Scaling Geological Fracture Network from a Micro to a Macro Scale

Characterizing fracture systems at various scales, modeling fracture distributions, and clarifying scale relations that correlate total fracture systems are of paramount importance in geology, mining, civil engineering, and petroleum engineering. In this paper, the conditions of fracture network geometry are investigated in a field scale (about 100 m) and a core sample scale (several centimeters). To achieve this purpose, field surveys and coring of rock outcrops were performed in the Asmari Formation of Iran. Fractures were manually sampled from rock outcrops on the field scale while micro-fractures were surveyed using CT-scan images of core samples on a small scale. To compare the fracture network geometry, two perspectives of fractal dimensions and orientation of fractures were used. The results showed that the fractal dimension has the same value in both field and core scales and the orientation of the fractures is similar in both scales. Therefore, it can be claimed that in the Asmari Formation of Iran the structure of the fracture network is similar in two studied scales

Geomechanical key parameters of the process of hydraulic fracturing propagation in fractured medium

International audienceHydraulic Fracturing (HF) is a well-stimulation technique that creates fractures in rock formations through the injection of hydraulically pressurized fluid. Because of the interaction between HF and Natural Fractures (NFs), this process in fractured reservoirs is different from conventional reservoirs. This paper focuses mainly on three effects including anisotropy in the reservoir, strength parameters of discontinuities, and fracture density on HF propagation process using a numerical simulation of Discrete Element Method (DEM). To achieve this aim, a comprehensive study was performed with considering different situations of in situ stress, the presence of a joint set, and different fracture network density in numerical models. The analysis results showed that these factors play a crucial role in HF propagation process. It also was indicated that HF propagation path is not always along the maximum principal stress direction. The results of the numerical models displayed that the affected area under HF treatment is decreased with increasing the strength parameters of natural fracture and decreasing fracture intensity

PREGLED PRIMJENE KAISEROVA EFEKTA U MJERENJU IN SITU NAPREZANJA U STIJENAMA

Knowledge of in-situ rock stress is one of the significant issues in many engineering problems. There are various methods for determining in-situ stress. Most of the common methods used for the determination of in-situ stress are time and cost consuming, and in many cases need specific accessibility. Therefore, attention to core-based methods is increasing. One of these methods is the acoustic emission technique based on the Kaiser effect. This method is among the stress stressing-destressing methods and is based on observing rock behaviour without having any important impact on it. Knowledge of the acoustic emission principles and acoustic signal parameters is the first step to use the Kaiser effect method for in-situ stress determination. Also, using the Kaiser effect method requires knowledge on the mechanism and theory associated with the Kaiser effect. In this research, different methods for determining the Kaiser effect in parametric (tangent method and maximum slope, etc.) and signal processing (Fourier transform, wavelet transform, etc.) terms were reviewed. The results obtained from the Kaiser effect method were compared to other common methods used for in-situ stress measurement, like over-coring and hydraulic fracturing methods, and based on the results, there was a good agreement between them. Also, the effective parameters on determining the Kaiser effect stress point were investigated. The important parameters were the testing procedure, confining pressure, physical properties of rock, delay time and retention time, direction and amount of loading, anisotropy angle and loading rate.Poznavanje in situ naprezanja u stijenskoj masi jedan je od važnijih aspekata u mnogim inženjerskim problemima. Postoje različite metode za određivanje in situ naprezanja. Većina uobičajenih metoda koje se koriste za određivanje in situ naprezanja vremenski su i financijski zahtjevne te je u mnogim slučajevima potrebna posebna pristupačnost. Stoga se sve više pažnje posvećuje metodama temeljenim na ispitivanjima provedenim na jezgrama. Jedna je od tih metoda i metoda akustičke emisije koja se temelji na Kaiserovu efektu. Ubraja se u metode naprezanja postupcima opterećenja i rasterećenja, a temelji se na promatranju ponašanja stijena bez znatnoga utjecaja na nju. Poznavanje principa akustičke emisije i parametara akustičkoga signala prvi je korak u primjeni metode za određivanje in situ naprezanja na temelju Kaiserova efekta. Nadalje, primjena metode Kaiserova efekta zahtijeva razumijevanje mehanizma i teorije povezane s Kaiserovim efektom. U ovome istraživanju razmatrane su različite metode za određivanje Kaiserova efekta u okviru parametara (metoda tangente i maksimalni nagib itd.) i obrade signala (Fourierova transformacija, valićna transformacija i sl.). Pored navedenog, rezultati dobiveni metodom Kaiserova efekta uspoređeni su s drugim uobičajenim metodama koje se koriste za mjerenje in situ naprezanja, kao što su overcoring metoda i metoda hidrauličkoga frakturiranja te je na temelju rezultata ustanovljena znatna podudarnost između njih. Također su istraženi ključni parametri za određivanje točke naprezanja kod Kaiserova efekta. Ključni parametri bili su postupak ispitivanja, ograničavajući tlak, fizička svojstva stijene, vrijeme kašnjenja i vrijeme zadržavanja, smjer i količina opterećenja, kut anizotropije i brzina opterećenja

Tomografia pasywna pola prędkości i symulacje geostatystyczne w obrębie pola ścianowego

Generally, the accurate determination of the stress in surrounding rock mass of underground miningarea has an important role in stability and ground control. In this paper stress redistribution around thelongwall face has been studied using passive seismic velocity tomography based on Simultaneous IterativeReconstructive Technique (SIRT) and Sequential Gaussian Simulation (SGS). The mining-induced microseismicevents are used as a passive source. Since such sources are used, the ray coverage is insufficientand in order to resolve this deficiency, the wave velocity is estimated in a denser network and by the SGSmethod. Consequently the three-dimensional images of wave velocity are created and sliced into the coalseam. To analyze the variations of stress around the panel during the study period, these images are interpreted.Results show that the state of stress redistribution around the longwall panel can be deduced fromthese velocity images. In addition, movements of the stressed zones, including front and side abutmentsand the goaf area, along the longwall face are evident. The applied approach illustrated in this paper canbe used as a useful method to monitoring the stress changes around the longwall face continuously. Thiscan have significant safety implications and contribute to improvements in operational productivity

Determination of the Fractal Dimension of the Fracture Network System Using Image Processing Technique

Fractal dimension (FD) is a critical parameter in the characterization of a rock fracture network system. This parameter represents the distribution pattern of fractures in rock media. Moreover, it can be used for the modeling of fracture networks when the spatial distribution of fractures is described by the distribution of power law. The main objective of this research is to propose an automatic method to determine the rock mass FD in MATLAB using digital image processing techniques. This method not only accelerates analysis and reduces human error, but also eliminates the access limitation to a rock face. In the proposed method, the intensity of image brightness is corrected using the histogram equalization process and applying smoothing filters to the image followed by revealing the edges using the Canny edge detection algorithm. In the next step, FD is calculated in the program using the box-counting method, which is applied randomly to the pixels detected as fractures. This algorithm was implemented in different geological images to calculate their FDs. The FD of the images was determined using a simple Canny edge detection algorithm, a manual calculation method, and an indirect approach based on spectral decay rate. The results showed that the proposed method is a reliable and fast approach for calculating FD in fractured geological media

Discrete Element Simulation of Interaction between Hydraulic Fracturing and a Single Natural Fracture

Hydraulic fracturing (HF) treatment is performed to enhance the productivity in the fractured reservoirs. During this process, the interaction between HF and natural fracture (NF) plays a critical role by making it possible to predict fracture geometry and reservoir production. In this paper, interaction modes between HF and NF are simulated using the discrete element method (DEM) and effective parameters on the interaction mechanisms are investigated. The numerical results also are compared with different analytical methods and experimental results. The results showed that HF generally tends to cross the NF at an angle of more than 45° and a moderate differential stress (greater than 5 MPa), and the opening mode is dominated at an angle of fewer than 45°. Two effects of changing in the interaction mode and NF opening were also found by changing the strength parameters of NF. Interaction mode was changed by increasing the friction coefficient, while by increasing the cohesion of NF it was less opened under a constant injection pressure

Shield Support Selection Based on Geometric Characteristics of Coal Seam

The most initial investment in longwall face equipping is the cost of powered support. Selection of proper shields for powered supports is based on load, geometric characterization of coal seams and economical considerations