to illustrate interactive process among the deformation, pore pressure, and hydraulic head. In general, the SNWDP and the GMP has restored the pore pressure of aquifer, reduced the land subsidence, and alleviated the groundwater storage depletion of Tianjin.

Migration characteristics and local capillary trapping mechanism after the CO2 leakage out of saline aquifers

CO2 may leak from the wellbore, cross-section of faults, fractures, and pore space due to geological tectonic activity or human-induced activity during the CO2 geological storage in deep saline aquifers. To evaluate the CO2 leakage risk and select safe saline aquifers, it is necessary and crucial to predict the CO2 migration characteristics and trapping mechanism after its leakage. Although some previous studies have shown that heterogeneous capillary pressure can change CO2 migration path, its impact on the migration and storage after CO2 leakage is still unclear. Therefore, a simulation model of CO2 leakage and migration in saline aquifers is established by adopting stochastic modeling in geology and two-phase seepage modeling in the fluid. Based on the model, the influence of capillary pressure heterogeneity and caprock factors (thickness and leakage area) on the leaked CO2 migration, distribution, and trapping is analyzed quantitatively. The research result shows that heterogeneous capillary pressure in the saline aquifers can effectively trap the leaked CO2 and enlarge the lateral swept volume of leaked CO2. With the increase of capillary pressure and its heterogeneity, the upward migration of CO2 slows down but the local capillary trapping increases. Both of these can weaken the risk of CO2 leakage to the surface. Under the local capillary trapping above and below the caprock, the increasing of caprock thickness and leakage area have little effect on CO2 leakage amount, but may increase the local capillary trapping and make the formation safer. Therefore, priority should be given to selecting saline aquifers with thicker caprock to reduce the potential risk of CO2 leakage. In order to ensure the safety of CO2 storage, a saline aquifer with strong capillary pressure heterogeneity and good trapping performance should be selected.

Study Makes Value Proposition for Carbon-Neutral Fuel From Light Tight Oil

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper URTeC 2021-5437, “Carbon-Neutral Fuel From Light Tight Oil: A Value Proposition,” by Christine A. Ehlig-Economides, SPE, University of Houston. The paper has not been peer reviewed. _ Emissions for light tight oil often begin with associated gas. Rather than flare the stranded methane, an option may be to use it to power capture of CO2 from the air using direct air capture (DAC) technologies. This study catalogs global gas/oil ratio (GOR) data to identify currently produced light crude oils that could be rendered carbon-neutral through the DAC mechanism. It then considers operational aspects, including proximity to a suitable location for a DAC unit and proximity to subsurface utilization or storage opportunities, that favor or discourage this approach. Introduction Previous work by the author defined carbon-neutral crude oil (CNCO) as crude oil that has offset the CO2 to be emitted by its combustible products before its arrival at the sale point. Other authors refer to enhanced oil recovery (EOR) using anthropogenic CO2 and including monitoring, verification, and reporting as EOR+. They describe three EOR+ categories: conventional, advanced, and maximum storage. Conventional storage minimizes CO2 use to reduce cost, resulting in net usage of 0.3 t CO2/bbl. Advanced-storage miscible flooding follows current best practices for optimized oil recovery and also may involve some “second-generation” approaches that boost CO2 use, resulting in net usage of 0.6 t CO2/bbl. Maximized storage effectively uses the reservoir pore space for additional CO2 storage without additional oil production and may include removing water, resulting in net usage of 0.9 t CO2/bbl. CO2 Capacity The pore-space requirements for saline aquifer CO2 storage depend on whether primary or secondary storage is envisioned for the short term. Primary storage entails bulk CO2 injection into an aquifer storage volume effectively limited by interference with neighboring injection wells, while secondary storage entails producing the same volume from the reservoir as is injected as CO2. The bulk rock volume required to store CO2 dissolved in brine is approximately 200 times the original coal volume (Fig. 1). Limiting the injection pressure below the fracturing pressure to avoid breaching the caprock, however, further limits the CO2 storage capacity to approximately 6% of the pore space. This makes the multiplier nearly 120 times the original coal volume without CO2 dissolution. Generally, CO2 captured from stationary point sources and stored through multiwell bulk injection in a saline aquifer has a primary storage capacity of approximately 4.2 t CO2/acre-ft without increasing the original aquifer pressure. Three tables in the complete paper list inputs for these estimations and parameters derived from these inputs for cases corresponding to primary and secondary storage without aquifer pressurization. If CO2 injection elevates the aquifer average pressure, this increases the risk of inducing earthquakes and fracturing through the caprock. Using a typical fracture gradient of 0.7 psi/ft, injectivity of 3.23 BOPD/ft/psi could enable elevating the aquifer average pressure to 6,297 psia, which would, in turn, enable increasing the primary storage capacity to 18 t CO2/acre-ft and secondary storage capacity to 135 t CO2/acre-ft, again without CO2 dissolution. It is useful to compare maximized EOR+ with secondary CO2 storage in a saline aquifer. Effectively, maximized storage is like secondary saline aquifer storage in a sealed and trapped reservoir volume. Elevating pressure can increase the storage efficiency in either case, though with associated risks.

Progressive Evolution Model of Fault Water Inrush Caused by Underground Excavation Based on Multiphysical Fields

Underground fault water inrushes are frequent hydrogeological disasters associated with underground mining and tunnel construction projects. In this study, we analyze the water inrush mechanism of underground engineering by building a numerical simulation model to evaluate the process of water inrush, analyze water inrush changes under various working conditions, and consider the fluid-solid coupling effect of rock mass and water. These analyses provide effective suggestions for preventing water inrush from faults. The study establishes a two-dimensional numerical model based on Darcy’s law and plane strain field to analyze water inrush from faults in underground engineering. The analysis shows that factors such as aquifer pressure, permeability between the aquifer and fault zone, and permeability sensitivity coefficient are important considerations that affect the occurrence of water-inrush disasters. The study also identifies the sudden change in water inrush speed at the fault zone and the roadway when the working condition is changed as an indication of the nature of water inrush at the fault. Additionally, the study presents preventive measures such as drainage grouting to ensure the safety of underground engineering constructions. Overall, this research provides important insights into the causes and effects of water inrush from faults and can inform practical measures to mitigate the risks associated with underground engineering.

Carbon Dioxide Sequestration in Low Porosity and Permeability Deep Saline Aquifer: Numerical Simulation Method

Abstract The saline aquifer is the most reliable place where anthropogenic carbon dioxide gas storage has shown a promising future. This paper evaluates and predicts the capacities of different carbon dioxide storage trapping mechanisms in storing carbon dioxide gas in low porosity and permeability deep saline aquifers by using commercial reservoir simulator software i.e., Computer modeling group (CMG). Four carbon dioxide storage trapping modeled and simulated were structural or stratigraphic trapping mechanisms, residual trapping mechanisms, solubility trapping mechanisms, and mineral trapping mechanisms. Carbon dioxide gas was injected into a deep saline aquifer for 15 years, followed by 833 years of post-injection. To reflect the real field reality and have a reasonable approximation of the amount of carbon dioxide which can be stored in an aquifer, this paper included water vaporization effects that occur during carbon dioxide injection and water injection operations so as to optimize residual and solubility trapping mechanisms as the most important trapping mechanisms. Furthermore, the effects of different important parameters such as salinity, vertical-to-horizontal permeability ratio, injection rate, bottom hole pressure, and temperature on each carbon dioxide trapping mechanism were analyzed. Results revealed that each carbon dioxide trapping mechanism has a different capacity for storing carbon dioxide and could be either affected linearly or nonlinearly with various parameters. Higher aquifer temperatures are not recommended for carbon dioxide storage because most of the carbon dioxide gas is stored as free gas, which increases the risk of leakage in case of mechanical failure or imbalance. Excess salinity is the only factor that reduces aquifer storage capacity. Furthermore, it was found that an aquifer with a lower vertical-to-horizontal permeability ratio is recommended for carbon dioxide storage because it increases carbon dioxide stored in an immobile phase, which avoids risk leakages. There was an increase of 43.2% and a decrease of 16.84% for minimum and maximum vertical-to-horizontal permeability (kv/kh) ratios, respectively, compared to the base for residual trapping mechanisms. Also, there was a decrease of carbon dioxide dissolved by 19% at maximum kv/kh ratios and an increase of 58% at minimum kv/kh ratios, compared to the base case. Further, there was an increase of carbon dioxide trapped by 96.4% and dissolved by 97% when water was injected at a higher rate compared to the base case (no water injection). Thus, a high injection rate is suggested to enhance residual and solubility trapping mechanisms. It is recommended that the carbon dioxide injection rate and bottom hole pressure be kept at optimal levels to avoid mechanical failure due to aquifer pressures building up, which can increase the risk of leakages and must be monitored and controlled at the surface using pressure gauges or sensor technology.

What makes them pump? Factors influencing groundwater extraction for cattle grazing in a semi-arid region

Quantifying groundwater extraction and understanding the factors that affect groundwater extraction are critical for improving water productivity and effectively managing groundwater resources. Groundwater extraction for cattle grazing is particularly poorly quantified and understood, despite groundwater being an important water source in many grazing regions globally. To address this knowledge gap, a world-first voluntary program of bore (well) metering in the Surat Basin, Queensland, Australia, collected extraction data for 54 bores that are used fully or partially for cattle grazing. Data on factors potentially affecting extraction were gathered for each bore through landholder interviews and government database searches. Multiple regression analysis examined 18 variables and found strong evidence that 5 factors influence annual extraction from the metered bores: cattle numbers, bore purpose, type of water storage infrastructure, aquifer pressure, and annual rainfall. Together these factors explained more than 78% of the variance of the annual extraction estimates. The results provide the basis for generalising extraction estimates to bores without meters in the same region and indicate the priority variables to explore in other regions. The results also provide evidence of the importance of cattle breed and water storage infrastructure in improving water efficiency.

Groundwater control for construction projects in chalk

Abstract Construction excavations and tunnels in chalk can encounter groundwater challenges, including high water flow rates, instability of excavations in weathered chalk and basal instability in overlying aquitards caused by high groundwater pressures in deeper chalk aquifers. In hydrogeological settings where the chalk has been exposed to periglacial weathering during the Quaternary Period the upper zones may be degraded to structureless chalk which can potentially be of very low hydraulic conductivity (putty chalk) or very high hydraulic conductivity (chalk bearings). In deeper, structured chalk groundwater flow tends to be concentrated along fissures associated with pre-existing geological structures such as bedding planes, flint beds or faults. A range of groundwater control strategies can be deployed, including open pumping, pre-drainage pumping, shallow and deep cut-off walls, ground treatment and, for tunnels and shafts, application of fluid counter pressures to exclude groundwater. The strategy appropriate to a given site must be selected based on a thorough understanding of the hydrogeological setting and chalk weathering profile. This requires a ground investigation of appropriate scope, using suitable techniques to characterize the chalk. Borehole geophysics can play a key role in identifying discrete zones of inflow.

Evaluation Method of Water Hazard Control Effect of Coal Seam Floor in Deep Mining: Sequence Verification Evaluation

Deep, highly pressurized aquifers seriously threaten the exploitation of resources of North China coalfield. At present, there is no particular guidance system to evaluate the prevention and control effect of water inrush disaster in deep coal seam mining. Therefore, this research focuses on the evaluation of the method for confined water disaster prevention and control effect of deep coal seam mining. Based upon various methods, such as effective thickness and water resistance capacity of aquifuge, water inrush coefficient, GIS (Geographic Information System) analysis of grouting data, dense drilling verification, and professional analysis system based on integrated sequence evaluation, the evaluation of the method for confined water disaster prevention and control effect of deep coal seam mining was put forward for the first time. The proposed method was called the “sequence verification evaluation (SVE)” method. The prevention and control of the effects of Ordovician water inrush disaster in deep mining of Huanghebei coalfield was evaluated using the proposed SVE method. The results show that the SVE method can effectively reflect the prevention and control of the effects of water inrush from confined water and provide scientific basis for the safe production of deep coal seam mining.

The Hydrogeological Characteristics of Thick Alluvium with High Water Level and the Influence on Zhaogu Mining Area, Henan Province, China

In parts of North China Plain, there are several aquifers in the thick alluvium over the coal seam, which overlays a large amount of coal resources. Roof water inrush accident occurred during coal mining often caused by multiple reasons. Based on the case of roof water inrush in Zhaogu Mining Area, by hydrologic supplementary exploration of the north flank, the stratum data were obtained; combined with previous geological drilling data of the mine, the structural characteristics of five main aquifers and five main aquicludes in the alluvium of the north flank were classified. By pumping test of the No. 1 and the No. 2 supplementary exploration drill holes, the water richness of the 5th aquifer of north flank is identified as medium level. Three thin layers of gravel aquifer beneath the 5th aquifer were found in No. 1 drill hole, and the water richness of the lowest thin gravel (2.95 m thickness) was identified as medium by the followed pumping test, which reveals an important factor leading to the insufficient size of the safety pillar. The leakage self-closed aperture test on the sand soil of the 5th aquiclude clarifies that in case of fracture connection, it cannot resist water and sand crushing under high water pressure of the 5th aquifer. Based on the mechanism of high-pressure water on the working face supports, the calculation method of support resistance under high water pressure alluvial layer is put forward, and the main reason of roof support failure and water inrush is the insufficient support resistance, which leads to abnormal height caving. The methods of experiment and calculation are clarified for preventing water and sand inrush accident of working faces with the similar mining background.

Consideration regarding the aquatic ecosystems developed in the open pit lakes from Oltenia mining region

Formation of so-called open-pit lakes has become an increasingly common practice in Romania (more precisely in the Oltenia Mining Basin, where the lignite deposit contained in Dacian and Romanian formations was and is extracted through open-pits). The flooding process of the remaining gaps is in general a natural one, as water comes from underground sources (phreatic and pressurized aquifers) and from precipitations (including runoff waters from the surrounding terrains). This practice is considered to be a type of ecological reconstruction of former exploitation areas and has been applied worldwide for quite a time (in Germany, Poland, Czech Republic, Spain, USA, Australia, etc.). The present paper aims to present some ecological aspects regarding these lakes, and, as case study, we chose the lake formed in the remaining gap of Urdari open-pit (within the Rovinari Mining Basin). However, the authors consider that given the multitude of similar conditions (bioclimate, geology, topography, etc.) in which they form, the preliminary conclusions of this study can be extrapolated for other lakes in the area (existing, such South Peșteana and Moi, or planned, such as Roșia and North Peșteana). This study presents data on the existing biotope and biocenoses (restricted to species identification), without going into specific details about populations, ecosystem production, energy and matter flows within it, as these investigations demand a longer period of time and more resources and are foreseen for a future development stage of this study.

Mitigating Water Production from High Viscosity Oil Wells in Unconsolidated Sandstone Formations

Summary Large reserves of high-viscous oil in Kuwait call for improved oil recovery scenarios. In Kuwaiti unconsolidated sandstone formations, the sandstone intervals represent extensive reservoir intervals of sand separated by laterally extensive nonreservoir intervals that comprise finer-grained argillaceous sands, silts, and muds. The reservoir is shallow with high permeability (above 1,000 md) and under bottom aquifer pressure support. Due to strong viscosity contrast between oil and water, after breakthrough, the water cut rises quickly, resulting in strong loss of production efficiency. Mitigating water production is thus mandatory to improve production conditions. The candidate wells have two to three open intervals in different rock facies with comingle production. The total perforated length is between 38 and 48 ft. Production is through progressing cavity pumps (PCP) at a rate of around 300 B/D, and basic sediment and water is between 71 and 87%. The technology applied utilizes pregelled size-controlled product [small microgels (SMG Microgels)] having relative permeability modification (RPM) properties; that is, inducing a strong drop of relative permeability to water without affecting oil relative permeability. The size is chosen to selectively treat the high-permeability water producing zones while preserving the lower-permeability oil zones. The chemical can also withstand downhole harsh conditions such as salinity of around 170,000 ppm and presence of 2% H2S. The treatment consisted of bullhead injection of 300 bbl of pregelled chemical through-tubing. The first results seem very favorable, since for two wells, the water cut has dropped from 80 to 40% with almost same gross production rate. The incremental oil is more than 100 BOPD. The third well did not show marked change after water system operation (WSO) treatment. The wells are under continuous monitoring to assess long-term performance. Such result, if confirmed, may lead to high possibilities for the improvement of heavy oil reservoir production under aquifer support by mitigating water production with simple chemical bullhead injection.