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27 pages, 27729 KiB

Open AccessFeature PaperArticle

Simulation of the Infiltration of Fractured Rock in the Unsaturated Zone

by Luat Khoa Tran and Stephan Konrad Matthai

Appl. Sci. 2021, 11(19), 9148; https://doi.org/10.3390/app11199148 - 01 Oct 2021

Cited by 2 | Viewed by 1619

Abstract

We study infiltration of rainwater into fractured rock and the accompanying capillary exchange processes between fractures and matrix, hereafter referred to as fracture–matrix transfer (FMT). Its influence on the velocity of the wetting front for uniform and variable aperture fractures is of prime [...] Read more.

We study infiltration of rainwater into fractured rock and the accompanying capillary exchange processes between fractures and matrix, hereafter referred to as fracture–matrix transfer (FMT). Its influence on the velocity of the wetting front for uniform and variable aperture fractures is of prime interest because it determines the penetration depth of infiltration pulses. FMT is modelled explicitly in a discrete fracture and matrix (DFM) framework realised using a hybrid finite element–finite volume discretisation with internal boundaries. The latter separate the fracture mesh from the rock matrix mesh with the benefit that the flow that occurs within the minute fracture subvolume can be tracked with great accuracy. A local interface solver deals with the transient nonlinear aspects of FMT, including spontaneous imbibition of the rock matrix. Two- and three-dimensional heuristic test cases are used to illustrate how FMT affects infiltration. For the investigated scenario, we find that—beyond a critical fracture aperture around 5–10-mm—infiltration rate is no longer affected by FMT. Fracture aperture variations promote in-fracture-plane fingering, with counter-current flow of water (downward) and air (upward). Fracture flow interacts with FMT in a complex fashion. For systems with a small fracture porosity (≤0.01%), our results suggest that intense, hour-long rainfall events can give rise to tens-of-meter-deep infiltration, depending on fracture/matrix properties and initial saturation of the fractured rock mass. Full article

(This article belongs to the Special Issue Fractured Reservoirs)

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12 pages, 4105 KiB

Open AccessFeature PaperArticle

Study of Rock Mass Rating (RMR) and Geological Strength Index (GSI) Correlations in Granite, Siltstone, Sandstone and Quartzite Rock Masses

by Gabor Somodi, Neil Bar, László Kovács, Marco Arrieta, Ákos Török and Balázs Vásárhelyi

Appl. Sci. 2021, 11(8), 3351; https://doi.org/10.3390/app11083351 - 08 Apr 2021

Cited by 13 | Viewed by 57726

Abstract

A comprehensive understanding of geological, structural geological, hydrogeological and geotechnical features of the host rock are essential for the design and performance evaluation of surface and underground excavations. The Hungarian National Radioactive Waste Repository (NRWR) at Bátaapáti is constructed in a fractured granitic [...] Read more.

A comprehensive understanding of geological, structural geological, hydrogeological and geotechnical features of the host rock are essential for the design and performance evaluation of surface and underground excavations. The Hungarian National Radioactive Waste Repository (NRWR) at Bátaapáti is constructed in a fractured granitic formation, and Telfer Gold Mine in Australia is excavated in stratified siltstones, sandstones and quartzites. This study highlights relationships between GSI chart ratings and calculated GSI values based on RMR rock mass classification data. The paper presents linear equations for estimating GSI from measured RMR₈₉ values. Correlations between a and b constants were analyzed for different rock types, at surface and subsurface settings. Full article

(This article belongs to the Special Issue Fractured Reservoirs)

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17 pages, 4431 KiB

Open AccessArticle

Comparison of DFN Modelled Microfracture Systems with Petrophysical Data in Excavation Damaged Zone

by Risto Kiuru, Dorka Király, Gergely Dabi and Lars Jacobsson

Appl. Sci. 2021, 11(7), 2899; https://doi.org/10.3390/app11072899 - 24 Mar 2021

Cited by 3 | Viewed by 1645

Abstract

Physical and petrographic properties of drill core specimens were determined as a part of investigations into excavation damage in the dedicated study area in the ONKALO^® research facility in Olkiluoto, Western Finland. Microfractures in 16 specimens from two drillholes were analysed and [...] Read more.

Physical and petrographic properties of drill core specimens were determined as a part of investigations into excavation damage in the dedicated study area in the ONKALO^® research facility in Olkiluoto, Western Finland. Microfractures in 16 specimens from two drillholes were analysed and used as a basis for fractal geometry-based discrete fracture network (DFN) modelling. It was concluded that the difference in resistivity between pegmatoid granite (PGR) and veined gneiss (VGN) specimens of similar porosity was likely due to differences in the types of microfractures. This hypothesis was confirmed from microfracture analysis and simulation: fractures in gneiss were short and mostly in one preferred orientation, whereas the fractures in granite were longer and had two preferred orientations. This may be due to microstructure differences of the rock types or could suggests that gneiss and granite may suffer different types of excavation damage. No dependencies on depth from the excavated surface were observed in the geometric parameters of the microfractures. This suggests that the excavation damaged zone cannot be identified based on the changes in the parameters of the microfracture networks, and that the disturbed layer observed by geophysical methods may be caused by macro-scale fractures. Full article

(This article belongs to the Special Issue Fractured Reservoirs)

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33 pages, 2803 KiB

Open AccessArticle

Fracturing Fluids and Their Application in the Republic of Croatia

by Nediljka Gaurina-Međimurec, Vladislav Brkić, Matko Topolovec and Petar Mijić

Appl. Sci. 2021, 11(6), 2807; https://doi.org/10.3390/app11062807 - 21 Mar 2021

Cited by 16 | Viewed by 4699

Abstract

Hydraulic fracturing operations are performed to enhance well performance and to achieve economic success from improved production rates and the ultimate reserve recovery. To achieve these goals, fracturing fluid is pumped into the well at rates and pressures that result in the creation [...] Read more.

Hydraulic fracturing operations are performed to enhance well performance and to achieve economic success from improved production rates and the ultimate reserve recovery. To achieve these goals, fracturing fluid is pumped into the well at rates and pressures that result in the creation of a hydraulic fracture. Fracturing fluid selection presents the main requirement for the successful performance of hydraulic fracturing. The selected fracturing fluid should create a fracture with sufficient width and length for proppant placement and should carry the proppant from the surface to the created fracture. To accomplish all those demands, additives are added in fluids to adjust their properties. This paper describes the classification of fracturing fluids, additives for the adjustment of fluid properties and the requirements for fluid selection. Furthermore, laboratory tests of fracturing fluid, fracture stimulation design steps are presented in the paper, as well as a few examples of fracturing fluids used in Croatia with case studies and finally, hydraulic fracturing performance and post-frac well production results. The total gas production was increased by 43% and condensate production by 106% in selected wells including wellhead pressure, which allowed for a longer production well life. Full article

(This article belongs to the Special Issue Fractured Reservoirs)

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18 pages, 3515 KiB

Open AccessArticle

Simulation of Groundwater Flow in Fractured-Karst Aquifer with a Coupled Model in Maling Reservoir, China

by Jinbang Cai, Yue Su, Huan Shen and Yong Huang

Appl. Sci. 2021, 11(4), 1888; https://doi.org/10.3390/app11041888 - 21 Feb 2021

Cited by 2 | Viewed by 1803

Abstract

A coupled model has been developed to simulate groundwater flow in fractured karst systems according to the complex geological and karst hydrogeological conditions of the dam site, where a 3D mathematical model based on Boussinesq equation was used to describe the movement of [...] Read more.

A coupled model has been developed to simulate groundwater flow in fractured karst systems according to the complex geological and karst hydrogeological conditions of the dam site, where a 3D mathematical model based on Boussinesq equation was used to describe the movement of groundwater flow in fractured medium, and a 1D conduit model for karst medium. The model was solved with the continuous hydraulic heads at the common boundaries. The hydraulic conductivities of karst medium were determined by geometrical parameters and flux of pipes. Furthermore, the permeability parameters for fractured medium were calibrated by the measured and calculated groundwater levels. The calibrated model was employed to predict the variation of groundwater flow field and leakage from the karst pipes and underground powerhouse during the reservoir operation. The simulated results showed that the groundwater level of the powerhouse had decreased by about 2–5 m. The water level of conveyance pipeline had risen by 10–20 m, and the water level on both banks had risen by 15–25 m. The leakage of karst conduits for impervious failure was larger than that for normal seepage control. In addition, the leakage of the powerhouse was estimated to be about 1000–3000 m³/d, and the seepage control of karst pipes had little influence on the leakage of underground powerhouse. Full article

(This article belongs to the Special Issue Fractured Reservoirs)

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20 pages, 5777 KiB

Open AccessArticle

Localisation of Ancient Migration Pathways inside a Fractured Metamorphic Hydrocarbon Reservoir in South-East Hungary

by Tivadar M. Tóth, László Molnár, Sándor Körmös, Nóra Czirbus and Félix Schubert

Appl. Sci. 2020, 10(20), 7321; https://doi.org/10.3390/app10207321 - 19 Oct 2020

Cited by 3 | Viewed by 1576

Abstract

Numerous fractured hydrocarbon reservoirs exist in the metamorphic basement of the Pannonian Basin in Hungary. Many decades of experience in production have proven that these reservoirs are highly compartmentalised, resulting in a complex mosaic of permeable and impermeable domains situated next to each [...] Read more.

Numerous fractured hydrocarbon reservoirs exist in the metamorphic basement of the Pannonian Basin in Hungary. Many decades of experience in production have proven that these reservoirs are highly compartmentalised, resulting in a complex mosaic of permeable and impermeable domains situated next to each other. Consequently, in most fields, only a small amount of the total hydrocarbon reserve can be extracted. This paper aims to locate the potential migration pathways inside the most productive basement reservoir of the Pannonian Basin, using a multiscale approach. To achieve this, evaluation well-log data, DFN modelling and a composition analysis of fluid trapped in a vein-filling zeolite phase are combined. Data on a single well are presented as an example. The results of the three approaches indicate the presence of two highly fractured intervals separated by a barely fractured amphibolite. The two zones are probably part of the communicating fracture system inside the single metamorphic mass. The gas analysis further specifies the migrated fluids and indicates hydrocarbons of a composition similar to that of the recently produced oil. Consequently, we conclude that the two zones do not only form an ancient migration pathway but are also members of a more recent hydrocarbon system. Full article

(This article belongs to the Special Issue Fractured Reservoirs)

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25 pages, 9326 KiB

Open AccessArticle

Application of Unconventional Seismic Attributes and Unsupervised Machine Learning for the Identification of Fault and Fracture Network

by Umar Ashraf, Hucai Zhang, Aqsa Anees, Hassan Nasir Mangi, Muhammad Ali, Zaheen Ullah and Xiaonan Zhang

Appl. Sci. 2020, 10(11), 3864; https://doi.org/10.3390/app10113864 - 02 Jun 2020

Cited by 67 | Viewed by 5036

Abstract

The identification of small scale faults (SSFs) and fractures provides an improved understanding of geologic structural features and can be exploited for future drilling prospects. Conventional SSF and fracture characterization are challenging and time-consuming. Thus, the current study was conducted with the following [...] Read more.

The identification of small scale faults (SSFs) and fractures provides an improved understanding of geologic structural features and can be exploited for future drilling prospects. Conventional SSF and fracture characterization are challenging and time-consuming. Thus, the current study was conducted with the following aims: (a) to provide an effective way of utilizing the seismic data in the absence of image logs and cores for characterizing SSFs and fractures; (b) to present an unconventional way of data conditioning using geostatistical and structural filtering; (c) to provide an advanced workflow through multi-attributes, neural networks, and ant-colony optimization (ACO) for the recognition of fracture networks; and (d) to identify the fault and fracture orientation parameters within the study area. Initially, a steering cube was generated, and a dip-steered median filter (DSMF), a dip-steered diffusion filter (DSDF), and a fault enhancement filter (FEF) were applied to sharpen the discontinuities. Multiple structural attributes were applied and shortlisted, including dip and curvature attributes, filtered and unfiltered similarity attributes, thinned fault likelihood (TFL), fracture density, and fracture proximity. These shortlisted attributes were computed through unsupervised vector quantization (UVQ) neural networks. The results of the UVQ revealed the orientations, locations, and extensions of fractures in the study area. The ACO proved helpful in identifying the fracture parameters such as fracture length, dip angle, azimuth, and surface area. The adopted workflow also revealed a small scale fault which had an NNW–SSE orientation with minor heave and throw. The implemented workflow of structural interpretation is helpful for the field development of the study area and can be applied worldwide in carbonate, sand, coal, and shale gas fields. Full article

(This article belongs to the Special Issue Fractured Reservoirs)

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Review

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30 pages, 8054 KiB

Open AccessReview

Drilling Fluid and Cement Slurry Design for Naturally Fractured Reservoirs

by Nediljka Gaurina-Međimurec, Borivoje Pašić, Petar Mijić and Igor Medved

Appl. Sci. 2021, 11(2), 767; https://doi.org/10.3390/app11020767 - 14 Jan 2021

Cited by 17 | Viewed by 10215

Abstract

For years, drilling engineers have been faced with the challenge of drilling wells through naturally fractured reservoirs that are present around the world. During drilling, the pressure at the bottomhole of a well is frequently intentionally higher than formation pressure, which can result [...] Read more.

For years, drilling engineers have been faced with the challenge of drilling wells through naturally fractured reservoirs that are present around the world. During drilling, the pressure at the bottomhole of a well is frequently intentionally higher than formation pressure, which can result in the loss of mud in surrounding rocks. During well cementing, the bottomhole pressure is even higher than it is during drilling, because the cement slurry density is higher than the density of the mud. Therefore, if natural or induced fractures in the surrounding rocks are not plugged during drilling, the cement slurry can be lost to them, reducing their permeability which is undesirable in the case of a pay zone. To prevent the loss of circulation and the related consequences, it is necessary to apply good drilling and cementing practices and to use adequate methods and carefully selected materials for plugging the loss zones. The aim of this article is to give an overview of the preventive and corrective methods that can be applied in drilling and cementing through fractured zones as well as improvements in drilling and cementing technology to avoid lost circulation issues (e.g., aerated drilling fluid, casing while drilling, managed pressure drilling, expandable tubulars, lightweight cement slurries, etc.). Full article

(This article belongs to the Special Issue Fractured Reservoirs)

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