Volumetric Fracture Intensity Research Articles

Curvature analysis and its correlation with faults and fractures in presalt carbonates, Santos Basin, Brazil

Presalt lacustrine carbonate reservoirs in the Santos Basin, Brazil, can have a high degree of fracturing. Properly characterizing such properties is important due to their influence on the porosity–permeability system. It may play a key role in the so-called excess permeability features and the flow behaviour, especially in the Aptian Alagoas Stage. Therefore, 3D characterization of the fracture system improves the reservoir model. For this purpose, correctly identifying the fault and fracture zones in seismic data is crucial. This task is generally performed manually and guided by amplitude interpretation or geometric seismic attributes. However, due to the seismic resolution of the pre-salt seismic datasets, many faults and fractured zones are not identified in the seismic amplitude domain or by standard geometric attributes such as coherence. Improving the resolution of seismic data and its geometric attributes can positively impact the delineation of fractured zones, structural lineaments, and their connectivity. Curvature (k) attributes have a high potential to reveal vertical to subvertical subseismic-scale structures, which is the main type of strain generated from the tectonic style in the Pre-salt rocks. Among the curvature attributes, the most-positive (k1) and the most-negative (k2) attributes are some of the most commonly used. The present study evaluates the use of seismic inverse Q-filter and three different dip estimation techniques based on Fast Fourier Transform (FFT), Principal Component Analysis (PCA), and Phase Gradient (PG). It also adds two postfiltering techniques (median and recursive Gaussian filters) to obtain a consistent most-positive curvature. We then analyse two analytical models that correlate the volumetric fracture intensity, measured on Borehole Image Logs (BHI), with the curvature. Then, we perform a comprehensive study to understand the role of seismic-scale strain (translated into curvature) and mechanical stratigraphy in the development of fractures in the presalt, where we address that in addition to seismic-scale deformation, the contents of silica, clay, and organic matter are also relevant variables. These results can help to guide future studies of fracture characterization to prospect naturally fractured reservoirs or to estimate fracture intensity for dual-porosity reservoir modelling on the Santos presalt carbonates.

Discrete Fracture Network (DFN) Analysis to Quantify the Reliability of Borehole-Derived Volumetric Fracture Intensity

Volumetric fracture intensity (P32) is a parameter that plays a major role in the mechanical and hydraulic behaviour of rock masses. While methods such as Ground Penetrating Radar (GPR) are available to map the 3D geometrical characteristics of the fractures, the direct measurement of P32 at a resolution compatible with geotechnical applications is not yet possible. As a result, P32 can be estimated from the borehole and surface data using either simulation or analytical solutions. In this paper, we use Discrete Fracture Network (DFN) models to address the problem of estimating P32 using information from boreholes (1D data). When calculating P32 based on Terzaghi Weighting, it is common practice to use drill run lengths and limit the minimum angle between the borehole and the intersected fractures. The analysis presented in this paper indicated that limiting the minimum angle of intersection would result in an underestimation of the calculated P32. Additionally, the size of the interval has a significant impact on the variability of the calculated P32. We propose a methodology to calculate P32 using variable lengths, depending on the angle between the fractures and the borehole. This methodology allows the capture of the spatial variation in intensity and simultaneously avoids artificially increasing or decreasing the intensity sampled along borehole intervals. Additionally, this work has addressed the impact of boundary effects in DFN models and proposes a methodology to mitigate them.

CO2 and heat energy transport by enhanced fracture permeability in the Monterotondo Marittimo-Sasso Pisano transfer fault system (Larderello Geothermal Field, Italy)

Carbon dioxide is one of the most important gasses naturally released from geothermal systems. Establishing the processes and pathways that regulate the CO2 diffuse degassing can provide valuable information for exploration and exploitation purposes of geothermal reservoirs. In this work, a high-resolution CO2 flux (with records up to 2927 g m–2 d–1) and soil temperature (with records up to 98.8 °C) survey was carried out along with detailed fracture parameters measurements in a selected area of the Monterotondo Marittimo-Sasso Pisano transfer fault (Larderello geothermal system, Tuscany, Italy). The main aim was to define the behavior of diffuse CO2 through the fault system and investigate how the soil CO2 flux and steam change with respect to the architecture of the fault damage zone (i.e., volumetric fracture intensity, permeability, and persistence of the fractures). The presence of multiple populations of CO2 flux suggested that three different transport mechanisms control soil degassing: i) purely diffusive, ii) mixed diffusive-advective, and iii) purely advective, characterized by efflux values of <20, between 20 and 300 and >300 g m–2 d–1, respectively. The spatial distribution of these fluxes well agrees with the fracture distribution and features of the Jurassic radiolarite (Diaspri Fm) dissected by NNE-striking faults. Areas with high CO2 emissions are indeed able to reveal major upflow areas from deep reservoirs through deep-reaching permeable fault zones. The interaction between pre-existing fractures and fracture-related fault-zone locally enhances the secondary rock permeability as highlighted by the correlation between Discrete Fracture Network (DFN) modeling and advective flux. Eventually, by normalizing the CO2 output to the fault strip (1350 m2), a release of CO2 equal to ∼155 t d–1 km–2 was estimated.

A hybrid predictive model of unstable rock blocks around a tunnel based on estimated volumetric fracture intensity and circular variance from borehole data sets

Several failure modes may occur around underground excavations, one of the most common being block failure. The size distribution and occurrence probabilities of unstable blocks depend on discontinuity parameters such as size, shape, intensity, and orientation. The knowledge about the size and the number of blocks provides critical information to optimize site characterization and design procedures, or at least to accomplish this more efficiently. In this work we introduce a new attribute, Circular Variance, to measure the degree of dispersion of the fracture poles in angular domain based on borehole data. In our study the final objective is to present a predictive model of the mean rate of the formation of unstable rock blocks and their uncertainty. To fulfill this goal, we introduce a hybrid approach based on the combination of probabilistic discrete fracture network modeling, rock block failure instability analysis and supervised vector machine concepts. The present research studies the impact of P32, circular variance, and mean radius of the fractures on the mean generation rate of unstable blocks in a tunnel at El Teniente mine, Chile. A two and three factors supervised Poisson regression models are compared against each other for the prediction of the mean generation rate of unstable blocks. The results showed a good agreement between the real number of unstable blocks and the estimated ones with supervised Poisson regression models.

Investigation of geometrical representative elementary volumes based on sampling directions and fracture sets

This paper reports that the geometrical representative elementary volume (REV) of fractured rock masses is anisotropic, and the geometric shape of an REV is of a complex, irregular geometry. A 3D fracture network model of 98 m × 80 m × 80 m was created using the field information collected from tunnel PD254 at the Maji dam site in China. One hundred random cubes of various sizes with the same sampling direction were selected to assess the volumetric fracture intensity (P32). Changing the sampling direction along the dip direction and dip angle at a 10° increment, the REV size of each sampling direction was inferred using the Wald–Wolfowitz runs test and P32 values. The REV sizes of entire rock masses and each fracture set were calculated, and the results indicated that the REV sizes varied with the different sampling directions. The anisotropy ratio and standard deviation were employed to quantify the degree of the anisotropy of REVs. Based on the results, the anisotropy of REVs for fracture set 2 is the most significant, and that of fracture set 3 is the worst. The relationship between the anisotropy of REVs and fracture sets was discussed; one fracture set, two fracture sets, and three fracture sets were selected randomly from these simulated rock masses to evaluate the characteristics of the anisotropy of REVs. The mean value and the anisotropy of REVs increased with the number of fracture sets. The maximum REV size with a sampling direction was selected as an REV for each fracture set. Four cubes with different directions and different sizes were assembled into an irregular geometry. This geometry can represent the geometric shape of the REV of entire rock masses.

A new and unified approach to improved scalability and volumetric fracture intensity quantification for GSI and rockmass strength and deformability estimation

Several investigators have attempted to quantify the Geological Strength Index (GSI) chart, with the latest modification of the chart (2013) utilizing RQD as a measure of blockiness. This approach has limitations where discontinuity spacing is wide (typically greater than 0.3 m) and RQD alone cannot adequately characterize the degree of blockiness, since it remains static at 100%. This paper introduces a new approach to quantifying widely spaced jointed rockmasses that is not dependent on RQD alone. At the core of the approach is a bias free volumetric fracture count (VFC) parameter (fractures/m3), that is integrated into the newly defined GSI chart as an aid to alleviate scalability and bias concerns related to the use of RQD in the quantification process. While the new GSI chart builds on the work of many, it is unique in the sense that not only is it fully quantifiable for a full range of block sizes, including block sizes much larger that possible with RQD alone, but it provides a unique approach linking the VFC parameter with P32, a parameter frequently used in DFN modelling. The correlation of VFC with P32 in particular, is possible because the VFC parameter has no constraints of a limited number and/or assumed orthogonality of discontinuity sets or rectilinear block shapes or the need for block shape correction factors. The new chart also includes correlated scales on both the vertical and horizontal axis using both the RMR and the Q-systems, providing a unified approach that is both scalable and easily quantifiable and allows for the use of all three major rock mass classification systems along with P32 within one chart, something not attempted before. Data from a dam foundation rockmass in the Lesotho Highlands are introduced and are used to validate the quantification process for the overall GSI ratings for the foundation rock mass. These ratings have been used to estimate strength and deformability parameters for the foundation rock mass using the Hoek-Brown empirical failure criteria equations and then they were compared to the large-scale in-situ test results to validate the use of the V-GSI chart and system as a new tool for use in rock engineering.

Computational framework for obtaining volumetric fracture intensity from 3D fracture network models using Delaunay triangulations

Fracture intensity, the degree of fracturing in rock masses, is one of the fundamental parameters used in characterizing rock mass as a fracture network. Among fracture intensity measures, volumetric fracture intensity (P32) is the most useful, since it directly reflects the degree of rock mass fracturing, independent of fracture orientation and size distribution. P32 represents the total area of fractures per cubic meter, and it can simultaneously reflect the spacing and size component of fractures. Because P32 cannot be directly measured in the field, previous works have developed various inference methods based on available 1D and 2D measures collected from field surveys. At present, discrete fracture network (DFN) models are deemed to be a suitable method for the characterization of fractured rock mass. Hence, calculating P32 from a DFN model is also a widely accepted approach. However, the geometry-based method cannot effectively address the irregular simulation space, and the methods employed are ill-suited if polygonal fractures appear in the DFN model. To solve these deficiencies, a computational framework based on the Delaunay triangulation of 3D uniformly distributed random points is introduced, and a detailed description is presented. The proposed computational framework is suitable for various types of DFN models, such as Baecher disk models, elliptical disk models and Dershowitz polygon models. A simulated example was generated to verify the proposed computational framework. Finally, a DFN model is built based on fracture data acquired from the Songta hydropower station, and the proposed computational framework was used to calculate P32 for the DFN model.

Determination of the geometrical REV based on fracture connectivity: a case study of an underground excavation at the Songta dam site, China

This paper presents a technique for estimating the geometrical representative elementary volume (REV) of a fractured rock mass based on fracture connectivity. A three-dimensional fracture network model, of dimensions 78 m × 50 m × 50 m, was generated and validated for a granite rock mass using joint data collected from an exploration tunnel at the Songta dam site in southwest China. Two hundred and forty cubic specimens with edge lengths ranging from 1 to 20 m were sampled from the model, and connectivity parameters of each specimen, including the number of intersections per unit volume (C 1) and the intersection length per unit volume (L 1), were calculated. The REV was determined using the connectivity parameters and a series of statistical tests. The results demonstrate that both the C 1- and L 1-based REVs are equal to 11 m × 11 m × 11 m. In this work, the volumetric fracture intensity (P32) was also used to calculate the REV for comparison. The REV obtained by P32 is 9 m × 9 m × 9 m, which is smaller than that calculated by C 1 and L 1. Finally, the geometrical REV for the rock mass by integrating C 1, L 1, and P32 is 11 m × 11 m × 11 m.

Characterization of Rock Fracturing for Vertical Boreability

Boreability can be defined as the ability of a bore to penetrate a rock mass. Understanding the factors influencing boreability is critical for enhanced project planning and reduce geotechnical risk in an offshore shaft boring environment. Large diameter drills are used for offshore shaft boring, which can be up to 7 m in diameter, and therefore more akin to tunnel boring machines due to the scale of the excavation and extent of ground interaction. With increases in bore diameter, there is a need to properly define and evaluate the effect of the degree of rock mass fracturing on machine performance for improved estimates of boreability. Discrete Fracture Network (DFN) simulation has been used as an innovative approach for stochastic realisation of rock mass fracturing by determination of the P32 volumetric fracture intensity in the context of boreability. P32 shows positive trend to specific penetration (SP), with maximum SP being achieved at moderate to high fracturing levels (20 - 25m-1). However, in this case, P32 shows a similar positive trend to P10, but with peak SP appearing at higher intensity levels. Increased RQD values result in reduced SP, with peak SP reached at moderate fracturing levels, similar to P10.

Estimation of REV Size for Fractured Rock Mass Based on Damage Coefficient

Estimation of representative elementary volume (REV) is significant to analyze fractured rock mass in the framework of continuum mechanics. Engineers can therefore simplify the analysis by using an equivalent rock block with an average property, and the influence of fractures can be neglected in finite element modelling. The indicators to determine the REV size based on the joint geometrical parameters include the volumetric fracture intensity (P 32) and the fracture tensor, but this type of calculation generally provides a lower bound evaluation. A novel conceptual framework of damage coefficient is introduced in this paper to consider the mechanical properties of fractures, such as joint aperture and roughness. A parametric study has been performed to establish the correlation between the proposed dimensionless damage coefficient and the traditional derived P 32 value. The effectiveness of the developed method is demonstrated by a case study, where a larger mechanical REV size is indeed calculated based on the damage coefficient.

Volumetric Fracture Intensity Measurement for Improved Rock Mass Characterisation and Fragmentation Assessment in Block Caving Operations

Recent discrete fracture network (DFN) related analysis of a number of block caving projects has demonstrated the role that the 3D volumetric fracture intensity measure (P32) plays on controlling a number of rock mass properties critical to caving operations. P32 represents the fracture area per unit volume and as such represents a non-directional intrinsic measure of the degree of rock mass fracturing, incorporating both a frequency measure and a fracture size component. Preliminary results suggest that the P32 intensity of a DFN model would strongly control the overall fragmentation of the rock mass. The implication would be that by taking the overall distribution of P32, the in situ fragmentation of a large rock mass volume could be determined in a computationally efficient way. With P32 also being shown to be one of the dominant controls on DFN derived directional stiffness measures, increasingly these DFN related work flows are being shown to be central to an improved rock mass characterisation process and ultimately the more accurate capturing of the caving process.

Estimating geometrical and mechanical REV based on synthetic rock mass models at Brunswick Mine

This paper uses a case study from Brunswick Mine in Canada to determine a representative elementary volume (REV) of a jointed rock mass in the vicinity of important underground infrastructure. The equivalent geometrical and mechanical property REV sizes were determined based on fracture systems modeling and numerical experiments on a synthetic rock mass. Structural data collected in massive sulphides were used to generate a large fracture system model (FSM), 40 m×40 m×40 m. This FSM was validated and subsequently sampled to procure 40 cubic specimens with a height to width ratio of 2 based on sample width from 0.05 to 10 m. The specimens were introduced into a 3D particle flow code (PFC3D) model to create synthetic rock mass (SRM) samples. The geometrical REV of the rock mass was determined based on the number of fractures in each sampled volume (P 30) and the volumetric fracture intensity (P 32) of the samples. The mechanical REV was estimated based on the uniaxial compressive strength (UCS) and elastic modulus ( E) of the synthetic rock mass samples. The REV size of the rock mass was determined based on a series of statistical tests. The T-test was used to assess whether the means of the samples were statistically different from each other and the F-test to compare the calculated variance. Finally, the coefficient of variation, for the synthetic rock mass geometrical and mechanical properties, was plotted against sample size. For this particular site the estimated geometrical REV size of the rock mass was 3.5 m×3.5 m×7 m, while the mechanical property REV size was 7 m×7 m×14 m. Consequently, for engineering purposes the largest volume (7 m×7 m×14 m) can be considered as the REV size for this rock mass.

Geo-descriptive Modeling of Water Conducting Features Characterized in Sedimentary Formations in Horonobe Area of Japan

AbstractA three-dimensional (3-D) discrete fracture network (DFN) geo-descriptive model is developed for water conducting features (WCFs) in the sedimentary formations of Horonobe underground research laboratory (URL) in Japan. Fracturing and faulting system in/around the URL area, which is the main investigation area of the Horonobe URL project, is characterized by taking into account borehole geophysical logging data, regional geologic/structural data, and fracture/fault data (orientation, intensity, size) obtained from the surface-based investigations. Volumetric fracture intensity potential is estimated by the correlation and the multi-linear regression analysis of observed data, and is used as one of controls for 3-D DFN model. A regional scale 3-D geo-descriptive DFN model is constructed based on the analyzed fracturing system identified for the WCFs. The current 3-D geo-descriptive model could be utilized explicitly to derive performance assessment (PA) parameters for the hypothetical repository of the high-level radioactive wastes in Japan, and to assist optimization of the safe repository design.

Evaluating rock mass geometry from photographic images

The importance of discontinuities in controlling rock mass behaviour in any engineering project involving excavations calls for a sound and spatial characterization of the discontinuity structure present. Such a characterization necessitates field work that requires sufficient rock exposures in order to obtain an adequate number of data, time and considerable cost. Photoanalysis techniques can help in overcoming the above difficulties equally well or better than other techniques. This paper refers to simple photographic techniques and their implementation for computer aided analyses for the characterization of the rock mass fracturing features. In particular attention is focused on scale problems and on reconstruction of fracture density stereoplots on the basis of data collected from one or two images according to different lithologies and outcropping conditions. The methodology for evaluating the volumetric fracture intensity follows in a slightly modified way a technique previously suggested in the literature. Certain reported examples allow to validate the photoanalytical technique used and the proposed method of analysis. Furthermore, analyses on planar density, spacing, frequency, terminations in solid rock or against other discontinuities and spatial correlation have been implemented in a software to yield a more complete rock mass characterization. At the same time input data and analysis results are produced in data files available as input for numerical analyses.