ABSTRACT: Due to low natural gas prices, low production rates, and increased development costs, many operators have shifted operations from shale gas to liquid-rich shale plays. One means to make shale gas plays more attractive is to enhance well production through stimulation optimization. In numerous previous works, the authors have highlighted the geomechanical causes and important parameters for hydraulic fracture optimization in naturally fractured shale plays. The authors have, for example, emphasized the impact that stress shadows, from multiple hydraulic fractures, has on increasing the resistance of natural fractures and weakness planes to shear stimulation. The authors have also shown the critical role that in-situ pressure and pressure changes have on natural fracture shear stimulation. In this paper, we present the results of a discrete element model numerical study of both hydraulic fracture spacing and hydraulic fracture timing in a fully hydro-mechanical coupled fashion. The pressure changes in the natural fracture system of an unconventional play, due to hydraulic fracturing, often follow a diffusion-type process, which means the pressure changes are time dependent. As shown in previous works of the authors and others, the time-dependent changes in the in-situ pressure can have a marked impact on shear stimulation. The study performed quantitatively looked at the impact of hydraulic fracture spacing as a function of in-situ pressure change and time for key parameters such as the in-situ stress ratio, natural fracture characteristics, and natural fracture mechanical properties. The results of the study help improve the understanding of in-situ pressure and hydraulic fracture timing on stimulation optimization and enhanced hydrocarbon production. The study also provides a means to optimize hydraulic fracture spacing and increase shear stimulation for unconventional wells.
ABSTRACT: Fracture set classification is an important task in the areas of mining, hydrocarbon industry, coal-bed methane production, carbon capture and storage (CCS), geothermal energy, nuclear waste storage. Traditional classification methods are supported by visual inspection of stereonets, which is subjective and sometimes error-prone. Recently several clustering techniques have been developed using data analysis methods; however, one important parameter that must be defined in order to perform clustering algorithms is the selection of the number of clusters present in the analyzed data. This parameter is defined subjectively by an expert or by forcing a search for a limited number of clusters (e.g. from 2 to n). For both, the strategy consists in performing a finite number of clustering runs (each with different number of clusters) and choosing the one with the best performance based on an index. In this paper, we propose a preprocessing method in order to choose the number of clusters and also to initialize clusters centers for the K-means and Fuzzy K-means algorithms. The method is based on calculating the poles density in the stereonet, and defining a threshold value in order to select the clusters prototypes. Additionally, a post process to estimate area of influence for candidates is performed in order to deal with large areas of high-density poles.
Huang, Yu-Hsiang (Institute of Nuclear Energy Research) | Chang, Chieh-Chun (Institute of Nuclear Energy Research) | Chiou, Yi-Fu (Institute of Nuclear Energy Research) | Tseng, Han-Hsiang (Institute of Nuclear Energy Research) | Chang, Shu-Jun (Institute of Nuclear Energy Research)
ABSTRACT: Spent nuclear fuel final disposal is a crucial issue for countries which use nuclear power for electricity generation. The purpose of publishing “The Technical Feasibility Assessment Report on Spent Nuclear Fuel Final Disposal (SNFD2017 Report)” is to demonstrate whether the SNFD in Taiwan is feasible. The aim of this study is to carry out the safety assessment works by applying the flow-related data generated from the hydrogeological model. The preliminary local data obtained from the test site at the offshore granitic islands in western Taiwan was adopted as the reference case practices to develop the technical feasibility of safety assessment. The scenario of canister failure due to corrosion which is most relevant to the geochemical and hydrogeological condition was selected in this work for demonstration. The hydro-DFN modeling software DarcyTools and radionuclide transport calculation software GoldSim were adopted in this study, and the variant sets for hydrogeological property of model were carried out to evaluate the uncertainties caused due to data and modeling simplification. This study demonstrated a preliminary safety assessment work by applied flow-related data generated by hydrogeological DFN model for spent nuclear final disposal in Taiwan.
As computing power increases, numerical modeling of large-scale engineering problems of a discrete nature becomes more feasible. While there are many applications where a fractured rock mass can be represented as a continuum with equivalent properties, there are applications in which explicit representation of a discrete fracture network (DFN) is key to correct assessment of rock mass response. Engineering of unconventional resources is an application area where natural fractures and computational methods that can explicitly represent DFNs is gaining attention. Gas extraction from tight shale formations is often controlled by the interaction of hydraulic fractures with the preexisting DFN (Walton and McLennan, 2013). In Enhanced Geothermal Reservoirs (EGS), success of the stimulation and production phases depends on understanding the role and response of fractures to cold water injection. In both applications, fracture heterogeneity and flow channeling is key to reservoir performance.
That is, how much fluid can be injected or extracted from the reservoir without triggering the mechanical failure of the reservoir and surrounding rocks. In an enhanced geothermal system, such as Horstberg in North German Basin (NGB), an induced hydraulic fracture is created by massive water injection to enhance the wellbore productivity and to increase the surface area of heat transfer. A safe operation in Horstberg geothermal system requires that induced hydraulic fracture and surrounding faults mechanically stay stable. Particularly, the further extension of the induced hydraulic fracture and reactivation of the preexisting faults should be prevented. On one hand, fault mechanics and stability of the faults are traditionally described by Mohr-Coulomb theory where a frictional instability along a preexisting fault surface may occur (Ellsworth, 2013) and on the other hand, the onset of fracture extension is described by fracture mechanics and a critical stress intensity factor which often regarded as a material property, fracture toughness (Anderson, 1991).
ABSTRACT: We developed a coupled thermal-hydraulic-mechanical-chemical (THMC) simulator, iPSACC (interface for Pressure Solution Analysis under Coupled Conditions) that can consider the coupled processes including change of physical properties of the rocks due to the cracking by incorporating elastic damage theory. Especially, modeling the relationships between geochemical reactions such as pressure solution within rock fracture and rock damage is most important characteristic of this simulator. By using the developed simulator, long-term prediction of rock permeability was conducted by assuming subsurface environment near the radioactive waste repository. The predictions show that in EDZ many fractures occur near the disposal cavity and permeability increases in damaged zone during the excavation, and after excavation the permeability of the damaged zone decreases with time due to pressure solution at contacting asperities within fractures. It is concluded that pressure solution within the fractures has significant impact on the change of the permeability in EDZ area by cavity excavation.
Current reservoir modeling strategies attempt to characterize the discrete fracture network (DFN) around producing wellbores to better predict both short-and long-term production levels and estimated ultimate recovery. A variety of data sources are used in describing the DFN, including image logs, petrophysical logs, geologically mapped fractures in the region (when available), and regional stress information. For hydraulic fracture stimulations, there is also microseismic data recorded during the stimulation of some wells. The event distribution obtained through microseismic monitoring gives a sense of where fracturing is occurring and how the stimulation progresses from the treatment zone into the reservoir. By using a multi-array distribution of sensors, seismic moment tensor inversion (SMTI) analysis may be performed for microseismic data, providing direct evidence of the DFN stimulated during completion activities. By performing this advanced analysis, a microseismic dataset includes the location, size, and orientation of stimulated fractures, allowing for detailed characterization of the DFN. This paper describes a methodology for characterizing a DFN observed through microseismic monitoring, which is illustrated by application to an example dataset from a North American shale play. By examining relationships between fractures and extracting statistical trends from the distribution of fractures, we arrive at a useful multifaceted description of the DFN which provides improved input data for reservoir modeling and allows a better understanding of the changes in the reservoir due to stimulation.
ABSTRACT: In open pit mines, repetitive blast-induced ground vibration can increase the risk of pit wall instability due to strain accumulation along discontinuities. Presence of discrete fractures or fracture networks in a rock mass can influence the propagation of blast-induced shock waves in the rock and consequently the degradation of shear strength of the jointed rock mass. Quantification of blast-induced rock mass degradation is essential for prediction of potential risk of pit slope failure. This paper presents the results of a series of numerical experiments that examine the effects of ground vibration from a single row blast on a jointed rock mass, simulated using the Particle Flow Code (PFC2D). Discrete Fracture Network (DFN) was used to generate two joint sets in a rock block of 4 m x 8 m. Two different scenarios were considered: a) two orthogonal joint sets (one horizontal and one vertical set), b) two inclined joint sets. For each joint orientation scenario, five different fracture intensities (P21) were generated, varying between 0.4 and 5 m-1. Recorded blast vibration history from a quarry was applied to the 2D jointed rock mass samples and wave propagation was monitored along the rock blocks. Results show that the first fractures along the wave propagation path have experienced more degradation (damage) in the form of micro-crack generation along the fractures. Damage was only developed along the horizontal joint set for the rock mass model with orthogonal sets, whereas in the rock mass with two inclined joint sets, damage was accumulated on both joint sets. Rock mass degradation increases as the fracture intensity increase.
Chuang, Po-Yu (Sinotech Engineering Consultants, Inc.) | Huang, Yin-Chung (Sinotech Engineering Consultants, Inc.) | Ke, Chien-Chung (Sinotech Engineering Consultants, Inc.) | Teng, Mao-Hua (National Taiwan University) | Chia, Yeeping (National Taiwan University) | Chiu, Yung-Chia (National Taiwan Ocean University)
ABSTRACT: A novel approach using nanoscale zero-valent iron (nZVI) as a tracer was developed for detecting fracture flow paths directly. This approach was examined at a hydrogeological research station in central Taiwan. Heat-pulse flowmeter tests were performed to delineate the vertical distribution of permeable fractures in two boreholes, providing the design basis of tracer test. A magnet array was placed in the observation well to attract arriving nZVI particles for identifying the location of incoming tracer. Then, the nZVI slurry was released in the injection well. The arrival of the slurry in the observation well was detected by an increase in electrical conductivity. The position where the maximum weight of attracted nZVI particles coincides with the depth of a permeable fracture zone delineated by the heat-pulse flowmeter. In addition, a saline tracer test produced comparable results with the nZVI tracer test. Numerical simulation was performed using multi-porosity approaches to estimate the hydraulic properties of the connected fracture zones between the two wells. The study results indicate that the nZVI particle could be a promising tracer for the characterization of flow paths in fractured rock.
However, successful hydraulic stimulation treatments can be challenging to implement, and require considerable forethought. Compositional variation, rock fabric, geomechanical stratigraphy, and natural fracture systems all interact to influence and complicate hydraulic fracture treatments in shale reservoirs (Gale et al., 2006; Passey et al., 2010). Previously published work has highlighted the interaction between natural and induced fractures in the Horn River Basin (Dunphy and Campagna, 2011). This indicates that effective well completions require the efficient utilization of natural fracture systems to enhance permeability and drainage volume. Since natural fracture systems are a significant factor controlling the response of shale reservoirs to hydraulic fracturing, it is essential to identify and understand the key parameters of natural fracture networks that influence the effectiveness of hydraulic fracturing treatments. This paper combines results from natural fracture network characterization with discrete fracture network (DFN) modelling to identify the key parameters that influence hydraulic fracture geometry in the Horn River Basin.