scholarly journals The Role of In Situ Stress in Organizing Flow Pathways in Natural Fracture Networks at the Percolation Threshold

Geofluids ◽  
2019 ◽  
Vol 2019 ◽  
pp. 1-14 ◽  
Author(s):  
Chuanyin Jiang ◽  
Xiaoguang Wang ◽  
Zhixue Sun ◽  
Qinghua Lei
Keyword(s):  
Fluid Flow ◽  
Percolation Threshold ◽  
Fractured Reservoirs ◽  
Fracture Network ◽  
In Situ Stress ◽  
Natural Fracture ◽  
Fracture Aperture ◽  
Dilation Effect ◽  
Stress Dependent

We investigated the effect of in situ stresses on fluid flow in a natural fracture network. The fracture network model is based on an actual critically connected (i.e., close to the percolation threshold) fracture pattern mapped from a field outcrop. We derive stress-dependent fracture aperture fields using a hybrid finite-discrete element method. We analyze the changes of aperture distribution and fluid flow field with variations of in situ stress orientation and magnitude. Our simulations show that an isotropic stress loading tends to reduce fracture apertures and suppress fluid flow, resulting in a decrease of equivalent permeability of the fractured rock. Anisotropic stresses may cause a significant amount of sliding of fracture walls accompanied with shear-induced dilation along some preferentially oriented fractures, resulting in enhanced flow heterogeneity and channelization. When the differential stress is further elevated, fracture propagation becomes prevailing and creates some new flow paths via linking preexisting natural fractures, which attempts to increase the bulk permeability but attenuates the flow channelization. Comparing to the shear-induced dilation effect, it appears that the propagation of new cracks leads to a more prominent permeability enhancement for the natural fracture system. The results have particularly important implications for predicting the hydraulic responses of fractured rocks to in situ stress fields and may provide useful guidance for the strategy design of geofluid production from naturally fractured reservoirs.

scholarly journals Influence of initial aperture field under varied in-situ stress conditions on incipient karst formation in carbonate rocks

2020 ◽  
Author(s):  
Xiaoguang Wang ◽  
Mohammed Aliouache ◽  
Qinghua Lei ◽  
Hervé Jourde
Keyword(s):  
Fractured Rocks ◽  
Reactive Transport ◽  
Flow Direction ◽  
Onset Time ◽  
Fracture Network ◽  
In Situ Stress ◽  
Stress Conditions ◽  
Natural Fracture ◽  
Stress Dependent

<p>We use numerical simulations to investigate the role of initial aperture heterogeneity under varied in-situ stress loadings in the early-time karstification in an anisotropic natural fracture network. We found that the importance of the stress-dependent initial aperture effect on karstification depends on the relative relationship between the flow direction and structural hierarchy/anisotropy of the fracture network. When the flow occurs in the direction of the dominant fracture set with more through-going discontinuities, karst conduits only develop locally along a few large fractures with a preferential orientation for frictional sliding under the differential stress due to enhanced transmissivity caused by the important shear-induced dilation. In contrast, when flow is in the direction transverse to the dominant fracture set, the far-field stress loading has a negligible impact on the emergent dissolution pattern while only somewhat impact on the onset time of breakthrough. In this case, the developed conduits are much more tortuous with numerous branches. In both cases, the presence of initial aperture variability enhances the stress effects and significantly changes the dissolution pattern and delays the breakthrough time. Our results demonstrate that the flow heterogeneity induced by geometrical complexities and in-situ stress conditions seems to play an essential role in the karstification processes in fractured rocks.</p><p>The proposed reactive transport model based on realistic fracture networks may be used to investigate the spatial relationship between tectonic structures and karst cavities. Our results demonstrate that the heterogeneity induced by geometrical complexities and in-situ stress conditions may play a decisive role in the karstification processes in fractured rocks. Thus, they must be properly considered in reactive transport simulations to make reliable designs for practical engineering applications.</p><p><strong>Keywords</strong>: discrete fracture network, karst, network topology, reactive flow, in-situ stress</p>

Modelling large-scale fractured reservoirs efficiently for geothermal energy and groundwater flow

2020 ◽  
Author(s):  
Simon Oldfield ◽  
Mikael Lüthje ◽  
Michael Welch ◽  
Florian Smit
Keyword(s):  
Fluid Flow ◽  
Geothermal Energy ◽  
Large Scale ◽  
Fractured Reservoirs ◽  
Fracture Network ◽  
Natural Fracture ◽  
Fracture Networks ◽  
Persistent Problem ◽  
Fracture Evolution ◽  
Rapid Generation

<p>Large scale modelling of fractured reservoirs is a persistent problem in representing fluid flow in the subsurface. Considering a geothermal energy prospect beneath the Drenthe Aa area, we demonstrate application of a recently developed approach to efficiently predict fracture network geometry across an area of several square kilometres.</p><p>Using a strain based method to mechanically model fracture nucleation and propagation, we generate a discretely modelled fracture network consisting of individual failure planes, opening parallel and perpendicular to the orientation of maximum and minimum strain. Fracture orientation, length and interactions vary following expected trends, forming a connected fracture network featuring population statistics and size distributions comparable to outcrop examples.</p><p>Modelled fracture networks appear visually similar to natural fracture networks with spatial variation in fracture clustering and the dominance of major and minor fracture trends.</p><p>Using a network topology approach, we demonstrate that the predicted fracture network shares greater geometric similarity with natural networks. Considering fluid flow through the model, we demonstrate that hydraulic conductivity and flow anisotropy are strongly dependent on the geometric connection of fracture sets.</p><p>Modelling fracture evolution mechanically allows improved representation of geometric aspects of fracture networks to which fluid flow is particularly sensitive. This method enables rapid generation of discretely modelled fractures over large areas and extraction of suitable summary statistics for reservoir simulation. Visual similarity of the output models improves our ability to compare between our model and natural analogues to consider model validation.</p>

Quantitative Multiparameter Prediction of Fractured Tight Sandstone Reservoirs: A Case Study of the Yanchang Formation of the Ordos Basin, Central China

SPE Journal ◽  
2021 ◽  
pp. 1-32
Author(s):  
Jingshou Liu ◽  
Wenlong Ding ◽  
Haimeng Yang ◽  
Yang Liu
Keyword(s):  
Fractured Reservoirs ◽  
In Situ Stress ◽  
Stress Sensitivity ◽  
Central China ◽  
Fracture Aperture ◽  
Sensitivity Index ◽  
Tight Sandstone ◽  
Porosity And Permeability ◽  
Sandstone Reservoirs

Summary Fractured reservoirs account for more than one-half of the global oil and gas output and thus play a pivotal role in the world’s energy structure. Under diagenesis, rocks become dense, and tectonic fractures easily form under subsequent tectonic movement. These tectonic fractures are the main seepage conduits of tight sandstone reservoirs and are important determinants of whether a tight sandstone reservoir can have high, stable oil and gas production. The influence of multistage tectonic movement has led to well-developed fractures in the Ordos Basin in central China. In the process of reservoir development, the effective stress on the fracture surface increases because of the decrease in pore pressure, and the fracture aperture, porosity, and permeability also change accordingly. Therefore, modeling of the dual porosity and dual permeability of fractured reservoirs requires a dynamic 4D modeling process related to time. In this paper, we propose a 4D modeling method of dual porosity and dual permeability in fractured tight sandstone reservoirs. First, the porosity and permeability distribution of the reservoir matrix are established based on reservoir modeling. Based on geomechanical modeling, the density and occurrence of natural fractures are predicted by the paleostress field. The in-situ stress field is used to analyze the fracture aperture, and the variation in the fracture aperture during the development process is analyzed along with the variation in the in-situ stress in the development process to realize 4D modeling of the porosity and permeability of fractured reservoirs. The total porosity of the fracture is 0 to 8 × 10−3%, and the principal value of the planar permeability of the fracture is 0 to 3 × 10−3 µm2; the principal value of the fracture permeability is concentrated in the direction of 65 to 70° east-northeast. The simulated fracture porosity stress sensitivity index is distributed between 0 and 0.2, and the fracture permeability stress sensitivity index is distributed between 0 and 0.4. The Young’s modulus of the rock, in-situ stress parameters, and sound velocity in the rock are important factors affecting the fracture stress sensitivity.

scholarly journals A new methodology to train fracture network simulation using Multiple Point Statistic

2018 ◽  
Author(s):  
Pierre-Olivier Bruna ◽  
Julien Straubhaar ◽  
Rahul Pranhakaran ◽  
Giovanni Bertotti ◽  
Kevin Bisdom ◽  
...  
Keyword(s):  
Fluid Flow ◽  
Fracture Network ◽  
Natural Fracture ◽  
Multiple Point ◽  
Fracture Aperture ◽  
Strong Impact ◽  
Data Sampling ◽  
Natural Fractures ◽  
Training Images ◽  
Network Geometry

Abstract. Natural fractures have a strong impact on flow and storage properties of reservoirs. Their distribution in the subsurface is largely unknown mainly due to their sub-seismic scale and to the scarcity of available data sampling them (borehole). Outcrop can be considered as analogues where natural fracture characteristics can be extracted. However, acquiring fracture data on outcrops may produce a large amount of information that needs to be processed and efficiently interpreted to capture the key parameters defining fracture network geometry. Outcrops thus become a natural laboratory where the interpreted fracture network can be tested mechanically (fracture aperture, distribution of strain/stress) and dynamically (fluid flow simulations (Bisdom et al., 2017). The goal of this paper is to propose the multiple point statistics (MPS) method as a new tool to quickly predict the geometry of a fracture network in both surface and subsurface conditions. This sequential simulation method is based on the creation of small and synthetic training images representing fracture distribution parameters observe in the field. These training images represent the complexity of the geological object or processes to be simulated and can be simply designed by the user. In this paper we chose to use multiple training images and a probability map to represent the fracture network geometry and its potential variability in a non-stationary manner. The method was tested on a fracture pavement (2D flat surface) acquired using a drone in the Apodi area in Brazil. Fractures were traced manually on images of the outcrop and constitute the reference on which the fracture network simulations will be based. A sensitivity analysis emphasizing the influence of the conditioning data, the simulation parameters and the used training images was conducted on the obtained simulations. Stress-induced fracture aperture calculations were performed on the best realisations and on the original outcrop fracture interpretation to qualitatively evaluate the accuracy of our simulations. The method proposed here is innovative and adaptable. It can be used on any type of rocks containing natural fractures in any kind of tectonic context. This workflow can also be applied to the subsurface to predict the fracture arrangement and its fluid flow efficiency in water, heat or hydrocarbon reservoirs.

Characterization of Critically Stressed Fractures Using Fluid-Flow Models for Naturally Fractured Reservoirs

2021 ◽  
Author(s):  
Osman H. Hamid ◽  
Reza Sanee ◽  
Gbenga Folorunso Oluyemi
Keyword(s):  
Fluid Flow ◽  
Stress Analysis ◽  
Critical Stress ◽  
Fractured Reservoirs ◽  
In Situ Stress ◽  
Fractured Reservoir ◽  
Stress Regime ◽  
Flow Paths ◽  
Naturally Fractured

Abstract Fracture characterization, including permeability and deformation due to fluid flow, plays an essential role in hydrocarbon production during the development of naturally fractured reservoirs. The conventional way of characterization of the fracture is experimental, and modeling approaches. In this study, a conceptual model will be developed based on the structural style to study the fracture distributions, the influence of the fluid flow and geomechanics in the fracture conductivity, investigate the stress regime in the study area. Understanding the fracture properties will be conducted by studying the fracture properties from the core sample, image log interpretation. 3D geomechanical models will be constructed to evaluate the fluid flow properties; the models consider the crossflow coefficient and the compression coefficient. According to the model results, the fracture permeability decreases with increasing effective stress. The degree of decline is related to the crossflow coefficient and the compression coefficient. Most of these reservoirs are mainly composed of two porosity systems for fluid flow: the matrix component and fractures. Therefore, fluid flow path distribution within a naturally fractured reservoir depends on several features related to the rock matrix and fracture systems' properties. The main element that could help us identify the fluid flow paths is the critical stress analysis, which considers the in-situ stress regime model (in terms of magnitude and direction) and the spatial distributions of natural fractures fluid flow path. The critical stress requires calculating the normal and shear stress in each fracture plane to evaluate the conditions for critical and non-critical fractures. Based on this classification, some fractures can dominate the fluid-flow paths. To perform the critical stress analysis, fracture characterization and stress analysis were described using a 3D stress tensor model capturing the in-situ stress direction and magnitude applied to a discrete fracture model, identifying the fluid flow paths along the fractured reservoir. The results show that in-situ stress rotation observed in the breakouts or drilling induce tensile fractures (DITFs) interpreted from borehole images. The stress regime changes are probably attributed to some influence of deeply seated faults under the studied sequence. the flow of water-oil ratio through intact rock and fractures with/without imbibition was modeled based on the material balance based on preset conceptual reservoir parameters to investigate the water-oil ratio flow gradients

Defining structural permeability in Australian sedimentary basins

2015 ◽  
Vol 55 (1) ◽  
pp. 119 ◽  
Author(s):  
Adam Bailey ◽  
Rosalind King ◽  
Simon Holford ◽  
Joshua Sage ◽  
Martin Hand ◽  
...  
Keyword(s):  
Regional Scale ◽  
Sedimentary Basins ◽  
In Situ Stress ◽  
Natural Fracture ◽  
Enhanced Geothermal Systems ◽  
Structural Development ◽  
Geothermal Systems ◽  
Stress Regime ◽  
Orientation Data

Declining conventional hydrocarbon reserves have triggered exploration towards unconventional energy, such as CSG, shale gas and enhanced geothermal systems. Unconventional play viability is often heavily dependent on the presence of secondary permeability in the form of interconnected natural fracture networks that commonly exert a prime control over permeability due to low primary permeabiliy of in situ rock units. Structural permeability in the Northern Perth, SA Otway, and Northern Carnarvon basins is characterised using an integrated geophysical and geological approach combining wellbore logs, seismic attribute analysis and detailed structural geology. Integration of these methods allows for the identification of faults and fractures across a range of scales (millimetre to kilometre), providing crucial permeability information. New stress orientation data is also interpreted, allowing for stress-based predictions of fracture reactivation. Otway Basin core shows open fractures are rarer than image logs indicate; this is due to the presence of fracture-filling siderite, an electrically conductive cement that may cause fractures to appear hydraulically conductive in image logs. Although the majority of fractures detected are favourably oriented for reactivation under in situ stresses, fracture fill primarily controls which fractures are open, demonstrating that lithological data is often essential for understanding potential structural permeability networks. The Carnarvon Basin is shown to host distinct variations in fracture orientation attributable to the in situ stress regime, regional tectonic development and local structure. A detailed understanding of the structural development, from regional-scale (hundreds of kilometres) down to local-scale (kilometres), is demonstrated to be of importance when attempting to understand structural permeability.

scholarly journals Artificial Interference of Stress Field in a Near-Fracture show="" $6#?=""Zone by Water Injection in a Preexisting Crack

2019 ◽  
Vol 2019 ◽  
pp. 1-15 ◽  
Author(s):  
Yongxiang Zheng ◽  
Jianjun Liu ◽  
Bohu Zhang
Keyword(s):  
Oil And Gas ◽  
Fractured Reservoirs ◽  
Water Injection ◽  
Fluid Pressure ◽  
In Situ Stress ◽  
Stress Intervention ◽  
Oil And Gas Resources ◽  
Favorable State ◽  
Artificial Stress

The in situ stress has an important influence on fracture propagation and fault stability in deep formation. However, the development of oil and gas resources can only be determined according to the existing state of in situ stress in most cases. It is passive acceptance of existing in situ stress. Unfortunately, in some cases, the in situ stress conditions are not conducive to resource development. If the in situ stress can be interfered in some ways, the stress can be adjusted to a more favorable state. In order to explore the method of artificial interference, this paper established the calculation method of the in situ stress around the cracks based on fracture mechanics at first and obtained the redistribution law of the in situ stress. Based on the obtained redistribution law, attempts were made to interfere with the surrounding in situ stress by water injection in the preexisting crack. On this basis, the artificial stress intervention was applied. The results show that artificial interference of stress can effectively be achieved by water injection in the fracture. And changing the fluid pressure in the crack is the most effective way. By stress artificial intervention, critical pressure for water channelling in fractured reservoirs, directional propagation of cracks in hydraulic fracturing, and stress adjustment on the structural plane were applied. This study provides guidance for artificial stress intervention in the exploitation of the underground resource.

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