This paper briefly introduces a series of workflows for automated digital geological mapping based on 3D photogrammetry. The 3D photogrammetry technique can produce a detailed 3D digital model of the area of interest including point cloud data to characterize the geometric features of geological structures in the scene and image data to characterize the visual features in the scene. The proposed workflow uses double-nested mean-shift clustering and region growth to extract discontinuity surfaces from point clouds. Discontinuities traces can be extracted from image data using a hybrid global and local threshold method and integrating a series of image-processing algorithms. The trace’s corresponding coordinates are acquired by linking the pixel locations corresponding to the identified traces in an image to the 3D coordinates in the point cloud based on fusing the point cloud data and image data. Finally, geological analysis can be performed to accurately measure the spatial geometry characteristics of the mapped discontinuities.
Discontinuity mapping is a fundamental task for rock mass characterization (Barton et al., 1974; ISRM, 1978; Priest, 1993; Kulatilake and Wu, 1984; Mouldon, 1998; Zhang and Einstein, 1998; Li et al., 2014; Zhu et al., 2014). Rock discontinuities in outcrops can appear in the form of planar surfaces or embedded traces as shown in Fig. 1. Measuring or mapping the properties of rock discontinuities is difficult, time-consuming, and often dangerous when using traditional field mapping and hand-held direct measuring devices (Ferrero et al., 2009).
Several recent techniques enable the construction of 3D point clouds to rapidly obtain 3D geometric information and 2D image data to obtain 2D texture information about inaccessible terrain and rock exposures, such as photogrammetry (Roncella et al., 2004; Sturzenegger and Stead, 2009; Gigi and Casagli, 2011; Tannant 2015; Li et al., 2016). The virtual geometry and texture of a rock face represented by point cloud data and image data allow an engineer to extract discontinuity information on a computer with the aid of mathematical algorithms. However, an intractable technical bottleneck is that improvements in automated extraction of rock discontinuity parameters are needed to gain maximum value from these surface models (Vöge et al., 2013).
Renteria, Alondra (University of British Columbia) | Maleki, Amir (University of British Columbia) | Frigaard, Ian (University of British Columbia) | Lund, Bjornar (SINTEF Industry) | Taghipour, Ali (SINTEF Industry) | Ytrehus, Jan David (SINTEF Industry)
Anywhere between 0%-80% of cemented wells have integrity failures, suggesting both geological and operational factors. One way geology affects cementing is via irregular wellbores, e.g. washouts. Here we study the effects of washouts on mud removal in strongly inclined wellbores, experimentally and via 2-D computational simulations, with aim of identifying key control parameters.
Experiments were performed with 2 fluids with properties representative of drilling mud and cement (or spacer), displaced at constant flow rate through a 10 m long annular flow loop. A downstream "washout" section of the annulus had an enlarged outer diameter. Twenty-four conductivity probes tracked the arrival times of the displacing fluid by measuring the conductivity of fluids as they pass. The experimental matrix includes 8 experiments with 2 eccentricities (standoff = 1, 0.58), 4 angles of inclination and slightly variable rheology. The simulation study covered wider ranges of variables. The results of simulations and experiments agree qualitatively on the main effects, as shown in [
Here we extend the study using the 2D simulation to study the effects of washout length and diameter, for both concentric and eccentric wells, all oriented horizontally. The simulations provide detailed information on the evolution of the fluid-fluid interfaces as they pass through the washout, as well as information on the velocity fields and stresses. In any near-horizontal section there is a delicate balance of buoyancy and eccentricity influences, in both regular and irregular geometries. Under some circumstances an irregular section seems to have a positive stabilizing effect on the interface. However, this positive message is balanced practically by uncertainty of washout size and in-situ mud properties and the positive effects are not universal. We find that increasing the washout diameter always appears to decrease the displacemnt efficiency, for both concentric and eccentric annuli. Increasing the washout length is less clear in its effects. In all cases, the main risk from residual drilling mud in isolated washouts is that it can contaminate the cement slurry as it passes.
Accurately simulating electromagnetic fields for oil reservoirs where conductive casings are present requires special care when building parameter models. Because casings and geological structures present in the reservoir occur at significantly different scales, from millimeters to kilometers, mesh sizes can grow to be computationally infeasible. There are many techniques currently in development to tackle this problem, such as homogenization, upscaling, multiscale and integral equations methods. Here, we present a method which couples a simple homogenization-by-averaging scheme with parameter optimization for cells which are intersected by casings. This takes into account that not all details of a borehole are known in practice and that parameter optimization is required to invert field data in any case. The proposed method maintains the low cost of homogenization schemes but improves the approximation accuracy. We demonstrate our method for a synthetic borehole model and also illustrate the dramatic effect of steel susceptiblity on the EM data.
Presentation Date: Tuesday, October 16, 2018
Start Time: 1:50:00 PM
Location: 213A (Anaheim Convention Center)
Presentation Type: Oral
3D voxel inversion sometimes produces a mysterious “ring” model when inverting strong data from a compact conductor in a resistive background. Motivated by an airborne TEM data set from a kimberlite exploration site, we design a mathematical example to understand why and when such erroneous models appear. For a time domain coincident loop-loop survey, the high sensitivity for a uniform half-space follows the shape of the well-known “smoke ring” encircling the loop location. If an inversion starts from a model that has a sensitivity ring exceeding the width of data anomaly, a ring-shaped conductor can be constructed in the model. Our remedy is to warm-start the inversion by a conductor-seeded initial model in conjunction with a uniform reference model. We find that initial models can be as simple as a vertically infinite conductive prism of a conductivity and width roughly guessed from data. At early inversion iterations, the seeding prism helps sensitivity concentrate at the location of peak data anomaly; because the reference model is set to uniform, the inversion gradually wipes out the unnecessary structure of the prism while building new structure required by the data. Eventually, the prism fades out, and the model only contains data-supported features. Field data tests show that our dual-model warm-start approach is straightforward and robust, as the final result does not change with a wild variety of the size, location, and conductivity of the seeding prism.
Presentation Date: Thursday, October 18, 2018
Start Time: 8:30:00 AM
Location: 213B (Anaheim Convention Center)
Presentation Type: Oral
ABSTRACT: The challenges of integrating a discrete fracture network (DFN) model with a geomechanical analysis increase with the complexity of the DFN model, since a DFN model can include thousands of fractures and result in very complex configurations. Although it is now relatively easy to generate DFN models, to date, comparatively limited attention has been given to the solution of the challenging issues related to optimal mesh generation routines required for geomechanical simulation. For any integrated DFNGeomechanical approach to be efficient and reliable, there is a need to carefully consider the way in which the structural data are embedded in the geomechanical model at the required engineering scale. In this context, this paper introduces a novel method (DFNCleaner), to simplify fracture networks, while maintaining their characteristic properties. We also present, an automated tool (DFNQuality) that incorporates various metrics to measure the DFN quality, and outputs a proposed “Meshability Index”, which is then used to further constrain the cleaning of the original DFN model. This paper shows example of the ability of the proposed algorithm to clean pillar DFN model, and then compares the simulation results to those obtained manually by both intermediate and experienced users of hybrid Finite-Discrete Element method codes.
ABSTRACT: One of the important applications of DFN models is the quantification of rock mass quality through rock mass classification systems such as Geological Strength Index (GSI, Hoek et al., 1995). For instance, Cai et al. (2004) presented a quantitative method to assist in the use of the GSI system for rock mass classification, introducing the concept of equivalent block volume and considering the impact of fracture persistence. However, the analysis was limited to a simple conceptual rock mass and did not account for the complex configurations in term of fracture intersections and connectivity that would arise when considering natural fracture networks. In this context, DFN models allow more realistic representation of 3D network of fractures and better prediction of block fragmentation of rock mass. This paper discusses an alternative method of quantifying the variation of GSI by using the quantification method of Cai et al. (2004) in combination with the block volume cumulative density function (CDF) plots obtained from DFN models. Since DFN approaches exploits the use of fracture data collected from mapping of exposed surfaces and boreholes, including direct physical mapping and indirect mapping by remote sensing techniques, the authors believe the proposed method would provide a better and more objective quantification of GSI variability.
ABSTRACT: The stochastic nature of the DFN process is such that there is an infinite, but equally probable, number of possible realisations of the 2D fracture systems based on the specified input parameters. Ideally, synthetic rock mass modelling should include the numerical testing of multiple DFN realizations to ensure better characterisation of the variability related to the input DFN network. Such an approach potentially requires large computational runtimes to test a minimum representative number of synthetic rock masses based on the same DFN statistics. To aid in both selection and reduction in the number of synthetic rock mass geomechanical models required, we have developed a web-based tool, DFNAnalyzer, which allows extraction of detailed information about each DFN realization including for example, areal facture intensity and density, number of fracture intersections, and fracture connectivity. As there exists in the published literature no accepted standard as to what would constitute a minimum number of DFN realizations to be used in synthetic rock mass models, the authors believe the proposed DFNAnalyzer can assist the engineer in characterizing the similarity of 2D DFN networks. For example, by plotting connectivity maps, practitioners can easily compare 2D DFN realizations and select those with the low similarity for testing within geomechanical models and thereby provide a range for the potential mechanical behavior of the rock mass. Use of the developed DFNAnalyzer will reduce the subjectivity in the selection of DFN models used in numerical analysis and hence lead to more representative modelling/design.
ABSTRACT: In deep underground mining, fault slip and associated rockbursting present a significant hazard to the safety of personnel, equipment and mine infrastructure. Recently, hydraulic injection has been proposed as a method of mitigating fault slip risk as it has the potential to trigger slip and release the built-up stress and strain energy driving slip events in advance of mining; however, injection field trials conducted to date have been met with varying degrees of success. Laboratory testing has been performed to investigate the effect of injection on the slip behavior of granite specimens containing through-going saw cuts of various geometries. Specimens containing larger asperities were observed to respond less effectively to injection treatment: shear stress releases of up to 5.9 MPa were triggered on planar fault surfaces compared to 0.7 MPa on surfaces with various sizes of asperities. 3-D imaging of post-test specimen surface characteristics indicates that variations in stress release are linked to the breakdown of surface asperities and accompanying loss of strength that occurs during shearing. As a result, field-scale fault structures that contain large cohesive elements, such as interlocking elements or rock bridges, are predicted to respond less favorably to injection treatment when compared to persistent planar structures.
Fault slip and associated rockbursting present a significant hazard to the safety of personnel and infrastructure in deep underground mines. When the advancement of a mine excavation alters the stresses acting on pre-existing geological structures, sudden, unexpected episodes of shear movement (i.e., slip) can be triggered. The energy release resulting from these events occurs much like a natural earthquake, in some cases producing seismicity with local magnitudes of up to Ml = 5.5 (Grobbelaar et al., 2017) that can result in extensive damage to mine workings.
Hydraulic injection has recently been proposed as a method of fault slip hazard mitigation, whereby pressurized water is pumped into the rock mass and used to intentionally pre-trigger shear movement, in much the same way as wastewater injection has been observed to trigger fault slip and induced seismicity (van der Elst et al., 2016). In the current application, pre-triggering of slip is performed in an effort to release the built up stress and strain energy responsible for driving future fault slip events in a controlled manner, in order to reduce the likelihood of events occurring unpredictably when personnel or equipment may be present. Several field trials have been conducted to test this effect, notably by Board et al. (1992). However, although numerical modeling conducted prior to the 1992 field trial indicated a good prognosis for the proposed injection treatment, no slip events larger than magnitude M = 0 were recorded in the field, suggesting that injection would not have a meaningful impact on reducing the frequency and severity of future slip events in this case. Hence, although hydraulic injection has the potential to activate fault movement, its effectiveness as a mine-scale hazard remediation tool remains unclear.
ABSTRACT: The excessive deformations that accompany the brittle fracturing and dilation of a highly stressed rock mass, also referred to as bulking, is a major concern for support design as it can lead to overloading and failure of the support system. Initial understanding of the dilation potential is obtained by performing conventional triaxial compression tests on intact rock. Then, guided by empirical data and engineering experience, the intact rock values are scaled to fit the dilative characteristics of the rock mass in situ. A variety of dilation models (also called flow rules in plasticity theory) have been suggested having the common assumption of σ2-independency. Key inputs for these models are largely derived from empirical curve fitting of laboratory data. Most dilation models are formulated using ψ, but ψ does not have true physical meaning. In this paper, we suggest a new model that uses the Plastic Strain Increments Ratio (PSIR) of (equation), instead of ψ. We will show that (equation) is a physically meaningful attribute that provides a mechanistic means to understand and explain the dilative characteristics of a rock mass susceptible to stress-induced fracturing and spalling. Our results suggest that the proposed flow rule is more convenient for modelling rock mass dilation, minimizing sources of parameter uncertainty, and guiding numerical modellers as to how the dilation parameters used affects the modelled rock mass behaviour, tunnel convergence, and displacement-based support design.
Yielding brittle rock around an excavation not only experiences post-peak strength loss, it experiences significant dilation. More importantly, because the brittle failure process involves the opening of stress-induced fractures propagating parallel to the excavation boundary, this dilation has a significant directional component. As evaluation of the convergence of the excavation is key to successful support design, characterization of the rock mass dilatancy and the underlying failure mechanisms become critical for modelling rock mass behaviour. Dilation Models (also known as flow rules) establish a relation between plastic strain increments in the direction of principal stresses. More specifically, when (equation) and (equation) are the components of plastic strain increments in the direction of o1, σ2 and σ3, respectively, the dilation model gives the relative incremental plastic straining in the direction of principal stresses, (equation). Dilation models assuming σ2-independence with (equation) (for asymmetric stress states of σ1 > σ2 > σ3), have been the most common dilation models in numerical investigations and practice, meaning that they only require defining the relative incremental plastic straining (equation). Instead of the ratio of (equation) dilation models use a quantity called dilation angle ψ that is related to the ratio of (equation).
ABSTRACT: Numerical methods and computing techniques are now integrated components in rock mechanics and rock engineering design, providing an opportunity to increase our fundamental understanding of the factors governing rock mass behavior. It is increasingly evident that models of rock mass behavior should incorporate realistic representation of fracture networks as well as should constitute an effective aid for the evaluation of scale-effects for those engineering problem where performing field tests at different scales is not technically or economically viable. This paper provides a discussion on proposed theoretical approach broadly adopted to study the stability of slopes that include intermittent joints. Limitations of the approach are demonstrated by showing the results of numerical analyses carried out with discontinuum (ELFEN) and continuum (PLAXIS) codes applied to the study of conceptual slope and foundation problems in fractured rock masses. The paper highlights the importance in rock engineering design of applying numerical modeling for rock bridge related problems, and emphasis is given to methods to account for rock bridge strength at the desired engineering scale.
The importance of scale effects in rock engineering design is well recognized. It is possible to directly study scale-effects associated with randomly distributed flaws in an otherwise intact rock specimen at the laboratory scale. For instance, Bieniawski and Van Heerden (1975) indicated that the unconfined compressive strength of different rock materials such as iron ore, quartz diorite and coal decreases with size, reaching constant values for samples of approximately 1.5 m edge length. This behaviour was initially explained in terms of the larger sample containing more flaws in so-called critical locations (Goodman, 1980). Cundall (2008) summarised the effect of size on strength by referring to studies by Bazant and Chen (1997) and mentioning examples of basic theories of scaling that may be responsible for the observed size effects.
However, the problem of scale-effects become quite a complex problem when dealing with larger rock mass volumes containing natural discontinuities, since it is not possible (or at least not economical) to perform field tests at different scales. In this context, numerical models provide a useful alternative to test the variation of rock mass strength with increasing sample size, as demonstrated by Elmo (2012) and Elmo et al. (2016) for compressive, indirect tension and shear loading conditions. However, numerical models may introduce an indirect form of scale effects due to the simplification required when modelling fractured rock masses, independently of whether a continuum or discontinuum modelling approach is used.